Coexpression of GABAA receptor gamma 1 and gamma 2 subunits in the rat trigeminal ganglion

Attenuation of Sensory Transmission Through the Rat Trigeminal Ganglion by GABA Receptor Activation

While the trigeminal ganglion is often considered a passive conduit of sensory transmission, neurons and satellite glial cells (SGCs) within it can release neurotransmitters and express neuroreceptors. Some trigeminal ganglion neurons contain the neurotransmitter γ-aminobutyric acid (GABA) and express GABA receptors. There is behavioral evidence that increased GABA levels in the trigeminal ganglion decreases nociception, while a loss of GABA receptors results in hyperalgesia, although the neural mechanisms for this remain to be investigated. In this study, the expression of GABA receptors by trigeminal ganglion neurons that innervate rat labial skin and masseter muscle was compared using immunohistochemistry. The effect of intraganglionic administration of GABA receptor agonists was investigated by single unit recording of trigeminal brainstem and ganglion neuron responses to stimulation of the labial skin and/or masseter muscle in anesthetized rats. The mean frequency of expression of GABA_A and GABA_B receptors by masseter and labial skin ganglion neurons was 62.5% and 92.7%, and 55.4% and 20.3%, respectively. The expression of both GABA receptors was significantly greater in skin ganglion neurons. Masticatory muscle evoked brainstem trigeminal neuron responses were significantly attenuated by intraganglionic injection of muscimol (GABA_A) but not baclofen (GABA_B). The mechanical sensitivity of slow and fast conducting masticatory muscle afferent fibers was decreased and increased, respectively, by intraganglionic injection of both muscimol and baclofen. Activation of GABA_A receptors may exert a gating effect on sensory transmission through the trigeminal ganglion by decreasing putative nociceptive input and enhancing innocuous sensory input.

Trigeminal ganglion neurons of mice show intracellular chloride accumulation and chloride-dependent amplification of capsaicin-induced responses

Intracellular Cl⁻ concentrations ([Cl⁻]_i) of sensory neurons regulate signal transmission and signal amplification. In dorsal root ganglion (DRG) and olfactory sensory neurons (OSNs), Cl⁻ is accumulated by the Na⁺-K⁺-2Cl⁻ cotransporter 1 (NKCC1), resulting in a [Cl⁻]_i above electrochemical equilibrium and a depolarizing Cl⁻ efflux upon Cl⁻ channel opening. Here, we investigate the [Cl⁻]_i and function of Cl⁻ in primary sensory neurons of trigeminal ganglia (TG) of wild type (WT) and NKCC1^−/− mice using pharmacological and imaging approaches, patch-clamping, as well as behavioral testing. The [Cl⁻]_i of WT TG neurons indicated active NKCC1-dependent Cl⁻ accumulation. Gamma-aminobutyric acid (GABA)_A receptor activation induced a reduction of [Cl⁻]_i as well as Ca²⁺ transients in a corresponding fraction of TG neurons. Ca²⁺ transients were sensitive to inhibition of NKCC1 and voltage-gated Ca²⁺ channels (VGCCs). Ca²⁺ responses induced by capsaicin, a prototypical stimulus of transient receptor potential vanilloid subfamily member-1 (TRPV1) were diminished in NKCC1^−/− TG neurons, but elevated under conditions of a lowered [Cl⁻]_o suggesting a Cl⁻-dependent amplification of capsaicin-induced responses. Using next generation sequencing (NGS), we found expression of different Ca²⁺-activated Cl⁻ channels (CaCCs) in TGs of mice. Pharmacological inhibition of CaCCs reduced the amplitude of capsaicin-induced responses of TG neurons in Ca²⁺ imaging and electrophysiological recordings. In a behavioral paradigm, NKCC1^−/− mice showed less avoidance of the aversive stimulus capsaicin. In summary, our results strongly argue for a Ca²⁺-activated Cl⁻-dependent signal amplification mechanism in TG neurons that requires intracellular Cl⁻ accumulation by NKCC1 and the activation of CaCCs.

