It has been demonstrated that the antinociception induced by it or icv administration of
endomorphins is mediated through μ-opioid receptors. Moreover, though endomorphins do
not have appreciable affinity for κ-opioid receptors, pretreatment with the κ-opioid receptor
antagonist nor-binaltorphimine markedly blocks the antinociception induced by icv-or it-
injected endomorphin-2, but not endomorphin-1. These evidences propose the hypothesis
that endomorphin-2 may initially stimulate the μ-opioid receptors, which subsequently …