Substrate specificity and developmental aspects of a presynaptic GABA receptor regulating glutamate release in the rat cerebellum

Modulation of glutamate release from parallel fibers by mGlu4 and pre-synaptic GABA(A) receptors

The regulation of pre-synaptic glutamate release is important in the maintenance and fidelity of excitatory transmission in the nervous system. In this study, we report a novel interaction between a ligand-gated ion channel and a G-protein coupled receptor which regulates glutamate release from parallel fiber axon terminals. Immunocytochemical analysis revealed that GABA(A) receptors and the high affinity group III metabotropic glutamate receptor subtype 4 (mGlu4) are co-localized on glutamatergic parallel fiber axon terminals in the cerebellum. GABA(A) and mGlu4 receptors were also found to co-immunoprecipitate from cerebellar membranes. Independently, these two receptors have opposing roles on glutamate release: pre-synaptic GABA(A) receptors promote, while mGlu4 receptors inhibit, glutamate release. However, coincident activation of GABA(A) receptors with muscimol and mGlu4 with the agonist (2S)-S-2-amino-4-phosphonobutanoic acid , increased glutamate release from [(3) H]glutamate-loaded cerebellar synaptosomes above that observed with muscimol alone. Further support for an interaction between GABA(A) and mGlu4 receptors was obtained in the mGlu4 knockout mouse which displayed reduced binding of the GABA(A) ligand [(35) S]tert-butylbicyclophosphorothionate, and decreased expression of the α1, α6, β2 GABA(A) receptor subunits in the cerebellum. Taken together, our data suggest a new role for mGlu4 whereby simultaneous activation with GABA(A) receptors acts to amplify glutamate release at parallel fiber-Purkinje cell synapses.

Functional evidence for two native GABAA receptor subtypes in adult rat hippocampus and cerebellum

Studies of molecular cloning predict great heterogeneity for the GABAA receptor; however, evidence for functionally and pharmacologically distinct native GABAA receptors is relatively scarce. In this work we have compared some of the functional and pharmacological properties of two GABAA receptors previously shown to be present in the adult rat central nervous system. In superfused hippocampal synaptosomes activation of GABAA receptors increased the basal release of [3H]noradrenaline (EC50 for GABA = 3.2 microM). In contrast, the overflow evoked by depolarization with high-K+ (12 or 35 mM) was not affected. Conversely, GABAA receptor activation led to potentiation of the K(+)-evoked overflow of [3H]D-aspartate from cerebellar synaptosomes (EC50 for GABA = 1.3 microM) whereas the basal release remained unchanged. GABA and muscimol also potentiated the K(+)-evoked overflow of endogenous glutamate in cerebellum. Diazepam enhanced the GABA (3 microM)-evoked [3H]noradrenaline release (EC50 = 65 nM). The diazepam potentiation of the GABA- or muscimol-evoked release of [3H]noradrenaline was inversely related to the agonist concentration. The effect of diazepam was reversed by the benzodiazepine antagonist flumazenil. Zolpidem mimicked diazepam (EC50 = 14 nM). The increase of the K(+)-evoked overflow of [3H]D-aspartate (or of endogenous glutamate) elicited by GABA or muscimol in cerebellar synaptosomes was not affected by benzodiazepines (diazepam or clonazepam) or by zolpidem. On the other hand, Ro 15-4513, an inverse agonist at the benzodiazepine site, strongly inhibited (EC50 = 7 nM) the enhancement by GABA (3 microM) of the K(+)-evoked [3H]D-aspartate overflow in cerebellar synaptosomes; the effect of Ro 15-4513 was reversed by flumazenil. These results suggest the existence in the central nervous system of the adult rat of two native pharmacological-subtypes of the GABAA receptor having different function, regional distribution and neuronal location; the receptors require different membrane potential to be activated and display different sensitivity to benzodiazepines and to drugs acting at benzodiazepine sites.

