Antibodies to the human gamma 2 subunit of the gamma-aminobutyric acidA/benzodiazepine receptor

RGS14414-Mediated Activation of the 14-3-3ζ in Rodent Perirhinal Cortex Induces Dendritic Arborization, an Increase in Spine Number, Long-Lasting Memory Enhancement, and the Prevention of Memory Deficits

The remedy of memory deficits has been inadequate, as all potential candidates studied thus far have shown limited to no effects and a search for an effective strategy is ongoing. Here, we show that an expression of RGS14414 in rat perirhinal cortex (PRh) produced long-lasting object recognition memory (ORM) enhancement and that this effect was mediated through the upregulation of 14-3-3ζ, which caused a boost in BDNF protein levels and increase in pyramidal neuron dendritic arborization and dendritic spine number. A knockdown of the 14-3-3ζ gene in rat or the deletion of the BDNF gene in mice caused complete loss in ORM enhancement and increase in BDNF protein levels and neuronal plasticity, indicating that 14-3-3ζ-BDNF pathway-mediated structural plasticity is an essential step in RGS14414-induced memory enhancement. We further observed that RGS14414 treatment was able to prevent deficits in recognition, spatial, and temporal memory, which are types of memory that are particularly affected in patients with memory dysfunctions, in rodent models of aging and Alzheimer's disease. These results suggest that 14-3-3ζ-BDNF pathway might play an important role in the maintenance of the synaptic structures in PRh that support memory functions and that RGS14414-mediated activation of this pathway could serve as a remedy to treat memory deficits.

14-3-3ζ is crucial for the conversion of labile short-term object recognition memory into stable long-term memory

The consolidation of new memories into long-lasting memories is multistage process characterized by distinct temporal dynamics. However, our understanding on the initial stage of transformation of labile memory of recent experience into stable memory remains elusive. Here, with the use of rats and mice overexpressing a memory enhancer called regulator of G protein signaling 14 of 414 amino acids (RGS14₄₁₄ ) as a tool, we show that the expression of RGS14₄₁₄ in male rats' perirhinal cortex (PRh), which is a brain area crucial for object recognition memory (ORM), enhanced the ORM to the extent that it caused the conversion of labile short-term ORM (ST-ORM) expected to last for 40 min into stable long-term ORM (LT-ORM) traceable after a delay of 24 hr, and that the temporal window of 40 to 60 min after object exposure not only was key for this conversion but also was the time frame when a surge in 14-3-3ζ protein was observed. A knockdown of 14-3-3ζ gene abrogated both the increase in 14-3-3ζ protein and the formation of LT-ORM. Furthermore, this 14-3-3ζ upregulation increased brain-derived growth factor (BDNF) levels in the time frame of 60 min and 24 hr and 14-3-3ζ knockdown decreased the BDNF levels, and a deletion of BDNF gene produced loss in mice ability to form LT-ORM. Thus, within 60 min of object exposure, 14-3-3ζ facilitated the conversion of labile ORM into stable ORM, whereas beyond the 60 min, it mediated the consolidation of the stable memory into long-lasting ORM by regulating BDNF signaling.

RGS14414 treatment induces memory enhancement and rescues episodic memory deficits

Memory deficits affect a large proportion of the human population and are associated with aging and many neurologic, neurodegenerative, and psychiatric diseases. Treatment of this mental disorder has been disappointing because all potential candidates studied thus far have failed to produce consistent effects across various types of memory and have shown limited to no effects on memory deficits. Here, we show that the promotion of neuronal arborization through the expression of the regulator of G-protein signaling 14 of 414 amino acids (RGS14₄₁₄) not only induced robust enhancement of multiple types of memory but was also sufficient for the recovery of recognition, spatial, and temporal memory, which are kinds of episodic memory that are primarily affected in patients or individuals with memory dysfunction. We observed that a surge in neuronal arborization was mediated by up-regulation of brain-derived neurotrophic factor (BDNF) signaling and that the deletion of BDNF abrogated both neuronal arborization activation and memory enhancement. The activation of BDNF-dependent neuronal arborization generated almost 2-fold increases in synapse numbers in dendrites of pyramidal neurons and in neurites of nonpyramidal neurons. This increase in synaptic connections might have evoked reorganization within neuronal circuits and eventually supported an increase in the activity of such circuits. Thus, in addition to showing the potential of RGS14₄₁₄ for rescuing memory deficits, our results suggest that a boost in circuit activity could facilitate memory enhancement and the reversal of memory deficits.-Masmudi-Martín, M., Navarro-Lobato, I., López-Aranda, M. F., Delgado, G., Martín-Montañez, E., Quiros-Ortega, M. E., Carretero-Rey, M., Narváez, L., Garcia-Garrido, M. F., Posadas, S., López-Téllez, J. F., Blanco, E., Jiménez-Recuerda, I., Granados-Durán, P., Paez-Rueda, J., López, J. C., Khan, Z. U. RGS14₄₁₄ treatment induces memory enhancement and rescues episodic memory deficits.

