Multiple T-type Ca2+ current subtypes in electrophysiologically characterized hamster dorsal horn neurons: possible role in spinal sensory integration

Abstract

Whole cell patch-clamp recordings were used to investigate the contribution of transient, low-threshold calcium currents ( IT) to firing properties of hamster spinal dorsal horn neurons. IT was widely, though not uniformly, expressed by cells in Rexed's laminae I–IV and correlated with the pattern of action potential discharge evoked under current-clamp conditions: IT in neurons responding to constant membrane depolarization with one or two action potentials was nearly threefold larger than IT in cells responding to the same activation with continuous firing. IT was evoked by depolarizing voltage ramps exceeding 46 mV/s and increased with ramp slope (240–2,400 mV/s). Bath application of 200 μM Ni2+ depressed ramp-activated IT. Phasic firing recorded in current clamp could only be activated by membrane depolarizations exceeding ∼43–46 mV/s and was blocked by Ni2+ and mibefradil, suggesting IT as an underlying mechanism. Two components of IT, “fast” and “slow,” were isolated based on a difference in time constant of inactivation (12 ms and 177 ms, respectively). The amplitude of the fast subtype depended on the slope of membrane depolarization and was twice as great in burst-firing cells than in cells having a tonic discharge. Post hoc single-cell RT-PCR analyses suggested that the fast component is associated with the CaV3.1 channel subtype. IT may enhance responses of phasic-firing dorsal horn neurons to rapid membrane depolarizations and contribute to an ability to discriminate between afferent sensory inputs that encode high- and low-frequency stimulus information.