Anxiolytic-like effects of mGlu1 and mGlu5 receptor antagonists in rats

The purpose of the present study was to compare anxiolytic activity of the metabotropic glutamate receptor 1 (mGlu) antagonist, EMQMCM ((3-ethyl-2-methyl-quinolin-6-yl)-(4-methoxy-cyclohexyl)-methanone methanesulfonate) and the mGlu5 receptor antagonist MTEP ([(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine) and MPEP (2-methyl-6-(phenylethynyl)pyridine) in animal models of anxiety. In the elevated plus maze, diazepam (1 mg/kg), but not the mGlu1 or mGlu5 receptor antagonists induced anxiolytic-like effects. Meanwhile, MTEP (2.5 and 5 mg/kg), EMQMCM (5 mg/kg), and diazepam (2 mg/kg) all significantly inhibited fear potentiated startle. In the contextual fear conditioning test, MTEP (1.25 and 2.5 but not 5 mg/kg) and EMQMCM (0.6 to 5 mg/kg) attenuated freezing responding. In the Geller-Seifter conflict test, MPEP (1 and 3 mg/kg), MTEP (3 mg/kg), chlordiazepoxide (10 and 20 mg/kg) and midazolam (1 mg/kg) all facilitated punished responding, while ECMQCM failed to produce any significant effects up to 3 mg/kg dose. To summarise, the present data further support a significant anxiolytic potential of group I mGlu receptor antagonists, while suggesting the effects of mGlu1 receptor antagonists may depend on the experimental procedure and may be qualitatively different from those of mGlu5 receptor antagonists.     

The role of group I metabotropic glutamate receptors in acquisition and expression of contextual and auditory fear conditioning in rats - a comparison

Glutamatergic neurotransmission in the CNS plays a predominant role in learning and memory. While NMDA receptors have been extensively studied, less is known about the involvement of group I metabotropic glutamate receptors in this area. The purpose of the present study was to evaluate the contribution of mGluR1 and mGluR5 to both acquisition and expression of behaviours in contextual and auditory fear conditioning models. The effects of both receptor types were tested using selective antagonists: (3-ethyl-2-methyl-quinolin-6-yl)-(4-methoxy-cyclohexyl)-methanone methanesulfonate (EMQMCM) for mGluR1, and [(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP) for mGluR5. Their effects on acquisition were compared to those of the NMDA receptor antagonist (+)MK-801, and the unselective muscarinic antagonist scopolamine, while diazepam and citalopram served as reference compounds in the expression experiments. EMQMCM (1.25 to 5 mg/kg) impaired acquisition of contextual fear conditioning (CFC), but not auditory fear conditioning (AFC). Similarly, administration of MTEP during the acquisition phase impaired learning in CFC at doses of 2.5 to 10 mg/kg, but was ineffective in AFC. When given before the retention test, both EMQMCM (1 and 3 mg/kg) and MTEP (3 mg/kg) impaired expression of CFC. In contrast, MTEP (2.5 and 5 mg/kg) blocked the expression of AFC, while EMQMCM was ineffective. In conclusion, group I mGlu receptors are shown to be involved in the acquisition of hippocampus-dependent CFC, but not hippocampus-independent AFC. Unlike mGluR5, mGluR1 does not seem be involved in expression of AFC.     

Functional interaction of NMDA and group I metabotropic glutamate receptors in negatively reinforced learning in rats

