Rat dorsal root ganglia express m1-m4 muscarinic receptor proteins.

The distribution of m1-m4 muscarinic acetylcholine receptors was investigated by immunolight and electron microscopy in rat dorsal root ganglia, by using subtype-specific antibodies. The light microscopic observations show that both the intensity and the localisation of immunolabeling of the four muscarinic subtypes are different. The m2 receptor is the most represented; m2 and m4 receptor proteins are principally expressed in the small-medium type neurones, while m1 and m3 receptor proteins are more homogeneously distributed among the neuronal population of the ganglion. On the contrary, there are no relevant differences in the subcellular distribution. Ultrastructural analysis shows that immunolabeling is associated with the Nissl bodies, budding Golgi cisterns, and as far as the m2 receptor is concerned, just beneath the plasmalemma with discrete loci along the neuronal plasma membrane. Unmyelinated nerve fibres are always immunopositive for all muscarinic receptor subtypes, suggesting an active transport for all subtypes. Occasionally perineuronal satellite cells surrounding large neurones are immunopositive for the m2 receptor, while both myelinating and non-myelinating Schwann cells faintly express all muscarinic receptor subtypes. These observations, together with previous data concerning pharmacological and functional characterisation of muscarinic acetylcholine receptors in dorsal root ganglion, suggest that sensory neurones are able to respond to acetylcholine. The presence of m2 and m4 immunoreactivity in the small type neurones and in their fibres involved in nociceptive response, suggest that these receptors may participate in the modulation of transduction of noxious stimuli from the periphery to the spinal cord. m1 and m3 receptors, having a more homogeneously distribution, may be involved in transduction of different kinds of stimuli.