Role of α1-GABAA receptors in the serotonergic dorsal raphe nucleus in models of opioid reward, anxiety, and depression.

BACKGROUND: The serotonin (5-hydroxytryptamine (5-HT))-mediated system plays an important role in stress-related psychiatric disorders and substance abuse. Our previous studies showed that stress and drug exposure can modulate the dorsal raphe nucleus (DRN)-5-HT system via γ-aminobutyric acid (GABA)A receptors. Moreover, GABAA receptor-mediated inhibition of serotonergic DRN neurons is required for stress-induced reinstatement of opioid seeking. AIM/METHODS: To further test the role of GABAA receptors in the 5-HT system in stress and opioid-sensitive behaviors, our current study generated mice with conditional genetic deletions of the GABAA α1 subunit to manipulate GABAA receptors in either the DRN or the entire population of 5-HT neurons. The GABAA α1 subunit is a constituent of the most abundant GABAA subtype in the brain and the most highly expressed subunit in 5-HT DRN neurons. RESULTS: Our results showed that mice with DRN-specific knockout of α1-GABAA receptors exhibited a normal phenotype in tests of anxiety- and depression-like behaviors as well as swim stress-induced reinstatement of morphine-conditioned place preference. By contrast, mice with 5-HT neuron-specific knockout of α1-GABAA receptors exhibited an anxiolytic phenotype at baseline and increased sensitivity to post-morphine withdrawal-induced anxiety. CONCLUSIONS: Our data suggest that GABAA receptors on 5-HT neurons contribute to anxiety-like behaviors and sensitivity of those behaviors to opioid withdrawal.

The ventral hippocampus and nucleus accumbens as neural substrates for cocaine contextual memory reconsolidation.

Drug craving triggered by cues that were once associated with drug intoxication is a major contributor to continued drug-seeking behaviors. Addictive drugs engage molecular pathways of associative learning and memory. Reactivated memories are vulnerable to disruption by interference with the process of reconsolidation, hence targeting reconsolidation could be a strategy to reduce cue-induced drug craving and relapse. Here we examined the circuitry of cocaine contextual memory reconsolidation and explored neuroplasticity following memory reactivation. Mice underwent chemogenetic inhibition of either nucleus accumbens (NA) neurons or the glutamatergic projection neurons from the ventral hippocampus (vHPC) to NA using inhibitory designer receptors exclusively activated by designer drugs (iDREADD). Mice underwent cocaine conditioned place preference followed by reactivation of the cocaine contextual memory. Clozapine-N-oxide (CNO) was administered after memory reactivation to inhibit either NA neurons or the accumbens-projecting vHPC neurons during the reconsolidation period. When retested 3 days later, a significant reduction in the previously established preference for the cocaine context was found in both conditions. FosTRAP2-Ai14 mice were used to identify neurons activated by cocaine memory recall and to evaluate plasticity in NA medium spiny neurons (MSNs) and vHPC pyramidal neurons upon recall of cocaine memories. Results indicate a significant increase in dendritic spine density in NA MSNs activated by cocaine memory recall, particularly of the thin spine type. Sholl analysis indicated longer dendritic length and more branching of NA MSNs after cocaine memory recall than without memory reactivation. vHPC neurons showed increased spine density, with the most robust change in stubby spines. These results implicate a circuit involving glutamatergic projections from the vHPC onto NA neurons which is necessary for the reconsolidation of cocaine memories. Interruption of cocaine memory reconsolidation reduced drug-seeking behavior.

Reactivation of cocaine contextual memory engages mechanistic target of rapamycin/S6 kinase 1 signaling.

