Involvement of N-methyl-D-aspartate (NMDA) receptors in noncompetitive NMDA receptor antagonist-induced hyperlocomotion in mice

A systematic scoping review of rodent models of catatonia: Clinical correlations, translation and future approaches

Catatonia is a psychiatric disorder, which subsumes a plethora of affective, motor and behavioral symptoms. In the last two decades, the number of behavioral and neuroimaging studies on catatonia has steadily increased. The majority of behavioral and neuroimaging studies in psychiatric patients suggested aberrant higher-order frontoparietal networks which, on the biochemical level, are insufficiently modulated by gamma-aminobutyric acid (GABA)-ergic and glutamatergic transmission. However, the pathomechanisms of catatonic symptoms have rarely been studied using rodent models. Here, we performed a scoping review of literature available on PubMed for studies on rodent models of catatonia. We sought to identify what we could learn from pre-clinical animal models of catatonia-like symptoms, their underlying neuronal correlates, and the complex molecular (i.e. genes and neurotransmitter) mechanisms by which its modulation exerts its effects. What becomes evident is that although many transgenic models present catatonia-like symptoms, they have not been used to better understand the pathophysiological mechanisms underlying catatonia so far. However, the identified neuronal correlates of catatonia-like symptoms correlate to a great extent with findings from neuroscience research in psychiatric patients. This points us towards fundamental cortical-striatal-thalamocortical and associated networks modulated by white matter inflammation as well as aberrant dopaminergic, GABAergic, and glutamatergic neurotransmission that is involved in catatonia. Therefore, this scoping review opens up the possibility of finally using transgenic models to help with identifying novel target mechanisms for the development of new drugs for the treatment of catatonia.

Repeated dextromethorphan administration in adolescent rats produces long-lasting behavioral alterations

Initiation of non-medical dextromethorphan (DXM) use often occurs in adolescence, yet little is known about the consequences when use begins during this developmental period. The current experiments examined the acute response and the effects of repeated exposure to DXM in adolescence on behavior in adulthood. We examined locomotor activity, locomotor sensitization, and cognitive function, in rats that received repeated administration of DXM. Groups of adolescent (PND 30) and adult (PND 60) male rats were treated with DXM (60 mg/kg) once daily for 10 days. Locomotor activity in response to DXM was assessed following the first injection, on the 10th day of injection (adolescent - PND 39; adult - PND 69), and following 20 days of abstinence (adolescent - PND 60; adult - PND 90). Acute locomotor effects and locomotor sensitization were compared in adolescents and adults; cross-sensitization to ketamine, another dissociative with abuse potential, was also examined. In a separate group of rodents cognitive deficits were assessed following a 20 day abstinence period (adolescent - PND 60; adult - PND 90) in spatial learning and novel object recognition tasks. The locomotor stimulant effect of DXM was much greater in adolescents than adults. Also, only adolescent rats that were repeatedly administered DXM demonstrated locomotor sensitization at the end of 10 days of injection. However, sensitization occurred after the abstinence period in all rats regardless of age. Nonetheless, cross-sensitization to ketamine was only evident in adolescent-treated rats. DXM also led to an increase in perseverative errors in reversal learning only in the adolescent-treated group. We conclude that repeated use of DXM produces long-lasting neuroadaptations that may contribute to addiction. Deficits in cognitive flexibility occur in adolescents, although further work is necessary to confirm these findings. The results extend the understanding of potential long-term consequences of DXM use in adolescents and adults.

N-methyl-d-aspartate receptors and glycinergic transmission, respectively, mediate muscle relaxation and immobility of pentobarbital in mice

Pentobarbital-induced anesthesia is believed to be mediated by enhancement of the inhibitory action of γ-aminobutyric acid (GABA)ergic neurons in the central nervous system. However, it is unclear whether all components of anesthesia induced by pentobarbital, such as muscle relaxation, unconsciousness, and immobility in response to noxious stimuli, are mediated only through GABAergic neurons. Thus, we examined whether the indirect GABA and glycine receptor agonists gabaculine and sarcosine, respectively, the neuronal nicotinic acetylcholine receptor antagonist mecamylamine, or the N-methyl-d-aspartate receptor channel blocker MK-801 could enhance pentobarbital-induced components of anesthesia. Muscle relaxation, unconsciousness, and immobility were evaluated by grip strength, the righting reflex, and loss of movement in response to nociceptive tail clamping, respectively, in mice. Pentobarbital reduced grip strength, impaired the righting reflex, and induced immobility in a dose-dependent manner. The change in each behavior induced by pentobarbital was roughly consistent with that in electroencephalographic power. A low dose of gabaculine, which significantly increased endogenous GABA levels in the central nervous system but had no effect on behaviors alone, potentiated muscle relaxation, unconsciousness, and immobility induced by low pentobarbital doses. A low dose of MK-801 augmented only the masked muscle-relaxing effects of pentobarbital among these components. Sarcosine enhanced only pentobarbital-induced immobility. Conversely, mecamylamine had no effect on any behavior. These findings suggest that each component of anesthesia induced by pentobarbital is mediated through GABAergic neurons and that pentobarbital-induced muscle relaxation and immobility may partially be associated with N-methyl-d-aspartate receptor antagonism and glycinergic neuron activation, respectively.

Antinociceptive and Analgesic Effects of (2R,6R)-Hydroxynorketamine

Commonly used pain therapeutics, such as opioid medications, exert dangerous side effects and lack effectiveness in treating some types of pain. Ketamine is also used to treat pain, but side effects limit its widespread use. (2R,6R)-hydroxynorketamine (HNK) is a ketamine metabolite that potentially shares some beneficial behavioral effects of its parent drug without causing significant side effects. This study compared the profile and potential mechanisms mediating the antinociception activity of ketamine and (2R,6R)-HNK in C57BL/6J mice. Additionally, this study compared the reversal of mechanical allodynia by (2R,6R)-HNK with gabapentin in a model of neuropathic pain. Unlike the near-immediate and short-lived antinociception caused by ketamine, (2R,6R)-HNK produced late-developing antinociception 24 hours following administration. Pharmacological blockade of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors with 2,3-dioxo-6-nitro-7-sulfamoyl-benzo[f]quinoxaline (NBQX) prevented the initiation and expressionof (2R,6R)-HNK antinociception, suggesting the involvement of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor-dependent glutamatergic mechanisms in the pain reduction-like responses. Blockade of opioid receptors with naltrexone partially prevented the antinociceptive effect of ketamine but was ineffective against (2R,6R)-HNK. Furthermore, (2R,6R)-HNK did not produce dystaxia, even when tested at doses five times greater than those needed to produce antinociception, indicating a superior safety profile for (2R,6R)-HNK over ketamine. Additionally, (2R,6R)-HNK reversed mechanical allodynia in a spared nerve injury model of neuropathic pain with similar short-term efficacy to gabapentin (within 4 hours) while outperforming gabapentin 24 hours after administration. These findings support the further study of (2R,6R)-HNK as a potentially valuable agent for treating different types of pain and establish certain advantages of (2R,6R)-HNK treatment over ketamine and gabapentin in corresponding assays for pain. SIGNIFICANCE STATEMENT: The ketamine metabolite (2R,6R)-HNK produced antinociception in male and female mice 24 hours after administration via activation of AMPA receptors. The effects of (2R,6R)-HNK differed in time course and mechanism and presented a better safety profile than ketamine. (2R,6R)-HNK also reversed allodynia in SNI-operated animals within 4 hours of treatment onset, with a duration of effect lasting longer than gabapentin. Taken together, (2R,6R)-HNK demonstrates the potential for development as a non-opioid analgesic drug.

