Skip to main content
NCBI home page
Journal of Neurophysiology logoLink to Journal of Neurophysiology
. 2020 Apr 15;123(5):1944–1954. doi: 10.1152/jn.00038.2020

Responses of neurons in the primary somatosensory cortex to itch- and pain-producing stimuli in rats

Sergey G Khasabov 1,✉, Hai Truong 2, Victoria M Rogness 1, Kevin D Alloway 3,4, Donald A Simone 1, Glenn J Giesler Jr 2
PMCID: PMC7444912  PMID: 32292106

Abstract

Understanding of cortical encoding of itch is limited. Injection of pruritogens and algogens into the skin of the cheek produces distinct behaviors, making the rodent cheek a useful model for understanding mechanisms of itch and pain. We examined responses of neurons in the primary somatosensory cortex by application of mechanical stimuli (brush, pressure, and pinch) and stimulations with intradermal injections of pruritic and algesic chemical of receptive fields located on the skin of the cheek in urethane-anesthetized rats. Stimuli included chloroquine, serotonin, β-alanine, histamine, capsaicin, and mustard oil. All 33 neurons studied were excited by noxious mechanical stimuli applied to the cheek. Based on mechanical stimulation most neurons were functionally classified as high threshold. Of 31 neurons tested for response to chemical stimuli, 84% were activated by one or more pruritogens/partial pruritogens. No cells were activated by all five substances. Histamine activated the greatest percentage of neurons and evoked the greatest mean discharge. Importantly, no cells were excited exclusively by pruritogens or partial pruritogens. The recording sites of all neurons that responded to chemical stimuli applied to the cheek were located in the dysgranular zone (DZ) and in deep laminae of the medial border of the vibrissal barrel fields (VBF). Therefore, neurons in the DZ/VBF of rats encode mechanical and chemical pruritogens and algogens. This cortical region appears to contain primarily nociceptive neurons as defined by responses to noxious pinching of the skin. Its role in encoding itch and pain from the cheek of the face needs further study.

NEW & NOTEWORTHY Processing of information related to itch sensation at the level of cerebral cortex is not well understood. In this first single-unit electrophysiological study of pruriceptive cortical neurons, we show that neurons responsive to noxious and pruritic stimulation of the cheek of the face are concentrated in a small area of the dysgranular cortex, indicating that these neurons encode information related to itch and pain.

Keywords: nociception, pruriception, rats, single-unit recording, somatosensory cortex

INTRODUCTION

Pruritus refers to chronic, pathological itch and is associated with many dermatological conditions such as eczema (dermatitis) and psoriasis, as well as nondermatological conditions including kidney failure, postherpetic neuralgia, AIDS, Hodgkin’s disease, and allergic reactions (Kremer et al. 2014; Lavery et al. 2016a; Reich et al. 2009). Severe pruritis is known to have an adverse impact on the quality of life (Anand 2003; Halvorsen et al. 2012; Lavery et al. 2016b; Mollanazar et al. 2016; Patel and Yosipovitch 2010). Although progress has been made in understanding how itch is signaled and encoded in the peripheral and central nervous systems (LaMotte et al. 2014), the underlying neurophysiological mechanisms remain incompletely understood. Pruriceptive primary sensory neurons and dorsal horn neurons have been identified with molecular markers including Mrgpr (Dong et al. 2001; Liu et al. 2009; Zhu et al. 2017), brain-derived neurotrophic factor (BDNF) (Dembo et al. 2018), and gastrin-releasing peptide (Mishra and Hoon 2013; Sun et al. 2009, 2017; Sun and Chen 2007). Electrophysiological studies show that itch is encoded by primary afferent nociceptors (Johanek et al. 2007, 2008; Namer et al. 2008; Pereira et al. 2015; Ringkamp and Meyer 2014), nociceptive dorsal horn neurons (Akiyama et al. 2012, 2013, 2015; Akiyama and Carstens 2014; Duan et al. 2018; Tsuda 2018), and spinal projection neurons including the spinothalamic tract (STT) (Davidson et al. 2007, 2009; Simone et al. 2004), the spinoparabrachial tract (SPbT) (Jansen and Giesler 2015), and the trigeminothalamic tract (Lipshetz and Giesler 2016; Moser and Giesler 2014a). We recently showed that many nociceptive thalamic neurons in the posterior triangular (PoT) and ventral posterior medial (VPM) nuclei were excited by pruritic stimuli (Lipshetz et al. 2018).

Human fMRI studies demonstrate that blood flow in the primary somatosensory cortex (SI) is increased after pruritic stimulation of the skin (Andersen et al. 2015; Ikoma et al. 2006; Lavery et al. 2016a; Mochizuki et al. 2017), suggesting that itch sensation is encoded in the SI along with other somesthetic modalities. Previous electrophysiological studies examined response properties of individual SI neurons activated by tactile, noxious mechanical, thermal, and algesic chemical stimuli in monkeys (Kaas et al. 1979; Kenshalo et al. 2000; Kenshalo and Isensee 1983; Merzenich et al. 1978), cats (Dykes et al. 1980; Rubel 1971), raccoons (Johnson et al. 1982), opossums (Pubols et al. 1976), squirrels (Sur et al. 1978), and rats (Chapin and Lin 1984; Guilbaud et al. 1992; Wang et al. 2010; Welker 1971). Responses of SI neurons to pruritic stimuli, however, have not been investigated.

In the present study, we characterized and compared responses of single neurons in SI to chemical pruritogenic and nociceptive stimuli applied to the skin of the cheek. We chose this area of the skin because pruriceptive and nociceptive stimuli applied to the cheek produce distinct and easily recognized behavioral responses: pruriceptive stimuli produce scratching of the cheek with the hindlimb, whereas algesic stimuli produce wiping of the cheek with the forelimbs (Klein et al. 2011; Moser and Giesler 2014b; Shimada and LaMotte 2008).

Our results show that nociceptive neurons in SI were often excited by multiple pruritogens, and many were located in the dysgranular zone (DZ) of SI (Chapin and Lin 1984), an area that had been previously described as the “unresponsive zone” (Welker 1971). Thus we located a population of SI neurons in the DZ that respond both to pruritogens and to noxious mechanical and chemical stimuli. These neurons likely contribute to production of both itch and pain.

METHODS

Adult male Sprague-Dawley rats (300–450 g) were maintained on a 12:12-h light-dark schedule and had access to food and water ad libitum. All procedures were approved by the University of Minnesota Institutional Animal Care and Use Committee.

Electrophysiological recording from neurons in SI.

