Animals restrict the time of birth of offspring to the most advantageous time of year, usually spring or summer. This is achieved by controlling the preceding period of fertility and, in some cases, by delaying implantation of the zygote. Seasonal changes in daylength, the principal, though not the only cue, regulate pulsatile release of hypothalamic releasing factors that in turn activates the pituitary-gonadal axis. The role of the neuroendocrine system is therefore to translate the photoperiodic stimulus into an endocrine signal (Figure 12). The measurement of day length is a function of the circadian system, environmental light being sampled on a 24-hour basis. Experimental manipulations of the photoperiodic response have revealed the existence of a rhythm of sensitivity to the presence of light that is entrained by the prevailing photoperiod. Light falling within the period of maximal sensitivity results in an LD type response. It is important to note that although different species measure day length in a similar manner, the gonadal response to a given photoperiod will vary between species depending upon the nature of their seasonal reproductive strategy. Photic information is conveyed from the retina to the pineal gland by way of the suprachiasmatic nuclei of the hypothalamus and the cervical sympathetic trunk. The central connections between these structures are poorly understood. The pineal is an essential mediator of the photoperiodic response. The effects of pinealectomy vary between species, but in all cases the responses to changes in day length are blocked. The gland is neither anti- nor progonadotrophic; it merely provides a signal. This signal is probably the nocturnal release of melatonin. Studies on in vivo melatonin production and the responses of photoperiodic species to timed administration of exogenous melatonin have suggested that the duration of nocturnal melatonin production by the pineal is read by the CNS as an indicator of the length of darkness. This model for PTM provides a physiological basis to the observed rhythm of sensitivity to light. This period of sensitivity is probably a parallel to the nocturnal rhythm of melatonin production. Light falling in this phase blocks melatonin production, truncates the pineal signal, and hence produces an LD response by the CNS. The site of the signal detector is not known, although the anterior hypothalamus may be involved. How the pineal signal triggers changes in the hypothalamic LHRH pulse generator is not known. The endogenous opioids, however, especially beta-END may have a major role in exercising photoperiodic control over pituitary action.
