Severe spinal cord injuries that remove all supraspinal input to lumbosacral spinal circuits lead to permanent paralysis of the legs in adult rodents1–3
and humans. Nevertheless, networks of neurons in the lumbosacral spinal cord retain an intrinsic capability to oscillate and generate coordinated rhythmic motor outputs. Circuits underlying such rhythmic and oscillatory outputs are commonly referred to as central pattern generators (CPGs) and are found in all invertebrate and vertebrate animals4,5
. Although the anatomical architecture of locomotor CPGs remains poorly understood, especially in mammals5
, the functional phenomenon, central pattern generation, has been documented extensively. Indirect evidence suggests that CPGs are present in human spinal cord6,7
. These observations offer the possibility of directly accessing and activating spinal cord CPGs to facilitate locomotor recovery after a severe spinal cord injury (SCI).
Several experimental strategies have been tested to activate locomotor circuits in mammals after a complete spinal cord transection, including pharmacological treatments8–10
electrical stimulation, and motor training1,2,8,15,16
. Serotonin or agonists of 5-HT2A
receptors can activate the quiescent locomotor circuitry in neonatal rodent fictive locomotion preparations17,18
and can facilitate treadmill stepping with limited weight bearing in adult rats9
with SCI. Epidural electrical stimulation (EES) applied dorsally at the lumbar (L2)2,11
or sacral (S1)12,19
spinal segments induces rhythmic hindlimb movements2,12
. Locomotor training, notably in conjunction with pharmacological8,9
or electrical stimulation2
interventions, can promote use-dependent plastic changes in sensorimotor circuits below the injury16,20,21
that lead to specific improvements of stepping patterns. These interventions, however, have shown limited potential for promoting weight-bearing capacities and there have been few attempts to correlate the specific functional states induced pharmacologically9,10
or by locomotor training2,20
with distinct characteristics of stepping motor patterns. When studied in sufficient statistical detail, analyses of kinematics and electromyographic (EMG) features revealed that such induced spinal locomotion differed from voluntary stepping in many important aspects2,9,13
. In addition, it remains unknown whether lumbosacral neuronal networks in the absence of brain input could sustain full weight-bearing locomotion that resembles nondisabled stepping. Considering the diffusely distributed5
character of the spinal locomotor system, it is likely that multiple complementary approaches, both acute and chronic, would be required to attain the full possible expression of effective stepping in the absence of supraspinal input.
We tested the hypothesis that combinations of specific pharmacological and electrical stimulation interventions, together with locomotor training, may interact synergistically to activate and functionally remodel spinal locomotor circuits, possibly enabling a coordinated and context-dependent function of the paralyzed hindlimbs of adult rats after a complete spinal cord transection. We tested combinations of 5-HT2A and 5-HT1A/7 serotonin agonists and EES at two different positions distal to the lesion, each of which individually exerted some facilitating effects on hindlimb function. Using detailed kinematics, EMG and anatomical analyses, we found that such combinatorial interventions induced unique functional states that correlated with distinct patterns of locomotion in paralyzed rats. We demonstrate for the first time, to the best of our knowledge, the ability of rats with SCI to generate full weight-bearing bipedal treadmill locomotion that is almost indistinguishable from voluntary stepping recorded in the same rats prior to injury. We also found that sensory input determines the formation of adaptive motor patterns in the absence of supraspinal influences.