Co-administration of cannabidiol and ketamine induces antidepressant-like effects devoid of hyperlocomotor side-effects

BACKGROUND AND PURPOSE

Although useful as a rapid-acting antidepressant drug, ketamine is known to induce psychotomimetic effects, which may interfere with its therapeutic use. Cannabidiol (CBD) is a non-psychostimulant compound from Cannabis sativa, which has shown promising antidepressant effects without inducing hyperlocomotion. AMPA receptor activation is involved in the antidepressant effect induced by ketamine, but its relevance for the effects of CBD is not known. Moreover, given that CBD has antipsychotic and antidepressant properties, it is unknown whether adding CBD to ketamine could potentiate the antidepressant properties of ketamine while also attenuating its psychostimulant effects.

EXPERIMENTAL APPROACH

S-Ketamine (2.5, 3, 5, 10, 30 mg/kg) and cannabidiol (3, 10, 30 mg/kg) were administered alone or in combination to male Swiss mice. Independent groups received NBQX (AMPA receptor antagonist) 5 min before administration of CBD or S-ketamine. The antidepressant-like effect was assessed in the forced swimming test (FST), and the open field test (OFT) evaluated the psychostimulant effect.

KEY RESULTS

CBD induced significant dose-dependent antidepressant effects without causing hyperlocomotion in the OFT. S-ketamine produced an antidepressant effect associated with hyperlocomotion in the higher dose. NBQX inhibited the antidepressant effect of both ketamine and CBD. Pretreatment with CBD (10 mg/kg) attenuated the ketamine-induced hyperlocomotion while preserving its antidepressant effect.

CONCLUSION

AND IMPLICATIONS: Similar to ketamine, the antidepressant-like effect elicited by CBD involves AMPA receptor activation. Additionally, CBD prevents the hyperlocomotion induced by S-ketamine without affecting its antidepressant-like effect. Our findings suggest that CBD and ketamine's combined administration can be a promising therapeutic strategy for achieving an appropriate antidepressant effect without unwanted side-effects. This article is part of the special issue on 'Cannabinoids'.

NMDA receptors promote hippocampal sharp-wave ripples and the associated coactivity of CA1 pyramidal cells

Hippocampal sharp-wave ripples (SWRs) support the reactivation of memory representations, relaying information to neocortex during "offline" and sleep-dependent memory consolidation. While blockade of NMDA receptors (NMDAR) is known to affect both learning and subsequent consolidation, the specific contributions of NMDAR activation to SWR-associated activity remain unclear. Here, we combine biophysical modeling with in vivo local field potential (LFP) and unit recording to quantify changes in SWR dynamics following inactivation of NMDAR. In a biophysical model of CA3-CA1 SWR activity, we find that NMDAR removal leads to reduced SWR density, but spares SWR properties such as duration, cell recruitment and ripple frequency. These predictions are confirmed by experiments in which NMDAR-mediated transmission in rats was inhibited using three different NMDAR antagonists, while recording dorsal CA1 LFP. In the model, loss of NMDAR-mediated conductances also induced a reduction in the proportion of cell pairs that co-activate significantly above chance across multiple events. Again, this prediction is corroborated by dorsal CA1 single-unit recordings, where the NMDAR blocker ketamine disrupted correlated spiking during SWR. Our results are consistent with a framework in which NMDA receptors both promote activation of SWR events and organize SWR-associated spiking content. This suggests that, while SWR are short-lived events emerging in fast excitatory-inhibitory networks, slower network components including NMDAR-mediated currents contribute to ripple density and promote consistency in the spiking content across ripples, underpinning mechanisms for fine-tuning of memory consolidation processes.

Synthesis and pharmacological evaluation of piperidine (piperazine)-amide substituted derivatives as multi-target antipsychotics

We report the optimisation of a series of novel amide-piperidine (piperazine) derivatives using the multiple ligand approach with dopamine and serotonin receptors. Of the derivatives, compound 11 exhibited high affinity for the D₂, 5-HT_1A, and 5-HT_2A receptors, but low affinity for the 5-HT_2C and histamine H₁ receptors and human ether-a-go-go-related gene (hERG) channels. In vivo, compound 11 reduced apomorphine-induced climbing, MK-801-induced hyperactivity and DOI-induced head twitching without observable catalepsy, even at the highest dose tested. In addition, it exhibited suppression in a CAR test. Furthermore, in a novel object recognition task, it displayed procognition properties. Therefore, compound 11 is a promising candidate multi-target antipsychotic.

Classical conditioning of antidepressant placebo effects in mice

INTRODUCTION

Placebo effects in human clinical trials for depression treatment are robust and often comparable to drug effects. Placebo effects are traditionally difficult to study in rodents due to the slow-onset action of classical antidepressant drugs. We hypothesized that the rapid antidepressant actions of ketamine would allow modeling antidepressant placebo effects in rodents.

METHODS

Male and female CD-1 mice received either ketamine or saline injections with concomitant exposure to specific environmental conditioning stimuli, for a total of three drug/conditioning sessions each 2 weeks apart. Two weeks later, during an evocation phase, mice were exposed to the drug-paired conditioning stimuli or no conditioned stimuli followed by testing for motor stimulatory actions and antidepressant-like effects using the forced swim test. Negative (no ketamine administration at any time) and positive (acute ketamine administration prior to evocation testing) control groups were included as comparators.

RESULTS

Both male and female mice exhibited increased locomotor activity following ketamine administration during the conditioning phase, which was not observed following exposure to the conditioning stimuli. Exposure to the conditioning stimuli previously paired with ketamine, similar to an acute ketamine administration, reduced immobility time in the forced swim test both 1 and 24 h after administration in male, but not female, mice.

CONCLUSIONS

These results represent the first evidence of antidepressant-like placebo-conditioned effects in an animal model. The developed approach can be used as a model to explore the neurobiological mechanisms of placebo effects, their possible sexually dimorphic effects, and relevance to mechanisms underlying antidepressant action.

Depletion of dietary phytoestrogens reduces hippocampal plasticity and contextual fear memory stability in adult male mouse

Introduction: Phytoestrogens are non-steroidal estrogen analogues and are found primarily in soy products. They have received increasing attention as dietary supplements for estrogen deficiency and as modulators of endogenous estrogen functions, including cognition and emotion. In addition to modifying the levels of circulating sex hormones, phytoestrogens also exert direct effects on estrogen and androgen receptors in the brain and thus effectively modulate the neural circuit functions.Objective: The aim of this study was to investigate the long-term effects of low phytoestrogen intake (∼6 weeks) on the hippocampal plasticity and hippocampus-dependent memory formation in the adult C57BL/6 male mice.Methods and Results: In comparison to mice on a diet with normal phytoestrogen content, mice on low phytoestrogen diet showed a significant reduction in the phosphorylation of NR2B subunit, a molecular correlate of plasticity in the Schaffer collateral-CA1 synapse. We observed a profound decrease in long-term potentiation (LTP) in the ventral hippocampus, whereas no effect on plasticity was evident in its dorsal portion. Furthermore, we demonstrated that acute perfusion of slices with an estrogen analogue equol, an isoflovane metabolized from daidzein produced by the bacterial flora in the gut, was able to rescue the observed LTP deficit. Examining potential behavioral correlates of the plasticity attenuation, we found that mice on phytoestrogen-free diet display decreased contextual fear memory at remote but not at recent time points after training.Conclusions: Our data suggests that nutritional phytoestrogens have profound effects on the plasticity in the ventral hippocampus and ventral hippocampus-dependent memory.

