Prolactin inhibiting activity of dopamine-free subcellular fractions from rat mediobasal hypothalamus.

In order to check the hypothesis of an identity of dopamine (DA) and prolactin inhibiting activity (PIF), their subcellular distribution was studied in the mediobasal hypothalamus (MBH) and the striatum, which served as a control structure. PIF was tested both in vivo and on pituitary incubates. Fractions were also assayed after adsorption of their catecholamine content on alumina, as well as in presence of haloperidol or alpha-flupentixol, potent DA receptor inhibitors. In the MBH, PIF was evenly distributed in the 17,000 g supernatant (S2) and in the crude mitochondrial fraction (P2) which contains synaptosomes. PIF activity was completely removed by alumina adsorption of S2, but not of P2 in spite of an over 99.9% elimination of DA. In contrast, striatal PIF activity was detected only in P2, and disappeared completely upon alumina adsorption, thus indicating that, in this structure, it is entirely due to DA. Addition of haloperidol (10--5M) or alpha-flupentixol (10--6M) reduced PIF activity of crude MBH homogenates, but no longer affected it after alumina adsorption. Quantitative studies suggest that only half of the total MBH PIF activity is accounted for by DA. It is concluded that the MBH contains dopamine-free PIF, which, as already shown for several other neurohormones, is exclusively distributed in nerve-endings.