Reduced GABA(A) receptor α6 expression in the trigeminal ganglion alters inflammatory TMJ hypersensitivity

Trigeminal ganglia neurons express the GABA(A) receptor subunit alpha 6 (Gabrα6) but the role of this particular subunit in orofacial hypersensitivity is unknown. In this report the function of Gabrα6 was tested by reducing its expression in the trigeminal ganglia and measuring the effect of this reduction on inflammatory temporomandibular joint (TMJ) hypersensitivity. Gabrα6 expression was reduced by infusing the trigeminal ganglia of male Sprague Dawley rats with small interfering RNA (siRNA) having homology to either the Gabrα6 gene (Gabrα6 siRNA) or no known gene (control siRNA). Sixty hours after siRNA infusion the rats received a bilateral TMJ injection of complete Freund's adjuvant to induce an inflammatory response. Hypersensitivity was then quantitated by measuring meal duration, which lengthens when hypersensitivity increases. Neuronal activity in the trigeminal ganglia was also measured by quantitating the amount of phosphorylated ERK. Rats in a different group that did not have TMJ inflammation had an electrode placed in the spinal cord at the level of C1 sixty hours after siRNA infusion to record extracellular electrical activity of neurons that responded to TMJ stimulation. Our results show that Gabrα6 was expressed in both neurons and satellite glia of the trigeminal ganglia and that Gabrα6 positive neurons within the trigeminal ganglia have afferents in the TMJ. Gabrα6 siRNA infusion reduced Gabrα6 gene expression by 30% and significantly lengthened meal duration in rats with TMJ inflammation. Gabrα6 siRNA infusion also significantly increased p-ERK expression in the trigeminal ganglia of rats with TMJ inflammation and increased electrical activity in the spinal cord of rats without TMJ inflammation. These results suggest that maintaining Gabrα6 expression was necessary to inhibit primary sensory afferents in the trigeminal pathway and reduce inflammatory orofacial nociception.

Role of γ-aminobutyric acid and its receptors in carcinogenesis

Gamma-aminobutyric acid (GABA) is a major inhibitory neurotransmitter in the adult mammalian brain and it is also expressed in the central nervous system, peripheral nerves and peripheral non-neural tissues. Recent studies have shown that GABA is involved in the proliferation and migration of tumor cells and other processes of tumor development. According to different sensitivity to agonists and antagonists, GABA receptors have been classified into three types: A, B and C. GABA receptors and their receptor subunits are involved in complicated regulation of tumor cells. Many studies have demonstrated that GABA binding to its receptors can activate or inhibit the cAMP signaling pathway and the MAPK/ERK pathway, and regulate cancer cell proliferation and migration. The potential value of GABA in cancer diagnosis, prognostic prediction and biotherapy has been gradually revealed. In the present article, we reviewed the recent progress in understanding the role of GABA and its receptors in carcinogenesis.

Prokineticin 2 suppresses GABA-activated current in rat primary sensory neurons

Prokineticin 2 (PK2) is a newly identified regulatory protein, which is involved in a wide range of physiological processes including pain perception in mammals. However, the precise role of PK2 in nociception is yet not fully understood. Here, we investigate the effects of PK2 on GABA(A) receptor function in rat trigeminal ganglion neurons using whole-cell patch clamp technique. PK2 reversibly depressed inward currents produced by GABA(A) receptor activation (I(GABA)) with an IC₅₀ of 0.26 ± 0.02 nM. PK2 appeared to decrease the efficacy of GABA to GABA(A) receptor but not the affinity. The maximum response of the GABA dose-response curve decreased to 71.2 ± 7.0% of control after pretreatment with PK2, while the threshold value and EC₅₀ of curve did not alter significantly. The effects of PK2 on I(GABA) were voltage independent. The PK2-induced inhibition of I(GABA) was removed by intracellular dialysis of either GDP-β-S (a non-hydrolyzable GDP analog), EGTA (a Ca²+ chelator) or GF109203X (a selective protein kinase C inhibitor), but not by H89 (a protein kinase A inhibitor). These results suggest that PK2 down-regulates the function of the GABA(A) receptor via G-protein and protein kinase C dependent signal pathways in primary sensory neurons and this depression might underlie the hyperalgesia induced by PK2.

