Differential immunocytochemical localization of GABAA receptor gamma 1 and gamma 2 subunits in the rat brain

Alterations of GABA(A) and glutamate receptor subunits and heat shock protein in rat hippocampus following traumatic brain injury and in posttraumatic epilepsy

Traumatic brain injury (TBI) can result in the development of posttraumatic epilepsy (PTE). Recently, we reported differential alterations in tonic and phasic GABA(A) receptor (GABA(A)R) currents in hippocampal dentate granule cells 90 days after controlled cortical impact (CCI) (Mtchedlishvili et al., 2010). In the present study, we investigated long-term changes in the protein expression of GABA(A)R α1, α4, γ2, and δ subunits, NMDA (NR2B) and AMPA (GluR1) receptor subunits, and heat shock proteins (HSP70 and HSP90) in the hippocampus of Sprague-Dawley rats evaluated by Western blotting in controls, CCI-injured animals without PTE (CCI group), and CCI-injured animals with PTE (PTE group). No differences were found among all three groups for α1 and α4 subunits. Significant reduction of γ2 protein was observed in the PTE group compared to control. CCI caused a 194% and 127% increase of δ protein in the CCI group compared to control (p<0.0001), and PTE (p<0.0001) groups, respectively. NR2B protein was increased in CCI and PTE groups compared to control (p=0.0001, and p=0.011, respectively). GluR1 protein was significantly decreased in CCI and PTE groups compared to control (p=0.003, and p=0.001, respectively), and in the PTE group compared to the CCI group (p=0.036). HSP70 was increased in CCI and PTE groups compared to control (p=0.014, and p=0.005, respectively); no changes were found in HSP90 expression. These results provide for the first time evidence of long-term alterations of GABA(A) and glutamate receptor subunits and a HSP following CCI.

Steroid modulation of GABAA receptor-mediated transmission in the hypothalamus: effects on reproductive function

The hypothalamus, the seat of neuroendocrine control, is exquisitely sensitive to gonadal steroids. For decades it has been known that androgens, estrogens and progestins, acting through nuclear hormone receptors, elicit both organizational and activational effects in the hypothalamus and basal forebrain that are essential for reproductive function. While changes in gene expression mediated by these classical hormone pathways are paramount in governing both sexual differentiation and the neural control of reproduction, it is also clear that steroids impart critical control of neuroendocrine functions through non-genomic mechanisms. Specifically, endogenous neurosteroid derivatives of deoxycorticosterone, progesterone and testosterone, as well and synthetic anabolic androgenic steroids that are self-administered as drugs of abuse, elicit acute effects via allosteric modulation of gamma-aminobutyric acid type A receptors. GABAergic transmission within the hypothalamus and basal forebrain is a key regulator of pubertal onset, the expression of sexual behaviors, pregnancy and parturition. Summarized here are the known actions of steroid modulators on GABAergic transmission within the hypothalamus/basal forebrain, with a focus on the medial preoptic area and the supraoptic/paraventricular nuclei that are known to be central players in the control of reproduction.

Drug interactions at GABA(A) receptors

Neurotransmitter receptor systems have been the focus of intensive pharmacological research for more than 20 years for basic and applied scientific reasons, but only recently has there been a better understanding of their key features. One of these systems includes the type A receptor for the gamma-aminobutyric acid (GABA), which forms an integral anion channel from a pentameric subunit assembly and mediates most of the fast inhibitory neurotransmission in the adult vertebrate central nervous system. Up to now, depending on the definition, 16-19 mammalian subunits have been cloned and localized on different genes. Their assembly into proteins in a poorly defined stoichiometry forms the basis of functional and pharmacological GABA(A) receptor diversity, i.e. the receptor subtypes. The latter has been well documented in autoradiographic studies using ligands that label some of the receptors' various binding sites, corroborated by recombinant expression studies using the same tools. Significantly less heterogeneity has been found at the physiological level in native receptors, where the subunit combinations have been difficult to dissect. This review focuses on the characteristics, use and usefulness of various ligands and their binding sites to probe GABA(A) receptor properties and to gain insight into the biological function from fish to man and into evolutionary conserved GABA(A) receptor heterogeneity. We also summarize the properties of the novel mouse models created for the study of various brain functions and review the state-of-the-art imaging of brain GABA(A) receptors in various human neuropsychiatric conditions. The data indicate that the present ligands are only partly satisfactory tools and further ligands with subtype-selective properties are needed for imaging purposes and for confirming the behavioral and functional results of the studies presently carried out in gene-targeted mice with other species, including man.