Molecular and functional interaction between protocadherin-γC5 and GABAA receptors

We have found that the γ2 subunit of the GABA(A) receptor (γ2-GABA(A)R) specifically interacts with protocadherin-γC5 (Pcdh-γC5) in the rat brain. The interaction occurs between the large intracellular loop of the γ2-GABA(A)R and the cytoplasmic domain of Pcdh-γC5. In brain extracts, Pcdh-γC5 coimmunoprecipitates with GABA(A)Rs. In cotransfected HEK293 cells, Pcdh-γC5 promotes the transfer of γ2-GABA(A)R to the cell surface. We have previously shown that, in cultured hippocampal neurons, endogenous Pcdh-γC5 forms clusters, some of which associate with GABAergic synapses. Overexpression of Pcdh-γC5 in hippocampal neurons increases the density of γ2-GABA(A)R clusters but has no significant effect on the number of GABAergic contacts that these neurons receive, indicating that Pcdh-γC5 is not synaptogenic. Deletion of the cytoplasmic domain of Pcdh-γC5 enhanced its surface expression but decreased the association with both γ2-GABA(A)R clusters and presynaptic GABAergic contacts. Cultured hippocampal neurons from the Pcdh-γ triple C-type isoform knock-out (TCKO) mouse (Pcdhg(tcko/tcko)) showed plenty of GABAergic synaptic contacts, although their density was reduced compared with sister cultures from wild-type and heterozygous mice. Knocking down Pcdh-γC5 expression with shRNA decreased γ2-GABA(A)R cluster density and GABAergic innervation. The results indicate that, although Pcdh-γC5 is not essential for GABAergic synapse formation or GABA(A)R clustering, (1) Pcdh-γC5 regulates the surface expression of GABA(A)Rs via cis-cytoplasmic interaction with γ2-GABA(A)R, and (2) Pcdh-γC5 plays a role in the stabilization and maintenance of some GABAergic synapses.

A dynamic expression pattern of sGalpha(i2) protein during early period of postnatal rat brain development

The function of sGalphai2 protein in central nervous system is not well understood. Therefore to explore the possible role of this protein in postnatal brain development, we have analyzed the protein expression pattern of brain obtained from rats of postnatal day 0 (P0) to P90 by dot-blots and immunocytochemistry techniques. In dot-blots, both nuclear and membrane fractions showed a gradual decrease from P0 to P60. Highest protein level was observed at the age of P0. There was also a trend of decline in the sGalphai2 protein from P0 to P90 in brain sections stained by immunocytochemistry method. At P0, the protein labeling was highest in cerebral cortex, hippocampus, cerebellum and mitral cell layer. In cerebral cortex, a drop in the immunolabeling of sGalphai2 protein was observed at P3, which was significantly increased at the age of P5. However, in striatum and olfactory tubercle, it was maintained through P0-P10 and P0-P5, respectively. Thalamus was one of the areas where labeling was not as strong as cortex, hippocampus or striatum. In contrary to other areas, immunostaining of sGalphai2 in corpus-callosum and lacunosum-molecular was not seen at P0 and appeared in advanced postnatal ages. A detectable level of sGalphai2 protein was observed at P5 in carpus-callosum and at P20 in lacunosum-molecular. A high level of sGalphai2 protein in the period when cellular layer organization and synaptic innervations, synaptic connections and maturation take place, suggests for a potential role of this protein in the early postnatal brain development.

Effect of chronic administration of ethanol on the regulation of the delta-subunit of GABA(A) receptors in the rat brain

In the present study, we investigated the effect of chronic ethanol (CE) administration on the polypeptide levels of the delta-subunit of GABA(A) receptors and [(3)H]muscimol binding to the immunoprecipitated delta-subunit-containing GABA(A) receptor assemblies in the rat brain. CE administration resulted a down-regulation of polypeptide levels of the delta-subunit of GABA(A) receptors in the rat cerebellum and hippocampus, whereas there were no changes in the delta-subunit polypeptide levels in the rat cerebral cortex. Further, CE administration caused a down-regulation of native delta-subunit-containing GABA(A) receptor assemblies in the rat cerebellum as determined by [(3)H]muscimol binding to the immunoprecipitated receptor assemblies. These results indicate that the delta-subunit-containing GABA(A) receptors may play a role in chronic ethanol-induced tolerance and dependence.

Role of a Galphai2 protein splice variant in the formation of an intracellular dopamine D2 receptor pool

Treatment of D2-receptor-expressing cells with specific drugs upregulates the receptor number at the cell surface independently of protein synthesis, leading to the concept of an intracellular receptor pool. However, how this pool is operating is still an enigma. Here, we report that a splice variant of the Galphai2 protein, protein sGalphai2, plays a crucial role in the maintenance of this D2-receptor pool. Co-expression of sGi2 with D2 receptor reduced receptor localization to cell surface by one-third. This effect is associated with specific intracellular protein-protein interaction and the formation of a sGi2-D2-receptor complex. It has been suggested that the formation of this complex serves to prevent D2 receptors from reaching the cell membrane. Treatment of D2-receptor-expressing cells with agonists increased the number of cell surface D2 receptors and coincided with a reduction in these receptors from intracellular complexes, suggesting that agonist treatment released D2 receptors from the complex allowing them to localize to the cell membrane. Thus, in addition to elucidating how the intracellular pool of D2 receptor functions, our findings uncover a novel mechanism regulating the density of cell surface D2 receptors.

Localization of the GoLoco motif carrier regulator of G-protein signalling 12 and 14 proteins in monkey and rat brain

Regulator of G-protein signalling (RGS)12 and -14 proteins possess the RGS domain, Ras-binding domains and the GoLoco motif. Emerging evidence suggests that these proteins are involved in several cellular functions in addition to stimulation of GTPase activity of G-protein alpha subunits. However, our understanding of the role of the two proteins in brain function remains marginal. Here, we have studied the expression pattern of RGS12 and RGS14 proteins in brain at regional, cellular and subcellular levels. Both proteins were expressed throughout the brain regions, including cortex, hippocampus, striatum, thalamus and substantia nigra. The most intense immunostaining for RGS12 was seen in cortex and that of RGS14 was found in striatum. In cortex, RGS12 and RGS14 proteins were associated with pyramidal and nonpyramidal cell types. Apical dendrites of pyramidal cells were also labelled. RGS12 was found in both nuclear and cytoplasmic compartments. In contrast to RGS12 protein, RGS14 was localized in astrocytes in addition to neurons. Pyramidal cells in the CA1 area showed labelling for both RGS proteins. The presence of RGS12 was predominantly nuclear in the striatum of rat brain; however, the labelling of this protein was non-nuclear in adult monkey brain. To our surprise, in 1-month-old monkey brain the immunostaining pattern of the same protein was changed to nuclear. Non-nuclear staining for RGS12 was also evident in thalamus of adult monkey brain; however, in 1-month-old monkey brain, it was seen into two different populations, one with nuclear and the other with cytoplasmic staining. Both RGS12 and RGS14 were exclusively localized at postsynaptic sites of excitatory synapses. Our results demonstrate a highly dynamic expression pattern of RGS12 and RGS14 proteins in the central nervous system, and support the view that these proteins may participate not only in G-protein receptor signalling pathways but also in other cellular activities.

