Effects of M‐current modulators on the excitability of immature rat spinal sensory and motor neurones

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Abstract
M‐currents have been shown to control neuronal excitability in a variety of central and peripheral neurones. Here we studied the effects of specific M‐current modulators on the excitability of spinal neurones and their response to synaptic activation. Experiments were performed in vitro using the hemisected spinal cord from 7‐ to 11‐day‐old rats. Intracellular recordings were obtained from lumbar deep dorsal horn and motor neurones. Neuronal excitability was assessed by applying outward current pulses and synaptic responses were elicited by activation of a lumbar dorsal root. The M‐current antagonist 10,10‐bis(4‐pyridinylmethyl)‐9(10H)‐anthracenone (XE‐991) and the agonist retigabine were superfused at 10 µm. Retigabine produced hyperpolarization and a large decrease in the excitability of motor (7/7) and dorsal horn neurones (11/12). The effects of retigabine were fully reversed by XE‐991. XE‐991 induced depolarization of most neurones tested and a large increase in the excitability of motor neurones (7/7) but only a weak increase in the excitability of a proportion of dorsal horn neurones (4/10). The effects of XE‐991 were partly reversed by retigabine. Consistent with their effects on neuronal excitability, retigabine showed a general depressant effect on synaptic transmission, whereas XE‐991 showed the opposite tendency to potentiate responses to dorsal root stimulation, particularly in motor neurones. The results show that retigabine can depress spinal excitability and the transmission of nociceptive information. Results also indicate a post‐synaptic expression of functional M‐currents in most motor neurones and a considerable proportion of deep dorsal horn neurones.