1-aminocyclopropanecarboxylates exhibit antidepressant and anxiolytic actions in animal models

The imidazodiazepine, KRM-II-81: An example of a newly emerging generation of GABAkines for neurological and psychiatric disorders

GABAkines, or positive allosteric modulators of γ-aminobutyric acid-A (GABA_A) receptors, are used for the treatment of anxiety, epilepsy, sleep, and other disorders. The search for improved GABAkines, with reduced safety liabilities (e.g., dependence) or side-effect profiles (e.g., sedation) constituted multiple discovery and development campaigns that involved a multitude of strategies over the past century. Due to the general lack of success in the development of new GABAkines, there had been a decades-long draught in bringing new GABAkines to market. Recently, however, there has been a resurgence of efforts to bring GABAkines to patients, the FDA approval of the neuroactive steroid brexanolone for post-partum depression in 2019 being the first. Other neuroactive steroids are in various stages of clinical development (ganaxolone, zuranolone, LYT-300, Sage-324, PRAX 114, and ETX-155). These GABAkines and non-steroid compounds (GRX-917, a TSPO binding site ligand), darigabat (CVL-865), an α2/3/5-preferring GABAkine, SAN711, an α3-preferring GABAkine, and the α2/3-preferring GABAkine, KRM-II-81, bring new therapeutic promise to this highly utilized medicinal target in neurology and psychiatry. Herein, we also discuss possible conditions that have enabled the transition to a new age of GABAkines. We highlight the pharmacology of KRM-II-81 that has the most preclinical data reported. KRM-II-81 is the lead compound in a new series of orally bioavailable imidazodiazepines entering IND-enabling safety studies. KRM-II-81 has a preclinical profile predicting efficacy against pharmacoresistant epilepsies, traumatic brain injury, and neuropathic pain. KRM-II-81 also produces anxiolytic- and antidepressant-like effects in rodent models. Other key features of the pharmacology of this compound are its low sedation rate, lack of tolerance development, and the ability to prevent the development of seizure sensitization.

Role of FMRP in rapid antidepressant effects and synapse regulation

Rapid antidepressants are novel treatments for major depressive disorder (MDD) and work by blocking N-methyl-D-aspartate receptors (NMDARs), which, in turn, activate the protein synthesis pathway regulated by mechanistic/mammalian target of rapamycin complex 1 (mTORC1). Our recent work demonstrates that the RNA-binding protein Fragile X Mental Retardation Protein (FMRP) is downregulated in dendrites upon treatment with a rapid antidepressant. Here, we show that the behavioral effects of the rapid antidepressant Ro-25-6981 require FMRP expression, and treatment promotes differential mRNA binding to FMRP in an mTORC1-dependent manner. Further, these mRNAs are identified to regulate transsynaptic signaling. Using a novel technique, we show that synapse formation underlying the behavioral effects of Ro-25-6981 requires GABABR-mediated mTORC1 activity in WT animals. Finally, we demonstrate that in an animal model that lacks FMRP expression and has clinical relevance for Fragile X Syndrome (FXS), GABABR activity is detrimental to the effects of Ro-25-6981. These effects are rescued with the combined therapy of blocking GABABRs and NMDARs, indicating that rapid antidepressants alone may not be an effective treatment for people with comorbid FXS and MDD.

Convergent Mechanisms Underlying Rapid Antidepressant Action

Traditional pharmacological treatments for depression have a delayed therapeutic onset, ranging from several weeks to months, and there is a high percentage of individuals who never respond to treatment. In contrast, ketamine produces rapid-onset antidepressant, anti-suicidal, and anti-anhedonic actions following a single administration to patients with depression. Proposed mechanisms of the antidepressant action of ketamine include N-methyl-D-aspartate receptor (NMDAR) modulation, gamma aminobutyric acid (GABA)-ergic interneuron disinhibition, and direct actions of its hydroxynorketamine (HNK) metabolites. Downstream actions include activation of the mechanistic target of rapamycin (mTOR), deactivation of glycogen synthase kinase-3 and eukaryotic elongation factor 2 (eEF2), enhanced brain-derived neurotrophic factor (BDNF) signaling, and activation of α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptors (AMPARs). These putative mechanisms of ketamine action are not mutually exclusive and may complement each other to induce potentiation of excitatory synapses in affective-regulating brain circuits, which results in amelioration of depression symptoms. We review these proposed mechanisms of ketamine action in the context of how such mechanisms are informing the development of novel putative rapid-acting antidepressant drugs. Such drugs that have undergone pre-clinical, and in some cases clinical, testing include the muscarinic acetylcholine receptor antagonist scopolamine, GluN2B-NMDAR antagonists (i.e., CP-101,606, MK-0657), (2R,6R)-HNK, NMDAR glycine site modulators (i.e., 4-chlorokynurenine, pro-drug of the glycineB NMDAR antagonist 7-chlorokynurenic acid), NMDAR agonists [i.e., GLYX-13 (rapastinel)], metabotropic glutamate receptor 2/3 (mGluR2/3) antagonists, GABAA receptor modulators, and drugs acting on various serotonin receptor subtypes. These ongoing studies suggest that the future acute treatment of depression will typically occur within hours, rather than months, of treatment initiation.

Engaging homeostatic plasticity to treat depression

Major depressive disorder (MDD) is a complex and heterogeneous mood disorder, making it difficult to develop a generalized, pharmacological therapy that is effective for all who suffer from MDD. Through the fortuitous discovery of N-methyl-D-aspartate receptor (NMDAR) antagonists as effective antidepressants, we have gained key insights into how antidepressant effects can be produced at the circuit and molecular levels. NMDAR antagonists act as rapid-acting antidepressants such that relief from depressive symptoms occurs within hours of a single injection. The mode of action of NMDAR antagonists seemingly relies on their ability to activate protein-synthesis-dependent homeostatic mechanisms that restore top-down excitatory connections. Recent evidence suggests that NMDAR antagonists relieve depressive symptoms by forming new synapses resulting in increased excitatory drive. This event requires the mammalian target of rapamycin complex 1 (mTORC1), a signaling pathway that regulates synaptic protein synthesis. Herein, we review critical studies that shed light on the action of NMDAR antagonists as rapid-acting antidepressants and how they engage a neuron's or neural network's homeostatic mechanisms to self-correct. Recent studies notably demonstrate that a shift in γ-amino-butyric acid receptor B (GABA_BR) function, from inhibitory to excitatory, is required for mTORC1-dependent translation with NMDAR antagonists. Finally, we discuss how GABA_BR activation of mTORC1 helps resolve key discrepancies between rapid-acting antidepressants and local homeostatic mechanisms.

Antidepressant-like effects of long-term sarcosine treatment in rats with or without chronic unpredictable stress

Sarcosine, an N-methyl-d-aspartate receptor enhancer, can improve depression-like behavior in rodent models and depression in humans. We found that a single dose of sarcosine exerted antidepressant-like effects with rapid concomitant increases in the mammalian target of rapamycin (mTOR) signaling pathway activation and enhancement of α-amino-3-hydroxy-5-methylisoxazole-4-propionate receptor (AMPAR) membrane insertion. Sarcosine may play a crucial role in developing novel therapy for depression. For a detailed understanding of sarcosine, this study examined the effects of long-term sarcosine treatment on the forced swim test (FST), mTOR signaling, and AMPAR membrane insertion in rats. The effects of long-term sarcosine treatment were examined in naive rats and rats exposed to chronic unpredictable stress (CUS). Long-term sarcosine treatment (560mg/kg/d for 21 d) significantly ameliorated the increased immobility induced by CUS in the FST, reaffirming the potential role of sarcosine as an antidepressant for depressed patients. The same long-term treatment exhibited no such effect in naive rats despite increased mTOR activation and AMPAR membrane insertion in both groups. Our findings clearly show CUS-exposed rats are sensitive to long-term sarcosine treatment in FST and the response at the same dose is absent in naïve rats. Nevertheless, the distinct sensitivity to long-term sarcosine treatment in rats with or without CUS is not associated with the activated mTOR signaling pathway or increased AMPAR membrane insertion. Additionally, understanding the behavioral and molecular basis of distinct responses is vital important for developing personalized treatment programs to increase the probability of success when treating depression.

