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.

No abstract

Of the different types of synaptic contacts with the mammalian spinal motoneuron, the synapse made by the cholinergic, so-called C-terminal of unknown origin and function has special morphological characteristics. Thus, in this synapse, there is no postsynaptic density but rather a large subsynaptic cistern in the motoneuron. To see whether this particular arrangement imposes special demands on the transmitter release machinery, we examined the presence of nerve terminal proteins in the C-terminal by using immunohistochemistry. Cholinergic nerve fibers and terminals in the spinal cord ventral horn were identified with an antiserum to the vesicular acetylcholine transporter (VAChT) protein. Immunohistochemistry in combination with confocal laser microscopy showed the presence of synaptosomal-associated protein of 25 kDa (SNAP-25)-, syntaxin-, cysteine string protein (CSP)-, synuclein-, synapsin I-, synapsin I/II-, synaptotagmin I-, synaptotagmin I/II-, synaptophysin-, and synaptobrevin-2-like immunoreactivity (-LI) in VAChT-containing C-terminals. Synaptotagmin III and synaptobrevin 1 could not be demonstrated in this type of terminal. VAChT-containing varicosities in the Renshaw cell area, with a probable origin from motoneuron axon collaterals, exhibited CSP, synapsin I/II, and synaptobrevin-1-LI, but not SNAP-25-, syntaxin-, synuclein-, synapsin I-, synaptotagmin I-, synaptotagmin I/II-, synaptophysin- and synaptobrevin-2-LI. The results suggest a differential content of nerve terminal proteins and their isoforms in cholinergic C-terminals apposing motoneurons and in the Renshaw cell area. It is concluded that C-terminals contain synaptic proteins necessary for fast transmitter release, and their origin should not be the motoneurons themselves.