Central serotonergic nerves project to the pial vessels of the brain.

Serotonin is strongly implicated in the aetiology of several cerebrovascular (circulatory) diseases, including stroke, migraine and vasospasm. Previous studies have suggested the existence of an indoleaminergic system of perivascular nerves in large cerebral arteries of the lamprey. In addition, some authors have observed that cerebral arteries (such as the vertebrobasilar system of the rabbit) and microvessels may take up serotonin and 5-hydroxytryptophan in various species. However, neither large cerebral arteries nor microvessels (primarily capillaries) directly control, or change, cerebral blood flow; as in other vascular beds, it is the arterioles and small arteries that are the major resistance elements. We report here on the presence of a central serotonergic innervation of pial arteries and arterioles in the rat, using immunocytochemical and neurochemical techniques. The fibres seem to have a central neuronal origin, emanating from both median and dorsal raphé nuclei. This perivascular serotonergic innervation may have a role both in the normal regulation of the cerebral circulation and in pathological conditions.