Structure and distribution of multiple GABAA receptor subunits with special reference to the cerebellum

Effects of progesterone on T-type-Ca2+-channel expression in Purkinje cells

Plasticity of cerebellar Purkinje cells (PC) is influenced by progesterone via the classical progesterone receptors PR-A and PR-B by stimulating dendritogenesis, spinogenesis, and synaptogenesis in these cells. Dissociated PC cultures were used to analyze progesterone effects at a molecular level on the voltage-gated T-type-Ca²⁺-channels Ca_v3.1, Ca_v3.2, and Ca_v3.3 as they helped determine neuronal plasticity by regulating Ca²⁺-influx in neuronal cells. The results showed direct effects of progesterone on the mRNA expression of T-type-Ca²⁺-channels, as well as on the protein kinases A and C being involved in downstream signaling pathways that play an important role in neuronal plasticity. For the mRNA expression studies of T-type-Ca²⁺-channels and protein kinases of the signaling cascade, laser microdissection and purified PC cultures of different maturation stages were used. Immunohistochemical staining was also performed to characterize the localization of T-type-Ca²⁺-channels in PC. Experimental progesterone treatment was performed on the purified PC culture for 24 and 48 hours. Our results show that progesterone increases the expression of Ca_v3.1 and Ca_v3.3 and associated protein kinases A and C in PC at the mRNA level within 48 hours after treatment at latest. These effects extend the current knowledge of the function of progesterone in the central nervous system and provide an explanatory approach for its influence on neuronal plasticity.

Expression Pattern of T-Type Ca2+ Channels in Cerebellar Purkinje Cells after VEGF Treatment

T-type Ca2+ channels, generating low threshold calcium influx in neurons, play a crucial role in the function of neuronal networks and their plasticity. To further investigate their role in the complex field of research in plasticity of neurons on a molecular level, this study aimed to analyse the impact of the vascular endothelial growth factor (VEGF) on these channels. VEGF, known as a player in vasculogenesis, also shows potent influence in the central nervous system, where it elicits neuronal growth. To investigate the influence of VEGF on the three T-type Ca2+ channel isoforms, Cav3.1 (encoded by Cacna1g), Cav3.2 (encoded by Cacna1h), and Cav3.3 (encoded by Cacna1i), lasermicrodissection of in vivo-grown Purkinje cells (PCs) was performed, gene expression was analysed via qPCR and compared to in vitro-grown PCs. We investigated the VEGF receptor composition of in vivo- and in vitro-grown PCs and underlined the importance of VEGF receptor 2 for PCs. Furthermore, we performed immunostaining of T-type Ca2+ channels with in vivo- and in vitro-grown PCs and showed the distribution of T-type Ca2+ channel expression during PC development. Overall, our findings provide the first evidence that the mRNA expression of Cav3.1, Cav3.2, and Cav3.3 increases due to VEGF stimulation, which indicates an impact of VEGF on neuronal plasticity.

Chloride Homeostasis in Neurons With Special Emphasis on the Olivocerebellar System: Differential Roles for Transporters and Channels

The intraneuronal ionic composition is an important determinant of brain functioning. There is growing evidence that aberrant homeostasis of the intracellular concentration of Cl- ([Cl-]i) evokes, in addition to that of Na+ and Ca2+, robust impairments of neuronal excitability and neurotransmission and thereby neurological conditions. More specifically, understanding the mechanisms underlying regulation of [Cl-]i is crucial for deciphering the variability in GABAergic and glycinergic signaling of neurons, in both health and disease. The homeostatic level of [Cl-]i is determined by various regulatory mechanisms, including those mediated by plasma membrane Cl- channels and transporters. This review focuses on the latest advances in identification, regulation and characterization of Cl- channels and transporters that modulate neuronal excitability and cell volume. By putting special emphasis on neurons of the olivocerebellar system, we establish that Cl- channels and transporters play an indispensable role in determining their [Cl-]i and thereby their function in sensorimotor coordination.

Expression of GABAA α2-, β1- and ε-receptors are altered significantly in the lateral cerebellum of subjects with schizophrenia, major depression and bipolar disorder

There is abundant evidence that dysfunction of the γ-aminobutyric acid (GABA)ergic signaling system is implicated in the pathology of schizophrenia and mood disorders. Less is known about the alterations in protein expression of GABA receptor subunits in brains of subjects with schizophrenia and mood disorders. We have previously demonstrated reduced expression of GABA(B) receptor subunits 1 and 2 (GABBR1 and GABBR2) in the lateral cerebella of subjects with schizophrenia, bipolar disorder and major depressive disorder. In the current study, we have expanded these studies to examine the mRNA and protein expression of 12 GABA(A) subunit proteins (α1, α2, α3, α5, α6, β1, β2, β3, δ, ε, γ2 and γ3) in the lateral cerebella from the same set of subjects with schizophrenia (N=9-15), bipolar disorder (N=10-15) and major depression (N=12-15) versus healthy controls (N=10-15). We found significant group effects for protein levels of the α2-, β1- and ε-subunits across treatment groups. We also found a significant group effect for mRNA levels of the α1-subunit across treatment groups. New avenues for treatment, such as the use of neurosteroids to promote GABA modulation, could potentially ameliorate GABAergic dysfunction in these disorders.

