BETA-CASOMORPHIN CAUSES HYPOALGESIA IN 10-DAY-OLD RATS - EVIDENCE FORCENTRAL MEDIATION

Two experiments determined behavioral effectiveness of beta-casomorphi ns (beta-CM) in 10-d-old rats by evaluating changes in heat escape lat ency from a 48 degrees C stimulus applied to a forepaw. In one study r ats were injected systemically with beta-CM4, -5, or -7 at a dose rang e of 0.1-2.5 mg/kg. Only beta-CM5 was effective, and the dose-response relationship was graded. The second study evaluated the locus of acti on of beta-CM5 through two experimental manipulations: first, by injec ting it (0.25 mu g) into the lateral ventricles and by attempting to b lock its effects with systemic injections of naloxone. Second, rats re ceived intracerebroventricular injections of naloxone (0.25 mu g) and systemic injections of beta-CM. beta-CM was effective centrally, sugge sting central detection of the drug. Naloxone injected into the latera l ventricles blocked the effects of systemic administration of beta-CM , implying that circulating beta-CM or their precursors cause behavior al change through central mechanisms.