The effect of axotomy on posttetanic potentiation of group Ia synapses in the cat

Posttetanic potentiation (PTP) of composite Ia excitatory postsynaptic potentials (EPSPs) has been studied in normal cat alpha-motoneurons and in motoneurons axotomized 2-3 wk earlier by ventral root section. The maximal amount of PTP of EPSP amplitude (expressed relative to unpotentiated amplitude) was considerably less in the axotomized population compared with the normal population. The decrease in PTP provoked by axotomy occurs in association with a postaxotomy increase of input resistance, the net effect being that PTP in axotomized cells was much the same as that observed by others in normal motoneurons possessing similarly high input resistance. In agreement with previous results, EPSP peak amplitudes were decreased after axotomy. This decrease seemed to be largely related to an absence of the largest EPSPs, since otherwise the EPSP distributions of normal and axotomized motoneurons showed considerable overlap. It is suggested that the observed decrease in PTP after axotomy is related to a change in synaptic release properties and not secondary to changes in the electrical properties of motoneurons. A previous analysis has suggested that axotomy causes an alteration of the distribution of passive electrical properties among motoneurons such that axotomized cells resemble normal high-resistance motoneurons. The present results suggest that axotomy may affect the distribution of Ia synaptic release properties in a similar manner, since PTP in axotomized motoneurons resembles that observed in normal high-resistance motoneurons.