5-Hydroxytryptamine-containing fibers in cerebral arteries of the cat, rat and guinea pig.

The distribution of serotonin-like immunoreactive (5-HT-LI) fibers in pial arteries and arteries of the choroid plexus in the cat, rat and guinea pig was examined. Weak 5-HT-LI fibers were observed in cerebral arteries when these arteries were fixed by immersion after they had been dissected from the exsanguinated animals. The 5-HT-LI fibers, however, were not detected in arteries when they were fixed in situ or in vitro after they had first been perfused with phosphate-buffered saline (PBS) or Krebs solution in situ to flush the blood components from the lumen prior to dissection. In these arteries, 5-HT-LI fibers with intense immunofluorescence could however be demonstrated following incubation with serotonin (5-HT) in vitro. The distribution patterns of 5-HT-LI fibers and catecholamine fluorescence fibers were found to be similar. Following bilateral superior cervical ganglionectomy, 5-HT-LI fibers were not detected in pial vessels even after incubation with 5-HT. Light-microscopic autoradiographic examination of [3H]-5-HT uptake in pial arteries showed that silver grain granules were concentrated only in the unmyelinated nerve profile in the adventitial layer. No uptake of [3H]-5-HT into smooth muscle cells and endothelial cells was found. The neuronal uptake of [3H]-5-HT was blocked by imipramine. Results from ultrastructural studies also showed that 5-HT-like immunoreactivities were found only in the axoplasm and were never detected following chronic superior cervical ganglionectomy. These results indicate that the presence of 5-HT-containing nerves in pial vessels in various species examined is due to the uptake of 5-HT into sympathetic nerves, and further support our hypothesis that cerebral pial vessels do not receive authentic serotonergic innervation. The present study also identifies the heterogeneity in characteristics of smooth muscle and endothelial cells among cerebral and peripheral arteries.