Relative potency of dopamine agonists on autoreceptor function in various brain regions of the rat

The effect of six dopamine agonists including apomorphine, epinine, dopamine, piribedil, lergotrile and bromocriptine on the incorporation of [3H]tyrosine into dopamine was studied in slices and synaptosomes prepared from various brain areas containing dopamine terminals including striatum, nucleus accumbens, olfactory tubercle and medial basal hypothalamus. It was observed that all of these drugs were active in causing a decrease in dopamine synthesis in these various brain areas. The catecholamine agonists apomorphine, epinine and dopamine were more potent in inhibiting dopamine synthesis in the mesolimbic structures than in the striatum. On the other hand, apomorphine and epinine were less potent while dopamine was more potent in the medial basal hypothalamus. The ergoline drugs were weak agonists in all structures studied. It is concluded that autoreceptor regulation of dopamine synthesis is more active in the mesolimbic compared with the nigrostriatal dopamine pathway while autoreceptors may be absent in the median eminence.