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Many studies have demonstrated that physical or psychological stress can increase Fos expression in brainstem monoaminergic nuclei. Little is known, however, about the extent to which stress increases the expression of Fos in monoaminergic and nonmonoaminergic neurons in the brainstem. We examined the effects of conditioned-fear (CF) stress following mild footshock (FS) as unconditioned stress on Fos expression in the monoaminergic and GABAergic neurons of the ventral tegmental area (VTA), locus coeruleus (LC), and dorsal raphe nucleus (DR) in rats. The CF stress significantly increased the number of Fos-positive (Fos+) cells in both the LC and DR, whereas it did not increase the number in the VTA. Using a double-labeling technique, we combined Fos immunostaining with that for tyrosine hydroxylase (TH), serotonin (5-HT), or GABA for histochemical identification of the CF stress-induced Fos+ neurons. The percentage of TH/Fos double-labeled cells resulting from CF stress was 63% of the Fos+ cells in the LC, whereas 52% of the Fos+ cells contained 5-HT in the DR. We also found that approximately 60% of the CF stress-induced Fos+ cells were GABAergic neurons in these brain regions. These results indicate that CF stress induces intense Fos expression in the noradrenergic LC and serotonergic DR neurons, but not in the dopaminergic VTA neurons. They also indicate that not only monoaminergic neurons but also GABAergic neurons within the LC and DR are activated by the stress.

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