Role of locus coeruleus in foot shock-evoked Fos expression in rat brain

The robust activation of locus coeruleus neurons in response to a variety o f stressors, in conjunction with the widespread outputs of the locus coerul eus, suggest that the locus coeruleus may be important in mediating respons es to stress. Previous studies in rats have demonstrated that exposure to f oot shock elicits Fos expression, a marker of neuronal activation, in the l ocus coeruleus and other brain sites. In order to evaluate the involvement of the locus coeruleus in foot shock-induced activation of other brain site s, shock-induced Fos expression was examined in the locus coeruleus and oth er brain areas known to be activated by foot shock, following direct inhibi tion of the locus coeruleus by local infusion of muscimol, a GABA agonist, prior to foot shock. Control rats received infusions of artificial cerebros pinal fluid into the locus coeruleus or muscimol into areas outside of locu s coeruleus. Rats infused with artificial cerebrospinal fluid and then expo sed to foot shock had significant increases in Fos expression in several br ain areas, including locus coeruleus, nucleus O, several subdivisions of th e hypothalamus, subnuclei of amygdala, bed nucleus of the stria terminalis and cingulate cortex. Inhibition of the locus coeruleus prior to foot shock significantly inhibited Fos expression in the locus coeruleus, nucleus O, some subdivisions of the hypothalamus including the magnocellular and media l parvicellular paraventricular hypothalamic nucleus, subnuclei of amygdala , and cingulate cortex. In contrast, inhibition of the locus coeruleus did not affect shock-induced Fos expression in other areas, including certain s ubdivisions of the hypothalamus and bed nucleus of the stria terminalis. We suggest that foot shock may activate multiple pathways, with activation of certain discrete nuclei requiring input from the locus coeruleus and act ivation of others occurring independently of locus coeruleus input. (C) 200 0 IBRO. Published by Elsevier Science Ltd. All rights reserved.