On the effect of minocycline on the depressive-like behavior of mice repeatedly exposed to malathion: interaction between nitric oxide and cholinergic system

This study was performed to investigate the antidepressant-like effect of minocycline in mice exposed to organophosphate pesticide malathion and possible involvement of nitric oxide/cGMP pathway in this paradigm. Mice were administered specific doses of malathion once daily for 7 consecutive days. After induction of depression, different doses of minocycline were daily injected alone or combined with non-specific NOS inhibitor, L-NAME, specific inducible NOS inhibitor, AG, NO precursor, L-arginine, and PDE5I, sildenafil. After locomotion assessment in open-field test, immobility times were recorded in the FST and TST. Moreover, hippocampal nitrite concentrations and acetylcholinesterase activity were measured. The results showed that repeated exposure to malathion induces depressive-like behavior at dose of 250 mg/kg. Minocycline (160 mg/kg) significantly reduced immobility times in FST and TST (P < 0.001). Combination of sub-effective doses of minocycline (80 mg/kg) with either L-NAME (3 mg/kg) or AG (25 mg/kg) significantly exerted a robust antidepressant-like effect in FST and TST (P < 0.001). Furthermore, minocycline at the same dose which has antidepressant-like effect, significantly reduced hippocampal nitrite concentration. The investigation indicates the essential role for NO/cGMP pathway in malathion-induced depressive-like behavior and antidepressant-like effect of minocycline. Moreover, the interaction between nitrergic and cholinergic systems are suggested to be involved in malathion-induced depression.

17α-ethinyl estradiol attenuates depressive-like behavior through GABAA receptor activation/nitrergic pathway blockade in ovariectomized mice

OBJECTIVES

This study was performed to investigate the antidepressant-like effect of 17α-ethinyl estradiol (EE2) in ovariectomized (OVX) mice and the possible role of nitrergic and gamma aminobutyric acid (GABA)ergic pathways in this paradigm.

METHODS

Bilateral ovariectomy was performed in female mice, and different doses of EE2 were intraperitoneally injected either alone or combined with GABAA agonist, diazepam, GABAA antagonist, flumazenil, non-specific nitric oxide synthase (NOS) inhibitor, N(ω)-nitro-L-arginine methyl ester (L-NAME), specific nNOS inhibitor, 7-nitroindazole (7-NI), a nitric oxide (NO) precursor, L-arginine, and selective PDE5I, sildenafil. After locomotion assessment, immobility times were recorded in the forced swimming test (FST) and tail suspension test (TST). Moreover, hippocampal nitrite concentrations were measured in the examined groups.

RESULTS

Ten days after ovariectomy, a significant prolonged immobility times were observed. EE2 (0.3 and 1μg/kg and 0.03, 0.1, and 1mg/kg) caused antidepressant-like activity in OVX mice in FST and TST. Diazepam (1 and 5mg/kg), L-NAME (30mg/kg), and 7-NI (100mg/kg) significantly reduced the immobility times. Co-administration of minimal and sub-effective doses of EE2 and diazepam (0.3μg/kg and 0.5mg/kg, respectively) exerted a significant antidepressant-like effect. The same effect was observed in combination of minimal and sub-effective doses of EE2 and either L-NAME or 7-NI. Moreover, combination of minimal and sub-effective doses of EE2, diazepam either L-NAME, or 7-NI emphasized the significant robust antidepressant-like activity.

CONCLUSIONS

The study has demonstrated that lowest dose of EE2 exerts a significant antidepressant-like behavior. It is suggested that suppression of NO system, as well as GABAA activation, may be responsible for antidepressant-like activity of EE2 in OVX mice. Moreover, GABAA activation may inhibit nitrergic pathway.

Role of calcium, glutamate and NMDA in major depression and therapeutic application

Major depression is a common, recurrent mental illness that affects millions of people worldwide. Recently, a unique fast neuroprotective and antidepressant treatment effect has been observed by ketamine, which acts via the glutamatergic system. Hence, a steady accumulation of evidence supporting a role for the excitatory amino acid neurotransmitter (EAA) glutamate in the treatment of depression has been observed in the last years. Emerging evidence indicates that N-methyl-D-aspartate (NMDA), group 1 metabotropic glutamate receptor antagonists and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) agonists have antidepressant properties. Indeed, treatment with NMDA receptor antagonists has shown the ability to sprout new synaptic connections and reverse stress-induced neuronal changes. Based on glutamatergic signaling, a number of therapeutic drugs might gain interest in the future. Several compounds such as ketamine, memantine, amantadine, tianeptine, pioglitazone, riluzole, lamotrigine, AZD6765, magnesium, zinc, guanosine, adenosine aniracetam, traxoprodil (CP-101,606), MK-0657, GLYX-13, NRX-1047, Ro25-6981, LY392098, LY341495, D-cycloserine, D-serine, dextromethorphan, sarcosine, scopolamine, pomaglumetad methionil, LY2140023, LY404039, MGS0039, MPEP, 1-aminocyclopropanecarboxylic acid, all of which target this system, have already been brought up, some of them recently. Drugs targeting the glutamatergic system might open up a promising new territory for the development of drugs to meet the needs of patients with major depression.

The role of glutamatergic, GABA-ergic, and cholinergic receptors in depression and antidepressant-like effect

Depression is one of the most common mental disorders and social issue worldwide. Although there are many antidepressants available, the effectiveness of the therapy is still a serious issue. Moreover, there are many limitations of currently used antidepressants, including slow onset of action, numerous side effects, or the fact that many patients do not respond adequately to the treatment. Therefore, scientists are searching for new compounds with different mechanisms of action. Numerous data indicate the important role of glutamatergic, GABA-ergic, and cholinergic receptors in the pathomechanism of major depressive disorder. This review presents the role of glutamatergic, GABA-ergic, and cholinergic receptors in depression and antidepressant-like effect.

Effect of NMDAR antagonists in the tetrabenazine test for antidepressants: comparison with the tail suspension test

OBJECTIVE

The N-methyl-d-aspartate receptor (NMDAR) antagonist ketamine, produces rapid and enduring antidepressant effect in patients with treatment-resistant depression. Similar dramatic effects have not been observed in clinical trials with other NMDAR antagonists indicating ketamine may possess unique pharmacological properties. Tetrabenazine induces ptosis (a drooping of the eyelids), and the reversal of this effect, attributed to a sympathomimetic action, has been used to detect first-generation antidepressants, as well as ketamine. Because the actions of other NMDAR antagonists have not been reported in this measure, we examined whether reversal of tetrabenazine-induced ptosis was unique to ketamine, or a class effect of NMDAR antagonists.

METHODS

The effects of ketamine and other NMDAR antagonists to reverse tetrabenazine-induced ptosis were examined and compared with their antidepressant-like effects in the tail suspension test (TST) in mice.

RESULTS

All the NMDAR antagonists tested produced a partial reversal of tetrabenazine-induced ptosis and, as expected, reduced immobility in the TST. Ketamine, memantine, MK-801 and AZD6765 were all about half as potent in reversing tetrabenazine-induced ptosis compared to reducing immobility in the TST, while an NR2B antagonist (Ro 25-6981) and a glycine partial agonist (ACPC) were equipotent in both tests.

CONCLUSION

The ability to reverse tetrabenazine-induced ptosis is a class effect of NMDAR antagonists. These findings are consistent with the hypothesis that the inability of memantine, AZD6765 (lanicemine) and MK-0657 to reproduce the rapid and robust antidepressant effects of ketamine in the clinic result from insufficient dosing rather than a difference in mechanism of action among these NMDAR antagonists.

