Estrogen regulates galanin but not tyrosine hydroxylase gene expression in the rat locus ceruleus

The neuropeptide galanin (GAL) is coexpressed by the majority of noradrenergic neurons in the rat locus ceruleus (LC) and may function as an inhibitory modulator of noradrenergic transmission. Because estrogen has been shown to induce GAL expression in other brain regions and modulate noradrenergic transmission, we used in situ hybridization histochemistry to assess the effects of chronic estrogen treatment on GAL and tyrosine hydroxylase (TH) gene expression in the LC of ovariectomized female rats. We found that GAL mRNA levels were significantly elevated in rats implanted with a Silastic capsule containing estradiol compared to sham-implanted controls. Both the average optical density (P < or = 0.05) and the labelling area (P < or = 0.007) differed significantly between the groups. In contrast, TH gene expression measured in alternate brain sections did not differ between the groups. If GAL functions as an inhibitory modulator of noradrenergic transmission as postulated, these findings suggest that chronic estrogen treatment could reduce the noradrenergic tone of the brain in the absence of significant alterations in TH expression by enhancing the level of cosecreted GAL.