Enhancement of Ia synaptic transmission to motoneurons after weight-lifting training in rats

Resistance training, involving repeated short-duration high-intensity exercises, evokes adaptive changes in muscle fibers and the electrophysiological properties of motoneurons (MNs). This study investigated whether weight-lifting exercises evoke changes in afferent synaptic transmission from muscle spindles to MNs. Male Wistar rats (<i>n</i> = 15) performed 5 wk of progressive weight-lifting training, whereas rats in the control group (<i>n</i> = 15) were restricted to standard cage activity. In vivo intracellular recordings were obtained under general anesthesia from two pools of MNs, innervating either the medial gastrocnemius (MG; <i>n</i> = 188) or lateral gastrocnemius and soleus (LG-Sol; <i>n</i> = 187) muscles. Basic membrane properties of MNs were measured, and monosynaptic Ia excitatory postsynaptic potentials (EPSPs) were recorded after stimulation of homonymous or heteronymous afferents from synergistic muscles. A generalized linear mixed model was used to analyze differences between groups. Homonymous EPSP amplitudes were an average of 28% higher in the weight-trained group compared with controls for both MG and LG-Sol MNs. Heteronymous EPSPs were larger for fast-type MNs in the weight-lifting group compared with controls by 45% and 46% for MG and LG-Sol MNs, respectively. Enhanced synaptic transmission from muscle spindles to MNs after weight-lifting training suggests synaptic plasticity; adaptive changes in MN membrane properties and presynaptic modulation of Ia fibers are also discussed. Adaptations were predominantly observed in fast MNs, which reflects their potent recruitment during weight-lifting training.<b>NEW & NOTEWORTHY</b> We demonstrate using electrophysiology that the synaptic transmission of sensory information from muscle spindles to spinal motoneurons is potentiated after 5 wk of weight-lifting training in rats. In trained animals, monosynaptic excitatory postsynaptic potentials (EPSPs) evoked from both homonymous and synergistic muscle afferents have higher amplitudes compared with controls. Adaptive changes are more pronounced in motoneurons innervating fast motor units, reflecting their more frequent recruitment during training.