Ionic currents and ion channels of lobster olfactory receptor neurons

The role of the soma of spiny lobster olfactory receptor cells in generating odor-evoked electrical signals was investigated by studying the ion channels and macroscopic currents of the soma. Four ionic currents; a tetrodotoxin-sensitive Na⁺ current, a Ca⁺⁺ current, a Ca⁺⁺-activated K⁺ current, and a delayed rectifier K⁺ current, were isolated by application of specific blocking agents. The Na⁺ and Ca⁺⁺ currents began to activate at -40 to -30 mV, while the K⁺ currents began to activate at - 30 to - 20 mV. The size of the Na⁺ current was related to the presence of a remnant of a neurite, presumably an axon, and not to the size of the soma. No voltage-dependent inward currents were observed at potentials below those activating the Na⁺ current, suggesting that receptor potentials spread passively through the soma to generate action potentials in the axon of this cell. Steady-state inactivation of the Na⁺ current was half-maximal at -40 mV. Recovery from inactivation was a single exponential function that was half-maximal at 1.7 ms at room temperature. The K⁺ currents were much larger than the inward currents and probably underlie the outward rectification observed in this cell. The delayed rectifier K⁺ current was reduced by GTP-γ-S and AlF₄⁻, agents which activate GTP-binding proteins. The channels described were a 215-pS Ca⁺⁺-activated K⁺ channel, a 9.7-pS delayed rectifier K⁺ channel, and a 35-pS voltage-independent Cl⁻ channel. The Cl⁻ channel provides a constant leak conductance that may be important in stabilizing the membrane potential of the cell.