Selective antagonists at the opiate delta-receptor

Abstract Classical opiate antagonists such as naloxone act predominantly at the opiate mu-receptor and exhibit only weak antagonism at the delta-receptor. Using two in vitro models - the field-stimulated mouse vas deferens and guinea pig ileum preparations - we have identified two novel peptide antagonists which selectively and competitively reverse the effects of opiates at the delta-receptor, with little or no activity at either mu or kappa receptors.