DJ Laurie

The expression patterns of 13 GABAA receptor subunit encoding genes (alpha 1-alpha 6, beta 1-beta 3, gamma 1-gamma 3, delta) were determined in adult rat brain by in situ hybridization. Each mRNA displayed a unique distribution, ranging from ubiquitous (alpha 1 mRNA) to narrowly confined (alpha 6 mRNA was present only in cerebellar granule cells). Some neuronal populations coexpressed large numbers of subunit mRNAs, whereas in others only a few GABAA receptor-specific mRNAs were found. Neocortex, hippocampus, and caudate-putamen displayed complex expression patterns, and these areas probably contain a large diversity of GABAA receptors. In many areas, a consistent coexpression was observed for alpha 1 and beta 2 mRNAs, which often colocalized with gamma 2 mRNA. The alpha 1 beta 2 combination was abundant in olfactory bulb, globus pallidus, inferior colliculus, substantia nigra pars reticulata, globus pallidus, zona incerta, subthalamic nucleus, medial septum, and cerebellum. Colocalization was also apparent for the alpha 2 and beta 3 mRNAs, and these predominated in areas such as amygdala and hypothalamus. The alpha 3 mRNA occurred in layers V and VI of neocortex and in the reticular thalamic nucleus. In much of the forebrain, with the exception of hippocampal pyramidal cells, the alpha 4 and delta transcripts appeared to codistribute. In thalamic nuclei, the only abundant GABAA receptor mRNAs were those of alpha 1, alpha 4, beta 2, and delta. In the medial geniculate thalamic nucleus, alpha 1, alpha 4, beta 2, delta, and gamma 3 mRNAs were the principal GABAA receptor transcripts. The alpha 5 and beta 1 mRNAs generally colocalized and may encode predominantly hippocampal forms of the GABAA receptor. These anatomical observations support the hypothesis that alpha 1 beta 2 gamma 2 receptors are responsible for benzodiazepine I (BZ I) binding, whereas receptors containing alpha 2, alpha 3, and alpha 5 contribute to subtypes of the BZ II site. Based on significant mismatches between alpha 4/delta and gamma mRNAs, we suggest that in vivo, the alpha 4 subunit contributes to GABAA receptors that lack BZ modulation.

The embryonic and postnatal expression of 13 GABAA receptor subunit genes in the rat CNS was studied by in situ hybridization. Each transcript exhibited a unique regional and temporal developmental expression profile. For example, in both embryonic and early postnatal cortex and thalamus, expression of the alpha 2, alpha 3, alpha 5, and beta 3 mRNAs was pronounced. In particular, the alpha 5 gene expression underwent a prominent peak in early brain. Subsequently, the thalamocortical expression of these four genes substantially diminished and was superseded in the adult by the alpha 1, alpha 4, beta 2, and delta subunit mRNAs. Similarly, gamma 1 and gamma 3 gene expression also dropped markedly during development, their initial stronger expression being restricted to relatively few structures. In contrast, gamma 2 gene expression was widespread and mostly remained constant with increasing age. The medial septum and globus pallidus were regions expressing few subunits in both early postnatal and adult stages, allowing clear developmental combinatorial changes to be inferred (alpha 2/alpha 3 beta 2 gamma 2 to alpha 1 beta 2 gamma 2, alpha 2/alpha 3 beta 2 gamma 1 to alpha 1 beta 2 gamma 1/gamma 2, respectively). In contrast, cerebellar Purkinje cells exhibited no developmental switch, expressing only the alpha 1, beta 2, beta 3, and gamma 2 mRNAs from birth to adult. Certain GABAA transcripts were also detected in germinal zones (e.g., beta 1, beta 3, gamma 1) and in embryonic peripheral tissues such as dorsal root ganglia (e.g., alpha 2, alpha 3, beta 3, gamma 2) and intestine (gamma 3). Some parallels in regional and temporal CNS expression were noted (e.g., alpha 1 beta 2, alpha 2 beta 3, alpha 4/alpha 6 delta), whereas the alpha 5 and beta 1 regional mRNA expressions converted over time. The changes of GABAA receptor subunit gene expression suggest a molecular explanation for earlier observations on changing ligand binding affinities. Thus, the composition, and presumably properties, of embryonic/early postnatal rat GABAA receptors differs markedly from those expressed in the adult brain.