Many of the methods used for in vivo electrophysiological experiments were described previously (Lipshetz et al. 2018). Rats were anesthetized with urethane (1.5 mg/kg ip; Sigma), and supplemental doses of urethane were given during the experiments as needed to maintain areflexia, which was monitored periodically by applying noxious pinches to the tail. A tracheotomy was performed to aid in breathing, both cheeks on the face were shaved, and the head was secured in a stereotaxic head holder. Cervical spinal segments C1–C2 were exposed by a laminectomy to allow placement of an electrode for orthodromic stimulation in the spinal nucleus and tract of the trigeminal nerve (V). A craniotomy was performed over the somatosensory cortex contralateral to the stimulating electrode, the dura was removed, and the cerebral cortex was covered with warm mineral oil. The method used to perform recording from a single cortical neuron is shown in Fig. 1. A stainless steel microelectrode was lowered into the area of the spinal tract (SpTV) or nucleus (SpNV) in the caudal medulla. Unipolar cathodal pulses (40–60 µA, 200 µs) were delivered as search stimuli to evoke orthodromic responses in cortical neurons. Either a glass-coated carbon fiber (0.4–1.2 MΩ; Kation Scientific, Minneapolis, MN) or a stainless steel microelectrode (~10 MΩ; FHC, Bowdoin ME) was lowered vertically in SI in the area of the contralateral DZ (Chapin and Lin 1984) and in close proximity to the medial border of the vibrissal barrel fields (VBF) in 3-µm steps with an electronic micropositioner (model 2660; Kopf, Tujunga, CA). We explored an area in the caudal DZ based on somatotopic organization of neurons that responded to stimulation of VBF and forelimb (Chapin and Lin 1984). Although the area of input from the cheek was not mapped previously, we extrapolated that an area responsive to cheek stimulation was located between VBF and the area responsive to stimulation of the forelimb [anterior-posterior (A-P): 0 to −4 mm, medial-lateral (M-L): 3.4–5 mm; (Paxinos and Watson 1998)]. Only neurons that exhibited orthodromic responses to electrical stimulation of SpTV/SpNV were studied. Criteria for the orthodromic activation were described previously (Lipshetz et al. 2018). After a neuron that responded to orthodromic activation was identified, the stimulating electrode was moved mediolaterally and dorsoventrally to determine the point at which minimal current elicited orthodromic activity. Extracellularly recorded action potentials were filtered below 100 and above 5,000 Hz, amplified (model DAM80; WPI, Worcester, MA), and digitized. Waveforms were discriminated with DAPSYS data acquisition software (http://www.dapsys.net).

Fig. 1.

Fig. 1.

Open in a new tab

Method used for identification of cortical neurons. Tip of orthodromic stimulating electrodes were placed in spinal nucleus/tract of the trigeminal nerve (V). Single units were recorded in the primary somatosensory cortex (SI). SpTV, spinal tract V; TG, trigeminal ganglion; Vc, nucleus caudalis.

Search criteria and characterization of cortical neurons.

Only orthodromically activated cortical neurons that had a mechanosensitive cutaneous receptive field located on the contralateral cheek were examined. Neurons that were excited exclusively by movement of vibrissae were not studied. The borders of the receptive fields were determined with mild pinch stimuli and sketched onto a drawing of the rat’s face. Neurons were classed functionally with mechanical stimuli. Stimuli included brushing the skin of the cheek with a soft-bristled brush, pressure produced by a weak arterial clip (7.4 bars), and noxious pinching of the skin with a stronger clip (15.4 bars). Each stimulus was applied for 10 s. Neurons that were maximally excited by brushing were classified as low-threshold (LT) neurons; neurons that were activated by brushing (>1.5 Hz over baseline activity) and maximally by pinching were classified as wide dynamic range (WDR) neurons; and neurons that were weakly activated by brushing <1.5 Hz and maximally by pinching (Dado et al. 1994) were classified as high-threshold (HT) neurons. We recently demonstrated that many thalamic neurons that have afferent input from the cheek are also activated by noxious pinch delivered over the ipsilateral forelimb and other areas of the body (Lipshetz et al. 2018). Here we determined whether cortical neurons have similar complex receptive fields that included the four paws.

Chemical stimuli.

Solutions were injected intradermally into the cheek within the receptive field of studied neurons in a volume of 10 µL with insulin syringes and a 29-gauge needle. The first solution injected was the vehicle (0.9% NaCl). The size and location in the receptive field of the bleb produced by each intradermal injection were drawn on a diagram of the rat face. All subsequent injections were given in the areas of the receptive field that did not overlap with blebs from preceding injections. Concentrations of pruritogens were as follows: serotonin (5-hydroxytryptamine, 5-HT; 47 mM), chloroquine (CQ; 100 mM), and β-alanine (β-al; 50 mM). Intradermal injection of these pruritogens in the cheek produced exclusively scratching bouts in behavioral experiments in rodents (Klein et al. 2011; Moser and Giesler 2014b; Shimada and LaMotte 2008). Injections of histamine (HA; 900 mM) and capsaicin (CAP; 1%) were also given. These pruritogens evoke both scratching and wiping in rats and are therefore considered partial pruritogens (Klein et al. 2011). In addition, we also injected mustard oil (MO; 10 µmol) in some experiments. MO induces wiping behavior, and not scratching (Klein et al. 2011), and is therefore considered a specific algogen. All substances were used in concentrations that produced maximal scratching and/or wiping behavior. In experiments in which MO was used, it was administered before CAP, which was given last. In experiments in which MO was not used, the final injection was CAP. The order of injection of the other four pruritogens/partial pruritogens was varied so that each was given as the first, second, third, or fourth injection in approximately equal numbers of experiments.

Criteria for classifying responses.

A neuron was considered to be activated by a test substance by criteria established in our previous studies of itch-responsive neurons (Davidson et al. 2007, 2009; Jansen and Giesler 2015; Lipshetz et al. 2018; Lipshetz and Giesler 2016; Moser and Giesler 2014a; Simone et al. 2004). First, the mean firing rate during one or more of five consecutive 60-s intervals after removal of the injection needle exceeded the mean spontaneous discharge rate (during the 60-s interval before insertion of the injection needle) by ≥50%. Second, the discharge rate after injection exceeded by ≥50% the mean discharge rate following injection of a vehicle solution (saline) during the corresponding 60-s period.

Activity following vehicle, CQ, β-al, HA, CAP, and MO was recorded for 5 min after the removal of the injection needle. Responses to 5-HT were determined for 15 min after injection since in our previous studies 5-HT induced the most prolonged responses of trigeminothalamic (Moser and Giesler 2014a), trigeminoparabrachial (Jansen and Giesler 2015), and thalamic (Lipshetz et al. 2018) neurons. After injection of 5-HT, discharge rates during the 6th through the 14th 60-s intervals were compared to the firing rate of the 5th 60-s interval following vehicle injection. Only one cortical neuron was studied in each experiment.

Histology.

At the end of each experiment, a small lesion was made by passing constant current through the recording electrode in the cortex and the stimulating electrode in the spinal cord for histological localization of recording and stimulation sites. Both anodal and cathodal 25-µA currents were passed for 25 s each through the stainless steel stimulating electrode in the SpNV/SpTV and for 60 s through stainless steel electrodes used for recording in the cortex. When a carbon fiber microelectrode was used for recording, 60-µA current was passed for 60 s. Rats were then euthanized by an overdose (360 mg/kg ip) of Fatal-Plus and perfused through the heart with 0.9% normal saline followed by 10% formalin containing 1% ferrocyanide solution.

The brains with two cervical spinal segments were dissected, placed in a 10% formalin-1% ferrocyanide solution for several days to mark iron deposits with the Prussian blue reaction, and cryoprotected in 30% sucrose. Coronal sections of the spinal cord (50 µm) were mounted on glass slides, and stimulating sites were identified by the location of the Prussian blue mark. The cortex was sectioned in either the coronal (n = 13) or horizontal (n = 20) plane at 75 µm. Recording sites were identified by the Prussian blue mark (recording by stainless steel microelectrode) or a small lesion (recording with carbon fiber microelectrode). Coronal sections were counterstained with cresyl violet. To map the region of DZ in preliminary anatomical studies, horizontal sections were stained by histochemical reaction for cytochrome oxidase with diaminobenzidine and cytochrome c type III according to the method described previously (Alloway et al. 2006; Land and Simons 1985; Wong-Riley 1979). For better visualization of VBF, tissue was counterstained with cresyl violet. Neurons in layer IV are a prominent feature of VBF (Land and Simons 1985; Woolsey and Van der Loos 1970; Zhang and Alloway 2006). Therefore, in horizontal sections the cortical layer was determined based on the distance between the easily recognized ventral limit of VBF layer IV and the Prussian blue mark or lesion. In coronal sections, the layer in which the recording site was located was determined based on a stereotaxic atlas of the rat brain (Paxinos and Watson 1998).

Data analysis.

