Evidence for dopamine autoreceptors in mesocortical dopamine neurons

The effect of different treatments which are thought to modify dopamine (DA) synthesis by an action on DA autoreceptors was compared in the caudate nucleus and two frontal cortical areas: the medial prefrontal cortex and dorsolateral frontal cortex, having the highest and lowest DA concentration, respectively, but having equal concentrations of norepinephrine (NE); the NE to DA ratio being 3:2 and 8:1, respectively. DA synthesis was measured by the rate of DOPA accumulation after inhibition of DOPA decarboxylase. Gamma-butyrolactone (GBL) (750 mg/kg) increased DOPA accumulation by 200% in the caudate nucleus but only by 40% in the medial prefrontal cortex and was ineffective in the dorsolateral frontal cortex. Apomorphine (25-100 micrograms/kg) decreased DOPA accumulation by 7-30% in the medial prefrontal cortex and by 20-40% in the caudate nucleus in a dose-dependent manner. N-n-propylnorapomorphine (NPA) produced a similar effect within the dose range of 2.5-10 micrograms/kg. Both DA agonists were completely ineffective in the dorsolateral frontal cortical area. Haloperidol (0.5 mg/kg) increased DOPA accumulation by 80 and 220% in the medial prefrontal cortex and the caudate nucleus, respectively. It is concluded that DA autoreceptors regulate DA synthesis in the medial prefrontal cortex as in the caudate nucleus. Moreover, it was found that DOPA accumulation was approximately equal in the medial prefrontal cortex, with dense dopaminergic innervation, as in the dorsolateral area, devoid of dopaminergic terminals, suggesting that only a small fraction of cortical DA synthesis takes place in dopaminergic neurons, while the major part occurs in noradrenergic neurons.(ABSTRACT TRUNCATED AT 250 WORDS)