Potentiation by WIN 55,212-2 of GABA-activated currents in rat trigeminal ganglion neurones

BACKGROUND AND PURPOSE

Although both natural and synthetic cannabinoid compounds have been shown to exert an antinociceptive effect on acute and persistent pain, the anatomical locus of the target of cannabinoid-induced analgesia has not been fully elucidated. Here, we investigated the effects of the cannabinoid agonist WIN 55,212-2 on GABA-activated currents (I(GABA)) in rat primary sensory neurones.

EXPERIMENTAL APPROACH

In the present study, experiments were performed on neurones freshly isolated from rat trigeminal ganglion (TG) by using whole-cell patch clamp and repatch techniques.

KEY RESULTS

GABA-evoked inward currents were potentiated by pretreatment with WIN 55,212-2 in a concentration-dependent manner (10(-10)-10(-8) M). WIN 55,212-2 shifted the GABA concentration-response curve upwards, with an increase of 30.3 +/- 3.7% in the maximal current response but with no significant change in the EC(50) (agonist concentration producing a half-maximal response) value. WIN 55,212-2 potentiated the responses to GABA in a manner independent of holding potential and in the absence of any change in the reversal potential of the current. This potentiation of I(GABA) induced by WIN 55,212-2 was almost completely blocked by AM 251 (3 x 10(-8) M), a CB(1) receptor antagonist, and, using the repatch technique, was found to be abolished after intracellular dialysis with the protein kinase A (PKA) activator cAMP or the PKA inhibitor H89.

CONCLUSIONS AND IMPLICATIONS

The potentiation by WIN 55,212-2 of I(GABA) in primary sensory neurones may help to elucidate the mechanism underlying the modulation of analgesia by cannabinoids in the spinal dorsal horn.

Migraine: an overview

The pathophysiology of migraine is not completely understood and continues to be investigated. The complexity of interactions taking place in the sensory neuronal network with the mediation of all different neurotransmitters involved gives the measure of the extreme difficulty connected with the knowledge of migraine pathogenesis and in particular of its cardinal sign. Neuronal components are relevant in migraine pathophysiology: there could be a generalized interictal abnormal excitability of the cerebral cortex in migraine, possibly favoring the occurrence of spreading depression with consequent activation of the trigeminal system. Many theories have been formulated in these last sixty years about the pathogenesis of migraine and other forms of primary headache, but the problem is still far to be fully clarified. The present review is focused on the description of different theories on the migraine pathogenesis. This review is dedicated to the memory of Prof. Alfredo Bianchi.

Immobilizing doses of halothane, isoflurane or propofol, do not preferentially depress noxious heat-evoked responses of rat lumbar dorsal horn neurons with ascending projections

BACKGROUND

The spinal cord is an important site where volatile anesthetics decrease sensation and produce immobility. Beyond this knowledge, our understanding of a site of anesthetic action is limited. Previous evidence suggests that dorsal horn neurons with ascending projections may be more susceptible to depression by general anesthetics than local spinal interneurons. In this study we evaluated the effects of volatile and injectable general anesthetics on lumbar dorsal horn neurons with and without ascending projections.

METHODS

Thirty-seven adult male rats underwent laminectomies at C1, for placement of a stimulating electrode, and T13/L1, for extracellular recording from the spinal cord dorsal horn. Neuronal responses to heat were evaluated under two doses of halothane, isoflurane, or propofol anesthesia.

RESULTS

Under both halothane and isoflurane anesthesia, increasing the dose from 0.8 to 1.2 minimum alveolar concentration (MAC) had no significant effect on heat-evoked responses in neurons that had ascending projections identified via antidromic stimulation (AD) or those without ascending projections (nAD). Heat responses in AD neurons 1 min after i.v. administration of 3 and 5 mg/kg of propofol were reduced to 60% +/- 18% (mean +/- SE) and 39% +/- 14% of control respectively. Similarly, in nAD neurons responses were reduced to 56% +/- 14% and 50% +/- 10% of control by 3 and 5 mg/kg propofol respectively.

