Low micromolar concentrations of 4-aminopyridine facilitate fictive locomotion expressed by the rat spinal cord in vitro

Abstract

Upregulating the operation of spinal locomotor networks is one mechanism to restore, at least partially, lesion-impaired locomotion. We investigated if the K + channel blocker 4-aminopyridine (4-AP) could facilitate spinal locomotor networks in addition to its well-known effect on motor nerve conduction. Fictive locomotor patterns were recorded from ventral roots (VRs) of the isolated spinal cord of the neonatal rat. 4-AP (0.1–50 μM) produced synchronous VR oscillations which did not develop into fictive locomotion. These oscillations had network origin, required intact glutamatergic transmission and were probably amplified via electrotonic coupling because of their depression by the selective gap junction blocker carbenoxolone. 4-AP (5 μM) slightly increased input resistance of lumbar motoneurons without affecting their action or resting potentials. Dorsal root (DR) evoked synaptic responses were enhanced (217±65%) by 5 μM 4-AP without changes in axon conduction. 4-AP (5 μM) accelerated fictive locomotion induced by N-methyl- d-aspartate (NMDA) and serotonin (5-HT) without altering cycle amplitude and facilitated the onset of fictive locomotion in the presence of sub-threshold concentrations of NMDA and 5-HT. Furthermore, in the presence of 4-AP, weak DR stimuli, previously insufficient to activate locomotor patterns, generated alternating VR discharges. Thus, although 4-AP per se could not directly activate the locomotor network of the spinal cord, it could strongly facilitate the locomotor program initiated by neurochemicals or electrical stimuli. These data suggest that the reported improvement by 4-AP in locomotor activity of spinal-injury patients may include activation of locomotor networks when low concentrations of this drug are administered in coincidence with appropriate stimuli.