Transgenic overexpression of galanin in the dorsal root ganglia modulates pain-related behavior

Abstract

The neuropeptide galanin is expressed in the dorsal root ganglia (DRG) and spinal cord and is thought to be involved in the modulation of pain processing. However, its mechanisms of action are complex and poorly understood, as both facilitatory and inhibitory effects have been described. To understand further the role played by galanin in nociception, we have generated two transgenic lines that overexpress galanin in specific populations of primary afferent DRG neurons in either an inducible or constitutive manner. In the first line, a previously defined enhancer region from the galanin locus was used to target galanin to the DRG (Gal-OE). Transgene expression recapitulates the spatial endogenous galanin distribution pattern in DRG neurons and markedly overexpresses the peptide in the DRG after nerve injury but not in the uninjured state. In the second line, an enhancer region of the c-Ret gene was used to constitutively and ectopically target galanin overexpression to the DRG (Ret-OE). The expression of this second transgene does not alter significantly after nerve injury. Here, we report that intact Ret-OE, but not Gal-OE, animals have significantly elevated mechanical and thermal thresholds. After nerve damage, using a spared nerve-injury model, mechanical allodynia is attenuated markedly in both the Gal-OE and Ret-OE mice compared with WT controls. These results support an inhibitory role for galanin in the modulation of nociception both in intact animals and in neuropathic pain states.

The neuropeptide galanin is widely distributed within the somatosensory system, where it has been implicated in the modulation of nociception (reviewed in refs. 1–3). In the adult dorsal root ganglia (DRG), galanin is expressed at relatively low levels in <5% of neurons that are predominantly small-diameter C fiber type (4). Expression of the peptide also is detected in the primary afferent terminals of the dorsal spinal cord and in subsets of dorsal horn interneurons (5). Galanin mRNA and peptide levels markedly increase in the DRG after axotomy and in several neuropathic pain models of sciatic nerve injury (4, 6–9). After axotomy, galanin is expressed in ≈40–50% of all DRG neurons (4, 10). In the dorsal horn, axotomy-induced galanin immunoreactivity increases in the afferent terminals, with a concurrent enhancement of synaptic release of the peptide (11).

The role of galanin in pain signaling is complex (reviewed in refs. 2, 12, and 13). Electrophysiological and behavioral studies in the intact, uninjured peripheral nervous system have demonstrated both facilitatory (14–21) and inhibitory (22, 23) effects of exogenously applied galanin on nociception, sometimes in a modality-specific manner (14, 16, 20, 21), tending toward inhibition at higher doses.

A number of groups have used rodent models of neuropathic pain to study the role of galanin in the modulation of chronic pain behavior. The oldest and least understood pain model involves the scoring of autotomy behavior (self-mutilation of the denervated limb) after sciatic nerve axotomy (24). More recently, several neuropathic pain models have been developed in which the sciatic nerve is only partially denervated, including chronic constriction injury (the Bennett model; ref. 25), partial nerve ligation (the Seltzer model; ref. 26), spinal nerve ligation (the Chung model; ref. 27), photochemically induced nerve ischemia (28, 29), and, most recently, spared nerve injury (SNI) (30). Pain behavior in these models is analogous to that observed in a number of human neuropathic pain conditions, including allodynia, hyperalgesia, and expansion of the receptive field. Electrophysiological experiments on axotomized (23, 31), chronic constriction injury (32), and spinal nerve ligation (15) animals have shown that intrathecal administration of galanin reduces spinal cord excitability. Furthermore, intrathecal administration of the galanin antagonist M35 (31) significantly potentiates the frequency and severity of axotomy-induced autotomy behavior (33). Similarly, several studies have shown that intrathecal galanin significantly reduces allodynia induced by chronic constriction injury (6, 34, 35). In addition, Eaton et al. (36) have shown that a spinal cord implant of genetically modified cells that secrete galanin significantly reduces allodynia after chronic constriction injury.