Distribution of C-terminal splice variant of G alpha i2 in rat and monkey brain

The significance of Galphai2 in neural signal transmission is well defined. However, the function of its alternative splice variant named sGi2 is unknown. Therefore here, we have studied the localization of sGi2 protein in rat and monkey brain at light and electron microscopy level. We found that this novel protein is widely expressed in rat and monkey brain regions, which are known to play crucial role in brain functions. Hippocampus, cerebral cortex, amygdala, thalamus, striatum, nucleus accumbens, olfactory tubercle and dopaminergic cell groups of substantia nigra, hypothalamus and olfactory bulb showed strong labeling with anti-sGi2. At subcellular level, sGi2 protein was localized in intracellular compartments, including endoplasmic reticulum, Golgi complex, mitochondria and nucleus. This protein was also found localized extra-synaptically in both axons and spines, which were making excitatory as well as inhibitory synaptic contacts. Moreover, the frequent localization of sGi2 protein in neck of spines further suggests that this protein may not engage directly in neuronal signal transmission but could influence other participating proteins of this process.

Alterations in the expression of GABAA receptor subunits in cerebellar granule cells after the disruption of the alpha6 subunit gene

Any given subunit of the heteromultimeric type-A gamma-aminobutyric acid (GABA) GABAA receptor may be present in several receptor subtypes expressed by individual neurons. Changes in the expression of a subunit may result in differential changes in the expression of other subunits depending on the subunit composition of the receptor subtype, leading to alterations in neuronal responsiveness to GABA. We used the targeted disruption of the alpha6 subunit gene to test for changes in the expression of other GABAA receptor subunits. Immunoprecipitation and ligand binding experiments indicated that GABAA receptors were reduced by approximately 50% in the cerebellum of alpha6 -/- mice. Western blot experiments indicated that the alpha6 subunit protein completely disappeared from the cerebellum of alpha6 -/- mice, which resulted in the disappearance of the delta subunit from the plasma membrane of granule cells. The amount of beta2, beta3 and gamma2 subunits was reduced by approximately 50%, 20% and 40%, respectively, in the cerebella of alpha6 -/- mice. A comparison of the reduction in the level of alpha1, beta2, beta3, gamma2, or delta-subunit-containing receptors in alpha6 -/- cerebellum with those observed after removal of alpha6-subunit-containing receptors from the cerebella of alpha6 +/+ mice by immuno-affinity chromatography demonstrated the presence of a significantly higher than expected proportion of receptors containing beta3 subunits in alpha6 -/- mice. The receptors containing alpha1, beta2, beta3 and gamma2 subunits were present in the plasma membrane of granule cells of alpha6 -/- mice at both synaptic and extrasynaptic sites, as shown by electron microscopic immunocytochemistry. Despite the changes, the alpha1 subunit content of Golgi-cell-to-granule-cell synapses in alpha6 -/- animals remained unaltered, as did the frequency of alpha1 immunopositive synapses in the glomeruli. Furthermore, no change was apparent in the expression of the alpha1, beta2 and gamma2 subunits in Purkinje cells and interneurons of the molecular layer. These results demonstrate that in alpha6 -/- mice, the cerebellum expresses only half of the number of GABAA receptors present in wild-type animals. Since these animals have no gross motor deficits, synaptic integration in granule cells is apparently maintained by alpha1-subunit-containing receptors with an altered overall subunit composition, and/or by changes in the expression of other ligand and voltage gated channels.

An update on GABAA receptors

Recent advances in molecular biology and complementary information derived from neuropharmacology, biochemistry and behavior have dramatically increased our understanding of various aspects of GABAA receptors. These studies have revealed that the GABAA receptor is derived from various subunits such as alpha1-alpha6, beta1-beta3, gamma1-gamma3, delta, epsilon, pi, and rho1-3. Furthermore, two additional subunits (beta4, gamma4) of GABAA receptors in chick brain, and five isoforms of the rho-subunit in the retina of white perch (Roccus americana) have been identified. Various techniques such as mutation, gene knockout and inhibition of GABAA receptor subunits by antisense oligodeoxynucleotides have been used to establish the physiological/pharmacological significance of the GABAA receptor subunits and their native receptor assemblies in vivo. Radioligand binding to the immunoprecipitated receptors, co-localization studies using immunoaffinity chromatography and immunocytochemistry techniques have been utilized to establish the composition and pharmacology of native GABAA receptor assemblies. Partial agonists of GABAA receptors are being developed as anxiolytics which have fewer and less severe side effects as compared to conventional benzodiazepines because of their lower efficacy and better selectivity for the GABAA receptor subtypes. The subunit requirement of various drugs such as anxiolytics, anticonvulsants, general anesthetics, barbiturates, ethanol and neurosteroids, which are known to elicit at least some of their pharmacological effects via the GABAA receptors, have been investigated during the last few years so as to understand their exact mechanism of action. Furthermore, the molecular determinants of clinically important drug-targets have been investigated. These aspects of GABAA receptors have been discussed in detail in this review article.

