Morphine Reward Research Articles

Role of mGlu7 receptor in morphine rewarding effects is uncovered by a novel orthosteric agonist

Opiate dependence is a major health issue and despite the existence of opioid substitution treatment, relapse frequently occurs. Group III metabotropic glutamate (mGlu) receptors has received much attention as a putative target in ethanol and cocaine addiction, but no data on opiate addiction exist. So we investigated the role of group III mGlu receptors in morphine rewarding effects through the expression and the reinstatement of conditioned place preference (CPP) using a newly synthesized mGlu4/mGlu7 receptor orthosteric agonist, LSP2-9166. We found that LSP2-9166 blocked morphine CPP expression and reinstatement after extinction. Blockade of CPP expression with LSP2-9166 was abolished when using XAP044, a mGlu7 antagonist. We also found that LSP2-9166 at the dose active for blocking morphine reward was devoid of any effect on locomotion, hedonic state, spatial memory, anxiety or depression. Altogether our data demonstrated that group III mGlu receptors, and more specifically mGlu7, might be a valuable target in opiate addiction.

Association of contextual cues with morphine reward increases neural and synaptic plasticity in the ventral hippocampus of rats

Drug addiction is associated with aberrant memory and permanent functional changes in neural circuits. It is known that exposure to drugs like morphine is associated with positive emotional states and reward-related memory. However, the underlying mechanisms in terms of neural plasticity in the ventral hippocampus, a region involved in associative memory and emotional behaviors, are not fully understood. Therefore, we measured adult neurogenesis, dendritic spine density and brain-derived neurotrophic factor (BDNF) and TrkB mRNA expression as parameters for synaptic plasticity in the ventral hippocampus. Male Sprague Dawley rats were subjected to the CPP (conditioned place preference) paradigm and received 10mg/kg morphine. Half of the rats were used to evaluate neurogenesis by immunohistochemical markers Ki67 and doublecortin (DCX). The other half was used for Golgi staining to measure spine density and real-time quantitative reverse transcription-polymerase chain reaction to assess BDNF/TrkB expression levels. We found that morphine-treated rats exhibited more place conditioning as compared with saline-treated rats and animals that were exposed to the CPP without any injections. Locomotor activity did not change significantly. Morphine-induced CPP significantly increased the number of Ki67 and DCX-labeled cells in the ventral dentate gyrus. Additionally, we found increased dendritic spine density in both CA1 and dentate gyrus and an enhancement of BDNF/TrkB mRNA levels in the whole ventral hippocampus. Ki67, DCX and spine density were significantly correlated with CPP scores. In conclusion, we show that morphine-induced reward-related memory is associated with neural and synaptic plasticity changes in the ventral hippocampus. Such neural changes could underlie context-induced drug relapse.

Role of orexin receptors in the ventral tegmental area on acquisition and expression of morphine-induced conditioned place preference in the rats

The orexins are hypothalamic neuropeptides and their role in reward processing and drug addiction has been demonstrated. The extent of involvement of each orexin receptor in the acquisition and expression of conditioned place preference (CPP) for morphine is still a matter of controversy. We investigated the functional differences between orexin-1 and -2 receptor blockade in the ventral tegmental area (VTA) on the acquisition and expression of morphine CPP. A total of 86 adult male Wistar rats weighing 250±30g (age 7–8weeks) received intra-VTA microinjection of either SB334867 (0.1, 1 and 10nM), a selective orexin-1 receptor (OX1R) antagonist, or TCS-OX2-29 (1, 5 and 25nM), a selective orexin-2 receptor (OX2R) antagonist. To measure the acquisition, the animals received each antagonist (SB334867 or TCS-OX2-29) 5min prior to subcutaneous injection of morphine (5mg/kg) during the conditioning phase. To measure the CPP expression, the animals received each antagonist on the post-conditioning phase. The CPP conditioning score was recorded by Ethovision software. Data showed that intra-VTA microinjection of OX1-R antagonist significantly attenuated morphine CPP acquisition, during the conditioning phase, and expression, during the post-conditioning phase. Intra-VTA microinjection of OX2-R antagonist also significantly attenuated morphine CPP acquisition and expression in the mentioned phases. Our results showed the orexin role in learning and memory and indicate that orexin receptors (OX1R and OX2R) function in the VTA is essential for both acquisition and expression of morphine reward in rats in the CPP model.

