Rescue of motoneuron and muscle afferent function in cats by regeneration into skin. II. Ia-motoneuron synapse.

1. In this study we describe application of high-frequency stimulation to the group Ia afferent-to-motoneuron synapse of cats to determine the extent to which regeneration of axotomized muscle afferents and motoneurons into skin or into muscle rescues their ability to generate excitatory postsynaptic potentials (EPSPs). 2. The medial gastrocnemius (MG) muscle nerve was transected and 1) left chronically axotomized, 2) cross-united to the caudal cutaneous sural (CCS) nerve, or 3) self-united. The ability of the operated MG muscle afferents to generate EPSPs in normal lateral gastrocnemius-soleus (LGS) motoneurons and of normal LGS muscle afferents to generate EPSPs in the operated MG motoneurons was tested 5 wk-30 mo later. 3. EPSPs were generated by bursts of 32 shocks at 167 Hz and averaged in register. In normal cats, EPSP amplitude decreased (negative modulation) during these bursts in type S motoneurons and could increase or decrease in type F motoneurons (positive or negative modulation). 4. After axotomy, EPSPs generated both in axotomized motoneurons and by axotomized afferents showed only negative modulation during the burst, and the negative modulation was much greater than in normal animals. Regeneration of the muscle nerve into skin significantly decreased the negative modulation relative to axotomy. Regeneration of the muscle nerve into muscle restored the EPSP modulation behaviors even more, to essentially normal values. 5. We conclude that the ability of muscle afferents to generate EPSPs in motoneurons in response to high-frequency stimulation, and the ability of motoneurons to express those EPSPs, are both influenced by the target innervated by those neurons. Synaptic efficacy is severely reduced by target deprivation (axotomy), partially rescued by cross-regeneration into skin, and rescued virtually completely by regeneration into the native muscle. We speculate on the role of target-derived neurotrophins in these effects.