The selective mGlu2/3 receptor antagonist LY341495 exacerbates behavioral signs of morphine withdrawal and morphine-withdrawal-induced activation of locus coeruleus neurons

Glutamatergic Systems and Memory Mechanisms Underlying Opioid Addiction

Glutamate is the main excitatory neurotransmitter in the brain and is of critical importance for the synaptic and circuit mechanisms that underlie opioid addiction. Opioid memories formed over the course of repeated drug use and withdrawal can become powerful stimuli that trigger craving and relapse, and glutamatergic neurotransmission is essential for the formation and maintenance of these memories. In this review, we discuss the mechanisms by which glutamate, dopamine, and opioid signaling interact to mediate the primary rewarding effects of opioids, and cover the glutamatergic systems and circuits that mediate the expression, extinction, and reinstatement of opioid seeking over the course of opioid addiction.

N-acetylcysteine mitigates acute opioid withdrawal behaviors and CNS oxidative stress in neonatal rats

BACKGROUND

Neonatal abstinence syndrome (NAS) is a significant problem. Opioid withdrawal induces oxidative stress and disrupts glutamate and glutathione homeostasis. We hypothesized that N-acetylcysteine (NAC) administered during acute opioid withdrawal in neonatal rats would decrease withdrawal behaviors and normalize CNS glutathione and glutamate.

METHODS

Osmotic minipumps with methadone (opioid dependent, OD) and saline (Sham) were implanted into Sprague Dawley dams 7 days prior to delivery. Pups were randomized to receive either naloxone plus saline or NAC (50-100 mg/kg), administered on postnatal day (PND) 7. We performed MR spectroscopy on PND6-7 before, 30 min, and 120 min after withdrawal. On PND7, we assessed withdrawal behaviors for 90 min after naloxone administration and summed scores during peak withdrawal period.

RESULTS

Mean summed behavioral scores were significantly different between groups (χ² (2) = 10.49, p = 0.005) but not different between NAC/NAL/OD and Sham (p = 0.14): SAL/NAL/OD = 17.2 ± 4.2 (n = 10); NAC/NAL/OD = 11.3 ± 5.6 (n = 9); Sham = 6.5 ± 0.6 (n = 4). SAL/NAL/OD pups had decreased glutathione at 120 min (p = 0.01), while NAC/NAL/OD pups maintained pre-withdrawal glutathione (p = 0.26).

CONCLUSION

In antenatal OD, NAC maintains CNS glutathione and mitigates acute opioid withdrawal in neonatal rats. This is the first study to demonstrate acute opioid withdrawal neurochemical changes in vivo in neonatal OD. NAC is a potential novel treatment for NAS.

Deletion of the type 2 metabotropic glutamate receptor increases heroin abuse vulnerability in transgenic rats

Opioid abuse is a rapidly growing public health crisis in the USA. Despite extensive research in the past decades, little is known about the etiology of opioid addiction or the neurobiological risk factors that increase vulnerability to opioid use and abuse. Recent studies suggest that the type 2 metabotropic glutamate receptor (mGluR2) is critically involved in substance abuse and addiction. In the present study, we evaluated whether low-mGluR2 expression may represent a risk factor for the development of opioid abuse and addiction using transgenic mGluR2-knockout (mGluR2-KO) rats. Compared to wild-type controls, mGluR2-KO rats exhibited higher nucleus accumbens (NAc) dopamine (DA) and locomotor responses to heroin, higher heroin self-administration and heroin intake, more potent morphine-induced analgesia and more severe naloxone-precipitated withdrawal symptoms. In contrast, mGluR2-KO rats displayed lower motivation for heroin self-administration under high price progressive-ratio (PR) reinforcement conditions. Taken together, these findings suggest that mGluR2 may play an inhibitory role in opioid action, such that deletion of this receptor results in an increase in brain DA responses to heroin and in acute opioid reward and analgesia. Low-mGluR2 expression in the brain may therefore be a risk factor for the initial development of opioid abuse and addiction.

