Administration of melatonin into the mediobasal hypothalamus as a continuous or intermittent signal affects the secretion of follicle stimulating hormone and prolactin in the ram.

The biological effects of administering melatonin into the mediobasal hypothalamus (MBH) was documented in adult Soay rams using two delivery systems: (1) microimplants in the MBH delivering melatonin continuously and (2) microdialysis probes in the MBH delivering melatonin intermittently as a daily timed infusion. The experimental protocol was to precondition rams to long days (LD 16:8) for 10 to 12 weeks, and then introduce the exogenous source of melatonin by implantation or infusion. Sixteen rams were divided equally into four treatment groups: (a) microimplants in the MBH, (b) microdialysis probes in the MBH, (c) empty microimplants in the MBH to act as sham-operated controls, and (d) no surgery to act as unoperated controls. The microimplants consisted of 22-gauge stainless steel cannulae with melatonin fused inside the tip and were placed bilaterally in the brain for 14 weeks. These implants had previously been shown to release melatonin at a relatively constant rate when incubated in buffered saline at 37 degrees C (3.42 +/- 0.42 micrograms/24 hr, mean +/- SEM, 1-10 weeks) and to produce a localised concentration of melatonin when implanted in the brain (localised to within 1 mm of the center of the implant). The microdialysis probes were also 22-gauge cannulae with a 3 mm membrane (Biotech). They were placed bilaterally into the MBH, connected to two portable syringe drivers secured to a backpack. Melatonin was infused daily for 10 hr (estimated delivery: 0.5 microgram/hr) starting in the mid-light phase to produce a long-duration intermittent melatonin signal. Technical problems limited the period of infusions to 8-10 weeks with minor interruptions. Animals from all groups were maintained on long days, and the observations extended for a period of 28 weeks. The melatonin implants placed in the MBH induced a premature increase in the blood concentrations of FSH and growth of the testes. This treatment also induced a marked decrease in the plasma concentrations of prolactin and the earlier development of the long winter pelage. These changes were reversed after the end of treatment with a decline in the plasma concentrations of FSH and regression of the testes, and an increase in the concentrations of prolactin and moult of the winter pelage. Daily infusions of melatonin from the microdialysis probes in the MBH produced qualitatively similar, but less marked responses. The overall results illustrate that the administration of melatonin into the MBH, either continuously or intermittently, to extend the duration of the daily melatonin signal, induces multiple short-day responses.(ABSTRACT TRUNCATED AT 400 WORDS)