The effects of common input characteristics and discharge rate on synchronization in rat hypoglossal motoneurones

Synchronous discharges between a pair of concurrently active motoneurones are thought to arise from the spike-triggering effects of synaptic inputs shared by the pair. Although there are a number of quantitative indices that have been developed to estimate the strength of this common input, there is still some debate as to whether motoneurone discharge rate affects the values of these indices. The aim of the present study was to test the effects of motoneurone discharge rate on these synchronization indices using known common inputs. To achieve this aim we elicited repetitive discharge in rat hypoglossal motoneurones by combining a suprathreshold injected current step with superimposed noise to mimic the synaptic drive likely to occur during physiological activation. The amplitude of the current step was varied in different trials to achieve discharge rates from 5 to 22 Hz. We first examined the effect of discharge rate on the spike-triggering efficacy of individual EPSPs. Motoneurones were more responsive to large EPSPs delivered at a low rate when their background discharge rate was relatively low and the probability of the EPSPs evoking an extra spike decreased with increasing discharge rate. However, the opposite dependence was found for small, high-frequency EPSPs. We then compared the discharge records obtained in several trials in which the same EPSP train was applied repeatedly to the same cell firing at different background discharge rates. The effect of this 'common input' on motoneurone discharge probability was determined by compiling cross-correlation histograms (CCHists) between the discharges of the same cell at different times. The common inputs induced synchronous discharge that gave rise to large central peaks in the CCHists. The relationship between the discharge rate and the level of synchronization changed depending on the synchronization indices used and the amplitude of the common EPSPs. When large EPSPs were used as the common input, the normalized probability of synchronous spikes declined as the discharge rate increased, regardless of the method of normalization used. In contrast, when the common input was composed of a large number of small EPSPs, similar to that likely to occur during physiological activation of motoneurones, different synchronization indices exhibited a positive, a negative or no dependence on the background discharge rate. Indices based on normalizing the number of synchronous spikes by either the number of discharges in the lower frequency train (E), or by the total number of discharges in both trains (S) showed no dependence on background discharge rate and therefore may be the most suitable for quantifying motoneurone synchrony over a range of background discharge rates.