On the cholinergic c-bouton
Abstract: The cholinergic C-bouton is a large nerve terminal type exclusively contacting motoneuron cell somata and proximal dendrites. C-boutons are characterized by subsurface cisterns found beneath the postsynaptic membrane, which is an exclusive feature for these nerve terminals. The origin of the C-boutons is unknown. Synaptic protein expression in C-boutons was investigated. C-boutons contained proteins necessary for fast Ca2+-sensitive neurotransmission. C-boutons contained VAMP-2, which distinguished them from motoneuron derived nerve terminals that expressed VAMP-1. C-boutons were not evenly distributed on cell somata of spinal motoneurons. Motoneurons innervating a distal foot sole muscle received only half the number of C-boutons as did motoneurons innervating the more proximal gastrocnemius muscle. These data imply that there is a diverse functional need of C-bouton input in subsets of motoneurons. In the brainstem, C-boutons were identified in the trigeminal, facial and hypoglossal motor nuclei. No C-boutons were seen on motoneurons in ocular motor nuclei, or on preganglionic autonomic neurons in brainstem and spinal cord. The mixed somatic and autonomic nucleus ambiguus exhibited C-boutons on somatic motoneurons in the caudal regions, while rostral autonomic neurons did not receive this type of input. Thus, probably all somatic non-ocular motoneurons are contacted by Cboutons. The muscarinic type 2 (m2) receptor was identified as postsynaptic target for Cboutons. Expression of m2 protein in plasma membranes of motoneurons was seen in nuclei containing Cboutons, but not in nuclei lacking this nerve terminal type. m2 was also seen in a subpopulation of neurons in nucleus abducens. Among the known cholinergic neurons in the spinal cord, partition cells in Rexed's lamina VII and preganglionic sympathetic neurons in the intermediolateral column expressed significant levels of VAMP-2 mRNA. Central canal cells in lamina X did not exhibit significant levels of VAMP-2 mRNA. It is thus unlikely that these cells are the origin of the cholinergic C-boutons. Spinal cord lamina IX contained medium sized cell profiles that expressed both VAMP-2 and VAChT mRNAs. A few cholinergic cells in lamina IX did not project into ventral roots. These data suggest that lamina IX may contain cholinergic interneurons. Studies of VAChT mRNA distribution in the spinal cord also showed that the dorsal laminae were devoid of VAChT mRNA and that lamina IX probably contained large non-cholinergic neurons not previously described. Lesions transecting fibers projecting from different sources, and anterograde tracing of fibers in the spinal cord showed that the origin of the C-boutons does not derive from descending pathways and that the putative VAMP-2 mRNA expressing interneurons in lamina IX constitute plausible candidates as cells of origin for the spinal C-boutons.
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