Dopaminergic agents including 3-PPP and its enantiomers on medial septal self-stimulation

The effects of several dopamine agonists were determined on medial septal self-stimulation in rats and compared with selected dopamine antagonists and with the psychostimulants, d-amphetamine and nomifensine. Apomorphine, 3-PPP, TL-99, N,N-dipropyl-5,6-ADTN and N,N-dipropyl-6,7-ADTN inhibited self-stimulation at dose ranges selective for the dopamine autoreceptor as indicated by biochemical studies. Haloperidol and molindone produced dose-related inhibition but sulpiride increased self-stimulation. D-amphetamine and nomifensine also increased responding. The agonist-induced inhibition differed from neuroleptic-induced inhibition of self-stimulation. Both (+) and (-) 3-PPP inhibited responding by a similar amount over the dose range 0.25-1.0 mg/kg. At higher doses, (-) 3-PPP further decreased responding whereas the effects of (+) 3-PPP plateaued at approximately 55% of controls. These studies show that dopamine agonists, like neuroleptics, inhibit medial septal self-stimulation. This effect appears to be mediated via autoreceptor activation. Differences between neuroleptic- and agonist-induced inhibition and the 3-PPP stereoisomer data accord with the hypothesis that behavioural inhibitory effects caused by autoreceptor activation are less severe than those caused by dopamine postsynaptic blockade.