Large cholinergic nerve terminals on subsets of motoneurons and their relation to muscarinic receptor type 2

The cholinergic C-bouton is a large nerve terminal found exclusively apposing motoneuron cell somata and proximal dendrites. The origin and function of the C-bouton is not known. An antiserum against the vesicular acetylcholine transporter was used to identify large cholinergic nerve terminals putatively of the C-type in close apposition to motoneuron cell somata. This type of nerve terminal was present in the rat spinal cord ventral horn, but only in some cranial motor nuclei. Fluoro-Gold tracing showed that subsets of spinal motoneuron cell somata were contacted by different numbers of putative C-boutons. Thus, motoneurons innervating an intrinsic foot muscle were contacted by about half the number of cholinergic terminals found on motoneurons of the predominantly fast-twitch gastrocnemius muscle. Slow-twitch soleus motoneurons showed an intermediate innervation. There was a strong correlation between the presence of putative C-boutons and muscarinic receptor 2 (m2)-like immunoreactivity (-LI) within a motor nucleus. By using confocal laser microscopy, the m2-LI appeared to be confined to the motoneuron cell membrane and strongly enriched beneath the C-type nerve terminal. Thus, our results suggested a differential distribution of large cholinergic C-boutons, depending on motoneuron type, and that the presence of this nerve terminal type is associated with m2-LI in the postsynaptic membrane.