In summary, the precise role played by galanin in the intact peripheral nervous system remains unclear, with both facilitatory and inhibitory effects described. After nerve injury, when levels of endogenous galanin are high, the general consensus is that the peptide plays a predominantly inhibitory function in spinal cord transmission and acts to reduce neuropathic pain behavior.

To study further the role played by galanin in the adaptive response of the nervous system to injury, we previously generated and characterized mice carrying a targeted deletion of the galanin gene (16, 37, 38). Uninjured galanin mutant animals have decreased thermal and mechanical thresholds, supporting the proposed inhibitory role for galanin in nociception. Unexpectedly, the galanin mutant mice have a complete absence of the autotomy behavior that normally is seen in WT animals after axotomy, apparently contradicting the previous findings by Verge et al. in rats when using the M35 galanin antagonist (33). Further, we have shown that these mutants have markedly reduced thermal and mechanical allodynia in the partial nerve ligation model when compared with WT controls (16). Most recently, electrophysiological studies have shown that the development of central sensitization is impaired in the mutants; both windup and facilitation of spinal reflexes after conditioning stimulation are attenuated significantly after peripheral nerve injury (39). These data raise the possibility that galanin may play an excitatory role in states of nerve injury, despite the previous body of evidence to the contrary.

Here, we describe two lines of transgenic mice. The first line inducibly overexpresses the peptide in the DRG after nerve injury and the second constitutively and ectopically overexpresses galanin in the DRG and spinal cord. Nociceptive behavior in intact transgenic animals and after a SNI model of neuropathic pain (30) is described and compared with the galanin mutant mice. These results support an inhibitory role for galanin in the modulation of nociception both in intact animals and in neuropathic pain states.

Methods

Generation of Mice.

The generation of galanin null mutant mice (maintained on the inbred 129OlaHsd strain) has been described (38). The mutation results in the deletion of five of the six exons from the galanin gene, including the entire coding region of the peptide.

Transgenic mice (CBA/B6 strain) that inducibly overexpress galanin in the DRG after nerve injury were generated by using a transgene corresponding to an ≈20-kb upstream enhancer region of the murine galanin locus and the 4.6-kb galanin-coding region, as described recently (40).

Transgenic mice (CBA/B6 strain) that constitutively overexpress galanin in the DRG were generated by using the previously described 12-kb flanking regulatory sequences of the murine c-Ret gene (41) to drive expression of the 4.6-kb galanin-coding region, inserted at the unique SacII site.

In all experiments, mutant and transgenic animals were compared with WT controls on the identical strain background.

Surgery.

Age-matched adult mice (10–12 weeks old, 25–30 g) were used in all experiments (n = 8 per genotype). Animals were fed standard chow and water ad libitum. All experiments were carried out in accordance with the United Kingdom Animals (Scientific Procedures) Act 1986.

Mice were anesthetized with Hypnorm (0.315 mg/ml fentanyl citrate + 10 mg/ml fluanisone; Janssen)/Hypnovel (5 mg/ml midazolam; Roche Applied Sciences)/water at a ratio of 1:1:2 at 4 μl/g. A modification of the spared nerve injury model, originally described by Decosterd and Woolf (30), was used as a model of neuropathic pain. An incision was made in the lateral right hind leg just above the level of the knee, exposing the three terminal branches of the sciatic nerve: the common peroneal, tibial, and sural nerves. The common peroneal and sural nerves were ligated tightly with 7/0 silk and sectioned distal to the ligation removing ≈2 mm of distal nerve stump. The tibial branch remained untouched during the procedure. The overlying muscle and skin was sutured, and the animals were allowed to recover. In sham-operated animals, the sciatic nerve branches were exposed but not lesioned.

Behavioral Testing.

In all tests, the examiner was blind to the genotype of the mice. Thermal thresholds were measured according to the method of Hargreaves et al. (42). Animals were habituated to the testing environment for at least 2 h, and testing was performed on two separate occasions before surgery. The hind paws were exposed to a beam of radiant heat through a transparent perspex holding box (Ugo Basile, Varese, Italy), and the latency of withdrawal was recorded automatically. Each hind paw was measured three times, with an interval of at least 5 min between measurements.

