Early pre- and postsynaptic decrease in glutamatergic and cholinergic signaling after spinalization is not modified when stimulating proprioceptive input to the ankle extensor α-motoneurons: Anatomical and neurochemical study

Alpha-motoneurons (MNs) innervating ankle extensor muscles show reduced peripheral inputs from Ia proprioceptive afferents and cholinergic afferents after chronic spinalization (SCT). That phenomenon is not observed on ankle flexor MNs, indicating a smaller vulnerability of the latter MNs circuit to SCT. Locomotor training of spinal rats which partially restored those inputs to extensor MNs tended to hyper innervate flexor MNs, disclosing a need for selective approaches. In rats with intact spinal cord 7-days of low-threshold proprioceptive stimulation of the tibial nerve enriched glutamatergic Ia and cholinergic innervation of lateral gastrocnemius (LG) MNs, suggesting usefulness of selective stimulation for restoration of inputs to extensor MNs after SCT. Accordingly, to examine its effectiveness after SCT, tibial nerves and soleus muscles were implanted bilaterally, and for MN identification fluorescence tracers to LG and tibialis anterior (TA) muscles were injected two weeks prior to spinalization. Stimulation of tibial nerve, controlled by H-reflex recorded in the soleus muscle, started on the third post-SCT day and continued for 7 days. Nine days post-SCT the number and volume of glutamatergic Ia and of cholinergic C-boutons on LG MNs was decreased, but stimulation affected neither of them. Postsynaptically, a threefold decrease of NMDAR NR1 subunit and thirtyfold decrease of M2 muscarinic receptor transcripts caused by SCT were not counteracted by stimulation, whereas a threefold decrease of AMPAR GluR2 subunit tended to deepen after stimulation. We conclude that LG MNs, supported with proprioceptive stimuli after SCT, do not transcribe the perceived cues into compensatory response at the transcriptional level in the early post-SCT period.