Rationale The role of glutamatergic system in learning and memory has been extensively studied, and especially N-methyl-d-aspartate (NMDA) receptors have been implicated in different learning and memory processes. Less is known, however, about group I metabotropic glutamate (mGlu) receptors in this field. Recent studies indicated that the coactivation of both NMDA and group I mGlu receptors is required for the induction of long-term potentiation (LTP) and learning. Objective The purpose of the study is to evaluate if there is a functional interaction between NMDA and group I mGlu receptors in two different models of aversive learning. Methods Effects of NMDA, mGlu1, and mGlu5 receptor antagonists on acquisition were tested after systemic coadministration of selected ineffective doses in passive avoidance (PA) and fear-potentiated startle (FPS). Results Interaction in aversive learning was investigated using selective antagonists: (3-ethyl-2-methyl-quinolin-6-yl)-(4-methoxy-cyclohexyl)-methanone methanesulfonate (EMQMCM) for mGlu1, [(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP) for mGlu5, and (+)-5-methyl-10,11-dihydro-5H-dibenzocyclohepten-5,10-imine maleate [(+)MK-801] for NMDA receptors. In PA, the coapplication of MTEP at a dose of 5 mg/kg and (+)MK-801 at a dose of 0.1 mg/kg 30 min before training impaired the acquisition tested 24 h later. Similarly, EMQMCM (2.5 mg/kg) plus (+)MK-801 (0.1 mg/kg), given during the acquisition phase, blocked the acquisition of the PA response. In contrast, neither the combination of MTEP (1.25 mg/kg) nor EMQMCM (5 mg/kg) plus (+)MK-801 (0.05 mg/kg) was effective on the acquisition assessed in the FPS paradigm. Conclusion The findings suggest differences in the interaction of the NMDA and mGlu group I receptor types in aversive instrumental conditioning vs conditioning to a discrete light cue.     

Neuroprotective activity of selective mGlu1 and mGlu5 antagonists in vitro and in vivo

The neuroprotective potential of allosteric mGlu5 and mGlu1 antagonists such as 6-methyl-2-(phenylethynyl)-pyridin (MPEP)/[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP) and (3-ethyl-2-methyl-quinolin-6-yl)-(4-methoxy-cyclohexyl)-methanone methanesulfonate (EMQMCM), was tested in vitro in organotypic hippocampal cultures and in the middle cerebral artery occlusion model of stroke in vivo. Both classes of agent have high selectivity toward mGlu sub-types and are active in animal models of various diseases indicating satisfactory CNS penetration. In organotypic hippocampal cultures MPEP showed high neuroprotective potency against sub-chronic (12 days) insult produced by 3-NP with an IC50 of c.a. 70 nM. In contrast, although the mGlu1 antagonist EMQMCM was also protective, it seems to be weaker yielding an IC50 of c.a. 1 microM. Similarly, in the transient (90 min) middle cerebral artery occlusion model of ischaemia in rats, MTEP seems to be more effective than EMQMCM. MTEP, at 2.5 mg/kg and at 5 mg/kg provided 50 and 70% neuroprotection if injected 2 h after the onset of ischaemia. At a dose of 5 mg/kg, significant (50%) neuroprotection was also seen if the treatment was delayed by 4 h. EMQMCM was not protective at 5 mg/kg (given 2 h after occlusion) but at 10 mg/kg 50% of neuroprotection was observed. The present data support stronger neuroprotective potential of mGlu5 than mGlu1 antagonists.     

Analgesic effects of mGlu1 and mGlu5 receptor antagonists in the rat formalin test

mGlu1 and mGlu5 receptors have been implicated in pain associated with inflammation. In the present study, the formalin test was used to measure sustained pain with components of tissue injury. The aims of the present study were to assess: (i) the role of mGlu1 and mGlu5 receptors in inflammatory pain using selective antagonist EMQMCM, 1.25-5 mg/kg, as the mGlu1 receptor antagonist, and MPEP or MTEP, 2.5-10 mg/kg, as mGlu5 receptor antagonist; (ii) the possible interaction between mGlu1 and mGlu5 receptor antagonists and morphine; and (iii) whether tolerance develops to the analgesic effects of these antagonists after prolonged treatment. EMQMCM, MTEP and MPEP significantly reduced the manifestation of both phases of formalin response. However, all these mGlu receptor antagonists did not affect the withdrawal latencies in a model of acute pain (Hargreaves test), which has a different underlying mechanism. In the present study, the suppressive effect on formalin-induced pain behaviour was much stronger when mGlu1 and mGlu5 receptor antagonists were co-injected compared to administration of a single antagonist, but this effect was not seen when mGlu receptor antagonist was co-administered with morphine. This is in contrast to the pronounced inhibitory effects after co-treatment with morphine and the uncompetitive NMDA receptor antagonist memantine. The present study also provides the first direct in vivo evidence that prolonged administration of MTEP (5 mg/kg) over 7 days leads to the development of tolerance to its antinociceptive effects. Such tolerance was not observed when EMQMCM (5 mg/kg) was administered in the same manner. In conclusion, these results provide additional arguments for the role of group I mGlu receptors in pain with inflammatory conditions.     

