Tyrosine hydroxylase in the stalk-median eminence and posterior pituitary is inactivated only during the plateau phase of the preovulatory prolactin surge

Abstract

This study examined changes in the activity of tyrosine hydroxylase (TH) in the stalk-median eminence (SME) and posterior pituitary (PP) during the preovulatory PRL surge. Immature female rats were injected with PMSG on day 28. Blood PRL levels were low on the morning of day 30, rose to a peak from 1400-1600 h, remained at a lower plateau from 1800-2400 h, and declined to basal levels on the morning of day 31. SME, PP, and striatum were removed from PMSG-treated rats at selected times during the periovulatory period and from age-matched control rats. TH activity was determined in tissue homogenates by a coupled hydroxylation-decarboxylation assay. Apparent Km and maximum velocity values with respect to 6-methyl tetrahydropterine were estimated from substrate saturation curves. The kinetic parameters for TH in either the SME or the PP of control rats were similar at 1100 and 1800 h on day 30. However, the apparent Km in both tissues was significantly lower than that in the striatum. The affinity of TH in the SME and PP was unchanged before and during the peak phase of the PRL surge, reduced significantly during the late plateau, and returned to presurge levels in the morning of day 31. TH activity in the striatum was similar at all times examined. To determine the state of activation of the enzyme, tissue homogenates were preincubated with cAMP, ATP, and magnesium. TH activity in the SME during the peak phase was unchanged by cAMP, and that in the PP was modestly increased. The relatively inactive enzyme in both tissues during the plateau phase was markedly activated by a cAMP-dependent mechanism. The low affinity of striatal TH was greatly increased by cAMP at both times. These data suggest that TH in the SME and PP exists in an activated state most of the time and is transiently inactivated during the plateau phase of the PRL surge. In contrast, TH in the striatum is relatively inactive in the basal state and is not affected by hormonal changes induced by PMSG. We conclude that the peak PRL surge occurs in spite of active dopamine (DA) neurons, suggesting that it is generated by a nondopaminergic mechanism. Decreased TH activity in DA neurons in the SME and PP may prolong the PRL surge during the plateau phase, whereas increased DA activity coincides with the termination of the surge.