An inward rectifier is present in presynaptic nerve terminals in the chick ciliary ganglion

Inwardly rectifying voltage-sensitive channels have been detected in the cell bodies and axons of a number of excitable cells. The question of whether similar channels exist at axon terminals has been a matter of speculation for some time. We now report the first direct evidence for the existence of inward rectifiers in vertebrate presynaptic nerve terminals. Following impalement with intracellular electrodes, the large calyciform nerve terminals innervating chick ciliary ganglion neurons exhibit pronounced inward rectification upon hyperpolarization that increases with increasing current strength. The response is blocked by 2 mM Cs+, but is insensitive to Ba2+, tetraethylammonium and tetrodotoxin. The inward rectifier exhibits dependence on both Na+ and K+, but is unaffected by altering extracellular Ca2+. Ciliary neurons innervated by these nerve terminals display inward rectification with similar properties. We conclude that the inward rectifier present in these presynaptic nerve terminals resembles the H-current previously described in sensory ganglion neurons and the Q-current found in hippocampal pyramidal neurons. The presence of channels that are activated by hyperpolarization may serve to enhance the excitability of the calyciform nerve terminals, which are capable of relatively high frequencies (greater than 100 Hz) of discharge.