We used whole-cell patch-clamp techniques on presynaptic terminals and postsynaptic neurons of the glutamatergic magnocellularis synapse in the chick auditory brainstem to study the effects of N, P, and L-type calcium channel blockers directly on presynaptic calcium currents and transmitter release. Presynaptic calcium currents and transmitter release were unaffected by nifedipine, blocked partially by omega- agatoxin IVA and completely by omega-conotoxin GVIA. The presynaptic calcium current is a low voltage-activated, noninactivating current and its block by omega-agatoxin IVA was not reversed by repeated depolarization of the presynaptic terminal. Thus, the presynaptic calcium current that underlies transmitter release at the chick magnocellularis synapse differs in some respects from N and P-type calcium currents described in vertebrate neuronal cell bodies