The role of group I mGlu receptors in the expression of ethanol-induced conditioned place preference and ethanol withdrawal seizures in rats

In animal models, N-methyl-d-aspartate (NMDA) receptors antagonists inhibit physical dependence and the reinforcing effects of ethanol. The group I metabotropic glutamate (mGlu) receptors antagonists (mGlu1 and mGlu5) attenuate excitatory effect of glutamate by functional modulation of the glutamate/NMDA receptors. The objective of the present study was to evaluate the effects of a selective mGlu5 receptors antagonist—MTEP, and mGlu1 receptors antagonist—EMQMCM, on two processes relevant to alcohol addiction: the expression of ethanol-induced conditioned place preference (CPP) paradigm, and ethanol withdrawal audiogenic seizures in rats. Our experiments indicated that EMQMCM at the doses of 5 and 10 mg/kg, and MTEP at the doses of 2.5 and 5 mg/kg, significantly attenuated the expression of ethanol CPP. Furthermore, both group I mGlu receptor antagonists, i.e. EMQMCM at the dose of 10 mg/kg and MTEP at the dose of 5 mg/kg, attenuated audiogenic seizures induced by the sound stimulus 12 h after withdrawal of ethanol in dependent rats. Our study shows the importance of mGlu5 and mGlu1 receptors for the expression of ethanol-induced CPP and withdrawal seizures, although mGlu5 receptors antagonist (MTEP) was more potent than the antagonist of mGlu1 receptors (EMQMCM).     

Preconditioning with acute and chronic lithium administration reduces ischemia/reperfusion injury mediated by cyclooxygenase not nitric oxide synthase pathway in isolated rat heart

The aim of the present study was to determine whether various glutamate receptor antagonists could affect ethanol withdrawal-induced anxiety-like behavior measured in the elevated plus-maze test in rats. In our study, memantine (8 and 12 mg/kg), a noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonist, did not show any effect on ethanol withdrawal anxiety. Acamprosate (NMDA and metabotropic glutamate5 (mGlu5) receptor antagonist), at a dose of 400 mg/kg showed anxiolytic-like effect, thus increasing the percent of time spent in open arms and open arms entries. Antagonists of group I mGlu receptors, such as MTEP ([(2-methyl-1,3-thiazol-4-yl) ethynyl] pyridine, mGlu5 receptor) or EMQMCM (3-ethyl-2-methyl-quinolin-6-yl-(4-methoxy-cyclohexyl)-methanone methanesulfonate, mGlu1 receptor), caused similar effects to acamprosate. In contrast to acamprosate and MTEP, EMQMCM (5 mg/kg) elevated the ethanol withdrawal-induced decrease in locomotion. When given alone to the saline-treated group, EMQMCM indicated anxiolytic-like effect. Our results imply a crucial role of mGlu5 receptor in an anxiety-like effect of ethanol withdrawal because MTEP (a selective mGlu5 receptor antagonist) and acamprosate (which also indirectly inhibits mGlu5 receptor) attenuated ethanol withdrawal anxiety-like behavior without influence on ethanol withdrawal hypolocomotion and did not show any effect in the saline-treated groups. However, difference in anxiolytic-like potency between both these group I mGlu receptors antagonists may be due to the recent experimental design. Therefore, taking into account a positive correlation between ethanol withdrawal-induced anxiety and relapse to ethanol drinking, our results suggest that mGlu receptor antagonists of group I (similarly to acamprosate) could prevent relapse to drinking and, therefore they might be useful in therapy of alcoholism.     

mGlu1 and mGlu5 receptor antagonists lack anticonvulsant efficacy in rodent models of difficult-to-treat partial epilepsy

