Periventricular and suprachiasmatic lesion effects on photoperiodic responses of the hamster hypophyseal-gonadal axis.

We examined the involvement of neural mechanisms within the suprachiasmatic nucleus (SCN) and periventricular area (PVA), and the role of prolactin (Prl) in control of endocrine function in short day-exposed Syrian hamsters. Hamsters bearing lesions of the SCN or PVA, hamsters implanted with an anterior pituitary under the kidney capsule to provide sustained Prl levels, and sham-operated hamsters were exposed to either 14L:10D or 8L:16D. After 9 wk, hamsters were sacrificed, and their testes and pituitaries were studied in vitro to assess their secretory capacity. SCN lesions and large periventricular lesions impinging on the paraventricular nucleus prevented testicular regression during short-day exposure. Small periventricular lesions and pituitary implants did not prevent gonadal regression in hamsters exposed to short days. Testis weights were positively correlated with basal and luteinizing hormone (LH)-stimulated androgen production in the control and lesioned groups; pituitary implants prevented the decline in androgen production in vitro in gonadally regressed animals. The relative in vitro pituitary response to gonadotropin-releasing hormone (GnRH) stimulation in control and lesioned groups was not reduced by short-day exposure. These data indicate that either axons coursing dorsally from the SCN or extra-SCN structures in the periventricular/paraventricular area are necessary for testicular regression in short photoperiods.