Metabotropic glutamate receptor ligands as possible anxiolytic and antidepressant drugs

Shining Light on an mGlu5 Photoswitchable NAM: A Theoretical Perspective

Metabotropic glutamate receptors (mGluRs) are important drug targets because of their involvement in several neurological diseases. Among mGluRs, mGlu5 is a particularly high-profile target because its positive or negative allosteric modulation can potentially treat schizophrenia or anxiety and chronic pain, respectively. Here, we computationally and experimentally probe the functional binding of a novel photoswitchable mGlu5 NAM, termed alloswitch-1, which loses its NAM functionality under violet light. We show alloswitch-1 binds deep in the allosteric pocket in a similar fashion to mavoglurant, the co-crystallized NAM in the mGlu5 transmembrane domain crystal structure. Alloswitch-1, like NAM 2-Methyl-6-(phenylethynyl)pyridine (MPEP), is significantly affected by P655M mutation deep in the allosteric pocket, eradicating its functionality. In MD simulations, we show alloswitch-1 and MPEP stabilize the co-crystallized water molecule located at the bottom of the allosteric site that is seemingly characteristic of the inactive receptor state. Furthermore, both NAMs form H-bonds with S809 on helix 7, which may constitute an important stabilizing interaction for NAM-induced mGlu5 inactivation. Alloswitch-1, through isomerization of its amide group from trans to cis is able to form an additional interaction with N747 on helix 5. This may be an important interaction for amide-containing mGlu5 NAMs, helping to stabilize their binding in a potentially unusual cis-amide state. Simulated conformational switching of alloswitch-1 in silico suggests photoisomerization of its azo group from trans to cis may be possible within the allosteric pocket. However, photoexcited alloswitch-1 binds in an unstable fashion, breaking H-bonds with the protein and destabilizing the co-crystallized water molecule. This suggests photoswitching may have destabilizing effects on mGlu5 binding and functionality.

Expression of group III metabotropic glutamate receptors in the reproductive system of male mice

Although the presence of metabotropic glutamate (mGlu) receptors in the central nervous system is well documented, they have recently been found in peripheral and non-neuronal tissues. In the present study we investigated the expression of group III mGlu receptors in the reproductive system of male mice. Reverse transcription–polymerase chain reaction analysis revealed the presence of mGlu6, mGlu7 and mGlu8 (but not mGlu4) receptor transcripts in testes and epididymides from adult mice. In addition, expression of mGlu6 (Grm6) and mGlu8 receptor (Grm8) mRNA was detected in spermatozoa isolated from the vas deferens. The vas deferens was found to contain only mGlu7 receptor (Grm7) mRNA, which was particularly intense in 21-day-old male mice. In penile homogenates, only the mGlu7 receptor signal was detected. Genetic ablation of the mGlu7 receptor in males led to fertility disorders manifested by decreased insemination capability as well as deterioration of sperm parameters, particularly sperm motility, vitality, sperm membrane integrity and morphology, with a simultaneous increase in sperm concentration. These results indicate that constitutively expressed mGlu receptors in the male reproductive system may play an important role in ejaculation and/or erection processes, as well as in the formation and maturation of spermatozoa.

mGlu2 Receptor Agonism, but Not Positive Allosteric Modulation, Elicits Rapid Tolerance towards Their Primary Efficacy on Sleep Measures in Rats

G-protein-coupled receptor (GPCR) agonists are known to induce both cellular adaptations resulting in tolerance to therapeutic effects and withdrawal symptoms upon treatment discontinuation. Glutamate neurotransmission is an integral part of sleep-wake mechanisms, which processes have translational relevance for central activity and target engagement. Here, we investigated the efficacy and tolerance potential of the metabotropic glutamate receptors (mGluR2/3) agonist LY354740 versus mGluR2 positive allosteric modulator (PAM) JNJ-42153605 on sleep-wake organisation in rats. In vitro, the selectivity and potency of JNJ-42153605 were characterized. In vivo, effects on sleep measures were investigated in rats after once daily oral repeated treatment for 7 days, withdrawal and consecutive re-administration of LY354740 (1–10 mg/kg) and JNJ-42153605 (3–30 mg/kg). JNJ-42153605 showed high affinity, potency and selectivity at mGluR2. Binding site analyses and knowledge-based docking confirmed the specificity of JNJ-42153605 at the mGluR2 allosteric binding site. Acute LY354740 and JNJ-42153605 dose-dependently decreased rapid eye movement (REM) sleep time and prolonged its onset latency. Sub chronic effects of LY354740 on REM sleep measures disappeared from day 3 onwards, whereas those of JNJ-42153605 were maintained after repeated exposure. LY354740 attenuated REM sleep homeostatic recovery, while this was preserved after JNJ-42153605 administration. JNJ-42153605 enhanced sleep continuity and efficiency, suggesting its potential as an add-on medication for impaired sleep quality during early stages of treatment. Abrupt cessation of JNJ-42153605 did not induce withdrawal phenomena and sleep disturbances, while the initial drug effect was fully reinstated after re-administration. Collectively, long-term treatment with JNJ-42153605 did not induce tolerance phenomena to its primary functional effects on sleep measures, nor adverse effects at withdrawal, while it promoted homeostatic recovery sleep. From the translational perspective, the present rodent findings suggest that mGluR2 positive allosteric modulation has therapeutic potential based on its superior long term efficacy over agonists in psychiatric disorders, particularly of those commonly occurring with REM sleep overdrive.

Characterization of brain mGluR5 binding in a pilot study of late-life major depressive disorder using positron emission tomography and [¹¹C]ABP688

The metabotropic glutamate receptor subtype 5 (mGluR5) has been implicated in the pathophysiology of mood and anxiety disorders and is a potential treatment target in major depressive disorder (MDD). This study compared brain mGluR5 binding in elderly patients suffering from MDD with that in elderly healthy volunteers using positron emission tomography (PET) and [(11)C]ABP688. Twenty elderly (mean age: 63.0 ± 6.3) subjects with MDD and twenty-two healthy volunteers in the same age range (mean age: 66.4 ± 7.3) were examined with PET after a single bolus injection of [(11)C]ABP688, with many receiving arterial sampling. PET images were analyzed on a region of interest and a voxel level to compare mGluR5 binding in the brain between the two groups. Differences in [(11)C]ABP688 binding between patients with early- and late-onset depression were also assessed. In contrast to a previously published report in a younger cohort, no significant difference in [(11)C]ABP688 binding was observed between elderly subjects with MDD and healthy volunteers. [(11)C]ABP688 binding was also similar between subgroups with early- or late-onset depression. We believe this is the first study to examine mGluR5 expression in depression in the elderly. Although future work is required, results suggest potential differences in the pathophysiology of elderly depression versus depression earlier in life.

Design of 4-Oxo-1-aryl-1,4-dihydroquinoline-3-carboxamides as Selective Negative Allosteric Modulators of Metabotropic Glutamate Receptor Subtype 2

Both orthosteric and allosteric antagonists of the group II metabotropic glutamate receptors (mGlus) have been used to establish a link between mGlu2/3 inhibition and a variety of CNS diseases and disorders. Though these tools typically have good selectivity for mGlu2/3 versus the remaining six members of the mGlu family, compounds that are selective for only one of the individual group II mGlus have proved elusive. Herein we report on the discovery of a potent and highly selective mGlu2 negative allosteric modulator 58 (VU6001192) from a series of 4-oxo-1-aryl-1,4-dihydroquinoline-3-carboxamides. The concept for the design of this series centered on morphing a quinoline series recently disclosed in the patent literature into a chemotype previously used for the preparation of muscarinic acetylcholine receptor subtype 1 positive allosteric modulators. Compound 58 exhibits a favorable profile and will be a useful tool for understanding the biological implications of selective inhibition of mGlu2 in the CNS.

