Role of relaxin-3/RXFP3 forebrain networks in the descending control of pain in the mouse.

Pain is a complex biological phenomenon that is beneficial and necessary for our survival, warning of changes and hazards in the environment that compromise optimal function. However, continuous activation of pain signalling systems results in maladaptive changes characterized by altered tissue and organ structure and activity. Pain lasting more than 3 months is termed chronic pain and it is under these conditions that it becomes a major burden for affected individuals. Chronic pain is also accompanied by serious social and economic burdens, making research in this field a high priority globally.The central nervous system (CNS) acts as a major control centre for nociceptive signal transmission, decoding pain for its sensory-discriminative and aversive components, i.e., deciphering the type of pain (pinch, burn, etc.), its location in the body, and its associated hedonic value, respectively. Once the information is decrypted, neural signals from the brain to the periphery act in accordance with the provided stimulus based on past and current experiences.The nociceptive signal is modulated at every step of this process by an abundance of neurochemical signals, including neuropeptides. The presence of neuropeptides and/or their receptors in areas linked to nociceptive processing and transmission suggests putative roles for these systems in the control of nociception.Relaxin-3 is a neuropeptide that is mainly synthesized in a hindbrain region known as the nucleus incertus (NI). Since its discovery, relaxin-3 has been linked to the control of a wide range of behaviours such as anxiety-like behaviours, arousal, and reward-seeking, through activation of the Gi/o-protein-coupled receptor, RXFP3. These putative roles of relaxin-3/RXFP3 signalling suggest a possible link between RXFP3 activation and modulation of pain sensitivity.Therefore, my initial studies assessed the effect of RXFP3 activation and inhibition on mechanical and thermal pain sensitivity in normal and persistent pain conditions. These studies demonstrated that central administration of the RXFP3 agonist peptide, RXFP3-A2, via intracerebroventricular (icv) injection, produced relief of mechanical, but not thermal, pain sensation. Moreover, icv injection of the RXFP3 antagonist peptide, R3(B1-22)R, augmented mechanical and thermal pain sensitivity. These data suggest that relaxin-3/RXFP3 signalling has a tonic action in maintaining mechanical and thermal pain thresholds, and the potential for activation of RXFP3 to produce pain relief.Additionally, I examined the possible involvement of different brain areas in these effects, by assessing the number of c-Fos-positive cell nuclei under different conditions. However, these studies revealed no difference in the number of c-Fos-positive cell nuclei or staining intensity in the vehicle- and RXFP3 agonist-treated mice.Further characterization of pain circuit-related brain areas using multiplex in situ hybridization revealed that RXFP3 mRNA is expressed within discrete populations of neurons in these areas. I also evaluated possible co-expression of RXFP3 mRNA with somatostatin and parvalbumin mRNA, and determined the relative proportion of RXFP3 mRNA expression in populations of neurons that express these transcripts.Finally, I examined the possible presence of comorbid anxiety in mice subjected to the persistent pain protocol. However, anxiety-like behaviour was not altered in mice with persistent hindpaw pain, suggesting this model does not display produce anxiety, and that effects of RXFP3 modulation observed specifically targeted nociceptive transmission.Overall, my findings implicate the relaxin-3/RXFP3 system in control of pain transmission, providing new opportunities for the development of therapeutic tools for pain management, by targeting a neuropeptide system that impacts several behaviours that are altered in chronic pain conditions.