Paraventricular nucleus efferents mediating photoperiodism in male golden hamsters.

Knife cuts were placed around the paraventricular nucleus of the hypothalamus (PVN) in an effort to identify the pathways mediating gonadal regression in male golden hamsters housed in a short photoperiod. Horizontal cuts dorsal to the PVN only affected photoperiodic responsiveness if they actually damaged the PVN. Coronal cuts caudal to the PVN had no effect on testes regression. Parasagittal cuts adjacent to the medial PVN had little or no effect on gonadal regression induced by short photoperiod. We conclude that efferents of the PVN arising dorsally or medially are not necessary for photoperiodic control of testes function, whereas projections from or through the lateral PVN region descending towards the spinal cord are important.