Summary. Two experiments were performed to test the hypothesis that the seasonal suppression of gonadotrophin pulse frequency in anoestrous horse mares reflects inhibitory neural mechanisms. In a preliminary experiment (Exp. 1) conducted in February, 4 anoestrous mares were sedated by repeated intravenous injections of xylazine, an α2-adrenergic receptor agonist. On the day of treatment, 1–2 LH pulses were observed in xylazine-treated mares. In contrast, during a 12-h period only 1/8 untreated control mares exhibited a LH pulse. In Exp. 2, the effect of xylazine-induced sedation on pulsatile gonadotrophin release was examined in 4 anoestrous mares on two occasions before (18 November and 9 December) and after (23 December and 6 January) an abrupt, artificial increase in day length. Treatment with xylazine was associated with an overall increased FSH (P < 0·01) and LH (P < 0·05) pulse frequency, compared with that observed during 12-h pretreatment periods. To evaluate an effect of treatment at the various time during the experimental period, the change in FSH pulse frequency was analysed, since occasionally FSH pulses were unaccompanied by a change in serum LH values indicative of a LH pulse. On two occasions before increased daylength only 1/4 and 3/4 mares exhibited an increase in FSH pulses; in contrast, 14 days after increased daylength (23 December), 4/4 mares exhibited increased FSH pulse frequency associated with treatment. After 27 days of increased daylength (6 January), endogenous FSH pulse frequency was greater than before increased daylength and treatment with xylazine was unaccompanied by a further increase.
The results support the hypothesis that an inhibitory catecholaminergic neural system may play a role in the seasonal suppression of pulsatile gonadotrophin secretion in the anoestrous mare. The activity of this inhibitory system appears to decrease towards the approach of the breeding season.
Keywords: gonadotrophins; pulse frequency; seasonal inhibition; xylazine; anoestrous mares