The analgesically effective peptide deprolorphin exhibits cross-tolerance to morphine in rats

The potent analgesically active beta-casomorphin derivative, D-Pro4-beta-casomorphin 1-5 (= Deprolorphin), was tested for its capacity to induce tolerance. In animals tolerant to morphine, a previously analgesically effective test dose of the peptide was without any antinocipetive effect, demonstrating a cross-tolerance between morphine and deprolorphin.