Role of prolactin feedback in the semicircadian rhythm of tuberoinfundibular dopaminergic neuronal activity during early pregnancy in the rat.

Abstract

During the first half of pregnancy rats exhibit a diurnal-nocturnal pattern of tuberoinfundibular dopaminergic (TIDA) neuronal activity, which is directly out of phase with the diurnal-nocturnal surges of prolactin secretion. To determine if the pattern of TIDA neuronal activity is maintained by a prolactin feedback mechanism the serum concentrations of this hormone were manipulated in the 6-day pregnant female by pharmacological means. TIDA neuronal activity was estimated in vivo by measuring the rate of dopamine synthesis (the rate of dihydroxyphenylalanine (DOPA) accumulation after the administration of an inhibitor of aromatic L-amino acid decarboxylase) in the median eminence, the region that contains the terminals of these neurons. In vehicle-treated pregnant rats cyclical changes in serum prolactin concentrations were observed, with peaks occurring at 06.00 and 21.00 h and a nadir (basal level) at 12.00 h. At the same time there was a diurnal-nocturnal pattern in the rate of DOPA accumulation in the median eminence with low values occurring at 03.00–06.00 and 18.00–21.00 h and a higher ‘basal’ value at 12.00 h. Pretreatment with bromocriptine, a long-acting dopaminergic agonist 24 h prior to sacrifice, maintained serum prolactin concentrations at very low values (<25 ng/ml) throughout day 6 of pregnancy. As a result of the low circulating concentrations of prolactin the rate of DOPA accumulation in the median eminence was reduced so that the diurnal-nocturnal pattern of TIDA neuronal activity was lost. On the other hand, pretreatment with haloperidol, a dopaminergic antagonist, 24 h prior to sacrifice increased serum concentrations of prolactin throughout the day (>700 ng/ml) such that the diurnal-nocturnal pattern was eliminated. As a result of the haloperidol-induced increase of serum prolactin concentrations the rate of DOPA accumulation in the median eminence was increased, but the diurnal-nocturnal pattern was maintained. Similarly, an intracerebroventncular injection of prolactin 12 h prior to sacrifice increased the rate of DOPA accumulation in the median eminence, but did not disrupt the characteristic pattern of activity in the TIDA neurons. The results of the latter two experiments suggest that the cyclical pattern of TIDA neuronal activity which occurs in day 6 pregnant rats is not the result of the diurnal-nocturnal surges of prolactin which occur in these-animals. On the other hand, the ‘basal’ activity of TIDA neurons in the early pregnant rat is ‘set’ by the serum concentration of prolactin just as it is in male and nonpregnant female rats.