GAD and GABA in an enriched population of cultured GABAergic neurons from rat cerebral cortex

Development of the GABA-ergic signaling system and its role in larval swimming in sea urchin

The present study aimed to elucidate the development and γ-amino butyric acid (GABA)-ergic regulation of larval swimming in the sea urchin Hemicentrotus pulcherrimus by cloning glutamate decarboxylase (Hp-gad), GABAA receptor (Hp-gabrA) and GABAA receptor-associated protein (Hp-gabarap), and by performing immunohistochemistry. The regulation of larval swimming was increasingly dependent on the GABAergic system, which was active from the 2 days post-fertilization (d.p.f.) pluteus stage onwards. GABA-immunoreactive cells were detected as a subpopulation of secondary mesenchyme cells during gastrulation and eventually constituted the ciliary band and a subpopulation of blastocoelar cells during the pluteus stage. Hp-gad transcription was detected by RT-PCR during the period when Hp-Gad-positive cells were seen as a subpopulation of blastocoelar cells and on the apical side of the ciliary band from the 2 d.p.f. pluteus stage. Consistent with these observations, inhibition of GAD with 3-mercaptopropioninc acid inhibited GABA immunoreactivity and larval swimming dose dependently. Hp-gabrA amplimers were detected weakly in unfertilized eggs and 4 d.p.f. plutei but strongly from fertilized eggs to 2 d.p.f. plutei, and Hp-GabrA, together with GABA, was localized at the ciliary band in association with dopamine receptor D1 from the two-arm pluteus stage. Hp-gabarap transcription and protein expression were detected from the swimming blastula stage. Inhibition of the GABAA receptor by bicuculline inhibited larval swimming dose dependently. Inhibition of larval swimming by either 3-mercaptopropionic acid or bicuculline was more severe in older larvae (17 and 34 d.p.f. plutei) than in younger ones (1 d.p.f. prism larvae).

Added astroglia promote greater synapse density and higher activity in neuronal networks

Astroglia are known to potentiate individual synapses, but their contribution to networks is unclear. Here we examined the effect of adding either astroglia or media conditioned by astroglia on entire networks of rat hippocampal neurons cultured on microelectrode arrays. Added astroglia increased spontaneous spike rates nearly two-fold and glutamate-stimulated spiking by six-fold, with desensitization eliminated for bath addition of 25 microM glutamate. Astrocyte-conditioned medium partly mimicked the effects of added astroglia. Bursting behavior was largely unaffected by added astroglia except with added glutamate. Addition of the GABA(A) receptor antagonist bicuculline also increased spike rates but with more subtle differences between networks without or with added astroglia. This indicates that networks without added astroglia were inhibited greatly. In all conditions, the log-log distribution of spike rates fit well to linear distributions over three orders of magnitude. Networks with added astroglia shifted consistently toward higher spike rates. Immunostaining for GFAP revealed a linear increase with added astroglia, which also increased neuronal survival. The increased spike rates with added astroglia correlated with a 1.7-fold increase in immunoreactive synaptophysin puncta, and increases of six-fold for GABA(Abeta), two-fold for NMDA-R1 and two-fold for Glu-R1 puncta, with receptor clustering that indicated synaptic scaling. Together, these results indicate that added astroglia increase the density of synapses and receptors, and facilitate higher spike rates for many elements in the network. These effects are reproduced by glia-conditioned media, with the exception of glutamate-mediated transmission.

Independence of synaptic specificity from neuritic guidance

Neuronal circuits are interconnected with a high degree of specificity. While axonal guidance has been demonstrated to be crucial for the choice of the correct target region, its role in specificity at the level of individual cells remains unclear. Specificity of synapse formation may either result from precise guidance of axonal outgrowth onto the target or depend on a molecular "match" between pre- and postsynapse. To distinguish between these possibilities, an in vitro system was used in which neuritic outgrowth of rat cortical neurons is accurately guided along the narrow pathways of a surface micropattern. The micropattern consisted of a blend of extracellular matrix molecules applied to a cell repellent background of polystyrene by microcontact printing. The system reproduces guidance by attractant and repellent surface cues while no other signals that may influence synapse formation, like gradients of trophic factors or accumulations of signaling molecules, are provided. While the number of contact points between neighboring cells was strongly reduced on patterned substrates due to the geometrical restrictions, frequency of synapse formation was not different from homogeneous cultures. Thus it was unaffected by stringent guidance onto the target cell or by the number of cell-cell contacts. Moreover, a statistically significant enrichment of reciprocal contacts between mixed pairs of excitatory and inhibitory neurons over probabilistic predictions was found, which has similarly been shown by others in dissociated neuronal cultures. Our results indicate that precise axonal guidance is insufficient for target-specific synapse formation and suggest that instead recognition between individual cells is required.

