EXPRESSION OF ESTROGEN RECEPTOR-LIKE IMMUNOREACTIVITY BY DIFFERENT SUBGROUPS OF BASAL FOREBRAIN CHOLINERGIC NEURONS IN GONADECTOMIZED MALE AND FEMALE RATS

Recent studies have demonstrated that estrogen administration can prod uce significant increases in relative levels of choline acetyltransfer ase (ChAT) mRNA and protein in specific regions of the female, but not the male, rat basal forebrain. In the present study immunocytochemica l techniques were used to identify and compare relative numbers of cho linergic neurons containing estro en receptors a within the medial sep tum, horizontal limb of the diagonal band of Broca, nucleus basalis ma gnocellularis, and striatum of gonadectomized male and female rats to determine whether there are differences in the percentage of cholinerg ic neurons expressing estrogen receptors which might contribute to the different regional- and sex-specific effects of estro en which have b een described. Counts of a choline acetyltransferase-immunoreactive ce lls revealed significant regional differences in the average number of cholinergic neurons/section; however, no difference between males and females in the numbers of cholinergic neurons in each of the four reg ions analyzed was observed. Fifty to eighty percent of the cholinergic neurons detected in both males and females contained estro en a recep tor-like immunoreactivity. A small but significant difference between males and females was detected with females having slightly more (10.5 %) double-labeled cells than males overall. Individual comparisons rev ealed that significantly more (18-33%) double-labeled cells were detec ted in the horizontal limb of the diagonal band, but not in the medial septum, nucleus basalis, or striatum of females vs. males. There was also a small but significant regional difference in the percentage of double-labeled cells with the highest percentage (74.2%) detected in t he striatum and the lowest percentage (63.4%) detected in the horizont al limb. None of these differences appear to account for the regional- and sex-specific effects of estrogen on cholinergic neurons which hav e been observed. We conclude that differences in the effects of estrog en on cholinergic neurons in males vs. females and in different subreg ions of the female basal forebrain are not due to differences in the p ercentage of cholinergic neurons expressing estrogen receptors.