Effects of mGlu1 and mGlu5 receptor antagonists on negatively reinforced learning

Effects on aversive learning of the novel highly selective mGlu5 receptor antagonist [(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP) and mGlu1 receptor antagonist (3-ethyl-2-methyl-quinolin-6-yl)-(4-methoxy-cyclohexyl)-methanone methanesulfonate (EMQMCM) were tested, after systemic administration, in the passive avoidance (PA) and fear potentiated startle (FPS) paradigms. Both MTEP at 10 mg/kg and EMQMCM at 5 and 10 mg/kg, given 30 min before training, impaired acquisition of the passive avoidance response (PAR). Co-administration of MTEP and EMQMCM at doses ineffective when administered alone, produced anterograde amnesia when given 30 min before the acquisition phase. Neither EMQMCM (5 mg/kg) nor MTEP (10 mg/kg) impaired retention of the PAR after direct post-training injections. EMQMCM (5 mg/kg), but not MTEP (10 mg/kg) blocked the PAR when given 30 min before testing. Pre-training administration of MTEP at doses of 2.5 and 5 mg/kg inhibited fear conditioning in the FPS when tested 24 h later. In contrast, EMQMCM was ineffective. Our findings suggest diverse involvement of mGlu1 and mGlu5 receptors in negatively reinforced learning.     

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.     

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.     

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.     

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).     

The mGluR5 Antagonist MTEP Dissociates the Acquisition of Predictive and Incentive Motivational Properties of Reward-Paired Stimuli in Mice

An environmental stimulus paired with reward (a conditioned stimulus; CS) can acquire predictive properties that signal reward availability and may also acquire incentive motivational properties that enable the CS to influence appetitive behaviors. The neural mechanisms involved in the acquisition and expression of these CS properties are not fully understood. The metabotropic glutamate receptor, mGluR5, contributes to synaptic plasticity underlying learning and memory processes. We examined the role of mGluR5 in the acquisition and expression of learning that enables a CS to predict reward (goal-tracking) and acquire incentive properties (conditioned reinforcement). Mice were injected with vehicle or the mGluR5 antagonist, MTEP (3 or 10 mg/kg), before each Pavlovian conditioning session in which a stimulus (CS+) was paired with food delivery. Subsequently, in the absence of the primary food reward, we determined whether the CS+ could reinforce a novel instrumental response (conditioned reinforcement) and direct behavior toward the place of reward delivery (goal-tracking). MTEP did not affect performance during the conditioning phase, or the ability of the CS+ to elicit a goal-tracking response. In contrast, 10 mg/kg MTEP given before each conditioning session prevented the subsequent expression of conditioned reinforcement. This dose of MTEP did not affect conditioned reinforcement when administered before the test, in mice that had received vehicle before conditioning sessions. Thus, mGluR5 has a critical role in the acquisition of incentive properties by a CS, but is not required for the expression of incentive learning, or for the CS to acquire predictive properties that signal reward availability.     

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.     

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.     

The antinociceptive and anxiolytic-like effects of the metabotropic glutamate receptor 5 (mGluR5) antagonists, MPEP and MTEP, and the mGluR1 antagonist, LY456236, in rodents: a comparison of efficacy and side-effect profiles

Rationale Modulation of metabotropic glutamate receptor (mGluR) subtypes represents a novel approach for the treatment of neurological and psychiatric disorders.Objectives This study was conducted to investigate the role of the mGluR5 and mGluR1 subtypes in the modulation of pain and anxiety.Methods The mGluR5 antagonists, 2-methyl-6-(phenylethynyl)pyridine (MPEP) and 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP), and the mGluR1 antagonist, (4-methoxy-phenyl)-(6-methoxy-quinazolin-4-yl)-amine HCl (LY456236), were tested in models of pain [mouse formalin test, rat spinal nerve ligation (SNL)] and anxiety [Vogel conflict, conditioned lick suppression (CLS)], and their efficacious effects were compared to any associated side effects.Results The systemic administration of MPEP, MTEP, and LY456236 reduced hyperalgesia induced by formalin and mechanical allodynia following SNL. However, only LY456236 completely reversed the allodynia. In the anxiety models, MPEP (3–30 mg/kg), MTEP (3–10 mg/kg), and LY456236 (10–30 mg/kg) produced anxiolytic-like effects similar to the benzodiazepine, chlordiazepoxide (CDP, 6 mg/kg). However, only MPEP and MTEP were able to produce a level of anxiolysis comparable to CDP. In a series of tests examining potential side effects, MPEP and MTEP reduced body temperature and locomotor activity and impaired operant responding for food and rotarod performance at doses of 3–30 and 1–30 mg/kg, respectively. LY456236 reduced operant responding at 30 mg/kg.Conclusion Both mGluR5 and mGluR1 antagonists are effective in models of pain and anxiety. However, an mGluR1 antagonist was more efficacious than the two mGluR5 antagonists in the pain models, which, conversely, appeared more efficacious in the anxiety models. These findings support the potential utility of mGluR5 and mGluR1 antagonists for both the treatment of chronic pain and as novel anxiolytics.     

