NT-3 increases amplitude of EPSPs produced by axotomized group Ia afferents.

We tested the hypothesis that neurotrophin-3 (NT-3) in adult cats can rescue the central synapses made by muscle afferents from the effects of peripheral axotomy. The medial gastrocnemius (MG) muscle nerve in cats was axotomized and capped or axotomized and the distal end provided with either saline or NT-3 by mini-osmotic pump. Four to five weeks later monosynaptic excitatory postsynaptic potentials (EPSPs) elicited by electrical stimulation of the axotomized MG nerve were recorded in intact lateral gastrocnemius/soleus (LGS) motoneurons. The axotomized MG afferents without NT-3 treatment generated EPSPs averaging one-half of the amplitude of those generated by normal intact MG afferents. Axotomized MG afferents treated with NT-3 elicited EPSPs averaging 2.5 times normal amplitude and 5 times the amplitude of those from afferents axotomized but not treated. The very large EPSPs generated by NT-3-treated afferents remained as susceptible to depression during high-frequency stimulation (32 shocks at 167 Hz) as those elicited by untreated axotomized afferents. The arrival of the afferent volley of the cord dorsum potential and the onset of EPSPs were both delayed by axotomy of the group Ia afferents and were both restored by exposure to NT-3. This result suggests that the conduction velocity and thus the caliber of group Ia afferents are also controlled by NT-3. We conclude that the neurotrophin NT-3 has a continuing role in the maintenance of physiological function of muscle afferents in adult mammals.