Horizontal knife cuts either ventral or dorsal to the hypothalamic paraventricular nucleus block testicular regression in golden hamsters maintained in short days.

The possible involvement of efferent pathways from the suprachiasmatic nucleus (SCN) in the photoperiodic regulation of reproduction was studied by measuring the testis size of hamsters bearing a horizontal knife cut either ventral or dorsal to the hypothalamic paraventricular nucleus (PVN) that were transferred from photostimulatory long days (light:dark (L:D) 14:10 h) to non-stimulatory short days (L:D 6:18 h). Knife cuts placed either ventral or dorsal to the PVN blocked testicular regression induced by exposure to short days. These results indicate that efferent fibers running dorsally from the SCN to the PVN are involved in relaying photoperiodic information from the SCN to the PVN. Furthermore, recently-defined efferents that leave the PVN dorsally and terminate in the spinal cord appear to be responsible for relaying seasonal information about day-length to the pineal-reproductive axis of hamsters.