Cerebrovascular nerve fibers immunoreactive for tryptophan-5-hydroxylase in the rat: distribution, putative origin and comparison with sympathetic noradrenergic nerves

Abstract

The distribution of serotonergic nerves in major basal and isolated small pial arteries (diameter≥ 50 μm) was investigated immunohistochemically using an antibody directed against tryptophan-5-hydroxylase (TPOH), the rate-limiting enzyme in the synthesis of 5-hydroxytryptamine (5-HT or serotonin), and compared to that of the noradrenergic system labeled for the selective noradrenaline (NA) synthesizing enzyme, dopamine-β-hydroxylase (DBH). In addition, the possible peripheral and/or central origins of the cerebrovascular serotonergic (TPOH-positive) nerve fibers were examined. Strongly labeled TPOH-immunoreactive (TPOH-I) fiber bundles were observed in major basal arteries and gave rise to small varicose organized in a meshwork pattern. The highest density of TPOH-I fibers was found in the middle cerebral artery followed by the anterior cerebral and the anterior communicating arteries, with a moderate to low density in the internal carotid and the vertebro-basilar trunk. Of the isolated pial arteries, only the larger ones (diameter> 75 μm) were significantly endowed with TPOH-I varicose fibers. However, free floating TPOH-I nerves were observed coursing through the pia-arachnoid membranes and reaching small pial vessels. In contrast, DBH-I nerve fibers were fine and were visualized primarily as numerous varicosities distributed in a circumferential manner around the vessel wall. A very high density of DBH-I varicosities was seen in the rostral part of the circle of Willis, with the internal carotid being the most richly supplied followed by the anterior cerebral and the anterior communicating arteries; comparatively, the middle cerebral artery was moderately innervated. The differences in distribution pattern and density between TPOH-I and DBH-I cerebrovascular fibers clearly suggest that these two innervation systems are not exactly superimposable. Superior cervical ganglionectomy caused an almost complete disappearance of TPOH-I nerves in all vascular segments, with some residual fibers in selected vessels. Lesion of the central serotonergic component with the neurotoxin 5,7-dihydroxytryptamine had virtually no effect on the TPOH-I fibers in the major basal and isolated pial arteries. These results strongly suggest that the serotonergic innervation of major cerebral as well as pial arteries has a prominent peripheral origin closely related to the sympathetic system. Processing of superior cervical ganglion slices for TPOH immunocytochemistry, however, failed to unequivocally detect TPOH-I neurons.