Electrophysiological characterization of spino-sciatic and cortico-sciatic associative plasticity: modulation by trans-spinal direct current and effects on recovery after spinal cord injury in mice.

Associative stimulation causes enduring changes in the nervous system based on the Hebbian concept of spike-timing-dependent plasticity. The present study aimed to characterize the immediate and long-term electrophysiological effects of associative stimulation at the level of spinal cord and to test how trans-spinal direct current stimulation (tsDC) modulates associative plasticity. The effect of combined associative stimulation and tsDC on locomotor recovery was tested in a unilateral model of spinal cord injury (SCI). Two associative protocols were tested: (1) spino-sciatic associative (SSA) protocol, in which the first stimulus originated from the sciatic nerve and the second from the spinal cord; and (2) cortico-sciatic associative (CSA) protocol, in which the first stimulus originated from the sciatic nerve and the second from the motor cortex. In addition, those two protocols were repeated in combination with cathodal tsDC application. SSA and CSA stimulation produced immediate enhancement of spinal and cortical outputs, respectively, depending on the duration of the interstimulus interval. Repetitive SSA or CSA stimulation produced long-term potentiation of spinal and cortical outputs, respectively. Applying tsDC during SSA or CSA stimulation markedly enhanced their immediate and long-term effects. In behaving mice with unilateral SCI, four consecutive 20 min sessions of CSA + tsDC markedly reduced error rate in a horizontal ladder-walking test. Thus, this form of artificially enhanced associative connection can be translated into a form of motor relearning that does not depend on practice or experience.