Eagle Yi-Kung Huang

Co-administration of dextromethorphan with morphine attenuates morphine rewarding effect and related dopamine releases at the nucleus accumbens.

Morphine is one of the most effective analgesics in clinic to treat postoperative pain or cancer pain. A major drawback of its continuous use is the development of tolerance and dependence. In our previous study we found that a widely used antitussive agent in clinics, dextromethorphan [(DM); also known as a non-competitive N-methyl-d-aspartate (NMDA) antagonist], could prevent the development of morphine tolerance. In the present study, we further investigated its effect on morphine addiction. Conditioned place preference (CPP) test and behavioral sensitization of locomotor activity were used to investigate the drug-seeking related behaviors, which were in correlation with psychological dependence. Our results showed that co-administered DM was able to abolish completely the CPP effect induced by morphine, but had no effect on morphine-induced behavioral sensitization. By employing the microdialysis technique in free-moving animals, we also determined the extracellular level of dopamine and serotonin metabolites in the shell region of the nucleus accumbens (NAc) in its response to morphine with/without DM. A significant increase in dopamine metabolites following morphine administration was demonstrated in the NAc. This increase by morphine could be attenuated by co-administered DM, whereas DM itself did not show any effect. Based on our results, it is speculated that DM may effectively attenuate morphine-induced psychological dependence. Neurochemical analysis revealed that the effect of DM could be through its action on the dopaminergic mesolimbic pathway, which could be activated by morphine and attributed to the cause of rewarding.

Modulations of spinal serotonin activity affect the development of morphine tolerance.

To test whether modulations of spinal serotonin (5-HT) levels would affect the development of morphine tolerance, we treated rats with either intrathecal 5-HT or 5,7-dihydroxytryptamine (5,7-DHT; a 5-HT neurotoxin) in addition to systemic infusion with morphine (2 mg · kg−1 · h−1). Continuous infusion of 5-HT (10 &mgr;g · 6 &mgr;L−1 · h−1) into the lumbar subarachnoid space of rats for 9 h accelerated the development of morphine tolerance. The area under the curve for the tail-flick latency test was 454.1 ± 35.1 in the Sham Control group vs 327.6 ± 41.0 in the 5-HT-Infused group. &mgr;-opioid receptor binding in the lumbar spinal cord showed a decrease in the Bmax (maximal binding −46.5%), but not the binding affinity (Kd), in 5-HT-infused rats. However, intrathecal injection of 5,7-DHT (50 &mgr;g), which resulted in a 48% reduction in 5-HT and 51% reduction in 5-hydroxyindoleacetic acid concentrations, led to an attenuation of morphine tolerance (the area under the curve was 613.0 ± 24.7 in the 5,7-DHT-Lesioned group). The binding study indicated that the affinity of lumbar &mgr;-opioid receptors decreased 196% in 5-HT-depleted rats, whereas there was no effect on apparent binding. The infusion of 5-HT (10 &mgr;g · 6 &mgr;L−1 · h−1) was not analgesic and the 5,7-DHT-induced lesion did not affect acute morphine-induced analgesia. We conclude that activity of spinal 5-HT-containing neurons plays a crucial role during the development of morphine tolerance.

Leptin action on nonneuronal cells in the CNS: potential clinical applications.

Leptin, an adipocyte‐derived cytokine, crosses the blood–brain barrier to act on many regions of the central nervous system (CNS). It participates in the regulation of energy balance, inflammatory processes, immune regulation, synaptic formation, memory condensation, and neurotrophic activities. This review focuses on the newly identified actions of leptin on astrocytes. We first summarize the distribution of leptin receptors in the brain, with a focus on the hypothalamus, where the leptin receptor is known to mediate essential feeding suppression activities, and on the hippocampus, where leptin facilitates memory, reduces neurodegeneration, and plays a dual role in seizures. We will then discuss regulation of the nonneuronal leptin system in obesity. Its relationship with neuronal leptin signaling is illustrated by in vitro assays in primary astrocyte culture and by in vivo studies on mice after pretreatment with a glial metabolic inhibitor or after cell‐specific deletion of intracellular signaling leptin receptors. Overall, the glial leptin system shows robust regulation and plays an essential role in obesity. Strategies to manipulate this nonneuronal leptin signaling may have major clinical impact.