Nortriptyline Enhances Morphine-Conditioned Place Preference in Neuropathic Rats: Role of the Central Noradrenergic System.

Combination drug therapy is commonly used to treat chronic pain conditions such as neuropathic pain, and antidepressant is often used together with opioid analgesics. While rewarding is an intrinsic property of opioid analgesics, it is unknown whether the use of an antidepressant would influence opioid reward, which may contribute to opioid addiction. In this study, we examined whether nortriptyline (a tricyclic antidepressant and a first-line medication for neuropathic pain) would enhance the morphine rewarding property in both naive and chronic constriction sciatic nerve injury (CCI) rats. The rewarding effect of these drugs was assessed using conditioned place preference (CPP). The real-time polymerase chain reaction, western blot, and enzyme-linked immunosorbent assay analysis were used to investigate the function of central noradrenergic system. In naive rats, coadministration of nortriptyline with morphine did not change the acquisition of morphine-induced CPP. However, nortriptyline enhanced the acquisition, delayed the extinction, and augmented the reinstatement of morphine-induced CPP in CCI rats. In CCI rats treated with both nortriptyline and morphine, the expression of α2A-adrenergic receptors, norepinephrine transporter, and tyrosine hydroxylase was markedly decreased in the locus coeruleus, whereas the norepinephrine concentration in the nucleus accumbens was remarkably increased. These results demonstrate that nortriptyline enhanced morphine reward when both drugs were used to treat neuropathic pain in rats and that this behavioral phenotype is likely to be mediated by upregulation of the central noradrenergic system. These findings may have implications in opioid therapy commonly used for chronic pain management.

Atg5- and Atg7-dependent autophagy in dopaminergic neurons regulates cellular and behavioral responses to morphine

ABSTRACTThe molecular basis of chronic morphine exposure remains unknown. In this study, we hypothesized that macroautophagy/autophagy of dopaminergic neurons would mediate the alterations of neuronal dendritic morphology and behavioral responses induced by morphine. Chronic morphine exposure caused Atg5 (autophagy-related 5)- and Atg7 (autophagy-related 7)-dependent and dopaminergic neuron-specific autophagy resulting in decreased neuron dendritic spines and the onset of addictive behaviors. In cultured primary midbrain neurons, morphine treatment significantly reduced total dendritic length and complexity, and this effect could be reversed by knockdown of Atg5 or Atg7. Mice deficient for Atg5 or Atg7 specifically in the dopaminergic neurons were less sensitive to developing a morphine reward response, behavioral sensitization, analgesic tolerance and physical dependence compared to wild-type mice. Taken together, our findings suggested that the Atg5- and Atg7-dependent autophagy of dopaminergic neurons contributed to cellular and behavioral responses to morphine and may have implications for the future treatment of drug addiction.

Effects of Cannabidiol on Morphine Conditioned Place Preference in Mice.

This study sought to determine whether the cannabis constituent cannabidiol attenuates the development of morphine reward in the conditioned place preference paradigm. Separate groups of mice received either saline or morphine in combination with one of four doses of cannabidiol using three sets of drug/no-drug conditioning trials. After drug-place conditioning, morphine mice displayed robust place preference that was attenuated by 10 mg/kg cannabidiol. Further, when administered alone, this dose of cannabidiol was void of rewarding and aversive properties. The finding that cannabidiol blocks opioid reward suggests that this compound may be useful in addiction treatment settings.

TRPV1 modulates morphine-induced conditioned place preference via p38 MAPK in the nucleus accumbens