Modulation of metabotropic glutamate (mGlu) receptors represents an interesting new approach for the treatment of a range of neurological and psychiatric disorders. Several lines of evidence suggest that functional blockade of group I (mGlu1 and mGlu5) receptors may be beneficial for treatment of epileptic seizures. This study was conducted to investigate whether mGlu1 or mGlu5 receptor antagonists have the potential to block partial or secondarily generalized seizures as occurring in partial epilepsy, the most common and difficult-to-treat type of epilepsy in patients. For this purpose, we systemically administered novel highly selective and brain penetrable group I mGlu receptor antagonists, i.e., the mGlu1 receptor antagonist EMQMCM [3-ethyl-2-methyl-quinolin-6-yl-(4-methoxy-cyclohexyl)-methanone methanesulfonate] and the mGlu5 receptor antagonist MTEP ([(2-methyl-1,3-thiazol-4-yl) ethynyl] pyridine), at doses appropriate for mGlu1 or mGlu5 receptor-mediated effects in rodent models of partial seizures. Two models were used: the 6-Hz electroshock model of partial seizures in mice and the amygdala-kindling model in rats. Clinically established antiepileptic drugs were included in the experiments for comparison. Antiepileptic drugs exerted significant anticonvulsant effects in both models, while EMQMCM and MTEP were ineffective in this regard, although both compounds were administered up to doses associated with essentially full receptor occupancy and with typical mGlu receptor-mediated effects in rodent models of anxiety or pain. Brain microdialysis for determining extracellular levels of MTEP following i.p. administration in rats substantiated that effective brain concentrations were reached at times of our experiments in seizure models. The present results do not support a significant anticonvulsant potential of group I mGlu receptor antagonists in rodent models of difficult-to-treat partial epilepsy.     

mGluR5, but not mGluR1, antagonist modifies MK-801-induced locomotor activity and deficit of prepulse inhibition

Hypoglutamatergic theory of schizophrenia is substantiated by observation that high affinity uncompetitive antagonists of NMDA receptors such as PCP can induce psychotic symptoms in humans. Recently, metabotropic glutamate receptors of the mGluR5 type have also been discussed as possible players in this disease. However, less is known about the potential contribution of mGluR1 in schizophrenia. Therefore, the aim of the present study was to compare the effect of selective mGluR1 antagonist EMQMCM, (3-ethyl-2-methyl-quinolin-6-yl)-(4-methoxy-cyclohexyl)-methanone methanesulfonate) and mGluR5 antagonist (MTEP ([(2-methyl-1, 3-thiazol-4-yl) ethynyl] pyridine) either alone or in combination with (+)MK-801 in a prepulse inhibition (PPI) model and locomotor activity tests. Additionally, the effect of both mGluR1 and mGluR5 antagonists on (+)MK-801-evoked ataxia was tested. In contrast to (+)MK-801, which induced disruption of PPI, neither MTEP (1.25-5 mg/kg) nor EMQMCM (0.5-4 mg/kg) altered the PPI. However, MTEP, but not EMQMCM, enhanced disruption of PPI induced by (+)MK-801. Although neither mGluR1 nor mGluR5 antagonists given alone changed locomotor activity of rats, MTEP at 5 mg/kg potentiated the effect of (+)MK-801 while EMQMCM (up to 4 mg/kg) turned out to be ineffective. On the other hand, EMQMCM, but not MTEP, enhanced ataxia evoked by MK-801. The present results demonstrate that blockade of mGluR1 and mGluR5 evokes different effects on behavior induced by NMDA receptor antagonists.     

Antidepressant-like effects of mGluR1 and mGluR5 antagonists in the rat forced swim and the mouse tail suspension tests.

