Changes of the release of luteinizing hormone (LH) on the day of proestrus after lesions or stimulation of the raphe nuclei in rats

The effect of stimulation or lesion of the raphe nuclei on ovulation and on the release of luteinizing hormone (LH) on the day of proestrus was studied in unanesthetized, unrestrained rats. Electrochemical stimulation (anodic DC or 100 μA during 30 s) was applied at 12.00 h on the day of proestrus through chronically implanted stainless steel electrodes. Lesions were made by passing a cathodic current of 1 mA for 20 s through nichrome electrodes stereotaxically implanted and the rats were used 15–30 days later. Blood samples were obtained hourly from the freely behaving rats through a plastic cannula inserted into the external jugular vein. Stimulation in the medial raphe nucleus (MRn) resulted in blockade of ovulation and of the preovulatory LH release. On the contrary, no change of the normal pattern of LH surge nor in the number of ovulating rats was seen after stimulation of the dorsal raphe nucleus (DRn) or in rats stimulated in the mesecephalon outside these nuclei. Injection of p-chloropheylalamine (PCPA) into the MRn to block 5-hydroxytryptamine (5-HT) synthesis, prevented the effect of MRn stimulation, whereas injection of saline solution did not. Rats bearing lesions destroying the DRn showed decreased proestrous LH surge and blockade of ovulation whereas those with lesions of MRn ovulated normally. Rats with transverse cuts placed just behind the DRn exhibited normal LH release indicating that the effects of destroying the DRn is not due to the interruption of ascending fibers crossing the nucleus. Injections of PCPA in the DRn but not saline solution mimicked the effect of lesions. It is concluded that the serotonergic system influences the proestrus surge of LH, with the DRn playing a facilitatory role and the MRn an inhibitory role.