Naloxone exacerbates intestinal and systemic anaphylaxis in the rat.

Following sensitization to ovalbumin (OA), male Wistar rats were pretreated with naloxone (20 mg/kg i.p.) and subjected to antigen challenge (3 mg OA i.p.). Naloxone exacerbated both systemic and intestinal anaphylaxis when injected 10 and 90 min before the antigen challenge. This was evidenced by a more pronounced drop in rectal temperature, higher hematocrit values, and by an enhanced elevation of basal short-circuit current (an indication of the secretory tone of the small intestine studied in Ussing chambers). Pretreatment with an equipotent does of methylnaloxone (200 mg/kg i.p.), a peripherally acting opiate antagonist, exacerbated the indices of intestinal anaphylaxis but had no apparent effect on indices of systemic anaphylaxis. Thus, our data strongly suggest that in the rat, components of the systemic hypersensitivity reaction are mediated through central opioid receptors, whereas the changes in gut function characterizing intestinal anaphylaxis are mediated through peripheral opioid receptors.