NR2B subunit of NMDA receptor at nucleus accumbens is involved in morphine rewarding effect by siRNA study

BACKGROUND Chronic use of morphine causes rewarding effects and behavioral sensitization, which may lead to the development of craving for morphine. A number of studies indicate that the NMDA receptors may be involved in these effects, especially the NR2B-containing NMDA receptors. It is also well recognized that the nucleus accumbens (NAc) and the ventral tegmental area (VTA) are involved in drug addiction, including morphine addiction. AIMS In this study, we further investigate the role of the NR2B subunit of NMDA receptors at NAc or VTA in morphine rewarding effects and behavioral sensitization. METHODS AND RESULTS The siRNA against the NR2B subunit of NMDA receptors was locally injected to decrease the expression of NR2B at NAc or posterior VTA in male Sprague-Dawley (S.D.) rats in the present study. The rats were then treated with morphine chronically. A conditioned place preference (CPP) test was used to examine the rewarding effect, and locomotor activity was measured to determine the behavioral sensitization induced by chronic morphine treatment. Results showed that morphine-induced rewarding behavior but not behavioral sensitization was abolished when the NR2B subunit of NMDA receptors at the NAc were significantly decreased. The dopamine turnover rate was not altered by the decrease of NR2B subunit at NAc. CONCLUSION These findings suggest that the NR2B subunit of NMDA receptors at the NAc is involved in morphine-induced rewarding effect and may not be through directly interacting with dopamine neurons.

Dextromethorphan potentiates morphine antinociception at the spinal level in rats.

PurposeMorphine is an effective analgesic, but adverse effects limit its clinical use in higher doses. The non-opioid antitussive, dextromethorphan (DM), can potentiate the analgesic effect of morphine and decrease the dose of morphine in acute postoperative pain, but the underlying mechanism remains unclear. We previously observed that DM increases the serum concentration of morphine in rats. Therefore, we investigated the effects of drugs administered at the spinal level to exclude possible pharmacokinetic interactions. As DM has widespread binding sites in the central nervous system [such as N-methyl-D-aspartate (NMDA) receptors, sigma receptors and α3ß4 nicotinic receptors], we investigated whether the potentiation of morphine antinociception by DM at the spinal level is related to NMDA receptors.MethodsWe used MK-801 as atool to block the NMDA channel first, and then studied the interaction between intrathecal (i.t.) morphine and DM. The tail-flick test was used to examine the antinociceptive effects of different combinations of morphine and other drugs in rats.ResultsDM (2–20 μg) or MK-801 (5–15 μg) showed no significant antinociceptive effect by themselves. The antinociceptive effect of morphine (0.5 μg, i.t.) was significantly enhanced by DM and reached the maximal potentiation (43.7%–50.4%) at doses of 2 to 10 μg. Pretreatment with MK-801 (5 or 10μg, i.t.) significantly potentiated morphine antinociception by 49.9% or 38.7%, respectively. When rats were pretreated with MK-801, DM could not further enhance morphine antinociception (45.7%vs 50.5% and 43.3%).ConclusionOur results suggest that spinal NMDA receptors play an important role in the effect of DM to potentiate morphine antinociception.RésuméObjectifAnalgésique efficace, la morphine a cependant des effets indésirables qui en limite l’usage clinique à fortes doses. L’antitussif, dextrométhorphane (DM), peut augmenter l’effet analgésique de la morphine et permettre d’en utiliser une dose postopératoire plus faible, mais le mécanisme responsable de cette action est encore inconnu. Nous savons que le DM augmente la concentration sérique de la morphine chez les rats. Nous avons donc voulu explorer les effets de médicaments administrés au niveau rachidien afin d’exclure des interactions pharmacocinétiques possibles. Les sites de fixation du DM étant nombreux dans le système nerveux central [comme les récepteurs N-méthyl-D-aspartate (NMDA), les récepteurs sigma et les récepteurs nicotiniques α3ß4], nous avons vérifié si la potentialisation de l’antinociception de la morphine par le DM au niveau rachidien est reliée aux récepteurs NMDA.MéthodeNous avons utilisé le MK-801 pour bloquer d’abord le canal NMDA et étudier ensuite l’interaction entre la morphine intrathécale (i.t.) et le DM. Le test de latence de rétraction de la queue a été utilisé pour vérifier les effets antinociceptifs de différentes combinaisons de morphine et d’autres médicaments chez les rats.RésultatsLe DM (2– 20 μg) ou le MK-801 (5– 15 μg) n’ont montré aucun effet antinociceptif significatif par euxmêmes. L’effet antinociceptif de la morphine (0,5 μg, i.t.) a été significativement accru par le DM et a atteint sa potentialisation maximale (43,7 % –50,4 %) aux doses de 2 à 10 μg. Un prétraitement avec MK-801 (5 ou 10 μg, i.t.) a significativement augmenté l’effet antinociceptif de la morphine de 49,9% ou de 38,7 %, respectivement. Chez les rats prétraités avec MK-801, le DM ne pouvait augmenter davantage l’antinociception de la morphine (45,7% vs 50,5% et 43,3%). Conclusion : Nos résultats indiquent que les récepteurs NMDA jouent un rôle important dans l’effet de potentialisation de l’antinociception de la morphine par le DM.

