Tail Pinching-Induced Hindlimb Movements Are Suppressed by Clonidine in Spinal Cord Injured Mice

Experiments in completely spinal cord transected (Tx) cats have provided compelling evidence that clonidine combined with tail stimulation can promote locomotor function recovery. However, clonidine has generally failed to induce locomotor activity in other comparable animal models suggesting the existence of species- or condition-specific effects. This study aimed at investigating the effects of clonidine administered (0.25 or 5.0 mg/kg, i.p.) in mice during tail pinching in early (6-7 days post-Tx) or late (41-42 days post-Tx) paraplegic animals (Th9/10 level). Comparisons were made with the effects induced by 8-OH-DPAT (1.0 mg/kg, i.p.), a 5-HTIA/7 receptor agonist known to display prolocomotor effects. Clonidine with or without tail pinching failed to induce hind limb movements and even suppressed the frequency of spontaneously occurring nonlocomotor (NLM) and locomotor-like movements (LM) whereas tail pinching alone (prior to clonidine administration) increased the frequency of spontaneous movements specifically in late chronic animals. In turn, 8-OH-DPAT clearly induced hind limb movements that remained relatively unchanged during tail pinching. Altogether, the results suggest that the prolocomotor effects of clonidine reported elsewhere must depend upon stimuli or factors that remain to be identified.