Action potentials were collected and binned in 1-s intervals for display and analysis of responses following injections. The baseline rate of spontaneous activity was defined as the mean rate for a 60-s period before an injection. The time before each injection during which the skin was held with forceps to perform the intradermal injection (~20 s) was not used for analysis. Responses to chemical injections were analyzed by one-way ANOVA with Bonferroni correction.

RESULTS

Results from 33 cortical neurons, which were obtained from 33 rats, are included in this study. Each was activated orthodromically by electrical stimulation within the SpNV/SpTV and by mechanical stimulation applied to the skin of the contralateral cheek. Figure 2A shows the locations at which minimal-amplitude current pulses elicited orthodromic activity. Almost all of the responsive sites were in SpNV. Figure 2B shows the localization of cutaneous receptive fields of studied neurons. All receptive fields were located on the cheek, and only five of them extended on the vibrissa area. No receptive fields that extended on the ear or neck were found.

Fig. 2.

Fig. 2.

Open in a new tab

A: locations of lowest threshold points for orthodromic activation of cortical neurons in spinal tract V (SpTV) and nucleus (SpNV) of the contralateral medulla and spinal segment C1. Pyr Dec, pyramidal decussation. B: locations of all 33 receptive fields on the face of rats where mechanical and chemical stimuli were applied.

Functional classification of cortical neurons.

Neurons were classified functionally according to their responses evoked by innocuous and noxious mechanical stimulation of the receptive field on the contralateral cheek. All 33 neurons were nociceptive. Twenty five neurons (76%) were classed as HT (Fig. 3, A–C) and eight neurons (24%) as WDR (Fig. 3, D–F). No LT neurons were encountered.

Fig. 3.

Fig. 3.

Open in a new tab

Peristimulus histogram of responses in high-threshold (HT) and wide dynamic range (WDR) neurons to brushing (A and D), pressure (B and E), and pinch (C and F) applied on the skin of the cheek. Responses are represented as mean ± SE frequency (impulses/s) during every 1-s bin. Horizontal lines indicate 10-s period of mechanical stimulations.

Responses of cortical neurons to injection of pruritogens and partial pruritogens.

Among the 33 cortical neurons studied, we examined responses of 31 neurons evoked by injections of pruritogens and partial pruritogens into the contralateral cheek. Two HT neurons were tested by using only mechanical stimuli without injections into the cheek. Figure 4 shows the responses of a nociceptive neuron located in layer IV near the border of DZ and VBF (Fig. 4A). The receptive field was located on the contralateral cheek (Fig. 4B). The neuron was activated by brushing (mean firing rate 3.4 Hz), exhibited an increased response to pressure (5.1 Hz), responded maximally to pinch (16.0 Hz), and was classified as a WDR neuron (Fig. 4C). Figure 4, D–I, demonstrate histograms of discharge frequency before and after subcutaneous injections of vehicle, pruritogens, and partial pruritogens (1-s bins). Figure 4D demonstrates that this neuron produced little, if any, response to injection of vehicle. The neuron was activated by injections of CQ (Fig. 4E), HA (Fig. 4F), and β-al (Fig. 4G), in each case for at least 5 min after injection. It was activated for 60 s after injection of CAP (Fig. 4H) and for at least 15 min after injection of 5-HT. Injection of 5-HT produced a prolonged response that persisted for >20 min (Fig. 4I).

Fig. 4.

Fig. 4.

Open in a new tab

Example of a pruriceptive wide dynamic range (WDR) neuron recorded in dysgranular zone (DZ). A: photomicrograph of coronal section with the recording site. The lesion of the recording site is indicated by an arrow. Arrowheads point out medial and lateral borders of DZ. B: striped area indicates receptive field on the contralateral cheek. Injection sites for each pruritogen are shown by circles with colors corresponding to colors of individual histograms in D–I. C: periods of 10 s brushing, pressure, and pinching of the skin on the receptive field are indicated by horizontal lines, with mean discharge frequencies during stimulations below lines. D–I: histograms of responses induced by injections of vehicle (D) or pruritogens/partial pruritogens (E–I) into the skin of the cheek. Needle insertion and removal are indicated by 2 vertical arrows, where the discharge rates are displayed as a lighter color in each histogram. Because placement on the tip of the needle in subcutaneous space in some injection sites was more challenging than in others, the duration of injections were not constant. The “plus” sign in parentheses indicates response to agent following injection. In each histogram, 5 superimposed action potentials are shown at the time point indicated by the diagonal arrow. CAP, capsaicin; CQ, chloroquine; HA, histamine; 5-HT, 5-hydroxytryptamine (serotonin); Veh, vehicle; β-al, β-alanine.

Of the 31 neurons tested with pruritogens/partial pruritogens, 25 (81%) were pruriceptive, including 20 of 23 HT neurons (87% of all tested HT units) and 5 of 8 WDR neurons (63% of all tested WDR units). Six neurons (19%) failed to respond to any of the pruritic or algesic chemical stimuli (3 HT and 3 WDR neurons). We evaluated whether responses to various agents were affected by the order of presentation. Figure 5A shows that the mean response frequency evoked by the different chemical injections did not vary with the order of presentation. Responses to the partial pruritogen CAP were not included in the analysis since it was always injected last (fifth), or second to last in experiments in which MO was tested.

Fig. 5.

Fig. 5.

Open in a new tab

Responses to chemical stimuli. A: mean (±SE) response frequency evoked by all pruritogens/partial pruritogens (excluding capsaicin) when they were given first, second, third, and fourth in order of injections. No significant differences were detected, indicating that the order of presentation did not affect responses (1-way ANOVA with Bonferroni correction). B: % of all 31 cortical neurons that were activated after injections of each agent. C: % of neurons that responded to any pruritogens/partial pruritogens; 0 indicates proportion of nonpruriceptive neurons and 1, 2, 3, 4, or 5 proportions of neurons that responded to corresponding number of pruritogens/partial pruritogens.

The partial pruritogen HA activated the greatest percentage of cortical neurons (49%). Injection of β-al excited 39% of neurons tested, CQ activated 32%, CAP activated 29%, and 5-HT excited 23% (Fig. 5B). Nearly one quarter of pruriceptive neurons (7 cells) responded to injection of only one pruritogen. Of these seven cells, three were activated exclusively by injection of HA, two were excited only by injection of CQ, and two were excited only by injection of β-al.

Most neurons (77%) were activated by more than one pruritogen/partial pruritogen. Among the 31 neurons tested, 16% (5 neurons) did not respond to any pruritogen/partial pruritogen, 23% (7 neurons) responded to one pruritogen/partial pruritogen, 39% (12 neurons) responded to two substances, 10% (3 neurons) responded to three pruritogens/partial pruritogens, and 13% (4 neurons) were excited by four substances. None of the examined neurons responded to all five substances (Fig. 5C).

Responses of cortical neurons to injection of the specific algogen MO.

Nineteen cortical neurons (15 HT and 4 WDR units) were examined for their responses to injection of MO into the skin of the cheek. Of these neurons, six (32%) were activated (see Fig. 5B). Among all algogen-responsive neurons, five (83%) were also activated by one or more pruritogens/partial pruritogens, which were injected before MO. Only one neuron (17%) responded to MO but not to any other chemical.

Mean responses of neurons that were activated by each chemical were compared to responses evoked after injection of vehicle. Ongoing spontaneous activity before each injection was subtracted from each response. Mean responses evoked by the chemical stimuli were compared to a response evoked in the same neuron by vehicle by two-way ANOVA with repeated measures. All substances evoked significantly stronger activation of SI neurons compared with vehicle. Significance of statistical comparisons is indicated for each histogram in Fig. 6.

Fig. 6.

Fig. 6.

