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Published in final edited form as: Neuropharmacology. 2011 May 27;61(4):747–752. doi: 10.1016/j.neuropharm.2011.05.020

The wake-promoting transmitter histamine preferentially enhances alpha-4 subunit-containing GABAA receptors

Matt T Bianchi 1, Alison G Clark 2, Janet L Fisher 2
PMCID: PMC3679895  NIHMSID: NIHMS304435  PMID: 21640733

Abstract

Histamine is an important wake-promoting neurotransmitter that activates seven-transmembrane G-protein coupled histamine receptors. However, histamine demonstrates target promiscuity, including direct interaction with the structurally unrelated glutamate (NMDA) and GABAA receptor channels. Previous work showed that histamine enhances the activity of recombinant GABAA receptor isoforms typically found in synaptic locations, although co-release of histamine and GABA is not known to occur in vivo. Here we used patch clamp recordings of various recombinant GABAA receptor isoforms (α1-6, β1-3, γ1-3, δ) to test the hypothesis that histamine might show subunit preference under low GABA concentration (extrasynaptic) conditions. We found that histamine potentiated the whole cell responses to GABA for all tested subunit combinations. However, the magnitude of enhancement was largest (~400% of EC10 GABA-evoked currents) with α4β3 and α4β3X isoforms, where X could be γ or δ. In contrast, histamine (1mM) had small effects on prolonging deactivation of α4β3γ2 receptors following brief (5ms) pulses of 1mM GABA. These findings suggest GABA-histamine cross-talk may occur preferentially at low GABA concentrations, which could theoretically be inhibitory (via enhancing tonic inhibition), directly excitatory (via enhancing presynaptic GABAergic signaling), or indirectly excitatory (via inhibiting GABAergic interneurons).

Keywords: sleep, wake, extrasynaptic, modulation

1. Introduction

The naming of neurotransmitter receptors implies specificity for a particular agonist or transmitter, but growing evidence suggests promiscuity both from the perspective of the receptors (which may recognize diverse agonists and modulators) and the neurotransmitters (which may act through a variety of molecular targets). Promiscuity may have functional importance by expanding the signaling armamentarium of neurons. Promiscuity also has implications for understanding the pharmacology of neuro-active therapeutics, such as anticonvulsants, antidepressants, neuroleptics, and anti-dementia drugs, all of which interact with multiple targets (Bianchi, 2008, 2010; Bianchi et al., 2009b; Millan, 2006; Rammes and Rupprecht, 2007). As a result, it may be important to consider the potential utility of network pharmacology and/or ‘magic shotgun’ strategies in drug development (Bianchi and Botzolakis, 2010; Hopkins, 2007; Morphy et al., 2004; Roth et al., 2004; Yildirim et al., 2007).

Although the concept of transmitter promiscuity runs counter to the specificity engendered by synaptic specializations, non-synaptic signaling may provide a physiological stage for promiscuous interactions. In non-synaptic communication, receptors lack the protective compartment of the synapse, and thus may be exposed to a wide variety of signaling molecules over a range of times scales and durations (Agnati et al., 2010; Farrant and Nusser, 2005; Kullmann et al., 2005; Mody et al., 2007; Richerson and Wu, 2003). This may be particularly true for GABAergic and histaminergic neurotransmission. Extra-synaptic GABAA receptors are well-known to play a prominent role in neuronal inhibition (Belelli et al., 2009; Farrant and Nusser, 2005; Mody et al., 2007). In addition, the majority of histamine receptors may be non-synaptic(Mizuguchi et al., 1991), and several studies suggest the importance of non-synaptic histamine signaling(Hayashi et al., 1984; Uhlrich et al., 1993). These findings raise the possibility of histamine-GABA cross-talk at the level of extrasynaptic signaling.