Emerging evidence suggests that the transient receptor potential vanilloid type 1 channel (TRPV1) is a novel target for the treatment of drug addiction, such as cocaine and morphine. Previously we reported that TRPV1 inhibition reduced morphine reward in the dorsal striatum (DSt) of mice and morphine self-administration through a decrease in accumbal activity in rats. However, the role of TRPV1 on morphine-conditioned reward in addiction-related brain regions, such as the nucleus accumbens (NAc), has not been previously established. Here, we investigated the effects of TRPV1 on morphine conditioned place preference (CPP) and intracellular mechanisms of TRPV1 using Western blot analysis and immunohistochemistry (IHC) in morphine-administered mice. TRPV1 knockout mice did not exhibit morphine reward responses, and both i.p. and intra-NAc injections of SB366791, a selective TRPV1 antagonist, reduced morphine-induced CPP in wild-type mice. Furthermore, i.p. injection of SB203580, a selective p38 MAPK inhibitor, also dampened morphine-induced CPP. To determine the molecular mechanisms of the TRPV1/p38 MAPK pathway in morphine CPP, we investigated the expression of adenylyl cyclase type 1 (AC1) and phospho-p38 mitogen-activated protein kinase (MAPK) and nuclear factor-kappa B (NF-κB) in the NAc. Either SB366791 or SB203580 decreased the protein expression levels of phospho-p38 MAPK, phosphor-NF-κB, and AC1 in the NAc of morphine CPP mice. Taken together, our findings suggest that TRPV1 may modulate morphine-induced conditioned reward effects via the p38 MAPK signaling pathway in the NAc. Therefore, blockade of TRPV1 may provide a novel therapeutic approach for the prevention and treatment of opioid addiction.

Alternatively spliced mu opioid receptor C termini impact the diverse actions of morphine.

Extensive 3' alternative splicing of the mu opioid receptor gene OPRM1 creates multiple C-terminal splice variants. However, their behavioral relevance remains unknown. The present study generated 3 mutant mouse models with truncated C termini in 2 different mouse strains, C57BL/6J (B6) and 129/SvEv (129). One mouse truncated all C termini downstream of Oprm1 exon 3 (mE3M mice), while the other two selectively truncated C-terminal tails encoded by either exon 4 (mE4M mice) or exon 7 (mE7M mice). Studies of these mice revealed divergent roles for the C termini in morphine-induced behaviors, highlighting the importance of C-terminal variants in complex morphine actions. In mE7M-B6 mice, the exon 7-associated truncation diminished morphine tolerance and reward without altering physical dependence, whereas the exon 4-associated truncation in mE4M-B6 mice facilitated morphine tolerance and reduced morphine dependence without affecting morphine reward. mE7M-B6 mutant mice lost morphine-induced receptor desensitization in the brain stem and hypothalamus, consistent with exon 7 involvement in morphine tolerance. In cell-based studies, exon 7-associated variants shifted the bias of several mu opioids toward β-arrestin 2 over G protein activation compared with the exon 4-associated variant, suggesting an interaction of exon 7-associated C-terminal tails with β-arrestin 2 in morphine-induced desensitization and tolerance. Together, the differential effects of C-terminal truncation illustrate the pharmacological importance of OPRM1 3' alternative splicing.

Effects of unilatral- and bilateral inhibition of rostral ventral tegmental area and central nucleus of amygdala on morphine-induced place conditioning in male Wistar rat.

The rostral ventral tegmental area (VTAR) and central nucleus of amygdala (CeA) are considered the main regions for induction of psychological dependence on abused drugs, such as morphine. The main aim of this study was to investigate the transient inhibition of each right and left side as well as both sides of the VTAR and the CeA by lidocaine (2%) on morphine reward properties using the conditioned place preference (CPP) method. Male Wistar rats (250±20g) 7days after recovery from surgery and cannulation were conditioned to morphine (7.5mg/kg) in CPP apparatus. Five minutes before morphine injection in conditioning phase, lidocaine was administered either uni- or bilaterally into the VTAR (0.25μL/site) or CeA (0.5μL/site). The results revealed that lidocaine administration into the left side, but not the right side of the VTAR and the CeA reduced morphine CPP significantly. The reduction was potentiated when lidocaine was injected into both sides of the VTAR and the CeA. The number of compartment crossings was reduced when lidocaine was injected into both sides of the VTAR and the CeA as well as the left side. Rearing was reduced when lidocaine was injected into the right, but not the left side of the VTAR. Sniffing and rearing increased when animals received lidocaine in the right side and reduced in the group that received lidocaine in the left side of the CeA. It was concluded that the right and the left side of VTAR and the CeA play different roles in morphine-induced activity and reward.

