Yao-Ying Ma

The role of NR2B containing NMDA receptor in place preference conditioned with morphine and natural reinforcers in rats

It has been reported that N-methyl-D-aspartate (NMDA) receptor is implicated in drug addiction and antagonists of the NMDA receptor complex can inhibit the development and expression of conditioned place preference (CPP) induced by several addictive drugs, implying that this class of compounds might be considered as candidate for the treatment of substance abuse. To explore this possibility, it is important to evaluate whether the inhibitory effect of NMDA receptor antagonists would be confined to behaviors produced by drugs of abuse only, but not by natural reinforcers. According to the quantitative changes of NMDA receptor subunits, including NR1, NR2A, and NR2B, induced by diverse types of reinforcers, we chose NR2B subunit as the target of research. Experimental results showed that (1) an augmented expression of NR2B subunit was revealed by Western blotting in the nucleus accumbens (NAc) and the hippocampus in rats with CPP induced by morphine, but not by natural rewards such as food, novel environment and social interaction. (2) Ifenprodil, an antagonist highly selective for NR2B subunit of the NMDA receptor, produced a dose-dependent reduction in CPP induced by morphine and novel environment, but not that by food consumption and social interaction. Taking together, these findings suggested that NR2B containing NMDA receptor may be more involved with morphine reward rather than natural rewards, and that antagonism of NR2B may have a potential for the treatment of morphine abuse.

NR2B-containing NMDA receptor is required for morphine-but not stress-induced reinstatement

Glutamate receptors are known to be densely distributed in the forebrain rewarding circuits, and glutamatergic transmission is actively involved in the regulation of rewarding and reinstating effects of drugs of abuse. Here we investigated the possible involvement of the N-methyl-D-aspartate (NMDA) receptors in the reinstatement of extinguished morphine conditioned place preference (CPP) in rats. We found that previously extinguished morphine (3 mg/kg, i.p.) CPP was markedly reinstated by a priming injection of morphine (2 mg/kg, i.p.) or an acute environmental stressor (forced swim for 10 min), but not by the stress induced by a 24-h food deprivation. Parallel with this, protein levels of the NMDA receptor 2B subunit (NR2B) were elevated in the nucleus accumbens (NAc) and the hippocampus, but not the prefrontal cortex, of reinstated rats. Systemic administration of an NR2B selective antagonist ifenprodil (1, 3, 10 mg/kg, i.p.) attenuated the reinstatement induced by a priming morphine injection, although not by the forced swim. Ifenprodil (2.0 microg/rat) directly injected into the NAc shell or the CA1 region of the dorsal hippocampus produced a similar effect. These results indicate that the NR2B-containing NMDA receptors in the NAc and the dorsal hippocampus play a significant role in mediating the reinstatement of rewarding responses to morphine.

Morphine-induced conditioned place preference in rats is inhibited by electroacupuncture at 2 Hz: Role of enkephalin in the nucleus accumbens

Our previous studies have demonstrated that morphine-induced conditioned place preference (CPP) can be inhibited by 2 Hz electroacupuncture (EA). This inhibition can be blocked by either the opioid receptor antagonist naloxone (i.p.) or lesion in the nucleus accumbens (NAc), providing evidence that endogenous opioid system in the NAc mediates the effects of EA. Here we report that 1) A single session of 2 Hz EA produced a significant increase of the content of enkephalin in the NAc of morphine-induced CPP rats, and this effect was stronger in three consecutive sessions of EA; 2) Intracerebroventricular injection of the mu-opioid receptor antagonist CTAP or delta-opioid receptor antagonist NTI, but not kappa-opioid receptor antagonist nor-BNI, dose-dependently reversed the inhibitory effects of 2 Hz EA on the expression of morphine-induced CPP; 3) Three consecutive sessions of 2 Hz EA up-regulated the mRNA level of preproenkephalin in the NAc of morphine-induced CPP rats. The results suggest that the inhibitory effects of 2 Hz EA on the expression of the morphine CPP is mediated by mu- and delta-, but not kappa-opioid receptor, possibly via accelerating both the release and synthesis of enkephalin in the NAc. These findings support the possibility of using 2 Hz EA for the treatment of opiate addiction.