Open in a new tab

Peristimulus time histograms illustrate mean responses of cortical neurons to each tested agent compared with vehicle injection in the same experiment (black). Mean baseline activity during 60 s before injections of an agent or vehicle is subtracted from respective response: chloroquine (A), histamine (B), β-alanine (C), capsaicin (D), mustard oil (E), and serotonin (F). Numbers of neurons that responded to each agent are indicated above histograms. Bin size = 5 s. Each agent induced responses that were statistically significantly higher than vehicle (2-way repeated-measures ANOVA with Bonferroni correction). F and P values are indicated above each histogram. In F, serotonin was compared with vehicle during first 5 min after injection. Dashed line indicates mean firing rate during 5th min after injection of vehicle.

Location of recording sites for cortical neurons.

Neurons in SI that met our search criteria were located in DZ and in deep laminae of the medial portion of VBF. Stereotaxic coordinates for all neurons are indicated in Table 1. The cortex containing recording sites from the initial 20 experiments was sectioned in the horizontal plane. We reasoned that sectioning the cortex in this plane would help us in determining the locations of recording sites in relation to the VBF and the rest of the rat somatotopic cortex. Figure 7 shows the localization of recording sites in the horizontal plane for neurons located in DZ and the pruritogen to which each cell responded or did not respond. The approximate depth of recording sites below layer IV is shown in the individual pie charts. Figure 8 shows the functional classification and recording location of these neurons, also in the horizontal plane.

Table 1.

Location of recording site for each neuron according to stereotaxic coordinates from bregma

Coordinates, mm
Neuron Number Neuron Type A-P M-L
1 HT 0.0 5.0
2 WDR −4.0 5.0
3 HT −2.0 4.6
4 HT −1.8 5.0
5 HT −2.0 4.5
6 WDR −1.1 4.5
7 HT −1.0 4.2
8 WDR −1.6 4.6
9 HT −1.8 4.0
10 HT −2.9 3.8
11 HT −1.8 4.1
12 HT −1.5 4.4
13 WDR −1.5 4.0
14 HT −1.0 4.0
15 HT −1.8 4.0
16 HT −1.5 3.6
17 WDR −1.8 3.6
18 HT −1.5 4.0
19 HT −1.5 4.0
20 WDR −1.8 4.0
21 HT −1.5 3.4
22 HT −1.6 3.7
23 HT −1.6 3.8
24 WDR −1.2 4.0
25 HT −2.0 3.7
26 HT −1.8 4.0
27 HT −1.8 4.0
28 WDR −1.9 4.0
29 HT −1.0 4.0
30 HT −1.5 3.6
31 HT −1.2 4.0
32 HT −1.2 3.7
33 HT −1.8 3.9
Open in a new tab

A-P, anterior-posterior; HT, high threshold; M-L, medial-lateral; WDR, wide dynamic range.

Fig. 7.

Fig. 7.

Open in a new tab

Location of recording points in the dysgranular zone (DZ) as determined in horizontal sections of the cortex (n = 20). A: photomicrograph of horizontal section of cortex stained for cytochrome oxidase and counterstained with cresyl violet. Vibrissal barrel fields (VBF) can be seen in center of image, lateral to DZ. B: reconstruction illustrating location of recording points. Each recording point is represented by a pie chart. Each portion of the pie charts represents that individual cell’s ability to respond to injection of pruritogen/partial pruritogen or algogen into contralateral cheeks. C: enlarged image of area indicated by rectangle in B. Note that the recording depth in or below layer IV is reflected in the diameter of the pie chart (smallest represents those recorded in layer IV, largest in layer VI). Spatial arrangement of the cortex surface: medial is lower part of the figure; rostral is the left portion of the figure. In C, a–e refer to the rows of vibrissal barrels. CAP, capsaicin; CQ, chloroquine; HA, histamine; 5-HT, 5-hydroxytryptamine (serotonin); MO, mustard oil; β-al, β-alanine.

Fig. 8.

Fig. 8.

Open in a new tab

Localization of recording sites for high-threshold (HT) and wide dynamic range (WDR) neurons. Nociceptive neurons were in deep layers (IV–V). DZ, dysgranular zone; VBF, vibrissal barrel fields. a–e refer to the rows of vibrissal barrels. Scale is as in Fig. 7.

We found it difficult to determine the precise recording depth or cortical layer with this histological approach. Therefore, the cortex from the final 13 experiments was sectioned in the coronal plane. This approach allowed a more precise determination of the cortical layer in which recordings were made (Fig. 9). All recording sites that were identified histologically were located in deep cortical layers: 8 (27%) were in layer IV and 22 (73%) in layers V–VI.

Fig. 9.

Fig. 9.

Open in a new tab

Reconstruction of recording sites in coronal section. A–C: photomicrographs showing recording points for 3 examined neurons. Arrowheads indicate medial borders of dysgranular zone in each example. Arrows indicate recording points. D and E: reconstructions illustrating location of recording sites and responses to pruritogens and algogen injected into the contralateral cheek and symbolized by pie charts. Key for pie chart is at bottom. Each portion of the pie charts represents the ability of the neuron to respond to an agent or not. F and G: reconstructions of recording points of functionally classified neurons. In D–G, arrowheads indicate medial and lateral borders of vibrissal barrel fields. CAP, capsaicin; CQ, chloroquine; HA, histamine; HT, high-threshold neuron; 5-HT, 5-hydroxytryptamine (serotonin); MO, mustard oil; WDR, wide dynamic range neuron; β-al, β-alanine.

DISCUSSION

The present study is a continuation of our project to determine and compare neural encoding of itch and pain at different levels of the central nervous system. Previously we evaluated ascending transmission of itch to the parabrachial nuclei and thalamus by recording electrophysiological responses of single trigeminoparabrachial (Jansen and Giesler 2015), spinothalamic (Davidson et al. 2007, 2009, 2012) and trigeminothalamic tract (Moser and Giesler 2014a) neurons evoked by pruritic stimuli. We also recently determined responses of thalamic neurons to pruritic stimuli (Lipshetz et al. 2018). Using our search approach and criteria we have found that the majority of examined neurons in the spinal cord and thalamus were excited by both pruritic and nociceptive stimuli.

We determined properties of responses of SI neurons evoked by injection of pruritogens and an algogen into the cheek. Neurons located in the caudal part of DZ and deep laminae of the medially adjusted portion of VBF in the SI were excited by pruritogens and partial pruritogens. All 33 cortical neurons studied that were orthodromically activated by electrical stimulation in the contralateral SpNV or SpTV were nociceptive, since they were excited by noxious mechanical stimuli. All neurons were located in deep cortical layers (IV–VI), with the majority (73%) located in layers V–VI. These findings are in line with previously published data, indicating that nociceptive neurons in SI that have receptive fields on body, limbs, or tail of the rat are located exclusively in deep layers (Guilbaud et al. 1992; Lamour et al. 1982). The majority (25 or 75.8%) of the neurons examined in this study were functionally classified as HT neurons, whereas 8 (24.2%) were WDR neurons based on their responses to graded mechanical stimuli applied to their receptive fields. The majority of examined nociceptive neurons also responded to intradermal injection of pruritogens. Of 31 cortical neurons tested with chemical injections, 83% (26 neurons) were excited by both mechanical and pruriceptive chemical stimuli. Only five (3 HT and 2 WDR neurons) were insensitive to all chemical stimuli employed in this study. Thus the neurons studied in SI, like those in the spinal cord and thalamus, did not exhibit specificity in their responses to pruritic stimuli. It is unlikely that changing the concentrations of pruritogens or the algogen (MO) would have altered the proportions of cortical neurons excited because the concentrations used were those that elicited the longest duration and highest frequencies of scratching and/or wiping behavior in awake rats (Klein et al. 2011). Therefore, the strength of these behavioral responses suggests that higher doses would not affect the proportions of neurons excited or other properties of their responses. We acknowledge that our sample is relatively small and sampling a greater number of neurons, or using a different search strategy, may in the future uncover neurons that are excited selectively by pruritic stimuli. However, neurons that respond exclusively to itch-producing stimuli have not been identified in the peripheral or central nervous system, arguing against a specificity theory for the sensation of itch.

