CALCIUM-CHANNEL SUBTYPES IN LAMPREY SENSORY AND MOTOR-NEURONS

Pharmacologically distinct calcium channels have been characterized in dissociated cutaneous sensory neurons and motoneurons of the larval l amprey spinal cord. To enable cell identification, sensory dorsal cell s and motoneurons were selectively labeled with fluorescein-coupled de xtran amine in the intact spinal cord in vitro before dissociation. Ca lcium channels present in sensory dorsal cells, motoneurons, and other spinal cord neurons were characterized with the use of whole cell vol tage-clamp recordings and specific calcium channel agonist and antagon ists. The results show that a transient low-voltage-activated (LVA) ca lcium current was present in a proportion of sensory dorsal cells but not in motoneurons, whereas high-voltage-activated (HVA) calcium curre nts were seen in all neurons recorded. The different components of HVA current were dissected pharmacologically and similar results were obt ained for both dorsal cells and motoneurons. The N-type calcium channe l antagonist omega-conotoxin-GVIA (omega-CgTx) blocked >70% of the KVA current. A large part of the omega-CgTx block was reversed after wash out of the toxin. The L-type calcium channel antagonist nimodipine blo cked similar to 15% of the total HVA current. The dihydropyridine agon ist (+/-)-BayK 8644 markedly increased the amplitude of the calcium ch annel current. The BayK-potentiated current was not affected by omega- CgTx, indicating that the reversibility of the omega-CgTx effect is no t due to a blockade of L-type channels. Simultaneous application of om ega-CgTx and nimodipine left similar to 15% of the HVA calcium channel current, a small part of which was blocked by the P/Q-type channel an tagonist omega-agatoxin-IVA. In the presence of the three antagonists, the persistent residual current (similar to 10%) was completely block ed by cadmium. Our results provide evidence for the existence of HVA c alcium channels of the N, L, and P/Q types and other HVA calcium chann els in lamprey sensory neurons and motoneurons. In addition, certain t ypes of neurons express LVA calcium channels.