Subcellular distribution of L-type Ca2+ channels responsible for plateau potentials in motoneurons from the lumbar spinal cord of the turtle

L-type calcium channels mediate the persistent inward current underlying plateau potentials in spinal motoneurons. Electrophysiological analysis shows that plateau potentials are generated by a persistent inward current mediated by low threshold L-type calcium channels located in the dendrites. As motoneurons express L-type calcium channels of the Ca-V 1.2 and Ca-V 1.3 subtypes, we have investigated the subcellular distribution of these channels using antibody labelling. The plateau generating a persistent inward current is modulated by the activation of metabotropic receptors. For this reason, we also examined the relationship between Ca-V 1.2 and Ca-V 1.3 subunits in motoneurons and presynaptic terminals labelled with antibodies against synapsin 1a. Motoneurons in the spinal cord of the adult turtle were identified as large neurons, immunopositive for choline acetyltransferase, located in the ventral horn. In these neurons, Ca-V 1.2 subunits were present in the cell bodies and axons. Patches of Ca-V 1.3 subunits were seen in association with the cell membrane of the somata and both the proximal and distal dendrites. Double labelling with an antibody against synapsin 1a showed that Ca-V 1.3 subunits, but not Ca-V 1.2 subunits, were always located at synaptic sites. The distribution of Ca-V 1.2 and Ca-V 1.3 strongly suggests that the persistent inward current underlying plateau potentials in spinal motoneurons is mediated by Ca-V 1.3 and not by Ca-V 1.2. Our findings also show that Ca-V 1.3 may be located in the somatic and dendritic membrane adjacent to particular presynaptic terminals.