Prenatal morphine exposure during late embryonic stage enhances the rewarding effects of morphine and induces the loss of membrane-bound protein kinase C-α in intermediate medial mesopallium in the chick

The susceptibility to drug abuse may be associated with the structural and/or functional changes in the reward-related brain regions induced by drug exposure during sensitive periods of embryonic development. Previously, we have found that prenatal morphine exposure during embryonic days 17–20 may be crucial for developing the susceptibility to morphine reward after hatching. However, the underlying structure and cellular mechanisms need further investigation. In the present study, the chicks of a few days old, which were prenatally exposed to morphine during E17-20, obviously showed higher preference for the morphine-paired chamber and hyperactivity during the expression of morphine conditioned place preference (CPP), and the reduction in membrane-bound of PKCα of the bilateral intermediate medial mesopallium (IMM) assayed immunologically. These results indicate that the decreased expression of PKCα in IMM may participate in the development of the susceptibility to the rewarding effects of morphine in chicks prenatally exposed to morphine, and provide further support for the cross-species evolutionary concordance among amniotes.

Effects of paclitaxel on mechanical sensitivity and morphine reward in male and female C57Bl6 mice.

This study evaluated the hypothesis that a paclitaxel treatment regimen sufficient to produce mechanical allodynia would alter sensitivities of male and female mice to the conditioned rewarding and reinforcing effects of morphine. Saline or paclitaxel were administered on Days 1, 3, 5, and 7 in male and female C57Bl/6 mice to induce morphine-reversible mechanical allodynia as measured by the Von Frey filament test. Paclitaxel treatment did not change sensitivity to morphine conditioned place preference (CPP) relative to saline treatment in either male or female mice. Morphine produced peak self-administration under a fixed ratio-1 (FR1) schedule of reinforcement for 0.03 mg/kg morphine per infusion in female mice and 0.1 mg/kg morphine per infusion in male mice. During the progressive ratio experiments, saline treatment in male mice decreased the number of morphine infusions for 12 days whereas the paclitaxel-treated male mice maintained responding for morphine similar to baseline levels during the same time period. However, paclitaxel did not have an overall effect on the reinforcing efficacy of morphine assessed over a limited dose range during the course of the repeated self-administration. These results suggest that the reward-related behavioral effects of morphine are overall not robustly altered by the presence of paclitaxel treatment under the current dosing regimen, with the exception of maintaining a small yet significant higher baseline than saline treatment during the development of allodynia in male mice. (PsycINFO Database Record

Impairment of opiate-mediated behaviors by the selective TRPV1 antagonist SB366791.

Transient receptor potential vanilloid type 1 (TRPV1), the archetypal member of the vanilloid TRP family, was initially identified as the receptor for capsaicin, the pungent ingredient in hot chili peppers. We previously demonstrated that TRPV1 in the dorsal striatum significantly contributes to morphine reward by using the conditioned place preference paradigm in mice; however, it is unknown whether TRPV1 has the same effect in other reward models. In this study, we investigated the role of TRPV1 in morphine reward by using a self-administration paradigm in rats. We found that treatment with a selective TRPV1 antagonist, SB366791, significantly decreased morphine self-administration on a fixed-ratio 1 schedule or a progressive ratio schedule of reinforcement. In addition, treatment with another selective TRPV1 antagonist, AMG9810, not only significantly prevented morphine self-administration but also prevented morphine-induced c-fos expression in the nucleus accumbens. Furthermore, administration of SB366791 decreased an anxiolytic-like effect during the morphine abstinence period. Moreover, treatment with SB366791 significantly decreased morphine-priming reinstatement. Taken together, our findings suggest that blockade of TRPV1 receptors could provide an approach to limiting morphine addiction.

Neurokinin 1 and opioid receptors: relationships and interactions in nervous system

Opioid receptors and neurokinin 1 receptor (NK1R) are found highly expressed in the central nervous system. The co-localization of these two kinds of receptors suggests that they might interact with each other in both the transmission and modulation of the pain signal. In this review, we explore the relationships between opioid receptors and NK1R. Substance P (SP) plays a modulatory role in the pain transmission by activating the NK1R. Opioid receptor activation can inhibit SP release. NK1R is found participating in the mechanisms of the side effects of the opioids, including opioid analgesic tolerance, hyperalgesia, anxiety behaviors of morphine reward and opioids related respiratory depression. A series of compounds such as NK1R antagonists and ligands works on both mu/delta opioid receptor (MOR/DOR) and NK1R were synthesized as novel analgesics that enhance the clinical pain management efficacy and reduce the dosage and side effects. The current status of these novel ligands and the limitations are discussed in this review. Although the working mechanisms of these ligands remained unclear, they could be used as research tool for developing novel analgesic drugs in the future.

