Ultrastructure of serotonin-containing nerve fibres in the middle cerebral artery of the rat and evidence for its localization within catecholamine-containing nerve fibres by immunoelectron microscopy.

This study has examined the ultrastructural characteristics of serotonin (5-HT)-like immunoreactive (5-HT-I) nerves in the perivascular plexus of a major cerebral blood vessel and their relationship with catecholaminergic (CA) nerves. This was achieved by immunohistochemistry for 5-HT used alone and in combination with simultaneous 5-hydroxydopamine (5-OHDA) false transmitter preloading to demonstrate catecholaminergic nerves examined by electron microscopy. About 15% of perivascular nerves showed some 5-HT-like immunoreactivity. These nerves were characterised by a predominance of small pleomorphic vesicles with a diameter of 40-50 nm and a few large granular vesicles about 100-150 nm diameter. 5-HT-I nerves were found at all levels of the perivascular plexus, from the outer adventitia where they were formed together with non-reactive nerves into bundles by Schwann cells and their processes, to more singly at the adventitia-media border, sometimes in close apposition to smooth muscle cells, separated by some 125 nm. Examination of animals treated by 5-OHDA preloading revealed that 5-HT-I nerves were almost invariably identified as catecholaminergic. However, CA nerves did not always show 5-HT-like immunoreactivity, some 25% only being doubly labelled in this study. Treatment of animals with the catecholaminergic nerve uptake blocker desmethylimipramine resulted in abolition of 5-HT-like immunoreactivity. These results provide direct ultrastructural evidence for the localisation of 5-HT within the perivascular catecholaminergic nerves of a major cerebral vessel and suggest that the contained 5-HT is mainly derived by uptake rather than synthesis within them.