Light-induced rhythms of prolactin secretion in the ram and the effect of cranial sympathectomy.

Four Soay rams cranially sympathectomised by removal of the superior cervical ganglia were housed alongside 4 control rams in an artificial lighting regime of alternating 16 week periods of short days (8L : 16D) and long days (16L : 8D) for 2 years. The variations in the plasma prolactin concentration revealed that while the control animals adjusted both the short term (diurnal) and long term (seasonal) rhythms in prolactin secretion in response to the changes in daylength, the ganglionectomised rams failed to respond. These sympathectomised animals continued to show long term changes in the circulating level of prolactin however and during the periods of hypersecretion a conspicuous diurnal rhythm in the hormone concentration was apparent.