Mechanistic target of rapamycin (mTOR) C1 and its downstream effectors have been implicated in synaptic plasticity and memory. Our prior work demonstrated that reactivation of cocaine memory engages a signaling pathway consisting of Akt, glycogen synthase kinase-3ß (GSK3ß), and mTORC1. The present study sought to identify other components of mTORC1 signaling involved in the reconsolidation of cocaine contextual memory, including eukaryotic translation initiation factor 4E (eIF4E)-eIF4G interactions, p70 S6 kinase polypeptide 1 (p70S6K, S6K1) activity, and activity-regulated cytoskeleton (Arc) expression. Cocaine contextual memory was established in adult CD-1 mice using conditioned place preference. After cocaine place preference was established, mice were briefly re-exposed to the cocaine-paired context to reactivate the cocaine memory and brains examined. Western blot analysis showed that phosphorylation of the mTORC1 target, p70S6K, in nucleus accumbens and hippocampus was enhanced 60 min following reactivation of cocaine memories. Inhibition of mTORC1 with systemic administration of rapamycin or inhibition of p70S6K with systemic PF-4708671 after reactivation of cocaine contextual memory abolished the established cocaine place preference. Immunoprecipitation assays showed that reactivation of cocaine memory did not affect eIF4E-eIF4G interactions in nucleus accumbens or hippocampus. Levels of Arc mRNA were significantly elevated 60 and 120 min after cocaine memory reactivation and returned to baseline 24 h later. These findings demonstrate that mTORC1 and p70S6K are required for reconsolidation of cocaine contextual memory.

Centrally Acting Angiotensin-Converting Enzyme Inhibitor Suppresses Type I Interferon Responses and Decreases Inflammation in the Periphery and the CNS in Lupus-Prone Mice.

Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by multi-organ damage. Neuropsychiatric lupus (NPSLE) is one of the most common manifestations of human SLE, often causing depression. Interferon-α (IFNα) is a central mediator in disease pathogenesis. Administration of IFNα to patients with chronic viral infections or cancers causes depressive symptoms. Angiotensin-converting enzyme (ACE) is part of the kallikrein-kinin/renin-angiotensin (KKS/RAS) system that regulates many physiological processes, including inflammation, and brain functions. It is known that ACE degrades bradykinin (BK) into inactive peptides. We have previously shown in an in vitro model of mouse bone-marrow-derived dendritic cells (BMDC) and human peripheral blood mononuclear cells that captopril (a centrally acting ACE inhibitor-ACEi) suppressed Type I IFN responsive gene (IRG) expression. In this report, we used the MRL/lpr lupus-prone mouse model, an established model to study NPSLE, to determine the in vivo effects of captopril on Type I IFN and associated immune responses in the periphery and brain and effects on behavior. Administering captopril to MRL/lpr mice decreased expression of IRGs in brain, spleen and kidney, decreased circulating and tissue IFNα levels, decreased microglial activation (IBA-1 expression) and reduced depressive-like behavior. Serotonin levels that are decreased in depression were increased by captopril treatment. Captopril also reduced autoantibody levels in plasma and immune complex deposition in kidney and brain. Thus, ACEi's may have potential for therapeutic use for systemic and NPSLE.

Glycogen Synthase Kinase 3ß in the Ventral Hippocampus is Important for Cocaine Reward and Object Location Memory.

The ventral hippocampus is a component of the neural circuitry involved with context-associated memory for reward and generation of appropriate behavioral responses to context. Glycogen synthase kinase 3 beta (GSK3ß) has been linked to the maintenance of synaptic plasticity, contextual memory retrieval, and is involved in the reconsolidation of cocaine-associated contextual memory. In this study, the effects of targeted downregulation of GSK3ß in the ventral hippocampus were examined on a series of behavioral tests for assessing drug reward-context association and non-reward related memory. The Cre/loxP site-specific recombination system was used to knockdown GSK3ß through bilateral stereotaxic delivery of an adeno-associated virus expressing Cre-recombinase (AAV-Cre) into the ventral hippocampus of adult mice homozygous for a floxed GSK3ß allele. GSK3ß floxed mice injected with AAV-Cre had a loss of 56-75% of GSK3ß in the ventral hippocampus and displayed diminished development of cocaine conditioned place preference, but not morphine place preference as compared with wild-type mice injected with AAV-Cre or GSK3ß floxed mice injected with a control virus, AAV-GFP. Impaired object location memory was observed in mice with GSK3ß downregulation in the ventral hippocampus, but novel object recognition remained intact. These results indicate that GSK3ß signaling in the ventral hippocampus is differentially involved in the formation of place-drug reward association dependent upon drug class. Additionally, ventral hippocampal GSK3ß signaling is important in detection of discrete spatial cues, but not recognition memory for objects.