Ketamine sensitization: Influence of dose, environment, social isolation and treatment interval

Ketamine is a dissociative anesthetic first developed in the 1960s but is increasingly used at subanesthetic doses for both clinical and non-clinical purposes. There is evidence from human recreational users of compulsive use and addiction. Sensitization is an increase in an effect of a drug with repeated use that is thought to be important in the development of addiction. Research on psychomotor stimulants has shown the development of sensitization in laboratory animals to be modified by factors that influence addiction. In the current paper we describe four experiments on the development of sensitization in laboratory rats aimed at determining if ketamine sensitization is also influenced by factors thought to be important in addiction. Adult, male Sprague-Dawley rats received ketamine (5, 10, 20 or 50 mg/kg i.p.) for five or more days and the development of locomotor sensitization was followed. Experiment 1 examined the ability of low doses of ketamine to produce sensitization and found sensitization at 5, 10 and 20 mg/kg. Experiment 2 examined the influence of environmental context and found that ketamine sensitization (20 mg/kg) was greater when administration occurred in a novel environment (the experimental apparatus) than in home cages. Experiment 3 found that ketamine sensitization (20 mg/kg) did not occur when animals were housed in social isolation but occurred readily in pair-housed animals. Finally, Experiment 4 found that ketamine sensitization (20 or 50 mg/kg) was similar whether drug was administered daily or at 3-day intervals. Together, the results demonstrate that ketamine sensitization is robust and reliable, occurring under a variety of circumstances. Moreover, ketamine sensitization is influenced by factors that influence the development of addiction in humans. The current results may lead to a better understanding of ketamine abuse and addiction and may help inform clinical use of the drug.

(R)-Ketamine exerts antidepressant actions partly via conversion to (2R,6R)-hydroxynorketamine, while causing adverse effects at sub-anaesthetic doses

BACKGROUND AND PURPOSE

(R)-Ketamine (arketamine) may have utility as a rapidly acting antidepressant. While (R)-ketamine has lower potency than (R,S)-ketamine to inhibit NMDA receptors in vitro, the extent to which (R)-ketamine shares the NMDA receptor-mediated adverse effects of (R,S)-ketamine in vivo has not been fully characterised. Furthermore, (R)-ketamine is metabolised to (2R,6R)-hydroxynorketamine (HNK), which may contribute to its antidepressant-relevant actions.

EXPERIMENTAL APPROACH

Using mice, we compared (R)-ketamine with a deuterated form of the drug (6,6-dideutero-(R)-ketamine, (R)-d2 -ketamine), which hinders its metabolism to (2R,6R)-HNK, in behavioural tests predicting antidepressant responses. We also examined the actions of intracerebroventricularly infused (2R,6R)-HNK. Further, we quantified putative NMDA receptor inhibition-mediated adverse effects of (R)-ketamine.

KEY RESULTS

(R)-d2 -Ketamine was identical to (R)-ketamine in binding to and functionally inhibiting NMDA receptors but hindered (R)-ketamine's metabolism to (2R,6R)-HNK. (R)-Ketamine exerted greater potency than (R)-d2 -ketamine in several antidepressant-sensitive behavioural measures, consistent with a role of (2R,6R)-HNK in the actions of (R)-ketamine. There were dose-dependent sustained antidepressant-relevant actions of (2R,6R)-HNK following intracerebroventricular administration. (R)-Ketamine exerted NMDA receptor inhibition-mediated behaviours similar to (R,S)-ketamine, including locomotor stimulation, conditioned-place preference, prepulse inhibition deficits, and motor incoordination, with approximately half the potency of the racemic drug.

CONCLUSIONS AND IMPLICATIONS

Metabolism of (R)-ketamine to (2R,6R)-HNK increases the potency of (R)-ketamine to exert antidepressant-relevant actions in mice. Adverse effects of (R)-ketamine require higher doses than those necessary for antidepressant-sensitive behavioural changes in mice. However, our data revealing that (R)-ketamine's adverse effects are elicited at sub-anaesthetic doses indicate a potential risk for sensory dissociation and abuse liability.

Long-lasting effects of repeated ketamine administration in adult and adolescent rats

Initiation of ketamine use often occurs in adolescence, yet little is known about long-term consequences when use begins in this developmental period. The current experiments were designed to examine the effects of repeated exposure to ketamine in adolescence on behavior in adulthood. We examined locomotor activity, as well as cognitive function, in animals that received repeated administration of ketamine. Groups of adolescent and adult male rats were treated with ketamine (25 mg/kg) once daily for 10 days. Locomotor activity was assessed following the first injection, following 10 days of injection, and following 20 days of abstinence. Acute locomotor effects and locomotor sensitization were compared in adolescents and adults; cross-sensitization to dextromethorphan, another dissociative with abusive potential, was also examined. In a separate group of animals cognitive deficits were assessed following the 20 day abstinence period in spatial learning and novel object recognition tasks. The locomotor stimulant effect of ketamine was much greater in adolescents than adults. Animals that were repeatedly administered ketamine demonstrated locomotor sensitization immediately after the final injection. However, sensitization only persisted after the abstinence period in animals treated as adults. No cross-sensitization to dextromethorphan was evident. Ketamine failed to produce statistically significant cognitive deficits in either age group, although drug-treated adults showed a trend towards deficits in spatial learning. Repeated use of ketamine produces long-lasting neuroadaptations that may contribute to addiction. Mild lasting memory deficits may occur in adults, although further work is necessary to confirm these findings. The results extend the understanding of potential long-term consequences of ketamine use in adolescents and adults.

(2R,6R)-hydroxynorketamine exerts mGlu2 receptor-dependent antidepressant actions

Currently approved antidepressant drugs often take months to take full effect, and ∼30% of depressed patients remain treatment resistant. In contrast, ketamine, when administered as a single subanesthetic dose, exerts rapid and sustained antidepressant actions. Preclinical studies indicate that the ketamine metabolite (2R,6R)-hydroxynorketamine [(2R,6R)-HNK] is a rapid-acting antidepressant drug candidate with limited dissociation properties and abuse potential. We assessed the role of group II metabotropic glutamate receptor subtypes 2 (mGlu₂) and 3 (mGlu₃) in the antidepressant-relevant actions of (2R,6R)-HNK using behavioral, genetic, and pharmacological approaches as well as cortical quantitative EEG (qEEG) measurements in mice. Both ketamine and (2R,6R)-HNK prevented mGlu_2/3 receptor agonist (LY379268)-induced body temperature increases in mice lacking the Grm3, but not Grm2, gene. This action was not replicated by NMDA receptor antagonists or a chemical variant of ketamine that limits metabolism to (2R,6R)-HNK. The antidepressant-relevant behavioral effects and 30- to 80-Hz qEEG oscillation (gamma-range) increases resultant from (2R,6R)-HNK administration were prevented by pretreatment with an mGlu_2/3 receptor agonist and absent in mice lacking the Grm2, but not Grm3 ^-/-, gene. Combined subeffective doses of the mGlu_2/3 receptor antagonist LY341495 and (2R,6R)-HNK exerted synergistic increases on gamma oscillations and antidepressant-relevant behavioral actions. These findings highlight that (2R,6R)-HNK exerts antidepressant-relevant actions via a mechanism converging with mGlu₂ receptor signaling and suggest enhanced cortical gamma oscillations as a marker of target engagement relevant to antidepressant efficacy. Moreover, these results support the use of (2R,6R)-HNK and inhibitors of mGlu₂ receptor function in clinical trials for treatment-resistant depression either alone or in combination.

