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

Abstract

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 CaV1.2 and CaV1.3 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 CaV1.2 and CaV1.3 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, CaV1.2 subunits were present in the cell bodies and axons. Patches of CaV1.3 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 CaV1.3 subunits, but not CaV1.2 subunits, were always located at synaptic sites. The distribution of CaV1.2 and CaV1.3 strongly suggests that the persistent inward current underlying plateau potentials in spinal motoneurons is mediated by CaV1.3 and not by CaV1.2. Our findings also show that CaV1.3 may be located in the somatic and dendritic membrane adjacent to particular presynaptic terminals.