SOMATIC SODIUM-CHANNELS OF FROG OLFACTORY RECEPTOR NEURONS ARE INACTIVATED AT REST

Membrane excitability of acutely isolated olfactory receptor neurones (ORNs) of the grass frog (R. pipiens) was studied with the use of the whole-cell "tight-seal" patch recording technique. ORNs of the frog had a mean resting membrane potential of -52 mV, a mean input resistance of 1 - 2 G-OMEGA, and a mean capacitance of 4.5 pF. In the majority of cells examined (over 70%), short duration (several milliseconds) action potentials were elicited at the end of a hyperpolarising pulse (off-spike) or following hyperpolarization of the membrane potential by injection of current. Under voltage-clamp conditions, a fast inward current followed by an outward current could be evoked upon depolarisation of the membrane. The fast inward current decayed with a time constant of 1 - 2 ms, with an e-fold decrease per 52 mV increase in voltage, and was blocked by the selective voltage-dependent sodium channel blocker tetrodotoxin (0.5 - 1-mu-M). Steady-state inactivation studies revealed that the mean voltage for half-inactivation (V1/2) was -82 mV (range -72 to -98 mV), which indicates that the voltage-dependent Na+ channels in the cell body or soma of frog ORNs are not available for conducting currents at the resting membrane potential. This finding raises the possibility that voltage-dependent Na+ channels may not play a significant role in sensory transduction at the soma. Our results indicate that ORNs of the frog are very efficient in transducing signals towards the brain since currents generated at the cilia will be directed towards depolarising the axon. The data also could account for the low number of single-cell action potentials encountered in extracellular recording experiments in vivo.