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 produce significant increases in relative levels of choline acetyltransferase (ChAT) mRNA and protein in specific regions of the female, but not the male, rat basal forebrain. In the present study immunocytochemical techniques were used to identify and compare relative numbers of cholinergic neurons containing estrogen receptors within the medial septum, horizontal limb of the diagonal band of Broca, nucleus basalis magnocellularis, and striatum of gonadectomized male and female rats to determine whether there are differences in the percentage of cholinergic neurons expressing estrogen receptors which might contribute to the different regional- and sex-specific effects of estrogen which have been described. Counts of choline acetyltransferase-immunoreactive cells 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 regions analyzed was observed. Fifty to eighty percent of the cholinergic neurons detected in both males and females contained estrogen receptor-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 revealed that significantly more (18-33%) double-labeled cells were detected 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 the striatum and the lowest percentage (63.4%) detected in the horizontal limb. None of these differences appear to account for the regional- and sex-specific effects of estrogen on cholinergic neurons which have been observed. We conclude that differences in the effects of estrogen on cholinergic neurons in males vs. females and in different subregions of the female basal forebrain are not due to differences in the percentage of cholinergic neurons expressing estrogen receptors.