Structural analysis of TrkA mutations in patients with congenital insensitivity to pain reveals PLCγ as an analgesic drug target.

Chronic pain is a major health issue, and the search for new analgesics has become increasingly important because of the addictive properties and unwanted side effects of opioids. To explore potentially new drug targets, we investigated mutations in the NTRK1 gene found in individuals with congenital insensitivity to pain with anhidrosis (CIPA). NTRK1 encodes tropomyosin receptor kinase A (TrkA), the receptor for nerve growth factor (NGF) and that contributes to nociception. Molecular modeling and biochemical analysis identified mutations that decreased the interaction between TrkA and one of its substrates and signaling effectors, phospholipase Cγ (PLCγ). We developed a cell-permeable phosphopeptide derived from TrkA (TAT-pQYP) that bound the Src homology domain 2 (SH2) of PLCγ. In HEK-293T cells, TAT-pQYP inhibited the binding of heterologously expressed TrkA to PLCγ and decreased NGF-induced, TrkA-mediated PLCγ activation and signaling. In mice, intraplantar administration of TAT-pQYP decreased mechanical sensitivity in an inflammatory pain model, suggesting that targeting this interaction may be analgesic. The findings demonstrate a strategy to identify new targets for pain relief by analyzing the signaling pathways that are perturbed in CIPA.

Electrical stimulation of the posterior insula induces mechanical analgesia in a rodent model of neuropathic pain by modulating GABAergic signaling and activity in the pain circuitry.

The insula has emerged as a critical target for electrical stimulation since it influences pathological pain states. We investigated the effects of repetitive electrical stimulation of the insular cortex (ESI) on mechanical nociception, and general locomotor activity in rats subjected to chronic constriction injury (CCI) of the sciatic nerve. We also studied neuroplastic changes in central pain areas and the involvement of GABAergic signaling on ESI effects. CCI rats had electrodes implanted in the left agranular posterior insular cortex (pIC), and mechanical sensitivity was evaluated before and after one or five daily consecutive ESIs (15 min each, 60 Hz, 210 µs, 1 V). Five ESIs (repetitive ESI) induced sustained mechanical antinociception from the first to the last behavioral assessment without interfering with locomotor activity. A marked increase in Fos immunoreactivity in pIC and a decrease in the anterior and mid-cingulate cortex, periaqueductal gray and hippocampus were noticed after five ESIs. The intrathecal administration of the GABAA receptor antagonist bicuculline methiodide reversed the stimulation-induced antinociception after five ESIs. ESI increased GAD65 levels in pIC but did not interfere with GABA, glutamate or glycine levels. No changes in GFAP immunoreactivity were found in this work. Altogether, the results indicate the efficacy of repetitive ESI for the treatment of experimental neuropathic pain and suggest a potential influence of pIC in regulating pain pathways partially through modulating GABAergic signaling.