Neuronal and non-neuronal cell populations of the avian dorsal root ganglia express muscarinic acetylcholine receptors

The distribution of muscarinic acetylcholine receptors was investigated by immuno-light and electron microscopy in the chick dorsal root ganglion during embryonic development (E12 and E18) and after hatching. The monoclonal antibody we used recognizes the acetylcholine binding site shared by all five muscarinic acetylcholine receptor subtypes. At E12, light microscopy reveals several immunopositive neurons with variable degrees of immunolabeling, heterogeneously distributed throughout the ganglion. Later in development and after hatching, the intensity of immunolabeling seems to decrease and the immunopositive neurons, of the small-medium-sized type, are located mostly in the medio-dorsal region of the ganglion. Under the electron microscope, the immunoreaction is associated with the Nissl bodies, budding Golgi cisterns and, especially at E12, with discrete loci along the neuronal plasma membrane. Unmyelinated nerve fibers, in both central and peripheral branches, are also immunopositive, suggesting that muscarinic acetylcholine receptors are transported towards the spinal cord and the periphery, respectively. A large number of perineuronal satellite cells and both myelinating and unmyelinating Schwann cells are intensely labeled. These observations, combined with previous data on the pharmacological and functional characterization of muscarinic acetylcholine receptors in the avian dorsal root ganglion, suggest that both sensory neurons and non-neuronal cells are able to respond to acetylcholine stimuli. Since muscarinic acetylcholine receptor-immunoreactivity is restricted to the small-medium-sized neurons and their unmyelinated fibers, of the nociceptive type, we suggest that these receptors are involved in modulating the transduction of noxious stimuli from the periphery.