Histamine is a wake promoting neurotransmitter released from the widely projecting tuberomamillary nucleus (TMN), an important component of the sleep-wake switch circuitry (Saper et al., 2005). Medications with anti-histamine properties induce sleepiness and are often used clinically as hypnotics (e.g., diphenhydramine, tricyclic anti-depressants). Decreased histamine levels in cerebrospinal fluid have been reported in disorders of hypersomnolence(Kanbayashi et al., 2009), and histamine levels inversely correlate with non-rapid eye movement sleep features such as slow wave and spindle activity(Valjakka et al., 1996). Although histamine operates through a family of G-protein coupled receptors(Haas et al., 2008), it has also been shown to act through allosteric sites on two structurally unrelated ionotropic receptors, the NMDA glutamate receptor(Bekkers, 1993; Vorobjev et al., 1993) and GABAA receptor channels(Saras et al., 2008). Although there is no current evidence for co-release of GABA and histamine, GABA is co-synthesized in the histamine neurons of the tuberomamillary nucleus(Airaksinen et al., 1992; Haas et al., 2008), and GABA modulates histamine release(Okakura-Mochizuki et al., 1996). Histamine has autocrine action on TMN neurons via H3 auto-receptors(Haas and Panula, 2003), raising the possibility of spillover influencing other targets such as GABAA receptors, which are also expressed in the TMN neurons. Given the wide histaminergic projections of the TMN, and the growing interest in non-synaptic and peri-synaptic GABAA receptor signaling, we raised the question of whether histamine might preferentially enhance GABAA receptor isoforms under “tonic” conditions (protracted activations by low GABA concentration), as has been shown for several other GABAA receptor modulators (Bai et al., 2001; Bianchi and Macdonald, 2003; Brown et al., 2002; Feng et al., 2004; Mody et al., 2007; Stell et al., 2003).

2. Methods

2.1 Transfection of mammalian cells

Human HEK-293T cells (GenHunter, Nashville TN) were maintained in Dulbecco’s modified Eagle medium (DMEM) supplemented with 10% fetal bovine serum, 100 IU/ml penicillin and 100 μg/ml streptomycin. Cells were passaged by a 5 min. incubation with 0.05% trypsin/0.02% EDTA solution in phosphate buffered saline (10 mM Na2HPO4, 150 mM NaCl, pH=7.3). Cells were transiently transfected with full-length cDNAs encoding rat or human GABAARs in pCMVNeo expression vectors (Dr. Robert Macdonald, Vanderbilt University) through calcium phosphate precipitation. Transfected cells were selected using the plasmid pHook™-1 (Invitrogen), which encodes the surface antibody sFv that recognizes antigen coated magnetic beads. Plasmids encoding GABAA receptor subunit cDNAs were added to the cells in 1:1:1 ratios of 2 μg each, plus 1 μg of pHook. After a 4–6 hr. incubation at 3% CO2, cells were treated for 30 seconds with a 15% glycerol solution in BBS buffer (50 mM BES(N,N-bis[2-hydroxyethyl]-2-aminoethanesulfonic acid), 280 mM NaCl, 1.5 mM Na2HPO4). Cells were mixed with 3–5 μl of antigen-coated magnetic beads 24–52 hrs later to isolate positively transfected cells (Chesnut et al., 1996). Selected cells were plated onto poly L-lysine and collagen coated glass coverslips, and used for recordings 18–28 hours later.

2.2 Electrophysiological recording solutions and techniques

The external solution consisted of (in mM): 142 NaCl, 8.1 KCl, 6 MgCl2, 1 CaCl2, and 10 HEPES (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid) with pH = 7.4 and osmolarity adjusted to 295–305 mOsm. The electrode (internal) solution consisted of (in mM); 153 KCl, 1 MgCl2, 5 K-EGTA (ethylene glycol-bis (β-aminoethyl ether N,N,N′N′-tetraacetate), 2 MgATP and 10 HEPES with pH = 7.4 and osmolarity adjusted to 295–305 mOsm. GABA and histamine were diluted into external solution from freshly made or frozen stocks (in water). Borosilicate filamented glass pipettes (World Precision Instruments, Sarasota FL) were pulled on a two-stage puller (Narishige, Japan) to a resistance of 5–10 MΩ. GABA was applied to cells in whole-cell patch clamp experiments using a stepper system with a complete exchange time of <50 msec at an open tip (SF-77B, Warner Instruments, Hamden CT). The 3-barrel square glass was pulled to a final size of ~200 μm for the concentration-jump experiments with excised patches. The 10–90% rise times of the open tip potential for excised patch experiments were consistently faster than 400 μsec. Currents were recorded with an Axon 200B (Foster City, CA) patch clamp amplifier.

