Multichannel electrophysiological recordings reveal independent modulatory actions of on-cells and off-cells in the rostral ventromedial medulla on pain processing

On-cells and Off-cells within the rostral ventromedial medulla (RVM) are critical neuronal populations involved in pain modulation. However, most previous electrophysiological studies used single-electrode recordings, which failed to reveal simultaneous activities of both neuronal types. Therefore, the potential correlations between On- and Off-cells remain controversial. To address this limitation, we employed multichannel electrode arrays to achieve synchronous recording of On- and Off-cell activity in the RVM under acute and chronic pain conditions in urethane-anesthetized mice. We further analyzed the millisecond-scale temporal sequence of their responses to noxious stimuli and applied cross-correlogram (CCG) analysis to examine the correlation between the firing activities of On- and Off-cells. Our results demonstrated that the electrode arrays successfully captured the synchronous activities of both neuronal types. These two neuronal types exhibited opposite stimulus-evoked firing patterns, with On-cells showing excitation and Off-cells inhibition. On- and Off-cells exhibited distinct baseline firing rates, which were differentially altered under chronic inflammatory pain conditions. The stimulus-evoked changes in both neuronal types were intensity-dependent. Under chronic pain conditions, these evoked responses became more pronounced and were consistent with nociceptive behaviors. Nevertheless, both neuronal types responded nearly simultaneously to the noxious stimulus. Following stimulus cessation, On-cells exhibited post-discharges, and Off-cells gradually resumed their activity only after On-cell firing terminated. Further CCG analysis of their firing patterns indicated the absence of direct monosynaptic connectivity. These findings suggest that On- and Off-cells do not exhibit direct monosynaptic connectivity, yet remain closely associated with nociceptive processes.