Parallel pain processing in freely moving rats revealed by distributed neuron recording

The present study was designed to examine the possible differential roles of the medial and lateral pain systems in pain perception. We used a microwire array recording technique to record the pain-evoked neural activity of multiple neurons in freely moving rats. Noxious radiant heat was delivered to either hind-paw in a randomized order. A total of 256 single units were recorded in primary somatosensory cortex (SI), anterior cingulate cortex (ACC), and medial dorsal (MD) and ventral posterior (VP) thalamus during the painful stimulation. The results showed that SI neurons displayed a strong pain-related excitatory response with short duration and significant contralateral bias; VP had very similar functional patterns to that of SI. This suggested that SI, together with VP, participate in the processing of the sensory-discriminative aspect of pain. In contrast, ACC and MD shared common characteristics of moderate and longer-lasting increase of neural activity, bilateral receptive fields without contralateral preference, as well as the anticipatory response at the start of a painful stimulus, corresponding to the specific role of ACC and MD in the affective-motivational aspects of pain. The results provide an initial demonstration of distributed activity patterns within different pain systems in awake and freely moving rats, hence, providing confirmation of the existence of the dual pain pathways.