2.3 Analysis of whole-cell and macropatch currents

Whole-cell currents were analyzed using Clampfit (pClamp8 suite, Axon Instruments, Foster City CA) and Prism (Graphpad, San Diego, CA) software. Concentration-response data was fit with a four-parameter logistic equation (Current = [Minimum current + (Maximum current - Minimum Current)]/1+(10^(log EC50 − log [agonist])*n) where n represents the Hill number. All fits were made to normalized data with current expressed as a percentage of the maximum response to GABA for each cell. The deactivation rate of excised patch currents was determined by fitting the decay phase with the Levenberg-Marquardt least squares method with two exponential functions. Student’s paired or unpaired t-tests, ANOVA, and Tukey-Kramer multiple comparison statistical tests, as appropriate, were performed using the Instat program (Graphpad) with a significance level of p<0.05.

3. Results

3.1. Effect of GABAA receptor α subunit subtype on positive modulation by histamine

The earlier study by Saras et al. (2008) showed that the activity of heteromeric GABAARs containing the α1 subunit could be modestly potentiated by 1 mM histamine. Sensitivity of the GABAARs to many agonists and modulators depends upon the identity of the α subtype. Therefore, we tested the effect of 1 mM histamine on GABA-evoked currents from cells expressing each of the six different α subtypes with β3 and γ2L (Figure 1). The response to a submaximal (EC5–10 for each isoform) GABA concentration was enhanced by histamine for all these receptor isoforms. 1 mM histamine caused a modest 1.5-2X increase in current amplitude for most of the receptors, similar to that reported previously for α1β2γ2 (Saras et al., 2008). However, we observed a clear preference for histamine activity at receptors containing the α4 subunit, with significantly greater enhancement of the current compared to all other α subtypes. The α4 subunit confers distinct kinetic properties to recombinant receptors (Lagrange et al., 2007), and its expression is enriched in extrasynaptic and perisynaptic membranes (Mody, 2008; Pirker et al., 2000; Sun et al., 2004). In combination with the δ or γ2 subunit, α4-containing receptors mediate tonic inhibition in hippocampus and thalamus (Cope et al., 2005; Hsu and Smith, 2003), and have been implicated in sleep, hypnotic action (Belelli et al., 2005; Chandra et al., 2006), stress, anxiety and seizures (Brooks-Kayal et al., 1998). These receptors are also primary mediators of the effects of steroid and reproductive hormones (Brown et al., 2002; Smith et al., 2004) and alcohol (Liang et al., 2008; Peng et al., 2004) on GABAergic neurotransmission. Therefore, given that histamine enhanced currents evoked by GABA concentrations typical of the extrasynaptic space (<1μM), the α4-containing receptors represent a physiologically distinct and clinically relevant receptor population.

Figure 1. Histamine enhancement of αβγ isoforms – effect of α subtype.

Figure 1

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Histamine (1mM) enhanced GABAA receptor currents evoked by a low concentration of GABA (EC5 – EC10). The GABA concentration used was adjusted for each isoform depending upon its GABA sensitivity (Picton and Fisher, 2007) and ranged from 0.1 to 3 μM. Each bar shows the mean enhancement, ± SEM, as a percentage of the amplitude of GABA alone (dashed line), for α1-α6 subtypes co-expressed with β3γ2L. The n is indicated by the number in parentheses. The α4β3γ2L isoform showed significantly greater enhancement (*, p<0.001) compared to all other receptors. Representative currents from receptors containing each of the α subtypes are shown above each bar, with GABA alone (gray) overlaid with GABA +1 mM histamine (black). All applications were 5 seconds in duration. The vertical bars indicate amplitude as follows: α1, 100pA; α2, 100pA; α3, 50pA; α4, 100pA; α5, 45pA; α6, 35pA,

