Variation in the ability of neuroleptics to block the inhibitory influence of dopaminergic neurons on the activity of cells in the rat prefrontal cortex.

The stimulation of the ventro-medial mesencephalic tegmentum (VMT) induces an inhibition of the spontaneous activity of prefrontal cortical cells and blocks the excitatory responses evoked by the stimulation of the medio-dorsal nucleus of the thalamus (MD). This effect is mediated by the activation of the mesocortical dopaminergic (DA) system. In the present study, the influence of the systemic administration of several neuroleptics on the inhibition of prefrontal cortical cells induced by VMT stimulation has been analyzed in ketamine anaesthetised rats. The acute IP administration of fluphenazine (2 mg/kg), spiroperidol (2 mg/kg) or (+/-)sulpiride (100 mg/kg) reversed the inhibitory responses. Moreover, the number of cortical cells inhibited by VMT stimulation was considerably decreased after these treatments. Surprisingly, neither haloperidol at any dose used (0.1 to 0.5 mg/kg IV or 0.5 to 5 mg/kg IP) nor levomepromazine (25 mg/kg IP) nor the long acting neuroleptic, pipotiazine palmitic ester (32 mg/kg SC) blocked the inhibitory effect of VMT stimulation and in fact they lengthened the duration of the inhibition. Finally, the inhibition of MD evoked spikes in prefrontal cortical cells produced by VMT stimulation was no longer observed after sulpiride but persisted after haloperidol administration. Our findings confirm that the mesocortico-prefrontal DA neurons exert an inhibitory influence on target cells but they reveal differences in the efficacy of neuroleptics in blocking this effect.