Enhancement of the contact hypersensitivity reaction by acute morphine administration at the elicitation phase

The present study investigated the effects of morphine on the irritant cont act sensitivity (ICS) and contact hypersensitivity (CHS) reaction. ICS was induced by croton oil application on the pinnae of naive rats, Morphine inj ected prior to croton oil application did not affect the ICS response when assessed by measurements of pinnae thickness, CHS was induced by applying t he antigen 2,4-dinitro-1-fluorobenzene (DNFB) to the pinnae of rats sensiti zed to DNFB. Rats received an injection of morphine prior to either initial antigen exposure (sensitization) or antigen reexposure (challenge). Morphi ne prior to challenge, but not sensitization, resulted in a pronounced enha ncement of the CHS response as measured by pinna thickness. Quantitative PC R also showed increased IFN-gamma mRNA levels in the inflamed tissue of mor phine-treated rats. Naltrexone blocked the morphine-induced enhancement of the CHS response. The differential effects of morphine suggest that opioids have a more pronounced effect on in vivo immune responses that involve imm unological memory. (C) 1999 Academic Press.