Pruriceptive spinothalamic tract neurons: Physiological properties

21 22 Itch of peripheral origin requires information transfer from the spinal cord to the brain for 23 perception. Here, primate spinothalamic tract (STT) neurons from lumbar spinal cord were 24 functionally characterized using in vivo electrophysiology to determine the role of these cells in 25 the transmission of pruriceptive information. 111 STT neurons were identified by antidromic 26 stimulation and then recorded while histamine and cowhage (a non-histaminergic pruritogen) 27 were sequentially applied to the cutaneous receptive field of each cell. 20% of STT neurons 28 responded to histamine, 13% responded to cowhage, and 2% responded to both. All 29 pruriceptive STT neurons were mechanically sensitive and additionally responded to heat, 30 intradermal capsaicin, or both. STT neurons located in the superficial dorsal horn responded 31 with greater discharge and longer duration to pruritogens than did STT neurons located in the 32 deep dorsal horn. Pruriceptive STT neurons discharged in a bursting pattern in response to the 33 activating pruritogen and to capsaicin. Micro-antidromic mapping was used to determine the 34 zone of termination for pruriceptive STT axons within the thalamus. Axons from histamine35 responsive and cowhage-responsive STT neurons terminated in several thalamic nuclei 36 including the ventral posterior lateral, ventral posterior inferior, and posterior nuclei. Axons from 37 cowhage-responsive neurons were additionally found to terminate in the suprageniculate and 38 medial geniculate nuclei. Histamine-responsive STT neurons were sensitized to gentle stroking 39 of the receptive field after the response to histamine, suggesting a spinal mechanism for 40 alloknesis. The results show that pruriceptive information is encoded by polymodal STT neurons 41 in histaminergic or non-histaminergic pathways and transmitted to the ventrobasal complex and 42 posterior thalamus in primates. 43 44