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Previous studies involving the placement of microimplants of melatonin in the brain in sheep exposed to long days have provided evidence that melatonin acts within or close to the mediobasal hypothalamus (MBH) to mediate the effects of daylength on cycles in reproduction, moulting and other seasonal characteristics. To extend these observations, groups of Soay rams have now been treated with micro-implants of melatonin placed in the pars tuberalis (PT) and pars distalis (PD) of the pituitary gland, and in the lateral septum of the forebrain (septum). The PT and septum are potential target sites for the action of melatonin based on the localized binding of iodomelatonin assessed by in situ autoradiography. The animals were initially exposed to alternating 16-week periods of long days (16 h light:8 h darkness; 16L:8D) and short days (8L:16D) to entrain the seasonal cycles. The treatments were started at 10 weeks into a period of long days when the animals had a physiology normally observed in summer (low blood plasma concentrations of FSH and high concentrations of prolactin),and they remained under long days throughout the experiments. In experiment 1, animals received micro-implants of melatonin placed in the PT (n=6) or PD (n=4), or received empty implants in similar sites (n=4) or no surgery (n=4; total control, n=8). In experiment 2, groups of animals received microimplants of melatonin placed in the lateral septum (septum, n=7) or received corresponding control treatments (total control, n=8). The micro-implants consisted of 22 gauge stainless-steel needles with melatonin fused inside the tip. They were inserted bilaterally in the selected sites and left in place for 14 weeks. The biological effects of the treatments were assessed by measuring the changes in the blood plasma concentrations of FSH and prolactin, growth of the testes and moulting of the pelage over a period of 28 weeks (14 weeks treatment and 14 weeks post-treatment).
The administration of melatonin in the PT, but not in the PD or septum, affected the photoperiodically induced cycle in the secretion of FSH and prolactin. In the PT group there was no significant change in the plasma concentrations of FSH during the treatment with melatonin, but there was a significant (P<0·001), ANOVA) decrease in the levels of FSH after the treatment associated with premature regression of the testes. The plasma concentrations of prolactin were significantly (P<0·001, ANOVA) decreased during the treatment with melatonin in the PT group and increased after the treatment with associated changes in the growth and moulting of the pelage in the most responsive animals. The effects of melatonin in the PT were qualitatively similar but less consistent than those previously observed following placement of micro-implants in the MBH (data included for comparison). The results support the conclusion that melatonin acts, at least in part, in the PT to mediate the inductive effects of photoperiod on the timing of seasonal cycles of reproduction and moulting in rams.
Journal of Endocrinology (1994) 142, 267–276
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