Pharmacological and functional implications of developmentally-regulated changes in GABA(A) receptor subunit expression in the cerebellum

The cerebellum undergoes many morphological, pharmacological, and electrophysiological changes during the first 3 weeks of postnatal development. The purpose of this review is to present the most up to date synopsis of the pharmacological and functional changes in, gamma-aminobutyric acid (GABA) type A receptors during this time of cerebellar maturation. Since most of the diversity in cerebellar, GABA(A) receptor pharmacology lies within the granule cell layer, research groups have focused on this area of the cerebellum to study the developmental changes in GABA(A) receptor subunit expression and the neurodifferentiating factors involved in regulating this expression. Thus, it is important to note that developmental changes in GABA(A) receptor composition and its corresponding pharmacology will be essential for determining the type of GABA-mediated transmission that occurs between neuronal contacts in the neonatal and subsequently in the mature cerebellum.

Prominence of the dopamine D2 short isoform in dopaminergic pathways

As a result of alternative splicing, the D2 gene of the dopamine receptor family exists in two isoforms. The D2 long is characterized by the insertion of 29 amino acids in the third cytoplasmic loop, which is absent in the short isoform. We have produced subtype-specific antibodies against both the D2 short and D2 long isoforms and found a unique compartmentalization between these two isoforms in the primate brain. The D2 short predominates in the cell bodies and projection axons of the dopaminergic cell groups of the mesencephalon and hypothalamus, whereas the D2 long is more strongly expressed by neurons in the striatum and nucleus accumbens, structures targeted by dopaminergic fibers. These results show that the splice variants of the dopamine D2 receptor are differentially distributed and possess distinct functions. The strategic localization of the D2 short isoform in dopaminergic cell bodies and axons strongly suggests that this isoform is the likely dopamine autoreceptor, whereas the D2 long isoform is primarily a postsynaptic receptor.

Coexistence of two beta subunit isoforms in the same gamma-aminobutyric acid type A receptor

Three novel subunit-specific antisera to the beta1, beta2, and beta3 subunits of rat gamma-aminobutyric acid type A (GABAA) receptors have been used to study the native receptor in the rat brain. Affinity-purified anti-beta1, anti-beta2, and anti-beta3 antibodies recognized in immunoblots protein bands of 57, 55, and 57 kDa, respectively. Quantitative immunoprecipitation of solubilized GABAA receptors from various rat brain regions showed that the beta2 subunit was the most abundant isoform in cerebellum (in 96% of the GABAA receptors) and cerebral cortex (64%) but that it was the least abundant isoform in hippocampus (44%). The beta3 subunit was found most abundant in hippocampus (64%) followed by cerebral cortex (48%) and cerebellum (33%). The beta1 subunit was present in a very small proportion of the cerebellar GABAA receptors (3%), but it was present in a high proportion of the GABAA receptors from the hippocampus (49%) and cerebral cortex (32%). Quantitative receptor immunoprecipitation or immunopurification followed by immunoblotting experiments have revealed the existence of colocalization of two different beta subunit isoforms in a significant proportion of the brain GABAA receptors. Thus, in the rat cerebral cortex 33% of the GABAA receptors have both beta2 and beta3 subunits, and 19% of the receptors have both beta1 and beta3 subunits. The extent of colocalization of beta subunit isoforms varied among brain regions, being highest in hippocampus and lowest in cerebellum. These and other results taken together suggest that the number of alpha, beta, and gamma subunits (stoichiometry) in the brain GABAA receptor pentamers might not be unique. It might vary depending on receptor type.

GABAA receptor subunit expression changes in the rat cerebellum and cerebral cortex during aging

Significant aging-related decreased expression of various GABAAR subunit mRNAs (alpha 1, gamma 2, beta 2, beta 3 and sigma) was found in both cerebellum and cerebral cortex using quantitative dot blot and in situ hybridization techniques. Contrary to the other subunits, the alpha 6 mRNA expression was significantly increased in the aged cerebellum. Parallel age-related changes in protein expression for gamma 2 and beta 2/3 (decrease) and alpha 6 (increase) were revealed in cerebellum by quantitative immunocytochemistry. However, no significant changes in alpha 1 protein expression nor in the number or affinity of [3H]zolpidem binding sites were detected in cerebellum even though alpha 1 mRNA expression was significantly decreased in the aged rat. Age-related increased expression of alpha 6 mRNA and protein in the cerebellum was accompanied by no significant changes in the number of diazepam-insensitive [3H]Ro15-4513 binding sites. In the cerebral cortex, no changes in the protein expression of the main GABAA receptor subunits (alpha 1, gamma 2 and beta 2/3) were observed which contrasted with the age-related decreased expression of the corresponding mRNAs. No significant changes in the number or affinity of [3H]zolpidem binding sites were observed in the cerebral cortex. Thus, age-related changes in the mRNA expression of a particular subunit does not necessarily lead to similar changes in protein or assembly into mature GABAA receptors. The results reveal the existence of complex regulatory mechanisms of GABAA receptor expression, at the transcriptional, translational and post-translational and/or assembly levels, which vary with the subunit and brain area.

GABAA receptor subunit expression and assembly in cultured rat cerebellar granule neurons

The assembly of multisubunit GABAA receptors in specific neuronal populations is a complex process which is poorly understood. To begin to examine receptor assembly, alpha 1, beta 2/3, and gamma 2 subunit polypeptide expression and association, as well as receptor binding, were examined in cultured rat cerebellar granule neurons. Western blots revealed two alpha 1-immunoreactive proteins. A 39 kDa species was maximal at 2 days in culture and subsequently declined. In contrast, a 51 kDa polypeptide, the anticipated size of the mature alpha 1 subunit, was first detected at 4 days and increased throughout the culture period. Additional studies demonstrated that the beta 2/3 and gamma 2 subunits were detectable at 2 days and attained maximal levels by 6 days. The level of [3H]Ro15-1788 binding, a measure of assembled receptors, rose in parallel with the increases in the 51 kDa alpha 1, beta 2/3 and gamma 2 subunits. Moreover, the 51 kDa alpha 1, beta 2/3, and gamma 2 subunits were associated in receptor complexes. However, immunohistochemical studies demonstrated the presence of substantial intracellular subunit staining. This finding suggest that only some of the subunits expressed in granule neurons contribute to functional GABAA receptors on the cell surface.

