The sensitivity of pituitaries from medial basal hypothalamus (MBH)-lesioned rats to the inhibitory effects of dopamine and apomorphine on prolactin (PRL) secretion were studied in vitro. Anterior pituitary halves from ovariectomized control rats or ovariectomized rats lesioned for either 1 or 14 days were incubated in medium 199 and showed a linear release of PRL over the 3-h incubation period. Dopamine (10-6 M) inhibited absolute or percentage of PRL release to the same extent from pituitaries of nonlesioned, 1-, and 14- day-lesioned animals. However, at 10-8 M dopamine, the percent inhibition of PRL release from the pituitaries of the 14-day-lesioned group was significantly greater than from the nonlesioned and 1-day-lesioned groups. Apomorphine (5 A- 10-8 M) produced a greater absolute and percentage inhibition of PRL release from pituitaries of the 14-day-than from the pituitaries of 1-day-lesioned animals. No concentration of dopamine had a measurable effect on the concentration of pituitary PRL. However, 10-8 M dopamine was effective in lowering total PRL (amount released plus amount remaining in the gland) only from pituitaries of 14-day-lesioned, but not of nonlesioned or 1-day-lesioned animals. This observation suggests that synthesis in addition to release might be involved in this supersensitivity phenomenon. The development of supersensitivity at the pituitary after long term removal of dopaminergic input, provides further evidence for the hypothesis that dopamine is a physiologic PRL release-inhibiting factor.
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