The pineal gland and seasonal reproductive adjustments

The pineal gland seems to play a major role in controlling and synchronizing circannual reproductive cycles in some mammals. The following scheme is based primarily on experimental evidence compiled using the golden hamster. However, it is probably applicable in one form or another to a number of long day breeding species. When hamsters are kept under natural photoperiodic conditions they exhibit a period of infertility followed by a period of fertility. The entire cycle encompasses approximately one year. The cycle has been divided into 4 distinctive phases: the inhibition phase, the sexually quiescent phase, the restoration phase, and the sexually active phase. During the inhibition phase the decreasing photoperiods in the fall of the year cause activation of the pineal gland and, as a consequence, gonadal regression. The sexually quiescent phase requires an intact pineal gland to maintain the gonads in a non-functional state. The restoration phase, which occurs in the spring of the year, allows the gonads to become recrudescent. This phase of the cycle seems to be light independent. The sexually active phase extends from spring until fall. During this phase the hypothalamo-pituitary-gonadal axis seems to be refractory to inhibition by the pineal gland. Some light is required during the summer months to interrupt the refractory period.