In vivo release of [3H]gamma-aminobutyric acid in the rat neostriatum--I. Characterization and topographical heterogeneity of the effects of dopaminergic and cholinergic agents.

The release of [3H]gamma-aminobutyric acid continuously synthesized from [3H]glutamine was studied in the striatum of halothane-anaesthetized rats superfused with a push-pull cannula. The levels of spontaneously released [3H]GABA were identical in all striatal regions examined, but were found to be higher at the junction between the striatum and the globus pallidus. Superfusion with a medium enriched in K+ ions induced a concentration-dependent increase in [3H]GABA release. Superfusion with a Ca2+-free medium did not affect the spontaneous outflow of [3H]GABA but sharply reduced the release of [3H]GABA evoked by 30 mM K+. Locally applied tetrodotoxin (50 microM) decreased slightly the spontaneous release of [3H]GABA (-22%). When acetylcholine (50 or 500 microM) was added to a superfusion medium containing eserine (50 microM), the spontaneous release of [3H]GABA was enhanced in the ventral but not in the dorsal region of the striatum. The local application of 2,3,4,5-tetrahydro, 7,8,-dihydroxy, 1-phenyl, 1-H, 3-benzazepine (10 microM), a dopaminergic agonist acting preferentially on D1 receptors increased the release of [3H]GABA in the dorsal striatum (+32%) but decreased it slightly (-19%) in the ventral striatum. 3-(2-(N-3 hydroxyphenylethyl)N-propylamino)ethyl-phenol (50 microM), a preferential D2 receptor agonist, decreased [3H]GABA release when it was applied dorsally (-23%) but not ventrally in the striatum. It is concluded that the regulation of the release of [3H]GABA by acetylcholine and dopaminergic drugs is different in the dorsal and ventral regions of the striatum. These differences may be related to the existence of subpopulations of GABA neurons and may well have functional implications as suggested by behavioural studies.