MUSCARINIC RECEPTOR STIMULATION INCREASES THE SPONTANEOUS [H-3] GABA RELEASE IN THE RAT SUBSTANTIA-NIGRA THROUGH MUSCARINIC RECEPTORS LOCALIZED ON STRIATONIGRAL TERMINALS

The effect of muscarinic agonists on the spontaneous release of [ H-3]GABA was investigated in vitro on rat substantia nigra slices. Acetylcholine (5 x 10(-5) M) in the presence of eserine (5 x 10- 5 M) induced a 12.3% increase of the spontaneous release of [H-3]GABA. Similarly, carbachol (5 x 10(-4) M) enhanced by 9% the release of [H-3]GABA. This effect was Ca2+-dependent, it was abolished in the presence of 0.4 mM Ca2+ and enhanced from 9 to 17% when Ca2+-concentration of the superfusion medium was increased from 1.3 to 2.4 mM. The carbachol effect was mediated by muscarinic receptors since it was abolished by atropine (2 x 10(-6) M). The pharmacologically M2 muscarinic receptor subtypes seems to be involved as the carbachol-induced effect was abolished by AF-DX384MS (10(-6) M), an M2 antagonist and was only partially reversed by pirenzepine (10(-5) and 10(-4) M), an MI antagonist which at these doses also block the M2 receptors. The absence of effect of SCH23390 (10(-6) M) a D-1 antagonist as well as the lack of effect of CNQX (10(-5) M) and dizocilpine maleate (10(-6) M), two glutamate antagonists, on the carbachol-induced effect indicated that neither dopamine (through D-1 receptors) nor glutamate (through ionotropic receptors) were involved in the response. In addition, the persistence of the carbachol-induced effect in the presence of tetrodotoxin (2 x 10(-7) M) suggests a direct muscarinic-medited modulation of [H-3]GABA. The localization of muscarinic receptors on striatonigral fibres was confirmed by autoradiographic studies showing a decrease of [H-3]pirenzepine binding in the substantia nigra after a unilateral striatal lesion induced by kainic add injection. This latter result provides evidence of the presence of M1 receptors on striatonigral terminals as the concentration of [H-3]pirenzepine used (10 NM) is M1-selective. These results indicate a cholinergic modulation of GABA release in the rat substantia nigra mediated by muscarinic receptors localized on striatonigral terminals. The involvement of the m4 muscarinic receptor subtype that have a M1/M2 pharmacology is discussed.