CONCLUSIONS

Our findings suggest, at peri-MAC concentrations, these general anesthetics do not preferentially depress lumbar dorsal horn neurons with ascending projections compared to those with no identifiable ascending projections.

Inflammatory pain upregulates spinal inhibition via endogenous neurosteroid production

Inhibitory synaptic transmission in the dorsal horn (DH) of the spinal cord plays an important role in the modulation of nociceptive messages because pharmacological blockade of spinal GABAA receptors leads to thermal and mechanical pain symptoms. Here, we show that during the development of thermal hyperalgesia and mechanical allodynia associated with inflammatory pain, synaptic inhibition mediated by GABAA receptors in lamina II of the DH was in fact markedly increased. This phenomenon was accompanied by an upregulation of the endogenous production of 5alpha-reduced neurosteroids, which, at the spinal level, led to a prolongation of GABAA receptor-mediated synaptic currents and to the appearance of a mixed GABA/glycine cotransmission. This increased inhibition was correlated with a selective limitation of the inflammation-induced thermal hyperalgesia, whereas mechanical allodynia remained unaffected. Our results show that peripheral inflammation activates an endogenous neurosteroid-based antinociceptive control, which discriminates between thermal and mechanical hyperalgesia.

A local GABAergic system within rat trigeminal ganglion cells

We investigated the GABAergic system within the Sprague-Dawley rat (2-3-weeks old) trigeminal ganglion (TG). Reverse transcription-polymerase chain reaction (RT-PCR) analysis revealed expression of glutamate decarboxylase (GAD) 65 and GAD67 mRNAs and mRNAs encoding GABA(A) receptor subunits alpha1-6, beta1-3, gamma1-3, and delta. In situ hybridization revealed that GAD65 and GAD67 mRNAs were expressed in neuronal cell bodies but not satellite cells. Immunohistochemical analysis showed that only GAD65 was expressed in all neuronal cell bodies, and approximately 70% of all neurons exhibited GABA immunoreactivity. Satellite cells were strongly immunopositive for GABA. GABA(A) receptor alpha1, alpha5, beta2/3 and gamma1/2/3 subunit immunoreactivities were observed in the majority of neurons, but no immunoreactivity for alpha2 was observed. Two types of cells were identified in TG based on cell size and morphology, type A and B. The percentage of cells expressing alpha3, alpha4, alpha6, and delta subunits appeared to be dependent on cell size, as delta and alpha6 expression were only observed in small (B-type) neurons. In whole-cell patch clamp experiments, GABA application induced inward Cl- currents in all neurons examined. The EC50 for GABA varied from 5.3 to 240 microm, and the Hill Coefficient (nH) varied between 0.98 and 2.6 at -60 mV. We found that GABA was released from TG cells by increasing extracellular K+ concentration to 100 mm. We speculate that GABA acts as a nonsynaptically released diffusible neurotransmitter, which may modulate somatic inhibition of neurons within the TG.

GABA and GABA receptors in the central nervous system and other organs

Gamma-aminobutyrate (GABA) is a major inhibitory neurotransmitter in the adult mammalian brain. GABA is also considered to be a multifunctional molecule that has different situational functions in the central nervous system, the peripheral nervous system, and in some nonneuronal tissues. GABA is synthesized primarily from glutamate by glutamate decarboxylase (GAD), but alternative pathways may be important under certain situations. Two types of GAD appear to have significant physiological roles. GABA functions appear to be triggered by binding of GABA to its ionotropic receptors, GABA(A) and GABA(C), which are ligand-gated chloride channels, and its metabotropic receptor, GABA(B). The physiological, pharmacological, and molecular characteristics of GABA(A) receptors are well documented, and diversity in the pharmacologic properties of the receptor subtypes is important clinically. In addition to its role in neural development, GABA appears to be involved in a wide variety of physiological functions in tissues and organs outside the brain.

