Melatonin rhythms in Pony mares and foals.

Melatonin concentrations in intact (N = 3) and sham-operated (N = 3) mares during March were greater (P less than 0 . 05) during the night than during the day, but this pattern was not seen in 3 mares from which the superior cervical ganglia had been removed bilaterally. When 4 Pony mares were exposed to a photoperiod of 10L:14D for 3 weeks and then to continuous darkness (0L:24D) for another 3 weeks, melatonin levels were greater (P less than 0 . 05) at the end of the 0L:24D period than during the earlier period and still displayed rhythmic fluctuations but were no longer co-ordinated with equivalent day/night rhythms or among mares. When melatonin rhythms were monitored in 3 mares and their foals housed in open pens exposed to natural lighting, significant time trends in melatonin concentrations were observed in mares when the foals were aged 1-3, 4-6 and 7-11 weeks, but foals did not display significant times trends in melatonin until they were 7-11 weeks old.

Effects of pinealectomy in Pony mares.

When mares were pinealectomized in summer or in winter there was no difference in the annual ovulation date of these animals and of pineal-intact controls during the first post-operative breeding season but in the second season the ovulation date of the winter pinealectomized mares was significantly delayed, suggesting a long-term but clear pineal involvement in seasonal reproductive patterns. Exposure of pinealectomized mares to a stimulatory photoperiod failed to advance the average date of first annual ovulation and was similar to that of intact mares receiving no stimulatory lighting. The results indicate that pinealectomy blocks the ability to respond to photostimulation. In intact mares plasma melatonin concentrations were significantly increased during darkness (P less than 0 . 003) but pinealectomized mares demonstrated no significant time trends.

Effects of pituitary stalk transection on endocrine function in Pony mares.

The pituitary stalk was transected in 10 Pony mares by a surgical approach that involved dorsal reflection of the brain and micro-dissection from the ventro-lateral aspect of the pituitary. Diabetes insipidus was the most immediate and marked result, requiring extensive electrolyte and antidiuretic therapy for approximately 48 h after operation. Fluid stasis then developed and no further supportive measures were necessary. Endocrine challenge tests with GnRH and TRH before and after stalk transection indicated a loss of responsiveness (GnRH) or suppressed responsiveness (TRH) after the operation. This technique permits isolation of the pituitary from its hypothalmic releasing and/or inhibiting hormones and therefore permits more refined study of the hypothalamic-pituitary axis.