Rapid antidepressants stimulate the decoupling of GABA(B) receptors from GIRK/Kir3 channels through increased protein stability of 14-3-3η

A single injection of N-methyl-D-aspartate receptor (NMDAR) antagonists produces a rapid antidepressant response. Lasting changes in the synapse structure and composition underlie the effectiveness of these drugs. We recently discovered that rapid antidepressants cause a shift in the γ-aminobutyric acid receptor (GABABR) signaling pathway, such that GABABR activation shifts from opening inwardly rectifiying potassium channels (Kir/GIRK) to increasing resting dendritic calcium signal and mammalian Target of Rapamycin activity. However, little is known about the molecular and biochemical mechanisms that initiate this shift. Herein, we show that GABABR signaling to Kir3 (GIRK) channels decreases with NMDAR blockade. Blocking NMDAR signaling stabilizes the adaptor protein 14-3-3η, which decouples GABABR signaling from Kir3 and is required for the rapid antidepressant efficacy. Consistent with these results, we find that key proteins involved in GABABR signaling bidirectionally change in a depression model and with rapid antidepressants. In socially defeated rodents, a model for depression, GABABR and 14-3-3η levels decrease in the hippocampus. The NMDAR antagonists AP5 and Ro-25-6981, acting as rapid antidepressants, increase GABABR and 14-3-3η expression and decrease Kir3.2. Taken together, these data suggest that the shift in GABABR function requires a loss of GABABR-Kir3 channel activity mediated by 14-3-3η. Our findings support a central role for 14-3-3η in the efficacy of rapid antidepressants and define a critical molecular mechanism for activity-dependent alterations in GABABR signaling.

1-aminocyclopropanecarboxylic acid (ACPC) produces procognitive but not antipsychotic-like effects in rats

RATIONALE

In addition to the negative and positive symptoms of schizophrenia, cognitive deficits, including prefrontal cortical dysfunction, are now recognized as core features of this disorder. Compounds increasing the NMDA receptor function via the strychnine-insensitive glycine receptors have been proposed as potential antipsychotics. Depending on the ambient concentrations of glutamate and glycine, 1-aminocyclopropanecarboxylic acid (ACPC) behaves as either a partial agonist or a functional antagonist at the strychnine-insensitive glycine receptors.

OBJECTIVES

We investigated the procognitive and antipsychotic-like effects of ACPC in rats treated with phencyclidine (PCP) or ketamine (KET), compounds that produce psychotic-like symptoms in humans and laboratory animals.

METHODS

Cognitive effects were investigated in the novel object recognition (NOR) and attentional set-shifting tests (ASST). In addition, the effects of ACPC were investigated in PCP-induced hyperactivity, conditioned avoidance response (CAR), and prepulse inhibition (PPI) tests. The effects on attention and impulsivity were measured in the five-choice serial reaction time task (5-CSRTT).

RESULTS

ACPC (200-400 mg/kg) inhibited memory fading in naive rats and like clozapine prevented PCP- and KET-induced amnesia in the NOR. In naive animals, ACPC at 400 but not 200 mg/kg enhanced cognitive flexibility in the ASST, as the animals required fewer trials to reach the criteria during the extra-dimensional phase. In contrast, ACPC did not affect PCP-induced hyperactivity, CAR, and PPI as well as attention and impulsivity in the 5-CSRTT.

CONCLUSION

The present study demonstrates that ACPC enhanced both object recognition memory and cognitive flexibility dependent on the prefrontal cortex, but did not affect impulsivity nor exhibit an antipsychotic-like profile.

The role of NMDA receptors in the pathophysiology and treatment of mood disorders

Mood disorders such as major depressive disorder and bipolar disorder are chronic and recurrent illnesses that cause significant disability and affect approximately 350 million people worldwide. Currently available biogenic amine treatments provide relief for many and yet fail to ameliorate symptoms for others, highlighting the need to diversify the search for new therapeutic strategies. Here we present recent evidence implicating the role of N-methyl-D-aspartate receptor (NMDAR) signaling in the pathophysiology of mood disorders. The possible role of NMDARs in mood disorders has been supported by evidence demonstrating that: (i) both BPD and MDD are characterized by altered levels of central excitatory neurotransmitters; (ii) NMDAR expression, distribution, and function are atypical in patients with mood disorders; (iii) NMDAR modulators show positive therapeutic effects in BPD and MDD patients; and (iv) conventional antidepressants/mood stabilizers can modulate NMDAR function. Taken together, this evidence suggests the NMDAR system holds considerable promise as a therapeutic target for developing next generation drugs that may provide more rapid onset relief of symptoms. Identifying the subcircuits involved in mood and elucidating the role of NMDARs subtypes in specific brain circuits would constitute an important step toward the development of more effective therapies with fewer side effects.

D-Serine in Neuropsychiatric Disorders: New Advances

D-Serine (DSR) is an endogenous amino acid involved in glia-synapse interactions that has unique neurotransmitter characteristics. DSR acts as obligatory coagonist at the glycine site associated with the N-methyl-D-aspartate subtype of glutamate receptors (NMDAR) and has a cardinal modulatory role in major NMDAR-dependent processes including NMDAR-mediated neurotransmission, neurotoxicity, synaptic plasticity, and cell migration. Since either over- or underfunction of NMDARs may be involved in the pathophysiology of neuropsychiatric disorders; the pharmacological manipulation of DSR signaling represents a major drug development target. A first generation of proof-of-concept animal and clinical studies suggest beneficial DSR effects in treatment-refractory schizophrenia, movement, depression, and anxiety disorders and for the improvement of cognitive performance. A related developing pharmacological strategy is the indirect modification of DSR synaptic levels by use of compounds that alter the function of main enzymes responsible for DSR production and degradation. Accumulating data indicate that, during the next decade, we will witness important advances in the understanding of DSR role that will further contribute to elucidating the causes of neuropsychiatric disorders and will be instrumental in the development of innovative treatments.

Rapid-acting glutamatergic antidepressants: the path to ketamine and beyond

Traditional antidepressants require many weeks to reveal their therapeutic effects. However, the widely replicated observation that a single subanesthetic dose of the N-methyl-D-aspartate glutamate receptor antagonist ketamine produced meaningful clinical improvement within hours, suggested that rapid-acting antidepressants might be possible. The ketamine studies stimulated a new generation of basic antidepressant research that identified new neural signaling mechanisms in antidepressant response and provided a conceptual framework linking a group of novel antidepressant mechanisms. This article presents the path that led to the testing of ketamine, considers its promise as an antidepressant, and reviews novel treatment mechanisms that are emerging from this line of research.

NMDA Receptor Antagonists for Treatment of Depression

Depression is a psychiatric disorder that affects millions of people worldwide. Individuals battling this disorder commonly experience high rates of relapse, persistent residual symptoms, functional impairment, and diminished well-being. Medications have important utility in stabilizing moods and daily functions of many individuals. However, only one third of patients had considerable improvement with a standard antidepressant after 2 months and all patients had to deal with numerous side effects. The N-methyl-d-aspartate (NMDA) receptor family has received special attention because of its critical role in psychiatric disorders. Direct targeting of the NMDA receptor could result in more rapid antidepressant effects. Antidepressant-like effects of NMDA receptor antagonists have been demonstrated in different animal models. MK-801 (a use-dependent channel blocker), and CGP 37849 (an NMDA receptor antagonist) have shown antidepressant properties in preclinical studies, either alone or combined with traditional antidepressants. A recent development is use of ketamine clinically for refractory depression. The purpose of this review is to examine and analyze current literature on the role of NMDA receptor antagonists for treatment of depression and whether this is a feasible route in drug discovery.