Drugs that act to reduce glutamatergic neurotransmission such as NMDA receptor antagonists exert antidepressant-like effects in a variety of experimental paradigms, but their therapeutic application is limited by undesired side effects. In contrast, agents that reduce glutamatergic tone by blocking type I metabotropic glutamate receptors have been suggested to have more a favorable side-effect profile. The present study aimed to compare the effects of mGluR1 antagonist (EMQMCM; JNJ16567083, 3-ethyl-2-methyl-quinolin-6-yl)-(4-methoxy-cyclohexyl)-methanone methanesulfonate, 0.156-10 mg/kg) and mGluR5 antagonist (MTEP, [(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine, 1.25-10 mg/kg) in two behavioral screening assays commonly used to assess antidepressant-like activity. In the modified forced swim test in rats, imipramine (used as a positive control) decreased immobility (MED 40 mg/kg) and increased the duration of escape-oriented (climbing and diving; MED 20 mg/kg) behaviors. Both EMQMCM and MTEP decreased the floating duration (MED 1.25 and 2.5 mg/kg) and increased the duration of mobile behaviors (paddling and swimming; MED 2.5 and 5 mg/kg). EMQMCM but not MTEP increased the duration of escape behaviors (climbing and diving; MED 1.25 mg/kg). In the mouse tail suspension test, EMQMCM (5 but not 2.5, 10 and 25 mg/kg), 2-methyl-6-(phenylethynyl)-pyridine (MPEP, 10 but not 1 mg/kg) and MTEP (MED 25 mg/kg) decreased immobility scores. For EMQMCM, the dose-effect relationship was biphasic. With the exception of EMQMCM (10 mg/kg), locomotor activity in mice was not affected by treatments. The present study therefore suggests that acute blockade of mGluR5 and also of mGluR1 exerts antidepressant-like effects in behavioral despair tests in rats and mice.     

Selective mGlu5 receptor antagonist MTEP attenuates naloxone-induced morphine withdrawal symptoms

Several lines of evidence suggest a crucial involvement of glutamate in the mechanism of drug addiction. The involvement of group I mGlu receptors in the mechanism of addiction has also been proposed. Given the recent discovery of selective and brain penetrable mGlu5 receptor antagonists, the effects of 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]-pyridine (MTEP) were evaluated in the naloxone-precipitated morphine withdrawal model. Experiments were performed on male C57BL/6J (20-25 g) mice. Mice were rendered morphine-dependent and withdrawal was precipitated with naloxone. Two hours and 15 min after the last dose of morphine, mice were injected with a mGlu5 receptor antagonist. MTEP (1-10 mg/kg) in a dose-dependent manner inhibited the naloxone-induced symptoms of morphine withdrawal in morphine-dependent mice, remaining without any effect on the locomotor activity of mice. The data suggest that selective mGlu5 receptor antagonists may play a role in the therapy of drug-dependence states.     

mGlu1 receptor blockade attenuates cue- and nicotine-induced reinstatement of extinguished nicotine self-administration behavior in rats

Glutamatergic neurotransmission is believed to be critically involved in the acquisition and maintenance of drug addiction. The present study evaluated the role of metabotropic glutamate (mGlu) 1 receptors in the reinstatement of nicotine-seeking behavior. Rats were trained to nose-poke to receive response-contingent intravenous infusions of nicotine (0.01 mg/kg/infusion, free base). Following the subsequent extinction phase, reinstatement tests were conducted in animals that were exposed either to response-contingent presentations of the nicotine-associated discrete light cues or to non-contingent nicotine priming injection (0.3mg/kg, s.c., salt) just prior to the test session. In a separate experiment, rats were subjected to the nearly identical response-reinstatement procedure but operant responding was established using food pellets instead of nicotine infusions. Pretreatment with the mGlu1 receptor antagonist EMQMCM (JNJ16567083, (3-ethyl-2-methyl-quinolin-6-yl)-(4-methoxy-cyclohexyl)-methanone methanesulfonate) significantly inhibited cue-induced reinstatement of nicotine-seeking behavior (5 and 10, but not 2.5 mg/kg). EMQMCM (5 mg/kg) also prevented nicotine priming-induced reinstatement of nicotine-seeking behavior. At the highest tested dose only (10 mg/kg), EMQMCM attenuated cue-induced reinstatement of food-seeking behavior. Taken together with the previous reports, the present findings further suggest that blockade of mGlu1 receptors may be beneficial for preventing relapse to tobacco smoking in nicotine-dependent individuals.     

