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 a cetylcholine receptor (mAChR) proteins, in the early postnatal rat bra in. The ontogeny of mAChR expression, exemplified on the parietal neoc ortex, was studied in a series of rat pups from postnatal days (PD) 1, 3, 7, 14 and 21. Immunoprecipitation in the parietal somatosensory co rtex was manifest in the population of pyramidal neurons during postna tal 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 a xonal and dendritic processes as observed in light and electron micros copic analysis. The immunoprecipitation in the apical dendrites yielde d dense labeling in layer 1 where the distal processes of the pyramida l dendrites branched extensively forming a plexus that intermingled wi th horizontal fibers in this superficial layer. At PD21, immunolabelin g in layer 1 and in axons of pyramidal cells was reduced compared to e arlier ages suggesting a transient expression of mAChRs in these neuro nal structures. The development of M35-ir in the cortex appeared to an tedate that of its cholinergic afferentation as indicated by AChE hist ochemical study.