Involvement of NMDA receptor complex in the anxiolytic-like effects of chlordiazepoxide in mice

In the present study, we demonstrated that low, ineffective doses of N-methyl-d-aspartic acid (NMDA) receptor antagonists [competitive NMDA antagonist, CGP 37849, at 0.312 mg/kg intraperitoneally (i.p.), antagonist of the glycine_B sites, L-701,324, at 2 mg/kg i.p., partial agonist of glycine_B sites, d-cycloserine, at 2.5 mg/kg i.p.] administered jointly with an ineffective dose of the benzodiazepine, chlordiazepoxide (CDP, 2.5 mg/kg i.p.), significantly increased the percentage of time spent in the open arms of the elevated plus-maze (index of anxiolytic effect). Furthermore, CDP-induced anxiolytic-like activity (5 mg/kg i.p.) was antagonized by NMDA (75 mg/kg i.p.) and by an agonist of glycine_B sites of the NMDA receptor complex, d-serine [100 nmol/mouse intracerebroventricularly (i.c.v.)]. The present study showed a positive interaction between γ-aminobutyric acid (GABA) and glutamate neurotransmission in the anxiolytic-like activity in the elevated plus-maze test in mice and this activity seems to particularly involve the NMDA receptors.

NMDA receptor antagonists augment antidepressant-like effects of lithium in the mouse forced swimming test

Although there is evidence of the involvement of N-methyl-D-aspartate receptors (NMDAR) in the action of lithium, its role in the antidepressant effects of lithium in a behavioural model remains unclear. In this study, we evaluated the effects of NMDAR antagonists on the antidepressant-like effects of lithium in the mouse forced swimming test. Lithium (30 and 100 mg/kg, i.p.) significantly (P < 0.01) reduced the immobility times of mice, whereas at lower doses (5 and 10 mg/kg) had no effect. NMDA antagonists ketamine (2 and 5 mg/kg, i.p.), MK-801 (0.1 and 0.25 mg/kg, i.p.) and ifenprodil (1 and 3 mg/kg, i.p.) significantly (P < 0.05) decreased the immobility time. Lower doses of ketamine (0.5 and 1 mg/kg), MK-801 (0.01 and 0.05 mg/kg) and ifenprodil (0.1 and 0.5 mg/kg) had no effect. Combined treatment of subeffective doses of lithium (10 mg/kg) and ketamine (1 mg/kg), MK-801 (0.05 mg/kg) or ifenprodil (0.5 mg/kg) robustly (P < 0.001) exerted an antidepressant-like effect. The noneffective dose of a NMDA agonist (NMDA, 75 mg/kg, i.p.) prevented the antidepressant-like effect of lithium (30 mg/kg). None of the drugs at subactive doses or in combination with lithium had significant effect on the locomotor activity in the open field test. We for the first time suggested a role for NMDAR signalling in the antidepressant-like effects of lithium, providing a new approach for treatment of depression.

NMDA receptor/nitrergic system blockage augments antidepressant-like effects of paroxetine in the mouse forced swimming test

OBJECTIVE

In this study, we evaluated the involvement of N-methyl-D-aspartate receptor (NMDAR)/nitric oxide (NO) system on the antidepressant-like effects of paroxetine in the mouse forced swimming test.

METHOD

Swim sessions were conducted by placing mice in individual glass cylinders filled with water for 6 min. The duration of behavioral immobility during the last 4 min of the test was evaluated.

RESULTS

Paroxetine (8 and 16 mg/kg, intraperitoneal [i.p.]) significantly reduced the immobility times of mice, whereas lower doses (2 and 4 mg/kg) had no effect. NMDA antagonists MK-801 (0.1 and 0.25 mg/kg, i.p.) and ifenprodil (1 and 3 mg/kg, i.p.) and the NO synthase inhibitor NG-L-arginine methyl ester (L-NAME; 30 and 100 mg/kg, i.p.) significantly decreased the immobility time. Lower doses of MK-801 (0.01 and 0.05 mg/kg), ifenprodil (0.1 and 0.5 mg/kg), and L-NAME (10 mg/kg) had no effect. Combined treatment of subeffective doses of paroxetine (4 mg/kg) and MK-801 (0.05 mg/kg), ifenprodil (0.5 mg/kg), and L-NAME (10 mg/kg) robustly exerted an antidepressant-like effect. The noneffective dose of a NO precursor L: -arginine (750 mg/kg, i.p.) prevented the antidepressant-like effect of paroxetine (30 mg/kg).

CONCLUSION

We suggested, for the first time, a possible role for NMDAR/NO signaling in the antidepressant-like effects of paroxetine, providing a new approach for the treatment of depression.

The discriminative stimulus effects of N-methyl-D-aspartate glycine-site ligands in NMDA antagonist-trained rats

RATIONALE

Many N-methyl-D-aspartate (NMDA) antagonists produce phencyclidine (PCP)-like side effects that limit their clinical utility. NMDA glycine-site antagonists may be less likely to produce these effects than other site-selective NMDA antagonists.

OBJECTIVES

The objective of the study is to compare the discriminative stimulus effects of novel NMDA glycine-site drugs to those of channel blocking and competitive NMDA antagonists.

MATERIALS AND METHODS

Drug discrimination studies were performed in separate groups of rats trained with saline vs. PCP (2 mg/kg i.p.) or the competitive antagonist NPC 17742 (4 mg/kg i.p.) using a standard two-lever operant conditioning procedure under an FR32.

RESULTS

Neither the partial glycine-site agonists aminocyclopropane carboxylic acid methyl ester and (+)-HA-966 nor the antagonists L701,324; MDL 100,458; MDL 100,748; MDL 103,371; MDL 104,472; MDL 105,519; MRZ 2/571; MRZ 2/576; and ACEA 0762 produced >50% PCP-lever selection, though all were tested over a sufficient dose range to produce response rate decreasing effects. All of the antagonists, except MDL 100,458 and MDL 100,748, were also tested for NPC 17742-like effects, producing somewhat more variable results than in PCP-trained rats. ACEA-0762 produced full substitution for NPC 17742, whereas MDL 105,519 produced no substitution. The remaining compounds engendered between 20% and 80% drug-lever selection.

CONCLUSION

These results provide evidence that NMDA glycine-site partial agonists and antagonists generally do not produce discriminative stimulus effects similar to those of representative NMDA channel blockers or competitive antagonists. This suggests that these NMDA glycine-site antagonists should be less likely to produce the undesirable behavioral side effects seen in clinical trials with many other NMDA antagonists.

Metabotropic glutamate receptors in the tripartite synapse as a target for new psychotropic drugs

Mental disorders, such as depression, anxiety and schizophrenia, has become a large medical and social problem recently. Studies performed in animal tests and early clinical investigations brought a new insight in the pharmacotherapy of these disorders. Latest investigations are focused mainly on the glutamatergic system, a main excitatory amino acid neurotransmitter in the brain. Evidence indicates that metabotropic glutamate receptors ligands have excellent antidepressant, anxiolytic and antipsychotic effects. Metabotopic glutamate receptors (mGlu) divaded into three groups (group I, II and III) are localized on nerve terminals, postsynaptic sites and glial cells and thus they can influence and modulate the action of glutamate on different levels in the synapse. Recent advances in the identification of selective and specific compounds (both ortho- and allosteric ligands), and the generation of transgenic animals enabled to have new insight into the pathophysiology and therapy of mood disorders. At present, the most potent seem to be negative allosteric modulators of the first group (mGlu1 and mGlu5), and positive allosteric modulators of the second (mGlu2 and mGlu3) and third (mGlu4/7/8) group of mGlu receptors.