In an effort to determine subunit compositions of in vivo GABAA receptors, the cellular localization of 13 subunit encoding mRNAs (alpha 1-alpha 6, beta 1-beta 3, gamma 2-gamma 3, delta) was determined in the rat olfactory bulb and cerebellum. Cerebellar granule cells expressed significant quantities of alpha 1, alpha 6, beta 2, beta 3, gamma 2, and delta mRNAs. They contained very much lower levels of alpha 4, beta 1, and gamma 3 mRNAs, and the alpha 2, alpha 3, alpha 5, and gamma 1 genes appeared to be silent. Purkinje cells contained only the alpha 1, beta 2, beta 3, and gamma 2 mRNAs. Putative Bergmann glial cells were found to contain the gamma 1 mRNA and possibly the alpha 2 mRNA. In the molecular layer, only the alpha 1, beta 2 and gamma 2 mRNAs were expressed in stellate/basket cells. The alpha 3 probe hybridized weakly to targets in the molecular layer. The inferior olivary nucleus contained significant quantities of alpha 2, alpha 4, and gamma 1 transcripts, with the alpha 1, alpha 3, beta 2, beta 3, and gamma 2 mRNAs also present. In the olfactory bulb, mitral cells were found to express the alpha 1, beta 1, beta 2, beta 3, and gamma 2 mRNAs strongly and the alpha 3 mRNA weakly. Tufted cells contained alpha 1, alpha 3, beta 2, beta 3, and gamma 2 mRNAs and, occasionally, the alpha 2 mRNA. In the internal granule cells the alpha 2, alpha 4, alpha 5, beta 3, and delta mRNAs were all present. Low levels of alpha 3, gamma 1, gamma 2, and gamma 3 mRNAs were also noted in these cells. Periglomerular cells expressed low levels of alpha 2, alpha 3, alpha 4, beta 2, beta 3, gamma 1, gamma 2, and gamma 3 mRNAs. No alpha 6 mRNA was present in the olfactory bulb. Correlations that are general ones from other brain regions are the colocalizations of alpha 1 beta 2, alpha 2 beta 3, and alpha 4 delta mRNAs. In both the olfactory bulb and cerebellum, alpha 1 beta 2 gamma 2 receptor cores are probably employed. The delta-subunit mRNA appears to codistribute with alpha-subunit mRNAs (alpha 4 and alpha 6) associated with GABAA subunits that fail to bind benzodiazepine agonists.

Developmental and regional alternative splicing of the NMDAR1 subunit gene transcript was examined by in situ hybridization in the developing and adult rat brain. NMDAR1 mRNA, barely detectable at embryonic day 14, increased gradually during development until the third postnatal week, after which it declined slightly to adult levels, when it was detected in every examined neuronal type. Each splice form of the primary NMDAR1 gene transcript was found to follow a parallel profile of abundance in the brain, but marked regional differences were observed in splicing at both 5' and 3' sequences. The individual regional distributions of splice forms appeared to be established around birth, with little change thereafter, except in the overall abundance. The NMDAR1-a and NMDAR1-2 splice forms occurred extensively and approximately homogeneously throughout brain gray matter. The NMDAR1-b variant was found primarily in the sensorimotor cortex, neonatal lateral caudate, thalamus, hippocampal CA3 field, and cerebellar granule cells, but was absent from adult caudate. The NMDAR1-1 and -4 splice forms were detected in almost complementary patterns; the former was concentrated in more rostral structures such as cortex, caudate, and hippocampus, while the latter was principally in more caudal regions such as thalamus, colliculi, and cerebellum. These two splice forms accounted for a greater proportion of the adult NMDAR1 mRNA than that of the neonate. The NMDAR1-3 mRNA variant was scarce, being detected only at very low levels in postnatal cortex and hippocampus. The different splice forms may generate regional differences in NMDA receptor properties during development and in the adult CNS.