A HYPERPOLARIZATION-ACTIVATED CATION CONDUCTANCE IN LOBSTER OLFACTORY RECEPTOR NEURONS

1. The current underlying inward rectification in lobster olfactory receptor neurons was investigated with the use of whole-cell patch-clamp techniques. Inward rectification could most likely result from an inwardly rectifying potassium conductance or a hyperpolarization-activated cation conductance. To distinguish between these possibilities, the current underlying inward rectification was examined with respect to its sensitivity to extracellular Cs+ and Ba2+, time course of activation, and reversal potential. 2. In current clamp, injection of negative current led to a hyperpolarization followed by a partial return (sag) toward the initial holding potential. The rate and magnitude of the sag depended on the magnitude of the hyperpolarizing current with larger currents leading to larger, faster depolarizing sags. In voltage clamp, hyperpolarizing steps elicited a slowly activating, noninactivating inward current that presumably underlies the sag observed in current clamp. Both the sag and the slow inward current were blocked reversibly by extracellular application of 5 mM CsCl but were unaffected by 2 mM BaCl2. 3. The rate of inward current activation was best approximated by a single exponential function with time constants that were voltage dependent, ranging from 7.8 s at -69 mV to 248 ms at -114 mV. 4. Cells normally exhibited an average input resistance of 0.99 G Omega over the range of -69 to -114 mV. With the hyperpolarization-activated inward current blocked by 5 mM CsCl, the average input resistance increased to 2.12 G Omega over the same range. 5. Analysis of tail currents revealed that the average predicted reversal potential of the hyperpolarization-activated inward current was 1.7 mV and was not affected significantly by a shift in E(Cl). 6. The combined findings of Cs+ sensitivity, Ba2+ insensitivity, slow activation, and a reversal potential near 0 mV that is unaffected by E(Cl) are indicative of a hyperpolarization-activated cation conductance. This conductance could play a number of roles in lobster olfactory receptor neurons including setting the resting membrane potential and input resistance, opposing hyperpolarizing inputs, and contributing to spike-frequency adaptation.