Interaction of zinc with antidepressants in the tail suspension test

The antidepressant-like effect of zinc has been shown in several animal models of depression. In this study, zinc chloride (ZnCl2) was given alone or in combination with different classes of antidepressants by oral route (p.o.) to mice and the behavioral response in the tail suspension test (TST), a predictive test of antidepressant action, was investigated. ZnCl2 at a dose of 10 and 30 mg/kg, p.o., reduced the immobility time in the TST, without affecting the locomotor activity in open-field test. The antidepressants fluoxetine, paroxetine, imipramine, desipramine and bupropion produced a significant reduction in the immobility time in TST at the doses of 10, 1, 1, 1 and 10 mg/kg, p.o., respectively. The combined treatment of sub-effective doses of ZnCl2 (1 mg/kg) with sub-effective doses of fluoxetine (5 mg/kg), paroxetine (0.1 mg/kg), desipramine (0.1 mg/kg), imipramine (0.1 mg/kg) or bupropion (1 mg/kg) induced a significant reduction in the immobility time in the TST when compared with the groups treated with ZnCl2 or with antidepressants alone. The treatment with sub-effective doses of ZnCl2 and antidepressants alone or in combination did not affect the locomotion in open-field test, except that desipramine alone reduced the ambulation. The results first indicate that ZnCl2 administered by p.o. route produces an antidepressant-like effect in the TST. Moreover, synergistic effects of zinc with antidepressants were shown in the TST, suggesting that an improvement in the response to the antidepressant therapy occurs when zinc is combined with different classes of antidepressants.

NMDA/glutamate mechanism of antidepressant-like action of magnesium in forced swim test in mice

Antidepressant-like activity of magnesium in forced swim test (FST) was demonstrated previously. Also, enhancement of such activity by joint administration of magnesium and antidepressants was shown. However, the mechanism(s) involved in such activity remain to be established. In the present study we examined the involvement of NMDA/glutamate pathway in the magnesium activity in FST in mice. In the present study we investigated the effect of NMDA agonists on magnesium-induced activity in FST and the influence of NMDA antagonists with sub-effective doses of magnesium in this test. Magnesium-induced antidepressant-like activity was antagonized by N-methyl-d-aspartic acid (NMDA). Moreover, low, ineffective doses of NMDA antagonists (CGP 37849, L-701,324, d-cycloserine, and MK-801) administered together with low and ineffective doses of magnesium exhibit significant reduction of immobility time in FST. The active in FST doses of examined agents did not alter the locomotor activity (with an exception of increased activity induced by MK-801). The present study indicates the involvement of NMDA/glutamate pathway in the antidepressant-like activity of magnesium in mouse FST and further suggests antidepressant properties of magnesium.

The role of glutamate in mood disorders: results from the ketamine in major depression study and the presumed cellular mechanism underlying its antidepressant effects

In this article, we first review a study showing that the N-methyl-D-aspartate (NMDA) receptor antagonist ketamine leads to rapid, robust, and relatively sustained antidepressant effects in patients with treatment-resistant major depression. We then discuss our hypothesis that the therapeutic effects of monoaminergic antidepressants and ketamine may be mediated by increased AMPA-to-NMDA glutamate receptor throughput in critical neuronal circuits. We hypothesize that ketamine directly mediates this throughput, whereas monoaminergic antidepressants work indirectly and gradually; this may explain, in part, the lag of onset of several weeks to months that is observed with traditional antidepressants.

Prevention of pro-depressant effect of l-arginine in the forced swim test by NG-nitro-l-arginine and [1H-[1,2,4]Oxadiazole[4,3-a]quinoxalin-1-one]

Previous studies have shown that l-arginine, the precursor of nitric oxide, has a dual effect (antidepressant and pro-depressant) in the forced swim test. The aim of the present study was to investigate whether nitric oxide-cGMP pathway was involved in this dual effect. Porsolt swim test was conducted to resemble the symptomatology of major depressive disorder. An open field locomotor activity test was also used. L-arginine exerted a U-shape effect in the forced swim test: doses of 30, 100, 300, and 1000 mg/kg caused no alteration, statistically significant reduction, no alteration, and non-significant enhancement, respectively. Neither N(G)-nitro-L-arginine (NNA) nor [1H-[1,2,4]Oxadiazole[4,3-a]quinoxalin-1-one] (ODQ) at doses of 3 mg/kg was found to be effective in the forced swim test, whereas 10 mg/kg ODQ significantly reduced the immobility time. In the presence of NNA, the antidepressant and pro-depressant effects of L-arginine disappeared, however, only the pro-depressant component of l-arginine effect was prevented by ODQ (3 and 10 mg/kg). Saline, the solvent of L-arginine and NNA, and dimethyl sulfoxide (15% in saline), the solvent of ODQ, had no effect on the duration of immobility. None of the drugs or solvents used in the present study had any effect on locomotor activity over the dose range applied. The results show that L-arginine exerts its paradoxical effects by producing nitric oxide and that cGMP seems to have a role only in the pro-depressant component.

Beyond monoamines: glutamatergic function in mood disorders

The monoamine theory has implicated abnormalities in serotonin and norepinephrine in the pathophysiology of major depression and bipolar illness and contributed greatly to our understanding of mood disorders and their treatment. Nevertheless, some limitations of this model still exist that require researchers and clinicians to seek further explanation and develop novel interventions that reach beyond the confines of the monoaminergic systems. Recent studies have provided strong evidence that glutamate and other amino acid neurotransmitters are involved in the pathophysiology and treatment of mood disorders. Studies employing in vivo magnetic resonance spectroscopy have revealed altered cortical glutamate levels in depressed subjects. Consistent with a model of excessive glutamate-induced excitation in mood disorders, several antiglutamatergic agents, such as riluzole and lamotrigine, have demonstrated potential antidepressant efficacy. Glial cell abnormalities commonly associated with mood disorders may at least partly account for the impairment in glutamate action since glial cells play a primary role in synaptic glutamate removal. A hypothetical model of altered glutamatergic function in mood disorders is proposed in conjunction with potential antidepressant mechanisms of antiglutamatergic agents. Further studies elucidating the role of the glutamatergic system in the pathophysiology of mood and anxiety disorders and studies exploring the efficacy and mechanism of action of antiglutamatergic agents in these disorders, are likely to provide new targets for the development of novel antidepressant agents.

Effects of mGlu1 receptor blockade on anxiety-related behaviour in the rat lick suppression test

RATIONALE

Group I metabotropic glutamate receptor antagonists, which block both the mGlu1 and mGlu5 receptors, have been shown to have anxiolytic effects in the lick suppression test in rats.

OBJECTIVE

The anxiolytic potential of the selective mGlu1 antagonist 3,4-dihydro-2H-pyrano[2,3]beta-quinolin-7-yl)(cis-4-methoxycyclohexyl)methanone (JNJ16259685) was investigated and compared with the mGlu5 antagonist MPEP.

METHODS

Anxiety-related behaviour was assessed in lick suppression and in the elevated zero maze in rats. Non-specific effects on pain threshold, water intake and locomotor activity were also measured.

RESULTS

Acute administration of JNJ16259685 or MPEP increased the number of licks (lowest active dose 2.5 mg/kg IP for each compound). JNJ16259685 did not increase water intake or reduce acute pain threshold, suggesting that the anxiolytic-like properties are specific. However, acute administration decreased locomotor activity. The effects of chronic administration of JNJ16259685 over 14 days (5 mg/kg bid) on lick suppression were comparable to those seen after acute administration, arguing against development of behavioural tolerance or sensitisation. Yet, there was a tendency for an increase in locomotor activity after cessation of chronic treatment. Acute co-administration of both JNJ16259685 and MPEP had additive effects on the number of licks. No anxiolytic-like properties of JNJ16259685 were observed in the elevated zero maze.

CONCLUSION

Our data suggest that the anxiolytic-like effects induced by group I metabotropic glutamate receptor antagonists are mediated through both mGlu1 and mGlu5 receptors. Rather than producing a general anxiolytic-like effect, the effects seen following mGlu1 antagonism seem task-dependent, as prominent effects were seen in a conflict procedure, but not in a task based on spontaneous exploration.

