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.