Calcium-dependent action potentials in the second-order neurones of cockroach ocelli

The ionic basis of the action potential in the large second-order neurones (L-neurones) of the ocellus of the cockroach, Periplaneta americana, was studied. L-neurones generated action potentials, usually once, at the off-set of hyperpolariz-ing light responses, or at the termination of hyperpolarizing current stimuli. The action potential was blocked by replacing saline Ca2+ with Mg2+ but maintained when Baz+ was substituted. A block was produced by 2mmoll~1 Cdz+ or ZOmmoll"1 Co2+. The peak amplitude of the action potential increased by 26 mV for a 10-fold increase in external Ca2+ concentration, at concentrations below l-8mmoll~'. The action potential was not affected by sodium-free saline or by 3xlO~6moll~1 tetrodotoxin (TTX). These observations suggest that calcium ions are the major carrier for the inward current of the action potential. This finding supports the suggestion that the off-set responses of hyperpolarizing visual neurones of both vertebrates and invertebrates have a common ionic mechanism, including voltage-sensitive calcium currents