Review paper
Opioid involvement in experimental peritonitis: minireview of comparative studies

Experimental peritonitis is a convenient model for investigations of the opioid action in the focus of inflammation, as both exudatory fluid and cells are easy for quantitative retrieval from the inflamed peritoneal cavity. During the course of inflammation the changes of Met-enkephalin, beta-endorphin, and dynorphin may be followed in the fluid while mRNAs for the precursor molecules of opioid peptides as well as opioid receptors may be detected in the inflammatory leukocytes. Endogenous opioid peptides deriving from the leukocytes recruited to the inflamed peritoneal cavity may participate in the control of visceral pain by the binding to the opioid receptors on the local endings of sensory neurons thus the pain symptoms are restricted to the early stages of peritonitis. Binding of opioid receptors by exogenous opioid, morphine, co-injected with the inflammation-inducing agent (zymosan), abolishes pain symptoms already at the low dose of morphine, while the high dose of morphine additionally inhibits intraperitoneal influx of leukocytes in the several strains of mice (except of CBA) and in the two fish species (salmon and goldfish), but not in the investigated frogs and toads; the putative reasons of the exceptions are being elucidated.