Acquisition deficit and time-dependent retrograde amnesia for contextual fear conditioning in agmatine-treated rats

Cumulative evidence indicates that the hippocampus plays a time-limited role in contextual learning paradigms. Pharmacological studies have indicated that acquisition of background contextual cues during Pavlovian fear conditioning is dependent upon hippocampal function, whereas early inactivation of the hippocampus after training produces retrograde amnesia. When administered prior to contextual fear conditioning, agmatine (5 and 10 mg/kg, i.p.), an endogenous polyamine and N-methyl-D-aspartate (NMDA) receptor ligand found at excitatory synapses in the hippocampus, impaired the acquisition of contextual fear (measured as defensive freezing 26 hours later) without a reduction in baseline motor activity during training. Furthermore, ascending doses of agmatine were found not to exert analgesic effects on response thresholds to peripheral shock. This negated the possibility that the observed learning deficit resulted from a difference in perceived shock intensity. Post-training agmatine treatment produced a time-dependent impairment of consolidation, with subjects approaching a level of fear equivalent to that of a reference group as the delay of treatment increased (up to 6 hours). Since physiologically high levels of agmatine are able to inhibit NMDA receptor activity, these results suggest that polyamine modulation of NMDA receptors, most likely within the hippocampus, is required for the acquisition and consolidation of contextual fear stimuli.     

NMDA but not AMPA glutamatergic receptors are involved in the antidepressant-like activity of MTEP during the forced swim test in mice

Several lines of evidence suggest an antidepressant-like activity for 3-[(methyl-1,3-thiazol-4-yl)ethynyl]-pyridine (MTEP), a highly selective, non-competitive antagonist of metabotropic glutamate receptors subtype 5 (mGluR(5)). This effect has been observed following both acute and chronic MTEP treatments in behavioral tests and experimental models of depression, such as the forced swim test (FST), the tail suspension test, and the olfactory bulbectomy model of depression. However, the mechanism of action for mGluR(5) antagonists remains unclear. The aim of this study was to investigate whether the antidepressant-like action of MTEPis dependent on ionotropic glutamatergic receptors. Male Albino Swiss mice were used, and antidepressant-like activity was evaluated using the FST. The antidepressant-like effect of MTEP (0.3 mg/kg) was significantly antagonized by pre-treatment with the NMDA receptor agonist N-methyl-D-aspartic acid (NMDA, 75 mg/kg, i.p.). The AMPA receptor antagonist NBQX (10 mg/kg, i.p.) did not affect the MTEP activity. Our results indicate that the antidepressant-like activity of MTEP in the FST involves NMDA but not AMPA receptors and suggest that the interaction between mGluR(5) and NMDA receptors plays an important role in the underlying antidepressant mechanism(s).     

Functional Reuniens and Rhomboid Nuclei Are Required for Proper Acquisition and Expression of Cued and Contextual Fear in Trace Fear Conditioning

BackgroundThe reuniens (Re) and rhomboid (Rh) nuclei (ReRh) of the midline thalamus interconnect the hippocampus and the medial prefrontal cortex. The hippocampus and medial prefrontal cortex are both involved in the acquisition of trace fear conditioning, in which a conditioned stimulus (tone) and an aversive unconditioned stimulus (footshock) are paired but separated in time with a trace interval. Earlier, we demonstrated that ReRh inactivation during trace conditioning impaired the acquisition of cued fear. In contrast, ReRh inactivation during both conditioning and test resulted in heightened fear to tones during retrieval. Because there was a generalized contextual fear on top of heightened fear to tones in the latter experiment, here we aimed to examine the specific importance of the functional ReRh in cued fear and contextual fear through introducing prolonged contextual exposure.MethodsThe ReRh were pharmacologically inactivated with muscimol (or saline as controls) before each experimental session.ResultsWe showed that although ReRh inactivation before trace fear conditioning impaired the acquisition of cued fear, the animals still acquired a certain level of fear to the tones. However, without the functional ReRh throughout the entire behavioral sessions, these animals showed heightened contextual fear that did not decline much with the passage of time, which generalized to the other context, and fear to tones reoccurred when the tones were presented.ConclusionsOur results suggested that functional ReRh are important for proper acquisition and expression of fear to context and tones acquired under trace procedure.     

MK-801 disrupts acquisition of contextual fear conditioning but enhances memory consolidation of cued fear conditioning

The effects of pre-training or post-training subcutaneous injections of multiple doses of the non-competitive NMDA-receptor antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801) on cued and contextual fear conditioning were examined in F344 rats. Pre-training injections of MK-801 (0.3 and 1.0 mg/kg) disrupted contextual fear conditioning but not cued fear conditioning. Post-training injections of MK-801 did not disrupt cued or contextual fear conditioning. In fact, the 0.3 mg/kg dose of MK-801 enhanced cued fear conditioning. Finally, rats were tested for MK-801-induced alterations in sensitivity to pain using the formalin test for nociception. MK-801 did not reduce sensitivity to pain. These results suggest that NMDA receptors are involved in acquisition of contextual fear conditioning but not in memory consolidation of the learned response.     

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.     

