Differential inhibition of dopamine and bromocriptine on induced prolactin release: multiple sites for the inhibition of dopamine.

Effects of dopamine and bromocriptine on TRH- or dibutyryladenosine 3',5'-cyclic monophosphate (dbcAMP)-induced prolactin release from primary cultured rat pituitary cells were studied using a perifusion system. TRH (100 nmol/l) stimulated prolactin release from basal concentrations of 33.8 +/- 0.5 to 151.2 +/- 28.0 ng/ml (net increase) or 447% increase. Dopamine inhibited the basal release of prolactin throughout the experiment, but TRH (100 nmol/l) was still able to stimulate prolactin release under the influence of dopamine. The increment in prolactin release was inversely proportional to the dopamine concentration. When TRH (100 nmol/l) was introduced during a perifusion period with bromocriptine 1 nmol/l, the prolactin concentration was increased to 110.9% of basal levels. The stimulatory effect of TRH under the influence of bromocriptine (1 nmol/l) was significantly lower than that without bromocriptine (control), although the higher concentrations of bromocriptine (10 and 100 nmol/l) did not further reduce the peak concentration of TRH-induced prolactin release. During a perifusion period with a low concentration of dopamine (1 nmol/l plus 0.1 mmol/l ascorbic acid), introduction of dbcAMP (3 mmol/l) stimulated prolactin release to 48% of basal concentration. A higher concentration of dopamine further reduced the stimulatory effect of prolactin release. Bromocriptine impeded the stimulatory effect of dbcAMP (3 mmol/l) on prolactin release in a similar manner as dopamine. Since a higher concentration of bromocriptine (10 and 100 nmol/l) did not further inhibit the TRH-induced prolactin release whereas a higher concentration of dopamine did, it is concluded that dopamine acts through additional mechanism(s) other than the D2 receptor transduction system.