N-methyl-D-aspartate antagonists for stroke and head trauma

The N-methyl-D-aspartate (NMDA) receptor is a ligand-gated ion channel which is widely distributed in the central nervous system (CNS), and which mediates most of the fast excitatory neuronal transmission in the CNS. As with other ligand-gated ion channels, the NMDA receptor is a macromolecular complex which possesses a number of intricate regulatory sites within and around a central ion channel. The key regulatory components for which prototypic antagonists have been developed are the competitive NMDA antagonist binding site, the non-competitive NMDA antagonist binding site within the ion channel, and the NMDA receptor-associated glycine antagonist site. The binding domains for each of these binding sites possess discrete and non-overlapping SAR with regard to the chemical series developed to date. The potential utility of NMDA antagonists in the treatment of stroke and traumatic brain injury was investigated soon after the synthesis of the first bioavailable NMDA antagonists. Efficacy in preclinical models was demonstrated with both competitive and non-competitive NMDA antagonists. However, preclinical testing also revealed potentially clinically-limiting side-effects which included phencyclidine (PCP)-like actions indicative of possible psychotomimetic activity, cerebral vacuolisation of limbic cortical neurones, low therapeutic indices relative to incapacitating motor side-effects and, in the case of non-competitive antagonists, hypertension. These limitations have led to the design of clinical trials that should define the therapeutic index for this type of compound in humans. Currently, the first competitive antagonist to enter clinical trials, selfotel, is on hold, while D-CPPene is still in development. The non-competitive antagonist, aptiganel, is currently in Phase III clinical trials and its therapeutic efficacy and index should be defined in 1997 and 1998. The well-defined limitations of competitive and non-competitive NMDA antagonists have been a key impetus in the investigation of alternative approaches to modulating the NMDA receptor complex. In the case of glycine site antagonists, these compounds have been shown in preclinical studies to be devoid of PCP-like actions and the neuronal vacuolisation associated with the competitive and non-competitive NMDA antagonists. This has induced the development of a number of chemical series with at least three compounds currently in Phase I and II clinical trials. These include ACEA 1021, GV150526A and ZD9379. Clinical efficacies and therapeutic indices of these compounds should be defined in 1998 and 1999. An alternative approach using a partial agonist of the glycine site (1-aminocyclopropane-carboxylic acid, ACPC) has been halted in Phase I. Another approach which has led to the development of NMDA receptor antagonists, selective for the NMDA receptor subunits 1A/2B (NR1A/2B subtype), was the discovery in early studies of the neuroprotective actions of ifenprodil. Structural analogues include eliprodil, CP-101,606 and lubeluzole. In the cases of eliprodil and lubeluzole, these compounds have demonstrated neuroprotection in preclinical models, but they possess the extremely dangerous side-effect of increasing cardiac repolarisation time (i.e., increased QTc interval). The therapeutic index for these compounds is low. This has led to the termination of eliprodil's development and has limited the current dosing strategy with lubeluzole. It has not been disclosed if CP-101,606 possesses this dose-limiting side-effect. In summary, strategies for drug design and development based on our knowledge of the NMDA receptor complex have led to the development of a new generation of compounds for the treatment of stroke and traumatic brain injury, which remain to be evaluated in the clinic. The success of this approach will be defined in the next two to three years.

Glutamate and Depression

The past decade has seen a steady accumulation of evidence supporting a role for the excitatory amino acid (EAA) neurotransmitter, glutamate, and its receptors in depression and antidepressant activity. To date, evidence has emerged indicating that N-methyl-d-aspartate (NMDA) receptor antagonists, group I metabotropic glutamate receptor (mGluR1 and mGluR5) antagonists, as well as positive modulators of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors have antidepressant-like activity in a variety of preclinical models. Moreover, antidepressant-like activity can be produced not only by drugs modulating the glutamatergic synapse, but also by agents that affect subcellular signaling systems linked to EAA receptors (e.g., nitric oxide synthase). In view of the extensive colocalization of EAA and monoamine markers in nuclei such as the locus coeruleus and dorsal raphe, it is likely that an intimate relationship exists between regulation of monoaminergic and EAA neurotransmission and antidepressant effects. Further, there is also evidence implicating disturbances in glutamate metabolism, NMDA, and mGluR1,5 receptors in depression and suicidality. Finally, recent data indicate that a single intravenous dose of an NMDA receptor antagonist is sufficient to produce sustained relief from depressive symptoms. Taken together with the proposed role of neurotrophic factors in the neuroplastic responses to stressors and antidepressant treatments, these findings represent exciting and novel avenues to both understand depressive symptomatology and develop more effective antidepressants.

Potential mechanisms of action of lamotrigine in the treatment of bipolar disorders

Based on the mood-stabilizing properties of carbamazepine and valproate, new anticonvulsants have been explored for use in bipolar disorders. One such agent, lamotrigine, has a novel clinical profile in that it may "stabilize mood from below," as it appears to maximally impact depressive symptoms in bipolar disorders. In this paper, we review the mechanisms of action of lamotrigine in an effort to understand the basis of its distinctive clinical use in the management of bipolar disorders as well as its diverse antiseizure effects. We consider lamotrigine mechanisms, emphasizing commonalities and dissociations among actions of lamotrigine, older mood stabilizers, and other anticonvulsants. Although ion channel effects, especially sodium channel blockade, may importantly contribute to antiseizure effects, such actions may be less central to lamotrigine thymoleptic effects. Antiglutamatergic and neuroprotective actions are important candidate mechanisms for lamotrigine psychotropic effects. Lamotrigine has a variable profile in kindling and contingent tolerance experiments and does not appear to have robust gamma-aminobutyric acid or monoaminergic actions. Lamotrigine intracellular signaling effects warrant investigation. Although lamotrigine mechanisms overlap those of other mood-stabilizing anticonvulsants, important dissociations suggest candidate mechanisms, which could contribute to lamotrigine's distinctive psychotropic profile.

The neurobiology and control of anxious states

Fear is an adaptive component of the acute "stress" response to potentially-dangerous (external and internal) stimuli which threaten to perturb homeostasis. However, when disproportional in intensity, chronic and/or irreversible, or not associated with any genuine risk, it may be symptomatic of a debilitating anxious state: for example, social phobia, panic attacks or generalized anxiety disorder. In view of the importance of guaranteeing an appropriate emotional response to aversive events, it is not surprising that a diversity of mechanisms are involved in the induction and inhibition of anxious states. Apart from conventional neurotransmitters, such as monoamines, gamma-amino-butyric acid (GABA) and glutamate, many other modulators have been implicated, including: adenosine, cannabinoids, numerous neuropeptides, hormones, neurotrophins, cytokines and several cellular mediators. Accordingly, though benzodiazepines (which reinforce transmission at GABA(A) receptors), serotonin (5-HT)(1A) receptor agonists and 5-HT reuptake inhibitors are currently the principle drugs employed in the management of anxiety disorders, there is considerable scope for the development of alternative therapies. In addition to cellular, anatomical and neurochemical strategies, behavioral models are indispensable for the characterization of anxious states and their modulation. Amongst diverse paradigms, conflict procedures--in which subjects experience opposing impulses of desire and fear--are of especial conceptual and therapeutic pertinence. For example, in the Vogel Conflict Test (VCT), the ability of drugs to release punishment-suppressed drinking behavior is evaluated. In reviewing the neurobiology of anxious states, the present article focuses in particular upon: the multifarious and complex roles of individual modulators, often as a function of the specific receptor type and neuronal substrate involved in their actions; novel targets for the management of anxiety disorders; the influence of neurotransmitters and other agents upon performance in the VCT; data acquired from complementary pharmacological and genetic strategies and, finally, several open questions likely to orientate future experimental- and clinical-research. In view of the recent proliferation of mechanisms implicated in the pathogenesis, modulation and, potentially, treatment of anxiety disorders, this is an opportune moment to survey their functional and pathophysiological significance, and to assess their influence upon performance in the VCT and other models of potential anxiolytic properties.

