Investigation into the role of dopamine and lysosomes in the impairment of prolactin transformation and release imposed by long periods of non-suckling in the rat.

The transformation of prolactin (PRL) within lactating rat hemipituitary glands incubated for 240 min and the release of the hormone into the incubation medium were progressively reduced by dopamine added to the medium over a 2.3-49 mumol/l range of concentration; the antilysosomal drug chloroquine did not alter these effects of dopamine. In related experiments, the short-term action of dopamine was manifested also upon in vitro transformation, repletion and release of in vivo labelled [3H]PRL, thus indicating that dopamine inhibits all phases of PRL secretion by the lactating rat anterior pituitary (AP). In other experiments, increasing the non-suckling period from 8 to 16 h reduced the pre-suckled concentration of PRL in the lactating rat AP, reduced the ability of the AP to transform PRL in response to suckling, and reduced the capability of the AP of such rats to secrete PRL in vitro. Injecting chloroquine (0.2 mmol/kg ip), haloperidol (0.27 mumol/kg ip) or providing 15-30 min of suckling midway during the 16-h non-suckling period restored each of these functions. Thus, frequent uncoupling of dopamine from its receptor appears necessary to prevent impairment of the suckling-induced transformation mechanism in the lactating rat AP and, presumably, to prevent PRL in the lactotrope from reaching an age where it becomes susceptible to lysosomal degradation.