Impact of repeated anesthesia with ketamine and xylazine on the well-being of C57BL/6JRj mice

Within the scope of the 3Rs of Russel and Burch, the number of laboratory animals can be reduced by repeated use of an animal. This strategy only becomes relevant, if the total amount of pain, distress or harm the individual animal experiences does not exceed the severity of a single manipulation. For example, when using imaging techniques, an animal can be examined several times during a study, but it has to be anesthetized each time imaging is performed. The severity of anesthesia is thought to be mild according to the Directive 2010/63/EU. However, the Directive does not differentiate between single and repeated anesthesia, although repeated anesthesia may have a greater impact on well-being. Hence, we compared the impact of single and repeated anesthesia (six times at an interval of three to four days) by injection of ketamine and xylazine (KX) on the well-being of adult female and male C57BL/6JRj mice. After anesthesia, well-being of mice was assessed according to a protocol for systematic assessment of well-being including nesting, the Mouse Grimace Scale (MGS), a test for trait anxiety, home cage activity, and the rotarod test for motor activity, food intake, and body weight, as well as corticosterone (metabolite) analysis. Repeated anesthesia increased the MGS in mice of both sexes and caused short-term effects on well-being of female mice in the immediate post-anesthetic period, indicated by longer lasting effects on trait anxiety-related behavior. However, corticosterone metabolite concentrations suggested that mice habituated to the stress induced by repeated KX administration. Hence, the mildly negative effects on well-being of repeated KX anesthesia do not seem to accumulate over time using the respective regimen. However, further observations for severity classification are warranted in order to more specifically determine the duration of mild distress and trait anxiety.

Histamine H3 receptor antagonists display antischizophrenic activities in rats treated with MK-801

BACKGROUND

Animal models based on N-methyl-d-aspartate receptor blockade have been extensively used for schizophrenia. Ketamine and MK-801 produce behaviors related to schizophrenia and exacerbated symptoms in patients with schizophrenia, which led to the use of PCP (phencyclidine)- and MK-801 (dizocilpine)-treated animals as models for schizophrenia.

METHODS

The study investigated the effect of subchronic dosing (once daily, 7 days) of histamine H3 receptor (H3R) antagonists, ciproxifan (CPX) (3 mg/kg, i.p.), and clobenpropit (CBP) (15 mg/kg, i.p.) on MK-801 (0.2 mg/kg, i.p.)-induced locomotor activity and also measured dopamine and histamine levels in rat's brain homogenates. The study also included clozapine (CLZ) (3.0 mg/kg, i.p.) and chlorpromazine (CPZ) (3.0 mg/kg, i.p.), the atypical and typical antipsychotic, respectively.

RESULTS

Atypical and typical antipsychotic was used to serve as clinically relevant reference agents to compare the effects of the H3R antagonists. MK-801 significantly increased horizontal locomotor activity, which was reduced with CPX and CBP. MK-801-induced locomotor hyperactivity attenuated by CPX and CBP was comparable to CLZ and CPZ. MK-801 raised striatal dopamine level, which was reduced in rats pretreated with CPX and CBP. CPZ also significantly lowered striatal dopamine levels, although the decrease was less robust compared to CLZ, CPX, and CBP. MK-801 increased histamine content although to a lesser degree. Subchronic treatment with CPX and CBP exhibited further increased histamine levels in the hypothalamus compared to MK-801 treatment alone. Histamine H3 receptor agonist, R-α methylhistamine (10 mg/kg, i.p.), counteracted the effect of CPX and CBP.

CONCLUSIONS

The present study shows the positive effects of CPX and CBP on MK-801-induced schizophrenia-like behaviors in rodents.

NMDAR inhibition-independent antidepressant actions of ketamine metabolites

Major depressive disorder afflicts ~16 percent of the world population at some point in their lives. Despite a number of available monoaminergic-based antidepressants, most patients require many weeks, if not months, to respond to these treatments, and many patients never attain sustained remission of their symptoms. The non-competitive glutamatergic N-methyl-D-aspartate receptor (NMDAR) antagonist, (R,S)-ketamine (ketamine), exerts rapid and sustained antidepressant effects following a single dose in depressed patients. Here we show that the metabolism of ketamine to (2S,6S;2R,6R)-hydroxynorketamine (HNK) is essential for its antidepressant effects, and that the (2R,6R)-HNK enantiomer exerts behavioural, electroencephalographic, electrophysiological and cellular antidepressant actions in vivo. Notably, we demonstrate that these antidepressant actions are NMDAR inhibition-independent but they involve early and sustained α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor activation. We also establish that (2R,6R)-HNK lacks ketamine-related side-effects. Our results indicate a novel mechanism underlying ketamine’s unique antidepressant properties, which involves the required activity of a distinct metabolite and is independent of NMDAR inhibition. These findings have relevance for the development of next generation, rapid-acting antidepressants.

Oral acetate supplementation attenuates N-methyl D-aspartate receptor hypofunction-induced behavioral phenotypes accompanied by restoration of acetyl-histone homeostasis

RATIONALE

Aberrations in cellular acetate-utilization processes leading to global histone hypoacetylation have been implicated in the etiology of neuropsychiatric disorders like schizophrenia.

OBJECTIVES

Here, we investigated the role of acetate supplementation in the form of glyceryl triacetate (GTA) for the ability to restore the N-methyl D-aspartate (NMDA) receptor-induced histone hypoacetylation and to ameliorate associated behavioral phenotypes in mice.

RESULTS

Taking cues from the studies in SH-SY5Y cells, we monitored acetylation status of specific lysine residues of histones H3 and H4 (H3K9 and H4K8) to determine the impact of oral GTA supplementation in vivo. Mice treated chronically with MK-801 (10 days; 0.15 mg/kg daily) induced hypoacetylation of H3K9 and H4K8 in the hippocampus. Daily oral supplementation of GTA (2.9 g/kg) was able to prevent this MK801-induced hypoacetylation significantly. Though MK-801-stimulated decreases in acetyl-H3K9 and acetyl-H4K8 were found to be associated with ERK1/2 activation, GTA seemed to act independent of this pathway. Simultaneously, GTA administration was able to attenuate the chronic MK-801-induced cognitive behavior phenotypes in elevated plus maze and novel object recognition tests. Not only MK-801, GTA also demonstrated protective effects against behavioral phenotypes generated by another NMDA receptor antagonist, ketamine. Acute (single injection) ketamine-mediated hyperactivity phenotype and chronic (10 days treatment) ketamine-induced phenotype of exaggerated immobility in forced swim test were ameliorated by GTA.

CONCLUSION

The signature behavioral phenotypes induced by acute and chronic regimen of NMDA receptor antagonists seemed to be attenuated by GTA. This study thus provides a therapeutic paradigm of using dietary acetate supplement in psychiatric disorders.

The Effect of Subchronic Dosing of Ciproxifan and Clobenpropit on Dopamine and Histamine Levels in Rats

The present study was designed to investigate the effect of once daily for 7-day (subchronic treatment) dosing of histamine H₃ receptor antagonists, ciproxifan (CPX) (3 mg/kg, i.p.), and clobenpropit (CBP) (15 mg/kg, i.p), including clozapine (CLZ) (3.0 mg/kg, i.p.) and chlorpromazine (CPZ) (3.0 mg/kg, i.p.), the atypical and typical antipsychotic, respectively, on MK-801(0.2 mg/kg, i.p.)-induced locomotor activity, and dopamine and histamine levels in rats. Dopamine and histamine levels were measured in striatum and hypothalamus, respectively, of rat brain. Atypical and typical antipsychotics were used to serve as clinically relevant reference agents to compare the effects of the H3 receptor antagonists. MK-801-induced increase of horizontal activity was reduced with CPX and CBP. The attenuation of MK-801-induced locomotor hyperactivity produced by CPX and CBP was comparable to CLZ and CPZ. MK-801 raised dopamine levels in the striatum, which was reduced in rats pretreated with CPX and CBP. CPZ also lowered striatal dopamine levels, though the decrease was less robust compared to CLZ, CPX and CBP. MK-801 increased histamine content although to a lesser degree. Subchronic treatment with CPX and CBP exhibited further increase in histamine levels in the hypothalamus compared to the MK-801 treatment alone. Histamine H₃ receptor agonist, R-α methylhistamine (10 mg/kg, i.p.) counteracted the effects of CPX and CBP. In conclusion, the subchronic dosing of CPX/CBP suggests some antipsychotic-like activities as CPX/CBP counteracts the modulatory effects of MK-801 on dopamine and histamine levels and prevents MK-801-induced hyperlocomotor behaviors.