Comparative analysis of the chemical neuroanatomy of the mammalian trigeminal ganglion and mesencephalic trigeminal nucleus

A characteristic peculiarity of the trigeminal sensory system is the presence of two distinct populations of primary afferent neurons. Most of their cell bodies are located in the trigeminal ganglion (TG) but part of them lie in the mesencephalic trigeminal nucleus (MTN). This review compares the neurochemical content of central versus peripheral trigeminal primary afferent neurons. In the TG, two subpopulations of primary sensory neurons, containing immunoreactive (IR) material, are identified: a number of glutamate (Glu)-, substance P (SP)-, neurokinin A (NKA)-, calcitonin gene-related peptide (CGRP)-, cholecystokinin (CCK)-, somatostatin (SOM)-, vasoactive intestinal polypeptide (VIP)- and galanin (GAL)-IR ganglion cells with small and medium-sized somata, and relatively less numerous larger-sized neuropeptide Y (NPY)- and peptide 19 (PEP 19)-IR trigeminal neurons. In addition, many nitric oxide synthase (NOS)- and parvalbumin (PV)-IR cells of all sizes as well as fewer, mostly large, calbindin D-28k (CB)-containing neurons are seen. The majority of the large ganglion cells are surrounded by SP-, CGRP-, SOM-, CCK-, VIP-, NOS- and serotonin (SER)-IR perisomatic networks. In the MTN, the main subpopulation of large-sized neurons display Glu-immunoreactivity. Additionally, numerous large MTN neurons exhibit PV- and CB-immunostaining. On the other hand, certain small MTN neurons, most likely interneurons, are found to be GABAergic. Furthermore, NOS-containing neurons can be detected in the caudal and the mesencephalic-pontine junction portions of the nucleus. Conversely, no immunoreactivity to any of the examined neuropeptides is observed in the cell bodies of MTN neurons but these are encircled by peptidergic, catecholaminergic, serotonergic and nitrergic perineuronal arborizations in a basket-like manner. Such a discrepancy in the neurochemical features suggests that the differently fated embryonic migration, synaptogenesis, and peripheral and central target field innervation can possibly affect the individual neurochemical phenotypes of trigeminal primary afferent neurons.

Involvement of GABA(A) receptor in modulation of jaw muscle activity evoked by mustard oil application to the rat temporomandibular joint

The effect of intrathecal administration of the GABA(A) receptor antagonist bicuculline methylbromide on jaw muscle electromyographic (EMG) activity evoked by mustard oil injection into the rat temporomandibular joint was studied. Bicuculline given prior to mustard oil augmented the EMG activity evoked by mustard oil, and "rekindling" of EMG activity was induced by bicuculline given 30 min after mustard oil. These results suggest that central GABA(A) receptors modulate reflex responses to noxious craniofacial stimuli.

Autoradiographic distribution of radioactivity from (14)C-GABA in the mouse

We investigated the distribution of radioactivity from (14)C-labeled gamma-aminobutyric acid (GABA) in the mouse by in vivo autoradiography to clarify the tissues that show GABA uptake and/or GABA binding. Male mice were injected intravenously with (14)C-GABA in both the absence and presence of an excess of unlabeled GABA, baclofen and isoguvacine. Whole-body autoradiography of (3)H-baclofen, a GABA(B) receptor agonist was also performed. At short intervals after (14)C-GABA injection ( 3 and 6 minutes), very high radioactivity was detected in the kidney cortex, liver, pineal gland, hypophysis, median eminence of the hypothalamus, and cervical ganglion. The hyaline cartilage and glandular part of the stomach showed moderate radioactivity. In the presence of an excess amount of unlabeled GABA, radioactivity in most of tissues decreased significantly, but no significant difference in radioactivity was observed in the presence of baclofen and isoguvacine, agonists of GABA(A) and GABA(B) receptors, respectively. Autoradiography of (3)H-baclofen showed that the kidney had high level of radioactivity, whereas the activity in other tissues and organs was similar or lower than in the blood except for the content of the urinary bladder and the pancreas at 15 minutes after injection. These data indicate that radioactivity from incorporated (14)C-GABA into a variety of cells is much higher than that from bound (14)C-GABA to the receptor sites. Our results suggest that GABA can be quickly localized in many organs of the mouse body after 3 minutes following injection, and GABA may serve multiple functions in those organs.