Biochemical characteristics of the gamma-aminobutyric acid system in the insulinoma cell lines HIT-T15, RIN-m5F, betaTC3, and comparison with rat brain

BACKGROUND

gamma-aminobutyric acid (GABA) is the most abundant inhibitory neurotransmitter in the mammalian brain. Both GABA and its synthesizing enzyme, L-glutamate decarboxylase (GAD), are also present in the insulin-secreting pancreatic beta cells, in which its physiologic role is unclear. We have studied several aspects of the GABA system in the insulinoma cell lines HIT-T15, RIN-m5F, and betaTC3 in comparison with rat brain tissue.

METHODS

Insulinoma cell lines and embryonic rat brain cortex neurons were cultured. GAD activity was determined by a radioenzymatic method and the presence of GAD(67) protein was assessed by immunocytochemistry. Amino acid content and the effect of different conditions on the release of endogenous GABA were measured by HPLC and fluorometric detection after o-phthaldialdehyde derivatization. [3H]GABA was used for measuring the uptake of the amino acid in the insulinoma cultures and in rat forebrain synaptosomes.

RESULTS

The three insulinoma lines possess GABA and GAD activity at levels of approximately 20% compared with adult rat brain cortex. Dissimilar from the latter, in insulinoma cultures enzyme activity was not enhanced by addition of an excess of the coenzyme pyridoxal-5'-phosphate. Immunocytochemical visualization of GAD showed that the cells in both neuronal cultures and insulinoma lines were GAD(67)-positive, similar to Purkinje cell somata of adult rat cerebellar cortex. [3H]GABA uptake in the cell lines was approximately 10% of that in rat forebrain synaptosomes and showed less ionic and temperature dependence. In both cultured cerebral neurons and RINm5F cells, the addition of arginine induced the release of GABA, whereas neither high K(+) concentration nor glucose had any effect.

CONCLUSIONS

The insulinoma cell lines studied possess the same GAD(67) form of the enzyme present in brain. RIN line cells are capable of transporting glutamate. In these cells as well as in cultured cortical neurons, arginine stimulates the release of GABA and glutamate probably as the result of its electrogenic transport. Insulinoma cell lines may therefore be useful to study GABA metabolism and function in pancreatic beta cells.

Central neuronal synapse formation on micropatterned surfaces

Controlling synapse formation is a key to patterning of neurons into functional circuits and networks in vitro. However, the process of synapse formation among neurons grown on artificial surfaces is relatively unstudied. We cultured embryonic hippocampal cells on trimethoxysilylpropyl-diethylenetriamine (DETA) and tridecafluoro-1, 1,2,2-tetrahydrooctyl-1-dimethylchlorosilane (13F), and on patterns composed of DETA lines separated by 13F spaces. For comparison, neurons were concurrently plated on surfaces coated with uniform poly-d-lysine (PDL). Pre- and postsynaptic specializations were identified by immunostaining for synapsin I and microtubule-associated protein-2 (MAP-2). Spontaneous (SPCs) and evoked (EPCs) postsynaptic currents were recorded using dual patch-clamp techniques. We found that DETA promoted synapse formation, whereas evidence for synapse formation on 13F was barely detected. MAP-2+ neuronal soma and rapidly growing dendrites were co-localized with synapsin I puncta faithfully along DETA lines. The expression of synapsin I puncta, and MAP-2+ soma and dendrites correlated well with the appearance of SPCs. Synapsin I, MAP-2 and SPCs emerged together at days 3-4 and increased at day 7, when EPCs appeared. Synaptic signals occurring during 4-7 days in culture were all GABAergic. These results indicate that fully functional synapses are formed on silane surfaces, demonstrating the suitability of patterned silane surfaces for organizing synapse formation in vitro.

