Changes in Activity of the Same Thalamic Neurons to Repeated Nociception in Behaving Mice

The sensory thalamus has been reported to play a key role in central pain sensory modulation and processing, but its response to repeated nociception at thalamic level is not well known. Current study investigated thalamic response to repeated nociception by recording and comparing the activity of the same thalamic neuron during the 1(st) and 2(nd) formalin injection induced nociception, with a week interval between injections, in awake and behaving mice. Behaviorally, the 2(nd) injection induced greater nociceptive responses than the 1(st). Thalamic activity mirrored these behavioral changes with greater firing rate during the 2(nd) injection. Analysis of tonic and burst firing, characteristic firing pattern of thalamic neurons, revealed that tonic firing activity was potentiated while burst firing activity was not significantly changed by the 2(nd) injection relative to the 1(st). Likewise, burst firing property changes, which has been consistently associated with different phases of nociception, were not induced by the 2(nd) injection. Overall, data suggest that repeated nociception potentiated responsiveness of thalamic neurons and confirmed that tonic firing transmits nociceptive signals.