ELECTROPHYSIOLOGICAL EFFECTS OF MONOAMINE-OXIDASE INHIBITION ON RAT MIDBRAIN DOPAMINERGIC-NEURONS - AN IN-VITRO STUDY

1 The effects of the inhibition of monoamine oxidase (MAO) type A and B have been evaluated on the spontaneous firing activity of the dopami nergic (principal) neurones of the rat midbrain intracellularly record ed from a slice preparation. 2 The non-specific MAO inhibitor, pargyli ne, superfused at a concentration of 10-100 mu M, decreased or abolish ed the spontaneous firing discharge of the principal neurones in the s ubtantia nigra pars compacta and ventral tegmental area. This effect h ad a slow onset and appeared to be sustained. 3 The administration of the dopamine D-2/3 receptor antagonist, sulpiride (100-300 nM), antago nized the pargyline-induced effect, while the superfusion of the dopam ine D-1 receptor antagonist, SCH 23390 (1-3 mu M) did not counteract t he induced inhibition of the firing rate. 4 The inhibitor for the MAO A, clorgyline (30-100 mu M), reduced the firing rate of the dopaminerg ic neurones. A similar depressant effect was also observed when a MAO B inhibitor, deprenyl (30-100 mu M), was applied. Lower concentrations of both drugs (300 nM-10 mu M) did not produce consistent effects on neuronal discharge. 5 Our data suggest that only the blockade of both types of MAO enzymes favours the inhibitory action of endogenous dopam ine on somato-dendritic D-2/3 autoreceptors.