Aging-related subunit expression changes of the GABAA receptor in the rat hippocampus

Aging-related changes in the subunit expression of some hippocampal GABAA receptors have been found. Quantitative in situ hybridization has revealed that alpha 1, subunit messenger RNA expression was significantly increased in the hippocampus (34%) of old rats. The largest increases were observed in the dentate gyrus (76%) and in the CA1 field (30%). Quantitative immunocytochemistry also showed increased protein expression of the alpha 1 subunit in the dentate gyrus (19%) and CA1 (14%) of old rats. The increased alpha 1 messenger RNA and protein expression led to increased proportions of assembled GABAA receptors that contained alpha 1 subunits, as revealed by quantitative immunoprecipitation of (3H)flunitrazepam and (3H)muscimol binding. In contrast, there were no significant changes in the expression of beta 2, beta 3 and total gamma 2 (gamma 2S + gamma 2L) subunits, although a slightly increased expression of gamma 2L peptide was detected in the hippocampus proper (7%), but not in the dentate gyrus. The results are consistent with the notion that in the rat hippocampus there is an aging-related change in the subunit composition of some GABAA receptors.

Immunocytochemical localization of the alpha 6 subunit of the gamma-aminobutyric acidA receptor in the rat nervous system

The localization in the rat central nervous system and retina of the alpha 6 subunit peptide of the gamma-aminobutyric acid (GABAA) receptor has been studied by light microscopy immunocytochemistry with a specific anti-alpha 6 antibody. The alpha 6 subunit was present in the granule cells of the cerebellum, the granule cells of the dorsal cochlear nucleus, axons of the olfactory nerve including the glomerular endings, layer II of the dorsal horn of the spinal cord, and in the retinal synaptic layers, particularly the inner plexiform layer. Thus, contrary to the general belief, the alpha 6 subunit is not exclusively localized in the granule cells of the cerebellum. It is also expressed in some sensory neurons and other neurons involved in the initial processing of sensory information.

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.

The alpha 4 subunit of the GABAA receptors from rat brain and retina

A novel anti-alpha 4 antibody has been used for the purification and characterization of the alpha 4-containing GABAA receptors in the rat brain and for studying the immunocytochemical distribution of the alpha 4 subunit peptide in rat brain and retina. The anti-alpha 4 antibody recognized a 66 kDa peptide in brain membranes and immunoprecipitated 10-28% of the brain GABAA receptors in various brain regions as determined by [3H]muscimol binding. The highest immunoprecipitation values were obtained in the thalamus and the lowest in the cerebellum. Surprisingly, the receptors immunoprecipitated by anti-alpha 4 showed little or no diazepam-insensitive or diazepam-sensitive [3H]Ro15-4513 binding sites in any brain region. In the cerebellum, where 25% of the [3H]Ro15-4513 binding is diazepam-insensitive, much of the latter was immunoprecipitated by an anti-alpha 6 antibody but not by the anti-alpha 4 antibody. Immunoblots of immunoaffinity-purified GABAA receptors from the cerebral cortex on immobilized anti-alpha 4 revealed molecular colocalization of alpha 4 and gamma 2. However, the absence of significant benzodiazepine binding in these GABAA receptors suggests that the assembly of the alpha 4 and gamma 2 subunits in the cerebral cortex and in other brain regions is such that they do not normally form diazepam-insensitive [3H]Ro15-4513 binding sites. This result contrasts with the presence of diazepam-insensitive [3H]Ro15-4513 binding sites in the GABAA receptors expressed in heterologous systems resulting from the combination of alpha 4, gamma 2 and beta 2 subunits. Immunocytochemistry has revealed the abundance of alpha 4 peptide immunoreactivity in the thalamus and dentate gyrus (mainly in the hilar neurons and the inner third of the granule cell layer). The alpha 4 immunoreactivity is also present in the external plexiform layer of the olfactory bulb and in all layers of the neocortex and pyriform cortex. In the retina, alpha 4 is concentrated on ganglion cells (including some giant ganglion cells), the inner plexiform layer and to a lesser extent in the outer plexiform layer.

GabaA Receptors: Pharmacology, Behavioral Roles, and Motor Disorders

τ-Aminobutyric acid (GABA), the most prevalent inhibitory neurotransmitter in the mammalian brain, exerts its main action through GABAA receptors. They belong to the superfamily of ligand-gated ion channels and respond to GABA by the opening of an intrinsic anion channel. Multiple GABAA receptor subtypes in the brain show differential regional and developmental expression patterns. The receptors have a pentameric structure and are formed from members of at least three different subunit families (α1–6, β1–3, and τ1–3). The regulation of functional properties by GABA and its analogs and by benzodiazepine (BZ) receptor ligands differs dramatically with the type of α variant in the receptor complex. Additional variations of GABAA receptors result from substitution of γ subunits. The role of the β subunits, which are essential for receptor assembly, is less well defined on a functional basis. Besides their involvement in anxiolysis and sedation, GABAA receptors clearly have an impact on motor coordination. However, with the possible exception of the alcohol-and BZ-sensitive alcohol non-tolerant (ANT) rat line, it is not well documented whether a genetic alteration in this receptor system is directly involved in the impairment of animal or human motor activity.

