Effects of common excitatory and inhibitory inputs on motoneuron synchronization

We compared the effects of common excitatory and inhibitory inputs on moton euron synchronization by simulating synaptic inputs with injected current t ransients. We elicited repetitive discharge in hypoglossal motoneurons reco rded in slices of rat brain stem using a combination of a suprathreshold in jected current step with superimposed noise to mimic the synaptic drive lik ely to occur during physiological activation. The effects of common inputs to motoneurons were simulated by the addition of a waveform composed of fro m 6 to 300 trains of current transients designed to mimic excitatory and/or inhibitory synaptic currents. We compared the discharge records obtained i n several trials in which the same "common input" waveform was applied repe atedly in the presence of different background noise waveforms. The effects of the common input on motoneuron discharge probability and discharge rate were determined by compiling a cross-correlation histogram (CCHist) and a perispike frequencygram (PSFreq) between the discharges of the same cell at different times. Both excitatory and inhibitory common inputs induced sync hronous discharge that was evident by a large central peak in the CCHist. T he CCHists produced by common excitatory inputs were characterized by large r and narrower central peaks than those generated by common inhibitory inpu ts. The PSFreqs produced by common excitatory inputs indicated an increase in the discharge rate of motoneurons around time 0 that coincided with the narrow and large central peak in the CCHist. On the other hand, inhibitory inputs often generated very little, if any, change in the discharge rate ar ound time 0 corresponding with the small and wide central peak in the CCHis t. These results suggest that the CCHist indicates the effective strength o f the net common input but not its sign. Although correlated changes in dis charge rate are often quite different for net excitatory and inhibitory com mon input, except in some restricted conditions, the PSFreq analysis also c annot be used to unambiguously distinguish net excitation from net inhibiti on.