The behavioral profile of the potent and selective mGlu5 receptor antagonist 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP) in rodent models of anxiety.

Previous reports have demonstrated the anxiolytic effect of the potent and systemically active metabotropic glutamate subtype 5 (mGlu5) receptor antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP) in rodents. Here, we present evidence for the anxiolytic activity of a novel mGlu5 receptor antagonist, 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP), in rats and compare its profile to the benzodiazepine receptor agonist diazepam. MTEP occupied mGlu5 receptors in a dose-dependent manner with essentially full receptor occupancy at the highest dose tested (10 mg/kg, i.p.). At doses appropriate for mGlu5 receptor-mediated effects, MTEP significantly reduced fear-potentiated startle and increased punished responding in a modified Geller-Seifter conflict model consistent with an anxiolytic-like profile. In both models, the magnitude of the anxiolytic-like response was similar to that seen with diazepam. In contrast, MTEP decreased unpunished responding to a lesser extent than diazepam and had no effect on rotarod performance when administered either alone or in combination with ethanol. Repeated dosing with MTEP in this model eliminated the increase in punished responding observed with acute dosing. The present results suggest that mGlu5 receptor antagonists lack the side effects seen with benzodiazepines, such as sedation and ethanol interaction, and provide insight into a possible role for mGlu5 receptor antagonists in the modulation of mood disorders.     

Effects of group I metabotropic glutamate receptor antagonists on the behavioral sensitization to motor effects of cocaine in rats

Rationale Metabotropic glutamate receptors (mGluRs) were reported to regulate various behavioral effects of addictive drugs.Objective The present study evaluated the role of group I mGluRs in the progressive augmentation (“sensitization”) of the behavioral effects observed after repeated, intermittent cocaine exposure.Materials and methods After habituation to handling and baseline activity measurement (days 1–2), rats received eight injections of cocaine (10 mg/kg) or saline on days 3–6, 8–11, and then, were tested twice with acute saline and cocaine given in a counterbalanced manner on days 13 and 15. Before the test sessions, subjects were pretreated with mGluR1 antagonist EMQMCM (JNJ16567083, (3-ethyl-2-methyl-quinolin-6-yl)-(4-methoxy-cyclohexyl)-methanone methanesulfonate) and mGluR5 antagonist MTEP ([(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine).Results Pretreatment with EMQMCM (2.5–10 mg/kg) but not MTEP (2.5–10 mg/kg) significantly reduced expression of the sensitized ambulatory motor activity of the cocaine-experienced animals acutely challenged with cocaine. Both EMQMCM and MTEP significantly reduced vertical motor activity across all cocaine/saline treatment conditions.Conclusions These findings indicate that the expression of behavioral sensitization to cocaine-induced stimulation of locomotor activity may be modulated by group I mGluR antagonists (mGluR1 rather than mGluR5), but these effects occur at the dose levels that attenuate vertical activity.     

Anxiolytic-like effects of MTEP, a potent and selective mGlu5 receptor agonist does not involve GABA(A) signaling

Several lines of evidence suggest a crucial involvement of glutamate in the mechanism of action of anxiolytic drugs including the involvement of group I metabotropic glutamate (mGlu) receptors. Given the recent discovery of a selective and brain penetrable mGlu5 receptor antagonists, the effect of 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]-pyridine (MTEP), i.e. the most potent mGlu5 antagonist, was evaluated in established models of anxiety after single or repeated administration. We also studied if the anxiolytic effect of MTEP is mediated by mechanism involving the GABA-benzodiazepine (BZD) receptor complex. Experiments were performed on male Wistar rats or male Albino Swiss mice. The anxiolytic-like effects of MTEP were tested in the conflict drinking test and the elevated plus-maze test in rats as well as in the four-plate test in mice. MTEP (0.3-3.0 mg/kg) induced anxiolytic-like effects in the conflict drinking test (after single and repeated administration) and in the elevated plus-maze test in rats. In the four-plate test in mice, it exerted anxiolytic activity at a dose of 20 mg/kg. MTEP had no effect on the locomotor activity of animals. The anxiolytic-like effect of MTEP was not changed by BZD antagonist flumazenil. Moreover, a synergistic interaction between non-effective doses of MTEP and diazepam was observed in the conflict drinking test. These data suggest that selective mGlu5 receptor antagonists mediated anxiolysis is not dependent on GABA-ergic system and that these agents may play a role in the therapy of anxiety.     