Mechanical thresholds were measured with a series of calibrated von Frey filaments (Stoelting) from 0.005 g to a maximum of 3.63 g. Animals were put in perspex enclosures placed on an elevated grid (Ugo Basile) and habituated for at least 2 h before testing. Mechanical sensitivity was assessed on each hind paw, employing the up-down testing paradigm to determine the threshold force required to elicit a withdrawal response to 50% of stimulations (43, 44).

Immunohistochemistry.

Mice were intracardially perfused with 4% paraformaldehyde/PBS, and the spinal columns were removed and postfixed for 4 h at room temperature. The spinal cord and L4 and L5 DRGs were dissected and equilibrated in 20% sucrose overnight at 4°C, embedded in OCT mounting medium, frozen on dry ice, and cryostat-sectioned (16-μM sections for DRG and 30-μM sections for spinal cord). Sections were blocked and permeabilized in 10% normal donkey serum/PBS 0.2% Triton X-100 (PBST) for 1 h at room temperature. Sections then were incubated in rabbit polyclonal antibody to galanin (Affiniti, Nottingham, U.K.) at 1:1,000 in PBST overnight at room temperature, washed 3 × 10 min in PBS, and incubated in donkey anti-rabbit cy3 (The Jackson Laboratory) at 1:800 for 3 h at room temperature. Where appropriate, sections were washed as before and then incubated in biotin-conjugated isolectin-B4 (IB4) at 10 μg/ml (Sigma) diluted in PBST + 0.1 mM CaCl₂, 0.1 mM MgCl₂, and 0.1 mM MnCl₂, followed by extravidin-FITC at 1:100 (Sigma). After washing, sections were mounted in VECTASHIELD (Vector Laboratories). Images were taken by using a Leica fluorescent microscope with RT Color Spot camera and spot advance image-capture system software (Diagnostic Instruments, Sterling Heights, MI).

Statistics.

Data are presented as the mean ± SEM. Student's t test was used to analyze the difference in baseline thermal withdrawal thresholds. ANOVAs and Dunnett's or nonparametric Mann–Whitney U post hoc tests were used as appropriate to analyze differences between genotypes and at different time points after SNI. A P value of <0.05 was considered to be significant.

Results and Discussion

WT Mice.

Mice were tested for allodynia-like pain behavior before and at 1, 2, 5, 7, 9, 12, and 14 days after SNI. After SNI, all WT mice on both the CBA/B6 and 129OlaHsd genetic backgrounds developed marked mechanical allodynia of the lateral surface of the hind paw ipsilateral to the injured nerve, which occurred within 1 day of surgery. The withdrawal threshold decreased from a mean baseline value of 1.02 ± 0.04 g to 0.36 ± 0.06 g (P < 0.001) for the CBA/B6 animals (Fig. 1) and 1.08 ± 0.11 g to 0.46 ± 0.09 g (P < 0.001) for the 129OlaHsd strain (Fig. 2A). Mechanical allodynia increased further by 2 days post-SNI and was maintained for the duration of the experiment (Figs. 1B and 2A). Nonoperated controls and sham-operated animals showed no significant change in withdrawal thresholds over the 14-day testing period (results not shown). The animals also were tested for thermal thresholds at 1, 7, and 14 days after SNI. Thermal latencies did not change appreciably from baseline measurements in the 129OlaHsd WT controls (Fig. 2B) or in the CBA/B6 WT animals (data not shown). This is consistent with previously published data acquired by using the rat SNI model (30). Because thermal latencies did not significantly change in mice after SNI, we did not measure this modality in subsequent SNI experiments.

Figure 1.

