Effect of lesions in the suprachiasmatic nucleus-retrochiasmatic area on the inhibition of pulsatile LH release induced by electrical stimulation of the midbrain dorsal raphe nucleus.

The present studies were designed to determine if the inhibition of pulsatile luteinizing hormone (LH) secretion reported to occur during midbrain dorsal raphe nucleus (DRN) stimulation involved some portion of the suprachiasmatic nucleus(SCN)-retrochiasmatic area (RCA). In animals having lesions that completely destroyed the SCN but did not extend into the RCA, dorsal raphe-induced inhibition of pulsatile LH release was still present. If, however, a significant portion (more than 40%) or all of the RCA was encompassed by the lesion (with variable or no SCN damage), the inhibition induced by DRN stimulation was prevented. This indicates that the SCN is not necessary for the inhibitory effect of DRN stimulation on pulsatile LH release to occur. However, the pathway responsible for this inhibition does appear to project to or through the RCA. Furthermore, pulsatile LH release was reduced, but present, during the control period prior to stimulation in rats with SCN lesions (and no damage to the RCA), and mean blood LH levels for this group were significantly decreased during the control bleeding period. Therefore, the neuronal pathway activating episodic LH release involves, but is not restricted solely to, the SCN.