REM sleep deprivation promotes a dopaminergic influence in the striatal MT2 anxiolytic-like effects

The aim of this study was to investigate the possible anxiolytic-like effects of striatal MT2 activation, and its counteraction induced by the selective blockade of this receptor. Furthermore, we analyzed this condition under the paradigm of rapid eye movement (REM) sleep deprivation (REMSD) and the animal model of Parkinson’s disease (PD) induced by rotenone. Male Wistar rats were infused with intranigral rotenone (12 μg/μL), and 7 days later were subjected to 24 h of REMSD. Afterwards the rats underwent striatal micro-infusions of selective melatonin MT2 receptor agonist, 8-M-PDOT (10 μg/μL) or selective melatonin MT2 receptor antagonist, 4-P-PDOT (5 μg/μL) or vehicle. Subsequently, the animals were tested in the open-field (OP) and elevated plus maze (EPM) tests. Results indicated that the activation of MT2 receptors produced anxiolytic-like effects. In opposite, the MT2 blockade did not show an anxiogenic-like effect. Besides, REMSD induced anxiolytic-like effects similar to 8-M-PDOT. MT2 activation generated a prevalent locomotor increase compared to MT2 blockade in the context of REMSD. Together, these results suggest a striatal MT2 modulation associated to the REMSD-induced dopaminergic supersensitivity causing a possible dopaminergic influence in the MT2 anxiolytic-like effects in the intranigral rotenone model of PD.

Metabotropic glutamate receptors as targets for new antipsychotic drugs: Historical perspective and critical comparative assessment

In this review, we aim to present, discuss and clarify our current understanding regarding the prediction of possible antipsychotic effects of metabotropic glutamate (mGlu) receptor ligands. The number of preclinical trials clearly indicates, that this group of compounds constitutes an excellent alternative to presently used antipsychotic therapy, being effective not only to positive, but also negative and cognitive symptoms of schizophrenia. Although the results of clinical trials that were performed for the group of mGlu2/3 agonists were not so enthusiastic as in animal studies, they still showed that mGlu ligands do not induced variety of side effects typical for presently used antipsychotics, and were generally well tolerated. The lack of satisfactory effectiveness towards schizophrenia symptoms of mGlu2/3 activators in humans could be a result of variety of uncontrolled factors and unidentified biomarkers different for each schizophrenia patient, that should be taken into consideration in the future set of clinical trials. The subject is still open for further research, and the novel classes of mGlu5 or mGlu2/3 agonists/PAMs were recently introduced, including the large group of compounds from the third group of mGlu receptors, especially of mGlu4 subtype. Finally, more precise treatment based on simultaneous administration of minimal doses of the ligands for two or more receptors, seems to be promising in the context of symptoms-specific schizophrenia treatment.

Discovery of a Selective and CNS Penetrant Negative Allosteric Modulator of Metabotropic Glutamate Receptor Subtype 3 with Antidepressant and Anxiolytic Activity in Rodents

Previous preclinical work has demonstrated the therapeutic potential of antagonists of the group II metabotropic glutamate receptors (mGlus). Still, compounds that are selective for the individual group II mGlus (mGlu2 and mGlu3) have been scarce. There remains a need for such compounds with the balance of properties suitable for convenient use in a wide array of rodent behavioral studies. We describe here the discovery of a selective mGlu3 NAM 106 (VU0650786) suitable for in vivo work. Compound 106 is a member of a series of 5-aryl-6,7-dihydropyrazolo[1,5-a]pyrazine-4(5H)-one compounds originally identified as a mGlu5 positive allosteric modulator (PAM) chemotype. Its suitability for use in rodent behavioral models has been established by extensive in vivo PK studies, and the behavioral experiments presented here with compound 106 represent the first examples in which an mGlu3 NAM has demonstrated efficacy in models where prior efficacy had previously been noted with nonselective group II antagonists.

Impaired 2-AG Signaling in Hippocampal Glutamatergic Neurons: Aggravation of Anxiety-Like Behavior and Unaltered Seizure Susceptibility

BACKGROUND

Postsynaptically generated 2-arachidonoylglycerol activates the presynaptic cannabinoid type-1 receptor, which is involved in synaptic plasticity at both glutamatergic and GABAergic synapses. However, the differential function of 2-arachidonoylglycerol signaling at glutamatergic vs GABAergic synapses in the context of animal behavior has not been investigated yet.

METHODS

Here, we analyzed the role of 2-arachidonoylglycerol signaling selectively in hippocampal glutamatergic neurons. Monoacylglycerol lipase, the primary degrading enzyme of 2-arachidonoylglycerol, is expressed at presynaptic sites of excitatory and inhibitory neurons. By adeno-associated virus-mediated overexpression of monoacylglycerol lipase in glutamatergic neurons of the mouse hippocampus, we selectively interfered with 2-arachidonoylglycerol signaling at glutamatergic synapses of these neurons.

RESULTS

Genetic modification of monoacylglycerol lipase resulted in a 50% decrease in 2-arachidonoylglycerol tissue levels without affecting the content of the second major endocannabinoid anandamide. A typical electrophysiological read-out for 2-arachidonoylglycerol signaling is the depolarization-induced suppression of excitation and of inhibition. Elevated monoacylglycerol lipase levels at glutamatergic terminals selectively impaired depolarization-induced suppression of excitation, while depolarization-induced suppression of inhibition was not significantly changed. At the behavioral level, mice with impaired hippocampal glutamatergic 2-arachidonoylglycerol signaling exhibited increased anxiety-like behavior but showed no alterations in aversive memory formation and seizure susceptibility.

CONCLUSION

Our data indicate that 2-arachidonoylglycerol signaling selectively in hippocampal glutamatergic neurons is essential for the animal's adaptation to aversive situations.

Quantification of the Serotonin 1A Receptor Using PET: Identification of a Potential Biomarker of Major Depression in Males

Multiple lines of research have implicated the serotonin 1A (5-HT1A) receptor in major depressive disorder (MDD). Despite this, quantification of 5-HT1A is yet to yield a clinically relevant MDD biomarker. One reason may be that reported sex differences in the serotonergic system confound the comparison between diagnostic groups. Therefore, this study sought to determine whether differences in 5-HT1A binding between depressed and control subjects are affected by sex. Using positron emission tomography (PET), serotonin 1A binding was quantified in 50 patients with MDD (34 female, 16 male) and 57 healthy controls (32 female, 25 male). The subjects' 5-HT1A density (BPF, equal to the product of the density of available receptors and tracer affinity), was determined by using the PET tracer [carbonyl-C-11]-WAY-100635, a selective 5-HT1A antagonist. Results indicated that male MDD subjects had a 67.0% higher BPF across 13 brain regions compared with male controls (df=103, p<0.0001). The greatest difference between MDD subjects and controls was in the raphe (132%, p=0.000). Furthermore, by using a threshold, male controls can be distinguished from depressed males with high sensitivity and specificity (both >80%). In females, the separation between diagnostic groups yields much lower sensitivity and specificity. This data therefore suggests a specific biosignature for MDD in males. Identification of such a biosignature could provide a deeper understanding of depression pathology, help identify those at highest risk, and aid in the development of new therapies. Further, these findings suggest that combining male and female cohorts may not be optimal for some MDD studies.