Differential Roles of Accumbal GSK3ß in Cocaine versus Morphine-Induced Place Preference, U50,488H-Induced Place Aversion, and Object Memory.

Previous research has demonstrated that activity of glycogen synthase kinase-3 (GSK3) is necessary for the rewarding effects of cocaine. In the present study, a conditional GSK3ß gene knockdown model was used to determine if GSK3ß activity specifically in the nucleus accumbens is important for cocaine conditioned reward. The roles of accumbal GSK3ß in morphine conditioned reward, trans-(±)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeneacetamide methanesulfonate salt (U50,488H)-induced conditioned place aversion, and cognitive function were also studied. Adult male and female GSK3ß-floxed or wild-type mice were injected with adeno-associated virus/Cre into the nucleus accumbens to reduce expression of GSK3ß and underwent behavioral testing 4 weeks later. The development of cocaine-induced conditioned place preference was significantly attenuated in mice with reduced levels of GSK3ß in the nucleus accumbens, whereas the development of morphine-induced place preference remained intact. Conditional knockdown of GSK3ß in the accumbens prevented the development of conditioned aversion produced by U50,488H, a κ-opioid receptor agonist. Cognitive memory tests revealed deficits in object location memory, but not novel object recognition in mice with accumbal GSK3ß knockdown. These data demonstrate that GSK3ß in the nucleus accumbens is required for cocaine conditioned place preference and U50,488H conditioned place aversion, as well as spatial memory in object location task, indicating differential roles of GSK3ß in the psychostimulant and opiate reward process, as well as in memory for spatial locations and object identity. SIGNIFICANCE STATEMENT: Knockdown of GSK3ß in the nucleus accumbens attenuated the development of cocaine-induced place preference, as well as conditioned place aversion to U50,488H, a κ-opioid receptor agonist. In contrast, the development of morphine place preference was not altered by GSK3ß knockdown. GSK3ß knockdown in nucleus accumbens impaired performance in the object location task, but not the novel object recognition task. These results elucidate different physiological roles of accumbal GSKß in conditioned reward, aversion, and memory.

Activation of GSK3ß induced by recall of cocaine reward memories is dependent on GluN2A/B NMDA receptor signaling.

Glycogen synthase kinase-3ß (GSK3ß) is a critical regulator of the balance between long-term depression and long-term potentiation which is essential for learning and memory. Our previous study demonstrated that GSK3ß activity is highly induced during cocaine memory reactivation, and that reconsolidation of cocaine reward memory is attenuated by inhibition of GSK3ß. NMDA receptors and protein phosphatase 1 (PP1) are activators of GSK3ß. Thus, this study investigated the roles of NMDA receptor subtypes and PP1in the reconsolidation of cocaine contextual reward memory. Cocaine contextual memories were established and evaluated using cocaine conditioned place preference methods. The regulation of GSK3ß activity in specific brain areas was assessed by measuring its phosphorylation state using immunoblot assays. Mice underwent cocaine place conditioning for 8 days and were tested for place preference on day 9. Twenty-four hours later, mice were briefly confined to the compartment previous paired with cocaine to reactivate cocaine-associated memories. Administration of the GluN2A- and GluN2B-NMDA receptor antagonists, NVP-AAM077 and ifenprodil, respectively, immediately following recall abrogated an established cocaine place preference, while preventing the activation of GSK3ß in the amygdala, nucleus accumbens, and hippocampus during cocaine memory reactivation. PP1 inhibition with okadaic acid also blocked the activation of GSK3ß and attenuated a previously established cocaine place preference. These findings suggest that the dephosphorylation of GSK3ß that occurred upon activation of cocaine-associated reward memories may be initiated by the activation of PP1 during the induction of NMDA receptor-dependent reconsolidation of cocaine mnemonic traces. Moreover, the importance of NMDA receptors and PP1 in reconsolidation of cocaine memory makes them potential therapeutic targets in treatment of cocaine use disorder and prevention of relapse.

