Ia excitatory postsynaptic potentials are potentiated in slow-type motoneurons after a 5-week treadmill endurance training in rats

The electrophysiological properties of motoneurons (MNs) are modified in response to motor activity; however, it remains unknown whether adaptations also involve peripheral input from muscle spindles, which are a potent source of excitatory connections to MNs. The aim of this study was to investigate whether endurance training evoked adaptive changes in Ia afferent synaptic transmission to MNs. Male Wistar rats (n = 15) underwent a 5-week treadmill running course. An electrophysiological experiment was performed on each rat under general anesthesia one day after the final training session. A corresponding control group of untrained rats (n = 15) was included. Lumbar spinal MNs innervating the medial gastrocnemius (MG, n = 178) or lateral gastrocnemius and soleus (LG-Sol, n = 186) muscles were investigated intracellularly to record passive membrane properties and parameters of monosynaptic Ia excitatory postsynaptic potentials (EPSPs) evoked from homonymous or heteronymous afferents from synergistic muscles. Between-group comparisons were performed using a generalized linear mixed model. Potentiation of Ia heteronymous EPSPs was observed in slow-type MNs. EPSPs were on average larger by 34% in MG and 44% in LG-Sol MNs; positive correlations of EPSP amplitudes with input resistance were observed. No significant changes were observed for parameters of homonymous EPSPs. The adaptations to endurance training may be attributable to a greater size and/or number of Ia synapses on MNs or altered levels of presynaptic inhibition of Ia fibers. Selective enhancement of synaptic transmission to slow MNs is likely related to the greater contribution of slow motor units during treadmill exercises.