Interaction between excitatory and inhibitory amino acids in the ventromedial nucleus of the hypothalamus in inducing hyper-running

We have previously shown that blockade of neurotransmission mediated by gamma-aminobutyric acid (GABA) in the ventromedial nucleus of the hypothalamus (VMH) by the microinjection of a GABAA receptor antagonist, bicuculline methiodide (BM), exclusively induced running activity in the rat. The purpose of the present study was to examine the role of receptors for excitatory amino acids (EAAs) in the VMH in inducing hyper-running and to clarify the interaction between GABA and EAAs in the VMH in controlling running activity. Although the injection of neither N-methyl-D-aspartate (NMDA) nor quisqualate into the VMH induced hyper-running, kainate (KA) produced running activity in a dose-dependent manner with similar features to that induced by BM. This effect of KA was blocked by a non-NMDA receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX). GABA injected simultaneously with KA into the VMH failed to affect hyper-running induced by KA. On the other hand, DNQX significantly suppressed the BM-induced running activity. These results suggest that endogenous EAAs acting on the KA-type receptor in the VMH facilitates running activity and that the release of such EAAs from the nerve terminal is presynaptically inhibited by GABA.

Effects of baclofen on amino acid release

This study showed the effects of baclofen on endogenous excitatory (Glu and Asp) and non-excitatory (Tau, Gly and Ala) amino acid release. (A) Release was stimulated by K+ 30 mM in rat frontal cortex slices in vitro (evoked release in ng/g tissue per 5 min): 3739 +/- 215 (Asp), 3429 +/- 357 (Glu), 763 +/- 181 (Tau), 945 +/- 71 (Ala), 468 +/- 44 (Gly). (B) Release was largely Ca(2+)-dependent for all amino acids but Ca(2+)-independent release was observed for Asp and Glu (29 and 32%, of total evoked release respectively). (C) Ca(2+)-dependent release was inhibited by baclofen 100 microM (% of inhibition taking as 100%, the Ca(2+)-dependent release in the absence of baclofen): 46 (Asp), 96 (Glu) (85% inhibition by baclofen 10 microM), 100 (Tau), 77 (Ala). Ca(2+)-independent release was inhibited: 87 and 88% (Asp), 85 and 95% (Glu) by baclofen 10 and 100 microM respectively. It is concluded that baclofen at a high concentration inhibits the evoked release of Glu, Asp, Tau and Ala due to inhibition of the Ca(2+)-dependent fraction and also of the Ca(2+)-independent component in the case of Glu and Asp. Baclofen at a low concentration inhibits only Glu- and Asp-evoked release (total release) due to inhibition of both Ca(2+)-dependent and -independent fractions in the case of Glu and to inhibition of the Ca(2+)-independent fraction in the case of Asp. The possibility that baclofen might affect the Ca(2+)-independent carrier-mediated release of Glu-Asp is discussed.

Guanfacine inhibits excitatory amino acid release from rat cerebral cortex through a non-adrenergic mechanism

The effect of guanfacine on the release of excitatory amino acids from rat cerebral cortex slices was assessed after preloading the slices with 3H-D-aspartate. It was found that guanfacine dose-dependently inhibited the veratridine stimulated release of the excitatory amino acid analogue; the ED50 of guanfacine being 6.4 +/- 2.3 microM (mean +/- S.E.M.) and the veratradine stimulated efflux being completely abolished at about 48 microM of guanfacine. Guanfacine also inhibited the veratridine stimulated release of endogenous aspartate and glutamate with similar potency to the inhibition of 3H-D-aspartate efflux. The inhibitory effect of guanfacine on the veratridine stimulated 3H-D-aspartate efflux was neither affected by high concentrations of the alpha 2-adrenoceptor blocker idazoxan nor by high concentrations of the alpha 1-adrenoceptor blocker prazosin. It is concluded that guanfacine inhibits the neural excitatory amino acid release in the rat cortex by a novel non-alpha 2/alpha 1-adrenergic mechanism.