Comparison of the effects of mGluR1 and mGluR5 antagonists on the expression of behavioral sensitization to the locomotor effect of morphine and the morphine withdrawal jumping in mice

The aim of the present study was to compare the influence of group I metabotropic glutamate receptor (mGluR) antagonists (mGluR1 and mGluR5) on the expression of sensitization to the locomotor effect of morphine. We also tested how these compounds affect the morphine withdrawal jumps in mice. In our study, the mGluR1 antagonist EMQMCM [3-ethyl-2-methyl-quinolin-6-yl-(4-methoxy-cyclohexyl)-methanone methanesulfonate] and the mGluR5 antagonist MTEP ([(2-methyl-1,3-thiazol-4-yl) ethynyl] pyridine) were used. Sensitization was induced by five intraperitoneal (i.p.) injections of morphine at the dose of 10 mg/kg, every 3 days. Morphine dependence was induced by subcutaneous (s.c.) implantation of pellets containing 37.5 mg of morphine base for three days. Our data indicate that pretreatment with EMQMCM (5, 10, 20 mg/kg) and MTEP (5, 10 mg/kg) on the challenge day, inhibited the expression of sensitization to the locomotor effect of morphine in mice. Antagonists of both subtypes of the group I mGlurs given alone, did not modify the acute locomotor effect of morphine. On the other hand, EMQMCM did not attenuate the morphine withdrawal jumps precipitated by naloxone (4 mg/kg). The results suggest that both subtypes of the group I mGluRs (mGluR1 and mGluR5) take part in the expression of morphine sensitization processes but mGluR1 is not involved in the expression of morphine withdrawal jumps in mice. These findings may have implications for the treatment of opiate addiction in future.     

Estrus variation in anticonflict-like effects of the mGlu5 receptor antagonist MTEP, microinjected into lateral septal nuclei of female Wistar rats

Anticonflict-like effects of the mGlu5 receptor antagonist MTEP (systemic administrations: 1.50, 3.0 or 6.0 mg/kg; i.p.; intra-lateral septal nuclei or intra-medial septal region infusions: 2.5 microg/microl, 5.0 microg/microl or 10.0 microg/microl) were assessed in Wistar rats during late proestrus or metestrus-diestrus. Results showed that control rats displayed an increased number of immediately punished reinforcers during late proestrus (P < 0.05), when compared to metestrus-diestrus. During late proestrus, systemic administrations (3.0 mg/kg, P < 0.05; 6.0 mg/kg P < 0.05) or intra-lateral septal nuclei infusions (5.0 microg/microl, P < 0.05; 10.0 microg/microl, P < 0.05) of MTEP increased the number of immediately punished reinforcers received. During metestrus-diestrus only the highest doses of MTEP (systemic administration: 6.0 mg/kg P < 0.05; intra-lateral septal nuclei infusions: 10.0 microg/microl, P < 0.05) increased the number of immediately punished reinforcers obtained. MTEP infusions into the medial septum produced neither of these anticonflict effects. In conclusion, data showed an estrus variation in those anticonflict-like effects of the mGlu5 receptor antagonist MTEP, systemically administered or microinjected into lateral septal nuclei of female Wistar rats.     

