Immunocytochemical demonstration of developmental distribution of muscarinic acetylcholine receptors in rat parietal cortex.

The present investigation reveals many cortical neurons immunopositive for M35, the monoclonal antibody raised against purified muscarinic acetylcholine receptor (mAChR) proteins, in the early postnatal rat brain. The ontogeny of mAChR expression, exemplified on the parietal neocortex, was studied in a series of rat pups from postnatal days (PD) 1, 3, 7, 14 and 21. Immunoprecipitation in the parietal somatosensory cortex was manifest in the population of pyramidal neurons during postnatal development. In particular during the early postnatal ages, until 2 weeks after birth, M35 immunoreactivity (M35-ir) was present in all neuronal compartments, indicating transportation of mAChR protein in axonal and dendritic processes as observed in light and electron microscopic analysis. The immunoprecipitation in the apical dendrites yielded dense labeling in layer 1 where the distal processes of the pyramidal dendrites branched extensively forming a plexus that intermingled with horizontal fibers in this superficial layer. At PD21, immunolabeling in layer 1 and in axons of pyramidal cells was reduced compared to earlier ages suggesting a transient expression of mAChRs in these neuronal structures. The development of M35-ir in the cortex appeared to antedate that of its cholinergic afferentation as indicated by AChE histochemical study.