Paraventricular neurons control hamster photoperiodism by a predominantly uncrossed descending pathway.

Exposure of Syrian hamsters (Mesocricetus auratus) to short daylengths triggers testicular regression through a pathway involving hypothalamic regulation of sympathetic control over melatonin production in the pineal gland. Destruction of the hypothalamic paraventricular nucleus (PVN) blocks this response. In order to determine whether descending paraventricular influences are crossed or uncrossed, we removed the superior cervical ganglion unilaterally and destroyed the PVN on the same or opposite side. Ipsilateral PVN lesions were consistent with gonadal regression and high nocturnal pineal melatonin levels, while contralateral lesions blocked regression and significantly reduced the melatonin content of the gland. In order to determine whether PVN cell bodies are critical to the photoperiodic response, we produced axon-sparing lesions by injection of ibotenic acid. Lesions which produced cell loss as verified by both Nissl staining and immunocytochemistry blocked gonadal regression in hamsters exposed to short days. These findings indicate that PVN cells regulate pineal function, and hence gonadal regression, by a predominantly uncrossed pathway.