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 axon 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 impoverished around microvessels within the cerebral cortex. At the EM level, most ChAT- or VIP-immunolabelled elements located within a 3 microns perimeter around vessel walls were axon terminals. These perivascular terminals were associated primarily with capillaries but also, to a lesser extent, with microarterioles. Even though ChAT and VIP terminals were frequently found in the immediate vicinity (< or = 0.25 microns) 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 microns 2) was significantly larger than that of their ChAT counterparts (0.32 +/- 0.02 microns 2; 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.

Preferential Localization of Muscarinic M1Receptor on Dendritic Shaft and Spine of Cortical Pyramidal Cells and Its Anatomical Evidence for Volume Transmission

“…Some studies reported that the significant part of cholinergic varicosities form conventional synapses with dendritic shafts and spines (Frotscher and Leranth, 1985;Smiley et al, 1997;Turrini et al, 2001), while others described the low incidence of synaptic specializations (Aoki and Kabak, 1992;Chedotal et al, 1994;Umbriaco et al, 1994Umbriaco et al, , 1995Vaucher and Hamel, 1995;Mechawar et al, , 2002. We revisited this issue by investigating the presence or absence of synapse-like specializations, including parallel membrane apposition of cholinergic varicosities and other neuronal elements, uniform cleft width between the apposed membranes, and thickening of the apposed membranes.…”

“…Some electron microscopic studies reported that cholinergic varicosities commonly make synaptic contact in the cerebral cortex (66% in rat, Turrini et al, 2001; 67% in human, Smiley et al, 1997), while other studies reported much lower incidence [6 -14% in rat hippocampus and cerebral cortex (Chedotal et al, 1994;Umbriaco et al, 1994Umbriaco et al, , 1995Vaucher and Hamel, 1995;Mechawar et al, 2000Mechawar et al, , 2002 21% in cat visual cortex (Aoki and Kabak, 1992)]. This large difference could be attributable to differences in animal species, cortical regions, and the criteria of synaptic contact.…”

“…Volume transmission is inferred from observations that cholinergic fibers are diffuse and rough in topographical organization (Zaborszky et al, 1999), are distributed at much lower densities than ACh receptor binding sites (reviewed by Herkenham, 1987), and make synaptic specialization only infrequently (Chedotal et al, 1994;Umbriaco et al, 1994Umbriaco et al, , 1995Vaucher and Hamel, 1995;Mechawar et al, 2000Mechawar et al, , 2002 for review, see Descarries and Mechawar, 2000). On the other hand, the possibility that the signaling mode is more specific than volume transmission is also suggested from phasic ACh rise in the prefrontal cortex of taskperforming rats (Parikh et al, 2007), transient mAChR-mediated modulation of cortical pyramidal cells (Gulledge and Stuart, 2005;Gulledge et al, 2009), and formation of "conventional" synapses by cholinergic fibers (Frotscher and Leranth, 1985;Smiley et al, 1997;Turrini et al, 2001).…”

Selective Excitation of Subtypes of Neocortical Interneurons by Nicotinic Receptors

“…First, VIPcontaining neuronal fibers are intimately associated with the intracortical blood vessels (Eckenstein and Baughman, 1984;Chédotal et al, 1994aChédotal et al, , 1994b. Second, the application of VIP induces a dilatation of pial arteries (Yaksh et al, 1987).…”

Interpreting BOLD: towards a dialogue between cognitive and cellular neuroscience

“…Furthermore, the release of neurotransmitters from monoaminergic and cholinergic subcortical afferents can evoke both a neural and CBF response concurrently. For instance, the release of acetylcholine from afferents originating in the basal forebrain [66,67] alters cortical neuronal activity, but also concurrently evokes increases in CBF [68]. The vascular response appears to be partly mediated by local somatostatin-or neuropeptide Y-positive or GABAergic interneurons [69] and/or astrocytes [67].…”