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 c holine acetyltransferase (ChAT) was used to estimate the areal and laminar densities of this innervation in the frontal (motor), parietal (somatosenso ry), and occipital (visual) cortex of adult rat. The number of ACh varicosi ties per length of axon (4 per 10 mum) appeared constant in the different l ayers and areas. The mean density of ACh axons was the highest in the front al cortex (13.0 m/mm(3) vs. 9.9 and 11.0 m/mm(3) in the somatosensory and v isual cortex, respectively), as was the mean density of ACh varicosities (5 .4 x 10(6)/mm(3) vs. 3.8 and 4.6 x 10(6)/mm(3)). In all three areas, layer I displayed the highest laminar densities of ACh axons and varicosities (e. g., 13.5 m/mm(3) and 5.4 x 10(6)/mm(3) in frontal cortex). The lowest were those of layer TV in the parietal cortex (7.3 m/mm(3) and 2.9 x 10(6)/mm(3) ). The lengths of ACh axons under a 1 mm(2) 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 x 10(6) ACh varicosities. In the parietal cortex, this meant a total of 1.2 x 10(6) synaptic ACh vari cosities under a 1 mm(2) surface, 48% of which in layer V alone, according to previous electron microscopic estimates of synaptic incidence. In keepin g with the notion that the synaptic component of ACh transmission in cerebr al cortex is preponderant in layer V, these quantitative data suggest a rol e for this innervation in the processing of cortical output as well as inpu t. Extrapolation of particular features of this system in terms of total ax on length and number of varicosities in whole cortex, length of axons and n umber of varicosities per cortically projecting neuron, and concentration o f ACh per axon varicosity, should also help in arriving at a better definit ion of its roles and functional properties in cerebral cortex. (C) 2000 Wil ey-Liss, Inc.