Effect of Photoperiod and Morphine on Plasma Prolactin Concentrations and Thyrotropin-Releasing Hormone Secretion in the Ewe

Abstract

This study investigated the effects of daily photoperiod length and morphine on thyrotropin-releasing hormone (TRH) and prolactin secretion in sheep. Two groups of adult ewes were kept under either 16 h L:8 h D or 8 h L:16 h D photoperiods for approximately 60 days. Then the photoperiods were reversed and approximately 60 days later push-pull cannulae were implanted into the hypothalamic stalk-median eminence. After a 7-day recovery period, the ewes were subjected to hypothalamic perfusion and blood was collected at 15-min intervals from the jugular vein. Perfusates also were collected into 15-min fractions. The first 3 h of perfusion served as an equilibrium period. During the next 2 h, saline was infused into one jugular vein. This was followed by a 2-hour morphine infusion (1 mg.kg-1.h-1). Data from 13 ewes were analyzed for effect of photoperiod and drug on TRH concentrations in the perfusates and prolactin in the serum. Prolactin was significantly (p less than 0.01) higher under 16 h L:8 h D than 8 h L:16 h D and was greatly increased (p less than 0.001) by morphine infusion. TRH only tended to be higher (p = 0.05) under 16 h L:8 h D than under 8 h L:16 h D, but morphine infusion induced a rapid and highly significant (p less than 0.01) increase in TRH secretion. There were no photoperiod and drug interactions for either TRH or prolactin.(ABSTRACT TRUNCATED AT 250 WORDS)