There are interesting differences in responses of neurons in the spinal cord, thalamus, and cortex to pruritogens. For example, whereas 83% of cortical neurons were excited by pruritogens, 67% and 55% of nociceptive trigeminoparabrachial tract (VcPbT) (Jansen and Giesler 2015) and trigeminothalamic tract (VcTT) (Moser and Giesler 2014a) neurons, respectively, were excited by pruritogens. Proportions of nociceptive thalamic neurons responsive to pruritogens (80%) were higher than those in the spinal cord and similar to those in cortex (Lipshetz et al. 2018). Among all pruritogens injected into the cheek, the partial pruritogen HA activated the greatest proportion of cortical neurons (49%), whereas HA excited a smaller proportion (27%) of VcTT neurons (Moser and Giesler 2014a) and a higher proportion (62%) of VcPbT neurons (Jansen and Giesler 2015). This suggests that VcPbT neurons play a greater role in ascending transmission of information related to HA compared with VcTT neurons. Injection of HA into the skin of the cheek activated 70% of neurons in thalamus (Lipshetz et al. 2018), proportions similar to VcPbT neurons (62%), but in the present study only 49% of cortical neurons responded to HA.

Although the proportion of neurons in thalamus responsive to pruritogens and the duration of their responses (Lipshetz et al. 2018) were similar to those in the DZ/VBF area of cortex, the maximal discharge rates evoked by pruritogens were lower in cortical neurons. For example, the peak discharge frequency of neurons in DZ/VBF evoked by injection of the partial pruritogen HA, which typically evoked the greatest discharge, was less than half of that observed in the thalamus (Lipshetz et al. 2018). Lower evoked discharge rates in DZ/VBF could be a result of anesthesia, which might have a greater effect on cortical neurons than on thalamic neurons. Urethane was used in the present study, but it was also used in our study of thalamic neurons (Lipshetz et al. 2018).

One of most important aspects of anesthesia is the inhibition of cortical responses to pain. Therefore, it is possible that anesthesia with urethane affected the intensity and/or duration of responses of cortical neurons. However, since we utilized high concentrations of pruritogens and algogen, we believe the likelihood that coding properties of individual neurons to different substances is preserved. Additional studies are needed in awake animals or using different anesthetics to determine whether the lower evoked discharge rates of cortical neurons were due to urethane anesthesia and how the anesthesia affects their response properties.

An interesting difference in responses between cortical neurons, thalamic neurons, and spinothalamic or trigeminothalamic tract neurons was their responses to CAP. Injection of CAP into the cheek produced relatively mild discharge of cortical neurons in DZ/VBF compared with that evoked by HA. This contrasts with activation of thalamic neurons (Lipshetz et al. 2018) and trigeminothalamic tract neurons in rats (Moser and Giesler 2014a) in which responses to CAP were comparable with another pruritogen under urethane anesthesia. In studies of primate spinothalamic tract neurons that were performed under nitrous oxide-halothane anesthesia, neurons were more excited by CAP than HA (Simone et al. 2004). Differences in responses evoked by different partial pruritogens are probably not attributed to anesthesia but rather reflect differences in encoding of the various chemical stimuli at the same level of the central nervous system.

Another interesting difference in responses of thalamic and cortical neurons to pruritogens was the functional subtypes of neurons that were excited by pruritogens. In thalamus, the proportions of HT and WDR neurons excited by pruritogens were similar (44% and 38%, respectively), whereas the majority of pruriceptive neurons in DZ/VBF were HT neurons (76%); only 24% of pruriceptive neurons were classed as WDR neurons. Moreover, 73% of nonnociceptive low-threshold (LT) neurons in thalamus were excited by pruritogens. We did not find LT neurons in DZ/VBF with our search strategy. Our rationale for electrical stimulation of SpNV/SpTV as a search stimulus was to avoid most afferent input from low-threshold neurons that convey information about whisker movement and are located in the principal trigeminal nucleus at the level of the pons (Jacquin et al. 2015; Minnery and Simons 2003; Veinante and Deschênes 1999; Xiang et al. 2014). Thus further studies are needed to determine whether LT neurons located in SI areas adjacent to DZ/VBF are responsive to pruritogens.

A particularly novel finding of our studies is the location of nociceptive/pruriceptive cortical neurons. All neurons studied were excited by noxious mechanical stimulation of the skin on the cheek and were located in the caudal part of DZ and in the adjacent lateral area of VBF cortex. DZ is characterized by the absence of granular aggregations (Chapin and Lin 1984), and neurons in it are unresponsive to vibrissal stimulation. Our data indicate that neurons in DZ encode noxious and pruritic information from the cheek. Thus DZ appears to represent an area that contains a high number of nociceptive neurons. This differs from earlier studies reporting that nociceptive cortical neurons, which receive afferent input from the body and limbs, were intermingled with LT neurons throughout the SI and were far fewer in number than LT neurons (Kenshalo and Isensee 1983). Additional studies are needed to further characterize the functions of neurons located in DZ.

Interestingly, in DZ/VBF area neurons with complex receptive fields were not identified. In contrast, 51% of thalamic neurons within VPM and PoT that received afferent input from the contralateral cheek were activated by noxious pinch applied to the ipsilateral cheek, forepaws, or tail (Lipshetz et al. 2018). It was suggested that the presence of complex receptive fields is attributed to the convergence of inputs from other cortical areas (Chapin et al. 1987). Despite the wide occurrence of cortical interconnections within the DZ (Chapin and Lin 1984), the absence of complex receptive fields indicates that nociceptive information from the cheek is encoded by a specific group of neurons in this zone that do not receive converging input from other cortical areas. Also, the finding that all examined neurons in DZ/VBF area had receptive fields that were restricted to the cheek suggests that they do not receive input from thalamic neurons with large complex receptive fields extending over the body, as described in our previous study (Lipshetz et al. 2018). It is possible that the examined cortical neurons received input exclusively from thalamic neurons with receptive fields restricted to the face. Such neurons constituted almost half of the examined neurons in our previous study.

In summary, our data represent the first single-unit study of encoding of pruritic stimuli in the SI and are consistent with imaging studies in humans (Leknes et al. 2007; Mochizuki et al. 2007; van de Sand et al. 2018). We found that most nociceptive neurons located in the DZ/VBF area of SI were excited by both pruritic and nociceptive stimuli, as in other areas of the spinothalamic and spinoparabrachial pathways. This raises an important question of how the responses evoked by noxious and pruritic stimuli produce distinct sensations and different behavioral responses. Several theories have been proposed to describe how the central nervous system differentiates itch from pain, including specificity, pattern, response frequency, and population coding theories (Handwerker 2014). Thus far, neurons that are specifically sensitive to pruritogens have not been identified in the peripheral or central nervous system, although it is possible that using a different search strategy could identify such neurons. Another possibility is that the responses of individual neurons to pruritogens could differ from those to algogens in the pattern or frequency of their responses. The small sample of neurons that responded to MO in this study did not allow us to make such a comparison. This possibility should be evaluated in further studies in which the order of injection of pruritogens/partial pruritogens and algogens is varied to ensure that the previous injection does not affect responses to subsequent injections.

Therefore, the neurophysiological coding mechanism(s) used to distinguish pruritic and painful stimuli remains to be determined.

