Naltrexone promotes mechanical allodynia in humans and rats

Mechanical allodynia has been studied in chronic naltrexone-treated people (N.T.P.) and rats (N.T.R.). After persistent naltrexone administration, patients acquired static and dynamic mechanical allodynia, as measured by von Frey filament (vFf) and brush stimulations. Pregabalin and levodopa administrations in N.T.P. significantly reduced allodynic behaviour, albeit these molecules did not completely stop it. As evidenced by the deployment of the vFf, subchronic treatment with Naltrexone delivered peripherally or intrathecally induced allodynic behaviour in rats. Increased expressions of two pain markers, pERK1/2 and PKCγ, in the spinal dorsal horn laminae were associated with naltrexone-induced allodynic behaviour. After vFf stimulation, pERK1/2 expression was substantially higher (p < 0.001) in superficial spinal dorsal horn laminae than in non-stimulated or naive non-stimulated rats. In addition, when compared to control rats, N.T.R. showed a substantial (p < 0.001) increase in PKCγ expression. PKCγ expression was found to be strong in lamina IIi and laminae III-IV. A cellular mechanism is proposed for the naltrexone effect. In both people and rats, Naltrexone induces static mechanical allodynia, according to this study.