A critical evaluation of the brain efflux index method as applied to the nitric oxide synthase inhibitor, aminoguanidine

The Brain Efflux Index (BEI) method is an in vivo procedure designed to quantitate saturable efflux mechanisms resident at the blood--brain barrier (BBB). The present work utilized the BEI method to assess the BBB efflux mechanisms of [(14)C]aminoguanidine, a nitric oxide synthase inhibitor. The BEI for [(14)C]aminoguanidine was >100% (relative to [(3)H]inulin diffusion) over a range of 41-184 pmol after 40 min. The unusually high retention (>100%) of [(14)C]aminoguanidine suggested brain parenchymal sequestration, either by neuronal uptake or tissue protein binding. The uptake of [(14)C]aminoguanidine in dendritic neuronal endings (synaptosomes) showed a saturable concentration dependency, consistent with a carrier-mediated process. Nonlinear least-squares regression yielded the following Michaelis--Menten and diffusional (k(ns)) parameters for synaptosomal [(14)C]aminoguanidine uptake: V(max)=118.50 +/- 28.77 pmol x mg protein(-1)/3 min; K(m)=58.34 +/- 8.33 muM; k(ns)=0.15 +/- 0.029 pmol x mg protein(-1)/3 min/muM; mean +/- SEM; n=3 concentration profiles). Protein binding studies using brain tissue showed negligible binding. In summary, this work identified three principle findings: (1) An apparent lack of quantifiable aminoguanidine BBB efflux; (2) a previously undescribed synaptosomal accumulation process for aminoguanidine; and (3) an interesting limitation of the BEI technique where unusual brain parenchymal sequestration yields values >100%.

Antidepressant activity and calcium signaling cascades

Although antidepressant treatments produce clear effects on monoaminergic neuronal function, the link between these effects and therapeutic response to treatment is controversial. Previous studies have demonstrated that antagonists of the NMDA receptor-gated calcium ionophore result in antidepressant-like responses in rodents and humans. Likewise, antidepressant treatments produce regionally selective adaptation of the NMDA receptor suggestive of diminished capacity to gate calcium into receptive neurons. Similarly, voltage-dependent calcium channel antagonists have been reported to produce antidepressant-like effects in rodents. A major target of increases in subcellular calcium concentration is nitric oxide synthase (NOS) which liberates NO in response to stimulation. Recently, we have demonstrated that nitric oxide synthase antagonists produced antidepressant-like response in both in vivo preclinical screening procedures and in post-mortem in vitro studies of beta-adrenoceptor density. We propose: 1) that interruption of the Ca(2+)-calmodulin-NOS-guanylyl cyclase subcellular signaling pathway at any point will produce antidepressant-like effects; 2) that the acute actions of antidepressants in preclinical screening procedures are a consequence of their ability to disrupt Ca(2+)-calmodulin-NOS-guanylyl cyclase signaling; 3) that chronic but, not acute treatment with antidepressants results in adaptation of the Ca(2+)-calmodulin-NOS-guanylyl cyclase signaling pathway; 4) that this adaptation is necessary for the achievement of the therapeutic actions of antidepressants and; 5) that major depression is accompanied by an alteration (hyperactivity?) of subcellular Ca(2+) signaling. Copyright 2001 John Wiley & Sons, Ltd.

Chronic treatment with antidepressants affects glycine/NMDA receptor function: behavioral evidence

Like the clinically effective benzodiazepine anxiolytic, chlordiazepoxide, the glycine/NMDA receptor antagonist L-701,324 (3, 7.5 and 10 mg/kg), produces dose-related increases in the percentage of time spent in the open arms and the percentage of entries into the open arms of an elevated plus maze in mice. Consistent with its proposed mechanism of action, these anxiolytic effects of L-701,324 (7.5 mg/kg) are reversed by pretreatment with glycine (500 and 800 mg/kg). Chronic treatment with citalopram (20 mg/kg for 21 days), imipramine (15 mg/kg for 21 days) and electroconvulsive shock (ECS, for 8 days), produced a reduction in the anxiolytic-like actions of L-701,324 (7.5 mg/kg) such that they could not be reversed by glycine. In contrast, the anxiolytic effects of L-701,324 and reversal of these effects by glycine were unaffected by acute treatment with imipramine, chronic administration with placebo or the neuroleptic chlorpromazine, or sham ECS. Further, imipramine administered for 21 days did not affect the anxiolytic effect of 5 mg/kg of chlordiazepoxide. The apparent reduction in the anxiolytic-like actions of a specific glycine/NMDA receptor antagonist following chronic treatment with a variety of antidepressants is consistent with previous neurochemical and molecular studies indicating that chronic antidepressant treatment can affect NMDA receptor function.

Ethological confirmatory factor analysis of anxiety-like behaviour in the murine elevated plus-maze

The elevated plus-maze has been used in animal research to measure anxiety since 1985 and is currently the most widely used animal model of anxiety. Since this paradigm has been the subject of several principal components analyses, it is well qualified for confirmatory factor analysis research. The current report builds on the substantial theoretical knowledge and empirical data obtained from these structural analyses with a view to obtain further progress in the evolution of our understanding of animal anxiety in the elevated plus-maze. The purpose of the present report was two-fold: (a) to test if the a piori imposition of a 3-factor model, or a competing 2-factor elevated plus-maze model, would fit our sample (n=200 CD-1 mice) data in each of two trials within an inferential confirmatory factor analytic framework; (b) provide a well-fitting model that confers indicator variables that can most effectively and parsimoniously measure underlying constructs of elevated plus-maze behaviour. Multiple model-fitting criteria were used, and issues related to data non-normality, outliers, replicability of the model, sampling error and error of approximation in the estimation of final model fit were addressed. The final 2-factor model, with estimated error covariance between two different pairs of indicator variables, was a good fit on the trial-1 data, although it was necessary to allow unprotected stretch attends to non-significantly cross-load on factor-2. A 2-factor model also fit the trial-2 data from the present analysis, although it was necessary to allow closed arm time ratio to negatively cross-load on factor-1. These results indicate that inferential hypothesis testing and model building procedures within a confirmatory factor analysis framework produces interpretable animal anxiety indices in the elevated plus-maze. Moreover, a 2-factor, rather than a 3-factor model, parsimoniously and unambiguously explained the underlying constructs of anxiety-like mouse behaviour in the elevated plus-maze in the present study. Taken together, a reduction in the growing number of behavioural indices reported in elevated plus-maze pharmacological studies is suggested.

The N-methyl-D-aspartate receptor, synaptic plasticity, and depressive disorder. A critical review

The roles of the N-methyl-D-aspartate (NMDA) receptor and NMDA receptor-mediated synaptic plasticity are reviewed in the context of depressive disorder and its treatment. The mode of action of antidepressant treatment is poorly understood. Animal studies have suggested that many antidepressant drugs show activity at the NMDA receptor and that NMDA antagonists have antidepressant profiles in preclinical models of depression. A post-mortem study in humans has suggested that certain binding characteristics of the NMDA receptor may be down-regulated in the brains of suicide victims. "Depressogenic" stressors in animals and chronic administration of antidepressant agents perturb NMDA-dependent synaptic plasticity in the hippocampus.

Potential anti-anxiety, anti-addictive effects of LY 354740, a selective group II glutamate metabotropic receptors agonist in animal models

Despite there being a lot of biochemical data about metabotropic glutamate (mGlu) receptors, our knowledge of the behavioural effects of mGlu receptor agonists/antagonists is still inadequate. LY 354740 is a systemically active agonist of group II mGlu receptors. After peripheral administration, LY 354740 produced anxiolytic-like effects in the conflict drinking test in rats and a four-plate test in mice. It was also found that LY 354740 decreased spontaneous locomotor activity in mice, but did not disturb motor coordination. In behavioural models of depression including the despair test and a tail suspension test, LY 354740 did not produce antidepressant-like effects. LY 354740 inhibited the naloxone-induced symptoms of morphine withdrawal in morphine-dependent mice. The above results indicate that agonists of group II mGlu receptors may play a role in the therapy of anxiety and/or drug-dependence states. The brain sites of action of LY 354740 need to be identified and the mechanism of both the above described effects remains to be elucidated.

