Action of estrogen and mechanical vaginocervical stimulation on the membrane excitability of hypothalamic and midbrain neurons

The effect of vaginocervical stimulation (VCS) and estrogen iontophoresis on the electrical activity recorded in urethane-anesthetized female rats from medial preoptic-septal (MPO-S) and midbrain central gray (MCG) neurons was studied during two phases of the estrous cycle, namely metestrus (M) and late proestrus-estrus (LP-E). The spontaneous discharge rate of both MPO-S and MCG neurons varied over the two stages of the estrous cycle. The spontaneous electrical activity of the MPO-S neurons was higher during M than during LP-E whereas MCG unit activity was low during M and higher during LP-E. The VCS-evoked changes in unit activity were specific, in that they were observed in response to mechanical genital stimulation and not in response to painful stimuli and/or nonspecific arousal. These responses were not dependent on the stage of estrous cycle. Finally, the iontophoresis of 17β-estradiol hemisuccinate evoked electrophysiological responses from MPO-S and MCG neurons. More MPO-S neurons were responsive to estrogen in LP-E than in M, while fewer MCG neurons were responsive to estrogen in LP-E than in M. The results clearly show that ongoing electrical activity of hypothalamic and midbrain nerve cells can fluctuate between M and LP-E phases of the estrous cycle and change with vaginocervical probing as well as iontophoretically applied estrogen. Furthermore, the results suggest the existence of a reciprocal relationship between the membrane activity of MPO-S and MCG neurons which may be related to CNS control of reproductive activities.