The effects of a sudden decrease or increase in daylength on prolactin secretion in the ram

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Abstract
Six adult Soay rams were preconditioned to an artificial lighting regimen of alternating 4 mo. periods of long (16L [light]:8D [dark]) and short days (8L:16D) for at least 10 mo. before blood samples were collected at hourly intervals for 24 h at various times. The abrupt change from long to short days resulted in a progressive decrease in plasma levels of prolactin, while that form short to long days had the reverse effect; the 1st response to the light changes was rapid, beginning within 6 days. During the periods of high secretion there was a 24 h cycle in plasma prolactin concentrations, with a peak in both the early dark and early light phases of each day. Changes in the relative magnitude of these peaks were observed in relation to the long-term alteration in prolactin secretion. Plasma levels of FSH [follicle stimulating hormone] were also measured and a close inverse relationship between gonadotropin in the prolactin secretion was observed.