Structural and pharmacological aspects of the GABAA receptor: involvement in behavioral pathogenesis

The gamma-aminobutyric acidA (GABAA) receptor is a complex hetero-oligomeric protein. It is composed of several subunits which assemble to form a functional chloride channel. The precise molecular organization of the receptor is as yet unknown. In the first part, we review recent literature dealing with the molecular and pharmacological aspects of the GABAA receptor, the second part will review some of the pathologies probably associated with gene defects and/or quantitative differential expression of transcripts encoding GABAA receptor subunits.

GABAA receptor subtypes differentiated by their gamma-subunit variants: prevalence, pharmacology and subunit architecture

Native GABAA receptors containing different gamma-subunit variants were distinguished immunobiochemically with antisera selectively recognizing the gamma 1-, gamma 2- and gamma 3-subunits. While GABAA receptors containing the gamma 2-subunits were confirmed to be rather ubiquitous in the adult brain, receptors characterized by the gamma 1- or gamma 3-subunit were of low abundance, as shown by immunoprecipitation. The three receptor populations differed strikingly in their benzodiazepine (BZ) site ligand binding profiles. The gamma 3-receptor population displayed reduced affinity for the full agonists clonazepam flunitrazepam and virtually lacked sensitivity to zolpidem. The gamma 1-receptor population displayed low affinity for all benzodiazepine site ligands tested, except for flunitrazepam, and could be differentiated from the gamma 2- and gamma 3-receptors by its low affinity for the inverse agonist beta CCM and its lack of affinity for the partial inverse agonist Ro 15-4513 and the antagonist flumazenil. Since flumazenil antagonizes all major effects of BZ agonists, gamma 1-receptors are not involved in mediating these actions in vivo. In immunopurified receptors, the gamma-subunit variants were found to be assembled with different variants of alpha- and beta-subunits, indicating that not only the gamma 2-subunit gamma 1- and gamma 3-subunits are part of various receptor subtypes. In addition, the gamma 2- and gamma 3-subunits can be co-assembled in native receptors, consistent with the subunit stoichiometry of two alpha-, one beta- and two gamma-subunits proposed previously for recombinant receptors.

Altered expression of gamma 2L and gamma 2S GABAA receptor subunits in the aging rat brain

Aging-related alterations in both protein and mRNA expression of gamma 2S and gamma 2L subunits of the GABAA receptors have been observed in several brain areas of Sprague-Dawley and Fischer 344 rats. Subunit-specific antibodies to gamma 2S and gamma 2L as well as a riboprobe to the large intracellular loop of gamma 2, which recognizes both gamma 2S and gamma 2L mRNAs, in conjunction with computerized image analysis were used for quantitative immunocytochemistry and in situ hybridization. In addition, specific oligonucleotide probes to gamma 2S or gamma 2L mRNA were used for quantitative dot blot hybridization. A large increase in the number of heavily immunostained neurons with the anti-gamma 2L antibody was detected in the cerebral cortex (115%) of old rats. However, only a small (but significant) aging-related increase in the density of gamma 2L immunostaining (7%) was observed throughout the cerebral cortex whereas no significant aging-related change in gamma 2L mRNA was detected in this brain region. Contrary to gamma 2L, the gamma 2S immunostaining did not show aging-related increased number of heavily immunostained neurons in cerebral cortex. Moreover, the density of gamma 2S immunostaining and the expression of gamma 2S mRNA were significantly decreased in the cerebral cortex (9-24%). Important aging-related changes were also found in the cerebellum of old rats where the expression of both gamma 2S and gamma 2L peptides was significantly decreased (24% and 23% respectively). This decrease in gamma 2 protein expression was accompanied by decreased expression of gamma 2S (16-38%) and gamma 2L (24%) mRNAs. Nevertheless, the most important decrease of gamma 2S (48%) and gamma 2L protein (20%) was revealed in the molecular layer of the cerebellum. In addition, the expression of gamma 2S protein was increased (14%) whereas the expression of gamma 2L was decreased (13%) in the granule cell layer. Therefore, the relative expression of gamma 2S protein in both layers was reversed in old animals. The observed aging-related changes in the expression of GABAA receptor subunits might lead to altered GABAA receptor/benzodiazepine receptor subunit composition.

Immunodetection of the large form of the gamma 2 subunit of mammalian GABAA receptor

Two forms of the GABAA receptor gamma 2-subunit, named short (gamma 2S) and long (gamma 2L), and generated by alternative RNA splicing, have been identified in mammalian brain by molecular cloning techniques. We have produced antibodies against a synthetic peptide containing the 8-amino acid insertion present in the long form but not in the short one. Using the antipeptide serum, we have identified the gamma 2L subunit in membrane preparations of GABAA receptors from rat and mouse cerebellum and its cellular location in cerebellum.

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.

Immunocytochemical localization of GABAA receptors in rat somatosensory cortex and effects of tactile deprivation

Immunocytochemical techniques were used to investigate the distribution of gamma-aminobutyric acidA (GABAA) receptors in the rat primary somatosensory cortex (SI). Monoclonal antibody 62-3G1 (de Blas et al., 1988; Victorica et al., 1988), which recognizes an epitope common to the beta 2 and beta 3 subunits of the GABAA receptor, produces staining of small punctate structures throughout the neuropil, and around somata and linear processes in all laminae of SI. Receptor immunostaining is relatively intense in upper lamina I and in lamina IV, where patches of intense receptor staining are interleaved with narrow zones of moderate immunoreactivity. Staining is lightest in lamina Vb, where stained puncta appear to be aligned with radially oriented processes, and moderate in the remaining laminae. Tangential sections through lamina IV reveal that each large cortical barrel encompasses several patches of intense receptor staining that are aligned with the corners or edges of individual barrels; interbarrel septa are moderately of intense cytochrome oxidase (CO) histochemical staining. A similar correspondence is apparent between a complex lattice of dense receptor clustering and a plexus of dark CO staining in the cortical trunk representation. Six to eight weeks of tactile deprivation produced by simple whisker trimming have no visible effect on GABAA receptor distribution. This is the case for rats whose whiskers were trimmed only during adulthood and for rats deprived from the day of birth until examination 6-8 weeks later. However, electrocautery ablation of whisker follicles leads to a marked decline in GABAA receptor immunoreactivity in cortical barrels associated with the ablated follicles. Our findings indicate that there is reasonable, though not perfect, correspondence between the distribution of GABAA receptors and the distribution of GABA-containing neurons and terminals in rat SI. These elements are associated with regions of intense oxidative metabolic activity revealed by CO staining. The density of GABAA receptors is reduced in lamina IV following complete loss of peripheral afferent input. However, less severe tactile deprivation, which is known to affect cortical neuron responsiveness, produces little or no change in receptor distribution.