Downregulation of mGluR2/3 receptors during morphine withdrawal in rats impairs mGluR2/3- and NMDA receptor-dependent long-term depression in the nucleus accumbens

Drugs of abuse modify synaptic long-term potentiation and long-term depression (LTD) in the nucleus accumbens, and the impairment of synaptic plasticity in this brain region may be a universal feature of drug addiction. It is unknown whether metabotropic glutamate receptors (mGluRs) play a role in synaptic plasticity induced by drugs such as morphine. The neurochemical, electrophysiological, and Western blotting experiments reported here reveal a novel form of LTD in synapses of the shell region of the nucleus accumbens induced in vivo by low-frequency stimulation of the medial prefrontal cortex. This plasticity required the activation of N-methyl-d-aspartate receptors and mGluR2/3 but not mGluR5. The expression of mGluR2/3 was downregulated during withdrawal from repeated morphine exposure (10 days after the last injection), resulting in impaired low-frequency stimulation-induced LTD. These results indicate that withdrawal-induced mGluR2/3 downregulation alters neural plasticity after morphine exposure, which may be a mechanism contributing to drug addiction.

Attenuation of Withdrawal Signs, Blood Cortisol, and Glucose Level with Various Dosage Regimens of Morphine after Precipitated Withdrawal Syndrome in Mice

Morphine withdrawal usually results in unsuccessful outcomes. Despite partial benefits from alternative substances such as methadone, its use may not lead to the desired result due to the lack of mental tranquility during the withdrawal period. In this study, by means of an animal model, morphine itself was used to manage morphine dependence. Forty mice were divided into 5 groups, in which 4 groups became dependent by increasing daily doses of morphine for 7 days (15-45 mg/kg). Afterwards, the animals received morphine for 14 days by either of the following regimens: Once daily 45 mg/kg (positive controls)Increasing the interval (each time 6 hours longer than the previous interval)Irregular interval in every 36, 12 and 24 hours until the 21(th) day12, 24, 36 hours decreasing doses (each time 2.5 mg/kg less than the former dosage). Negative controls received saline solution only. On day 22, total withdrawal index (TWI) was determined by injecting 3 mg/kg of naloxone. Thereafter, blood samples were taken for the measurement of cortisol and glucose levels. TWI significantly decreased in all test groups in comparison with the positive control animals (P<0.001). Cortisol levels significantly decreased when either the dosage or the administration frequencies were decreased on a regular and gradual basis (P<0.005). Blood glucose levels significantly decreased in animals that received decreasing doses of morphine (P<0.005). This study suggests that no other measures may be required in clinical practice except for changing the dosage regimen of morphine for the cessation of self-administration.

The role of memantine in the treatment of psychiatric disorders other than the dementias: a review of current preclinical and clinical evidence

Memantine, a non-competitive NMDA receptor antagonist approved for Alzheimer's disease with a good safety profile, is increasingly being studied in a variety of non-dementia psychiatric disorders. We aimed to critically review relevant literature on the use of the drug in such disorders. We performed a PubMed search of the effects of memantine in animal models of psychiatric disorders and its effects in human studies of specific psychiatric disorders. The bulk of the data relates to the effects of memantine in major depressive disorder and schizophrenia, although more recent studies have provided data on the use of the drug in bipolar disorder as an add-on. Despite interesting preclinical data, results in major depression are not encouraging. Animal studies investigating the possible usefulness of memantine in schizophrenia are controversial; however, interesting findings were obtained in open studies of schizophrenia, but negative placebo-controlled, double-blind studies cast doubt on their validity. The effects of memantine in anxiety disorders have been poorly investigated, but data indicate that the use of the drug in obsessive-compulsive disorder and post-traumatic stress disorder holds promise, while findings relating to generalized anxiety disorder are rather disappointing. Results in eating disorders, catatonia, impulse control disorders (pathological gambling), substance and alcohol abuse/dependence, and attention-deficit hyperactivity disorder are inconclusive. In most psychiatric non-Alzheimer's disease conditions, the clinical data fail to support the usefulness of memantine as monotherapy or add-on treatment However, recent preclinical and clinical findings suggest that add-on memantine may show antimanic and mood-stabilizing effects in treatment-resistant bipolar disorder.

