Effects of placing micro-implants of melatonin in the mediobasal hypothalamus and preoptic area on the secretion of prolactin and β-endorphin in rams

Abstract

ABSTRACT In a previous study, we showed that the local administration of melatonin into the mediobasal hypothalamus (MBH), but not the preoptic area (POA), caused a premature increase in the secretion of FSH and growth of the testes in sexually inactive Soay rams exposed to long days. To extend these observations, we have now measured blood concentrations of prolactin and β-endorphin and the associated peripheral responses in the same animals, to establish whether the treatments produced multiple endocrine changes such as those which occur following exposure to short days. Groups of rams were initially exposed to alternating 16 weekly periods of long days (16 h light: 8 h darkness; 16L:8D) and short days (8L:16D) for at least 9 months to entrain the seasonal cycles in the secretion of the pituitary hormones. The treatments were started at 10 weeks under long days, when the animals had a physiology characteristic of the early summer with high blood plasma concentrations of prolactin (associated with growth of the summer pelage), and low concentrations of β-endorphin (associated with low body weight). The animals were assigned at random to the following treatments: (i) micro-implants of melatonin in the MBH, (ii) microimplants of melatonin in the POA, (iii) empty implants in the MBH or POA to act as operated controls, and (iv) no surgery to act as unoperated controls (n=12 rams/treatment). The micro-implants consisted of 22-gauge stainless-steel needles with melatonin fused inside the tip. The implants were inserted bilaterally in the brain, and left in place for 12–14 weeks. The observations continued for a total of 28 weeks while the animals remained under long days. The administration of melatonin in the MBH induced a rapid decrease in plasma concentrations of prolactin while in the POA it induced a less marked but significant effect. The mean times to minimum concentrations of prolactin were 7·4±0·4, 17·3±2·8 and 26·0 ±0·3 weeks for the MBH, POA and combined control groups respectively (MBH vs control, P<0·001, POA vs control P<0·01. In the MBH group, the concentrations of prolactin subsequently increased to a maximum 6 weeks after the end of melatonin treatment. The changes in prolactin were accompanied by changes in growth and moulting of the pelage; only animals in the MBH group showed a conspicuous moult associated with the change from low to high prolactin secretion. There was also a marked effect of melatonin when given into the MBH, but not the POA, on plasma concentrations of β-endorphin, which was correlated with differences between groups in the cycle in body weight. In conclusion, continuous administration of melatonin into the MBH acts like a long-duration melatonin signal and induces a full sequence of biological responses normally evoked by transfer to short days. This is used as evidence that melatonin acts within or close to the MBH to induce the multiple effects of photoperiod in the ram. Journal of Endocrinology (1992) 134, 437–448