Antibodies to the rat beta 3 subunit of the gamma-aminobutyric acid A receptors

Polymerase chain reaction was used to amplify the cDNA region that codes for the large intracellular loop of the beta 3 subunit of the gamma-aminobutyric acidA/benzodiazepine receptors (GABAAR/BZDR) from rat brain. The amplified cDNA was inserted into the prokaryotic expression vector pGEX-3X and a fusion protein containing glutathione-S-transferase and beta 3 intracellular loop moieties was expressed in bacteria. The fusion protein was affinity-purified and it was used to raise a rabbit anti-beta 3 antiserum. The anti-beta 3 antiserum immunoprecipitated the gamma-aminobutyric acidA receptor from rat and bovine brain. Immunoblots of the affinity-purified GABAAR/BZDR from bovine brain revealed that the anti-beta 3 antiserum reacted with a 57 kDa peptide, whereas the monoclonal antibody 62-3G1 that recognized both beta 2 and beta 3 reacted with 55 and 57 kDa peptides. The anti-beta 3 antiserum showed specificity for the beta 3 subunit vs beta 2 and beta 1.

Immunocytochemical localization of the beta 2 subunit of the gamma-aminobutyric acidA receptor in the rat brain

An antiserum to the beta 2 subunit of the rat gamma-aminobutyric acid (GABAA) receptor was prepared by immunizing a rabbit with a fusion protein expressed in bacteria. The fusion protein had the large, intracellular loop expanding between the putative M3 and M4 transmembrane domains of the beta 2 subunit fused to staphylococcal protein A (SPA). The antiserum immunoprecipitated both the solubilized and the affinity-purified GABAA receptors. The anti-beta 2 antibodies were affinity purified on immobilized beta 2 intracellular loop peptide. The antibodies recognized a 55-57 kDa peptide in immunoblots of either crude membranes from rat cerebral cortex or affinity-purified GABAA receptors from bovine cerebral cortex. Immunocytochemistry with the affinity-purified antibody has revealed for the first time the localization of the beta 2 subunit in the rat brain. A comparative study of the regional and cellular immunoreactivities of the affinity-purified anti-beta 2 antibody and the monoclonal antibody 62-3G1 (which recognizes both beta 2 and beta 3 subunits) is presented. The procedure described for generating and preparing specific anti-beta 2 subunit antibodies that are valuable for immunocytochemistry could be extended to other GABAA receptor subunits.

GABAA receptor subunit polypeptides increase in parallel but exhibit distinct distributions in the developing rat cerebellum

The GABAA receptor, a multisubunit ligand-gated ion channel, plays a central role in cell-cell communication in the developing and adult nervous system. Although the developmental expression of mRNAs encoding many subunit isoforms has been extensively characterized throughout the central nervous system, little is known concerning the relationship between subunit mRNA and polypeptide expression. To address this issue, we examined the developmental expression of the alpha 1, beta 2/3, and gamma 2 subunit polypeptides, subunits that are thought to coassemble in many brain regions. Western blot analysis using subunit-specific antibodies revealed that the levels of these polypeptides in both the cerebral cortex and cerebellum increased severalfold during the second postnatal week. Whereas polypeptide expression in the cerebellum paralleled that of the corresponding subunit mRNAs, increases in beta 2/3 and gamma 2 polypeptide expression in the cerebral cortex occurred in the absence of detectable changes in the mRNA levels. To determine whether the increases in subunit polypeptide expression in the cerebellum were accompanied by changes in distribution, immunohistochemistry was performed. These studies demonstrated that the subunits exhibited different but partially overlapping distributions that remained constant throughout postnatal development. Our findings suggest that although GABAA receptor subunit polypeptide expression may be regulated primarily at the level of the mRNA, additional regulatory mechanisms may play a role. Furthermore, the observation that subunit distribution remains constant in the cell bodies of cerebellar Purkinje neurons, which express the alpha 1, beta 2, beta 3, and gamma 2 subunit mRNAs exclusively, suggests that GABAA receptor subunit composition in this cell population does not change during postnatal maturation.

