A Quantitative Description of Low- and High-threshold Ca2+ Spikes in Rat Sensory Neurons: A Perforated-patch Study

Action potentials generated by voltage-dependent Ca2+ conductances were studied at 25 degrees C with the perforated-patch technique, in freshly dispersed adult rat sensory neurons perfused with Na-free solutions containing tetraethylammonium. Brief depolarizing currents from membrane potentials negative to - 75 mV always elicited long (> 100 ms) plateau spikes which had different thresholds in different neurons: a low threshold around - 60/- 50 mV and a high-threshold at - 30/- 20 mV. Stimulations from potentials positive to - 55 mV, on the contrary, elicited spikes originating only in the high threshold region and sensitive to 25 microM Cd2+, designated high-threshold spikes. In neurons which showed spikes with low threshold, addition of 25 microM Cd2+ disclosed a smaller and shorter regenerative response, the low-threshold spike. Moreover, the classical 'anode-break' stimulation from - 50/- 60 mV uncovered isolated low-threshold spikes, indicating a time- and voltage-dependent de-inactivating process. From the properties of the low (LVA) and high (HVA) voltage-activated Ca2+ currents, recorded under the same extracellular conditions, a Hodgkin - Huxley model was derived and used to reconstruct all the features of the recorded spikes. The model was also able to simulate experimental blocking of LVA channels by amiloride, modulation of HVA channels by baclofen and induced oscillatory firing. This agreement between the behaviour of recorded spikes and their mathematical description led us to conclude that the LVA and HVA Ca2+ currents underlie the low- and high-threshold Ca2+ spikes, respectively. Furthermore, our data suggest that complex behaviour known to be typical of central nervous system neurons is also present in sensory peripheral neurons.