The delta-opioid receptor, also known as delta opioid receptor or simply delta receptor, abbreviated DOR, is an opioid receptor that has enkephalins as its endogenous ligands. Activation of delta receptors produces some analgesia, although less than that of mu-opioid agonists. Many delta agonists may also cause seizures at high doses, although not all delta agonists produce this effect. Evidence for whether delta agonists produce respiratory depression is mixed; high doses of the delta agonist peptide DPDPE produced respiratory depression in sheep, but in tests on mice the non-peptide delta agonist SNC-80 produced respiratory depression only at the very high dose of 40 mg/kg. In contrast both the peptide delta agonist Deltorphin II and the non-peptide delta agonist (+)-BW373U86 actually stimulated respiratory function and blocked the respiratory depressant effect of the potent mu-opioid agonist alfentanil, without affecting pain relief. It thus seems likely that while delta opioid agonists can produce respiratory depression at very high doses, at lower doses they have the opposite effect, a fact that may make mixed mu/delta agonists such as DPI-3290 potentially very useful drugs that might be much safer than the mu agonists currently used for pain relief.