Potential noradrenergic regulation of serotonergic neurons in the median raphe nucleus.

Pharmacological and morphological evidence suggests that the functional activity of serotonergic neurons may be regulated by catecholamines. We have attempted to reveal a potential pathway by which this interaction might occur. Rats received bilateral knife cut lesions of the ventral noradrenergic bundle which severed the A-1 and A-2 cell body contributions to this projection. Controls received a sham lesion into the cerebellum. Two weeks later all animals were sacrificed, and norepinephrine and serotonin levels were measured in discrete nuclei of the brain. Lesion placement was confirmed histofluorometrically. Serotonin levels in the median raphe nucleus were significantly reduced by 40%, but levels of serotonin were unaffected in the dorsal raphe nucleus and 8 serotonergic terminal regions. The lesions did not affect levels of norepinephrine in the locus coeruleus, cingulate cortex, or habenula. This study suggests that a noradrenergic projection to the median raphe nucleus from the A-1 and A-2 cell body groups may modulate serotonergic neuronal function.