Dextromethorphan differentially affects opioid antinociception in rats

1 Opioid drugs such as morphine and meperidine are widely used in clinical pain management, although they can cause some adverse effects. A number of studies indicate that N‐methyl‐D‐aspartate (NMDA) receptors may play a role in the mechanism of morphine analgesia, tolerance and dependence. Being an antitussive with NMDA antagonist properties, dextromethorphan (DM) may have some therapeutic benefits when coadministered with morphine. In the present study, we investigated the effects of DM on the antinociceptive effects of different opioids. We also investigated the possible pharmacokinetic mechanisms involved. 2 The antinociceptive effects of the μ‐opioid receptor agonists morphine (5 mg kg−1, s.c.), meperidine (25 mg kg−1, s.c.) and codeine (25 mg kg−1, s.c.), and the κ‐opioid agonists nalbuphine (8 mg kg−1, s.c.) and U‐50,488H (20 mg kg−1, s.c.) were studied using the tail‐flick test in male Sprague–Dawley rats. Coadministration of DM (20 mg kg−1, i.p.) with these opioids was also performed and investigated. 3 The pharmacokinetic effects of DM on morphine and codeine were examined, and the free concentration of morphine or codeine in serum was determined by HPLC. 4 It was found that DM potentiated the antinociceptive effects of some μ‐opioid agonists but not codeine or κ‐opioid agonists in rats. DM potentiated morphines antinociceptive effect, and acutely increased the serum concentration of morphine. In contrast, DM attenuated the antinociceptive effect of codeine and decreased the serum concentration of its active metabolite (morphine). 5 The pharmacokinetic interactions between DM and opioids may partially explain the differential effects of DM on the antinociception caused by opioids.

Distribution of neuropeptide FF (NPFF) receptors in correlation with morphine-induced reward in the rat brain

Neuropeptide FF (NPFF) exhibited anti-/pro-opioid effects when centrally injected. It was proved to bind to its own receptors, namely NPFF(1) and NPFF(2) receptors, but did not bind to opioid receptors. In our previous study, we found that i.c.v. injected NPFF suppressed morphine-induced conditioned place preference (CPP) in rats, which indicated that NPFF may play a role in the modulation of morphine-induced reward. In the present study, we further investigated the action site of NPFF to attenuate morphine-induced reward. Bilateral intra-VTA (ventral tegmental area) and intra-NAc (nucleus accumbens) injections of NPFF both blocked the CPP caused by morphine in rats. This suggests that NPFF may act at both VTA and NAc to inhibit the sensitization of the mesocorticolimbic dopaminergic pathway. Neurochemical analyses support that NPFF could be acting through the inhibition of the mesocorticolimbic dopaminergic activity increased by morphine. We also determined the distribution of NPFF receptors in rat brains. Our results showed that both NPFF receptors were abundantly expressed in VTA but with less content in NAc. In fluorescent immunohistochemical staining, our results revealed that NPFF(1) and NPFF(2) receptors could be expressed at the TH (tyrosine hydroxylase)- or GAD67 (glutamic acid decarboxylase-67)-positive neurons in VTA, whereas some of them were present in the negative neurons. This implied a possible function of NPFF to modulate dopaminergic neurons directly and a possible indirect action of NPFF on GABAergic neurons to modulate dopamine release. Taken together, our study should be helpful for clarifying the possible mechanisms of NPFF system to modulate morphine-induced reward.

