Presynaptic Ca-antagonist omega-conotoxin irreversibly blocks N-type Ca-channels in chick sensory neurons

The present studies on electrophysiological and pharmacological differences of the three types of Ca-currents (N-, L- and T-types) in whole-cell clamped, cultured embryonic chick sensory neurons revealed that the majority (94%) of the Ca-currents in the nerve cells were the N-type, omega-Conotoxin (omega CTX, 5 microM), a blocker of transmitter release at the presynaptic terminals, induced a complete and irreversible blockage of Ca-currents elicited from the resting membrane potential (-60 mV) in 29 cells among 58. The Ca-currents thus irreversibly blocked by the omega CTX were determined as the N-type (neuronal), as they were insensitive to nifedipine (5 microM) or were reduced in amplitude by Bay K 8644 (5 microM). A small fraction (12%) of the total Ca-currents, which were still present after the omega CTX treatment (in the rest of 29 cells), were pure L-type (long-lasting) Ca-currents, as they were enhanced by the Bay K and were blocked by the nifedipine. omega CTX was a partial and reversible blocker of the L-type Ca-currents. Furthermore, T-type (transient) Ca-currents elicited in the hyperpolarized membrane (at -100 mV) were blocked by omega CTX in an incomplete and reversible manner. The N-type Ca-currents thus separated in the nerve cells exhibited various differences in features of the voltage-dependence and ionic selectivity from the L- and T-type Ca-currents.