Presynaptic and postsynaptic effects of local cathodal DC polarization within the spinal cord in anaesthetized animal preparations

The main aim of the study was to compare presynaptic and postsynaptic actions of direct current polarization in the spinal cord, focusing on DC effects on primary afferents and motoneurons. In order to reduce the directly affected spinal cord region, weak polarizing direct current (0.1- 0.3 μA) was applied locally in deeply anaesthetized cats and rats; within the hindlimb motor nuclei in the caudal lumbar segments, or in the dorsal horn within the terminal projection area of low threshold skin afferents. Changes in excitability of primary afferents activated by intraspinal stimuli (20-50 μA), were estimated using increases or decreases in compound action potentials recorded from the dorsal roots or peripheral nerves as their measure. Changes in postsynaptic actions of the afferents were assessed from intracellularly recorded monosynaptic EPSPs in hindlimb motoneurons and monosynaptic extracellular field potentials (evoked by group Ia afferents in motor nuclei, or by low threshold cutaneous afferents in the dorsal horn). The excitability of motoneurons activated by intraspinal stimuli was assessed using intracellular records or motoneuronal discharges recorded from a ventral root or a muscle nerve. Cathodal polarization was found to affect motoneurons and afferents providing input to them to a different extent. Excitability of both was markedly increased during DC application but post-polarization facilitation was found to involve presynaptic afferents and some of their postsynaptic actions but only negligibly motoneurons themselves. Taken together the results indicate that long-lasting post-polarization facilitation of spinal activity induced by locally applied cathodal current primarily reflects facilitation of synaptic transmission.