Tolerance to anxiolytic- and antidepressant-like effects of a partial agonist of glycineB receptors

The present study examined effects of acute and repeated administration of 1-aminocyclopropanecarboxylic acid (ACPC), a partial agonist of glycineB receptors, in the conflict drinking test and the forced swim test in rats. Diazepam and imipramine were used, respectively, as reference drugs in those tests. In the conflict drinking test, acute administration of ACPC (200 mg/kg) increased fivefold the number of punished licks. A three- and fivefold increase in the number of punished licks was observed in rats treated repeatedly with ACPC (200 mg/kg daily; 14 days) and challenged with the same dose of the drug 24 h or 4 days later, respectively. A single injection of ACPC (400 mg/kg) reduced by 40% the immobility time in the forced swim test. In rats treated repeatedly with ACPC (400 mg/kg daily; 14 days) and challenged with the same dose 24 h or 4 days later, the drug either produced no significant effect or reduced the immobility time by 50%, respectively. On the other hand, no changes in anxiolytic- and antidepressant-like effects of chronically administered diazepam (10 mg/kg daily; 14 days) and imipramine (30 mg/kg daily; 14 days), respectively, were observed. The above results indicate that tolerance develops to the anxiolytic- and, particularly, to the antidepressant-like activity of ACPC.

The NMDA receptor partial agonist, 1-aminocyclopropanecarboxylic acid (ACPC), reduces ethanol consumption in the rat

The present studies assessed the effects of both systemic and intraaccumbens injections of 1-aminocyclopropanecarboxylic acid (ACPC), and NMDA partial agonist, on ethanol consumption in a limited access procedure in Wistar rats. Systemically administered ACPC reduced ethanol consumption in a dose-dependent manner, while a single dose of ACPC administered bilaterally into the nucleus accumbens also reversibly reduced ethanol consumption. Indirect measures of general appetitive behavior showed no effect of ACPC on weight or water intake, which suggests that this effect of ACPC may be specific to ethanol. These data are compatible with the role of NMDA receptors in modulating ethanol consumption and provide the first data showing that ACPC can reduce ethanol consumption. ACPC has neuroprotective effects and does not show the psychotomimetic effects observed with NMDA receptor agents. Thus, ACPC may be helpful in future clinical studies designed to reduce alcohol use.

Antidepressants for the new millennium

Despite a remarkable structural diversity, most conventional antidepressants may be viewed as 'monoamine based', increasing the synaptic availability of serotonin, norepinephrine, and/or dopamine. Both preclinical and recent clinical studies indicate that compounds which reduce transmission at N-methyl-D-aspartate (NMDA) receptors are antidepressant. Moreover, chronic administration of antidepressants to mice alters both the mRNA levels encoding N-methyl-D-aspartate receptor subunits and radioligand binding to these receptors within circumscribed areas of the central nervous system. It is hypothesized that these two different treatment strategies converge to produce an identical functional endpoint: a region-specific dampening of NMDA receptor function. The pathways leading to this convergence provide a rudimentary framework for discovering novel antidepressants.

Nitric oxide synthase inhibitors have antidepressant-like properties in mice. 2. Chronic treatment results in downregulation of cortical beta-adrenoceptors

Down-regulation of cortical beta-adrenoceptors is observed in rodents following chronic treatment with many clinically effective antidepressant therapies. [3H]dihydroalprenolol binding to cortical beta-adrenoceptors was examined in mice treated with the nitric oxide (NO) synthase antagonist N(G)-nitro-L-arginine (L-NNA). Administration of L-NNA (0.1, 0.3 mg/kg) for 21 days produced a significant reduction (28%, 31%, respectively, P<0.05) in [3H]dihydroalprenolol binding to cortical membranes without affecting Kd. Dose 1 mg/kg of L-NNA given chronically also produced a 20% decrease in beta-adrenoceptor density, but this effect was not statistically significant. While chronic treatment with imipramine (15 and 30 mg/kg) produced respectively a 30% and 25% (P<0.05) reduction in the density of [3H]dihydroalpenolol, single injection of either imipramine (15 and 30 mg/kg) or L-NNA (0.1, 0.3, and 1 mg/kg) had no effect on [3H]dihydroalprenolol binding. These findings are consistent with the hypothesis that drugs which can affect the Ca2+ -calmodulin/nitric oxide synthase/guanylyl cyclase signaling pathway may represent a novel approach to the treatment of depression and are congruent with our previous observation, which has demonstrated the antidepressant-like properties of NO synthase inhibitors in the forced swim test.

Nitric oxide synthase inhibitors have antidepressant-like properties in mice. 1. Acute treatments are active in the forced swim test

Previous studies have demonstrated that antagonists at the NMDA receptor are as efficacious as tricyclic antidepressants in pre-clinical antidepressant screening procedures and in blocking or reversing the behavioral deficits associated with animal analogs of major depressive symptomatology. The NMDA receptor complex gates Ca2+, which interacts with calmodulin to subsequently activate nitric oxide (NO) synthase. We hypothesized that NO synthase antagonists might display antidepressant-like properties, similar to NMDA receptor antagonists. We examined the effects of N(G)-nitro-L-arginine (L-NNA), its dextrorotatory enantiomer, D-NNA, N(G)-nitro-L-arginine methyl ester (L-NAME) and N(G)-monomethyl-L-arginine (L-NMMA) at doses from 1 to 30 mg/kg in the forced swim test in mice. We now report that NO synthase antagonists are as efficacious as imipramine (15 mg/kg) in reducing the duration of immobility in the mouse forced swim test. The effects of NO synthase antagonists, as well as those of imipramine were blocked by pre-treatment with L-arginine (L-Arg) (500 mg/kg). In contrast to imipramine, the NO synthase antagonists were without effect on locomotor activity over the dose range active in the forced swim test (3-10 mg/kg). Likewise, L-Arg was without effect on locomotor activity. These data support the hypothesis that NO synthase antagonists possess antidepressant properties and may represent a novel class of therapeutics for major depressive disorders.

Disposition of two tetramethylcyclopropane analogues of valpromide in the brain, liver, plasma and urine of rats

2,2,3,3-Tetramethylcyclopropane carboxamide (TMCD) and N-methyl TMCD (M-TMCD) are analogues of valpromide (VPD) or amide derivatives of valproic acid (VPA), one of the major antiepileptic drugs (AEDs). In rodent models both TMCD and M-TMCD are more potent as anticonvulsants than VPA. The present study investigates the pharmacokinetics (PK) of TMCD and M-TMCD in rats by monitoring the levels of these two amides in the brain, liver, plasma and urine of rats. The disposition of TMCD and M-TMCD was analyzed in a comparative manner with that of VPD and VPA, previously studied by us. The following similar PK parameters were obtained for TMCD and M-TMCD, respectively: clearance, 5 and 5.6 ml/min/kg; volume of distribution (Vss), 0.72 and 0.96 l/kg; half-life (t1/2), 1.1 and 1. 2 h; and mean residence time (MRT), 2.41 and 2.8 h. The ratio of AUCs of TMCD of liver to plasma and brain to plasma were 1.67 and 1. 13, respectively. The ratios of the AUCs of M-TMCD of liver to plasma and brain to plasma were 1.43 and 0.99, respectively. Thus, both compounds distribute evenly between plasma and brain, but their distribution into the liver is 50% larger than that in the plasma. Therefore, PK analysis of TMCD and M-TMCD brain levels gave major PK parameters similar to those obtained from the plasma data. The fraction metabolized of M-TMCD to TMCD was 32%. The brain was not found to be a metabolic site for the M-TMCD to TMCD biotransformation which occurred primarily in the liver as indicated by the high liver concentrations of TMCD as a metabolite of M-TMCD. Unlike VPD, TMCD and M-TMCD did not undergo amide-acid biotransformation to their corresponding inactive acid, 2,2,3, 3-tetramethylcyclopropane carboxylic acid (TMCA). Both M-TMCD and TMCD distribute better into the brain than VPA, a fact that may contribute to their better anticonvulsant activity.