Ultrastructural analysis of tryptophan hydroxylase immunoreactive nerve terminals in the rat cerebral cortex and hippocampus: their associations with local blood vessels.

Physiological evidence has indicated that serotonin (5-hydroxytryptamine) could be a regulator of cerebral blood flow in various regions of the brain. In the present study, tryptophan hydroxylase immunocytochemistry was used to characterize, both at the light and electron microscopic levels, serotonergic nerve terminals and primarily their relationships with intraparenchymal microarterioles and capillaries in the rat frontoparietal cortex, entorhinal cortex and hippocampus. Irrespective of the brain area, serotonergic varicosities were primarily apposed to either dendrites or nerve terminals, were on average 0.37 micron2 in surface area (0.69 micron calculated diameter) and 12-22% of them engaged in synaptic junctions, mostly with dendritic elements. Perivascular terminals (defined as immunolabelled varicosities located within a 3 micron perimeter around the vessel basal lamina) in the frontoparietal cortex represented 8-11% of all immunoreactive terminals counted, as determined by light and electron microscopy, respectively. In the entorhinal cortex and hippocampus, the proportion of perivascular terminals was only determined at the ultrastructural level and corresponded to 10% and 4%, respectively. In the frontoparietal cortex, serotonergic varicosities were located significantly closer (n = 250, 0.98 +/- 0.05 micron; P < 0.001) to the blood vessels than those of the entorhinal cortex (n = 116, 1.41 +/- 0.08 microns) or hippocampus (n = 105, 1.31 +/- 0.08 microns). Of all perivascular serotonergic terminals in the frontoparietal cortex, 26% were in the immediate vicinity (0-0.25 micron) of the vessel wall, with 2.8% directly abutting on the basement membrane, while 11.6% were separated from it only by a thin astrocytic leaflet. This situation contrasts with that observed in the entorhinal cortex and hippocampus, where no immunoreactive varicosity was ever seen directly contacting the vessel basal lamina and with only 10-13% of the terminals being within 0.25 micron from the vessels. The surface area of perivascular serotonergic terminals was comparable in all regions studied and corresponded to 0.22 micron2; these virtually never engaged in synaptic contacts with adjacent neuronal structures. Our results indicate that tryptophan hydroxylase-immunolabelled terminals are identical to previously characterized serotonin-containing varicosities. Furthermore, the present data show intimate associations between serotonergic terminals and microvessels in the three regions examined. However, perivascular terminals in the frontoparietal cortex were more frequent and/or located much closer to local microvessels than those in the other regions, and might be more directly involved in neurogenic control of local cerebral blood flow.