Potentiation of synaptic strength and intrinsic excitability in the nucleus accumbens after 10 days of morphine withdrawal

Neuroadaptations in the nucleus accumbens (NAc) are associated with the development of drug addiction. Plasticity in synaptic strength and intrinsic excitability of NAc medium spiny neurons (MSNs) play critical roles in addiction induced by different classes of abused drugs. However, it is unknown whether morphine exposure influences synaptic strength, intrinsic excitability or both in NAc. Here we show that chronic withdrawal (10 days after the last injection) from repeated morphine exposure elicited potentiation in both glutamatergic synaptic strength and intrinsic excitability of MSNs in NAc shell (NAcSh). The potentiation of synaptic strength was demonstrated by an increase in the frequency of miniature excitatory postsynaptic currents (mEPSCs), a decrease in the paired-pulse ratio (PPR), and an increase in the ratio of α-amino-3-hydroxy-5-methyl-isoxazole propionic acid receptors (AMPAR)- to N-methyl-D-aspartate receptors (NMDAR)-mediated currents. The potentiation of intrinsic excitability was mediated by inhibition of the sustained potassium currents via extrasynaptic NMDAR activation. The function of the presynaptic group II metabotropic glutamate receptors (mGluR2/3) was downregulated, enhancing the probability of glutamate release on synaptic terminals during chronic morphine withdrawal. Pretreatment with the mGluR2/3 agonist LY379268 completely blocked potentiation of both synaptic strength and intrinsic excitability. These results suggest that chronic morphine withdrawal downregulates mGluR2/3 to induce potentiation of MSN glutamatergic synapse via increased glutamate release, leading to potentiation of intrinsic excitability. Such potentiation of both synaptic strength and intrinsic excitability might contribute to neuroadaptations induced by morphine application.

Voluntary exercise ameliorates cognitive deficits in morphine dependent rats: the role of hippocampal brain-derived neurotrophic factor

Chronic exposure to opiates impairs spatial learning and memory. Given the well-known beneficial effects of voluntary exercise on cognitive functions, we investigated whether voluntary exercise would ameliorate the cognitive deficits that are induced by morphine dependence. If an effect of exercise was observed, we aimed to investigate the possible role of hippocampal brain-derived neurotrophic factor (BDNF) in the exercise-induced enhancement of learning and memory in morphine-dependent rats. The rats were injected with bi-daily doses (10mg/kg, at 12h intervals) of morphine over a period of 10 days of voluntary exercise. Following these injections, a water maze task was performed twice a day for five consecutive days, followed by a probe trial 2 days later. A specific BDNF inhibitor (TrkB-IgG chimera) was used to block the hippocampal BDNF action during the 10 days of voluntary exercise. We found that voluntary exercise blocked the ability of chronic morphine to impair spatial memory retention. A blockade of the BDNF action blunted the exercise-induced improvement of spatial memory in the dependent rats. Moreover, the voluntary exercise diminished the severity of the rats' dependency on morphine. This study demonstrates that voluntary exercise ameliorates, via a TrkB-mediated mechanism, the cognitive deficits that are induced by chronic morphine. Thus, voluntary exercise might be a potential method to ameliorate some of the deleterious behavioral consequences of the abuse of morphine and other opiates.