Role of the GABA(A) receptor gamma2 subunit in the development of gonadotropin-releasing hormone neurons in vivo

We have employed transgenic mouse models to examine the functional significance of the gamma2 subunit of the GABA(A) (gamma-aminobutyric acid) receptor to the correct development of gonadotropin-releasing hormone (GnRH) neurons in vivo. In the first experiment, the expression of gamma2 subunit protein by the GnRH phenotype was determined using transgenic mice in which GnRH gene sequences direct the expression of the LacZ reporter to the nucleus of the GnRH neurons. This greatly facilitates the immunocytochemical identification of non-nuclear-located antigens within GnRH neurons and revealed that approximately 25% of juvenile GnRH neurons were immunoreactive for the gamma2 subunit and that this increased to 40% in pubertal mice. In the second experiment, GnRH mRNA expression was examined in the brains of gamma2 subunit knockout mice (gamma2(0/0)) and their wild-type (gamma2+/+) littermates at embryonic day 15 and postnatal days (P) 0 and 11-16 using in situ hybridization. The distribution and numbers of cells expressing GnRH mRNA in gamma2+/+ and gamma2(0/0) mice were not found to differ at any age. However, the GnRH mRNA content of medial septal cells was significantly lower in gamma2(0/0) compared with gamma2+/+ mice at P11-16 (P<0.05) and the same trend was observed for preoptic area neurons. These results demonstrate that while the gamma2 subunit of the GABA(A) receptor is expressed by postnatal GnRH neurons, their embryonic development does not require a functional gamma2 subunit. In contrast, postnatal GnRH mRNA expression was found to be dependent upon signalling through the GABA(A) receptor.

Properties and sex-specific differences of GABAA receptors in neurons expressing gamma1 subunit mRNA in the preoptic area of the rat

Gamma-aminobutyric acid type A (GABAA) receptors expressed within the medial preoptic area (mPOA) are known to play a critical role in regulating sexual and neuroendocrine functions. In the rat brain, high levels of expression of the gamma1 subunit mRNA of the GABAA receptor are restricted to a limited number of regions that mediate sexual behaviors, including the mPOA. The biophysical and pharmacological profiles of native gamma1-containing receptors in neurons are unknown. Here, we have characterized the properties of GABAA receptor-mediated spontaneous inhibitory postsynaptic currents (sIPSCs) and currents elicited by fast perfusion of GABA to isolated mPOA neurons of juvenile male and female rats. No significant sex-specific differences were evident in the mean peak amplitude, distribution of event amplitudes, kinetics of current decay, or the frequency of sIPSCs. The profile of modulation of sIPSCs by diazepam, beta-CCM and zolpidem, allosteric modulators that act at the benzodiazepine (BZ) site of the GABAA receptor, support the assertion that mPOA neurons of both sexes express functional gamma1-containing receptors. The ability of zolpidem to modulate both sIPSC amplitude and currents elicited by rapid perfusion of GABA to mPOA neurons differed significantly between the sexes. Zolpidem reversibly induced negative modulation of currents in mPOA neurons isolated from male rats, but had no effect in mPOA neurons from female rats. Concentration-response analysis of responses in neurons acutely isolated from male rats indicated an IC50 of 58 nM with maximal decreases of approximately 50% of control peak current amplitude. In situ hybridization analysis demonstrated that levels of the gamma1 subunit mRNA are significantly higher in mPOA neurons from male than female rats. No significant sex-specific differences were detected in the levels of alpha1, alpha2, or alpha5 mRNAs. These results suggest that native gamma1-containing receptors are expressed in primary neurons of the mPOA and that sex-specific differences in the expression of this subunit may contribute to sexual dimorphism in GABAA receptor modulation by compounds acting at the BZ site.

