Spinal Caspase-6 Contributes to Intrathecal Morphine-induced Acute Itch and Contact Dermatitis-induced Chronic Itch Through Regulating the Phosphorylation of Protein Kinase Mζ in Mice

Abstract

Patients receiving neuraxial treatment with morphine for pain relief often experience a distressing pruritus. Neuroinflammation-mediated plasticity of sensory synapses in the spinal cord is critical for the development of pain and itch. Caspase-6, as an intracellular cysteine protease, is capable of inducing central nociceptive sensitization through regulating synaptic transmission and plasticity. Given the tight interaction between protein kinase Mζ (PKMζ) and excitatory synaptic plasticity, this pre-clinical study investigates whether caspase-6 contributes to morphine-induced itch and chronic itch via PKMζ. Intrathecal morphine and contact dermatitis were used to cause pruritus in mice. Morphine antinociception, itch-induced scratching behaviors, spinal activity of caspase-6, and phosphorylation of PKMζ and ERK were examined. Caspase-6 inhibitor Z-VEID-FMK, exogenous caspase-6 and PKMζ inhibitor ZIP were utilized to reveal the mechanisms and prevention of itch. Herein, we report that morphine induces significant scratching behaviors, which is accompanied by an increase in spinal caspase-6 cleavage and PKMζ phosphorylation (but not expression). Intrathecal injection of Z-VEID-FMK drastically reduces morphine-induced scratch bouts and spinal phosphorylation of PKMζ, without abolishing morphine analgesia. Moreover, intrathecal strategies of ZIP dose-dependently reduce morphine-induced itch-like behaviors. Spinal phosphorylation of ERK following neuraxial morphine is down-regulated by ZIP therapy. Recombinant caspase-6 directly exhibits scratching behaviors and spinal phosphorylation of ERK, which is compensated by PKMζ inhibition. Also, spinal inhibition of caspase-6 and PKMζ reduces the generation and maintenance of dermatitis-induced chronic itch. Together, these findings demonstrate that spinal caspase-6 modulation of PKMζ phosphorylation is important in the development of morphine-induced itch and dermatitis-induced itch in mice.