Cocaine-induced neuroadaptations in the dorsal striatum: glutamate dynamics and behavioral sensitization.

Recent evidence suggests that diminished ability to control cocaine seeking arises from perturbations in glutamate homeostasis in the nucleus accumbens. However, the neurochemical substrates underlying cocaine-induced neuroadaptations in the dorsal striatum and how these mechanisms link to behavioral plasticity is not clear. We employed glutamate-sensitive microelectrodes and amperometry to study the impact of repeated cocaine administration on glutamate dynamics in the dorsolateral striatum of awake freely-moving rats. Depolarization-evoked glutamate release was robustly increased in cocaine-pretreated rats challenged with cocaine. Moreover, the clearance of glutamate signals elicited either by terminal depolarization or blockade of non-neuronal glutamate transporters slowed down dramatically in cocaine-sensitized rats. Repeated cocaine exposure also reduced the neuronal tone of striatal glutamate. Ceftriaxone, a ß-lactam antibiotic that activates the astrocytic glutamate transporter, attenuated the effects of repeated cocaine exposure on synaptic glutamate release and glutamate clearance kinetics. Finally, the antagonism of AMPA glutamate receptors in the dorsolateral striatum blocked the development of behavioral sensitization to repeated cocaine administration. Collectively, these data suggest that repeated cocaine exposure disrupts presynaptic glutamate transmission and transporter-mediated clearance mechanisms in the dorsal striatum. Moreover, such alterations produce an over activation of AMPA receptors in this brain region leading to the sensitized behavioral response to repeated cocaine.

Reactivation of cocaine reward memory engages the Akt/GSK3/mTOR signaling pathway and can be disrupted by GSK3 inhibition.

RATIONAL: Memories return to a labile state following their retrieval and must undergo a process of reconsolidation to be maintained. Thus, disruption of cocaine reward memories by interference with reconsolidation may be therapeutically beneficial in the treatment of cocaine addiction. OBJECTIVE: The objectives were to elucidate the signaling pathway involved in reconsolidation of cocaine reward memory and to test whether targeting this pathway could disrupt cocaine-associated contextual memory. METHODS: Using a mouse model of conditioned place preference, regulation of the activity of glycogen synthase kinase-3 (GSK3), mammalian target of Rapamycin complex 1 (mTORC1), P70S6K, ß-catenin, and the upstream signaling molecule Akt, was studied in cortico-limbic-striatal circuitry after re-exposure to an environment previously paired with cocaine. RESULT: Levels of phosporylated Akt-Thr308, GSK3α-Ser21, GSK3ß-Ser9, mTORC1, and P70S6K were reduced in the nucleus accumbens and hippocampus 10 min after the reactivation of cocaine cue memories. Levels of pAkt and pGSK3 were also reduced in the prefrontal cortex. Since reduced phosphorylation of GSK3 indicates heightened enzyme activity, the effect of a selective GSK3 inhibitor, SB216763, on reconsolidation was tested. Administration of SB216763 immediately after exposure to an environment previously paired with cocaine abrogated a previously established place preference, suggesting that GSK3 inhibition interfered with reconsolidation of cocaine-associated reward memories. CONCLUSIONS: These findings suggest that the Akt/GSK3/mTORC1 signaling pathway in the nucleus accumbens, hippocampus, and/or prefrontal cortex is critically involved in the reconsolidation of cocaine contextual reward memory. Inhibition of GSK3 activity during memory retrieval can erase an established cocaine place preference.

The suppressive effect of an intra-prefrontal cortical infusion of BDNF on cocaine-seeking is Trk receptor and extracellular signal-regulated protein kinase mitogen-activated protein kinase dependent.

