Excitatory Superficial Dorsal Horn Interneurons Are Functionally Heterogeneous and Required for the Full Behavioral Expression of Pain and Itch

To what extent dorsal horn interneurons contribute to the modality specific processing of pain and itch messages is not known. Here, we report that loxp/cre-mediated CNS deletion of TR4, a testicular orphan nuclear receptor, results in loss of many excitatory interneurons in the superficial dorsal horn but preservation of primary afferents and spinal projection neurons. The interneuron loss is associated with a near complete absence of supraspinally integrated pain and itch behaviors, elevated mechanical withdrawal thresholds and loss of nerve injury-induced mechanical hypersensitivity, but reflex responsiveness to noxious heat, nerve injury-induced heat hypersensitivity, and tissue injury-induced heat and mechanical hypersensitivity are intact. We conclude that different subsets of dorsal horn excitatory interneurons contribute to tissue and nerve injury-induced heat and mechanical pain and that the full expression of supraspinally mediated pain and itch behaviors cannot be generated solely by nociceptor and pruritoceptor activation of projection neurons; concurrent activation of excitatory interneurons is essential. •Deletion of TR4 results in loss of excitatory interneurons in spinal dorsal horn•Modality specific processing of pain and itch messages in dorsal horn•Different interneurons mediate reflex and supraspinally processed pain behaviors•Excitatory interneurons required for full expression of pain and itch behavior Wang et al. show that mice with a deletion of the TR4 gene lose dorsal horn excitatory interneurons and appear not to experience pain or itch. Thus, interneuron-mediated facilitation of spinal projection neurons is required to engage sufficiently forebrain structures underlying pain and itch.