GRANTS

This work was supported by NIH Grants NS-089647 (G.J.G.) and NS-011471 (D.A.S.) and The Huck Institutes of Pennsylvania State University (K.D.A.).

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

S.G.K. and G.J.G. conceived and designed research; S.G.K., H.T., V.M.R., and G.J.G. performed experiments; S.G.K., H.T., V.M.R., and G.J.G. analyzed data; S.G.K., H.T., K.D.A., D.A.S., and G.J.G. interpreted results of experiments; S.G.K., H.T., and G.J.G. prepared figures; S.G.K., V.M.R., D.A.S., and G.J.G. drafted manuscript; S.G.K., K.D.A., D.A.S., and G.J.G. edited and revised manuscript; S.G.K., K.D.A., D.A.S., and G.J.G. approved final version of manuscript.

REFERENCES

  1. Akiyama T, Carstens E. Spinal coding of itch and pain. In: Itch: Mechanisms and Treatment, edited by Carstens E, Akiyama T. Boca Raton, FL: CRC Press, 2014. [PubMed] [Google Scholar]
  2. Akiyama T, Nguyen T, Curtis E, Nishida K, Devireddy J, Delahanty J, Carstens MI, Carstens E. A central role for spinal dorsal horn neurons that express neurokinin-1 receptors in chronic itch. Pain 156: 1240–1246, 2015. doi: 10.1097/j.pain.0000000000000172. [DOI] [PMC free article] [PubMed] [Google Scholar]
  3. Akiyama T, Tominaga M, Carstens MI, Carstens EE. Site-dependent and state-dependent inhibition of pruritogen-responsive spinal neurons by scratching. Eur J Neurosci 36: 2311–2316, 2012. doi: 10.1111/j.1460-9568.2012.08136.x. [DOI] [PMC free article] [PubMed] [Google Scholar]
  4. Akiyama T, Tominaga M, Davoodi A, Nagamine M, Blansit K, Horwitz A, Carstens MI, Carstens E. Roles for substance P and gastrin-releasing peptide as neurotransmitters released by primary afferent pruriceptors. J Neurophysiol 109: 742–748, 2013. doi: 10.1152/jn.00539.2012. [DOI] [PMC free article] [PubMed] [Google Scholar]
  5. Alloway KD, Lou L, Nwabueze-Ogbo F, Chakrabarti S. Topography of cortical projections to the dorsolateral neostriatum in rats: multiple overlapping sensorimotor pathways. J Comp Neurol 499: 33–48, 2006. doi: 10.1002/cne.21039. [DOI] [PubMed] [Google Scholar]
  6. Anand P. Capsaicin and menthol in the treatment of itch and pain: recently cloned receptors provide the key. Gut 52: 1233–1235, 2003. doi: 10.1136/gut.52.9.1233. [DOI] [PMC free article] [PubMed] [Google Scholar]
  7. Andersen HH, Elberling J, Arendt-Nielsen L. Human surrogate models of histaminergic and non-histaminergic itch. Acta Derm Venereol 95: 771–777, 2015. doi: 10.2340/00015555-2146. [DOI] [PubMed] [Google Scholar]
  8. Chapin JK, Lin CS. Mapping the body representation in the SI cortex of anesthetized and awake rats. J Comp Neurol 229: 199–213, 1984. doi: 10.1002/cne.902290206. [DOI] [PubMed] [Google Scholar]
  9. Chapin JK, Sadeq M, Guise JL. Corticocortical connections within the primary somatosensory cortex of the rat. J Comp Neurol 263: 326–346, 1987. doi: 10.1002/cne.902630303. [DOI] [PubMed] [Google Scholar]
  10. Dado RJ, Katter JT, Giesler GJ Jr. Spinothalamic and spinohypothalamic tract neurons in the cervical enlargement of rats. I. Locations of antidromically identified axons in the thalamus and hypothalamus. J Neurophysiol 71: 959–980, 1994. doi: 10.1152/jn.1994.71.3.959. [DOI] [PubMed] [Google Scholar]
  11. Davidson S, Zhang X, Khasabov SG, Moser HR, Honda CN, Simone DA, Giesler GJ Jr. Pruriceptive spinothalamic tract neurons: physiological properties and projection targets in the primate. J Neurophysiol 108: 1711–1723, 2012. doi: 10.1152/jn.00206.2012. [DOI] [PMC free article] [PubMed] [Google Scholar]
  12. Davidson S, Zhang X, Khasabov SG, Simone DA, Giesler GJ Jr. Relief of itch by scratching: state-dependent inhibition of primate spinothalamic tract neurons. Nat Neurosci 12: 544–546, 2009. doi: 10.1038/nn.2292. [DOI] [PMC free article] [PubMed] [Google Scholar]
  13. Davidson S, Zhang X, Yoon CH, Khasabov SG, Simone DA, Giesler GJ Jr. The itch-producing agents histamine and cowhage activate separate populations of primate spinothalamic tract neurons. J Neurosci 27: 10007–10014, 2007. doi: 10.1523/JNEUROSCI.2862-07.2007. [DOI] [PMC free article] [PubMed] [Google Scholar]
  14. Dembo T, Braz JM, Hamel KA, Kuhn JA, Basbaum AI. Primary afferent-derived BDNF contributes minimally to the processing of pain and itch. eNeuro 5: ENEURO.0402-18.2018, 2018. doi: 10.1523/ENEURO.0402-18.2018. [DOI] [PMC free article] [PubMed] [Google Scholar]
  15. Dong X, Han S, Zylka MJ, Simon MI, Anderson DJ. A diverse family of GPCRs expressed in specific subsets of nociceptive sensory neurons. Cell 106: 619–632, 2001. doi: 10.1016/S0092-8674(01)00483-4. [DOI] [PubMed] [Google Scholar]
  16. Duan B, Cheng L, Ma Q. Spinal circuits transmitting mechanical pain and itch. Neurosci Bull 34: 186–193, 2018. doi: 10.1007/s12264-017-0136-z. [DOI] [PMC free article] [PubMed] [Google Scholar]
  17. Dykes RW, Rasmusson DD, Hoeltzell PB. Organization of primary somatosensory cortex in the cat. J Neurophysiol 43: 1527–1546, 1980. doi: 10.1152/jn.1980.43.6.1527. [DOI] [PubMed] [Google Scholar]
  18. Guilbaud G, Benoist JM, Levante A, Gautron M, Willer JC. Primary somatosensory cortex in rats with pain-related behaviours due to a peripheral mononeuropathy after moderate ligation of one sciatic nerve: neuronal responsivity to somatic stimulation. Exp Brain Res 92: 227–245, 1992. doi: 10.1007/BF00227967. [DOI] [PubMed] [Google Scholar]
  19. Halvorsen JA, Dalgard F, Thoresen M, Bjertness E, Lien L. Itch and pain in adolescents are associated with suicidal ideation: a population-based cross-sectional study. Acta Derm Venereol 92: 543–546, 2012. doi: 10.2340/00015555-1251. [DOI] [PubMed] [Google Scholar]
  20. Handwerker HO. Itch hypotheses: from pattern to specificity and to population coding. In: Itch: Mechanisms and Treatment, edited by Carstens E, Akiyama T. Boca Raton, FL: CRC Press, 2014. [PubMed] [Google Scholar]
  21. Ikoma A, Steinhoff M, Ständer S, Yosipovitch G, Schmelz M. The neurobiology of itch. Nat Rev Neurosci 7: 535–547, 2006. doi: 10.1038/nrn1950. [DOI] [PubMed] [Google Scholar]