Exposure to morphine-associated cues increases mu opioid receptor mRNA expression in the nucleus accumbens of Wistar Kyoto rats

The Wistar-Kyoto (WKY) rat has been proposed as a model of anxiety vulnerability as it exhibits pronounced behavioral inhibition, passive avoidance, exaggerated startle response, enhanced HPA-axis activation, and active avoidance that is resistant to extinction. Accumulating evidence suggests that WKY rats respond differently to rewarding stimuli when compared to outbred strains of rat. Conditioned responding to drug-associated cues is linked with alterations in the activation of mu opioid receptors (MOR) and kappa opioid receptors (KOR) in the nucleus accumbens (NAc). Furthermore, alterations in KOR expression/activation in the NAc of WKY rats are implicated in the regulation of some of the components that make up the unique behavioral phenotype of this strain. The purpose of this study was to extend upon previous work from our laboratory by investigating conditioned morphine reward in adult male WKY and SD rats, and to examine levels of KOR mRNA and MOR mRNA in the NAc at baseline and after acquisition of morphine CPP. Our results demonstrate that SD rats displayed morphine-induced CPP to each of the six doses of morphine tested (0.5, 1.25, 2.5, 5, 7.5, or 10mg/kg). Interestingly, WKY rats demonstrated CPP for only the 1.25, 2.5, and 5mg/kg doses, yet no preference at the lowest (0.5mg/kg) or highest (7.5 and 10mg/kg) doses. qPCR analysis of MOR and KOR in the NAc revealed no strain differences in basal levels of MOR, but higher levels of KOR in WKY rats compared to those of SD rats. Interestingly, after completion of the CPP task, WKY rats had overall higher levels of NAc MOR mRNA compared to SD rats; the initial basal differences in NAc KOR levels persisted without change due to CPP in either strain. These results demonstrate that the WKY rat exhibits a unique pattern of behavioral responding to morphine and implicates differences in NAc KOR signaling as a potential source of aversion to higher doses of morphine. Additionally, the CPP-induced upregulation of NAc MOR mRNA in WKY rats warrants further investigation in terms of its potential role as a factor constituting a unique vulnerability to subsequent drug exposure.

The Effect of Ventral Tegmental Area Rictor Knockout on Susceptibility to Chronic Social Defeat Stress and Stress‐induced Changes in Morphine Reward

There is significant co‐morbidity of addiction and depression and this may be due to dysregulation of common brain circuits. For example, similar alterations in signaling in the ventral tegmental area (VTA) have been identified in response to both chronic morphine and chronic social defeat stress (CSDS), a rodent model of depression. We are particularly interested in target of rapamycin complex 2 (TORC2) signaling in the VTA as it has been established to play a critical role in morphine reward and morphine‐induced neuroadaptations. However, whether it plays a similar role in stress‐induced changes in morphine reward, or stress susceptibility itself, is unknown. To address these questions, we used viral and genetic approaches to decrease VTA TORC2 signaling by knocking out the expression of Rictor, a TORC2 component protein necessary for TORC2 function. Floxed‐Rictor mice were either crossed with a TH‐Cre mouse line to eliminate TORC2 signaling in all catecholaminergic neurons or underwent stereotaxic surgery to specifically decrease TORC2 signaling in the VTA via AAV‐Cre infusion. The mice were then subjected to chronic physical or emotional CSDS and their susceptibility to stress was measured using the social interaction (SI) test. Following SI testing, mice underwent a two‐bottle choice assay to assess voluntary morphine preference and consumption. In contrast to previous data that found that decreasing AKT signaling was sufficient to increase susceptibility to stress, we found that decreased VTA TORC2 signaling did not alter stress susceptibility. However, we did observe a difference in morphine consumption, as Rictor knockout mice (both TH‐Cre positive and AAV‐Cre) that underwent stress consumed significantly more morphine than stressed controls (intact TORC2 signaling). Additionally, following stress, Rictor knock‐out mice also exhibited an increase in water and sucrose consumption, suggestive of an overall increase in consummative drive. Importantly, these changes were only evident in mice following stress, suggesting a novel role for VTA TORC2 signaling in stress‐mediated changes in consumption.Support or Funding InformationThis work was supported by NIH T32 GM092715 (SK) and NIDA DA037426 (MMR).

