Spontaneous bursting and rhythmic activity in the cuneate nucleus of anaesthetized rats

Spontaneous and rhythmic neuronal activity in dorsal column nuclei has long been identified in anesthetized cats. Here, we have studied the spontaneous behavior of cuneate cells in anesthetized rats through extracellular recording, showing that most cuneate neurones recorded (155 of 185) fired spontaneously. Overall, 74% of these spontaneously firing neurones were single-spiking and 26% were bursting. Cells were considered "bursting" when more than 50% of the spontaneous spikes belonged to bursts. Nevertheless, occasional bursts were seen in 33% of spontaneous cuneate cells which were classified as single-spiking. Rhythmic firing was observed in about 14% of both spontaneously bursting and single-spiking cells, and these cells were located close to the obex (+/-0.5 mm). Although the spike-frequency was mostly in the range 0-15 spikes/s, spontaneous rhythmic activity was circumscribed mainly to the alpha/beta-like range, both in single-spiking (26.1+/-3.6 Hz, n=16) and bursting cells (19.5+/-4.1 Hz, n=6). Lemniscal stimulation often activated several antidromic units with the same latency. About 65% of cuneolemniscal cells were spontaneously active and of these, 83% were single-spiking and 11% rhythmic (all single-spiking). In cells that were not antidromically activated from the medial lemniscus, short latency orthodromic responses consistent with excitation by recurrent lemniscal collaterals were often observed following lemniscal activation. Interestingly, only cells completely unresponsive to lemniscal stimulation showed rhythmic bursting. Most spontaneous cells responded with a burst to natural receptive field stimulation, while rhythmic cells became temporally arrhythmic. These results demonstrate, for the first time, that rat cuneate neurones can fire bursts spontaneously. Besides, this bursting activity can be rhythmic. These two properties, and the fact that groups of cuneolemniscal cells share the same conduction velocity, probably imply the reinforcement of temporal and spatial summation at their targets when they are synchronously recruited by the stimulation of overlapping receptive fields.