Light and electron microscopic immunocytochemical analysis of the neurovascular relationships of choline acetyltransferase and vasoactive intestinal polypeptide nerve terminals in the rat cerebral cortex

Abstract

Acetylcholine or vasoactive intestinal peptide (VIP)nerve terminals closely related to intracortical blood vessels have previously been reported. Recent physiological evidence indicates that these central neuronal 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 azon terminals and intracortical microvessels were characterized by light (LM) and electron microscopic (EM) immunocytochemistry. In semithin sections, LM analysis of the distribution of ChAT‐and VIP‐immunostained puncta juxtaposed to small intraparenchymal blood vessels demonstrated that neither type of terminal was enriched or improverished around microvessels within the cerebral cortex. At the EM level, most ChAT‐ or VIP ‐Immunolabelled elements located within a 3μm perimeter around vessel walls were axon terminals. These perivascular terminals were associated primarily with capillaries but also, to a lesser extent, with microarterioles. Even thought ChAT and VIP terminals were frequently found in the immediate vicinity (≤0.25 μm) of microvessels, they almost never contacted the outer basal lamina, usually abutting onto perivascular astroglial leaflets. There were no membrane specializations at the site of contact between ChAT or VIP terminals and perivascular astroglia. In all cortical areas examined, the average size of VIP‐Immunolabelled varicosities (0.56 ± 0.04 μm²) was significantly larger than that of their ChAT counterparts (0.32 ± 0.02 μm²; P< 0.001). Perivascular VIP terminals were more frequently engaged in synaptic contact than those immunostained for ChAT, which rarely exhibited a synaptic junction even in serial thin sections. Neither VIP nor ChAT immunostaining was ever observed in endothelial cells. These results suggest that both acetylcholine and VIP exert their effects on intracortical microvessels through indirect, paracrine mechanisms. The marked difference in synaptic incidence and average size between both types of perivascular terminals indicates that these two vasoactive agents are primarily located in distinct neuronal populations. Further, our results show that the astrocytic glia is the major direct target for both ChAT and VIP perivascular terminals and suggest that neuronal/glial/vascular interactions are a key element in the neurogenic control of the intracortical microcirculation.