Activation of an inhibitory noradrenergic pathway projecting from the locus coeruleus to the cingulate cortex of the rat.

Repetitive stimulation of the locus coeruleus evoked strong inhibition of the firing rate of about 50% of cells of the cingulate rat cortex. Forty per cent of the cells were not affected and 9% were excited by stimulation of the locus coeruleus. Pretreatment of the rats with reserpine and alpha-methyl-p-tyrosine drastically reduced the percentage of cells inhibited by locus coeruleus stimulation. The cells inhibited in response to stimulation of the locus coeruleus as well as those not inhibited were depressed by microiontophoretically applied norepinephrine. This inhibitory action of NE was observed in rats anesthetized either with urethane, chloral hydrate or with Nembutal. The transsynaptically elicited, as well as the norepinephrine elicited, depression of the cells' discharge rate was antagonized by the microiontophoretically applied beta-receptor blocking drug MJ 1999. These data suggest that the inhibitory action on cingulate cortical cells of locus coeruleus stimulation is mediated by the dorsal ascending noradrenergic pathway.