DSR-98776, a novel selective mGlu5 receptor negative allosteric modulator with potent antidepressant and antimanic activity

Modulation of monoaminergic systems has been the main stream of treatment for patients with mood disorders. However, recent evidence suggests that the glutamatergic system plays an important role in the pathophysiology of these disorders. This study pharmacologically characterized a structurally novel metabotropic glutamate 5 (mGlu5) receptor negative allosteric modulator, DSR-98776, and evaluated its effect on rodent models of depression and mania. First, DSR-98776 in vitro profile was assessed using intracellular calcium and radioligand binding assays. This compound showed dose-dependent inhibitory activity for mGlu5 receptors by binding to the same allosteric site as 2-methyl-6-(phenylethynyl)-pyridine (MPEP), a known mGlu5 inhibitor. The in vivo therapeutic benefits of DSR-98776 were evaluated in common rodent models of depression and mania. In the rat forced swimming test, DSR-98776 (1-3mg/kg) significantly reduced rats immobility time after treatment for 7 consecutive days, while paroxetine (3 and 10mg/kg) required administration for 2 consecutive weeks to reduce rats immobility time. In the mouse forced swimming test, acute administration of DSR-98776 (10-30 mg/kg) significantly reduced immobility time. This effect was not influenced by 4-chloro-DL-phenylalanine methyl ester hydrochloride-induced 5-HT depletion. Finally, DSR-98776 (30 mg/kg) significantly decreased methamphetamine/chlordiazepoxide-induced hyperactivity in mice, which reflects this compound antimanic-like effect. These results indicate that DSR-98776 acts as an orally potent antidepressant and antimanic in rodent models and can be a promising therapeutic option for the treatment of a broad range of mood disorders with depressive and manic states.

Agomelatine: mechanism of action and pharmacological profile in relation to antidepressant properties

Agomelatine behaves both as a potent agonist at melatonin MT1 and MT2 receptors and as a neutral antagonist at 5-HT2C receptors. Accumulating evidence in a broad range of experimental procedures supports the notion that the psychotropic effects of agomelatine are due to the synergy between its melatonergic and 5-hydroxytryptaminergic effects. The recent demonstration of the existence of heteromeric complexes of MT1 and MT2 with 5-HT2C receptors at the cellular level may explain how these two properties of agomelatine translate into a synergistic action that, for example, leads to increases in hippocampal proliferation, maturation and survival through modulation of multiple cellular pathways (increase in trophic factors, synaptic remodelling, glutamate signalling) and key targets (early genes, kinases). The present review focuses on the pharmacological properties of this novel antidepressant. Its mechanism of action, strikingly different from that of conventional classes of antidepressants, opens perspectives towards a better understanding of the physiopathological bases underlying depression.

Fenobam promoted the neuroprotective effect of PEP-1-FK506BP following oxidative stress by increasing its transduction efficiency

We examined the ways in which fenobam could promote not only the transduction of PEP-1-FK506BP into cells and tissues but also the neuroprotective effect of PEP-1-FK506BP against ischemic damage. Fenobam strongly enhanced the protective effect of PEP-1-FK506BP against H₂O₂-induced toxicity and DNA fragmentation in C6 cells. In addition, combinational treatment of fenobam with PEP-1-FK506BP significantly inhibited the activation of Akt and MAPK induced by H₂O₂, compared to treatment with PEP-1-FK506BP alone. Interestingly, our results showed that fenobam significantly increased the transduction of PEP-1-FK506BP into both C6 cells and the hippocampus of gerbil brains. Subsequently, a transient ischemic gerbil model study demonstrated that fenobam pretreatment led to the increased neuroprotection of PEP-1-FK506BP in the CA1 region of the hippocampus. Therefore, these results suggest that fenobam can be a useful agent to enhance the transduction of therapeutic PEP-1-fusion proteins into cells and tissues, thereby promoting their neuroprotective effects. [BMB Reports 2013; 46(11): 561-566]

Diagnosis and early detection of CNS-SLE in MRL/lpr mice using peptide microarrays

Background

An accurate method that can diagnose and predict lupus and its neuropsychiatric manifestations is essential since currently there are no reliable methods. Autoantibodies to a varied panel of antigens in the body are characteristic of lupus. In this study we investigated whether serum autoantibody binding patterns on random-sequence peptide microarrays (immunosignaturing) can be used for diagnosing and predicting the onset of lupus and its central nervous system (CNS) manifestations. We also tested the techniques for identifying potentially pathogenic autoantibodies in CNS-Lupus. We used the well-characterized MRL/lpr lupus animal model in two studies as a first step to develop and evaluate future studies in humans.

Results

In study one we identified possible diagnostic peptides for both lupus and altered behavior in the forced swim test. When comparing the results of study one to that of study two (carried out in a similar manner), we further identified potential peptides that may be diagnostic and predictive of both lupus and altered behavior in the forced swim test. We also characterized five potentially pathogenic brain-reactive autoantibodies, as well as suggested possible brain targets.

Conclusions

These results indicate that immunosignaturing could predict and diagnose lupus and its CNS manifestations. It can also be used to characterize pathogenic autoantibodies, which may help to better understand the underlying mechanisms of CNS-Lupus.

Computational analysis of negative and positive allosteric modulator binding and function in metabotropic glutamate receptor 5 (in)activation

Metabotropic glutamate receptors (mGluRs) are high-profile G-protein coupled receptors drug targets because of their involvement in several neurological disease states, and mGluR5 in particular is a subtype whose controlled allosteric modulation, both positive and negative, can potentially be useful for the treatment of schizophrenia and relief of chronic pain, respectively. Here we model mGluR5 with a collection of positive and negative allosteric modulators (PAMs and NAMs) in both active and inactive receptor states, in a manner that is consistent with experimental information, using a specialized protocol that includes homology to increase docking accuracy, and receptor relaxation to generate an individual induced fit with each allosteric modulator. Results implicate two residues in particular for NAM and PAM function: NAM interaction with W785 for receptor inactivation, and NAM/PAM H-bonding with S809 for receptor (in)activation. Models suggest the orientation of the H-bond between allosteric modulator and S809, controlled by PAM/NAM chemistry, influences the position of TM7, which in turn influences the shape of the allosteric site, and potentially the receptor state. NAM-bound and PAM-bound mGluR5 models also reveal that although PAMs and NAMs bind in the same pocket and share similar binding modes, they have distinct effects on the conformation of the receptor. Our models, together with the identification of a possible activation mechanism, may be useful in the rational design of new allosteric modulators for mGluR5.