Presence of mRNA for glutamic acid decarboxylase in both excitatory and inhibitory neurons

Neurons in very low density hippocampal cultures that are physiologically identified as either GABAergic inhibitory or glutamatergic excitatory all contain mRNA for the gamma-aminobutyric acid (GABA) synthetic enzyme, glutamic acid decarboxylase (GAD), as detected by single cell mRNA amplification and PCR. However, consistent with the physiology, immunocytochemistry revealed that only a subset of the neurons stain for either GAD protein or GABA. A similar fraction hybridize with RNA probes for GAD65 and GAD67. Hippocampal CA1 pyramidal neurons in slice preparations, which are traditionally thought to be excitatory, also contain mRNA for GAD65 and GAD67. Hippocampal neurons in culture did not contain mRNA for two other neurotransmitter synthesizing enzymes, tyrosine hydroxylase, and choline acetyl transferase. These data suggest that in some neurons, presumably the excitatory neurons, GAD mRNA is selectively regulated at the level of translation. We propose that neurotransmitter phenotype may be posttranscriptionally regulated and neurons may exhibit transient phenotypic plasticity in response to environmental influences.

The level of GAD67 protein is highly sensitive to small increases in intraneuronal gamma-aminobutyric acid levels

Increases (> 2.5-fold) in GABA levels in rat brain lead to a large decrease in the level of the 67-kDa form of glutamate decarboxylase (GAD67) through a mechanism involving either a change in GAD67 protein stability or a change in GAD67 mRNA translation. In the present study, brain levels of GABA were manipulated by treating rats with various doses of gamma-vinyl-gamma-aminobutyric acid (GVG), and the dependence of total GAD activity and levels of GAD67 and GAD65 protein on the levels of GABA was analyzed. Initial studies showed that both GABA and GAD67 protein levels reached new steady-state levels after two to four daily injections; GABA increased 1.5- (30 mg of GVG/kg) and fourfold (150 mg of GVG/kg), and GAD67 protein content decreased by 30 and 70%. To assess the sensitivity of GAD67 to GABA, rats were injected with eight different doses of GVG (15-150 mg/kg) for 5 days. With increasing doses of GVG, we observed a gradual increase in both whole-tissue and synaptosomal GABA levels and a gradual decrease in GAD67 protein and GAD activity. The levels of GAD65 remained constant at all GVG doses. GAD67 was remarkably sensitive to GABA. The synaptosomal GAD67 level decreased approximately 12% and the whole-neuron GAD67 level decreased approximately 3% for each 1% increase in nerve terminal GABA content when it was close to its physiological level. Our results clearly demonstrate that GAD67 is tightly controlled by intraneuronal GABA, and we suggest that this regulatory mechanism has important implications for the physiological regulation of GABAergic function in the mammalian brain.

Increased intracellular gamma-aminobutyric acid selectively lowers the level of the larger of two glutamate decarboxylase proteins in cultured GABAergic neurons from rat cerebral cortex

The regulation of glutamate decarboxylase (GAD; EC 4.1.1.15) was studied by using cultures of cerebral cortical neurons from rat brain grown in serum-free medium. About 50% of the neurons in the cultures were gamma-aminobutyric acid (GABA)ergic as determined by two double-staining procedures. Immunoblotting experiments with four anti-GAD sera that recognize the two forms to varying degrees, demonstrated that the cultures contained the two forms of GAD that are present in rat brain (apparent molecular masses = 63 and 66 kDa). GAD activity was reduced by 60-70% when intracellular GABA levels were increased by incubating the cultures with the GABA-transaminase inhibitor gamma-vinyl-GABA for greater than 5-10 h or with 1 mM GABA itself. Neither baclofen nor muscimol (100 microM) affected GAD activity. Immunoblotting experiments showed that only the larger of the two forms of GAD (66 kDa) was decreased by elevated GABA levels. These results, together with previous results indicating that the smaller form of GAD is more strongly regulated by pyridoxal 5'-phosphate (the cofactor for GAD), suggest that the two forms of GAD are regulated by different mechanisms.