The effect of Gly–Gln [ß-endorphin30–31] on morphine-evoked serotonin and GABA efflux in the nucleus accumbens of conscious rats

Glycyl-L-glutamine (Gly–Gln; β-endorphin30–31) is an endogenous dipeptide synthesized through the post-translational processing of β-endorphin1–31. Central Gly–Gln administration inhibits the rewarding properties of morphine and attenuates morphine tolerance, dependence and withdrawal although it does not interfere with morphine analgesia. In an earlier study, we found that Gly–Gln inhibits morphine-induced dopamine efflux in the nucleus accumbens (NAc), consistent with its ability to inhibit morphine reward. To further investigate the mechanism responsible for its central effects we tested whether i.c.v. Gly–Gln administration influences the rise in extracellular serotonin and GABA concentrations evoked by morphine in the NAc. Conscious rats were treated with Gly–Gln (100nmol/5μl) or saline i.c.v. followed, 2min later, by morphine (2.5mg/kg) or saline i.p. and extracellular serotonin and GABA concentrations were analyzed by microdialysis and HPLC. Morphine administration increased extracellular serotonin and GABA concentrations significantly within 20min, as shown previously. Unexpectedly, Gly–Gln also increased extracellular serotonin concentrations significantly in control animals. Combined treatment with Gly–Gln+morphine also elevated extracellular serotonin concentrations although the magnitude of the response did not differ significantly from the effect of Gly–Gln or morphine, given alone suggesting that Gly–Gln suppressed morphine induced serotonin efflux. Gly–Gln abolished the morphine-induced rise in extracellular GABA concentrations but had no effect on extracellular GABA when given alone to otherwise untreated animals. These data show that Gly–Gln stimulates NAc serotonin efflux and, together with earlier studies, support the hypothesis that Gly–Gln inhibits the rewarding effects of morphine by modulating morphine induced dopamine, GABA and serotonin efflux in the NAc.

Differential effects of intra-accumbal orexin-1 and -2 receptor antagonists on the expression and extinction of morphine-induced conditioned place preference in rats

Orexinergic neurons originate from the hypothalamic nuclei, sending projections toward mesolimbic regions such as the nucleus accumbens (NAc). In this study, an attempt was made to determine the effects of intra-accumbal administration of SB334867 as an orexin-1 receptor (OX1R) antagonist and TCS OX2 29 as an orexin-2 receptor (OX2R) antagonist in the expression and maintenance of morphine-induced conditioned place preference (CPP) in rats. One hundred and five adult Wistar rats weighing 200–280g were bilaterally implanted with cannulae into the NAc. During the 3-day conditioning phase, animals received daily subcutaneous administration of morphine (5mg/kg). CPP score and locomotor activity of animals were recorded by Ethovision software. Different doses of bilateral injections of the OX1R and OX2R antagonists (3, 30 and 300μg/0.5μl DMSO) were administered just before the conditioning test or daily injection during extinction phase. Our finding revealed that intra-accumbal administration of OX1R not OX2R antagonist just before the CPP test attenuated the expression of the morphine-induced CPP. However, the blockade of these two kinds of receptors shortened the extinction phase in the rats. This effect was more significant in intra-NAc OX1R antagonist-treated animals. The results suggested that OX1R within the NAc may be necessary for the morphine-induced expression. Additionally, it seems that the existence of the orexin receptors in the NAc was important for the maintenance of morphine rewarding properties during the extinction phase. Therefore, orexins may be considered as a promising therapeutic agent in preventing the expression and maintenance of morphine rewarding effects on dependent individuals.

Altered nociception and morphine tolerance in neuropeptide FF receptor type 2 over-expressing mice.

