Responses of neurons in the primary somatosensory cortex to itch- and pain-producing stimuli in rats.

Understanding of cortical encoding of itch is limited. Injection of pruritogens and algogens into the skin of the cheek produces distinct behaviors, making the rodent cheek a useful model for understanding mechanisms of itch and pain. We examined responses of neurons in the primary somatosensory cortex by application of mechanical stimuli (brush, pressure, and pinch) and stimulations with intradermal injections of pruritic and algesic chemical of receptive fields located on the skin of the cheek in urethane-anesthetized rats. Stimuli included chloroquine, serotonin, β-alanine, histamine, capsaicin, and mustard oil. All 33 neurons studied were excited by noxious mechanical stimuli applied to the cheek. Based on mechanical stimulation most neurons were functionally classified as high threshold. Of 31 neurons tested for response to chemical stimuli, 84% were activated by one or more pruritogens/partial pruritogens. No cells were activated by all five substances. Histamine activated the greatest percentage of neurons and evoked the greatest mean discharge. Importantly, no cells were excited exclusively by pruritogens or partial pruritogens. The recording sites of all neurons that responded to chemical stimuli applied to the cheek were located in the dysgranular zone (DZ) and in deep laminae of the medial border of the vibrissal barrel fields (VBF). Therefore, neurons in the DZ/VBF of rats encode mechanical and chemical pruritogens and algogens. This cortical region appears to contain primarily nociceptive neurons as defined by responses to noxious pinching of the skin. Its role in encoding itch and pain from the cheek of the face needs further study.