Effects of GABA receptor antagonist on trigeminal caudalis nociceptive neurons in normal and neonatally capsaicin-treated rats

We have recently demonstrated that significant increases in cutaneous mechanoreceptive field (RF) size and spontaneous activity occur in nociceptive neurons of trigeminal subnucleus caudalis (Vc, the medullary dorsal horn) of adult rats depleted of C-fiber afferents by neonatal treatment with capsaicin. These neuronal changes in capsaicin-treated (CAP) rats are suggestive of central neuroplasticity and involve N-methyl-D-aspartic acid (NMDA) receptor mechanisms. The present study examined whether the GABA(A) receptor antagonist bicuculline (BIC) or the GABA(B) receptor antagonist 2-hydroxysaclofen (SAC) can influence the RF properties and activity of Vc nociceptive neurons classified as either nociceptive-specific or wide-dynamic range in CAP adult rats or in neonatally vehicle-treated (CON) rats. C-fiber depletion was confirmed in the CAP rats by a significant decrease in plasma extravasation of Evans blue dye in a skin area receiving topical application of mustard oil, a small-fiber excitant and inflammatory irritant. As previously reported, marked increases in cutaneous RF size and spontaneous activity occurred in Vc nociceptive neurons of adult CAP rats, compared with CON rats. GABA(A) receptor blockade by BIC (i.t.) in CON rats produced a significant increase in spontaneous activity and in pinch RF size and tactile RF size (or appearance of a tactile area in the RF of nociceptive-specific neurons), as well as a significant lowering of the mechanical threshold and a significant enhancement of responses to pinch stimuli applied to the RF. In CAP rats, GABA(A) receptor blockade also produced significant changes similar to those documented in CON rats, except for a paradoxical and significant decrease in pinch RF size and no noticeable changes in responses to pinch stimuli. GABA(B) receptor blockade by SAC (i.t. ) did not produce any significant changes in Vc nociceptive neurons in either CON or CAP rats. These results suggest that GABA(A) receptor-mediated inhibition may be involved in maintaining the functional expression of Vc nociceptive neuronal properties in normal conditions, and that in animals depleted of their C-fiber afferents, some features of this GABA(A) receptor-mediated modulation may be disrupted such that a GABA(A) receptor-mediated excitation is manifested.

Activation of peripheral GABAA receptors inhibits temporomandibular joint-evoked jaw muscle activity

We have previously shown that injection of mustard oil or glutamate into rat temporomandibular joint (TMJ) tissues, an experimental model of acute TMJ injury, can reflexly induce a prolonged increase in the activity of both digastric (jaw-opener) and masseter (jaw-closer) muscles. In this study, GABA was applied to the TMJ region by itself or in combination with glutamate, and the magnitude of evoked jaw muscle electromyographic (EMG) activity was measured. Application of GABA alone to the TMJ region did not evoke significant jaw muscle EMG activity when compared with normal saline controls. In contrast, co-application of GABA and glutamate into the TMJ region decreased the magnitude of glutamate-evoked EMG activity. This GABA-mediated inhibition of glutamate-evoked EMG activity followed an inverse dose-response relationship with an estimated median inhibitory dose (ID50) of 0.17 +/- 0.05 (SE) micromol and 0.031 +/- 0.006 micromol for the digastric and masseter muscles, respectively. Co-administration of the GABAA receptor antagonist bicuculline (0.05 micromol) but not the GABAB receptor antagonist phaclofen (0.05 or 0. 15 micromol) reversed the suppressive actions of GABA, indicating that this action of GABA may be mediated by peripheral GABAA receptors located within the TMJ region. Our results suggest that activation of peripheral GABAA receptors located within the TMJ region could act to decrease the transmission of nociceptive information.

