Time- and voltage-dependent currents in locust ocellar L-neurones

Abstract

1. Current-clamp and voltage-clamp experiments were performed on L-neurones in the median ocellar nerve of locusts (Locusta migratoria). Constant-current steps applied at the normal resting potential (which ranges around −40 mV) elicited electrotonic potentials that displayed outward and inward rectification. Positive constant-current steps superimposed to a negative holding current elicited small-amplitude action potentials whose threshold was around −65 mV. 2. Under voltage-clamp two fast currents were observed. A fast inward current reflecting the action potentials in the current clamp experiments had a threshold of −65 mV. It could be blocked by TTX and its voltage characteristics were similar to those of I(Na) in giant cockroach axons. The steady-state inactivation curve shows that the channels of this current are completely inactivated at the L-neurones' resting potential. 3. A fast outward current partially shunting the fast inward current could be blocked with 4-AP. Its threshold was at about −50 mV, and at −40 mV about 20% of this current could be activated by depolarizing steps. The reversal potential was around −75 mV and the voltage characteristics were very similar to those of a transient potassium current (I(A)) inDrosophila flight muscle. 4. Additionally slowly activating time- and voltage-dependent currents were observed under voltage-clamp. These currents probably reflected the existence of further voltage-gated channels but the possibility of synaptically mediated conductance changes could not be excluded. 5. The results demonstrate that locust L-neurones are capable of generating action potentials of small amplitude. The currents underlying these action potentials are similar to those described in spiking neurones. Their voltage dependence shows that the ion channels of these currents are mostly in the inactivated state at the normal resting potential of the L-neurones. Inactivation can be removed by shifting the membrane to a more negative level. Possible reasons for the low resting potential, which is mainly responsible for the ‘nonspiking’ state of an L-neurone, are discussed.