The mixed antinociceptive agonist-antagonist activity of β-endorphin(1-27) in mice

β-Endorphin(1-27) (i.c.v.) has been reported to inhibit the antinociceptive activity of i.c.v. administered β-endorphin in mice. In this study the antagonist activity of β-endorphin(1-27) has been confirmed and the antagonism appears to be mediated at δ₁ opioid receptors. At higher doses than that used for antagonism, i.c.v. administered β-endorphin(1-27) was a full antinociceptive agonist. The antinociceptive activity of β-endorphin is attributed to the release of met-enkephalin in the spinal cord and is antagonized by the selective δ₂ opioid receptor antagonist, naltriben (NTB) but not by the selective δ₁ opioid receptor antagonist, 7-benzylidenenaltrexone (BNTX). In contrast, the antinociceptive activity of i.c.v. administered β-endorphin(1-27) was not affected by either NTB or BNTX administered i.c.v. or i.t. Also, the antinociceptive activity of β-endorphin(1-27) was unaffected by the selective μ opioid receptor antagonist, β-funaltrexamine (β-FNA) or the selective κ opioid receptor antagonist, norbinaltorphimine (norBNI). Thus, β-endorphin(1-27) appears to mediate antinociception supraspinally through the interaction of a unique receptor, i.e. a receptor that is different from μ, κ, δ₁ or δ₂ opioid receptors. Alternatively, a non-opioid mechanism may be considered.