Dextromethorphan potentiates morphine-induced antinociception at both spinal and supraspinal sites but is not related to the descending serotoninergic or adrenergic pathways

Morphine is a strong and widely used opioid analgesic in pain management, but some adverse effects limit its clinical use at high doses. The clinically available non-opioid antitussive, dextromethorphan (DM) can potentiate the analgesic effect of morphine and decrease the dose of morphine in acute postoperative pain. However, the mechanism underlying this synergistic phenomenon is still not clear. To examine if the potentiation by DM occurs through the descending pain-inhibitory pathways, ketanserin (a 5-HT2 receptor antagonist) and yohimbine (an alpha2-adrenergic receptor antagonist) were employed and found to have no significant effect on the potentiation by DM. Using local delivery of drugs in rats in the present study, potentiation of morphine-induced antinociception by DM was observed via both intrathecal and intracerebroventricular routes, suggesting that both spinal and supraspinal sites are involved. This suggests that the potentiation of morphine-induced antinociception by DM is not mediated by the serotoninergic or adrenergic descending pain-inhibitory pathways. The present results are consistent with findings in clinical studies, which showed that DM can effectively decrease the consumption of morphine in patients suffering from pain. Since DM has excellent clinical potential as a synergistic agent with morphine, further investigating and clarifying the possible pharmacological mechanism of DM are of great importance for future studies.

Attenuation by dextromethorphan on the higher liability to morphine-induced reward, caused by prenatal exposure of morphine in rat offspring

Co-administration of dextromethorphan (DM) with morphine during pregnancy and throughout lactation has been found to reduce morphine physical dependence and tolerance in rat offspring. No evidence was presented, however, for the effect of DM co-administered with morphine during pregnancy on morphine-induced reward and behavioral sensitization (possibly related to the potential to induce morphine addiction) in morphine-exposed offspring. Conditioned place preference and locomotor activity tests revealed that the p60 male offspring of chronic morphine-treated female rats were more vulnerable to morphine-induced reward and behavioral sensitization. The administration of a low dose of morphine (1 mg/kg, i.p.) in these male offspring also increased the dopamine and serotonin turnover rates in the nucleus accumbens, which implied that they were more sensitive to morphine. Co-administration of DM with morphine in the dams prevented this adverse effect of morphine in the offspring rats. Thus, DM may possibly have a great potential in the prevention of higher vulnerability to psychological dependence of morphine in the offspring of morphine-addicted mothers.

Supraspinal anti-allodynic and rewarding effects of endomorphins in rats

Two potent endogenous opioid peptides, endomorphin-1 (EM-1) and -2 (EM-2), which are selective micro-opioid agonists, have been identified from bovine and human brain. These endomorphins were demonstrated to produce a potent anti-allodynic effect at spinal level. In the present study, we further investigated their supraspinal anti-allodynic effects and rewarding effects. In a neuropathic pain model (sciatic nerve crush in rats), EM-1 and -2 (15 microg, i.c.v.) both showed significant suppressive effects in the cold-water allodynia test, but EM-1 showed a longer duration than EM-2. Naltrexone (NTX; 15 microg) and naloxonazine (NLZ; 15 microg) were both able to completely block the anti-allodynic effects of EM-1 and -2. In the tests of conditioned place preference (CPP), only EM-2 at the dose of 30 microg showed significant positive rewarding effect, whereas both endomorphins did not induce any reward at the dose of 15 microg. Due to the low solubility and the undesired effect (barrel rotation of the body trunk), EM-1 was not tested for the dose of 30 microg in the CPP tests. It was also found that acute EM-2 (30 microg) administration increased dopamine turnover in the shell of nucleus accumbens in the microdialysis experiments. From these results, it may suggest that EM-1 and -2 could be better supraspinal anti-allodynic agents compared with the other opioid drugs, although they may also induce rewarding.