The effects of microelectrophoretically applied estrogen, cortisol and acetylcholine on medial preoptic-septal unit activity throughout the estrous cycle of the female rat

The technique of microelectrophoresis was utilized to study the effects of 17β-estradiol hemisuccinate (17βE₂S), cortisol hemisuccinate (CS), and acetylcholine (Ach) on medial preoptic-septal (mPOA-S) neurons throughout the estrous cycle in urethane anesthetized rats. A total of 23 cells were identified antidromically as having their projections to the arcuate-median eminence region of the hypothalamus, and 18 of these were tested with 17βE₂S. Only four cells showed an inhibition to the gonadal steroid, while the remaining neurons did not respond. Twelve antidromically identified (AI) neurons were tested with CS, none of which responded, and 2 out of 16 AI neurons showed an excitation to Ach. Extracellular potentials were recorded from 273 non-antidromically identified neurons, 138 of which were successfully tested with 17βE₂S. Sixty-two of these responded to the steroid. The predominant response on late diestrus I was excitation, and the majority of estrogen-sensitive cells showed an inhibitory response on late diestrus 2, proestrus and estrus. A Chi-square analysis showed that the response varied significantly throughout the estrous cycle. Cortisol hemisuccinate was tested on 154 non-antidromically identified mPOA-S neurons. The response to the corticosteroid also varied significantly throughout the cycle, but further analysis determined that the response was different from that of 17βE₂S. Acetylcholine was also applied to 180 mPOA-S neurons, but no significant change in the response was found from one day of the cycle to the next. It is apparent from these studies that (1) 17βE₂S can bring about direct and rapid changes in the firing frequency of neurons, (2) the non-antidromically identified neuron is a site for these effects, and (3) the response differs significantly throughout the estrous cycle.