Cholinergic innervation in adult rat cerebral cortex: A quantitative immunocytochemical description

A method for determining the length of acetylcholine (ACh) axons and number of ACh axon varicosities (terminals) in brain sections immunostained for choline acetyltransferase (ChAT) was used to estimate the areal and laminar densities of this innervation in the frontal (motor), parietal (somatosensory), and occipital (visual) cortex of adult rat. The number of ACh varicosities per length of axon (4 per 10 μm) appeared constant in the different layers and areas. The mean density of ACh axons was the highest in the frontal cortex (13.0 m/mm3 vs. 9.9 and 11.0 m/mm3 in the somatosensory and visual cortex, respectively), as was the mean density of ACh varicosities (5.4 × 106/mm3 vs. 3.8 and 4.6 × 106/mm3). In all three areas, layer I displayed the highest laminar densities of ACh axons and varicosities (e.g., 13.5 m/mm3 and 5.4 × 106/mm3 in frontal cortex). The lowest were those of layer IV in the parietal cortex (7.3 m/mm3 and 2.9 × 106/mm3). The lengths of ACh axons under a 1 mm2 surface of cortex were 26.7, 19.7, and 15.3 m in the frontal, parietal, and occipital areas, respectively, for corresponding numbers of 11.1, 7.7, and 6.4 × 106 ACh varicosities. In the parietal cortex, this meant a total of 1.2 × 106 synaptic ACh varicosities under a 1 mm2 surface, 48% of which in layer V alone, according to previous electron microscopic estimates of synaptic incidence. In keeping with the notion that the synaptic component of ACh transmission in cerebral cortex is preponderant in layer V, these quantitative data suggest a role for this innervation in the processing of cortical output as well as input. Extrapolation of particular features of this system in terms of total axon length and number of varicosities in whole cortex, length of axons and number of varicosities per cortically projecting neuron, and concentration of ACh per axon varicosity, should also help in arriving at a better definition of its roles and functional properties in cerebral cortex. J. Comp. Neurol. 428:305–318, 2000. © 2000 Wiley‐Liss, Inc.