Estrogen modulates spontaneous alternation and the cholinergic phenotype in the basal forebrain

We report that a small population of neurons expresses both choline acetylt ransferase and classical estrogen receptor immunoreactivity and they are fo und primarily in the bed nucleus of the stria terminalis. In short-term ova riectomized ageing mice (24 months, n = 5) there were 41.0 +/- 4.1% fewer o f these double-labeled cells than in young (five months, n = 5) short-term ovariectomized C57BL/6J mice. To study cholinergic neuron estrogen responsi veness, young mice (n = 8) were ovariectomized at puberty (five weeks). Aft er three months half of the mice (n = 4) were given physiological levels of 17 beta estradiol for 10 days. Bed nucleus double-labeled neurons increase d by 32.9% (P less than or equal to 0.003) in the young mice given estrogen . In a gel shift assay, double-stranded oligonucleotides with putative estr ogen response elements from the choline acetyltransferase gene were used as competitors against estrogen receptor binding to consensus estrogen respon se elements. A sequence with 60% homology to the vitellogenin estrogen resp onse element was found to compete at 500- and 1 000-fold excess. Young mice (five months) with ovaries demonstrated significantly (P less than or equa l to 0.04) better performance in the spontaneous alternation T-maze test th an did old (19 month) mice with ovaries (young=66.3 +/- 3.3% correct choice s; vs old= 55.0 +/- 4.0% in old mice with ovaries). Young mice (five months old), ovariectomized for one month and treated with estrogen, showed signi ficantly more spontaneous alternation than ovariectomized controls (69.1 +/ - 2.8% vs 58.3 +/- 3.9%; P less than or equal to 0.04). Estrogen also incre ased spontaneous alternation in old, shortterm ovariectomized mice (61.5 +/ - 2.7% vs 48 +/- 3.3%; P less than or equal to 0.005). In either young or o ld ovariectomized mice, estrogen increased spontaneous alternation to level s seen in young animals with ovaries. Estrogen increases the number of choline acetyltransferase-immunoreactive a rid choline acetyltransferase/estrogen receptor-immunoreactive cells in old or young mice lacking estrogen, and enhances working memory in old or youn g mice lacking estrogen. Our data suggest that estrogen may act at the leve l of the choline acetyltransferase gene, but in view of the limited distrib ution of cholinergic cells expressing the classical estrogen receptor, it i s unlikely that these cells can account for a memory enhancing effect of es trogen replacement. (C) 1999 IBRO. Published by Elsevier Science Ltd.