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

Six adult Soay rams were preconditioned to an artificial lighting regimen of alternating 4-month periods of long (16L:8D) and short days (8L:16D) for at least 10 months 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 from short to long days had the reverse effect; the first 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 were also measured and a close inverse relationship between gonadotrophin and prolactin secretion was observed.