Spinal Cats on the Treadmill: Changes in Load Pathways

Abstract

Treadmill training and clonidine, an ␣-2 noradrenergic agonist, have been shown to improve locomotion after spinal cord injury. We speculate that transmission in load pathways, which are involved in body support during stance, is specifically modified by training. This was evaluated by comparing two groups of spinal cats; one group (n ϭ 11) was trained to walk until full-weight-bearing (3-4 weeks), and the other (shams; n ϭ 7) was not. During an acute experiment, changes in group I pathways, monosynaptic excitation, disynaptic inhibition, and polysynaptic excitation were investigated by measuring the response amplitude in extensor motoneurons before and after clonidine injection. Monosynaptic excitation was not modified by clonidine but was decreased significantly by training. Disynaptic inhibition was significantly decreased by clonidine in both groups, but more significantly in trained cats, and significantly reduced by training after clonidine. Also, clonidine could reverse group IB inhibition into polysynaptic excitation in both groups but more frequently in trained cats. We also investigated whether fictive stepping revealed additional changes. In trained cats, the phase-dependent modulation of all three responses was similar to patterns reported previously, but in shams, modulation of monosynaptic and polysynaptic responses was not. Overall, training appears to decrease monosynaptic excitation and enhance the effects of clonidine in the reduction of disynaptic inhibition and reversal to polysynaptic excitation. Because it is believed that polysynaptic excitatory group I pathways transmit locomotor drive to extensor motoneurons, we suggest that the latter changes would facilitate the recruitment of extensor muscles for recovering weight-bearing during stepping.

Step Training-Dependent Plasticity in Spinal Cutaneous Pathways

“…Moreover, modified properties of motoneurons could not explain increased monosynaptic reflexes in chronic spinal animals Hochman and McCrea, 1994). In this study, training did not modify AHP duration, which varies with input resistance and membrane time constant (Gustafsson and Pinter, 1984;Côté et al, 2003). Moreover, a general change in membrane responsiveness is unlikely to explain the simultaneous increase in R1 amplitude and decrease in R3 amplitude observed in the same motoneuron (Table 2).…”

“…In both humans and cats recovering from SCI, progressively increasing weight bearing improves stepping ability Edgerton et al, 1992;Harkema et al, 1997). Moreover, we recently showed that step training modified transmission from group I afferents of extensors in spinal cats (Côté et al, 2003). However, cutaneous afferents are also activated by locomotor movements and may participate in recovery.…”

“…After administration of preoperative medication, the cats were anesthetized (2% isoflurane; Abbott Laboratories, Montréal, Québec, Canada) and spinalized at T 13 under aseptic conditions. Protocols for spinalization procedures and subsequent postoperative care were analogous to those described previously (Chau et al, 1998a;Côté et al, 2003). A patch of fentanyl (Duragesic; 25 g; JanssenOrtho, Markham, Ontario, Canada) was sutured on the back of the cat for continuous and stable delivery of analgesic over a 2 d period.…”

“…It is remarkable that clonidine modified pathways mostly in shams (five of seven). This is surprising because we expected training to favor the responsiveness of spinal networks to clonidine (Côté et al, 2003). An alternative interpretation for these findings is that both training and clonidine target the same pathways so that clonidine only added a small effect because training already decreased cutaneous transmission.…”

Spinal Cord Injury: Time to Move?

“…As seen in previous sections of this review (e.g., the section by Schwab), such neuroplasticity may occur via regeneration of damaged fibers or sprouting of undamaged fibers (Raineteau and Schwab, 2001;Fouad et al, 2001;Raineteau et al, 2002;Bareyre et al, 2004;Ballermann and Fouad, 2006). Neuroplastic changes can also be manifest in the spinal cord itself below the lesion (Grillner, 1981;Rossignol, 1996Rossignol, , 2006Côté et al, 2003;Côté and Gossard, 2004;Frigon and Rossignol, 2006;Rossignol et al, 2006).…”