An endogenous neuropeptide and μ-opioid receptor agonist (Ki = 0.69 nM); selective for μ- over δ- and κ-opioid receptors (Kis = 9,233 and 5,240 nM, respectively); binds to mouse brain membranes and rat recombinant μ-opioid receptors expressed in CHO cells (Kis = 1.37 and 0.58 nM, respectively); inhibits forskolin-induced cAMP accumulation in CHO cells expressing rat recombinant μ-opioid receptors and in SH-SY5Y human neuroblastoma cells (IC50s = 7.08 and 7.76 nM, respectively); increases analgesia in a radiant heat tail-flick assay in mice following intracerebroventricular or intrathecal administration (ED50s = 1.99 and 3.81 μg, respectively); induces conditioned place aversion in mice at 10 μg, i.c.v.; decreases gastrointestinal transit time in mice at 3 μg, i.c.v.,