Endogenous µ-opioid receptor activity in the lateral and capsular subdivisions of the right central nucleus of the amygdala prevents chronic postoperative pain.

Tissue injury induces a long-lasting latent sensitization (LS) of spinal nociceptive signaling that is kept in remission by an opposing µ-opioid receptor (MOR) constitutive activity. To test the hypothesis that supraspinal sites become engaged, we induced hindpaw inflammation, waited 3 weeks for mechanical hypersensitivity to resolve, and then injected the opioid receptor inhibitors naltrexone, CTOP or ß-funaltrexamine subcutaneously, and/or into the cerebral ventricles. Intracerebroventricular injection of each inhibitor reinstated hypersensitivity and produced somatic signs of withdrawal, indicative of LS and endogenous opioid dependence, respectively. In naïve or sham controls, systemic naloxone (3 mg/kg) produced conditioned place aversion, and systemic naltrexone (3 mg/kg) increased Fos expression in the central nucleus of the amygdala (CeA). In LS animals tested 3 weeks after plantar incision, systemic naltrexone reinstated mechanical hypersensitivity and produced an even greater increase in Fos than in sham controls, particularly in the capsular subdivision of the right CeA. One third of Fos+ profiles co-expressed protein kinase C delta (PKCδ), and 35% of PKCδ neurons co-expressed tdTomato+ in Oprm1Cre ::tdTomato transgenic mice. CeA microinjection of naltrexone (1 µg) reinstated mechanical hypersensitivity only in male mice and did not produce signs of somatic withdrawal. Intra-CeA injection of the MOR-selective inhibitor CTAP (300 ng) reinstated hypersensitivity in both male and female mice. We conclude that MORs in the capsular subdivision of the right CeA prevent the transition from acute to chronic postoperative pain.

Cerebrolysin improves peripheral inflammatory pain: Sex differences in two models of acute and chronic mechanical hypersensitivity.

Chronic inflammatory pain is a major health problem worldwide with high prevalence in women. Cerebrolysin is a multimodal neuropeptide preparation that crosses the blood brain barrier and displays neuroprotective properties in aging and disease. Previously, we showed that cerebrolysin reduced mechanical allodynia in a model of persistent inflammation and pain. We aim to build upon the findings of our previous study by investigating the response to acute administration of cerebrolysin in two models of peripheral inflammation and assessing sex differences. We utilized the complete Freund's adjuvant (CFA) that produces maximal oedema and mechanical allodynia within days and carrageenan that produces similar effects within hours. Cerebrolysin reversed the mechanical allodynia in both sexes in CFA-treated rats. On the other hand, in rats treated with carrageenan, cerebrolysin was only effective in reducing mechanical allodynia in female rats. In conclusion, the present study shows that cerebrolysin effects may be sex-specific depending on different mechanisms that are at play in these two models of peripheral inflammatory pain. Further investigations are required to determine the factors contributing to sex differences.