  22. Jacquin MF, Arends JJ, Renehan WE, Waite PM, Shortland PJ. Whisker-related circuitry in the trigeminal nucleus principalis: Topographic precision. Somatosens Mot Res 32: 8–20, 2015. doi: 10.3109/08990220.2014.937414. [DOI] [PubMed] [Google Scholar]
  23. Jansen NA, Giesler GJ Jr. Response characteristics of pruriceptive and nociceptive trigeminoparabrachial tract neurons in the rat. J Neurophysiol 113: 58–70, 2015. doi: 10.1152/jn.00596.2014. [DOI] [PMC free article] [PubMed] [Google Scholar]
  24. Johanek LM, Meyer RA, Friedman RM, Greenquist KW, Shim B, Borzan J, Hartke T, LaMotte RH, Ringkamp M. A role for polymodal C-fiber afferents in nonhistaminergic itch. J Neurosci 28: 7659–7669, 2008. doi: 10.1523/JNEUROSCI.1760-08.2008. [DOI] [PMC free article] [PubMed] [Google Scholar]
  25. Johanek LM, Meyer RA, Hartke T, Hobelmann JG, Maine DN, LaMotte RH, Ringkamp M. Psychophysical and physiological evidence for parallel afferent pathways mediating the sensation of itch. J Neurosci 27: 7490–7497, 2007. doi: 10.1523/JNEUROSCI.1249-07.2007. [DOI] [PMC free article] [PubMed] [Google Scholar]
  26. Johnson JI, Ostapoff EM, Warach S. The anterior border zones of primary somatic sensory (S1) neocortex and their relation to cerebral convolutions, shown by micromapping of peripheral projections to the region of the fourth forepaw digit representation in raccoons. Neuroscience 7: 915–936, 1982. doi: 10.1016/0306-4522(82)90052-5. [DOI] [PubMed] [Google Scholar]
  27. Kaas JH, Nelson RJ, Sur M, Lin CS, Merzenich MM. Multiple representations of the body within the primary somatosensory cortex of primates. Science 204: 521–523, 1979. doi: 10.1126/science.107591. [DOI] [PubMed] [Google Scholar]
  28. Kenshalo DR Jr, Isensee O. Responses of primate SI cortical neurons to noxious stimuli. J Neurophysiol 50: 1479–1496, 1983. doi: 10.1152/jn.1983.50.6.1479. [DOI] [PubMed] [Google Scholar]
  29. Kenshalo DR, Iwata K, Sholas M, Thomas DA. Response properties and organization of nociceptive neurons in area 1 of monkey primary somatosensory cortex. J Neurophysiol 84: 719–729, 2000. doi: 10.1152/jn.2000.84.2.719. [DOI] [PubMed] [Google Scholar]
  30. Klein A, Carstens MI, Carstens E. Facial injections of pruritogens or algogens elicit distinct behavior responses in rats and excite overlapping populations of primary sensory and trigeminal subnucleus caudalis neurons. J Neurophysiol 106: 1078–1088, 2011. doi: 10.1152/jn.00302.2011. [DOI] [PMC free article] [PubMed] [Google Scholar]
  31. Kremer AE, Feramisco J, Reeh PW, Beuers U, Oude Elferink RP. Receptors, cells and circuits involved in pruritus of systemic disorders. Biochim Biophys Acta 1842: 869–892, 2014. doi: 10.1016/j.bbadis.2014.02.007. [DOI] [PubMed] [Google Scholar]
  32. LaMotte RH, Dong X, Ringkamp M. Sensory neurons and circuits mediating itch. Nat Rev Neurosci 15: 19–31, 2014. doi: 10.1038/nrn3641. [DOI] [PMC free article] [PubMed] [Google Scholar]
  33. Lamour Y, Willer JC, Guilbaud G. Neuronal responses to noxious stimulation in rat somatosensory cortex. Neurosci Lett 29: 35–40, 1982. doi: 10.1016/0304-3940(82)90360-3. [DOI] [PubMed] [Google Scholar]
  34. Land PW, Simons DJ. Cytochrome oxidase staining in the rat SmI barrel cortex. J Comp Neurol 238: 225–235, 1985. doi: 10.1002/cne.902380209. [DOI] [PubMed] [Google Scholar]
  35. Lavery MJ, Kinney MO, Mochizuki H, Craig J, Yosipovitch G. Pruritus: an overview. What drives people to scratch an itch? Ulster Med J 85: 164–173, 2016a. [PMC free article] [PubMed] [Google Scholar]
  36. Lavery MJ, Stull C, Kinney MO, Yosipovitch G. Nocturnal pruritus: the battle for a peaceful night’s sleep. Int J Mol Sci 17: 425, 2016b. doi: 10.3390/ijms17030425. [DOI] [PMC free article] [PubMed] [Google Scholar]
  37. Leknes SG, Bantick S, Willis CM, Wilkinson JD, Wise RG, Tracey I. Itch and motivation to scratch: an investigation of the central and peripheral correlates of allergen- and histamine-induced itch in humans. J Neurophysiol 97: 415–422, 2007. doi: 10.1152/jn.00070.2006. [DOI] [PubMed] [Google Scholar]
  38. Lipshetz B, Giesler GJ Jr. Effects of scratching and other counterstimuli on responses of trigeminothalamic tract neurons to itch-inducing stimuli in rats. J Neurophysiol 115: 520–529, 2016. doi: 10.1152/jn.00326.2015. [DOI] [PMC free article] [PubMed] [Google Scholar]
  39. Lipshetz B, Khasabov SG, Truong H, Netoff TI, Simone DA, Giesler GJ Jr. Responses of thalamic neurons to itch- and pain-producing stimuli in rats. J Neurophysiol 120: 1119–1134, 2018. doi: 10.1152/jn.00264.2018. [DOI] [PMC free article] [PubMed] [Google Scholar]
  40. Liu Q, Tang Z, Surdenikova L, Kim S, Patel KN, Kim A, Ru F, Guan Y, Weng HJ, Geng Y, Undem BJ, Kollarik M, Chen ZF, Anderson DJ, Dong X. Sensory neuron-specific GPCR Mrgprs are itch receptors mediating chloroquine-induced pruritus. Cell 139: 1353–1365, 2009. doi: 10.1016/j.cell.2009.11.034. [DOI] [PMC free article] [PubMed] [Google Scholar]
  41. Merzenich MM, Kaas JH, Sur M, Lin CS. Double representation of the body surface within cytoarchitectonic areas 3b and 1 in “SI” in the owl monkey (Aotus trivirgatus). J Comp Neurol 181: 41–73, 1978. doi: 10.1002/cne.901810104. [DOI] [PubMed] [Google Scholar]
  42. Minnery BS, Simons DJ. Response properties of whisker-associated trigeminothalamic neurons in rat nucleus principalis. J Neurophysiol 89: 40–56, 2003. doi: 10.1152/jn.00272.2002. [DOI] [PubMed] [Google Scholar]
  43. Mishra SK, Hoon MA. The cells and circuitry for itch responses in mice. Science 340: 968–971, 2013. doi: 10.1126/science.1233765. [DOI] [PMC free article] [PubMed] [Google Scholar]
  44. Mochizuki H, Sadato N, Saito DN, Toyoda H, Tashiro M, Okamura N, Yanai K. Neural correlates of perceptual difference between itching and pain: a human fMRI study. Neuroimage 36: 706–717, 2007. doi: 10.1016/j.neuroimage.2007.04.003. [DOI] [PubMed] [Google Scholar]
  45. Mochizuki H, Schut C, Nattkemper LA, Yosipovitch G. Brain mechanism of itch in atopic dermatitis and its possible alteration through non-invasive treatments. Allergol Int 66: 14–21, 2017. doi: 10.1016/j.alit.2016.08.013. [DOI] [PubMed] [Google Scholar]
