Interactions of Dopamine and Thyrotropin-Releasing Hormone in the Regulation of Prolactin Release in Lactating Rats*

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Abstract
Dopamine, secreted by the median eminence into hypophysial stalk blood, accounts for much of the inhibition of PRL release known to be exerted by the hypothalamus, and recent studies of lactating rats have revealed a brief decrease (60–70% for 3–5 min) in hypophysial stalk plasma dopamine concentrations immediately after the onset of a simulated suckling stimulus (electrical stimulation of a mammary nerve trunk). In the present study, we investigated the regulatory significance of this decrease for the increased PRL secretion evoked by this stimulus and, in addition, researched the effect of TRH on plasma PRL levels when it was administered before and after the decrease in dopamine. Control lactating rats anesthetized with urethane exhibited low and unvarying levels of plasma PRL (mean ± SEM 3.5 ± 0.6 ng/ml). Intraperitoneal injection of αmethyl- p-tyrosine (AMPT) resulted in a 120-fold elevation of plasma PRL levels. Dopamine infusions (1.0 μg-min/kg BW) into AMPT-treated rats, achieving stalk plasma dopamine concentrations in the range normally found there (8.4 ± 1.6 ng/ml), suppressed plasma PRL levels by 67%. However, a brief decrease (70% for 5 min) in the dopamine infusion rate to mimic the pattern of dopamine secretion in hypophysial stalk blood after a simulated suckling stimulus did not alter plasma PRL concentrations. TRH injection (300 ng/kg BW, iv) failed to acutely increase plasma PRL levels in control rats with low basal levels of plasma PRL, in AMPT-treated rats with high basal PRL levels, or in AMPT-treated dopamine-infused rats exhibiting intermediate plasma PRL levels; however, it did so in rats undergoing a 5-min 70% decrease in the rate of dopamine infusion. The magnitude of the increase was about 3-fold and lasted for 20–30 min. These results suggest that the brief decrease in dopamine secretion evoked by suckling does not, by itself, increase PRL secretion. However, it may condition the pituitary to respond to a putative PRL-releasing factor (TRH). Thus, these results suggest that the increased PRL secretion after suckling is due to a decrease in the secretion of dopamine and an increase in the secretion of a PRL-releasing factor.