To determine whether the enhanced currents were due to direct agonist action on GABAA receptor or to a modulatory action, we tested high concentrations of histamine (1mM and 10mM) at α4β3γ2L receptors. Histamine elicited direct currents of small amplitude in the absence of GABA, 2.8±1.0 % (1mM; n=3), or 8.3±1.6% (10mM; n=3), relative to the amplitude of current evoked by 1mM GABA (data not shown). Thus, although histamine does act as a weak direct agonist, the enhanced current seen with co-application of GABA and histamine was due to a modulatory effect. We also observed no effect of gabazine in blocking this direct action of histamine, providing additional evidence for a presumably allosteric effect.

3.2. Concentration-response relationship for histamine at α4β3γ2 receptors

Since the α4 subunit conferred higher sensitivity to histamine modulation, all further studies were performed with receptors containing this subunit. At α4β3γ2L receptors, histamine increased GABA-activated current in a concentration-dependent manner with an average EC50 of 521 ± 173 μM (N = 6) (Figure 2). This is similar to the EC50 for modulation of α1β2 receptors (965 μM) reported by Saras et al., (2008). However, the amount of potentiation was much greater for the α4-containing receptors, with an average maximal potentiation of 612 ± 160% (N=6) of the current evoked by GABA alone. In comparison, the maximum potentiation reported for the α1β2 isoform was near 200% in response to 10 mM histamine (Saras et al., 2008).

Figure 2. Concentration–response relationship for modulation of α4β3γ2L by histamine.

Figure 2

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Histamine enhanced currents evoked by 0.3 μM GABA applied to cells expressing α4β3γ2L receptors in a concentration-dependent manner. All current traces were obtained from the same cell and show the response to GABA alone or to GABA co-applied with the indicated concentration of histamine for 5 sec, as indicated by the line. Averaged data was fit with a four-parameter curve (solid line). From the fit shown, the histamine EC50 for modulation was 630.2 μM with a peak current 619% of the response to GABA alone (n = 6 cells).

3.3. Effect of β, γ and δ subunits on histamine modulation of α4-containing receptors

Since our results indicated that histamine activity was influenced by the α subtype, it could also depend upon the nature of the other subunit families. We therefore compared the response of receptors containing the α4 subunit in combination with different β and γ subunit subtypes. We also tested whether sensitivity required a γ subunit by examining the response of both δ-containing receptors and the α4β3 heterodimer (Figure 3). We found no difference in modulation by 1 mM histamine when γ1, γ3 or δ replaced the γ2 subunit. In contrast, replacing the β3 subtype with either β1 or β2 significantly reduced the amount of potentiation. The α4β3, α4β3γ2 and α4β3δ isoforms were all highly potentiated by histamine, while the α1β3δ isoform was significantly less sensitive, similar to the response of α1β3γ2L receptors. Together, these findings suggest that α4 and β3 provide the key substrate for histamine preference. Although there is clear potentiation in each combination, the largest effect depended on the joint presence of α4 and β3 subunits, and β3 containing receptors may undergo synaptic targeting.

Figure 3. Histamine modulation of α4-containng receptors in combination with different β or γ subtypes, or the δ subunit.

Figure 3

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Histamine (1mM) enhanced currents evoked by EC5-EC10 GABA concentration from cells expressing GABAA receptors of various subunit composition. Open bars indicate β subtype, gray bars indicate γ subtype, hatched bars indicate δ-containing isoforms, and the black bar indicates the α4β3 heterodimer. The number in parentheses indicates the n for each isoform. * indicates a significant difference from the α4β3γ2L response (p ≤ 0.001, ANOVA, Tukey-Kramer multiple comparisons test). The response of the α4β3 heterodimer was not different than α4β3γ or α4β3δ and the response of α1β3δ was not significantly different from α1β3γ2L. Representative current traces for δ-containing and the α4β3 heteromer are shown with the response to GABA alone (gray) superimposed upon the response to GABA + 1 mM histamine (black). All applications were 5 seconds in duration. The vertical bars indicate amplitude as follows: α1β3δ 50pA; α4β3δ, 50pA; α4β3, 40pA.