Developmental expression of functional GABAA receptors containing the gamma 2 subunit in neurons derived from embryonal carcinoma (P19) cells

The expression of the gamma 2 subunit into functional GABAA receptors has been examined in the embryonal carcinoma (EC) cell line P19, a pluripotent cell line which differentiates into a neuronal phenotype after exposure to retinoic acid. Whole-cell voltage-clamp recordings were used to examine the characteristics of the GABA receptors expressed in P19 cells at different times after exposure to retinoic acid. Messenger RNA for both the gamma 2L and gamma 2S splice variants of the GABAA receptor increased dramatically following differentiation of P19 EC cells with retinoic acid. By 12 days after retinoic acid treatment, while both mRNAs were present, there was an approximately 10-fold greater abundance of gamma 2S mRNA compared to gamma 2L. However, at this same time point neurons derived from P19 cells stained intensely with a polyclonal antibody raised against a peptide fragment specific for the gamma 2L subunit. A significant increase in both the affinity for GABA and the maximum current amplitude elicited by GABA occurred between 7 and 12 days after retinoic acid treatment. In contrast, the ability of the benzodiazepine agonist flurazepam to potentiate GABA-induced membrane current was the same at 7 and 12 days after retinoic acid treatment. These data suggest that the gamma 2 subunit of the GABAA receptor is expressed early following differentation of P19 cells into a neuronal phenotype, and that this subunit is incorporated into functional GABAA receptors. Moreover, the gamma 2S and gamma 2L splice variants of this subunit may be co-expressed in neurons derived from P19 cells. The observed affinity change for GABA may reflect a time-dependent change in the expression of alpha and/or beta subunits of the GABAA receptor, as occurs in developing neuronal tissue both in vitro and in vivo.

From ion currents to genomic analysis: recent advances in GABAA receptor research

The gamma-aminobutyric acid type A (GABAA) receptor represents an elementary switching mechanism integral to the functioning of the central nervous system and a locus for the action of many mood- and emotion-altering agents such as benzodiazepines, barbiturates, steroids, and alcohol. Anxiety, sleep disorders, and convulsive disorders have been effectively treated with therapeutic agents that enhance the action of GABA at the GABAA receptor or increase the concentration of GABA in nervous tissue. The GABAA receptor is a multimeric membrane-spanning ligand-gated ion channel that admits chloride upon binding of the neurotransmitter GABA and is modulated by many endogenous and therapeutically important agents. Since GABA is the major inhibitory neurotransmitter in the CNS, modulation of its response has profound implications for brain functioning. The GABAA receptor is virtually the only site of action for the centrally acting benzodiazepines, the most widely prescribed of the anti-anxiety medications. Increasing evidence points to an important role for GABA in epilepsy and various neuropsychiatric disorders. Recent advances in molecular biology and complementary information derived from pharmacology, biochemistry, electrophysiology, anatomy and cell biology, and behavior have led to a phenomenal growth in our understanding of the structure, function, regulation, and evolution of the GABAA receptor. Benzodiazepines, barbiturates, steroids, polyvalent cations, and ethanol act as positive or negative modulators of receptor function. The description of a receptor gene superfamily comprising the subunits of the GABAA, nicotinic acetylcholine, and glycine receptors has led to a new way of thinking about gene expression and receptor assembly in the nervous system. Seventeen genetically distinct subunit subtypes (alpha 1-alpha 6, beta 1-beta 4, gamma 1-gamma 4, delta, p1-p2) and alternatively spliced variants contribute to the molecular architecture of the GABAA receptor. Mysteriously, certain preferred combinations of subunits, most notably the alpha 1 beta 2 gamma 2 arrangement, are widely codistributed, while the expression of other subunits, such as beta 1 or alpha 6, is severely restricted to specific neurons in the hippocampal formation or cerebellar cortex. Nervous tissue has the capacity to exert control over receptor number, allosteric uncoupling, subunit mRNA levels, and posttranslational modifications through cellular signal transduction mechanisms under active investigation. The genomic organization of the GABAA receptor genes suggests that the present abundance of subtypes arose during evolution through the duplication and translocations of a primordial alpha-beta-gamma gene cluster. This review describes these varied aspects of GABAA receptor research with special emphasis on contemporary cellular and molecular discoveries.

