Currently available pain medications are limited by adverse side effects leading to enormous individual and socioeconomic costs. Therefore, the investigation of pathological receptor conformations and mechanisms involved in pain sensitization is urgently needed. I investigated the pain pathway from two perspectives. In the first part, I focused on mechanisms underlying the initiation of pain and sensitization. I concentrated on the identification of mechanisms that sensitize TRPV1. TRPV1 is an excitatory ion channel that plays a fundamental role in neuronal sensitization during tissue injury and inflammation. I found that the interaction of TRPV1 with other proteins like TRPA1 or ARMS leads to a cAMP and PKA dependent channel sensitization. The same signaling pathway is responsible for TRPV1-induced hyperalgesia during opioid withdrawal, leading to the conclusion that targeting PKA-induced TRPV1 sensitization could be a strategy to circumvent TRPV1 sensitization without direct TRPV1 blockade. In the second part, I concentrated on the investigation of inhibitory components of the pain pathway, particularly the opioid receptor system under pathological conditions. Results showed that decreased pH – a hallmark of tissue inflammation – can be used to design opioids that selectively activate opioid receptors under pathological conditions. Opioid ligands have to be protonated to bind and active their respective receptors. Classical opioids are protonated under both physiological and pathological conditions and therefore activate opioid receptors in both healthy and injured tissues. The reduction of the pKa of an opioid ligand close to the pH of inflamed tissue resulted in the selective activation of opioid receptors in injured, but not healthy environments. Our lead candidate NFEPP produced efficient injury-restricted analgesia in animal models of inflammatory, visceral, and neuropathic pain without inducing side effects like respiratory depression, sedation, constipation, or addiction.
