Immunohistochemical localization of delta- and mu-opioid receptors in primate spinal cord

Immunofluorescent techniques were used to examine the distribution of δ- and μ-opioid receptor-like immunoreactivity (ir) in primate spinal cord. At every segmental level examined, opioid receptor-ir formed a dense plexus of small profiles within the superficial dorsal horn. δ-Opioid receptor-ir was found in laminae I and II, whereas μ-opioid receptor-ir was confined mostly to lamina II. The distribution of opioid receptors within the superficial dorsal horn is appropriate for a localization to the central terminals of thinly myelinated and unmyelinated sensory neurons. Differences in the distribution of the δ- and μ-opioid receptors may reflect the presence of these two opioid receptor types on different functional classes of afferent neurons, or they may relate to differences in presynaptic and postsynaptic localization within the spinal gray matter.