Dose- and time-dependent, context-induced elevation of dopamine and its metabolites in the nucleus accumbens of morphine-induced CPP rats

The dopamine (DA) projections from the ventral tegmental area to the nucleus accumbens (NAc) are the key component of the brain reward circuitry. The encoded information by DA in reward-related memory within this circuit during opiate reinforcement requires further clarification. The present study was designed to explore the correlations between morphine dose, retention of morphine-induced conditioned place preference (CPP), morphine-induced changes in levels of DA and its metabolites in the NAc in expression and retention of CPP in Sprague-Dawley male rats. A dose-effect curve for morphine-induced CPP (0.01-10 mg/kg, i.p.) was obtained using 4-day conditioning sessions followed by a CPP test; the retention of morphine CPP was measured with CPP tests after the development of CPP. We found a dose-dependent effect of morphine (from 0.01 to 10.0 mg/kg, i.p.) on both the magnitude and the retention of CPP. During the retention of morphine-induced CPP, a morphine-dose- and time-dependent elevation of DA and its metabolites was observed in the NAc. These changes were absent if the same dose of morphine was injected outside of the conditioning environment (i.e., in the home cage). These results suggest that that the long-lasting elevation of DA and its metabolites in the NAc is attributable mainly to drug-associated context, rather than the residual effect of morphine.

NMDA Receptor in Nucleus Accumbens is Implicated in Morphine Withdrawal in Rats

The purpose of the present study is to elucidate whether ketamine, a non-competitive antagonist of the NMDA receptor, can suppress the morphine withdrawal syndrome in rats at a dose without affecting motor functions and to identify its site of action in the central nervous system. Rats were made dependent on morphine by multiple injections of morphine hydrochloride for 5 days. They were then given ketamine at the following doses and routes of administration: (a) intraperitoneal (i.p.) injections (2–16 mg/kg), (b) intracerebroventricular (i.c.v.) injections (4–100 μg), and (c) intra-nucleus accumbens (NAc) or intra-amygdalar microinjections (0.4–10 μg). Naloxone HCl (1 mg/kg, i.p.) was administered 3 h after the last ketamine injection to precipitate withdrawal syndrome, which was scored within a period of 30 min. Results showed that some of the precipitated withdrawal signs were dose-dependently suppressed by repeated injections of ketamine at 8 and 16 mg/kg, i.p. or 100 μg, i.c.v. Dose-dependent suppression was observed by repeated microinjections (0.4–10 μg) of ketamine to NAc, but not to amygdala. These results indicate that the NMDA receptor antagonist ketamine has the ability to suppress morphine withdrawal syndrome in experimental settings without motor interference, and NAc could be the critical CNS site mediating such effect.

Glutamatergic Projections from the Entorhinal Cortex to Dorsal Dentate Gyrus Mediate Context-Induced Reinstatement of Heroin Seeking

Reexposure to the context associated with heroin intake provokes relapse to drug taking after abstinence. The dorsal dentate gyrus (dDG) and entorhinal cortex (EC) have been implicated in contextual memory processing, but the underlying circuit mechanisms in context-induced relapse remain poorly understood. In this study, using a self-administration rat model, we found that activation and synaptic transmission of glutamatergic projections from the EC to the upper blade of dentate gyrus (dDGub) were significantly enhanced during context-induced reinstatement of heroin seeking. This effect was associated with increased of phosphorylation of GluN2B-containing NMDA receptors (GluN2B) at Y1472, ratio of GluN2B membrane/total protein levels, and expression of downstream extracellular signal-regulated kinase-1/2 (ERK1/2) in the dDG region. Furthermore, DREADD-mediated specific inactivation of the EC–dDG pathway or disconnection of the pathway with local postsynaptic GluN2B–ERK1/2 signaling both decreased context-induced reinstatement of heroin seeking. These experimental manipulations had no effect on saccharin-reinforced responding and general locomotor activity in rats. Our results indicate that the EC–dDG pathway mediates context-induced reinstatement of heroin seeking, via the activation of postsynaptic GluN2B–ERK1/2 signaling in the dDG.

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 PESs suppression of the rewarding effect of drug-associated environmental cues in the rat.