Naloxone-reversible and non-reversible shock-induced analgesia during development.

We evaluated the development of naloxone-reversible and naloxone-non-reversible analgesia induced by footshock in rats of different ages and correlated it with the concentrations of beta-endorphin and dynorphin in brain areas and the spinal cord. We observed that naloxone-non-reversible shock-induced analgesia appeared first and its appearance might be related to the early presence of high dynorphin concentrations in the spinal cord. Naloxone-reversible analgesia appeared later together with the reaching of adult concentrations of cerebral beta-endorphin.