Effects of omega-3 supplementation on interleukin and neurotrophin levels in an animal model of schizophrenia

ABSTRACTNew studies suggest that polyunsaturated fatty acids, such as omega-3, may reduce the symptoms of schizophrenia. The present study evaluated the preventive effect of omega-3 on interleukines (IL) and neurotrophin brain-derived neurotrophic factor (BDNF) levels in the brains of young rats subjected to a model of schizophrenia. Treatment was performed over 21 days, starting on the 30th day of rat's life. After 14 days of treatment with omega-3 or vehicle, a concomitant treatment with saline or ketamine (25 mg/kg) was started and maintained until the last day of the experiment. BDNF levels in the rat's prefrontal cortex were decreased at 1 h and 24 h after the last administration of ketamine, whereas the group administered with ketamine and omega-3 showed a decrease in BDNF levels only after 24 h. In contrast, both interventions induced similar responses in levels of IL-1β and IL6. These findings suggest that the similarity of IL-1β and IL6 levels in our experimental groups is due to the mechanism of action of ketamine on the immune system. More studies have to be carried out to explain this pathology. In conclusion, according to previous studies and considering the current study, we could suggest a prophylactic role of omega-3 against the outcome of symptoms associated with schizophrenia.

Selective blockade of N-methyl-D-aspartate channels in combination with dopamine receptor antagonism induces loss of the righting reflex in mice, but not immobility

RATIONALE

The selective N-methyl-D-aspartate (NMDA) channel blocker MK-801 is known to induce no loss of the righting reflex (LORR) and to stimulate catecholaminergic (CAergic) neurons in rodents, playing a crucial role in arousal.

OBJECTIVES

We examined whether MK-801 in combination with CA receptor ligands, which inhibit CAergic neuronal activities, could induce anesthesia including LORR.

METHODS

All drugs were administered systemically to mice. To assess anesthesia, three different behaviors were used: loss of nociceptive response (analgesia in the free-moving state without LORR), LORR, and loss of movement in response to noxious stimulation (immobility under LORR).

RESULTS

A very large dose of MK-801 (50 mg/kg) induced neither analgesia nor LORR. In contrast, MK-801 in combination with a small dose of the dopamine (DA) receptor antagonist haloperidol (0.2 mg/kg) dose-dependently produced LORR with a 50 % effective dose (ED50) of 1.6 (0.9-3.0; 95 % confidence limit) mg/kg, but not immobility. The α2-adrenoceptor agonist dexmedetomidine induced not only analgesia, but also immobility in animals treated with MK-801 (5 mg/kg) plus haloperidol (0.2 mg/kg), which then lost their righting reflex. The ED50 value of 0.26 (0.10-0.66) mg/kg (various doses of dexmedetomidine plus a fixed dose of MK-801 and haloperidol) for immobility was approximately three-fold larger than that of 0.09 (0.03-0.23) mg/kg (dexmedetomidine plus vehicle saline) for analgesia. This may occur, as LORR induced by MK-801 plus haloperidol inhibits the pain suppression system. The other ligands had little or no effect.

CONCLUSIONS

The DAergic stimulant actions of MK-801 may mask its LORR effects by NMDA channel blockade.

Clinical physiology and mechanism of dizocilpine (MK-801): electron transfer, radicals, redox metabolites and bioactivity

Dizocilpine (MK-801), an extensively investigated drug possessing secondary amine and benzenoid functions, displays a wide array of biological properties, including anticonvulsant and anesthetic. There is scant discussion of biomechanism. A relevant, important finding is formation of oxidative metabolites in the hydroxylamine and phenolic categories. Analogy to cocaine metabolites suggests participation of redox entities, such as, hydroxylamine, nitroxide and nitrosonium, which can lead to electron transfer and radical formation. There is also similarity to metabolism by 3,3′-iminodipropionitrile and phencyclidine. Alternatively, the phenolic metabolites are well-known precursors of ET quinones. The review documents various physiological effects, mainly involving the central nervous system. Also of interest are the pro- and anti-oxidant properties. Considerable attention has been paid to MK-801 as an antagonist of the N-methyl-D-aspartate receptor in the glutamate category. This aspect is often associated with effects on the central nervous system. The review also provides recent literature dealing with MK-801/NMDA receptor in various areas of bioactivity. Studies were made of MK-801 involvement in working memory processing. Deficits in behavior were noted after administration of the drug. Treatment of mice with dizocilpine induced learning impairment. The influence of MK-801 on fear has been investigated. The substance is known to exert an analgesic effect in pain control. A number of reports deal with anesthetic properties.

Sensitivity to MK-801 in phospholipase C-β1 knockout mice reveals a specific NMDA receptor deficit

Phospholipase C-β1 (PLC-β1) is a critical component of multiple signalling pathways downstream of neurotransmitter receptors. Mice lacking this enzyme display a striking behavioural phenotype with relevance to human psychiatric disease. Glutamatergic dysfunction is strongly associated with several abnormal behavioural states and may underlie part of the phenotype of the phospholipase C-β1 knockout (KO) mouse. A heightened response to glutamatergic psychotomimetic drugs is a critical psychosis-related endophenotype, and in this study it was employed as a correlate of glutamatergic dysfunction. Control (n=8) and PLC-β1 KO mice (n=6) were treated with MK-801, a NMDA receptor (NMDAR) antagonist, following either standard housing or environmental enrichment, and the motor function and locomotor activity thus evoked was assessed. In addition, MK-801 binding to the NMDAR was evaluated through radioligand autoradiography in post-mortem tissue (on a drug-naive cohort). We have demonstrated a significantly increased sensitivity to the effects of the NMDA antagonist MK-801 in the PLC-β1 KO mouse. In addition, we found that this mouse line displays reduced hippocampal NMDAR expression, as measured by radioligand binding. We previously documented a reversal of specific phenotypes in this mouse line following housing in an enriched environment. Enrichment did not alter this heightened MK-801 response, nor NMDAR expression, indicating that this therapeutic intervention works on specific pathways only. These findings demonstrate the critical role of the glutamatergic system in the phenotype of the PLC-β1 KO mouse and highlight the role of these interconnected signalling pathways in schizophrenia-like behavioural disruption. These results also shed further light on the capacity of environmental factors to modulate subsets of these phenotypes.

Effect of acetaldehyde on behavioral and neurochemical changes induced by MK-801 in rats

Alterations in motor activity related to dopamine changes in some brain regions have been described as consequences of the modifications produced by systemic administration of MK-801 (a noncompetitive NMDA receptor antagonist) in rats. Acetaldehyde (ACH), the main metabolite of ethanol, has been implicated in different alterations in the central nervous system after ethanol ingestion. ACH might exert some control on dopaminergic transmission through the formation of other compounds with dopamine, which eventually may modify dopamine content and its metabolism. In order to evaluate such a hypothesis, we used Wistar rats in the present study to evaluate the effect of ACH on locomotor alterations and dopamine metabolism changes induced by MK-801. Our results show that the MK-801-treated group had a significant increase in locomotor activity. In contrast, we did not find significant differences in locomotion tests after ACH administration. However, the group to which both drugs were administered showed a significant decrease in locomotor activity compared with those given MK-801 alone. Neurochemical analysis showed an increase in dopamine content in the striatum and frontal cortex after MK-801 administration, however; the increase was reversed by giving 200 mg/kg of ACH. These results indicate that ACH can produce an antagonic-like effect on locomotor alterations and dopamine content changes induced by MK-801, thus modulating the MK-801-induced hyperlocomotion by interfering with dopamine metabolism.

