β-Casomorphin Causes Hypoalgesia in 10-Day-Old Rats: Evidence for Central Mediation

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Abstract
Two experiments determined behavioral effectiveness of β-casomorphins(β-CM) in 10-d-old rats by evaluating changes in heat escape latency from a 48°C stimulus applied to a forepaw. In one study rats were injected systemically with β-CM4, -5, or -7 at a dose range of 0.1-2.5 mg/kg. Onlyβ-CM5 was effective, and the dose-response relationship was graded. The second study evaluated the locus of action of β-CM5 through two experimental manipulations: first, by injecting it (0.25 μg) 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 μg) and systemic injections of β-CM.β-CM was effective centrally, suggesting central detection of the drug. Naloxone injected into the lateral ventricles blocked the effects of systemic administration of β-CM, implying that circulating β-CM or their precursors cause behavioral change through central mechanisms.