LHRH effects on hippocampal neurons are modulated by estrogen in rats

The effect of luteinizing hormone releasing hormone (LHRH) on the neuronal activity of CA1 and CA3 regions of the hippocampus was studied by means of extracellular recordings in the castrated male, intact male, castrated female, and castrated female injected s.c. with 20 micrograms estradiol benzoate (EB) for 3 days. The basal firing rate of the CA3 neurons of castrated EB-treated female rats was significantly lower than that of the neurons in castrated EB-untreated female rats as well as male rats, either intact or castrated, whereas that of the CA1 neurons was not significantly different from castrated EB-untreated female rats. Iontophoretically applied LHRH predominantly resulted in facilitation of the neuronal firing in most of the animal groups. In the CA3 region of castrated EB-treated female rats, however, inhibition occurred in a large percentage of neurons, and the ratio of facilitation, inhibition and no response was significantly different from that in castrated EB-untreated female rats. In the CA1 region of castrated EB-treated female rats, the predominant effect of LHRH was facilitation and the ratio of facilitation, inhibition and no response was not significantly different from that in castrated EB-untreated female rats. These results suggest that LHRH is involved in increasing the neuronal activity of the CA3 region in the hippocampus, and estrogen reduces this LHRH action.