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 (I(T)) to firing properties of hamster spinal dorsal horn neurons. I(T) 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: I(T) in neurons responding to constant membrane depolarization with one or two action potentials was nearly threefold larger than I(T) in cells responding to the same activation with continuous firing. I(T) 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 Ni(2+) depressed ramp-activated I(T). Phasic firing recorded in current clamp could only be activated by membrane depolarizations exceeding ∼43-46 mV/s and was blocked by Ni(2+) and mibefradil, suggesting I(T) as an underlying mechanism. Two components of I(T), "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 Ca(V)3.1 channel subtype. I(T) 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.

Characterisation of rebound depolarisation in mice deep dorsal horn neurons in vitro

“…The T-type current found in our dorsal horn neurons may have little contributions from Ca v 3.2, according to their pharmacology, and a reduced contribution of Cav 3.3, according to kinetic parameters [4]. The present results are in line with a previous report showing a higher expression of Cav 3.1 subunit in spinal cord neurons using a single-cell PCR [16]. Here, T-type currents are related to the presence of fast rebounds of high amplitude associated to highfrequency firing.…”

“…We found that, applied at 100 μM, NiCl only reduced a ≈25 % of the current recorded. This observation is in line with a previous report showing a low efficacy of 200 μM NiCl and mibefradil at blocking I T recorded from dorsal horn neurons [16]. Increasing the concentration of NiCl to 1 mM produced a fast and reversible full block of the current, and application of the novel blocker TTA-P2 [6] also suppressed this current, supporting its identification as a T-type current.…”

“…Kinetic parameters of the calcium currents recorded match well those of previous descriptions of cloned Ca v 3 subunits [4] and native T-currents expressed in spinal neurons [16]. Here, recordings were performed using a preparation that maintained most of the neuronal processes where these channels may reside [24].…”

“…In fact, the I h was implicated in low-amplitude rebounds observed in neurons from the substantia gelatinosa [43]. In addition, the expression of different voltagedependent calcium currents, including the T-type, has been reported as well [10,16,31].…”

“…Recent reports have shown that Ca v 3.2 channels are expressed presynaptically at the spinal cord level and may be related to nociceptive transmission from small diameter primary afferents to neurons in superficial laminae of the cord [14]. Dorsal horn neurons also express low-threshold voltage-dependent calcium currents yielded by several Ca v 3 subunits [10,16,31]. The blockade of I T in different models of pain produced antinociceptive effects acting at the periphery [34], but also at a spinal level [5,22,41,42].…”