Knife cuts lateral but not dorsal to the hypothalamic paraventricular nucleus abolish gonadal responses to photoperiod in female hamsters (Mesocricetus auratus)

Horizontal and parasagittal knife cuts in the hypothalamus of female hamsters (Mesocricetus auratus) were employed to investigate the neural pathways that mediate gonadal responses to photoperiod. Bilateral horizontal knife cuts placed dorsal to the paraventricular nucleus (PVN) did not prevent short-day-induced acyclicity and uterine regression. On the other hand, regardless of photoperiod, animals with bilateral parasagittal knife cuts placed lateral to the PVN continued to exhibit regular 4-day estrous cycles and stimulated uteri. Thus, parasagittal cuts prevented the effects of short days on reproductive physiology. This finding suggests that the lateral efferent projections from the PVN represent an important component of the neural pathway mediating reproductive photoperiodism in female hamsters.