Evidence for coexistence of serotonin and noradrenaline in sympathetic nerves supplying brain vessels of guinea pig.

Nerve fibers containing 5-hydroxytryptamine (5-HT) were demonstrated immunohistochemically in the wall of pial vessels associated with the circle of Willis in the guinea pig. The fibers formed a network structure which was more dense in the rostral part of the arterial circle and its branches than in the caudal part. The 5-HT immunoreactive fibers disappeared in all arteries studied after bilateral superior cervical ganglionectomy, and unilateral ganglionectomy eliminated the 5-HT immunoreactivity in the ipsilateral part of the middle cerebral, posterior cerebral and superior cerebellar arteries. Decentralization of the superior cervical ganglion had no effect on the perivascular nerve plexus. Subsequent staining with dopamine-beta-hydroxylase (DBH) antiserum following elution of the first antibody revealed that 5-HT was present in the noradrenergic nerve fibers. Small intensive fluorescent cells with positive immunoreaction for 5-HT and DBH, respectively, were located in clusters within the ganglion, which showed no immunohistochemical evidence for the presence of serotonergic neurons. It is concluded that 5-HT is probably not synthesized in truly serotonergic fibers but rather taken up and stored together with noradrenaline in cerebrovascular sympathetic nerves originating in the superior cervical ganglia.