Hormone-dependent regulation of GABAA receptor gamma subunit mRNAs in sexually dimorphic regions of the rat brain

Transmission mediated by gamma-aminobutyric acid type A (GABAA) receptors expressed within the medial preoptic area (mPOA) and the ventromedial nucleus (VMN) of the hypothalamus is known to play critical, but contrasting, roles in regulating steroid-dependent sexual behaviours in rats. Previous studies have demonstrated a striking dichotomy in receptor composition between the two regions with regard to gamma, but not alpha or beta, subunit expression. To test if gonadal steroids regulate the expression of the gamma subunit genes within the mPOA and the VMN, in situ hybridization analysis for messenger RNAs encoding the gamma 1, gamma 2Short (gamma 2S) and gamma 2Long (gamma 2L) subunits was done in gonadectomized male and female rats and in gonadally intact females over the oestrous cycle. No significant differences in the expression of the gamma subunit mRNAs were observed in gonadectomized male versus female rats. Significant effects of gonadal state in female rats were observed for gamma 1 mRNA levels in the mPOA and gamma 2L levels in the VMN. These data demonstrate that gonadal hormones exert activational control of expression of GABAA receptor gamma subunit mRNAs and suggest that differences in receptor structure may contribute to the functional modulation of female sexual behaviours mediated by GABAergic transmission in these regions.

The gamma subunits of the native GABAA/benzodiazepine receptors

Subunit-specific antibodies to all the gamma subunit isoforms described in mammalian brain (gamma(1), gamma(2S), gamma(2L), and gamma(3) have been made. The proportion of GABA(A) receptors containing each gamma subunit isoform in various brain regions has been determined by quantitative immunoprecipitation. In all tested regions of the rat brain, the gamma(1) and gamma(3) subunits are present in considerable smaller proportion of GABA(A) receptor than the gamma(2) subunit. Immunocytochemistry shows that gamma(1) immunoreactivity concentrates in the stratum oriens and stratum radiatum of the CA1 region of the hippocampus. In the dentate gyrus, gamma(1) immunoreactivity concentrates on the outer 2/3 of the molecular layer coinciding with the localization of the axospinous synapses of the perforant pathway. In contrast, gamma(3) immunoreactivity concentrates on the basket cells and other GABAergic local circuit neurons of the hilus. These cells are also rich in gamma(2S). In the cerebellum, gamma(1)++ immunolabeling was localized on the Bergmann glia. The gamma(2S) and gamma(2L) subunits are differentially expressed in various brain regions. Thus the gamma(2S) is highly expressed in the olfactory bulb and hippocampus whereas the gamma(2L) is very abundant in inferior colliculus and cerebellum, particularly in Purkinje cells, as immunocytochemistry, in situ hybridization and immunoprecipitation techniques have revealed. The gamma(2S) and gamma(2L) coexist in some brain areas and cell types. Moreover, the gamma(2S) and gamma(2L) subunits can coexist in the same GABA(A) receptor pentamer. We have shown that this is the case in some GABA(A) receptors expressed in cerebellar granule cells. These GABA(A) receptors also have alpha and beta subunits forming the pentamer. Immunoblots have shown that the rat gamma(1), gamma(2S), gamma(2L) and gamma(3) subunits are peptides of 47, 45, 47 and 44 kDa respectively. Results also indicate that there are aging-related changes in the expression of the gamma(2S) and gamma(2L) subunits in various brain regions which suggest the existence of aging-related changes in the subunit composition of the GABA(A) receptors which in turn might lead to changes in receptor pharmacology. The results obtained with the various gamma subunit isoforms are discussed in terms of the high molecular and binding heterogeneity of the native GABA(A) receptors in brain.