Responses of WT (CBA/B6)- and galanin-overexpressing lines to mechanical stimuli. Presurgery 50% withdrawal thresholds are denoted as day 0. (A) Pre-SNI thresholds were not significantly different between genotypes. After SNI, CBA/B6 WT controls developed robust allodynia (Mann–Whitney U test; P < 0.001 at day 1). The Gal-OE initially developed allodynia, but thresholds returned to presurgery measurements by day 7 post-SNI. *, WT 0 group compared with WT groups at each time point; $, Gal-OE 0 group compared with Gal-OE groups at each time point; &&, comparison between Gal-OE groups at days 5 and 7. ** and *** denote P < 0.01 and P < 0.001, respectively. ## denotes to P < 0.01. $ and $$ denote P < 0.05 and P < 0.01, respectively. (B) Pre-SNI thresholds were significantly greater in Ret-OE mice (Mann–Whitney U test; P < 0.01). After SNI, mechanical withdrawal thresholds were reduced in Ret-OEs, although thresholds remained significantly higher than WT controls for the entire duration of the experiment. *, WT 0 group compared with WT groups at each time point; #, WT compared with Ret-OE at each time point; $, Ret-OE 0 group compared with Ret-OE groups at each time point. * and ** denote P < 0.05 and P < 0.01, respectively. # and ## denote P < 0.05 and P < 0.01, respectively. $ and $$ denote P < 0.05 and P < 0.01, respectively.

Figure 2.

Responses of WT (129OlaHsd) and galanin mutant mice to mechanical and thermal stimuli. (A) Presurgery 50% withdrawal thresholds are denoted as day 0. WT animals developed robust allodynia within 1 day of SNI (Mann–Whitney U test; P < 0.01), and this was maintained for the duration of the experiment. Mutants have significantly lower pre-SNI mechanical withdrawal thresholds than WT controls (Mann–Whitney U test; P < 0.01) and fail to develop allodynia after SNI. (B) Time course of thermal hyperalgesia after SNI. Presurgery withdrawal thresholds are denoted as day 0. Mutants had significantly lower pre-SNI thermal withdrawal thresholds than WT controls (Dunnett's P < 0.05). There was no significant change in thresholds for either genotype after SNI. *, WT 0 group compared with WT groups at each time point; #, WT compared with knockout at each time point. ** and *** denote P < 0.01 and P < 0.001, respectively. # and ## denote P < 0.05 and P < 0.01, respectively.

We have used an adaptation of the existing rat SNI model recently described by S. D. Shields, W. A. Eckert III, and A. I. Basbaum (unpublished work) to induce neuropathic pain behavior in mice. It has several advantages over other existing models: (i) it is technically straightforward to perform, (ii) the degree and time course of the mechanical allodynia are robust and reproducible, (iii) all animals develop allodynia to a similar extent (particularly important when comparing different genotypes), and (iv) the allodynia develops within 1 day and lasts for at least 4 weeks (data not shown).

We also studied the effects of the SNI on galanin expression in the DRG and spinal cord. Groups of WT animals from both strain backgrounds were subjected to SNI, and L4 and L5 DRG and lumbar spinal cord tissue was collected at 0, 3, 5, 7, 14, and 21 days after nerve injury. In control, uninjured animals, galanin was expressed only in a few cells in the DRG, as described (4). The number of galanin-positive neurons and the levels of galanin in the neurons in the DRG increased within 3 days of SNI, which increased further by 7 days after surgery (Fig. 3), and was maintained for the duration of the experiment (data not shown). Increased galanin immunoreactivity also was seen in the ipsilateral (right side) dorsal horn (Fig. 4).

Figure 3.

Time course of immunohistochemical localization of galanin in WT, Gal-OE, and Ret-OE DRG before (time 0) and at 3, 5, and 7 days post-SNI. In uninjured WT animals, galanin was expressed in a few neurons in the DRG and was up-regulated after SNI. No differences were noted in DRG staining between uninjured WT and Gal-OE animals. After SNI, DRG galanin immunoreactivity was greater in the Gal-OE than in SNI-injured WT controls at all time points. In the uninjured Ret-OE, galanin was expressed in various neurons. A majority of galaninergic neurons colocalized with IB4, although it is clear that a substantial number of large-diameter neurons that are positive for galanin are negative for IB4 binding (Lower). (Bar = 100 μM.)

Figure 4.

