Buprenorphine is a mixed opioid receptor agonist-antagonist used clinically for maintenance therapy in opiate addicts and pain management. Dose-response curves for buprenorphine-induced antinociception display ceiling effects or are bell shaped, which have been attributed to the partial agonist activity of buprenorphine at opioid receptors. Recently, buprenorphine has been shown to activate opioid receptor-like (ORL-1) receptors, also known as OP4 receptors. Here we demonstrate that buprenorphine, but not morphine, activates mitogen-activated protein kinase and Akt via ORL-1 receptors. Because the ORL-1 receptor agonist orphanin FQ/nociceptin blocks opioid-induced antinociception, we tested the hypothesis that buprenorphine-induced antinociception might be compromised by concomitant activation of ORL-1 receptors. In support of this hypothesis, the antinociceptive effect of buprenorphine, but not morphine, was markedly enhanced in mice lacking ORL-1 receptors using the tail-flick assay. Additional support for a modulatory role for ORL-1 receptors in buprenorphine-induced antinociception was that coadministration of J-113397, an ORL-1 receptor antagonist, enhanced the antinociceptive efficacy of buprenorphine in wild-type mice but not in mice lacking ORL-1 receptors. The ORL-1 antagonist also eliminated the bell-shaped dose-response curve for buprenorphine-induced antinociception in wild-type mice. Although buprenorphine has been shown to interact with multiple opioid receptors, mice lacking micro-opioid receptors failed to exhibit antinociception after buprenorphine administration. Our results indicate that the antinociceptive effect of buprenorphine in mice is micro-opioid receptor-mediated yet severely compromised by concomitant activation of ORL-1 receptors.

Opioid-receptor activation in cell lines results in phosphorylation of p42/44 mitogen-activated protein kinase (MAPK), which contributes to agonist-induced desensitization of adenylate cyclase signaling. In this study, morphine-induced MAPK modulation was examined in the mouse brain using antibodies against phosphorylated MAPK. Thirty minutes after systemic morphine, MAPK modulation was observed in brain areas associated with analgesia and reward. Activation of MAPK was increased in the anterior cingulate (Acc), somato-sensory and association cortices, and locus ceruleus (LC). In contrast, MAPK activation was decreased in the nucleus accumbens and central amygdala (CeA). Double-label confocal microscopy revealed that morphine-induced MAPK modulation occurred predominantly in cells not expressing mu-opioid receptors, with the exception of the LC. Furthermore, the NMDA receptor antagonist 3,3-(2-carboxypiperazine-4-yl)-propyl-1-phosphonate blocked morphine-induced MAPK modulation in several cortical areas including the Acc. We then examined morphine-induced MAPK modulation during expression of either analgesic tolerance or locomotor sensitization, which were differentiated by two repeated morphine regimens. Analgesic tolerance was accompanied by tolerance to morphine-induced MAPK modulation in all of the brain areas examined except the CeA. Locomotor sensitization resulted in sensitization to morphine-induced MAPK activation in the posterior basolateral amygdala. Additionally, a pronounced instatement of morphine-induced MAPK activation was observed in CA3 hippocampal processes. This instatement was observed during expression of tolerance; however, it was not significant during sensitization. In summary, these results provide distinct, region-specific mechanisms for morphine-induced MAPK modulation in the mouse brain and give insight into the brain circuitry involved in acute and adaptive opioid behaviors.

Regenerating optic nerves from fish produce a factor that is cytotoxic to oligodendrocytes. The cytotoxic factor is recognized by antibodies to interleukin-2 (IL-2) and has the apparent molecular size of a dimer of IL-2. An enzyme, identified as a nerve transglutaminase, was purified from regenerating optic nerves of fish and was found to catalyze dimerization of human IL-2. The dimerized IL-2, unlike monomeric IL-2, is cytotoxic to oligodendrocytes from rat brain in culture. The results suggest that posttranslational modification of a cytokine can alter its activity. Under conditions in which oligodendrocytes inhibit neuronal regeneration, dimerization of IL-2 might provide a mechanism to permit nerve growth.

Multiple studies demonstrate that coadministration of N-methyl-D-aspartate (NMDA) receptor antagonists with the opioid agonist morphine attenuates the development of analgesic tolerance. Sex differences in the effects of noncompetitive, but not competitive NMDA receptor antagonists on acute morphine analgesia, have been reported in mice, yet the role of sex in modulation of morphine tolerance by NMDA receptor antagonists has yet to be addressed. Therefore, we tested whether there is a sex difference in the effect of NMDA receptor antagonists on the development of morphine analgesic tolerance in C57BL/6J mice. Acutely, at a dose required to affect morphine tolerance in male mice, the noncompetitive NMDA receptor antagonist dizocilpine (MK-801) prolonged morphine analgesia similarly in both sexes in the hot plate and tail withdrawal assays. In the hot plate assay, coadministration of MK-801 or the competitive antagonist 3-(2-carboxpiperazin-4-yl)propyl-1-phosphanoic acid (CPP) with morphine attenuated the development of tolerance in male mice, while having no effect in females. Like normal and sham females, ovariectomized mice were similarly insensitive to the attenuation of morphine tolerance by MK-801 in the hot plate assay. Surprisingly, in the tail withdrawal assay, MK-801 facilitated the development of morphine-induced hyperalgesia and tolerance in males but not females. The results demonstrate that male mice are more sensitive to modulation of nociception and morphine analgesia after repeated coadministration of NMDA receptor antagonists. Furthermore, the underlying mechanisms are likely to be different from those mediating the sex difference in the modulation of acute morphine analgesia that has previously been reported.

Failure of axons of the central nervous system in adult mammals to regenerate spontaneously after injury is attributed in part to inhibitory molecules associated with oligodendrocytes. Regeneration of central nervous system axons in fish is correlated with the presence of a transglutaminase. This enzyme dimerizes interleukin-2, and the product is cytotoxic to oligodendrocytes in vitro. Application of this nerve-derived transglutaminase to rat optic nerves, in which the injury had caused the loss of visual evoked potential response to light, promoted the recovery of that response within 6 weeks after injury. Transmission electron microscopy analysis revealed the concomitant appearance of axons in the distal stump of the optic nerve.