Mu-opioid receptor redistribution in the locus coeruleus upon precipitation of withdrawal in opiate-dependent rats

Administration of mu-opioid receptor (MOR) agonists is known to produce adaptive changes within noradrenergic neurons of the rat locus coeruleus (LC). Alterations in the subcellular distribution of MOR have been shown to occur in the LC in response to full agonists and endogenous peptides; however, there is considerable debate in the literature whether trafficking of MOR occurs after chronic exposure to the partial-agonist morphine. In the present study, we examined adaptations in MOR after chronic opioid exposure using immunofluorescence and electron microscopy (EM), using receptor internalization as a functional endpoint. MOR trafficking in LC neurons was characterized in morphine-dependent rats that were given naltrexone at a dose known to precipitate withdrawal. After chronic morphine exposure, a subtle redistribution of MOR immunoreactivity from the membrane to the cytosol was detected within dendrites of LC neurons. Interestingly, an acute injection of naltrexone in rats exposed to chronic morphine produced a robust internalization of MOR, whereas administration of naltrexone failed to do so in naïve animals. These findings provide anatomical evidence for modified regulation of MOR trafficking after chronic morphine treatment in brain noradrenergic neurons. Adaptations in the MOR signaling pathways that regulate internalization may occur as a consequence of chronic treatment and precipitation of withdrawal. Mechanisms underlying this effect might include differential MOR regulation in the LC, or downstream effects of withdrawal-induced enkephalin (ENK) release from afferents to the LC.

Role of the glutamatergic system in nicotine dependence : implications for the discovery and development of new pharmacological smoking cessation therapies

Preclinical research findings in laboratory animals indicate that the glutamatergic system is critically involved in nicotine dependence. In animals, compounds that decrease glutamatergic neurotransmission, such as antagonists at postsynaptic NMDA receptors, antagonists at excitatory postsynaptic metabotropic glutamate (mGlu) 5 receptors, or agonists at inhibitory presynaptic mGlu(2) and mGlu(3) receptors, decreased nicotine self-administration or reinstatement of nicotine-seeking behaviour. These findings suggest that medications that decrease glutamatergic transmission overall may reduce the reinforcing effects of tobacco smoking and prevent relapse to tobacco smoking in humans. Furthermore, compounds that increase glutamate release, such as antagonists at mGlu(2) and mGlu(3) receptors, ameliorated reward deficits associated with nicotine withdrawal in animals, and thus may alleviate the depression-like symptoms associated with nicotine withdrawal in humans. Animal studies also showed that alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/kainate receptors did not appear to be involved in mediating the primary reinforcing effects of nicotine but that they may be involved in the development of nicotine dependence and withdrawal.Taken together, the preclinical data indicate that different glutamatergic receptors are involved in the mediation of different aspects of nicotine dependence. These findings have implications for the discovery and development of new pharmacotherapies that target the glutamatergic system to aid in smoking cessation. At present, very few clinical studies have addressed the effects of glutamatergic compounds on cigarette smoking. Clinical studies involving compounds that have actions at ionotropic glutamate receptors are briefly discussed in this review and suggest the potential of glutamatergic compounds as pharmacotherapies to aid in smoking cessation. Medications that target mGlu receptors have recently been tested in human phase II trials for various indications; however, the potential of these mGlu compounds as medications for nicotine dependence remains to be evaluated in humans. The preclinical data evaluated in this review indicate that such clinical trials for smoking cessation with mGlu compounds are clearly warranted and may reveal novel treatments for nicotine dependence.

Distinct neural mechanisms mediate olfactory memory formation at different timescales

Habituation is one of the oldest forms of learning, broadly expressed across sensory systems and taxa. Here, we demonstrate that olfactory habituation induced at different timescales (comprising different odor exposure and intertrial interval durations) is mediated by different neural mechanisms. First, the persistence of habituation memory is greater when mice are habituated on longer timescales. Second, the specificity of the memory (degree of cross-habituation to similar stimuli) also depends on induction timescale. Third, we demonstrate a pharmacological double dissociation between the glutamatergic mechanisms underlying short- and long-timescale odor habituation. LY341495, a class II/III metabotropic glutamate receptor antagonist, blocked habituation only when the induction timescale was short. Conversely, MK-801, an N-methyl-D-aspartate (NMDA) receptor antagonist, prevented habituation only when the timescale was long. Finally, whereas short-timescale odor habituation is mediated within the anterior piriform cortex, infusion of MK-801 into the olfactory bulbs prevented odor habituation only at longer timescales. Thus, we demonstrate two neural mechanisms underlying simple olfactory learning, distinguished by their persistence and specificity, mediated by different olfactory structures and pharmacological effectors, and differentially utilized based solely on the timescale of odor presentation.