Effects of glutamate-related drugs on marble-burying behavior in mice: implications for obsessive-compulsive disorder

Clinical evidence demonstrates altered glutamatergic neurotransmission in patients suffering from obsessive-compulsive disorder (OCD). We examined the effects of glutamate-related drugs on marble-burying behavior, which is an animal model of OCD. The uncompetitive N-methyl-d-aspartate (NMDA) antagonists memantine (10 mg/kg, i.p.) and amantadine (30 mg/kg, i.p.) significantly inhibited marble-burying behavior without affecting locomotor activity in mice. Similarly, the uncompetitive NMDA receptor antagonist 5R,10S-(+)-5-methyl-10,11-dihydro-5H-dibenzo(a,d)cyclohepten-5,10-imine hydrogen maleate (MK-801, 0.3 mg/kg, i.p.) inhibited marble-burying behavior. However, MK-801 at the same dose markedly increased locomotor activity. By contrast, the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor antagonist 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide (NBQX) and the glutamate release inhibitor riluzole showed no effect on marble-burying behavior and significant suppression of locomotor activity. MK-801 (0.3 mg/kg, i.p.) and memantine (10 mg/kg, i.p.) significantly disrupted prepulse inhibition as an operational measure of sensorimotor gating. By contrast, amantadine (30 mg/kg, i.p.) did not affect prepulse inhibition. These findings suggest that amantadine could be a useful drug for the treatment of OCD.

Involvement of NMDA Receptors in Thiopental-Induced Loss of Righting Reflex, Antinociception and Anticonvulsion Effects in Mice

Potentiation of GABA(A) receptor-mediated inhibitory neurotransmission contributes to the anesthetic action of thiopental. However, the inhibiting action of general anesthetic on excitatory neurotransmission also purportedly underlies its effects. The aim of the study was to elucidate the role of glutamate receptors (NMDA and AMPA receptors) in thiopental-induced anesthesia. Intracerebroventricular (i.c.v.) NMDA (50 ng) significantly increased the induction time of loss of righting reflex and decreased sleep time induced by intraperitoneal injection (i.p.) of thiopental (50 mg/kg). Furthermore, NMDA at 50 ng i.c.v. increased the 50% effective dose values for thiopental to produce loss of righting reflex and immobility in response to noxious tail clamp by 25% and 21% (p < 0.05), respectively. However, intrathecal (IT) administration of NMDA or both of i.c.v. or IT administration of AMPA did not show such antagonizing effects on thiopental action at subconvulsive dose. Finally, thiopental (25 mg/kg i.p.) inhibited convulsions induced by NMDA (0.4 microg i.c.v.) or bicuculline (0.6 microg i.c.v.). However, i.p. muscimol (1 mg/kg) blocked the convulsions induced by bicuculline, but not those induced by NMDA at 3 mg/kg. Similarly, i.p. MK-801 (0.1 mg/kg) antagonized NMDA-induced convulsions, but not bicuculline-induced convulsions at 0.3 mg/kg. Therefore, we suggest that the effects of the selective GABA(A) and NMDA receptors on convulsive behavior are special to their sites of action, and that the inhibitory action of thiopental on NMDA receptors is possibly not mediated by secondary effects of its GABA(A) receptors agonism. These results above indicate the involvement of NMDA receptors in thiopental-induced anesthesia in mice.

Ketamine, at a dose that disrupts motor behavior and latent inhibition, enhances prefrontal cortex synaptic efficacy and glutamate release in the nucleus accumbens

Noncompetitive N-methyl-D-aspartate (NMDA) antagonists such as ketamine represent useful pharmacological tools to model, in both healthy humans and rodents, behavioral and cerebral abnormalities of schizophrenia. These compounds are thought to exert some of their disruptive effects by impairing glutamatergic transmission in corticolimbic circuits including the nucleus accumbens (NAc). In this study, we investigated in freely moving rats behavioral changes as well as electrophysiological and neurochemical alterations in the NAc following acute systemic injection of a subanesthetic dose (25 mg/kg) of ketamine. We found that ketamine induced an immediate behavioral activation, characterized by hyperlocomotion, stereotypies and ataxia, and abolished latent inhibition in a conditioned-fear paradigm when injected at the pre-exposure stage. We also observed that during expression of motor effects which are thought to be related to the positive symptoms of schizophrenia, ketamine potentiated synaptic efficacy in the prefrontal-accumbens pathway and increased the extracellular levels of glutamate in the NAc. These results, taken together with previous findings, suggest that the psychotic-like effects of noncompetitive NMDA antagonists may be, in part, mediated by an increase in glutamate release in the NAc associated with synaptic changes in accumbens glutamatergic inputs including enhancement of synaptic efficacy in the prefrontal input.

Modulation of cholestasis-induced antinociception in rats by two NMDA receptor antagonists: MK-801 and magnesium sulfate

Acute cholestasis is associated with increased activity of the endogenous opioid system that results to changes including analgesia. N-methyl-d-aspartate (NMDA) receptors are involved in the nociceptive pathway and play a major role in the development of morphine induced analgesia. The magnesium acts as a non-competitive NMDA receptor antagonist by blocking the NMDA receptor channel. Considering the reported antinociceptive effect of magnesium sulfate as a NMDA receptor antagonist and the existence of close functional links between NMDA receptor antagonists and magnesium with the opioid system, we studied the effect of acute and chronic administration of MK-801 as a NMDA antagonist and magnesium sulfate on modulation of nociception in an experimental model of elevated endogenous opioid tone, acute cholestasis, using the tail-flick paradigm. Cholestasis was induced by ligation of the main bile duct using two ligatures and then transsection of the duct at the midpoint between them. A significant increase (P<0.001) in nociception threshold was observed in bile duct ligated rats compared to unoperated and sham-operated animals. In acute treatment, MK-801 (0.1 mg/kg, b.i.d), but not magnesium (150 mg/kg magnesium sulfate, i.e. 30 mg/kg of Mg(+2), i.p., b.i.d.) increased antinociception in cholestatic rats compared to saline treated cholestatics (P<0.05). In chronic treatment, administration of MK-801 or magnesium sulfate for 7 consecutive days, increased tail-flick latency (P<0.05, P<0.01) in cholestatic animals compared to saline treated cholestatics. These data showed that NMDA receptor pathway is involved in modulation of cholestasis-induced antinociception in rats and that repeated dosages of magnesium sulfate similar to MK-801 is able to modulate nociception in cholestasis.

Prolonged effect of single carnitine administration on fasted carnitine-deficient JVS mice regarding their locomotor activity and energy expenditure

Carnitine is an essential cofactor for the oxidation of fatty acid in the mitochondria and an efficient therapeutics for primary carnitine deficiency. We herein analyzed the prolonged effects of carnitine on the reduced locomotor activity and energy metabolism of fasted carnitine-deficient juvenile visceral steatosis (jvs(-/-)) mice. We found that a single carnitine administration to 24-h fasted jvs(-/-) mice in the morning increased both the locomotor activity and oxygen consumption at night not only on the same day, but also on the next day, when the carnitine levels in the blood and tissues were already as low as at the original carnitine-deficient state. We also found that fat utilization for energy production significantly increased under fasting even in jvs(-/-) mice and was stimulated in the carnitine-administrated fasted jvs(-/-) mice at night, in comparison to that observed in the saline-administered jvs(-/-) mice, at least for 2 days even under the low plasma and tissue carnitine levels. These results suggest that the low tissue carnitine levels are therefore not the sole rate-limiting factor of general fatty acid oxidation in carnitine-deficient jvs(-/-) mice.

