Photoperiodic control of breeding activity

Abstract

Annual variations in daylength or photoperiod are responsible for an alternation between a breeding and a rest season in most animal species. Depending on its length, the photoperiod can stimulate or inhibit reproductive activity. Animals deprived of photoperiodic information, however, exhibit an endogenous rhythm of reproduction and the main role of photoperiod under natural conditions appears to be a synchronisation of this internal rhythm.

Light information is perceived by the retina and, after neural processing, it is transformed into an endocrine signal, the rhythm of melatonin secretion. This hormone is only secreted at night. Melatonin acts on the central nervous system to control the pulsatile secretion of LHRH. Its action on LHRH neurons is not direct ; it involves the use of interneurons. In addition, the melatonin-induced changes in LHRH secretion are seen after a long latency relative to the time of the change in the rhythm of melatonin secretion, 40 to 60 days in the ewe. The changes in LHRH secretion cause modifications in gonadotropic hormone secretion which in turn control the activity of the gonads.