M-currents constitute a unique effector system to control neuronal excitability due to their voltage and ligand sensitivities. We have used M-channel openers and blockers to study the possible role of these currents in the modulation of nociceptive and non-nociceptive spinal reflexes using the in vitro hemisected cord.

Spinal cords were dissected from urethane-anaesthetised (2 g kg^-1 I.P.) Wistar rat pups, hemisected and superfused with standard oxygenated ACSF. Animals were then killed with an overdose of urethane. Simultaneous AC and DC recordings of responses to electrical stimulation of the L5 dorsal root were obtained from the ventral root using suction electrodes. The effects of bath-applied retigabine, an M-current opener (Rundfelt, 1997), and XE-991, an M-current blocker (Zaczek et al. 1998), were tested on responses to single (and repetitive) stimulation at A- and C-fibre intensities. Quantification of signals was made according to previous reports (Hedo & Lopez-Garcia, 2001). Data are expressed as means ± S.E.M.

Retigabine applied at 1, 3 and 10 µM (n = 5) produced a concentration-dependent inhibition of the integrated area of responses to single and repetitive C-fibre intensity stimulation of the dorsal root as well as of the action potential counts elicited by repetitive stimulation (overall ANOVA, P ²le³ 0.001). The minimum effective concentration was 3 µM (Dunnet’s post hoc test, P ²le³ 0.05). Retigabine applied at 10 µM did not change the monosynaptic reflex evoked by C-fibre intensity stimuli but increased the threshold intensity to obtain a ventral root response from 27 ± 5 to 38 ± 4 µA (n = 6; Student’s paired t test, P ²le³ 0.01). Superfusion of XE-991 (n = 8) produced an enhancement of spinal reflexes elicited by single and repetitive C-fibre stimulation which was reflected in an increased integrated area and in a larger number of action potential counts (minimum effective concentration was 10 µM, Dunnet’s post hoc test, P ²le³ 0.05). XE-991 applied at 10 µM reversed the inhibitory effects of retigabine 3 µM (n = 3).

These observations indicate the presence of functional M-currents within the mammalian spinal cord and suggest that their modulation can be relevant to controlling the flow of nociceptive signals through spinal circuits.

This work was supported by the Spanish Ministry of Science and Technology (SAF-2000-0199), the Madrid Regional Government (Contrato Programa, Principal investigator: Dr F. Cervero) and Instituto UPSA del Dolor.