Immunolocalization of galanin in dorsal spinal cord before and 7 days post-SNI. Galanin was present in primary afferent fibers projecting into lamina I and II of the dorsal horn. Immunoreactivity was increased after SNI ipsilateral to the injury side (Right). Some increase in dorsal horn staining was observed in the intact Gal-OE, which was increased further after SNI. In the Ret-OE, galanin immunoreactivity was stronger and extended further, into inner lamina II. Staining was increased after SNI ipsilateral to the injury side, which represents the increase in endogenous galanin expression. (Bar = 100 μM.)

Gal-OE Mice.

To obtain an expression cassette to inducibly target various biologically active molecules to the adult DRG, we have used transgenesis to map and characterize the enhancer regions of the murine galanin gene. We have defined a 20-kb transgene that fully recapitulates the endogenous expression pattern of galanin in both the adult intact DRG and in adult DRG and spinal cord after axotomy (A.B., N.C.H.K., F.E.H., and D.W., unpublished work). We now have used this 20-kb galanin enhancer to drive expression of the murine galanin gene in transgenic mice. We have shown recently that these transgenic animals have 4-fold-higher levels of galanin in the DRG 1 week after axotomy (measured by RIA) compared with injured WT controls but have normal peptide levels in the intact, uninjured DRG (40). Consistent with this, immunohistochemical staining for galanin in the intact DRG was unchanged, and no significant differences were observed between Gal-OE and matched WT controls in either mechanical or thermal thresholds (Fig. 1A and ref. 40). Somewhat higher levels of galanin staining were observed in the dorsal horn of the intact Gal-OE spinal cord than in the intact WT controls (Fig. 4). These data would imply that there may be increased galanin synthesis in the intact DRG of Gal-OE animals that is transported and stored in the afferent terminals of the dorsal horn. Presumably, the relatively minor increase in galanin expression in the dorsal horn is not sufficient to perturb nociceptive thresholds in intact Gal-OE animals.

After SNI, Gal-OE animals developed some degree of mechanical allodynia by day 1 after surgery (Fig. 1A) with a withdrawal threshold of 0.61 ± 0.12 g, although this was a significantly greater threshold than the WT controls on the same strain background at this time point (P < 0.05). By day 7 post-SNI, the withdrawal thresholds returned to presurgery values (1.16 ± 0.09 g) and remained at those values for the rest of the experiment (Fig. 1A).

Immunohistochemistry for galanin in the DRG ipsilateral to the lesion (right side) of the Gal-OE animals demonstrated that after SNI, galanin expression increased to a maximum, by day 7 after injury (Fig. 3). Stronger neuronal immunoreactivity in the DRG was observed in the SNI-operated Gal-OE animals compared with SNI-operated WT animals at all time points studied (Fig. 3). Galanin expression also was studied in the spinal cord, demonstrating that by day 7 after surgery there is a pronounced up-regulation in the dorsal horn in both WT and Gal-OE animals that is greater in the Gal-OE animals (Fig. 4). The reduction in mechanical thresholds observed in the Gal-OE after SNI may be related to the increasing levels of galanin in the DRG observed between days 3, 5, and 7 (Fig. 3). Further, it is possible that galanin overexpression in other tissues (as yet unknown) such as the autonomic nervous system also may contribute to the observed pain phenotype. These new data further support the hypothesis that galanin indeed does play an inhibitory role in pain processing after nerve injury, compatible with the previous published pharmacological data.

Ret-OE Mice.

In addition to the Gal-OE mice, we have also generated transgenic animals by using the previously described 12-kb c-Ret enhancer region (41) to drive the expression of the murine galanin gene (Ret-OE). This enhancer region recently has been demonstrated to target the IB4-binding neurons in the DRG (41), and expression of the transgene does not alter after axotomy (F.E.H., D.W., and V.P., unpublished data), consistent with the previously reported expression of c-Ret, a component of the GDNF receptor (45, 46).