Cocaine-mediated neuroadaptations in the prefrontal cortical-nucleus accumbens pathway underlie drug-seeking in animals with a cocaine self-administration (SA) history. Neuroplasticity in the cortico-accumbens pathway is regulated, in part, by the expression and availability of neurotrophic factors, such as BDNF. We have previously demonstrated that infusion of BDNF into the dorsomedial prefrontal cortex (dmPFC) immediately after the last of 10 cocaine SA sessions attenuates contextual, cue- and cocaine prime-induced reinstatement of cocaine-seeking (Berglind et al., 2007) and normalizes cocaine-induced disruption of glutamatergic transmission in the nucleus accumbens (Berglind et al., 2009). In the present study, the suppressive effect of intra-dmPFC BDNF on cocaine-seeking is shown to depend on Trk receptor-mediated activation of extracellular signal-regulated kinase (ERK) signaling in the dmPFC. The tyrosine kinase inhibitor, K252a, and the mitogen-activated protein/extracellular signal-regulated kinase kinase inhibitor, U0126 (1,4-diamino-2,3-dicyano-1,4-bis[2-aminophenylthio]butadiene), prevented BDNF's suppressive effects on cocaine-seeking. Vehicle-infused rats with a cocaine SA history showed significant decreases in ERK and cyclic AMP response element binding protein (CREB), but not Akt, phosphorylation after the final cocaine SA session that were reversed by intra-dmPFC BDNF. Additionally, BDNF's ability to normalize cocaine-mediated decreases in ERK and CREB phosphorylation was blocked by U0126, demonstrating that ERK/MAPK activation mediated the behavioral effects. This study elucidates a mechanism whereby BDNF/TrkB (tropomyosin receptor kinase B) activates ERK-regulated CREB phosphorylation in the dmPFC to counteract the neuroadaptations induced by cocaine SA and subsequent relapse to cocaine-seeking.

D1 and D2 dopamine receptors differentially mediate the activation of phosphoproteins in the striatum of amphetamine-sensitized rats.

RATIONALE: Extracellular signal-regulated kinase (ERK), cAMP response element binding protein (CREB), and protein kinase B (PKB or Akt) in the striatum are differentially activated by acute and repeated amphetamine (AMPH) administration. However, the dopamine receptor subtypes that mediate transient vs. prolonged phosphorylation changes in these proteins induced by AMPH challenge in AMPH-sensitized rats are unknown. OBJECTIVES: The role of the D1 and D2 class of dopamine receptors in the differential phosphorylation of striatal ERK, CREB, Thr308-Akt and Ser473-Akt and the expression of behavioral sensitization induced by AMPH challenge in AMPH-pretreated rats were determined. METHODS: D1 or D2 dopamine receptor antagonists were injected before an AMPH challenge in AMPH-sensitized rats. After behavioral activity was recorded, rats were euthanized either 15 min or 2 h after AMPH challenge and striatal phosphoprotein status was analyzed by Western blotting. RESULTS: The D1 receptor antagonist (SCH23390) decreased stereotypical behavior whereas the D2 receptor antagonist (eticlopride) decreased all behavioral activity induced by an AMPH challenge in AMPH-sensitized rats. SCH23390, but not eticlopride, significantly decreased ERK, CREB, and Thr308-Akt phosphorylation in the striatum 15 min, and ERK and CREB phosphorylation 2 h, after AMPH challenge in AMPH-sensitized rats. In contrast, eticlopride, but not SCH23390, prevented a decrease in Akt phosphorylation 2 h after AMPH challenge. CONCLUSIONS: These data indicate that the time course of phosphoprotein signaling is differentially regulated by D1 and D2 receptors in the striatum of AMPH-sensitized rats, suggesting that complex regulatory interactions are activated by repeated AMPH exposure.

Peripheral electrical stimulation-induced suppression of morphine-induced CCP in rats: a role for dopamine in the nucleus accumbens.

