Inhibitory influence of the mesocortical dopaminergic neurons on their target cells: electrophysiological and pharmacological characterization.

Both dopaminergic (DA) and noradrenergic (NA) systems exert an inhibitory influence on the activity of prefrontal cortical neurons (PFC). As NA-containing fibers run close to the dorsal ventral tegmental area (VTA), electrical stimulation of the VTA might coactivate both DA and NA systems. In the present study extracellular recordings and microiontophoresis were used in anesthetized rats to analyze first whether the inhibitory cortical responses to VTA stimulation and DA application were mediated by DA or adrenergic receptors. Inhibitory responses elicited by DA application or VTA stimulation were observed in PFC output neurons identified by antidromic activation from subcortical structures. Both types of inhibitory effects were reversed by the DA antagonist sulpiride, but not by the adrenergic antagonists prazosin (alpha-1), yohimbine (alpha-2) or propranolol (beta). NA and the beta agonist isoproterenol inhibited the activity of PFC cells and these effects were antagonized by propranolol, but neither by prazosin and yohimbine nor by the DA antagonist sulpiride. Thus, the inhibitory influence of the mesocortical DA system in the PFC involves DA, but not NA, recognition sites. The DA receptor subtype mediating the inhibitory effects of VTA stimulation and DA application in the PFC was analyzed further. VTA- and DA-evoked inhibitory responses were antagonized by the D2 selective antagonists (-)-sulpiride, LUR 2366 and RIV 2093, but not by the D1 selective antagonist SCH23390. In addition, the DA-induced inhibitory response was mimicked by the selective D2 agonist LY 171555 but not by the selective D1 agonist SKF 38393. Surprisingly, haloperidol, which is also a potent D2 antagonist, failed to consistently block DA- and VTA-induced inhibitory effects. The present results indicate that the inhibition of PFC cells by mesocortical DA neurons is mediated via a subtype of DA receptors which is particularly sensitive to benzamides.