Metabotropic glutamate receptors mediate lipopolysaccharide-induced fever and sickness behavior

Several mechanisms have been proposed for neuroimmune communication supporting the sickness syndrome (fever, anorexia, inactivity, and cachexia) following infection. We examined the role of glutamate as a neurochemical intermediary of sickness behavior induced by intraperitoneal lipopolysaccharide (LPS). Mice implanted with biotelemetry devices capable of detecting body temperature (Tb) were administered LPS (50 or 500 microg/kg i.p., serotype 0111:B4) with or without i.p. pretreatment with vehicle or broad-spectrum antagonists selective for N-methyl-d-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic (AMPA)/kainite, or metabotropic glutamate (mGlu) receptors. While NMDA and AMPA/kainate receptor antagonism failed to attenuate LPS-induced sickness behavior, antagonism of metabotropic receptors with l(+)-AP3 reduced the febrile (0-11h: control: 37.32+/-0.16 degrees C, l(+)-AP3: 36.66+/-0.27), anorexic (control: -87+/-5%, l(+)-AP3: 48+/-12% scotophase food intake), and cachexic (control: -8.9+/-0.4%, l(+)-AP3: -6.1+/-1.3% body weight) effects of 500 microg/kg LPS, and produced a biphasic Tb effect in response to 50 microg/kg LPS (1h: -0.90+/-0.26; 6h: 1.78+/-0.35 degrees C relative to baseline). At this dose the Tb of l(+)-AP3-treated mice was 1.18 degrees C lower than controls 2h post-injection, and 0.68 degrees C greater that controls 8h post-injection. These results suggest a role for mGlu receptors in mediating fever, anorexia, and cachexia possibly via activation of extra-vagal pathways, since the attenuating effect of l(+)-AP3 increased with increasing dosages of LPS. Given the critical role ascribed to mGlu receptors in neurotransmitter release and astrocytic processes, it is possible that these observations reflect an l(+)-AP3-induced attenuation of these systems.

Effect of 5-HT3 receptor antagonist MDL 72222 on behaviors induced by ketamine in rats and mice

Phencyclidine and ketamine (but not other NMDA channel blockers, such as memantine) produce psychotomimetic effects. Since unlike memantine, phencyclidine-like compounds show no significant affinity at 5-HT(3) receptors, we investigated if behavioral effects of ketamine could be reduced by 5HT(3) receptor blockade. Ketamine (3-40 mg/kg) produced ataxia, stereotypes and diminished exploratory activity in mice, and reduced prepulse inhibition of acoustic startle response, lowered accuracy in fixed consecutive number and in delayed non-matching-to-sample tasks in rats. The 5HT(3) receptor antagonist MDL 72222 (0.3-3 mg/kg) administration did not reverse any of these deficits and exerted no effects on discriminative stimulus properties of ketamine. In the tail suspension test, both ketamine and MDL 72222 produced anti-immobility effects when given alone (50-66 and 3 mg/kg, respectively) and together (12.5-25 and 1 mg/kg). The present data suggest that 5-HT(3) receptor blockade does not reverse the behavioral deficits of ketamine and may even enhance its certain effects, such as the antidepressant-like action.

Paradoxical effects of very low dose MK-801

Systemic injection of the noncompetitive NMDA (N-methyl-D-aspartate) receptor antagonist MK-801 (dizocilpine maleate) is known to cause increased locomotion and various stereotypic behaviors in rodents. However, the MK-801 dose ranges commonly examined usually begin at tenth of mg/kg and going higher, with the implicit assumption of lower doses being ineffective. We report here that very low dose MK-801, well below the commonly studied doses, exert distinct effects on rodent behaviors. In C57BL/6 mice, very low dose MK-801 (0.02 mg/kg) has strikingly different effects than higher doses commonly reported in the literature. Locomotion, rearing, grooming, and other behaviors are strongly inhibited, replaced by periods of immobility. This is in contrast to the mobility-enhancing effect of MK-801 at commonly reported dose ranges. The effects of very low dose MK-801 are qualitatively similar to those observed with moderate doses (0.1-0.2 mg/kg) of the typical antipsychotic haloperidol. These results highlight the complexity of the dose-response relation for MK-801-induced behaviors.

Fasting-induced reduction in locomotor activity and reduced response of orexin neurons in carnitine-deficient mice

We found reduced locomotor activity (LA) under fasting in systemic carnitine-deficient juvenile visceral steatosis (jvs(-/-)) mice. When food was withdrawn at 8:00 a.m. (lights-off at 7:00 p.m., 12h/cycle), the nocturnal LA of jvs(-/-) mice was much less than the control (jvs(+/+) and jvs(+/-)) mice. LA recovered under carnitine or sucrose administration, but not under medium-chain triglyceride. In addition, fasted jvs(-/-) mice, without any energy supply, were activated by modafinil, a stimulator of the dopamine pathway. These results suggest that the reduced LA is not adequately explained by energy deficit. As the fasted jvs(-/-) mice showed lower body core temperature (BT), we examined the central nervous system regulating LA and BT. We found lower percentage of c-Fos positive orexin neurons in the lateral hypothalamus and reduced orexin-A concentration in the cerebrospinal fluid of fasted jvs(-/-) mice. Sleep analysis revealed that fasted jvs(-/-) mice had disruption of prolonged wakefulness, with a higher frequency of brief episodes of non-REM sleep during the dark period than fasted jvs(+/+) mice. These results strongly suggest that the reduced LA in fasted jvs(-/-) mice is related to the inhibition of orexin neuronal activity.

Opposite effects of MK-801 on the expression of food and morphine-induced conditioned place preference in rats

Behavioural studies have provided strong evidence for common substrates in the rewards of natural and addictive substances, but it is still unclear whether there is a common glutamatergic NMDA receptor mechanism involved in the processing of reward for both. The present study was designed to investigate the effects of MK-801 (0.1mg/kg) on the expression of place preference conditioned with food and morphine (5.0mg/kg) in rats. The data indicates that MK-801 potentiates the expression of food-induced conditioned place preference (CPP) but retards that of morphine CPP. It also demonstrates that the opposite effects of MK-801 on food and morphine CPP expression were caused neither by hyperactivity nor by the impairment of memory retrieval. These results suggest that MK-801 enhances food craving and inhibits morphine craving in rats, and that the roles of glutamatergic NMDA receptor mechanisms in the reward processing of natural reinforcers and addictive drugs may be dissociable.

Dorsal hippocampus NMDA receptors differentially mediate trace and contextual fear conditioning

The dorsal hippocampus (DH) is critically involved in the acquisition and expression of trace and contextual fear conditioning. NMDA/glutamate receptor-mediated transmission is thought to be one mechanism mediating the plastic changes that support long-term memories in the DH. However, their precise involvement in acquisition and expression processes has not been defined. To examine this issue, the NMDA receptor antagonist, D,L-2-amino-5-phosphonovaleric acid (APV; 10 microg/microl; 0.5 microl), was infused into the DH prior to conditioning and/or testing, using a trace fear conditioning procedure. All rats were tested for freezing to both tone and context in separate, counterbalanced sessions. The three sessions (1 training and 2 test) were separated by approximately 24 h. Using this design, it was possible to assess the role for DH NMDA receptors in the acquisition versus expression of trace and contextual fear conditioning. APV disrupted acquisition, but not expression, of contextual fear conditioning. By contrast, APV attenuated both acquisition and expression of trace fear memories. Thus, DH NMDA receptors appear to contribute to retrieval of some, but not all, fear memories.

Mice lacking d-amino acid oxidase activity display marked attenuation of stereotypy and ataxia induced by MK-801

The behavioral effects produced by MK-801 (0.4 mg/kg) were compared in mutant DAO-/- mice lacking D-amino acid oxidase activity and normal DAO+/+ mice. Mutant mice display marked diminution of stereotypy and ataxia induced by MK-801 compared to normal mice. Because the D-serine level in the brain of mutant mice is significantly higher than that of normal mice, the elevated D-serine in the brain of mutant mice could antagonize MK-801-induced stereotypy and ataxia.