  46. Mollanazar NK, Smith PK, Yosipovitch G. Mediators of chronic pruritus in atopic dermatitis: getting the itch out? Clin Rev Allergy Immunol 51: 263–292, 2016. doi: 10.1007/s12016-015-8488-5. [DOI] [PubMed] [Google Scholar]
  47. Moser HR, Giesler GJ Jr. Characterization of pruriceptive trigeminothalamic tract neurons in rats. J Neurophysiol 111: 1574–1589, 2014a. doi: 10.1152/jn.00668.2013. [DOI] [PMC free article] [PubMed] [Google Scholar]
  48. Moser HR, Giesler GJ Jr. Itch elicited by intradermal injection of serotonin, intracisternal injection of morphine, and their synergistic interactions in rats. Neuroscience 274: 119–127, 2014b. doi: 10.1016/j.neuroscience.2014.05.025. [DOI] [PMC free article] [PubMed] [Google Scholar]
  49. Namer B, Carr R, Johanek LM, Schmelz M, Handwerker HO, Ringkamp M. Separate peripheral pathways for pruritus in man. J Neurophysiol 100: 2062–2069, 2008. doi: 10.1152/jn.90482.2008. [DOI] [PMC free article] [PubMed] [Google Scholar]
  50. Patel T, Yosipovitch G. Therapy of pruritus. Expert Opin Pharmacother 11: 1673–1682, 2010. doi: 10.1517/14656566.2010.484420. [DOI] [PMC free article] [PubMed] [Google Scholar]
  51. Paxinos G, Watson C. The Rat Brain in Stereotaxic Coordinates (4th ed.). New York: Academic Press, 1998. [Google Scholar]
  52. Pereira PJ, Machado GD, Danesi GM, Canevese FF, Reddy VB, Pereira TC, Bogo MR, Cheng YC, Laedermann C, Talbot S, Lerner EA, Campos MM. GRPR/PI3Kγ: partners in central transmission of itch. J Neurosci 35: 16272–16281, 2015. doi: 10.1523/JNEUROSCI.2310-15.2015. [DOI] [PMC free article] [PubMed] [Google Scholar]
  53. Pubols BH Jr, Pubols LM, DiPette DJ, Sheely JC. Opossum somatic sensory cortex: a microelectrode mapping study. J Comp Neurol 165: 229–245, 1976. doi: 10.1002/cne.901650208. [DOI] [PubMed] [Google Scholar]
  54. Reich A, Ständer S, Szepietowski JC. Drug-induced pruritus: a review. Acta Derm Venereol 89: 236–244, 2009. doi: 10.2340/00015555-0650. [DOI] [PubMed] [Google Scholar]
  55. Ringkamp M, Meyer R. Pruriceptors. In: Itch: Mechanisms and Treatment, edited by Carstens E, Akiyama T. Boca Raton, FL: CRC Press, 2014. [Google Scholar]
  56. Rubel EW. A comparison of somatotopic organization in sensory neocortex of newborn kittens and adult cats. J Comp Neurol 143: 447–480, 1971. doi: 10.1002/cne.901430404. [DOI] [PubMed] [Google Scholar]
  57. Shimada SG, LaMotte RH. Behavioral differentiation between itch and pain in mouse. Pain 139: 681–687, 2008. doi: 10.1016/j.pain.2008.08.002. [DOI] [PMC free article] [PubMed] [Google Scholar]
  58. Simone DA, Zhang X, Li J, Zhang JM, Honda CN, LaMotte RH, Giesler GJ Jr. Comparison of responses of primate spinothalamic tract neurons to pruritic and algogenic stimuli. J Neurophysiol 91: 213–222, 2004. doi: 10.1152/jn.00527.2003. [DOI] [PubMed] [Google Scholar]
  59. Sun S, Xu Q, Guo C, Guan Y, Liu Q, Dong X. Leaky gate model: intensity-dependent coding of pain and itch in the spinal cord. Neuron 93: 840–853.e5, 2017. doi: 10.1016/j.neuron.2017.01.012. [DOI] [PMC free article] [PubMed] [Google Scholar]
  60. Sun YG, Chen ZF. A gastrin-releasing peptide receptor mediates the itch sensation in the spinal cord. Nature 448: 700–703, 2007. doi: 10.1038/nature06029. [DOI] [PubMed] [Google Scholar]
  61. Sun YG, Zhao ZQ, Meng XL, Yin J, Liu XY, Chen ZF. Cellular basis of itch sensation. Science 325: 1531–1534, 2009. doi: 10.1126/science.1174868. [DOI] [PMC free article] [PubMed] [Google Scholar]
  62. Sur M, Nelson RJ, Kaas JH. The representation of the body surface in somatosensory area I of the grey squirrel. J Comp Neurol 179: 425–449, 1978. doi: 10.1002/cne.901790211. [DOI] [PubMed] [Google Scholar]
  63. Tsuda M. Astrocytes in the spinal dorsal horn and chronic itch. Neurosci Res 126: 9–14, 2018. doi: 10.1016/j.neures.2017.08.010. [DOI] [PubMed] [Google Scholar]
  64. van de Sand MF, Menz MM, Sprenger C, Büchel C. Nocebo-induced modulation of cerebral itch processing—an fMRI study. Neuroimage 166: 209–218, 2018. doi: 10.1016/j.neuroimage.2017.10.056. [DOI] [PubMed] [Google Scholar]
  65. Veinante P, Deschênes M. Single- and multi-whisker channels in the ascending projections from the principal trigeminal nucleus in the rat. J Neurosci 19: 5085–5095, 1999. doi: 10.1523/JNEUROSCI.19-12-05085.1999. [DOI] [PMC free article] [PubMed] [Google Scholar]
  66. Wang N, Zhang Y, Wang JY, Gao G, Luo F. Effects of pentobarbital anesthesia on nociceptive processing in the medial and lateral pain pathways in rats. Neurosci Bull 26: 188–196, 2010. doi: 10.1007/s12264-010-0150-x. [DOI] [PMC free article] [PubMed] [Google Scholar]
  67. Welker C. Microelectrode delineation of fine grain somatotopic organization of (SmI) cerebral neocortex in albino rat. Brain Res 26: 259–275, 1971. doi: 10.1016/S0006-8993(71)80004-5. [DOI] [PubMed] [Google Scholar]
  68. Wong-Riley M. Changes in the visual system of monocularly sutured or enucleated cats demonstrable with cytochrome oxidase histochemistry. Brain Res 171: 11–28, 1979. doi: 10.1016/0006-8993(79)90728-5. [DOI] [PubMed] [Google Scholar]
  69. Woolsey TA, Van der Loos H. The structural organization of layer IV in the somatosensory region (S I) of mouse cerebral cortex. The description of a cortical field composed of discrete cytoarchitectonic units. Brain Res 17: 205–242, 1970. doi: 10.1016/0006-8993(70)90079-X. [DOI] [PubMed] [Google Scholar]
  70. Xiang C, Arends JJ, Jacquin MF. Whisker-related circuitry in the trigeminal nucleus principalis: ultrastructure. Somatosens Mot Res 31: 141–151, 2014. doi: 10.3109/08990220.2014.905469. [DOI] [PubMed] [Google Scholar]
  71. Zhang M, Alloway KD. Intercolumnar synchronization of neuronal activity in rat barrel cortex during patterned airjet stimulation: a laminar analysis. Exp Brain Res 169: 311–325, 2006. doi: 10.1007/s00221-005-0152-5. [DOI] [PubMed] [Google Scholar]
  72. Zhu Y, Hanson CE, Liu Q, Han L. Mrgprs activation is required for chronic itch conditions in mice. Itch (Phila) 2: e9, 2017. doi: 10.1097/itx.0000000000000009. [DOI] [PMC free article] [PubMed] [Google Scholar]

Articles from Journal of Neurophysiology are provided here courtesy of American Physiological Society

RESOURCES