Peripheral GABAA receptor-mediated effects of sodium valproate on dural plasma protein extravasation to substance P and trigeminal stimulation

1. The GABA transaminase inhibitor and activator of glutamic acid decarboxylase, valproic acid is being used for the treatment of migraine. Its mechanism of action is unknown. We tested the effects of sodium valproate and GABAA-agonist muscimol on dural plasma protein ([125I]-bovine serum albumin) extravasation evoked by either unilateral trigeminal ganglion stimulation (0.6 mA, 5 ms, 5 Hz, 5 min) or substance P (SP) administration (1 nmol kg-1,i.v.) in anaesthetized Sprague-Dawley rats. 2. Intraperitoneal (i.p.) injection of sodium valproate or muscimol, but not baclofen (< or = 10 mg kg-1, i.p.) dose-dependently reduced dural plasma protein extravasation caused either by electrical trigeminal stimulation (ED50: 6.6 +/- 1.4 mg kg-1, i.p., and 58 +/- 18 micrograms kg-1, i.p. for valproate or muscimol, respectively) or by intravenous substance P administration (ED50: 3.2 +/- 1.4 mg kg-1, i.p. and 385 +/- 190 micrograms kg-1, i.p. for valproate or muscimol, respectively). 3. Valproate (6.6 mg kg-1, i.p.) or muscimol (58 micrograms kg-1, i.p.) had no effect on mean arterial blood pressure or heart rate when measured for 30 min after i.p. administration. 4. The GABAA-antagonist bicuculline (0.01 mg kg-1, i.p.) completely reversed the effect of valproate and muscimol on plasma extravasation following electrical stimulation or substance P administration, whereas the GABAB-receptor antagonist, phaclofen (0.01-1 mg kg-1, i.p.) did not. Bicuculline or phaclofen, given alone, did not alter the plasma extravasation response after either electrical stimulation or SP administration. 5. Valproate decreased plasma extravasation following substance P administration in adult animals, neonatally treated with capsaicin by a bicuculline-reversible mechanism. This suggests that GABAA receptors are not found primarily on those afferent neurones or fibres which are sensitive to capsaicin treatment in neonatal rats.6. We conclude that sodium valproate blocks plasma extravasation in the meninges through GABAA mediated postjunctional receptors probably within the meninges. The dosages required are comparable to those used clinically. Agonists and modulators at the GABAA receptor may become useful for the development of selective therapeutic agents for migraine and cluster headache.

Expression of glutamate (AMPA type) and gamma-aminobutyric acid (GABA)A receptors in the rat caudal trigeminal spinal nucleus

The localization of GABAA receptor gamma 1 and gamma 2 subunits and the AMPA-type glutamate receptor subunits GluR1 and GluR2/3 were identified in the caudal trigeminal spinal tract nucleus (TNC) by immunohistochemistry using specific antibodies. The receptor species on the projecting neurons to the thalamus in TNC were also examined. A retrograde tracer, Fluoro-gold (FG), was injected into the thalamus, and the sections were simultaneously labeled with the antibodies. Injection of FG into the ventral posteromedial nucleus of the thalamus resulted in labeling of scattered neurons contralaterally in the TNC. Most of the neurons labeled by retrograde tracing also showed gamma 1- and gamma 2-like immunoreactivity, while many of the neurons containing FG lacked GluR1- and GluR2/3-like immunoreactivity. These findings show that neurons projecting to the thalamus from the TNC receive GABAergic input via GABAA receptors containing gamma 1 and gamma 2 subunits, while many neurons expressing the AMPA-type receptor did not project to the thalamus.

Expression of GABAA receptor gamma 1 and gamma 2 subunits in the peripheral vestibular system of the rat

The distribution of GABAA receptor gamma 1 and gamma 2 subunits in the rat vestibular ganglion cells and end-organs was examined by using immunohistochemical techniques. Both gamma 1 and gamma 2 subunit-like immunoreactivities were observed in most vestibular ganglion cell bodies and peripheral terminal endings in the vestibular sensory epithelia. These results indicate that GABAA receptors are expressed in the vestibular afferent terminal endings and therefore suggest that GABA in addition to glutamate is a neurotransmitter which effects on vestibular afferents.