No abstract

Mechanical allodynia, a cardinal symptom of persistent pain, is associated with the unmasking of usually blocked local circuits within the superficial spinal or medullary dorsal horn (MDH) through which low-threshold mechanical inputs can gain access to the lamina I nociceptive output neurons. Specific interneurons located within inner lamina II (IIi) and expressing the gamma isoform of protein kinase C (PKCγ⁺) have been shown to be key elements for such circuits. However, their morphologic and electrophysiologic features are still unknown. Using whole-cell patch-clamp recordings and immunohistochemical techniques in slices of adult rat MDH, we characterized such lamina IIi PKCγ⁺ interneurons and compared them with neighboring PKCγ⁻ interneurons. Our results reveal that PKCγ⁺ interneurons display very specific activity and response properties. Compared with PKCγ⁻ interneurons, they exhibit a smaller membrane input resistance and rheobase, leading to a lower threshold for action potentials. Consistently, more than half of PKCγ⁺ interneurons respond with tonic firing to step current. They also receive a weaker excitatory synaptic drive. Most PKCγ⁺ interneurons express Ih currents. The neurites of PKCγ⁺ interneurons arborize extensively within lamina IIi, can spread dorsally into lamina IIo, but never reach lamina I. In addition, at least 2 morphologically and functionally different subpopulations of PKCγ⁺ interneurons can be identified: central and radial PKCγ⁺ interneurons. The former exhibit a lower membrane input resistance, rheobase and, thus, action potential threshold, and less PKCγ⁺ immunoreactivity than the latter. These 2 subpopulations might thus differently contribute to the gating of dorsally directed circuits within the MDH underlying mechanical allodynia.

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 i ) interneurons expressing the ␥ isoform of protein kinase C (PKC␥ ϩ ) 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 PKC␥ ϩ interneurons, but (3) depolarized resting membrane potential (RMP) in all lamina II i PKC␥ ϩ /PKC␥ Ϫ interneurons. Blocking MDH 5HT 2A receptors (5-HT 2A R) prevents facial mechanical allodynia and associated changes in the morphology of PKC␥ ϩ interneurons, but not depolarized RMP in lamina II i interneurons. Finally, activation of MDH 5-HT 2A R in naive animals is enough to reproduce the behavioral allodynia and morphological changes in PKC␥ ϩ interneurons, but not the electrophysiological changes in lamina II i interneurons, induced by facial inflammation. This suggests that inflammation-induced mechanical allodynia involves strong morphological reorganization of PKC␥ ϩ interneurons via 5-HT 2A R activation that contributes to open the gate for transmission of innocuous mechanical inputs to superficial SDH/MDH pain circuitry. Preventing 5-HT 2A R-induced structural plasticity in PKC␥ ϩ interneurons might represent new avenues for the specific treatment of inflammation-induced mechanical hypersensitivity.Inflammatory or neuropathic pain syndromes are characterized by pain hypersensitivity such as mechanical allodynia (pain induced by innocuous mechanical stimuli). It is generally assumed that mechanisms underlying mechanical allodynia, because they are rapid, must operate at only the level of functional reorganization of spinal or medullary dorsal horn (MDH) circuits. We discovered that facial inflammation-induced mechanical allodynia is associated with rapid and strong structural remodeling of specifically interneurons expressing the ␥ isoform of protein kinase C (PKC␥) within MDH inner lamina II. Moreover, we elucidated a 5-HT 2A receptor to PKC␥/ERK1/2 pathway leading to the behavioral allodynia and correlated morphological changes in PKC␥ interneurons. Therefore, descending 5-HT sensitize PKC␥ interneurons, a putative "gate" in allodynia circuits, via 5-HT 2A receptor-induced structural reorganization.