Mechanical allodynia, a widespread pain symptom that still lacks effective therapy, is associated with the activation of a dorsally directed polysynaptic circuit within the spinal dorsal horn (SDH) or medullary dorsal horn (MDH), whereby tactile inputs into deep SDH/MDH can gain access to superficial SDH/MDH, eliciting pain. Inner lamina II (II;) interneurons expressing the y isoform of protein kinase C (PKCy+) are key elements for allodynia circuits, but how they operate is still unclear. Combining behavioral, ex vivo electrophysiological, and morphological approaches in an adult rat model of facial inflammatory pain (complete Freund's adjuvant, CFA), we show that the mechanical allodynia observed 1 h after CFA injection is associated with the following (1) sensitization (using ERK1/2 phosphorylation as a marker) and (2) reduced dendritic arborizations and enhanced spine density in exclusively PKCy1 interneurons, but (3) depolarized resting membrane potential (RMP) in all lamina II; PKCy+/PKCy interneurons. Blocking MDH 5HT2A receptors (5-HT2AR) prevents facial mechanical allodynia and associated changes in the morphology of PKCy+ interneurons, but not depolarized RMP in lamina Ilj interneurons. Finally, activation of MDH 5-HT2AR in naive animals is enough to reproduce the behavioral allodynia and morphological changes in PKCy1 interneurons, but not the electrophysiological changes in lamina II; interneurons, induced by facial inflammation. This suggests that inflammation-induced mechanical allodynia involves strong morphological reorganization of PKCy1 interneurons via 5-HT2aR activation that contributes to open the gate for transmission of innocuous mechanical inputs to superficial SDH/MDH pain circuitry. Preventing 5-HT2AR-induced structural plasticity in PKCy 1 interneurons might represent new avenues for the specific treatment of inflammation-induced mechanical hypersensitivity. [ABSTRACT FROM AUTHOR]