The hypothalamic paraventricular nucleus mediates the photoperiodic control of reproduction but not the effects of light on the circadian rhythm of activity.

Photoperiods of less than 12.5 h of light/24 h induce gonadal regression in the golden hamster. Photic information is relayed from the retina to the hypothalamic suprachiasmatic nucleus (SCN), a structure responsible for the generation of many circadian rhythms including the circadian rhythms in locomotor activity and pineal melatonin synthesis and release. Although pineal melatonin mediates the photoperiodic-neuroendocrine response, the complete neural circuit from the SCN to the pineal gland is unknown. Complete destruction of the hypothalamic paraventricular nucleus (PVN) prevented short-day-induced testicular regression without affecting the circadian rhythm of locomotor activity. The results indicate that the PVN plays an important role in the photoperiodic-neuroendocrine circuit and is responsible for relaying information from the SCN to the pineal. A different efferent pathway connects the SCN to structures in the brain responsible for locomotor activity.