In the previous study we reported that morphine-induced conditioned place preference (CPP) could be suppressed by peripheral electric stimulation (PES), an effect related to the increased gene expression of opioid peptides in the central nervous system. Considering that opioids were known to elevate dopamine (DA) activity in the mesolimbic brain, the present study was designed to further analyze the possible involvement of the mesolimbic dopaminergic system (MLDS) in the suppressive effect of PES on the rewarding effects of morphine in SD male rats. We found that morphine-induced CPP can be successfully suppressed by PES, an effect accompanied by a reversal of the increased tissue contents of DA and its metabolites in the nucleus accumbens (NAc) of morphine-induced CPP rats. Our results suggest that MLDS seems to play important roles in the mechanisms underlying PES's suppression of the rewarding effect of drug-associated environmental cues in the rat.

Regulation of psychostimulant-induced signaling and gene expression in the striatum.

Amphetamine (AMPH) and cocaine are indirect dopamine agonists that activate multiple signaling cascades in the striatum. Each cascade has a different subcellular location and duration of action that depend on the strength of the drug stimulus. In addition to activating D1 dopamine-Gs-coupled-protein kinase A signaling, acute psychostimulant administration activates extracellular-regulated kinase transiently in striatal cells; conversely, inhibition of extracellular-regulated kinase phosphorylation decreases the ability of psychostimulants to elevate locomotor behavior and opioid peptide gene expression. Moreover, a drug challenge in rats with a drug history augments and prolongs striatal extracellular-regulated kinase phosphorylation, possibly contributing to behavioral sensitization. In contrast, AMPH activates phosphoinositide-3 kinase substrates, like protein kinase B/Akt, only in the nuclei of striatal cells but this transient increase induced by AMPH is followed by a delayed decrease in protein kinase B/Akt phosphorylation whether or not the rats have a drug history, suggesting that the phosphoinositide-3 kinase pathway is not essential for AMPH-induced behavioral sensitization. Chronic AMPH or cocaine also alters the regulation of inhibitory G protein-coupled receptors in the striatum, as evident by a prolonged decrease in the level of regulator of G protein signaling 4 after non-contingent or contingent (self-administered) drug exposure. This decrease is exacerbated in behaviorally sensitized rats and reversed by re-exposure to a cocaine-paired environment. A decrease in regulator of G protein signaling 4 levels may weaken its interactions with metabotropic glutamate receptor 5, Galphaq, and phospholipase C beta that may enhance drug-induced signaling. Alteration of these protein-protein interactions suggests that the striatum responds to psychostimulants with a complex molecular repertoire that both modulates psychomotor effects and leads to long-term neuroadaptations.

Repeated amphetamine treatment increases phosphorylation of extracellular signal-regulated kinase, protein kinase B, and cyclase response element-binding protein in the rat striatum.

Extracellular signal-regulated kinases, protein kinase B/Akt and cyclase response element-binding protein play important roles in drug-induced neuroadaptations. Acute psychostimulant exposure rapidly alters the phosphorylation of these proteins in the striatum but less is known about their responses to repeated stimulant administration. In this study the phosphorylated state of these proteins in rat striatum was analyzed by immunoblotting 15 min and 2 h after amphetamine (AMPH)-induced behavioral sensitization. Two weeks after the last dose of 5 mg/kg, i.p. AMPH once daily for 5 days, rats were challenged with 1 mg/kg, i.p. AMPH or saline and sacrificed 15 min or 2 h later. Sensitization to AMPH-induced behavioral activity was observed in AMPH pre-treated rats after AMPH on the challenge day. Phosphorylation of all three proteins was significantly greater 15 min after AMPH in AMPH-pre-treated than in saline-pre-treated rats. Two hours after AMPH challenge in AMPH-pre-treated rats, phospho-extracellular signal-regulated kinase and phospho-cAMP response element-binding protein immunoreactivity was still significantly elevated but not after AMPH injection in saline-pre-treated rats. In contrast, phospho-Akt was down-regulated to the same extent 2 h after acute AMPH or repeated AMPH with an AMPH challenge. These data implicate differential regulation of phospho-extracellular signal-regulated kinase, phospho-cAMP response element-binding protein versus phospho-Akt in sensitized responses to AMPH.