Glutamatergic substrates of drug addiction and alcoholism

The past two decades have witnessed a dramatic accumulation of evidence indicating that the excitatory amino acid glutamate plays an important role in drug addiction and alcoholism. The purpose of this review is to summarize findings on glutamatergic substrates of addiction, surveying data from both human and animal studies. The effects of various drugs of abuse on glutamatergic neurotransmission are discussed, as are the effects of pharmacological or genetic manipulation of various components of glutamate transmission on drug reinforcement, conditioned reward, extinction, and relapse-like behavior. In addition, glutamatergic agents that are currently in use or are undergoing testing in clinical trials for the treatment of addiction are discussed, including acamprosate, N-acetylcysteine, modafinil, topiramate, lamotrigine, gabapentin and memantine. All drugs of abuse appear to modulate glutamatergic transmission, albeit by different mechanisms, and this modulation of glutamate transmission is believed to result in long-lasting neuroplastic changes in the brain that may contribute to the perseveration of drug-seeking behavior and drug-associated memories. In general, attenuation of glutamatergic transmission reduces drug reward, reinforcement, and relapse-like behavior. On the other hand, potentiation of glutamatergic transmission appears to facilitate the extinction of drug-seeking behavior. However, attempts at identifying genetic polymorphisms in components of glutamate transmission in humans have yielded only a limited number of candidate genes that may serve as risk factors for the development of addiction. Nonetheless, manipulation of glutamatergic neurotransmission appears to be a promising avenue of research in developing improved therapeutic agents for the treatment of drug addiction and alcoholism.

Glutamate receptor subunit expression in the rhesus macaque locus coeruleus

The locus coeruleus (LC) is a major noradrenergic brain nucleus that regulates states of arousal, optimizes task-oriented decision making, and may also play an important role in modulating the activity of the reproductive neuroendocrine axis. Rodent studies have shown that the LC is responsive to glutamate receptor agonists, and that it expresses various glutamate receptor subunits. However, glutamate receptor subunit expression has not been extensively examined in the primate LC. We previously demonstrated expression of the NR1 NMDA glutamate receptor subunit in the rhesus macaque LC and now extend this work by also examining the expression of non-NMDA (AMPA and kainate) ionotropic glutamate receptor subunits. Using in situ hybridization histochemistry and immunohistochemistry, we confirmed the presence of the obligatory NR1 subunit in the LC. In addition, we demonstrated expression of the AMPA glutamate receptor subunits GluR1, GluR2, and GluR3. More extensive receptor profiling, using rhesus monkey gene microarrays (Affymetrix GeneChip), further corroborated the histological findings and showed expression of mRNA encoding ionotropic glutamate receptor subunits NR2A, NR2D, GluR4, and GluR6, as well as the metabotropic glutamate receptor subunits mGluR1, mGluR3, mGluR4, mGluR5, and mGluR7. These data provide a foundation for future examination of how changes in glutamate receptor composition contribute to the control of primate physiology.

