Synaptic currents at interpositorubral and corticorubral excitatory synapses measured by a new iterative single-electrode voltage-clamp method.

A new iterative single-electrode voltage clamp method was applied to the measurement of synaptic currents in the red nucleus (RN) neuron of the cat. Voltage clamp was attained within 10 repetitions with great stability and the new algorithm was demonstrated to be superior to the original algorithm of iterative voltage clamp. With a conventional microelectrode, it was possible to measure the synaptic current with the time resolution of 50 microseconds. The synaptic currents evoked by stimulation of the contralateral interpositus nucleus (IP) had time-to-peak ranging from 200 to 540 microseconds and fitted well to alpha functions. Corticorubral (CR) synaptic current was also measured by making use of synaptic plasticity. The stimulation of the ipsilateral cerebral peduncle in cats with chronic lesion of the contralateral IP evoked fast rising EPSPs, as reported previously. The CR-EPSPs with times-to-peak less than 1 ms were subjected to voltage clamp. The CR synaptic currents had times-to-peak ranging from 350 to 880 microseconds. Since most of the interpositorubral (IR) synapses and a part of the CR synapses in IP-lesioned cats are situated on the somatic membrane of RN neurons and some of the CR synaptic currents were as rapid as the IR synaptic currents, the observed synaptic currents evoked by stimulation of the IP and those of the fast-rising CR-EPSPs were taken to originate from the synaptic membrane under space-clamp, i.e. soma. The present study provided additional evidence for the sprouting of the CR fibers as well as the time course of the synaptic current at the dendritic synapses remote from the soma, for the first time.