Repeated peripheral electrical stimulations suppress both morphine-induced CPP and reinstatement of extinguished CPP in rats: accelerated expression of PPE and PPD mRNA in NAc implicated.

Previous studies have shown that peripheral electrical stimulation (PES) can suppress morphine-induced conditioned place preference (CPP) and the reinstatement of extinguished CPP in the rat. The present study was performed to elucidate if preproenkephalin (PPE) and preprodynorphin (PPD) mRNAs in the nucleus accumbens (NAc) play a role in this event. Rats were trained with morphine for 4 days to establish CPP paradigm. They were then given 15-min test once a day for eight consecutive days for extinction trial. Twenty-four hours after the 8th session of extinction trials, rats were given peripheral electrical stimulation (PES) at 2 or 100 Hz once a day for 3 days, then a morphine-priming injection at a dose of 1, 2, or 4 mg/kg to reinstate the extinguished CPP. At the end of the experiment, PPE and PPD mRNA levels in the nucleus acccumbens (NAc) were determined by the semiquantitative RT-PCR technique. The results showed that PES at 2- and 100-Hz administered 30 min a day for 3 days suppressed both the expression of morphine-induced CPP and the reinstatement of extinguished CPP. PES at 2 Hz increased preproenkephalin (PPE) mRNA levels, whereas PES of 100 Hz that of preprodynorphin (PPD) mRNA levels in the NAc. These findings suggest that enkephalin and dynorphin in NAc may play important roles in the mechanisms underlying the inhibitory effect of PES on the expression and reinstatement of morphine-induced CPP in rats.

[Electroacupuncture suppresses morphine--induced conditioned place preference (CPP) in rats].

OBJECTIVE: To examine the effects produced by electroacupuncture (EA) of different frequencies on the expression of morphine conditioned place preference (CPP) in rats. METHODS: SD rats were given 4 days consecutive trials in a computerized three-chamber "unbiased" CPP apparatus. Twenty-four hours later, the time spent on drug-pairing compartment of the rat was examined. Rats trained with CPP paradigms were then given EA of 2 Hz or 100 Hz once a day for 3 days. Twenty-four hours after the final EA session, they were again put to the CPP chamber, and the time spent on drug-pairing compartment was measured. RESULTS: Rats receiving morphine at a dose of 4 mg.kg-1 (i.p.) showed significantly enhanced preference scores in drug-pairing side than that of the control group. In other words, rats preferred the drug-pairing environment to the nondrug-pairing place. In addition, rats that received treatment with EA of 2 Hz or 100 Hz spent significantly less time on the drug pairing side than that of the CPP control group. CONCLUSION: Morphine-induced CPP paradigms were stably established in rats using a computer-controlled 3-chamber CPP experimental system. The expression of CPP could be significantly inhibited by multiple treatments with EA of either 2 Hz or 100 Hz.

Brain opioid-receptors are involved in mediating peripheral electric stimulation-induced inhibition of morphine conditioned place preference in rats.

Conditioned place preference (CPP) paradigm has been suggested as one of the animal models for drug craving. The present study was performed to examine the effect of 100 Hz peripheral electric stimulation (PES) on the expression of morphine-induced CPP. Rats were trained with morphine for 4 days to establish the CPP paradigm in a three-chamber "unbiased" apparatus. Morphine-induced CPP was maintained up to 4 weeks when tests were given once a week. PES of 100 Hz administered 30 min a day for 3 days significantly attenuated morphine-induced CPP (P<0.01). I.c.v. injection of the delta-opioid receptor antagonist naltrindole (NTI) or the kappa-antagonist norbinaltorphimine (nor-BNI) but not the mu-antagonist cyclic D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2) (CTAP), completely blocked the inhibitory effect of 100 Hz PES on the expression of morphine-induced CPP (P<0.05-0.01). These results indicate that the anti-craving effects induced by repeated PES of 100 Hz is mediated by the activation of supra-segmental delta- and kappa-opioid receptors in the central nervous system.