EMBRYONIC RAT MOTONEURONS EXPRESS A FUNCTIONAL P-TYPE VOLTAGE-DEPENDENT CALCIUM-CHANNEL

Only L- and N-type high voltage-activated calcium currents (HVA I-Ca) have been demonstrated in identified embryonic spinal motoneurons. However, pharmacological experiments suggest that other HVAI(Ca), including P-type, govern neurotransmitter release at the adult neuromuscular junction. We sought to analyse if embryonic motoneurons express these other I-Ca, using the whole-cell voltage-clamp method on motoneurons purified by a new metrizamide-panning technique from E15 rat embryos, In addition to L-type dihydropyridine-sensitive and N-type omega-GVIA-sensitive currents, motoneurons express two other HVAI(Ca). One has properties related to the P-type channel currents described in Purkinje cells: it is inhibited by the peptide omega-agatoxin-IVA with a maximal effect at 100-200 nM. The inhibited current has a characteristic sustained component during depolarizing test pulses. Furthermore, 50-100 nM concentrations of omega-agatoxin-IVA reduce the increase in cytoplasmic calcium concentration observed after depolarization. The other HVAI(Ca) is resistant to saturating concentrations of verapamil, omega-conotoxin GVIA and omega-agatoxin-IVA which block L, N and P-type HVAI(Ca), respectively. These results suggest that it is now possible to dissect, using a simple method of purification, the properties of the I-Ca in embryonic mammalian motoneurons and to provide pharmacological evidence for multiple calcium channels which may be involved in regulation of their activity during development.