Circadian and seasonal control of neuroendocrine-gonadal activity.

A circadian clock(s) located in the suprachiasmatic nucleus (SCN) of the hypothalamus plays an important role in regulating the timing of pituitary gonadotropin release in many mammalian species. The results from studies involving two unusual experimental paradigms are presented to demonstrate a role for this circadian clock in the regulation of the timing of the preovulatory LH surge in the golden hamster as well as in the measurement of the seasonal change in day length in the Djungarian hamster; information which is used in the regulation of neuroendocrine-gonadal activity on a seasonal basis. The data also demonstrate that the circadian rhythm of locomotor activity can be used as a "marker" rhythm for the clock involved in both ovulatory and seasonal cycles in hamsters.