Response of pre-pubertally castrated rams and ewes to artificial photoperiod--changes in plasma LH and prolactin

Castrate rams and ovariectomized ewes were maintained in the presence of entire rams and ewes and subjected to successive periods of alternating 6 h light:18 h darkness ('short' days) and 18 h light:6 h darkness ('long' days) preceded by a period of 12 h light:12 h darkness ('constant' light days). Plasma concentrations of LH and prolactin were measured in the castrate animals in order to determine how LH and prolactin secretion responded to the artificial light regime and corresponding periods of elevated or depressed testicular and ovarian activity in the entire rams and ewes. There was no variation in mean plasma LH concentrations or LH pulse frequency with either the changes in photoperiod or the phases of gonadal activity in the entire animals. However, there was a highly significant (P less than 0.001) relationship between prolactin secretion and the artificial photoperiod in both castrate groups with high and low levels coinciding with long and short days respectively. In addition, there was a marginally significant (P less than 0.1) relationship between prolactin secretion in the castrate ram and the stage of testicular activity in the entire rams with elevated levels associated with regressed activity. Prolactin secretion in the ovariectomized ewes was significantly (P less than 0.05) related to the phase of ovarian development with high levels associated with acyclic activity. It is concluded that LH secretion and pituitary responsiveness to exogenous GnRH were not modified by the artificial light regime. However, the changing light pattern was physiologically 'perceived' by the castrate animals as indicted by a concomitant variation in plasma prolactin concentrations.