Benzodiazepine-sensitive GABA(A) receptors limit the activity of the NMDA/NO/cyclic GMP pathway: a microdialysis study in the cerebellum of freely moving rats

In the cerebellum, infusion of NMDA (200 microM) for 20 min evoked a marked (200%) increase of extracellular cyclic GMP (cGMP) levels. The selective GABA(A) receptor agonist muscimol (0.01-100 microM) was able to counteract the NMDA effect with an EC(50) of 0.65 microM; the inhibitory effect of muscimol (10 microM) was prevented by bicuculline (50 microM). Diazepam (10 microM) significantly potentiated the muscimol (1 microM) inhibition; furthermore, when coinfused with 0.1 microM muscimol (a concentration not affecting, on its own, the cGMP response to NMDA), diazepam (10 microM) reduced the NMDA effect. Similar results were obtained with zolpidem (0.1-1 microM). Finally, local infusion of the benzodiazepine site antagonist flumazenil (10 microM), together with muscimol and diazepam, almost completely restored the effect of NMDA on extracellular cGMP levels. It is concluded that GABA(A) receptors potently control the NMDA/nitric oxide/cGMP pathway in the cerebellum in vivo. In terms of the alpha subunit composition, we can deduce that the cerebellar GABA(A) receptor does not contain alpha(6) or beta(4) subunits because it is diazepam-sensitive. Moreover, the observation that zolpidem is active at a rather low concentration, in combination with localization studies present in the literature, tend to exclude the presence of alpha(5) subunits in the receptor composition and suggest the involvement of an alpha(1) subunit.

Neuregulin induces GABA(A) receptor subunit expression and neurite outgrowth in cerebellar granule cells

Neuregulin (NRG), a growth and differentiation factor that signals via erbB receptor tyrosine kinases, has been shown to have biological effects in both the CNS and the peripheral nervous system. We report here that erbB4 is expressed in mature cerebellar granule cells, where it appears to be concentrated at the granule cell postsynaptic terminals. We also show that one form of NRG, Ig-NRG, plays a crucial role in aspects of cerebellar granule cell development in vitro. First, Ig-NRG treatment of granule cells in culture selectively induces the expression of the GABA(A) receptor beta2 subunit. This increase in subunit expression is paralleled by an increase in functional GABA(A) receptors. In contrast to its effects on GABA(A) receptor subunit expression, Ig-NRG does not upregulate NMDA receptor N2B and N2C subunit expression. Second, we demonstrate that Ig-NRG also enhances neurite outgrowth from cultured granule cells. Ig-NRG does not, however, act as a survival factor for the granule cells. We have compared the effect of Ig-NRG with the effects of brain-derived neurotrophic factor (BDNF), a neurotrophin that exerts specific effects on granule cells in culture, and found that BDNF does not mimic the effects of Ig-NRG on GABA(A) receptor subunit expression. Our results show that Ig-NRG has specific effects on granule cell development and maturation and may regulate these processes in vivo.

Composition of the GABA(A) receptors of retinal dopaminergic neurons

Transgenic technology, single-cell RT-PCR, and immunocytochemistry were combined to investigate the composition of the GABA(A) receptors of dopaminergic (interplexiform) amacrine (DA) cells. A mouse line was used in which these neurons were labeled with human placental alkaline phosphatase and could therefore be identified in vitro after dissociation of the retina. We performed single-cell RT-PCR on the isolated cells and showed that (1) DA cells contained the messages for alpha1, alpha3, alpha4, beta1, beta3, gamma1, gamma2(S), and gamma2(L) subunits; (2) this transcript repertory did not change on dissociation of the retina and throughout the time required for cell harvesting; and (3) all DA cells contained the entire transcript repertory. Immunocytochemistry with subunit-specific antibodies showed that all subunits were expressed and appeared homogeneously distributed throughout the cell membrane at a low concentration. In addition, with the exception of alpha4, the subunits formed clusters at the surface of the dendrites and on the inner pole of the cell body. Because of their size, shape, and topographic coincidence with GABAergic endings, the clusters were interpreted as postsynaptic active zones containing GABA(A) receptors. The composition of the synaptic receptors was not uniform: clusters distributed throughout the dendritic tree contained alpha3, beta3, and, less frequently, beta1 subunits, whereas clusters containing the alpha1 subunit were confined to large dendrites. Therefore, DA cells possess at least two types of GABA(A) receptors localized in different synapses. Furthermore, they exhibit multiple extrasynaptic GABA(A) receptors.

Role of cerebellar granule cell-specific GABAA receptor subtype in the differential sensitivity of [3H]ethynylbicycloorthobenzoate binding to GABA mimetics

Binding of [3H]ethynylbicycloorthobenzoate ([3H]EBOB) to the gamma-aminobutyric acid type A (GABAA) receptor of cultured cerebellar granule neurons is inhibited by GABA, muscimol and 3-aminopropanesulfonic acid with IC50 values of 69-250 nM. Sensitivity to these GABA mimetics is lower by 3-4-fold for cerebellum and 10-20-fold for cerebral cortex, midbrain, and pons and medulla, a differential sensitivity by brain region and cell type consistent with earlier findings using tert-[35S]butylbicyclophosphorothionate and GABA. In contrast, the inhibitory potencies of two chloride channel blockers, alpha-endosulfan and picrotoxinin, do not differ in these assays. The hypothesis that this pharmacological profile is conferred by the alpha 6 subunit specific to cerebellar granule cells is supported by the finding that forskolin (which downregulates the alpha 6 subunit) but not the inactive dideoxyforskolin markedly decreases the sensitivity of [3H]EBOB binding to GABA without affecting inhibition by alpha-endosulfan.

The cerebellum: a model system for studying GABAA receptor diversity

The basic unsolved questions concerning GABAA receptors are: "How many receptor subtypes exist?", "What subtypes are used by which types of neuron and where are they located on the cell?", and "What are the functions of the different subtypes?" As described in this Review, the cerebellum is an ideal vertebrate brain region for investigating these issues.