Withdrawal thresholds to mechanical and thermal stimulation were measured before SNI. Ret-OE mice had significantly higher baseline mechanical (1.833 ± 0.17 g compared with 0.84 ± 0.19 g, P < 0.001; Fig. 1B) and thermal (14.18 ± 0.57 s compared with 11.74 ± 0.6 s, P < 0.05) thresholds than corresponding WT controls. Two of the eight Ret-OE mice tested failed to respond to the highest von Frey filament used, so a maximum value of 3.63 g was assigned to them. These findings are analogous to those by Blakeman et al. (47), who described transgenic mice that ectopically overexpress galanin under the control of the platelet-derived growth factor β (PDGFβ) promoter (although the precise neuronal distribution of galanin in these PDGFβ-OE animals has not been documented). These mice are also hypoalgesic to thermal but not mechanical stimulation in the intact state. Thus, both the Ret-OE and PDGFβ-OE transgenic lines imply an antinociceptive role for galanin in intact animals, albeit in a modality-specific manner.

Immunohistochemistry demonstrates that intact Ret-OE animals abundantly expressed galanin in DRG neurons of various sizes, the majority of which colocalize with IB4 (Fig. 3). However, the neurons that expressed the highest levels of galanin were of large diameter and did not bind IB4. These data are consistent with that reported previously, demonstrating that 21% of c-Ret-positive neurons in the DRG do not bind IB4 and are predominantly large-diameter cells (45). Galanin expression in the intact Ret-OE spinal cord appears to extend deeper into lamina II than in WT controls (Fig. 4), consistent with its localization in the c-Ret neurons, which mostly terminate in inner lamina II (45). IB4-negative and IB4-positive neurons appear to be functionally distinct in that they have particular neurochemical phenotypes and corresponding electrophysiological characteristics that are relevant to the detection and processing of nociceptive stimuli (48–50). Furthermore, IB4-negative and IB4-positive nociceptors terminate in distinct regions of the superficial dorsal horn: IB4-negative afferents project mostly into lamina I and outer lamina II, whereas IB4-positive afferents terminate mostly in inner lamina II (45). The differing nociceptive behavioral phenotypes in the PDGFβ-OE and Ret-OE lines therefore may reflect differences in the neuronal populations that ectopically express galanin and its precise site of synaptic release within the dorsal horn as well as its potential pre- and postsynaptic targets. Alternatively, because our previous data have demonstrated that galanin plays a role in the developmental survival of DRG neurons (37), it is possible that the sustained overexpression of galanin in the DRG may result in developmental/compensatory changes that could contribute to the observed phenotypic changes in pain behavior in the adult transgenic animals of either line.

After SNI, the Ret-OE animals had reduced mechanical withdrawal thresholds when compared with baseline measurements from day 2 onward. At 14 days post-SNI, the threshold was 0.49 ± 0.12 g (Fig. 1B), which represents a decrease of 73% compared with a 79% decrease in the WT controls at the same time point. It should be noted, however, that at all time points tested after SNI, the mechanical thresholds in the Ret-OE animals remained higher than in the intact WT animals (Fig. 1B), and it is therefore debatable as to whether the reduction in threshold values after SNI can be classified as allodynia in the Ret-OE animals.

Galanin expression in the DRG of the Ret-OE animals did not significantly alter after SNI. The increase that was observed in the dorsal horn staining, ipsilateral to the side of injury (Fig. 3), is due to the injury-induced up-regulation of the endogenous galanin gene.

Galanin Mutant Mice.

Baseline withdrawal thresholds to noxious mechanical and thermal stimuli were measured and compared with WT controls. The mutant mice were significantly more sensitive to both modalities compared with WT controls, with mechanical withdrawal thresholds of 1.08 ± 0.11 g and 0.58 ± 0.09 g (P < 0.001; Fig. 2A) and thermal withdrawal latencies of 12.20 ± 0.98 and 7.79 ± 0.75 s (P < 0.05; Fig. 2B) for WT and mutants, respectively. These results are in accordance with those reported previously by Kerr et al. (16) and are once again consistent with the proposed inhibitory role played by galanin in the intact animal.

As predicted from our previous published data when using the partial nerve ligation model, the galanin mutant animals failed to develop significant mechanical allodynia after SNI on any of the test days (Fig. 2A). The withdrawal thresholds changed by a maximum of 0.13 g at 5 days post-SNI (which was not significantly different from the mutant baseline threshold) compared with 0.94 g in the WT controls at this time point.

