Different effects of spinalization and locomotor training of spinal animals on cholinergic innervation of the soleus and tibialis anterior motoneurons

Cholinergic input modulates excitability of motoneurons and plays an important role in the control of locomotion in both intact and spinalized animals. However, spinal cord transection in adult rats affects cholinergic innervation of only some hindlimb motoneurons, suggesting that specificity of this response is related to functional differences between motoneurons. Our aim was therefore to compare cholinergic input to motoneurons innervating the soleus (Sol) and tibialis anterior (TA) motoneurons following spinal cord transection at a low‐thoracic level. The second aim was to investigate whether deficits in cholinergic input to these motoneurons could be modified by locomotor training. The Sol and TA motoneurons were identified by retrograde labelling with fluorescent dyes injected intramuscularly. Cholinergic terminals were detected using anti‐vesicular acetylcholine transporter (VAChT) antibody. Overall innervation of motoneurons was evaluated with anti‐synaptophysin antibody. After spinalization we found a decrease in the number of VAChT‐positive boutons apposing perikarya of the Sol (to 49%) but not TA motoneurons. Locomotor training, resulting in moderate functional improvement, partly reduced the deficit in cholinergic innervation of Sol motoneurons by increasing the number of VAChT‐positive boutons. However, the optical density of VAChT‐positive boutons terminating on various motoneurons, which decreased after spinalization, continued to decrease despite the training, suggesting an impairment of acetylcholine availability in the terminals. Different effects of spinal cord transection on cholinergic innervation of motoneurons controlling the ankle extensor and flexor muscles point to different functional states of these muscles in paraplegia as a possible source of activity‐dependent signaling regulating cholinergic input to the motoneurons. Complete transection of the spinal cord causes pronounced decrease in the number of VAChT‐positive boutons on the soma of the soleus motoneurons but not on tibialis anterior motoneurons. This distinct effect points to different functional states of the ankle extensor and flexor muscles as a possible source of activity‐dependent signaling regulating this input. Importantly, locomotor training causes an increase of the VAChT‐positive boutons reducing their deficit on soleus motoneurons.