Enhancement by T-type Ca2+ currents of odor sensitivity in olfactory receptor cells

Mechanisms underlying action potential initiation in olfactory receptor cel ls (ORCs) during odor stimulation were investigated using conventional and dynamic patch-clamp recording techniques. Under current-clamp conditions, a ction potentials generated by a least effective odor-induced depolarization were almost completely blocked by 0.1 mM Ni2+, a T-type Ca2+ channel block er, but not by 0.1 mM Cd2+, a high voltage-activated Ca2+ channel blocker. Under voltage-clamp conditions, depolarizing voltage steps induced a fast t ransient inward current, which consisted of Na+ (I-Na) and T-type Ca2+ (I-C a, (T)) currents. The amplitude of I-Ca,I-T was approximately one- fourth o f that of I-Na (0.23 +/- 0.03, mean +/- SEM). Because both I-Na and I-Ca,(T ) are known to show rapid inactivation, we examined how much I-Na and I-Ca, (T) are activated during the gradually depolarizing initial phase of recept or potentials. The ratio of I-Ca,(T) /I-Na during a ramp depolarization at the slope of 0.5 mV/msec was 0.56 +/- 0.03. Using the dynamic patch-clamp r ecording technique, we also recorded I-Ca,I-T and I-Na during the generatio n of odor-induced action potentials. This ratio of I-Ca,I-T /I-Na was 0.54 +/- 0.04. These ratios were more than twice as large as that (0.23) obtaine d from the experiment using voltage steps, suggesting that I-Ca,I-T carries significant amount of current to generate the action potentials. We conclu de that I-Ca,I-T contributes to enhance odor sensitivity by lowering the th reshold of spike generation in ORCs.