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 metabotropic glutamate receptor antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) blocks fear conditioning in rats.

Glutamate receptors play an essential role in fear-related learning and memory. The present study was designed to assess the role of the group I metabotropic glutamate receptor (mGluR) subtype 5 in the acquisition and retrieval of conditioned fear in rats. The selective mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) was applied systemically (0.0, 0.3, 3.0, 30.0 mg/kg per os) 60 min before the acquisition training and before the expression of conditioned fear, respectively, in the fear-potentiated startle paradigm. MPEP dose-dependently blocked the acquisition of fear. This effect was not due to state-dependent learning. MPEP also prevented the expression of fear at a dose of 30.0 mg/kg. As a positive control for these effects, we showed that the benzodiazepine anxiolytic compound diazepam (1.25 mg/kg intraperitoneally) also blocked acquisition and expression of fear potentiated startle. MPEP did not affect the baseline startle magnitude, short-term habituation of startle, sensitisation of startle by footshocks or prepulse inhibition of startle. These data indicate a crucial role for mGluR5 in the regulation of fear conditioning. In the highest dose MPEP might exert anxiolytic properties.     

The CB1 inverse agonist AM251, but not the CB1 antagonist AM4113, enhances retention of contextual fear conditioning in rats

The effects of CB1 antagonist/inverse agonists on the acquisition and consolidation of conditioned fear remain uncertain. Recent studies suggest that the CB1 antagonist/inverse agonist AM251 affects acquisition or consolidation of both contextual and discretely cued fear memories. AM251 is frequently referred to as a CB1 antagonist; however in vitro signal transduction assays indicate that this drug also elicits inverse agonist activity at CB1 receptors. The present studies were undertaken to compare the effects of AM251 on conditioned fear with those produced by AM4113, a novel CB1 antagonist with minimal inverse agonist activity. All drugs were administered prior to conditioning. In retention tests conducted two weeks after conditioning, both AM251 (4.0 mg/kg) and AM4113 (6.0 mg/kg)-treated animals exhibited reduced freezing during a conditioned tone cue played within a novel context. In contextual fear retention tests, animals previously treated with 4.0 or 8.0 mg/kg AM251 exhibited enhanced freezing. By contrast, no dose of AM4113 had any significant effect on contextual fear memory, which is consistent with the lower signal transduction activity of AM4113 at CB1 receptors compared to AM251. These results suggest that CB1 neutral antagonists may be less likely than CB1 inverse agonists to facilitate the acquisition or consolidation of contextual fear that may contribute to some clinical disorders.     

Systemic administration of polyaminergic agents modulate fear conditioning in rats

Rationale The polyamines putrescine, spermine, and spermidine are a group of aliphatic amines that physiologically modulate the N-methyl-d-aspartate (NMDA) receptor, a glutamate receptor implicated in memory formation. Objectives Given the potential application of these drugs in the treatment of memory disorders, we investigated whether agonists and/or antagonists of the NMDA receptor polyamine binding site alters the memory of fear conditioning and determined the time window in which fear conditioning is modulated by polyaminergic agents given by the systemic route. Results Post-training intraperitoneal administration of spermidine (10–100 mg/kg) immediately after training increased, whereas arcaine (10 mg/kg) and MK-801 (0.01–0.1 mg/kg) decreased contextual and auditory fear conditioning. Arcaine and MK-801, at doses that had no effect per se, reversed the facilitatory effect of spermidine. Memory of fear conditioning was impaired by polyaminergic blockade up to 180 min but not at 360 min after training. Conclusion These results provide evidence that systemic administration of polyamine binding site ligands modulate early consolidation of fear conditioning.     

mGluR5 is necessary for maintenance of methamphetamine-induced associative learning

Conditioned place preference (CPP) reflects the significance of contextual cues that are associated with rewarding effects of abused drugs such as methamphetamine (Meth). Glutamate neurotransmission is augmented following exposure to stimulants and associated cues. Activation of group I metabotropic glutamate receptors (mGluR) is critical for the acquisition and expression of stimulant-induced CPP. We hypothesized that the maintenance of Meth-induced CPP would also require activated mGluR, and that the role of mGluR1 vs. mGluR5 group I subtypes may differ. To test this hypothesis, negative allosteric modulators (NAMs) of these receptors were administered following the development of Meth-induced CPP. NAMs exert their functional effects by displacing agonist from agonist-occupied receptors, thus NAMs selectively target brain regions with glutamate release. Conditioning with Meth every other day for six days resulted in significant preference for the Meth-paired compartment. Two once-daily injections of the mGluR1 NAM, JNJ16259685 (0.3 mg/kg, i.p.) or its vehicle on days 13 and 14 after Meth-conditioning did not influence the maintenance of Meth-induced CPP; however, administration of the mGluR5 NAMs MTEP (3 mg/kg, i.p.) and MPEP (30 mg/kg, i.p.) inhibited maintenance processes necessary for CPP to be expressed. These findings suggest a subtype-specific role of mGluR5 receptors in the maintenance of place preference memory and potential of mGluR5 NAMs as a useful target for Meth addiction therapy.