Blockade of the metabotropic glutamate (mGluR2) modulates arousal through vigilance states transitions: evidence from sleep-wake EEG in rodents

Accumulating data continue to support the therapeutic potential of glutamate metabotropic (mGluR2) receptors for treatment of psychiatric disorders such as depression, anxiety and schizophrenia. Glutamate neurotransmission is an integral component of sleep-wake mechanisms, which have translational relevance to assess on-target activity of drugs. Here, we investigated the influence of mGluR2 inactivation upon sleep-wake electroencephalogram (EEG) in rodents. Rats were administered with vehicle, the specific mGluR2 antagonist LY341495 (2.5, 5, 10mg/kg) or negative allosteric modulator (NAM) Ro-4491533 (2.5, 10 and 40 mg/kg) at lights onset. mGluR2 (-/-) mice were used to confirm the selectivity of functional response. Both LY341495 and Ro-4491533 induced an immediate and endured desynchronized cortical activity during 3-6h associated with enhanced theta and gamma oscillations and depressed slow oscillations during sleep. The arousal-promoting effect is consistent with the marked lengthening of sleep onset latency, an increased number of state transitions from light sleep to waking and the gradual increase in homeostatic compensatory sleep. The arousal response to mGluR2 blockade was not accompanied by sharp rebound hypersomnolence as found with the classical psycho-stimulant amphetamine. mGluR2 (-/-) mice and their WT littermates exhibited similar sleep-wake phenotype, while Ro-4491533 (40 mg/kg) enhanced waking associated with increased locomotor activity and body temperature in WT but not in mGluR2 (-/-) mice, which confirm the role of mGluR2 inactivation in the arousal response. Our results lend support for a role of mGluR2 blockade in promoting cortical arousal associated with theta/gamma oscillations as well as high thresholds transitions from sleep to waking.

Radiation Dosimetry of (18)F-FPEB in Humans

(18)F-3-fluoro-5-[(pyridin-3-yl)ethynyl]benzonitrile ((18)F-FPEB) is a potent and specific radioligand for the metabotropic glutamate receptor subtype 5 (mGluR5). Before undertaking clinical research studies with (18)F-FPEB, we performed studies of human radiation dosimetry.

METHODS

Serial whole-body scans were obtained in 9 healthy human subjects (5 men, 4 women) for 190-440 min after the intravenous administration of (18)F-FPEB. Radiation doses were estimated using the OLINDA/EXM software.

RESULTS

Peak organ doses were to the urinary bladder wall, 0.258 mGy/MBq (0.955 rad/mCi), and gallbladder wall, 0.193 mGy/MBq (0.716 rad/mCi). The effective dose was 0.025 mSv/MBq (0.0922 rem/mCi). The doses to the red marrow and spleen were 0.00797 mGy/MBq (0.0295 rad/mCi) and 0.00709 mGy/MBq (0.0262 rad/mCi), respectively. Reducing the urinary voiding interval to 60 or 90 min lowered the urinary bladder wall dose to 0.0885 mGy/MBq (0.327 rad/mCi) or 0.128 mGy/MBq (0.473 rad/mCi), respectively, and the effective dose to 0.0149 mSv/MBq (0.0551 rem/mCi) or 0.0171 mSv/MBq (0.0634 rem/mCi), respectively.

CONCLUSION

Urinary voiding should be performed during (18)F-FPEB studies to minimize radiation exposure to research subjects.

An open-label, pilot study evaluating the safety and antidepressant effects of Rellidep in major depressive disorder

BACKGROUND

In 4 clinical studies of a biological tissue isolate (BTI) similar to Rellidep, the majority of subjects reported improved well-being and mood. Based on these reports of positive effect on mood in trials, an antidepressant pilot study was undertaken with the BTI Rellidep.

METHOD

Twenty-three subjects (14 women and 9 men; mean age, 44.8 years) with major depressive disorder entered an 8-week trial of Rellidep with a 17-item Hamilton Depression Rating Scale (HAM-D) score of 18 or above. Three subjects dropped out before beginning treatment, leaving an intention-to-treat (ITT) sample of 20. Sixteen subjects completed the trial. All patients received Rellidep 1000 mg (500 mg × 2) twice a day orally daily for 8 weeks. No other treatments were permitted except for intermittent lorazepam for sleep if necessary. The primary response to treatment outcome was a 50% decline on the HAM-D. Secondary measures included the Beck Depression Inventory, Montgomery-Åsberg Depression Rating Scale, Clinical Global Impression, Hamilton Rating Scale for Anxiety, Medical Outcomes Study Short-Form 36, and Arizona Sexual Experience Scale.

RESULTS

The overall response rate was 75% (15/20). Almost 87% of those who responded did so by week 4. The data analysis of change scores indicates that for the ITT population the decrease in the HAM-D was statistically significant (P < 0.001). On all other measures, improvement from baseline was statistically significant (P < 0.001), except for the Arizona Sexual Experience Scale (ITT: P = 0.055). There were no significant adverse events.

CONCLUSIONS

The positive outcome of this open trial indicates that Rellidep may be an effective and safe antidepressant. Further placebo-controlled trials are indicated.

Preclinical evidence of rapid-onset antidepressant-like effect in Radix Polygalae extract

Radix Polygalae (the root of Polygala tenuifolia) is a herb widely used in traditional Asian medicine that is thought to exert a variety of neuropsychiatric effects. Radix Polygalae extract can protect against N-methyl D-aspartate (NMDA) neurotoxicity and induce brain-derived neurotrophic factor (BDNF) expression, suggesting modulatory roles at glutamatergic synapses and possible antidepressant action. In accordance with this hypothesis, Radix Polygalae extract demonstrated antidepressant-like effects in 8-week-old male C57Bl/6 mice by decreasing behavioral despair in the forced swim and tail suspension tasks and increasing hedonic-like behavior in the female urine sniffing test 30 minutes after a single oral administration of 0.1 mg/kg. Reduced latency to acquire a food pellet in the novely suppressed feeding paradigm, without change in anxiety-like behaviors suggested a rapid-onset nature of the antidepressant-like effect. In addition, it decreased the number of failed escapes in the learned helplessness paradigm after two oral administrations 24 hours and 30 minutes before the first test. Finally, it reversed anhedonia as measured by saccharin preference in mice exposed to the chronic stress model after two administrations of 0.1 mg/kg, in contrast to the repeated administration generally needed for similar effect by monoamergic antidepressants. Immobility reduction in tail suspension task was blocked by the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist NBQX, a pattern previously demonstrated by ketamine and other ketamine-like rapid-onset antidepressants. Also similarly to ketamine, Radix Polygalae appeared to acutely decrease phosphorylation of GluR1 serine-845 in the hippocampus while leaving the phosphorylation of hippocampal mTOR serine 2448 unchanged. These findings serve as preclinical evidence that Radix Polygalae extract exerts rapid-onset antidepressant effects by modulating glutamatergic synapses in critical brain circuits of depression and may be worthy of further evaluation as a safe substitute to other rapid-onset antidepressants known to have unacceptable side effects.

Rethinking metabotropic glutamate receptor 5 pathological findings in psychiatric disorders: implications for the future of novel therapeutics

BACKGROUND

Pharmacological modulation of metabotropic glutamate receptor 5 (mGluR5) is of marked interest as a novel therapeutic mechanism to treat schizophrenia and major depression. However, the status of mGluR5 in the pathophysiology of these disorders remains unknown.

