Voltage-dependent and odorant-regulated currents in isolated olfactory receptor neurons of the channel catfish

The electrical properties of olfactory receptor neurons, enzymatically dissociated from the channel catfish (Ictalurus punctatus), were studied using the whole-cell patch-clamp technique. Six voltage-dependent ionic currents were isolated. Transient inward currents (0.1-1.7 nA) were observed in response to depolarizing voltage steps from a holding potential of -80 mV in all neurons examined. They activated between -70 and -50 mV and were blocked by addition of 1 μM tetrodotoxin (TTX) to the bath or by replacing Na⁺ in the bath with N-methyl-D-glucamine and were classified as Na⁺ currents. Sustained inward currents, observed in most neurons examined when Na⁺ inward currents were blocked with TTX and outward currents were blocked by replacing K⁺ in the pipette solution with Cs⁺ and by addition of 10 mM Ba²⁺ to the bath, activated between -40 and -30 mV, reached a peak at 0 mV, and were blocked by 5 μM nimodipine. These currents were classified as L-type Ca²⁺ currents. Large, slowly activating outward currents that were blocked by simultaneous replacement of K⁺ in the pipette with Cs⁺ and addition of Ba²⁺ to the bath were observed in all olfactory neurons examined. The outward K⁺ currents activated over approximately the same range as the Na⁺ currents (-60 to -50 mV), but the Na⁺ currents were larger at the normal resting potential of the neurons (-45 ± 11 mV, mean ± SD, n = 52). Four different types of K⁺ currents could be differentiated: a Ca²⁺-activated K⁺ current, a transient K⁺ current, a delayed rectifier K⁺ current, and an inward rectifier K⁺ current. Spontaneous action potentials of varying amplitude were sometimes observed in the cell-attached recording configuration. Action potentials were not observed in whole-cell recordings with normal internal solution (K⁺ = 100 mM) in the pipette, but frequently appeared when K⁺ was reduced to 85 mM. These observations suggest that the membrane potential and action potential amplitude of catfish olfactory neurons are significantly affected by the activity of single channels due to the high input resistance (6.6 ± 5.2 GΩ, n = 20) and low membrane capacitance (2.1 ± 1.1 pF, n = 46) of the cells. Stimulation of voltage-clamped receptor neurons with a mixture of amino acids (100 μM each of L-arginine, L-alanine, and L-norleucine with or without L-glutamate), which act at independent classes of receptor sites, elicited receptor currents that were heterogeneous in terms of voltage and ion dependence, time course, reversal potential, and sensitivity to drugs, and appeared to result from at least two different processes. These different types of receptor cell responses to odorant amino acids may reflect activation of different transduction pathways.