3.4. Effect of GABA concentration on positive modulation by histamine

Finally, we determined the dependence of histamine modulation on GABA concentration. The GABA sensitivity of α4β3γ2L receptors was measured in the presence or absence of 1 mM histamine (Figure 4A). Modulation by histamine was clearly related to GABA concentration, such that higher concentrations showed progressively smaller enhancement and the maximal current in response to saturating GABA concentrations was unaffected by histamine. Co-application of 1 mM histamine caused a nearly 10-fold shift in GABA EC50, from an average of 11.2 ± 2.3 μM with GABA alone (N=5) to 1.22 ± 0.25 μM in the presence of histamine (N=6) (p ≤ 0.001, unpaired t-test comparison of log EC50 values).

Figure 4. Histamine preferentially enhances currents evoked by low GABA concentrations.

Figure 4

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A. GABA concentration response curves are shown for α4β3γ2L receptors with or without co-application of histamine (1mM). The curve is left shifted in the presence of histamine, with a decrease in EC50 from 10.2 μM to 1.2 μM. The inset shows an overlap of current responses to 1 mM GABA in the presence and absence of 1 mM histamine demonstrating the minimal effect of histamine on the peak amplitude.

B. Brief (5ms) pulses of GABA (1mM) were delivered with or without co-applied histamine (1mM) to excised patches containing α4β3γ2L receptors. Bars show the average duration of the fast and slow components of the fit to the current decay (n=7 patches). Histamine prolonged deactivation by significantly increasing the slow tau (* p ≤ 0.01, paired t-test). Histamine had no significant effect on the fast component, or on the relative contributions of the two components. The percent contribution of the fast tau was 91.7±2.7% for GABA alone and 90.7±2.2% for GABA + histamine. Overlaid traces of GABA alone (gray) and GABA+histamine (solid) demonstrate the modest impact on deactivation time course.

3.5. Effect of histamine on macroscopic kinetic properties

A similar “ceiling” phenomena to that described above is seen with other GABAA modulators such as benzodiazepines. Although minimal amplitude change is seen when receptors are maximally activated by GABA, the most relevant parameter under synaptic conditions is the deactivation time constant – the rate at which a synaptic current decays – which governs charge transfer and thus inhibitory impact. In order to determine whether histamine would enhance GABAA receptor function under synaptic conditions, we activated excised patches with brief pulses of GABA (1 mM) +/− histamine (1 mM) (Figure 4B). The α4β3γ2L receptors rapidly deactivate in response to a 5 msec application of 1 mM GABA, with an weighted mean decay τ of 20.1 ± 5.3 msec (n=7). Addition of 1 mM histamine caused a small but significant (p< 0.01, paired t-test) slowing of the average decay, to 26.7 ± 4.9 msec (n=7). The slowing of deactivation by histamine was due to a significant increase in the slower time constant, with no change in fast time constant or in the relative contributions of the two components (Figure 4B). These results are consistent with histamine having a greater impact on GABAA receptor function under non-synaptic than synaptic conditions.

4. Discussion

4.1 Diversity of histamine signaling

The only neuronal source of histamine is the TMN, which delivers synaptic and extrasynaptic histamine diffusely throughout the brain. Neuronal histamine receptor subtypes include the G-protein coupled receptors H1 (activates phospholipase C and inhibits potassium channels), H2 (activates adenylate cyclase) and H3 (inhibits adenylate cyclase, and is typically presynaptic). Histamine also acts as an agonist or allosteric modulator at ionotropic receptors. Histamine (and the metabolite 1-methylhistamine) enhances NMDA receptor currents (Bekkers, 1993; Vorobjev et al., 1993), in a subunit dependent manner, with a preference for the NR1/NR2B subunits (EC50 ~10μM)(Williams, 1994). This modulation was independent of histamine receptor antagonists, and therefore likely mediated by direct binding to the NMDA receptor. Histamine also enhances recombinant GABAA receptor channel activity, and our results show that this effect depends upon subunit composition. The cross-reactivity extends to histamine receptor modulators, as anti-histamine drugs have been shown to block recombinant GABAA receptor activity(Cannon et al., 2004). However, evidence for histamine-GABA cross talk in native neuronal contexts remains lacking.

