beta-Casomorphin causes hypoalgesia in 10-day-old rats: evidence for central mediation.

Two experiments determined behavioral effectiveness of beta-casomorphins (beta-CM) in 10-d-old rats by evaluating changes in heat escape latency from a 48 degrees C stimulus applied to a forepaw. In one study rats were injected systemically with beta-CM4, -5, or -7 at a dose range 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 action of beta-CM5 through two experimental manipulations: first, by injecting it (0.25 microgram) into the lateral ventricles and by attempting to block its effects with systemic injections of naloxone. Second, rats received intracerebroventricular injections of naloxone (0.25 microgram) and systemic injections of beta-CM. beta-CM was effective centrally, suggesting central detection of the drug. Naloxone injected into the lateral ventricles blocked the effects of systemic administration of beta-CM, implying that circulating beta-CM or their precursors cause behavioral change through central mechanisms.