Sustained L-type calcium currents in dissociated deep dorsal horn neurons of the rat: characteristics and modulation

Deep dorsal horn neurons present plateau properties involved in non-linear integration of nociceptive inputs, in the windup of the discharge, and in the expression of long-lasting afterdischarges. In vitro experiments using intracellular recordings in a slice preparation of the rat spinal cord have established that they are supported in part by voltage-dependent calcium currents, and positively modulated by metabotropic glutamate receptor activation. In the present study, whole-cell patch-clamp recordings in acutely isolated soma of dorsal horn neurons (n=48) were used to analyse the voltage-dependent calcium currents involved.Deep dorsal horn neurons expressed both inactivating and non-inactivating calcium currents with Ca(2+) or Ba(2+) used as a charge carrier. The non-inactivating component activated at intermediate threshold (-55mV), and was blocked mostly by nifedipine (61+/-6%). Although voltage-dependent facilitation of whole-cell calcium currents could be obtained by prepulses to +100mV, repetitive depolarization at potentials compatible with the plateau (-45mV and -10mV) failed to induce facilitation of calcium currents. No direct modulation of somatic calcium currents by application of (S)-3,5-dihydroxyphenylglycine, a selective group I metabotropic glutamate receptor agonist and 1S,3R-1-amino-1,3-cyclopentanedicarboxylic acid, a group I and II metabotropic glutamate receptor agonist, was found, while application of the metabotropic GABA(B) receptor agonist baclofen induced a significant decrease in calcium currents.Thus, the present voltage-clamp study shows that rat deep dorsal horn neurons express a non-inactivating, nifedipine sensitive, intermediate threshold (-55mV) calcium current which could provide the depolarizing drive to generate plateau potentials near threshold. Our results also indicate that calcium currents are not sensitized following repetitive stimulation, and not modulated by metabotropic glutamate receptor activation. They provide, however, the first evidence for a direct modulation of voltage-gated calcium channels in dorsal horn neurons by GABA(B) receptor activation, which may contribute to the mechanism of baclofen's antinociceptive activity.