DISCUSSION

The majority of studies in the schizophrenia post-mortem brain indicate that total mGluR5 expression is unaltered. However, close examination of the literature suggests that these findings are superficial, and in actuality, a number of critical factors have not yet been considered; alterations may be highly dependent on brain region, neuronal population or molecular organisation in specific cellular compartments. A number of genetic knockout studies (mGluR5, Norbin, Homer1 etc.) continue to lend support to a role of mGluR5 in the pathology of schizophrenia, providing impetus to explore the regulation of mGluR5 beyond total mGluR5 protein and mRNA levels. With regards to major depression, preliminary evidence to date shows a reduction in total mGluR5 protein and mRNA levels; however, as in schizophrenia, there are no studies examining mGluR5 function or regulation in the pathological state. A comprehensive understanding of mGluR5 regulation in major depression, particularly in comparison to schizophrenia, is crucial as this has extensive implications for mGluR5 targeting novel therapeutics, especially considering that opposing modulation of mGluR5 is of therapeutic interest for these two disorders.

SUMMARY

Despite the complexities, examinations of post-mortem human brain provide valuable insights into the pathologies of these inherently human disorders. It is important, especially with regards to the identification of novel therapeutic drug targets, to have an in depth understanding of the pathophysiologies of these disorders. We posit that brain region- and cell type-specific alterations exist in mGluR5 in schizophrenia and depression, with evidence pointing towards altered regulation of this receptor in psychiatric pathology. We consider the implications of these alterations, as well as the distinction between schizophrenia and depression, in the context of novel mGluR5 based therapeutics.

The antidepressant-like action of mGlu5 receptor antagonist, MTEP, in the tail suspension test in mice is serotonin dependent

RATIONALE

Numerous studies indicate the potential antidepressant actions of several mGlu5 receptor antagonists, including 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]-pyridine (MTEP). The explanation for the mechanism of these effects might be a key step in finding new antidepressant drugs (AD).

OBJECTIVES

The aim of the present study was to investigate the possible role of the serotonergic system in the antidepressant-like activity of MTEP in the tail suspension test (TST) in C57BL/6J mice, using selected antagonists of serotonergic receptors and by applying two different methods of serotonin (5-HT) depletion.

RESULTS

The results of our studies showed that the mGlu5 receptor antagonist, MTEP, similar to the fluoxetine used as reference AD, did not induce antidepressant-like effects in mice pretreated with tryptophan hydroxylase inhibitor, parachlorophenylalanine. On the other hand, MTEP worked as a potential AD in the TST in mice fed on a tryptophan-free (TRP-free) diet for 3 weeks. However, fluoxetine, which was used as a reference control was also active in this experiment, suggesting that a TRP-free diet was not sufficiently effective in reducing the 5-HT level. Furthermore, we showed that the 5HT2A/2C antagonist, ritanserin, yet not the 5-HT1A antagonist, WAY100635, 5HT1B antagonist, SB224289 or 5HT4 antagonist, GR125487, reversed the antidepressant-like effects of MTEP in the TST. Finally, a sub-effective dose ofMTEP coadministered with a sub-effective dose of citalopram induced an antidepressant-like effect in the TST in mice.

CONCLUSION

The results of our studies suggest the involvement of serotonergic system activation in the antidepressant-like effects of the mGlu5 antagonist, MTEP, in the TST in mice.

Xiaochaihutang prevents depressive-like behaviour in rodents by enhancing the serotonergic system

Objectives

Xiaochaihutang (XCHT) has been used in China for thousands of years to treat ‘Shaoyang syndrome’, which involves depressive-like symptoms. However, no studies were conducted to demonstrate its antidepressant effect and mechanism. This study was designed to confirm the antidepressant effect of XCHT and explore its mechanism using the pharmacological methods.

Methods

Ultra-HPLC and mass spectrometry was used to identify the chemical constituents of XCHT. Forced swimming test (FST) and tail suspension test (TST) were used to determine the antidepressant-like activity of XCHT in mice and rats. The possible mechanism of XCHT was elucidated by the reserpine-induced hypothermia and 5-hydroxytryptophan (5-HTP)-induced head-twitch in mice. The levels of serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were measured in prefrontal cortex and hippocampus tissue of both mice and rats. Moreover, the extracellular 5-HT in rat hippocampus was assessed by using microdialysis coupled to HPLC with electrochemical detection.

Key findings

Forty-four components were detected in XCHT. XCHT significantly reduced immobility time in the TST and the FST, antagonized reserpine-induced depressive-like behaviours, increased 5-HTP-induced head-twitches, elevated 5-HT and 5-HIAA levels, and increased 5-HT turnover at doses that did not affect general activity.

Conclusions

These data demonstrate that XCHT has therapeutic effects in animal models of depression by enhancing the serotoninergic system in the prefrontal cortex and hippocampus.

The metabotropic glutamate receptors: potential drug targets for the treatment of anxiety disorders?

Anxiety-related disorders are a common public health issue. Several lines of evidence suggest that altered glutamatergic neurotransmission underlies anxiety. Thus, novel molecules targeting glutamatergic neurotransmission, such as ligands of the metabotropic glutamate receptors (mGlurs) might be promising candidates for the treatment of anxiety disorders. To date, several ligands selective for each mGlu receptor (mGlur) have been synthesized, and pharmacological significances of these compounds have been demonstrated mainly in animal models. Here we critically review advances in research of these emerging molecular targets for the treatment of anxiety, discuss their advantages over currently used anxiolytics as well as remaining challenges.

Neuroprotective and symptomatic effects of targeting group III mGlu receptors in neurodegenerative disease

Neurodegenerative disorders possess common pathological mechanisms, such as protein aggregation, inflammation, oxidative stress (OS) and excitotoxicity, raising the possibility of shared therapeutic targets. As a result of the selective cellular and regional expression of group III metabotropic glutamate (mGlu) receptors, drugs targeting such receptors have demonstrated both neuroprotective properties and symptomatic improvements in several models of neurodegeneration. In recent years, the discovery and development of subtype-selective ligands for the group III mGlu receptors has gained pace, allowing further research into the functions of these receptors and revealing their roles in health and disease. Activation of this class of receptors results in neuroprotection, with a variety of underlying mechanisms implicated. Group III mGlu receptor stimulation prevents excitotoxicity by inhibiting glutamate release from neurons and microglia and increasing glutamate uptake by astrocytes. It also attenuates the neuroinflammatory response by reducing glial reactivity and encourages neurotrophic phenotypes. This article will review the current literature with regard to the neuroprotective and symptomatic effects of group III mGlu receptor activation and discuss their promise as therapeutic targets in neurodegenerative disease. We review the neuroprotective and symptomatic effects of targeting group III mGlu receptors in neurodegenerative disease: Excess extracellular glutamate causes overactivation of NMDA receptors resulting in excitotoxicity. Externalization of phosphatidylserine stimulates phagocytosis of neurons by activated microglia, which contribute to damage through glutamate and pro-inflammatory factor release. Reactive astrocytes produce cytotoxic factors enhancing neuronal cell death. Activation of group III mGlu receptors by glutamate and/or mGlu receptor ligands results in inhibition of glutamate release from presynaptic terminals and microglia, reducing excitotoxicity. Astrocytic glutamate uptake is increased and microglia produce neurotrophic factors.