The localization of GABAA receptors in mice with mutations affecting the structure and connectivity of the cerebellum

The distribution of cerebellar [3H]muscimol binding sites was studied autoradiographically in normal C57BL/6J mice and in the weaver, reeler, Purkinje cell degeneration and staggerer mutant mice. In the normal 79-day-old mouse cerebellum, the highest concentration of [3H]muscimol binding sites was observed in the granule cell layer. A much lower grain density was present over the Purkinje cell and molecular layers and negligible numbers of binding sites were seen over the deep cerebellar nuclei and white matter. A significant decrease in [3H]muscimol labeling was observed over the cerebellar cortex of the 81-86-day-old weaver mutant; this was most pronounced in the vermis where granule cell loss was the greatest. Over the hemispheres, where fewer granule cells degenerate, a higher density of binding sites remained. In the 27-29-old reeler cerebellum, where Purkinje cells are malpositioned, no labeling was seen over the deep Purkinje cell masses. In the quasi-normal superficial cortex, labeling density over the surviving granule cell layer was only slightly decreased. In the 54-57-day-old Purkinje cell degeneration mutant, where essentially all Purkinje cells have disappeared by day 45, a 29% decrease in grain density over the granule cell layer was observed, while labeling was still present in the molecular layer. Virtually no [3H]muscimol labeling was detected over any part of the cerebellar cortex of the 25-27-day-old staggerer mutant (which lacks parallel fiber-Purkinje cell synapses), although clusters of surviving granule cells were present in significant numbers in the lateral aspects of the cortex. Our autoradiographic data indicate that GABAA receptors are associated with granule cells in both the molecular and granule cell layers. Furthermore, our results raise the possibility that the maintenance of receptor levels may be dependent upon synaptic contacts between the granule cell and its main postsynaptic target, the Purkinje cell.

Effects in vitro of L-glutamate and kainic acid on the ATPase activities of synaptosomal membranes from different areas of rat brain

Changes in the activities of enzymes responsible for the active transport of Na+, K+, Ca2+, Mg2+ in synaptosomal membrane (SM) preparations from the cerebral cortex, hippocampus and thalamus with hypothalamus after incubation with L-glutamate (Glu) or kainic acid (KA) were investigated. Glu stimulated Ca,Mg- and Na,K-ATPase activities in cortex but reduced the activities of all the investigated ATPases, except Na,K-ATPase in the hippocampus and thalamus with hypothalamus. KA reduced distinctly the activity of ATPases in the cortex and only slightly in the thalamus with hypothalamus, but stimulated the enzyme activities in the hippocampus. Both, Glu and KA in vitro altered the processes of active transport of cations in SM preparations.

Residual benzodiazepine (BZ) binding in the cortex of pcd mutant cerebella and qualitative BZ binding in the deep cerebellar nuclei of control and mutant mice: an autoradiographic study

In mutant mice 'Purkinje cell degeneration' (pcd), there is an almost complete degeneration of Purkinje cells followed subsequently by a partial degeneration of granule cells. Recent neurochemical studies have revealed a 50% decrease in benzodiazepine (BZ) receptors in 45-day-old pcd mutants after degeneration of the Purkinje cells. At 300 days there is an 80% decrease in BZ receptors concomitant with granule cell losses. To determine the histological localization of these receptor changes this autoradiographic analysis was conducted. An in vitro autoradiographic technique was used to explore [3H]flunitrazepam binding. BZ receptors were found to be more concentrated in the molecular than the granular layer of mutant and control cerebellar cortices. There was, nonetheless, no statistically significant difference in grain counts between control and mutant mice in any layer. Substantial atrophy of cerebellar structures, particularly of the molecular layer, occurred in the mutant mice. It began even before 45 days of age but was extreme at 300 days. When the appropriate mathematical correction factor was introduced for the layer atrophy there was a 60% decrease in grain count in 45-day-old mutants in the molecular layer and a 84% decrease in 300-day-old mutants compared to controls. The initial decrease in total BZ receptors in the 45-day-old mutant animals is associated with a selective loss of Purkinje cells. The amount of receptor binding which persists in the 300-day-old mutants in the molecular layer would appear to reflect binding in the remaining parallel fibers from granule cells which remain.(ABSTRACT TRUNCATED AT 250 WORDS)