The neuropeptide FF system is thought to act as an anti-opioid modulator and plays a role in nociception, morphine antinociception and dependence. Two receptor subtypes, NPFFR1 and NPFFR2, have been identified, but their respective roles in these processes remain uncertain. In the present study, the role of NPFFR2 was investigated using transgenic mice over-expressing NPFFR2 in addition to a NPFFR2 agonist AC-263093. NPFFR2 Tg mice exhibited increased sensitivity to both mechanical and thermal noxious stimuli compared to the WT mice, while the antinociceptive effects of morphine at three different doses (6.25, 12.5 and 25mg/kg, s.c.) were similar in both strains. The development of tolerance to morphine antinociception after chronic morphine treatment (12.5mg/kg, s.c.; twice daily×5days) was attenuated in NPFFR2 Tg mice when compared to WT mice. Similarly, WT mice receiving AC-263093 pretreatment (2.5mg/kg, i.p.) showed attenuated morphine tolerance compared to vehicle controls. Most naloxone-precipitated morphine withdrawal symptoms were not attenuated in NPFFR2 Tg mice, with the exception of wet dog shake that was significantly reduced. Both NPFFR2 Tg and WT mice displayed similar degree of morphine rewarding. Our results suggest that neuropeptide FF R2 is mainly involved in the modulation of nociception and tolerance to morphine antinociception.

Regulator of G-Protein Signaling 7 Regulates Reward Behavior by Controlling Opioid Signaling in the Striatum

BackgroundMorphine mediates its euphoric and analgesic effects by acting on the μ-opioid receptor (MOR). MOR belongs to the family of G-protein coupled receptors whose signaling efficiency is controlled by the regulator of G-protein signaling (RGS) proteins. Our understanding of the molecular diversity of RGS proteins that control MOR signaling, their circuit specific actions, and underlying cellular mechanisms is very limited. MethodsWe used genetic approaches to ablate regulator of G-protein signaling 7 (RGS7) both globally and in specific neuronal populations. We used conditioned place preference and self-administration paradigms to examine reward-related behavior and a battery of tests to assess analgesia, tolerance, and physical dependence to morphine. Electrophysiology approaches were applied to investigate the impact of RGS7 on morphine-induced alterations in neuronal excitability and plasticity of glutamatergic synapses. At least three animals were used for each assessment. ResultsElimination of RGS7 enhanced reward, increased analgesia, delayed tolerance, and heightened withdrawal in response to morphine administration. RGS7 in striatal neurons was selectively responsible for determining the sensitivity of rewarding and reinforcing behaviors to morphine without affecting analgesia, tolerance, and withdrawal. In contrast, deletion of RGS7 in dopaminergic neurons did not influence morphine reward. RGS7 exerted its effects by controlling morphine-induced changes in excitability of medium spiny neurons in nucleus accumbens and gating the compositional plasticity of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid and N-methyl-D-aspartate receptors. ConclusionsThis study identifies RGS7 as a novel regulator of MOR signaling by dissecting its circuit specific actions and pinpointing its role in regulating morphine reward by controlling the activity of nucleus accumbens neurons.

Pleiotrophin modulates morphine withdrawal but has no effects on morphine-conditioned place preference

Pleiotrophin (PTN) is a neurotrophic factor with important functions in addiction and neurodegenerative disorders. Morphine administration induces an increase in the expression of PTN and Midkine (MK), the only other member of this family of cytokines, in brain areas related with the addictive effects of drug of abuse, like the Ventral Tegmental Area or the hippocampus. In spite of previous studies showing that PTN modulates amphetamine and ethanol rewarding effects, and that PTN is involved in morphine-induced analgesia, it was still unknown if the rewarding effects of morphine may be regulated by endogenous PTN. Thus, we aim to study the role of PTN in the reward and physical dependence induced by morphine.We used the Conditioned Place Preference (CPP) paradigm in PTN genetically deficient (PTN−/−) and wild type (WT) mice to assess the rewarding effects of morphine in absence of endogenous PTN. Second, to study if PTN may be involved in morphine physical dependence, naloxone-precipitated withdrawal syndrome was induced in PTN−/− and WT morphine dependent mice. Although the increase in the time spent in the morphine-paired compartment after conditioning tended to be more pronounced in PTN−/− mice, statistical significance was not achieved. The data suggest that PTN does not exert an important role in morphine reward. However, our results clearly indicate that PTN−/− mice develop a more severe withdrawal syndrome than WT mice, characterized as a significant increase in the time standing and in the total incidences of forepaw licking, forepaw tremors, wet dog shake and writhing. The data presented here suggest that PTN is a novel genetic factor that plays a role in morphine withdrawal syndrome.