The reversal of cognitive, but not negative or positive symptoms of schizophrenia, by the mGlu₂/₃ receptor agonist, LY379268, is 5-HT₁A dependent

mGlu(2/3) receptor agonists were shown to possess an antipsychotic-like potential in animal studies. Recent clinical investigations revealed that their antipsychotic potential might also manifest in humans. LY379268, the group II mGlu receptor orthosteric agonist, was previously shown to exhibit antipsychotic-like action in animal models of schizophrenia. However, the mechanism of its action is not fully recognized. Here, we decided to investigate the involvement of 5-HT1A receptors in the LY379268-induced antipsychotic effects. We used models of positive, negative and cognitive symptoms of schizophrenia, such as MK-801- and amphetamine-induced hyperactivity tests, DOI-induced head twitches, social interaction and novel object recognition. LY379268 was active in a wide range of doses (0.5-5 mg/kg), depending on the paradigm. The effects of the drug were not antagonized by 5-HT(1A) antagonist, WAY100635 (0.1 mg/kg) in the models of positive and negative symptoms. Conversely, in the novel object recognition test, which exerts cognitive disturbances, the action of LY379268 was antagonized by WAY100635. Concomitantly, the action of a sub-effective dose of the drug was enhanced by the administration of a sub-effective dose of 5-HT(1A) agonist, (R)-(+)-8-Hydroxy-DPAT. Altogether, we propose that the antipsychotic-like action of group II mGlu receptors' agonist is 5-HT(1A) independent in context of positive and negative symptoms, while the action toward cognitive disturbances seems to be 5-HT(1A) dependent.

mGluR5 ablation in cortical glutamatergic neurons increases novelty-induced locomotion

The group I metabotropic glutamate receptor 5 (mGluR5) has been implicated in the pathology of various neurological disorders including schizophrenia, ADHD, and autism. mGluR5-dependent synaptic plasticity has been described at a variety of neural connections and its signaling has been implicated in several behaviors. These behaviors include locomotor reactivity to novel environment, sensorimotor gating, anxiety, and cognition. mGluR5 is expressed in glutamatergic neurons, inhibitory neurons, and glia in various brain regions. In this study, we show that deleting mGluR5 expression only in principal cortical neurons leads to defective cannabinoid receptor 1 (CB1R) dependent synaptic plasticity in the prefrontal cortex. These cortical glutamatergic mGluR5 knockout mice exhibit increased novelty-induced locomotion, and their locomotion can be further enhanced by treatment with the psychostimulant methylphenidate. Despite a modest reduction in repetitive behaviors, cortical glutamatergic mGluR5 knockout mice are normal in sensorimotor gating, anxiety, motor balance/learning and fear conditioning behaviors. These results show that mGluR5 signaling in cortical glutamatergic neurons is required for precisely modulating locomotor reactivity to a novel environment but not for sensorimotor gating, anxiety, motor coordination, several forms of learning or social interactions.

Research review: the role of cytokines in depression in adolescents: a systematic review

BACKGROUND

While cytokines have been implicated in the pathophysiology of depression in adults, the potential role in younger age groups such as adolescents is less clear. This article therefore reviews the literature (a) to explore the relationship between cytokines and depression in adolescents, and (b) to examine how cytokines may be related to adolescent depression in the context of other neurobiological theories of depression.

METHOD

A systematic review of the scientific literature on the subject was conducted in February 2013, searching the Web of Knowledge, PubMed (Medline), PsycInfo and Cochrane electronic databases.

RESULTS

Eighteen studies were identified measuring both depression or depressive symptoms and cytokines or immune markers in adolescents. Adolescents with depression show age-specific characteristics of the immune and inflammatory system, specifically in NK cell activity and in pro-inflammatory cytokines (such as IL-1β and TNF-α). In addition, the role of cytokines in adolescent depression is influenced by neurodevelopment, hormonal changes, stress and trauma.

CONCLUSIONS

There may be differences in the neurobiology of adolescent major depressive disorder (MDD) compared with adult MDD. Increased understanding of the role of cytokines in adolescent MDD may lead to improved outcomes in the treatment of adolescent depression.

mGluR1/5 receptor densities in the brains of alcoholic subjects: a whole-hemisphere autoradiography study

Increased glutamatergic neurotransmission and hyper-excitability during alcoholic withdrawal and abstinence are associated with increased risk for relapse, in addition to compensatory changes in the glutamatergic system during chronic alcohol intake. Type 5 metabotropic glutamate receptor (mGlur5) is abundant in brain regions known to be involved in drug reinforcement, yet very little has been published on mGluR1/5 expression in alcoholics. We evaluated the densities of mGluR1/5 binding in the hippocampus and striatum of post-mortem human brains by using [(3)H]Quisqualic acid as a radioligand in whole hemispheric autoradiography of Cloninger type 1 (n=9) and 2 (n=8) alcoholics and healthy controls (n=10). We observed a 30-40% higher mGluR1/5 binding density in the CA2 area of hippocampus in type 1 alcoholics when compared with either type 2 alcoholics or healthy subjects. Although preliminary, and from a relatively small number of subjects from these diagnostic groups, these results suggest that the mGluR1/5 receptors may be increased in type 1 alcoholics in certain brain areas.

Glutamate-based anxiolytic ligands in clinical trials

INTRODUCTION

With regard to anxiety, the role of the balance between glutamatergic and GABAergic systems was pursued for many years. The majority of drugs used presently as effective anxiolytics enhance the GABAergic system activity, thus increasing inhibition within the central nervous system (CNS). On the other hand, decreasing the activity of glutamatergic neurotransmission may attenuate excitation in the CNS, thus resulting in anxiolysis.

AREAS COVERED

The present review focuses on clinical data of well-known and recently discovered glutamatergic and, to a lesser extent, GABAergic agents, which reached at least the Phase II criteria.

EXPERT OPINION

A variety of glutamatergic agents active at both N-acetylo-D-asparaginian and metabotropic glutamate (mGlu) receptors have been tested in humans to examine their potential anxiolytic activity. Many compounds acting on the glutamatergic system and approved for the treatment of other disorders than anxiety were shown to exert anxiolytic effects in clinical trials. Those are mainly voltage-dependent ion channel ligands as well as d-cycloserin and memantine. Also, ligands active at mGlu receptors, such as fenobam and LY354740, exhibited activity in controlled clinical trials. However, relatively few trials are found on the agents that are focused on GABAergic neurotransmission. Therefore, it seems that glutamatergic system may become a novel target for modern and effective anxiolytics.

Homer1 mediates acute stress-induced cognitive deficits in the dorsal hippocampus

In recent years, the glutamatergic system has been implicated in the development and treatment of psychiatric disorders. Glutamate signaling is processed by different receptors, including metabotropic glutamate receptors (mGluRs), which in turn interact with the scaffolding protein Homer1 to modulate downstream Ca(2+) signaling. Stress is a major risk factor for the incidence of psychiatric diseases, yet acute stress episodes may have diverging effects on individuals. Cognitive impairments have often been shown to occur after episodes of stress, however the specific role of mGluR5/Homer1 signaling in the interaction of stress and cognition has not yet been elucidated. In this study we show that a single episode of social defeat stress is sufficient to specifically induce cognitive impairments in mice 8 h after the stressor without affecting the animals' locomotion or anxiety levels. We also demonstrate that Homer1b/c levels as well as mGluR5/Homer1b/c interactions in the dorsal hippocampus are reduced up to 8 h after stress. Blockade of mGluR5 during the occurrence of social stress was able to rescue the cognitive impairments. In addition, a specific overexpression of Homer1b/c in the dorsal hippocampus also reversed the behavioral phenotype, indicating that both mGluR5 and Homer1b/c play a crucial role in the mediation of the stress effects. In summary, we could demonstrate that stress induces a cognitive deficit that is likely mediated by mGluR5/Homer1 signaling in the hippocampus. These findings help to reveal the underlying effects of cognitive impairments in patients suffering from stress-related psychiatric disorders.

