MUSCARINIC ACETYLCHOLINE RECEPTOR-EXPRESSION IN ASTROCYTES IN THE CORTEX OF YOUNG AND AGED RATS

The present report describes the cellular and subcellular distribution pattern of immunoreactivity to M35, a monoclonal antibody raised agai nst purified muscarinic acetylcholine receptor protein, in astrocytes in the cerebral cortex of young and aged rats. Most M35-positive astro cytes were localized in the superficial layers of the cortex and part of the corpus callosum. At the ultrastructural level, immunoprecipitat es were localized in the Golgi complexes, but the nucleus, rough endop lasmic reticulum, mitochondria, and microfilaments were generally free of labeling. Labeling was also present associated to the cell membran e, although without the characteristic immunoreactive postsynaptic mem brane thickening found in neuronal structures. In aging rats of 30-34 months, the number of M35-labeled astrocytes doubled, whereas the neur onal staining slightly decreased in the same region in half of the ani mals studied. Fluorescent double-labeling for M35 and GFAP, an astrocy tic microfilament protein, revealed that all M35-positive glial cells express GFAP and, conversely, that almost all GFAP glial cells were M3 5-immunostained. Based on the high incidence of coexpression of mAChRs and GFAP, both proteins may be functionally linked to each other. Rou gh semiquantitative estimates revealed that in young adult rats the GF AP/M35-immunoreactive astrocytes made up approximately one fifth of al l cortical astrocytes. An important aspect of the presently demonstrat ed immunoreactivity of astroglia to mAChR proteins is its labeling in situ instead of in tissue culture. This finding may further support in vestigation, e.g., on anatomical relations of astroglia with neuronal and vascular elements, and its reactivity in experimental conditions.