It is clear that the markedly diminished neuropathic pain behavior and decreased electrophysiological responses (39) in the galanin mutant animals are contrary to (i) that observed in the Gal-OE and Ret-OE animals and (ii) the majority of the existing pharmacological data. Further, the deficits in the mutant animals are present by the first day after the nerve lesion in the partial nerve ligation and SNI chronic pain models we have studied, whereas if the phenotype was due to the absence of galanin in the adult, then differences would be expected to occur 3–5 days after injury, which is the time course for the up-regulation of the endogenous galanin gene after injury. To explain the above disparities, we hypothesized that the chronic absence of galanin throughout sensory neuron development may cause the loss of a subset of neurons that is important for injury-related pain behavior. Consistent with this, we have demonstrated previously a 13% reduction in the total number of neurons in the adult DRG of galanin mutant animals (37). These deficits were associated with a 2.8-fold increase in the number of apoptotic cells at postnatal day 3 (37).

For each of the above transgenic and knockout lines, the effects of galanin on the modulation of pain behavior are mediated via the activation of three G protein-coupled receptor subtypes, designated GALR1, GALR2, and GALR3 (for review, see ref. 51). Binding of galanin to GALR1 has been shown to inhibit adenylyl cyclase (52). Activation of GALR2 stimulates both phospholipase C and ERK (53, 54). These studies raise the possibility that the receptor subtypes function in divergent molecular cascades in the DRG and may play differing roles in neuronal function in the DRG. In situ hybridization studies have shown that GALR1-expressing neurons in the DRG are larger than those that express GALR2, whereas only 20% of neurons express both receptor subtypes (55–57). GALR3 expression has not been demonstrated convincingly in the DRG or dorsal horn (58). In situ hybridization studies also have shown that high levels of GALR1 expression are detected within the interneurons of the superficial dorsal horn, whereas GALR2 is detected only at low levels throughout the dorsal horn (55). The DRG expression of both GALR1 and GALR2 is down-regulated markedly after axotomy (56, 57). Thus, the complement of galanin receptor subtypes both within particular populations of nociceptors and in the neurons in the dorsal horn with which they synapse is clearly crucial to the effects of galanin on nociceptive behavior. Recent studies have indicated that activation of GALR2 may be the principal receptor subtype that mediates the facilitatory effect of galanin in the intact animal, whereas GALR1 is predominantly responsible for the inhibitory actions of galanin in states of nerve injury (3, 17). Axotomy (or any of the neuropathic pain models) markedly increases endogenous galanin synthesis in the DRG, which then is transported to the terminal afferents of the primary sensory neurons. The enhanced release of galanin may predominantly activate GALR1, which is expressed postsynaptically on the dorsal horn interneurons, because presynaptic expression of GALR1 and GALR2 is down-regulated markedly after injury (56, 57).

It is also important to note that in both the overexpressing and mutant transgenic lines, it is possible that the chronic alterations in the levels of galanin in the DRG may modify the expression of one or more of the galanin receptor subtype(s) in the DRG and/or spinal cord. To date, we have failed to detect specific immunohistochemical staining in the mouse DRG or cord when using antisera raised against rat GALR1 or GALR2 (data not shown); thus, we have been unable to perform expression studies at the protein level in the DRG or spinal cord that would allow us to test the above hypothesis.

In summary, we have characterized two transgenic lines that either inducibly overexpress galanin after nerve injury, with a distribution that recapitulates that observed endogenously in WT animals (Gal-OE), or constitutively and ectopically overexpress galanin in both intact and nerve-injured animals (Ret-OE). Behavioral analysis of these transgenic lines and the existing galanin mutants coupled with the previous published data when using PDGFβ-OE animals supports the hypothesis that galanin indeed does play an inhibitory role in pain processing in the intact animal and in states of nerve injury.

Abbreviations

DRG

dorsal root ganglia

SNI

spared nerve injury

IB4

isolectin-B4