Brain GABAA receptors studied with subunit-specific antibodies

Brain GABAA/benzodiazepine receptors are highly heterogeneous. This heterogeneity is largely derived from the existence of many pentameric combinations of at least 16 different subunits that are differentially expressed in various brain regions and cell types. This molecular heterogeneity leads to binding differences for various ligands, such as GABA agonists and antagonists, benzodiazepine agonists, antagonists, and inverse agonists, steroids, barbiturates, ethanol, and Cl- channel blockers. Different subunit composition also leads to heterogeneity in the properties of the Cl- channel (such as conductance and open time); the allosteric interactions among subunits; and signal transduction efficacy between ligand binding and Cl- channel opening. The study of recombinant receptors expressed in heterologous systems has been very useful for understanding the functional roles of the different GABAA receptor subunits and the relationships between subunit composition, ligand binding, and Cl- channel properties. Nevertheless, little is known about the complete subunit composition of the native GABAA receptors expressed in various brain regions and cell types. Several laboratories, including ours, are using subunit-specific antibodies for dissecting the heterogeneity and subunit composition of native (no reconstituted) brain GABAA receptors and for revealing the cellular and subcellular distribution of these subunits in the nervous system. These studies are also aimed at understanding the ligand-binding, transduction mechanisms, and channel properties of the various brain GABAA receptors in relation to synaptic mechanisms and brain function. These studies could be relevant for the discovery and design of new drugs that are selective for some GABAA receptors and that have fewer side effects.

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.

Characterisation of GABAA receptor gamma subunit expression by magnocellular neurones in rat hypothalamus

Gamma-aminobutyric acid (GABA) is known to inhibit the electrical and secretory activity of oxytocin and vasopressin neurones located in the supraoptic and paraventricular nuclei following osmotic, cardiovascular or suckling stimuli. To understand fully the nature of GABA actions on these magnocellular neurones it is important to define the heteropentameric GABAA receptor proteins they express. In the present study, single and dual labelling in situ hybridisation and immunocytochemical experiments were undertaken to define the GABAA receptor gamma subunits expressed by these cells. In situ hybridisation with 35S-labelled antisense oligonucleotides showed that all magnocellular neurones in the supraoptic and paraventricular nuclei of the female rat expressed mRNA encoding the gamma 2 subunit of the GABAA receptor but not the gamma 1 or gamma 3 subunits. Immunocytochemical experiments using a specific polyclonal rabbit antibody directed against the gamma 2 subunit of the GABAA receptor showed that all hypothalamic magnocellular neurones were strongly immunoreactive for gamma 2 subunit protein. Dual in situ hybridisation experiments using the gamma 2 subunit 35 S-labelled oligonucleotide with alkaline phosphatase-labelled antisense oligonucleotides specific for either oxytocin or vasopressin revealed that essentially all oxytocin and vasopressin neurones in both the supraoptic and paraventricular nuclei expressed the gamma 2 subunit of the GABAA receptor. Similarly, sequential double immunoperoxidase staining revealed that all oxytocin and vasopressin neurones in both magnocellular nuclei of the hypothalamus were immunoreactive for the gamma 2 subunit. This study shows that only the gamma 2 subunit of the GABAA receptor gamma subunit family is expressed by hypothalamic oxytocin and vasopressin neurones. In conjunction with our previous results, these findings indicate that individual magnocellular neurones express a complement of alpha 1, alpha 2, beta 2, beta 3 and gamma 2 subunits of the GABAA receptor. The observation of strong gamma 2 subunit expression by neurones known to also express alpha 1 and alpha 2 subunit proteins suggests that these magnocellular cells may express GABAA receptors with both benzodiazepine type-1 and type-2 pharmacology.

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.

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.

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

We examined the expression of gamma-aminobutyric acid (GABA)A receptor gamma 1 and gamma 2 subunit mRNAs in the rat trigeminal ganglion using in situ hybridization histochemistry. Most ganglion cells expressed both gamma 1 and gamma 2 mRNAs simultaneously. These findings are a marked contrast to the findings in the central nervous system where areas expressing both subunits are rare. In addition, we demonstrated using immunohistochemistry that the gamma 1 subunit is also expressed at the protein level in trigeminal ganglion neurons and fibers in the trigeminal spinal nucleus.