Suppression of lordotic responsiveness in the female rat during mesencephalic electrical stimulation.

The effects of biphasic electrical stimulation of the mesencephalic median (MRN) and dorsal raphe (DRN) nuclei, as well as the adjacent periaqueductal gray (PAG), on lordotic behavior were investigated in ovariectomized rats primed with three daily injections of estradiol benzoate (2 micrograms). Animals were tested between 4-8 hours after a progesterone (0.5 mg) injection on day four which normally facilitates high levels of receptivity during this period. Although stimulation of the MRN had no significant effect on lordosis, DRN activation at 100 Hz (0.5 msec pulse duration) or 10 Hz (2 msec pulse duration) caused a marked and immediate suppression (53% and 56%, respectively) in receptivity. This suppression does not appear to be due to activation of serotoninergic neurons originating in the DRN since pretreatment with an inhibitor of serotonin synthesis (parachlorophenylalamine, 320 mg/kg) essentially did not modify the suppression, thus providing no evidence in support of an inhibitory role for serotonin in lordotic behavior. Activation within the PAG adjacent to the DRN at 10 Hz (0.5 or 2 msec pulse duration) produced an immediate, dramatic decrease (81% and 80%, respectively) in receptivity. The suppressions induced by DRN and PAG stimulation appear most likely to be due to activation of a descending pathway inhibitory to the lordosis reflex at medullary or spinal cord levels.