The effects of clonidine and yohimbine on locomotion and cutaneous reflexes in the adult chronic spinal cat.

The effects on the locomotor pattern of a noradrenergic agonist (clonidine) and an antagonist (yohimbine) were studied in 3 adult chronic spinal cats walking on a treadmill. In the early post-transection period, when the cat walked mainly on the tip of its feet, without supporting its own weight, it was observed that clonidine (150 micrograms/kg) could induce a good bilateral foot placement and intermittent complete weight support. When clonidine was given 1-3 months following the transection, at a time when the spinal cats had a stable and regular locomotor performance, the step length increased markedly, especially at low speeds. This was associated with an increase in the duration of the flexor and extensor bursts, as well as an increase of the angular excursion of all joints. These effects, seen during forward locomotion, were also observed during backward locomotion. In addition, the latter was more easily elicited after clonidine. Yohimbine (1.5-3 mg/kg) partially antagonized these effects. The threshold current needed to elicit a small flexion reflex through wires implanted in the dorsum of the paw was 2-3 times higher after clonidine. Trains of shocks in the animal, standing quietly, did not induce the prolonged late discharges normally found in acute spinal cats. Fast paw shaking, elicited by dipping one paw in water, was abolished by clonidine and reappeared after yohimbine. These results indicate that noradrenergic drugs may influence both spinal locomotion and the excitability of cutaneous reflexes. This class of substances could thus play a useful role in the recovery and/or maintenance of locomotor functions after spinal trauma.