β-Endorphin Secretion in Rams Related to Season and Photoperiod

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Abstract
A newly established RIA was used to measure changes in the concentration of .beta.-endorphin in peripheral blood and pituitary tissue from adult Soay rams living outside under natural conditions and housed indoors under artificial photoperiods. A pronounced seasonal cycle in plasma .beta.-endorphin immunoreactivity occurred in the outdoor animals, with low levels in spring and early summer (February-May; < 200 pg/ml plasma) and maximal levels 10-20 times higher in late summer and autumn (July-October). Seasonal changes in plasma levels of PRL,FSH, and cortisol, testis size; and body weight were also monitored; the seasonal cycle in the levels of immunoreactive .beta.-endorphin occurred in parallel with the cycle in plasma FSH and body weight. There were no significant seasonal changes in plasma cortisol concentrations. Marked changes in the plasma levels of .beta.-endorphin were also seen in rams kept under the artificial photoperiod regimen of alternating 12- to 16-week periods of long days (16 h of light and 8 h of darkness: 16L:8D) and short days (8L:16D). Transfer from long days to short days led to a greater than 20-fold increase in the levels of .beta.-endorphin, reaching a maximum after 4-8 weeks; the reverse switch in photoperiod led to a rapid decrease in the levels. There was no diurnal rhythm in the plasma levels of .beta.-endorphin based on hourly samples collected for 24 h under long and short days. The total content of immunoreactive .beta.-endorphin in the pituitary gland was lower in rams under short days than under long days, converse to the pattern in the blood. Sephadex chromatography of the plasma samples revealed that most of the .beta.-endorphin immunoreactivity coeluted with synthetic .beta.-endorphin-(1-31), and a small amount of activity eluted with .beta.-lipotropin. The seasonal and photoperiod-induced changes were largely due to changes in the levels of .beta.-endorphin. Extracts of pituitary tissue revealed a large proportion of .beta.-lipotropin to .beta.-endorphin compared to plasma, with no consistent change in ratio related to the photoperiod. The overall results illustrate that there are pronounced seasonal and photoperiod-induced changes in immunoreactive plasma .beta.-endorphin levels in the ram. Under artificaial photo periods, long days inhibit and short days stimulate .beta.-endorphin secretion. Under natural conditions, the development of refractoriness to both the hibibitory effects of long days and the stimulatory effects of short days may explain the timing of the annual cycle of .beta.-endorphin secretion.
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