DISTINCT CHOLINE-ACETYLTRANSFERASE (CHAT) AND VASOACTIVE INTESTINAL POLYPEPTIDE (VIP) BIPOLAR NEURONS PROJECT TO LOCAL BLOOD-VESSELS IN THERAT CEREBRAL-CORTEX

Innervation of rat intracortical cerebral blood vessels by acetylcholi ne (ACh) and vasoactive intestinal polypeptide (VIP) remains largely u nexplored and it is not known if the cells of origin are intra- or ext racortical nor if perivascular fibers colocalize ACh and VIP, Cortical cholinergic innervation arises primarily from the basal forebrain and to a small extent from intrinsic bipolar ACh neurons thought to be th e sole source of cortical VIP. In order to evaluate if intracortical p erivascular ACh terminals could be distinguished from those of the bas al forebrain by their colocalization with VIP, we performed a double i mmunofluorescence study and determined the percentage of colocalizatio n of choline acetyltransferase (ChAT) and VIP in cortical neurons, as well as in terminal fields associated with intracortical blood vessels . From a total of 2103 cells examined in all cortical areas, VIP neuro ns accounted for the largest population (58.3%) followed by ChAT-posit ive cells (28.2%) with only 13.5% of cells being double-labelled for V IP and ChAT. Of the cortical ChAT-immunostained cells (n = 878), 32.3% colocalized VIP whereas only 18.8% of VIP neurons (n = 1509) also con tained ChAT. In various cortical areas, ChAT cell bodies were seen to be contacted by VIP terminals which surrounded closely their cell soma and proximal dendrites. Perivascular fibers studied by double immunof luorescence and confocal microscopy were of three categories including cholinergic, VIPergic with a smaller population of fibers which costa ined for both ChAT and VIP. These results show that cortical VIP neuro ns are much more numerous than those containing ChAT, and that a major ity of VIP neurons do not colocalize with ChAT. This observation indic ates that ACh and VIP are primarily located in distinct neuronal popul ations and that VIP cannot be used as a marker of intracortical ACh ne urons and terminals. Our results further suggest that intracortical bl ood vessels are primarily under the influence of distinct ChAT and VIP perivascular fibers. Also, the presence of a subset of VIP and ChAT/V IP fibers in association with intracortical blood vessels strongly sug gests a role for cortical bipolar neurons in local cerebrovascular reg ulation. The origin of the perivascular ChAT fibers which do not coloc alize VIP, however, remains unknown.