Saras et al. (2008) showed that histamine can directly activate β homomeric GABAA channels (EC50 ~200μM), and enhance GABA-evoked α1β2γ2 channel responses to a small degree (50%) (Saras et al., 2008). They also found that the histamine precursor histidine and the metabolite methylhistamine were low affinity agonists at β3 homomers. Direct activation by histamine could be blocked by certain H1-4 subtype antagonists but were insensitive to others, suggesting a distinct pharmacological profile compared to G-protein-coupled histamine receptors. Histamine (1 mM) enhancement of α1β2γ2 receptors was inversely related to GABA concentration, resulting in a pure left shift of the GABA concentration response curve, similar to our results with α4β3γ2. They found the reverse pattern with α1β2 receptors, in which currents evoked by low GABA concentration were unaffected by histamine, whereas the current response to maximal GABA concentrations were potentiated to a small degree (~25%, with an EC50 of ~1mM). This suggests distinct mechanisms for modulation for α1β2 and α1β3γ2 isoforms. In our study, we found that histamine was an effective positive modulator of both α4β3 and α4β3γ2 receptors at low GABA concentrations.

4.2 Modulation of α4-containing GABAA receptors

Our results now elucidate the subunit-dependence of histamine modulation of GABAA receptors, with a clear preference for receptors containing the α4 subunit, independent of whether γ or δ subunits (or neither) are incorporated. Receptors containing the α4 subunit may participate in synaptic and extrasynaptic forms of inhibition, and are expressed in thalamus, hippocampus, and cortex (Cope et al., 2005; Liang et al., 2008; Pirker et al., 2000). α4 has been implicated as well in animal models of epilepsy(Brooks-Kayal et al., 1998), and its expression has been linked to neurosteroid hormone status(Hsu et al., 2003; Smith et al., 1998). The dependence on GABA concentration, with greater histamine enhancement at low GABA concentrations, strongly suggests that if histamine modulation of GABAA receptors occurs in vivo, it is likely to be most relevant at non-synaptic receptors. Although speculation has been raised as to the role of histamine modulation of GABAA receptors in rodent models of seizures and pain (as discussed in Cannon), no direct evidence has been reported, and thus the potential role of histamine in the regulation of extrasynaptic receptors remains speculative. Like the many other examples of drug-target promiscuity observed across a range of ion channels, establishing which, if any, has physiologic or therapeutic relevance remains a challenging problem. Nevertheless, demonstrating interactions in recombinant systems is an important step in formulating and testing hypotheses using native neuronal systems.

4.3 Potential functional impact of histamine-GABAA cross-talk

The histamine system has been linked to a multitude of normal and pathological processes in the brain, including promoting wakefulness, metabolism/feeding, stress/anxiety, epilepsy, dementia, neuro-inflammation, and hepatic encephalopathy(Haas et al., 2008). Predicting which of these diverse processes might be influenced through cross-talk with GABAA receptors (which have also been implicated in some of these processes) remains speculative. Furthermore, the concentration of histamine may depend on local release patterns, breakdown kinetics, and the presence of certain pathologies. Baseline histamine concentrations may be sub-μM(Strecker et al., 2002), but could increase over 10-fold in the pathological situations, such as reported in a rodent model of hepatic encephalopathy(Fogel et al., 2002). Interestingly about 1/3 of the GABAergic neurons in the basal forebrain are wake active(Hassani et al., 2009); it is not known whether GABAA receptor modulation might be involved in histamine-mediated alerting effects of BF activation. Other sleep-related substances interact with multiple targets, including extrasynaptic GABAA receptors, suggesting that such cross-talk may be a common signaling mechanism. Examples of such substances include nitric oxide(Wall, 2003), oleamide(Coyne et al., 2002), and neurosteroids(Mody, 2008).

