Inhibitory effect of dorsal cut of hypothalamus on ovulation delayed by pentobarbital in female rats

To clarify the role of the extrahypothalamic signals to the hypothalamus in regulating ovulation, the effect of interruption of the dorsal inputs of the hypothalamus on delayed ovulation induced by the treatment with pentobarbital (PB) was examined in female rats. A posterior half-circle horizontal cut at the dorsal of the hypothalamus (PDC) was made and spontaneous ovulation or ovulation delayed by PB were observed. When PDC was performed at 10:00-11:30 h on the day of proestrus, ovulation occurred normally the next morning in all rats. Treatment with PB at 16:00h on the day of proestrus, but not at 13:00 h, induced one-day-delayed ovulation in most rats without brain surgery. In contrast, ovulation was seen normally on estrus in 60.0% of PDC females, when treated with PB at 16:00 h. However when PB was injected at 13:00 h, ovulation was delayed one day in 88.9% of PDC females. Twenty days after making PDC, delayed ovulation was not induced by PB at 16:00 h in 54.5% of rats. These results suggest the possibility that PDC advances the effective period in which PB can cause delayed ovulation. Furthermore, the effect of the PDC continues for at least 20 days after the operation. Thus, dorsal inputs to the hypothalamus may be concerned with a control mechanism of the critical period for triggering of ovulation in female rats.