Fear learning induces persistent facilitation of amygdala synaptic transmission

In the maintenance phase of fear memory, synaptic transmission is potentiated and the stimulus requirements and signalling mechanisms are altered for long-term potentiation (LTP) in the cortico-lateral amygdala (LA) pathway. These findings link amygdala synaptic plasticity to the coding of fear memories. Behavioural experiments suggest that the amygdala serves to store long-term fear memories. Here we provide electrophysiological evidence showing that synaptic alterations in rats induced by fear conditioning are evident in vitro 10 days after fear conditioning. We show that synaptic transmission was facilitated and that high-frequency stimulation dependent LTP (HFS-LTP) of the cortico-lateral amygdala pathway remained attenuated 10 days following fear conditioning. Additionally, we found that the low-frequency stimulation dependent LTP (LFS-LTP) measured 24 h after fear conditioning was absent 10 days post-training. The persistent facilitation of synaptic transmission and occlusion of HFS-LTP suggests that, unlike hippocampal coding of contextual fear memory, the cortico-lateral amygdala synapse is involved in the storage of long-term fear memories. However, the absence of LFS-LTP 10 days following fear conditioning suggests that amygdala physiology 1 day following fear learning may reflect a dynamic state during memory stabilization that is inactive during the long-term storage of fear memory. Results from these experiments have significant implications regarding the locus of storage for maladaptive fear memories and the synaptic alterations induced by these memories.

Comparison of c-Fos induction in the brain by the mGlu2/3 receptor antagonist LY341495 and agonist LY354740: evidence for widespread endogenous tone at brain mGlu2/3 receptors in vivo

LY341495 and LY354740 are potent and selective antagonist and agonist, respectively, for Group II metabotropic glutamate (mGlu2/3) receptors. Here we demonstrate that LY341495 (3 mg/kg) significantly increased c-Fos expression in almost all brain regions analyzed (44 out of 52 regions) in animals that were prehandled and kept in home-cage environment to minimize stress. Robust c-Fos induction was observed in all cortical regions, hippocampal CA1 and CA3 subregions, amygdala and several other subcortical nuclei. In contrast to LY341495, changes in c-Fos expression following LY354740 were more modest and not generally widespread (decreased in 1 region, dentate gyrus; and increased in 13 out of 52 regions). Interestingly, although LY354740 is anxiolytic in animals, LY341495 did not increase c-Fos expression in the paraventricular nucleus of the hypothalamus which is usually activated by stress/fear and several anxiogenic compounds. To further investigate the behavioral consequences of mGlu2/3 receptor antagonism, LY341495 was administered to prehandled animals that were placed in the elevated plus maze test under low light (low stress) conditions. Here LY341495 increased mouse elevated plus maze (EPM)-anxiety in a dose-dependent manner, significantly decreasing the time spent in open arms, but not affecting total ambulations. The behavioral consequences and associated widespread pattern of brain neuronal activations following blockade of mGlu2/3 receptors suggest that there is considerable endogenous glutamate tone throughout the brain at negative feedback peri-synaptic mGlu2/3 receptors, even under low stress conditions where synaptic glutamate release spillover would be expected to be minimized.

Attenuation of morphine withdrawal signs by a D1 receptor agonist in the locus coeruleus of rats

In the present study, the effects of intra-locus coeruleus injection of a dopamine D(1) receptor agonist (SKF38393) on naloxone-induced withdrawal signs of morphine-dependent rats were examined. Twenty different withdrawal signs were assessed. The total withdrawal score was calculated and used as an index of withdrawal intensity for comparison. The D(1) agonist and antagonist were injected 15 and 30 min prior to expression of naloxone-induced withdrawal signs, respectively. SKF38393 (2 and 4 microg/site) decreased while SCH23390 (a D(1) antagonist) had no effect on the total withdrawal score. On the other hand, SCH23390 (25 ng/site) reversed the SKF38393 effect. It may be concluded that activation of dopamine D(1) receptors in the locus coeruleus attenuates naloxone-induced withdrawal.

2-MPPA, a selective glutamate carboxypeptidase II inhibitor, attenuates morphine tolerance but not dependence in C57/Bl mice

RATIONALE AND OBJECTIVES

We have recently reported that conditioned morphine reward and tolerance to its antinociceptive effect, but not expression of morphine dependence, were attenuated by 2-(phosphonomethyl)pentanedioic acid (2-PMPA), a prototypic inhibitor of glutamate carboxipeptidase II (GCP II), which is an enzyme responsible for the supply of glutamate. In the present study, we investigated in more detail the effects of GCP II inhibition on opioid dependence and tolerance to its antinociceptive effect in C57/Bl mice using a novel GCP II inhibitor.

