Further evidence of an opposite effect of dorsal and median raphe nuclei on the proestrous surge of LH.

The effect of stimulation or lesions of either the dorsal or the median raphe nucleus on the proestrous surge of LH and on ovulation was studied in rats kept under constant illumination. Electrochemical stimulation (anodic DC of 100 microA during 30 sec) was applied at noon on the day of proestrus through chronically implanted electrodes. Lesions of the raphe nuclei were made by passing a cathodic current of 1 mA for 20 sec through nichrome electrodes stereotaxically implanted. Blood samples from freely behaving rats were obtained hourly through cannulae inserted into the jugular vein. Rats under constant light from diestrous day 1 or diestrous day 2 showed a delay in the onset of the LH surge of the next proestrus. Stimulation applied into the median raphe nucleus inhibited proestrous LH release and blocked ovulation, whereas stimulation of the dorsal raphe nucleus resulted in enhanced LH release in rats under constant light from diestrous day 2 but not from diestrous day 1. In turn, in rats bearing lesions in the dorsal raphe nucleus LH surges were decreased and ovulation was blocked, but rats with lesions in the median raphe nucleus exhibited enhanced LH release. It is concluded that the dorsal and the median raphe nuclei exert opposite effects on the proestrous surge of LH and on ovulation.