Transcriptional profiling of the rat frontal cortex following administration of the mGlu5 receptor antagonists MPEP and MTEP

The development of selective type 5 metabotropic glutamate receptor (mGlu5) antagonists, such as 2-methyl-6-(phenylethynyl)-pyridine (MPEP) and 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]-pyridine (MTEP), has revealed an important role for these receptors in various disorders of the nervous system including depression, anxiety, epilepsy, Parkinson's disease, drug addiction, and alcoholism. In this study, we used microarray technology to examine changes in gene expression induced by repeated administration of the mGlu5 antagonists MPEP and MTEP. Male Wistar rats (n=5 per treatment group) were administered MPEP (10 mg/kg), MTEP (10 mg/kg) or vehicle intraperitoneally twice daily for 5 days. Approximately 30 min following the final drug administration, rats were sacrificed and frontal cortices were then dissected and examined for changes in gene expression by cDNA microarray analysis. Changes in gene expression with p-values less than 0.01 were considered to be statistically significant. The expression of 63 genes was changed by both MPEP and MTEP, with 58 genes down-regulated and 5 genes up-regulated. Quantitative PCR verified the magnitude and direction of change in expression of 9 of these genes (r2=0.556, p=0.017). Pathway analysis revealed that many of the biological processes altered by repeated MPEP and MTEP treatment were related to ATP synthesis, hydrolase activity, and signaling pathways associated with mitogen-activated protein kinase (MAPK). Our results demonstrate diverse effects of MPEP and MTEP gene expression in the frontal cortex, and these results may help elucidate the mechanisms by which these compounds produce beneficial effects in animal models of various disorders of the central nervous system.     

Anxiolytic-like action of MTEP expressed in the conflict drinking Vogel test in rats is serotonin dependent

The purpose of the present study was to investigate whether the anxiolytic-like action of a selective and brain penetrable group I metabotropic glutamate (mGlu5) receptor antagonist 3-[(2-methyl-1,3-tiazol-4-yl)ethynyl]-pyridine (MTEP) is dependent upon the serotonergic system. Experiments were performed on male Wistar rats. The Vogel conflict drinking test was used to detect anxiolytic-like activity. MTEP administered intraperitoneally at doses of 1, 3 and 6 mg/kg induced anxiolytic-like effect. The potential anxiolytic effect of MTEP (1 mg/kg) was inhibited by a nonselective 5-HT receptor antagonist metergoline (2 mg/kg i.p.) and 5-HT2A/2C receptor antagonist ritanserin (0.5 mg/kg i.p.), but not by a 5-HT1A receptor antagonist N-{2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl}-N-(2-pyridynyl)cyclohexane-carboxamide (WAY 100635) (0.1 mg/kg i.p). The anxiolytic effect of MTEP (6 mg/kg) was attenuated by ritanserin (1 mg/kg i.p.). Moreover, MTEP-induced a dose-dependent release of serotonin in the frontal cortex. The obtained results suggest that the potential anxiolytic effect of the mGlu5 receptor antagonist MTEP is due to the increased serotonin release with subsequent activation of 5-HT2A/2C receptors, most probably located postsynaptically, but not by the 5-HT1A receptors.     

The anxiolytic and analgesic properties of fenobam, a potent mGlu5 receptor antagonist, in relation to the impairment of learning

Fenobam [N-(3-chlorophenyl)-N′-(4,5-dihydro-1-methyl-4-oxo-1H-imidazole-2-yl)urea] was suggested to possess anxiolytic actions 30 years ago. Hoffmann-La Roche researchers recently reported that it is a selective and potent mGlu5 receptor antagonist, acting as a negative allosteric modulator. In the present study, we show that fenobam readily penetrates to the brain, reaching concentrations over 600 nM, clearly above the affinity for mGluR5 receptors. Fenobam (at 10, 30, and 100 mg/kg) did not affect horizontal locomotor activity in the open field test. Anxiolytic-like activity in the context freezing test was seen at 30 mg/kg, while fenobam was not active in the elevated plus maze test at the tested concentrations. Fenobam had antinociceptive actions in the formalin test at 10 and 30 mg/kg, but failed to attenuate mechanical allodynia in the chronic constriction injury model. Impairment of learning was revealed in the passive avoidance test at 30 mg/kg. Fenobam also impaired performance in both the Morris water maze and in the contextual fear conditioning test at the doses of 30 and 10 mg/kg, respectively. Prepulse inhibition, used as a model of psychomimetic activity, was not affected by fenobam at doses of up to 60 mg/kg. Our results indicate that the beneficial effects of fenobam occur in a similar dose range as the potential side-effects.