LIGHT AND ELECTRON-MICROSCOPIC IMMUNOCYTOCHEMICAL ANALYSIS OF THE NEUROVASCULAR RELATIONSHIPS OF CHOLINE-ACETYLTRANSFERASE AND VASOACTIVE INTESTINAL POLYPEPTIDE NERVE-TERMINALS IN THE RAT CEREBRAL CARTER

Acetylcholine or vasoactive intestinal peptide (VIP) nerve terminals c losely related to intracortical blood vessels have previously been rep orted. Recent physiological evidence indicates that these central neur onal systems are involved in the fine control of local cerebral blood flow. In the present study, the intimate associations between choline acetyltransferase (ChAT) and VIP axon terminals and intracortical micr ovessels were characterized by light (LM) and electron microscopic (EM ) immunocytochemistry. In semithin sections, LM analysis of the distri bution of ChAT- and VIP-immunostained puncta juxtaposed to small intra parenchymal blood vessels demonstrated that neither type of terminal w as enriched or impoverished around microvessels within the cerebral co rtex. At the EM level, most ChAT- or VIP-immunolabelled elements locat ed within a 3 mu m perimeter around vessel walls were axon terminals. These perivascular terminals were associated primarily with capillarie s but also, to a lesser extent, with microarterioles. Even though ChAT and VIP terminals were frequently found in the immediate vicinity (le ss than or equal to 0.25 mu m) of microvessels, they almost never cont acted the outer basal lamina, usually abutting onto perivascular astro glial leaflets. There were no membrane specializations at the site of contact between ChAT or VIP terminals and perivascular astroglia. In a ll cortical areas examined, the average size of VIP-immunolabelled var icosities (0.56 +/- 0.04 mu m(2)) was significantly larger than that o f their ChAT counterparts (0.32 +/- 0.02 mu m(2); P < 0.001). Perivasc ular VIP terminals were more frequently engaged in synaptic contact th an those immunostained for ChAT, which rarely exhibited a synaptic jun ction even in serial thin sections. Neither VIP nor ChAT immunostainin g was ever observed in endothelial cells. These results suggest that b oth acetylcholine and VIP exert their effects on intracortical microve ssels through indirect, paracrine mechanisms. The marked difference in synaptic incidence and average size between both types of perivascula r terminals indicates that these two vasoactive agents are primarily l ocated in distinct neuronal populations. Further, our results show tha t the astrocytic glia is the major direct target for both ChAT and VIP perivascular terminals and suggest that neuronal/glial/vascular inter actions are a key element in the neurogenic control of the intracortic al microcirculation. (C) 1994 Wiley-Liss, Inc.