Cholinoceptive cells in rat cerebral cortex: somatodendritic immunoreactivity for muscarinic receptor and cytoskeletal proteins.

Adult rat telecephalon was surveyed for cells demonstrating immunopositivity for muscarinic receptor (M35 antibody), microtubule-associated proteins, neurofilaments, and brain-spectrin. Neurons immunostained for muscarinic receptor were found in frontal, parietal, temporal, and occipital isocortex where they accounted for approximately 15-16% of all neurons. This labeling involved a large proportion of layer V pyramidal cells, some layer III pyramidal cells and a small proportion of non-pyramidal cells in layers II-VI. In the hippocampus, pyramidal cells, non-pyramidal cells and granular cells were immunoreactive, as were many pyramidal cells in subicular and entorhinal cortices. In every cortical region examined, cells demonstrating muscarinic receptor were morphologically identical to cells stained lightly to moderately for acetylcholinesterase following pretreatment with diisopropylfluorophosphate, and they were found in similar numbers and in a similar laminar distribution. These characteristics further corresponded to those of cells whose somatodendritic compartments were intensely immunostained by antibodies to microtubule-associated proteins (MAP): MAP-1, MAP-2, MAP-5; neurofilament proteins (NF): NF-68kD, NF-160kD, NF-200kD; and brain-spectrin. Double immunostaining using a fluorescence method followed by an avidin-biotin staining procedure revealed that cortical cells which possessed immunoreactivity for muscarinic receptor demonstrated an 80-85% overlap with cells that were immunoreactive for MAP-2 (and tau) or NF-200kD. Following unilateral ibotenic acid lesions of the nucleus basalis, MAP-2 immunostaining was reduced in the ipsilateral isocortex. This significant reduction was most evident in the parietal cortex, exactly where maximal loss of acetylcholinesterase-containing fibers occurred. The same lesion produced no significant difference in immunodensity of muscarinic receptor, MAP-1, MAP-5 NF-68kD, NF-160kD and NF-200kD. Thus, cortical cholinoceptive cells are enriched with cytoskeletal components and cholinergic afferents modulate cortical MAP-2.