EFFECT OF PHOTOPERIOD AND EXOGENOUS MELATONIN ON CORRELATES OF ESTRUSIN THE DOMESTIC RABBIT

It was the purpose of this study to test whether inhibition of estrus in the rabbit by short photoperiod could be simulated by subcutaneous implants of the pineal hormone melatonin. By measuring two non-invasiv e and readily quantifiable correlates of estrus, emission of the so-ca lled nipple-search pheromone and scent-marking behavior (chinning), it was found that: 1. In intact does kept in stimulatory long photoperio d, melatonin suppressed chinning but had no effect on emission of nipp le-search pheromone. 2. In intact, melatonin-treated does which were r eturned from inhibitory short day to stimulatory long day, chinning re mained suppressed, whereas pheromone emission increased as for the veh icle-treated, control does. 3. In ovariectomized does, estradiol admin istration stimulated both pheromone emission and chinning whereas mela tonin failed to suppress these effects. Thus, although the administrat ion of melatonin had an inhibitory effect on chinning it failed to sup press emission of the nipple-search pheromone and therefore failed to fully simulate the effect of short photoperiod on these two correlates of estrus. Several explanations for this discrepancy are considered, including the possibility that melatonin is not the sole mediary of th e photoperiodic response, and that at least in female rabbits, other p ineal products may also play a significant role.