18F-FPEB, a PET radiopharmaceutical for quantifying metabotropic glutamate 5 receptors: a first-in-human study of radiochemical safety, biokinetics, and radiation dosimetry

Identification of safe and valid PET radioligands for metabotropic glutamate receptor, type 5 (mGluR5), is essential to measure changes in brain mGluR5 in neuropsychiatric disorders, to confirm central mGluR5 occupancy of drug candidates, and to guide dose selection for obtaining an optimum therapeutic window. Here we present the results of a first-in-human study assessing the safety and effectiveness of a novel PET radiopharmaceutical, (18)F-3-fluoro-5-[(pyridin-3-yl)ethynyl]benzonitrile ((18)F-FPEB), for quantifying regional brain concentrations of mGluR5.

METHODS

Quantification of whole-body biokinetics was conducted in 6 healthy adults (3 men and 3 women). The radiation safety profile was estimated with OLINDA/EXM software. Subsequently, pairs of dynamic brain scans were obtained for 11 healthy men to identify optimal methods for derivation of regional distribution volume and binding potential and to determine the repeatability of measurement.

RESULTS

The whole-body effective radiation dose was approximately 17 μSv/MBq (62 mrem/mCi), with the gallbladder receiving the highest dose of 190 μSv/MBq. In brain studies, time-activity curves showed high accumulation in the insula/caudate nucleus, moderate uptake in the thalamus, and the lowest concentration in the cerebellum/pons. The plasma reference graphical analysis method appeared optimal for (18)F-FPEB; it showed acceptable test-retest variability of nondisplaceable binding potential (<10%) and identified the highest nondisplaceable binding potential values (from ∼0.5 in the globus pallidus to ∼3.5 in the insula) for target regions. Safety assessments revealed no clinically meaningful changes in vital signs, electrocardiogram, or laboratory values.

CONCLUSION

(18)F-FPEB is safe and well tolerated, and its regional cerebral distribution is consistent with previous reports in the literature for metabotropic glutamate receptors. The repeatability of measurement suggests that (18)F-FPEB is suitable for quantifying mGluR5 in humans.

Group II metabotropic glutamate receptor-mediated activation of G-proteins in rat hippocampal and striatal membranes

Stimulation of G-proteins coupled with metabotropic glutamate receptors (mGlu receptors) was investigated by means of guanosine-5'-O-(3-[(35)S]thio)triphosphate ([(35)S]GTPγS) binding assay in rat hippocampal and striatal membranes. The endogenous ligand l-glutamate increased specific [(35)S]GTPγS binding in a concentration-dependent manner with a mean EC50 values of around 10μM in both brain regions, The maximal % increase over the respective basal binding was highest in cerebral cortex, intermediate in hippocampus, and lowest in striatum. The pharmacological profiles of the responses investigated with a series of glutamatergic agonists and antagonists clearly indicated that they were mediated through group II mGlu receptors, particularly mGlu2 subtype, in both brain regions. The pEC50 and relative %Emax values for a series of agonists were essentially identical in both brain regions that were also correlated with those previously reported in cerebral cortical membranes. The selective allosteric potentiator of mGlu2 receptor subtype, LY487379, potentiated the increasing effects of l-glutamate at a maximally effective concentration of 1mM on specific [(35)S]GTPγS binding, without altering the basal unstimulated binding. It is concluded that [(35)S]GTPγS binding assay is applicable to rat hippocampal and striatal membranes to detect functional activation of Gαi/o proteins coupled with mGlu2 receptors.

Fragile X syndrome: from targets to treatments

Fragile X syndrome (FXS) is one of the most prevalent and well-studied monogenetic causes of intellectual disability and autism and, although rare, its high penetrance makes it a desirable model for the study of neurodevelopmental disorders more generally. Indeed recent studies suggest that there is functional convergence of a number of genes that are implicated in intellectual disability and autism indicating that an understanding of the cellular and biochemical dysfunction that occurs in monogenic forms of these disorders are likely to reveal common targets for therapeutic intervention. Fundamental research into FXS has provided a wealth of information about how the loss of function of the fragile X mental retardation protein results in biochemical, anatomical and physiological dysfunction leading to the discovery of interventions that correct many of the core pathological phenotypes associated with animal models of FXS. Most promisingly such strategies have led to development of drugs that are now in clinical trials. This review highlights how progress in understanding disorders such as FXS has led to a new era in which targeted molecular treatment towards neurodevelopmental disorders is becoming a reality. This article is part of the Special Issue entitled 'Neurodevelopmental Disorders'.

Involvement of mGlu5 and NMDA receptors in the antidepressant-like effect of acamprosate in the tail suspension test

Accumulating evidence supports the hypothesis that modulation of glutamatergic system via NMDA receptors and mGlu5 receptors might be an effective antidepressant therapy. However, clinical application of NMDA and mGlu5 antagonists in the therapy of depression is still an open question. In the present study we investigated potential antidepressant-like effect of a functional NMDA and mGlu5 receptor antagonist, acamprosate, which has been used in the therapy of human alcoholics as an anti-craving drug for more than 20 years and is considered as a safe substance. We have found potential antidepressant-like effect of acamprosate at doses of 100-400 mg/kg in the TST in C57BL/6J mice. Furthermore we have shown that the antidepressant-like effect of acamprosate used at a dose of 200 mg/kg was dependent on NMDA and mGlu5 receptor blockade, since NMDA (25 mg/kg) and mGlu5 receptor positive allosteric modulator, CDPPB (3 mg/kg), antagonized its activity in the TST. These data suggest that acamprosate may induce antidepressant-like effect and that NMDA and mGlu5 receptors are crucial targets of acamprosate in this action.

Metabotropic glutamate 2/3 receptor antagonists improve behavioral and prefrontal dopaminergic alterations in the chronic corticosterone-induced depression model in mice

Metabotropic glutamate 2/3 (mGlu2/3) receptor antagonists have an antidepressant-like effect, but the exact mechanism still remains unclear. This study examined the effects of mGlu2/3 receptor antagonists in chronic corticosterone-treated mice which could be used as an animal model of depression. In the forced swim test, the mGlu2/3 receptor antagonists MGS0039 (1.0 mg/kg, i.p.) and LY341495 (0.3 mg/kg, i.p) significantly reduced the increased immobility time of mice pretreated with corticosterone (20 mg/kg, s.c.) for 21 days, while desipramine (30 mg/kg, i.p.) and fluoxetine (30 mg/kg, i.p.) did not. The antidepressant-like effect of LY341495 was not blocked by the α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor antagonist NBQX (10 mg/kg, i.p.). Systemic administration of LY341495 did not affect basal release of glutamate, dopamine or serotonin in the prefrontal cortex of the control or chronic corticosterone-treated mice. Chronic corticosterone markedly enhanced high K(+)-induced release of dopamine, but not serotonin or glutamate, in the prefrontal cortex. This neurochemical change was blocked by systemic administration of MGS0039 and LY341495, but not desipramine or fluoxetine. These results suggest that chronic corticosterone-treated mice could be used as an animal model of treatment-resistant depression. This study also suggests that the prefrontal dopaminergic system is involved in the antidepressant-like effect of mGlu2/3 receptor antagonists in the chronic corticosterone-induced depression model.