Molecular characterization of type I GABAA receptor complex from rat cerebral cortex and hippocampus

The molecular composition of the native gamma-aminobutyric acidA (GABAA) receptor complex is actually unknown. In the present communication we report a novel approach to characterize the minimal molecular conformation of the native GABAA receptor complex. This novel approach is based on the combination of subunit specific antibodies and specific 3H-labeled ligands in immunoprecipitation experiments. We have determined the presence of beta 2/3 and gamma 2 subunits in the Type I GABAA receptor complex, from rat cerebral cortex and hippocampus, by using two antibodies, the monoclonal 62-3G1 (specific for beta 2/3) and the polyclonal anti-gamma 2 (to the large intracellular loop of the gamma 2 short form) together with the Type I-specific ligand [3H]zolpidem. The association of gamma 2 and beta 2/3 subunits with the GABAA receptor complex was also tested using [3H]flumazenil. The results indicated that both gamma 2 and beta 2/3 were the most abundant subunits associated to either Type I or total benzodiazepine receptors from both cortex and hippocampus. Between 70-80% of Type I or total benzodiazepine binding activity was immunoprecipitated by either antibody. In addition, we have also investigated the coexistence of both subunits as part of the same population of Type I GABAA receptor complex by cross-immunoprecipitation experiments with 62-3G1 and anti-gamma 2. The results indicated that, in cerebral cortex, both gamma 2 and beta 2/3 subunits were part of the same population of Type I receptors. In hippocampus, an additional 20% of Type I receptors displayed either gamma 2 or beta 2/3 but not both subunits.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential expression of the short and long forms of the gamma 2 subunit of the GABAA/benzodiazepine receptors

The distribution of the mRNAs encoding the gamma 2S and gamma 2L subunits of the GABAA receptor in the rat brain has been revealed by in situ hybridization, northern blot and dot blot analysis using specific antisense oligonucleotides. In addition, the quantitative distribution of the gamma 2S and gamma 2L subunit peptides participating in the fully assembled GABAA receptors/benzodiazepine receptors has been mapped by immunoprecipitation with specific anti-gamma 2S and anti-gamma 2L antibodies. Several neuronal types and brain regions are enriched in gamma 2L such as neurons of the layer II of striate cortex and cerebellar Purkinje cells as well as the inferior colliculus, superior colliculus, deep cerebellar nuclei, medulla and pons. Other neuronal types and regions are enriched in gamma 2S such as the mitral cells of the olfactory bulb, pyramidal neurons of the pyriform cortex, layer VI of the neocortex, granule cells of the dentate gyrus and pyramidal cells of the hippocampus. Other cortical areas and cerebellar granule cells express both gamma 2S and gamma 2L in comparable amounts. There is a good correlation between the relative expression of gamma 2S and gamma 2L mRNAs and the relative presence of these protein subunits in fully assembled and mature receptors in the studied brain regions. The differential distribution of gamma 2S and gamma 2L might result in differential ethanol sensitivity of the neurons expressing these GABAA receptor subunits.

Coincidental appearance of the alpha 1 subunit of the GABA-A receptor and the type I benzodiazepine receptor near birth in macaque monkey visual cortex

The expression of subtypes of the GABA-A/benzodiazepine receptor complex has been studied during pre- and postnatal development of Macaca monkey visual cortex using complementary radioligand and immunocytochemical labeling. Type I benzodiazepine receptors were labeled directly by [3H]zolpidem. Type II receptors were determined by the amount of binding for [3H]flunitrazepam (FZ) persisting in the presence of the type I-specific ligand CL218872. Monoclonal antibody bd24 was used to label alpha 1 subunits and bd17 to label beta 2 and beta 3 subunits of the GABA-A receptor. Radioligand binding data and bd17 immunoreactivity indicated that type II benzodiazepine receptors were present by fetal day (Fd) 74 (44% of gestation). Immunoreactivity for the beta 2/beta 3 subunits increased until 3-6 weeks after birth, and then declined somewhat into adulthood. Neither radioligand labeling for type I receptors nor immunocytochemical staining for the alpha 1 subunit were apparent until mid-gestation. Both markers appeared shortly before birth in layer 4C, and then in other cortical layers after birth. Immunoreactivity for the alpha 1 subunit increased steadily after birth until it became more intense than that for beta 2/3 subunits in the adult. Quantitative densitometry of CL218872 competition for [3H]FZ binding showed that type I/II distribution was 22%/78% at Fd103; 42%/58% at Fd131; 67%/33% at 9 months; and 61%/39% in adult visual cortex. This "switch" between benzodiazepine receptor subtypes overlaps the postnatal critical period for geniculostriate development, suggesting that the change from type II to type I receptors and the appearance of alpha 1 subunits may play a decisive role in the maturation of geniculocortical axon terminations and cortical response properties. It remains to be shown whether this "switch" is dependent on functional visual input.

Molecular heterogeneity of the type I GABAA/benzodiazepine receptor complex

We have investigated the presence of alpha 1, gamma 2 and beta 2-3 subunits as part of the type I (high affinity for [3H]zolpidem) GABAA/benzodiazepine receptor from rat cerebral cortex. Three subunit specific antibodies have been used (anti-alpha 1, to the C-terminal of rat alpha 1 subunit; anti-gamma 2, to the large intracellular loop of the gamma 2 subunit short form and the monoclonal 62-3G1, specific to beta 2-3 subunits) in immunoprecipitation experiments of the [3H]zolpidem binding activity or the 51,000 Da [3H]Ro 15-4513 photoaffinity labeled peptide (P51). The results indicated that alpha 1 subunit was present in the whole population (90%) of the GABAA/benzodiazepine receptors with high affinity for [3H]zolpidem. We cannot exclude the presence of other alpha subunits co-localized with alpha 1. On the one hand, 70-75% of type I GABAA/benzodiazepine receptor from rat cortex have co-existing alpha 1, beta 2-3 and gamma 2 subunits. We call this type Ia. On the other hand, 20-25% of the type I GABAA/benzodiazepine receptor complex should be constructed by the association of alpha 1 with subunits other than beta 2-3 and gamma 2. We call this type Ib. The identity of these subunits is currently unknown.

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

Subunit-specific polyclonal antisera against the GABAA receptor gamma 1 and gamma 2 subunits were raised in rabbits and used for immunoblotting and immunocytochemistry of the rat brain. Each subunit protein was differentially distributed even in the region like cerebellar cortex where mRNAs of both subunits were distributed in the same manner. This may indicate that GABAA receptor gamma 1 and gamma 2 subunit proteins are subject to a subunit-specific subcellular sorting mechanism.