RESULTS

The treatment with 2-(3-mercaptopropyl)pentanedioic acid (2-MPPA; 60 but not 10 or 30 mg/kg) prevented the development of morphine tolerance without affecting acute morphine antinociception. 2-MPPA at 30 and 60 mg/kg did not prevent the development of dependence induced by 10 and 30 mg/kg of morphine. The study on opioid withdrawal syndrome, i.e., expression of opioid dependence, demonstrated that 2-MPPA potentiated jumping behavior and teeth chattering but attenuated chewing and ptosis. None of these opioid withdrawal signs were affected by 2-MPPA in morphine nondependent mice. Pretreatment with the mGluR II antagonist LY341495 (1 mg/kg) reversed the 2-MPPA-induced increase or decrease in opioid withdrawal signs in morphine-dependent mice. 2-MPPA (60 mg/kg) administered for 7 days with morphine did not affect brain concentration of this opiate.

CONCLUSIONS

The present findings suggest complex effects of GCP II inhibition on morphine dependence and tolerance and imply a role of mGluR II in the actions of 2-MPPA.

Endogenous opiates and behavior: 2004

This paper is the 27th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over 30 years of research. It summarizes papers published during 2004 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Pharmacological rescue of synaptic plasticity, courtship behavior, and mushroom body defects in a Drosophila model of fragile X syndrome

Fragile X syndrome is a leading heritable cause of mental retardation that results from the loss of FMR1 gene function. A Drosophila model for Fragile X syndrome, based on the loss of dfmr1 activity, exhibits phenotypes that bear similarity to Fragile X-related symptoms. Herein, we demonstrate that treatment with metabotropic glutamate receptor (mGluR) antagonists or lithium can rescue courtship and mushroom body defects observed in these flies. Furthermore, we demonstrate that dfmr1 mutants display cognitive deficits in experience-dependent modification of courtship behavior, and treatment with mGluR antagonists or lithium restores these memory defects. These findings implicate enhanced mGluR signaling as the underlying cause of the cognitive, as well as some of the behavioral and neuronal, phenotypes observed in the Drosophila Fragile X model. They also raise the possibility that compounds having similar effects on metabotropic glutamate receptors may ameliorate cognitive and behavioral defects observed in Fragile X patients.

The mGlu5 receptor antagonists MPEP and MTEP attenuate behavioral signs of morphine withdrawal and morphine-withdrawal-induced activation of locus coeruleus neurons in rats

N-Methyl-D-aspartate (NMDA) antagonists have been demonstrated to suppress the signs of opiate withdrawal; however, side effects limit their clinical use. Since the metabotropic glutamate (mGlu) 5 receptor has been shown to affect glutamate release and modulate NMDA receptor function, we examined the effects of two selective mGlu5 receptor antagonists, 2-methyl-6-(phenyl-ethynyl)-pyridine (MPEP) and 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP), on morphine withdrawal. Pretreatment with MPEP or MTEP (1, 3, and 10 mg/kg, i.p.) significantly attenuated behavioral signs of morphine withdrawal. Specifically, both MPEP and MTEP attenuated the occurrence/severity of chews, digging, salivation, and weight loss, and increased the occurrence of erections. Neither compound changed the occurrence of wet-dog shakes, ptosis, irritability, or lacrimation. Both MPEP and MTEP produced a modest, but significant, attenuation of morphine-withdrawal-induced activation of locus coeruleus neurons in anesthetized rats. These results indicate a role for mGlu5 receptors in morphine withdrawal and suggest the potential for mGlu5 antagonists in the treatment of withdrawal from opiates and other drugs of abuse.