Pharmacological characterization and modeling of the binding sites of novel 1,3-bis(pyridinylethynyl)benzenes as metabotropic glutamate receptor 5-selective negative allosteric modulators

Metabotropic glutamate receptor subtype 5 (mGluR5) is a potential drug target in neurological and psychiatric disorders, and subtype-selective allosteric modulators have attracted much attention as potential drug candidates. In this study, the binding sites of three novel 2-methyl-6-(phenylethynyl)pyridine (MPEP)-derived negative allosteric modulators, 2-, 3-, and 4-BisPEB, have been characterized. 2-, 3-, and 4-BisPEB are 1,3-bis(pyridinylethynyl)-benzenes and differ only by the position of the nitrogen atoms in the pyridine rings. Despite their high structural similarity, 2-BisPEB [1,3-bis(pyridin-2-ylethynyl)-benzene, nitrogen atoms in ortho positions], with an IC(50) value in the nanomolar range, is significantly more potent than the 3- and 4-pyridyl analogs. Mutational analysis, directed by a previously published mGluR5 homology model, was used to determine key residues for the ligand-receptor interactions that may explain the potency differences of 2-, 3-, and 4-BisPEB. Residues Ile651, Pro655, Tyr659, Asn747, Trp785, Phe788, Tyr792, Ser809, and Ala810 were found to have critical roles for the activity of one or more of the three BisPEBs and the reference compound MPEP. The mutagenesis data suggest that the higher potency of 2-BisPEB is due to hydrogen bonding to Ser809 because the S809A mutation made 2-BisPEB equipotent to 3- and 4-BisPEB (IC(50), 1-2.5 μM). The potency of MPEP was also greatly affected by S809A (52-fold), suggesting that a Ser809-mediated hydrogen bond is also a key interaction between MPEP and mGluR5. Potential binding modes of 2-, 3-, and 4-BisPEB obtained by molecular docking to the mGluR5 homology model provide a structural context for the reported major mutational effects.

Psychopharmacologic treatment of children and adolescents with anxiety disorders

Over the last decade, psychopharmacologic treatments for pediatric anxiety disorders have been developed and increasingly subjected to randomized, controlled trials. The authors summarize the data concerning the use of tricyclic antidepressants (TCAs), selective serotonin reuptake inhibitors (SSRIs), serotonin norepinephrine reuptake inhibitors (SNRIs), atypical anxiolytics, and benzodiazepines. The extant data suggest that SSRIs--both as monotherapy and when combined with psychotherapy--are effective in the treatment of pediatric anxiety disorders. In addition, some TCAs and SNRIs are effective. However, randomized controlled trials do not suggest efficacy for benzodiazepines or the atypical anxiolytic, buspirone, for children and adolescents with anxiety disorders.

Influence of GRIA1, GRIA2 and GRIA4 polymorphisms on diagnosis and response to treatment in patients with major depressive disorder

The present study is aimed to exploring whether some single nucleotide polymorphisms (SNPs) within GRIA1, GRIA2 and GRIA4 could be associated with major depressive disorder (MDD) and whether they could predict clinical outcomes in Korean in-patients, respectively, treated with antidepressants. One hundred forty-five (145) patients with MDD and 170 healthy controls were genotyped for 17 SNPs within GRIA1, GRIA2 and GRIA4. Baseline and final clinical measures, including the Montgomery-Asberg Depression Rating Scale (MADRS) for patients with MDD, were recorded. No association was observed between alleles, genotypes and haplotypes under investigation and clinical and demographical variables. As a secondary finding, a marginal association was observed between rs4302506 and rs4403097 alleles within GRIA2 and age of onset in patients with MDD. Our findings provide evidence for a possible association between rs4302506 and rs4403097 SNPs and age of onset in patients with MDD. However, taking into account that the several limitations of our study including the moderately small sample size of our study, our findings should be considered with caution and further research is needed to draw more definitive conclusions.

Attenuation of methamphetamine seeking by the mGluR2/3 agonist LY379268 in rats with histories of restricted and escalated self-administration

Recent findings implicate group II metabotropic glutamate receptors (mGluR(2/3)) in the reinforcing effects of psychostimulants and have identified these receptors as potential treatment targets for drug addiction. Here, we investigated the effects of mGluR(2/3) stimulation on cue- and drug-primed reinstatement in rats with different histories of methamphetamine (METH) self-administration training, under two conditions: 16 daily sessions of short access (90 min/day, ShA), or 8 daily sessions of short access followed by 8 sessions of long access (6 h/day, LgA). Following self-administration and subsequent extinction training, rats were pretreated with the selective mGluR(2/3) agonist LY379268 (variable dose, 0-3 mg/kg), exposed to METH-paired cues or a priming injection of METH (1 mg/kg), and tested for reinstatement of METH-seeking behavior. LgA rats self-administered greater amounts of METH during the second half of training, but when pretreated with vehicle, ShA and LgA rats showed cue- and drug-primed reinstatement at equivalent response rates. However, LgA rats demonstrated greater sensitivity to mGluR(2/3) stimulation with attenuated responding during cue-induced reinstatement after 0.3 mg/kg and higher doses of LY379268, whereas ShA rats decreased cue-induced reinstatement behavior following 1.0 mg/kg and 3.0 mg/kg LY379268. Additionally, both LgA and ShA rats exhibited decreased METH-primed reinstatement behavior following 0.3 mg/kg and higher doses of LY379268. A separate group of control rats was trained to self-administer sucrose pellets, and demonstrated attenuated cue-induced sucrose-seeking behavior following 1.0 and 3.0 mg/kg LY379268. Together, the results indicate that LY379268 has differential attenuating effects on cue-induced reinstatement behavior in rats with different histories of METH intake. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

mGlu2/3 and mGlu5 receptors: potential targets for novel antidepressants

Major depressive disorder is among the most prevalent forms of mental illness. All currently available antidepressant medications have stemmed from study of the mechanisms of serendipitously discovered drugs, and only 30-50% of patients exhibit remission and frequently at least 3-4 weeks are required for manifestation of significant therapeutic effects. To overcome these drawbacks, discovering novel neuronal mechanisms of pathophysiology of depression as well as more effective treatments are necessary. This review focuses on the metabotropic glutamate (mGlu) receptors and their potential for drug targets for the treatment of depression. In particular, accumulating evidence has indicated the potential importance and usefulness of agents acting on mGlu2/3 and mGlu5 receptors. Preclinical and clinical evidence of mGlu2/3 receptor ligands and mGlu5 receptor antagonists are described. Moreover, their potential in clinic will be discussed in the context of neuronal mechanisms of ketamine, an agent recently demonstrated a robust effect for patients with treatment-resistant depression. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.