Inhibition of Rat Prolactin Release by Apomorphine<i>in Vivo</i>and<i>in Vitro</i>

Abstract

Studies were performed to determine if apomorphine (APO), a specific dopamine receptor stimulant, was able to influence serum prolactin levels in vivo and in vitro. Administration of APO (ip, 20 mg/kg) to rats with normal 4-day estrous cycles reduced serum prolactin levels during diestrus and prevented the proestrous rise of serum prolactin. The elevation of serum prolactin observed after chlorpromazine treatment (2.5 or 1 /mg) was significantly attenuated by APO. In addition, treatment: of lactating rats with 15 mg/kg of APO prevented the suckling-induced rise of serum prolactin. These observations demonstrate that dopamine receptor stimulation results in inhibition of prolactin secretion in vivo. Incubation of pituitary halves in vitro with 8 + 10−8M (25 ng/ml) APO consistently resulted in a significant decrease in the amount of prolactin released by the pituitary tissue. Concentrations of APO as low as 1.6 + 10−8M (5 ng/ml) were effective.Pimozide, a specific dopamine antagonist, was able to reduce the direct inhibitory action of APO on the pituitary gland in vitro. We conclude from these results that APO can stimulate dopamine receptors in the pituitary gland, and this dopamine receptor activation results in an inhibition of prolactin release. (Endocrinology95: 123, 1974)