Abstract
The aims of the experiments described in this thesis were 1) to investigate the role of endogenous opioid peptides (EOP) in the control of pulsatile LH release in the ram, and 2) to study the generation of pineal melatonin rhythms and their role in relaying the effects of photoperiod on seasonal reproductive cycles in rams.
A series of experiments was carried out on the effects of pharmacological opiate antagonists and agonists on LH secretion. Morphine significantly reduced LH pulse frequency in sexually active rams, and this effect was reversible by concurrent administration of the opiate antagonist naloxone, thus indicating the existence of specific opiate receptors. Naloxone injections alone increased LH pulse frequency in rams, showing that EOP mechanisms inhibit tonic LH secretion in a physiological situation. Effects of naloxone were studied at different stages of the seasonal reproductive cycle in rams maintained out -of- doors. Responses in intact rams were greater in the breeding season in September and December than when the rams were sexually quiescent in March and June. In pinealectomized rams which displayed a premature and attenuated reproductive cycle, LH responses to naloxone also correlated with testicular activity. These observations support the hypothesis that EOP mechanisms mediate the inhibitory effects of negative steroid feedback on LHRH secretion in the hypothalamus.
Radioimmunoassay techniques were developed and validated for the extraction and measurement of poendorphin in tissue and plasma. Substantial levels of immunoreactive poendorphin (p0EP) were found in the preoptic area and median eminence /arcuate nucleas area in the hypothalamus of the sexually active ram. This distribution is similar
Or) to that previously observed for LHRH, and is consistent with the view that the effects of naloxone on LH secretion result from antagonism of inhibitory EOP mechanisms on LHRH release. A considerable seasonal variation in peripheral blood plasma poEP concentrations was observed, levels being 5 -20 fold higher under short days than under long days. Studies suggest that this material is of pituitary origin, but its physiological function in peripheral target tissues is unknown.
Studies were also made of melatonin rhythms and sexual responses in rams housed for prolonged periods under constant illumination (LL) and constant darkness (DD). After eight weeks under DD melatonin levels were not constantly elevated to normal nocturnal levels, though no clear 24 hour periodicity was evident. Likewise, under LL melatonin levels were not constantly suppressed. DD was unable to prevent gonadal regression in rams previously housed under photostimulatory short days, thus rams do not show the phenomenon of relative photorefractoriness. Paradoxically, stimulated testicular growth occurred in rams transferred from inhibitory long days to LL.
A one hour light pulse per 24 hours re- established the melatonin rhythm in rams maintained in DD. The onset of the melatonin peak was shortly after the light pulse, suggesting that the pulse acts as an entrainment cue rather than by direct suppression of melatonin release. In a short term experiment a one hour light pulse was able to phase shift established melatonin rhythms free -running in constant darkness. A second light pulse given 7 hours after the first had only a transient suppressive effe ^t, thus the effect of light pulses on melatonin levels may depend on their temporal relationship to the endogenous melatonin rhythm. These observations are consistent with a model in which the
(V1) environmental light -dark cycle regulates the melatonin rhythm by entraining two or more endogenous oscillators. The role of circadian rhythms in photoperiodic time measurement would appear to be in the generation of melatonin rhythms rather than in their ultimate interpretation by the hypothalamus.