The impact of histamine on GABAergic neurotransmission may depend on the cellular location of the cross-talk. For example, enhancement of GABA-ergic inhibition mediated by low GABA concentrations may be a mechanism by which histamine could mediate neuronal inhibition. GABAA receptors containing the α4 subunit in the cortex, hippocampus, and thalamus would represent such potential targets. The anticonvulsant effects of histamine and the pro-convulsant action of anti-histamines (Haas et al., 2008) might operate, in part, via cross-talk with the GABA system. Alternatively, histamine action on GABAA receptors may be directly excitatory, as presynaptic GABAA receptors have been shown to be excitatory in many brain regions (Trigo et al., 2008). Although perisynaptic α6-containing GABAA receptors have been demonstrated near excitatory (glutamatergic) synapses in the cerebellum(Nusser et al., 1996), whether presynaptic GABAA receptors are also localized at histamine terminals remains to be demonstrated. Finally, indirect excitation could occur via histamine mediated inhibition of GABAergic neurons.

The need to consider the context of receptor modulation is emphasized by studies investigating drug action under synaptic versus extra-synaptic conditions(Bai et al., 2001). For example, open channel block by penicillin shows distinct features at high versus low GABA concentration(Feng et al., 2009), neurosteroid modulation is highly dependent on GABA concentration(Bianchi and Macdonald, 2003), and the effects of benzodiazepines on opening frequency depend on synaptic versus steady state conditions(Bianchi et al., 2009a). In each case, previously described drug actions could be (in part) explained by the experimental conditions. In the current study, the potentiation of α4β3γ2 receptors was greater with low compared to high GABA concentrations.

4.4 System-level effects of histaminergic signaling

Several lines of evidence support the wake promoting action of histamine. Decreased histamine concentrations in human cerebrospinal fluid have been linked to hypersomnolence(Kanbayashi et al., 2009), and anti-histamines are commonly employed in over-the-counter hypnotic medications. However, its true role may be more complex than this. A substantial fraction of TMN neurons are active during sleep (Ko et al., 2003), and near-complete lesions of the TMN do not appreciably alter sleep-wake percentages(Gerashchenko et al., 2004). H1 receptor knockout mice showed normal sleep-wake stage percentages, but with decreased fragmentation from brief awakenings(Huang et al., 2006) while H3 knockout mice also showed normal sleep-wake stage percentages, but had decreased locomotor activity(Toyota et al., 2002). Mice lacking histamine (due to histamine decarboxylase knockout) demonstrated increased fragmentary sleep-wake transitions, and 30% increase in the amount of REM sleep (Parmentier et al., 2002). These findings, together with the sparing of time awake (predicted to decrease without histamine) may be due to a mixture of histamine effects on the sleep-wake system.

Regarding possible inhibitory actions of histamine, electrophysiological studies show that histamine inhibits neurons in the thalamus(Lee et al., 2004), in the intralaminar thalamic nuclei(Sittig and Davidowa, 2001), and in the hypothalamus (via chloride conductance)(Hatton and Yang, 2001). Although the precise mechanism subserving these forms of histaminergic inhibition remain uncertain, they suggest that histamine may be employed for inhibitory reasons in vivo. Considering the diverse roles of tonic inhibition mediated by GABAA receptors, such as in the regulation of fast (gamma) oscillations liked to cognition (Hassani et al., 2009; Mann and Mody, 2010), the potential impact of histamine modulation of GABAergic inhibition mediated by low GABA concentrations raises many testable hypotheses. Given the broadly distributed projections of the TMN, the wide expression of GABAA receptors in non-synaptic locations, and the importance of tonic GABAA receptor signaling, the positive allosteric modulation of α4-containing receptors suggest a plausible mechanism by which histamine-GABA cross talk may occur.

  • Histamine is an important wake-promoting transmitter.

  • However, histamine exhibits cross-talk with other receptors.

  • We find that histamine enhances GABAA receptor activity.

  • This enhancement is most likely to be relevant for extrasynaptic signaling.

Acknowledgments

Funding: This work was supported by NIH-NINDS (RO1-NS045950) and the University of South Carolina School of Medicine summer research program.

Footnotes

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