Photoperiod action on equine reproduction

Abstract

Mares and stallions have their reproductive period during the season where the days are long. This annual reproductive rhythm is controlled by the length of the daylight. Treatment of mares during winter when their ovaries are normally inactive, with an artificial light during 14.5 h per day, beginning around the winter solstice, causes the first ovulation of the year to occur at an earlier date.

Studies have verified certain stages in the process by which equine species perceive light. The light message is transformed into nerve impulse by specialized retinal cells. These impulses are transmitted through the supra-chiasmatic nucleus and the cervical superior nucleus to the pineal gland. The pinealocytes respond to noradrenergic stimulation by secreting melatonin. The administration of exogenous melatonin by subcutaneous implants or by oral route suppresses the photostimulation effect. The studies on the use of melatonin implants in mares to obtain reproductive activity out of the breeding season are now in progress.

The GnRH secretion is regulated by such neuromediators as dopamine, noradrenalin, serotonin and opioids. Naloxone, an opioid antagonist, induces a GnRH secretion that is followed by LH and FSH release, in mares during the winter period of inactivity. The thyroid hormones also seem to act on the annual rhythm of GnRH secretion.

An alternation of one month of short days and one month of long days which suppresses the seasonal cycle of variation in small ruminant males has not yet been shown to have a similar effect on mares or on stallions.