Ketamine and amphetamine both enhance synaptic transmission in the amygdala-nucleus accumbens pathway but with different time-courses

Excitatory glutamatergic fibers from limbic structures, such as the hippocampus and the basolateral amygdala, are known to converge on the same neurons in the nucleus accumbens. We have recently shown that ketamine, at a dose (25 mg/kg) that produces psychosis-like behaviors in rats, decreases glutamatergic transmission between the hippocampus and the nucleus accumbens. Here we investigated whether ketamine also affects glutamatergic transmission between the basolateral amygdala and the nucleus accumbens. We also studied the effects of amphetamine (1.5 mg/kg), known to evoke psychosis-like behaviors in rats. We found that each drug produced a long-lasting (at least 30 min) potentiation of synaptic efficacy in the projection from the basolateral amygdala to the nucleus accumbens. However, while this synaptic potentiation developed shortly after ketamine injection (within 4 min), it occurred after a 30-min delay in rats injected with amphetamine. These data reveal, in freely behaving rats, that ketamine has a more rapid and powerful effect on projection targets of the basolateral amygdala than does amphetamine.

The pre-ischaemic neuroprotective effect of a novel polyamine antagonist, N1-dansyl-spermine in a permanent focal cerebral ischaemia model in mice

The polyamine sites on the NMDA receptor complex offer a therapeutic target for focal ischaemia, potentially devoid of most side effects associated with NMDA antagonists. In this study, we investigated the effect of a novel polyamine antagonist, N(1)-dansyl-spermine (0.5-10 mg kg(-1)) in a permanent focal cerebral ischaemia model in mice, and compared its effect to that of MK-801 (0.3-3 mg kg(-1)) following administration 30 min prior to ischaemia. A battery of histological and behavioural tests was employed following permanent middle cerebral artery occlusion to assess any neuroprotective effect. Following middle cerebral artery occlusion, N(1)-dansyl-spermine (1-5 mg kg(-1)) and MK-801 (1 or 3 mg kg(-1)) caused a comparable and significant reduction in the percentage hemisphere lesion volume. Similarly, both drugs significantly reduced oedema and neurological deficit score to a similar extent. Locomotor activity in MCAO mice was not significantly improved by MK-801 or N(1)-dansyl-spermine, although N(1)-dansyl-spermine induced a trend towards significant improvement. Significant improvement in rotarod performance was observed at neuroprotective doses with both drugs. Upon comparison of the profile of effects, N(1)-dansyl-spermine at least matched the effectiveness of MK-801 as a neuroprotective agent in this model. In addition, in sham-operated control mice, N(1)-dansyl-spermine was well tolerated, in contrast to the pronounced adverse effects of MK-801 on locomotor activity and rotarod performance. In conclusion, this study has shown that N(1)-dansyl-spermine is as effective a neuroprotective drug as MK-801 in this model. Moreover, in contrast to MK-801, N(1)-dansyl-spermine could be a promising therapeutic candidate for stroke as it is well tolerated at neuroprotective doses in sham-operated animals.

Involvement of NO/cGMP pathway in toluene-induced locomotor hyperactivity in female rats

RATIONALE

Nitric oxide (NO) is implicated in the acute locomotor activating effects of some addictive drugs such as amphetamine, caffeine, and PCP, but has not been investigated in the case of toluene.

OBJECTIVES

This study determined the contribution of the NO-cyclic GMP (cGMP) pathway to locomotor stimulant effects of toluene.

METHODS

Locomotor activity was measured for 90 min immediately following toluene (500-1,000 mg/kg, IP) or corn oil treatments in Sprague-Dawley female rats. A NO generator, sodium nitroprusside (SNP) (3 and 6 mg/kg), a NO precursor, L-arginine ( L-Arg) (250 mg/kg), a NO synthase inhibitor, N(G)-nitro- L-arginine methyl ester (L-NAME) (5-20 mg/kg, IP), and a soluble guanylyl cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) (10 mg/kg) were injected 5 min before toluene (750 mg/kg, IP) treatment. The combination effects of SNP with L-NAME, L-arginine with L-NAME, SNP with ODQ and L-arginine with ODQ on toluene-induced locomotor hyperactivity were also determined. RESULTS. The locomotor hyperactivity induced by toluene was significantly inhibited by SNP and L-arginine, but enhanced by L-NAME and ODQ. SNP and L-arginine completely reversed the combined effects of L-NAME and toluene to a basal level and abolished the enhancing effects of ODQ.

CONCLUSIONS

The results suggested that NO/cGMP-dependent mechanism might be involved in toluene-induced locomotor activity in rats.

Modulation of the locomotor activating effects of the noncompetitive NMDA receptor antagonist MK801 by dopamine D2/3 receptor agonists in mice

The noncompetitive NMDA receptor antagonist MK801 (dizocilpine) produces behavioral stimulation mediated, in part, through indirect activation of the dopamine (DA) system. Previous reports indicate that D2/3 agonists inhibit MK801-induced stereotypies; however, it is unclear if these agonists also attenuate MK801-induced locomotion. As such, the ability of the D2/3 agonists, quinelorane and quinpirole, and the partial D3 agonist, BP897, to attenuate the locomotor activating effects of MK801 was examined in mice. MK801 (0.1-1.0 mg/kg) produced a biphasic effect on total distance traveled with the intermediate dose of 0.3 mg/kg producing the greatest stimulation. The increase in MK801-induced total distance traveled was attenuated by the coadministration of quinelorane and quinpirole at doses that alone had no effect on activity. Similarly, the partial D3 agonist, BP897, blocked the effects of MK801. The D3-preferring antagonist, nafadotride, reversed the attenuation of quinelorane and partially reversed the attenuation of quinpirole. The D2-preferring antagonist, eticlopride, reversed the attenuating effects of quinelorane, but was not effective against quinpirole. Nafadotride and eticlopride were ineffective against the attenuating effects of BP897 on MK801-induced locomotion. Because BP897 is a partial agonist it was tested against quinelorane/MK801 and quinpirole/MK801 combinations. BP897 reversed the attenuating effects of quinelorane, but not those of quinpirole on MK801's effects. These results demonstrate that the DA system, through D2/3 receptor activation, modulates the locomotor activating effects produced by noncompetitive NMDA receptor blockade.

Propofol-induced anesthesia in mice is mediated by gamma-aminobutyric acid-A and excitatory amino acid receptors

To elucidate the role of gamma-aminobutyric acid (GABA)(A) receptor complex and excitatory amino acid receptors (N-methyl-D-aspartate [NMDA] and non-NMDA receptors) in propofol-induced anesthesia, we examined behaviorally the effects of GABAergic and glutamatergic drugs on propofol anesthesia in mice. All drugs were administered intraperitoneally. General anesthetic potencies were evaluated using a righting reflex assay. The GABA(A) receptor agonist muscimol potentiated propofol (140 mg/kg; 50% effective dose for loss of righting reflex) induced anesthesia. Similarly, the benzodiazepine receptor agonist diazepam and the NMDA receptor antagonist MK-801 augmented propofol anesthesia, but the non-NMDA receptor antagonist CNQX did not. In contrast, the GABA(A) receptor antagonist bicuculline antagonized propofol (200 mg/kg; 95% effective dose for loss of righting reflex) induced anesthesia. However, neither the benzodiazepine receptor antagonist flumazenil, the GABA synthesis inhibitor L-allylglycine, nor the NMDA receptor agonist NMDA reversed propofol anesthesia. Conversely, the non-NMDA receptor agonist kainate enhanced propofol anesthesia. These results suggest that propofol-induced anesthesia is mediated, at least in part, by both GABA(A) and excitatory amino acid receptors.

IMPLICATIONS

We examined behaviorally the effects of GABAergic and glutamatergic drugs on propofol-induced anesthesia in mice. The results suggest that propofol anesthesia is mediated, at least in part, by both GABA(A) and excitatory amino acid receptors.

Effects of prior apparatus experience and novelty of testing environment on locomotor activity following MK-801

Robust increases in locomotor activity are observed following administration of dizocilpine maleate (MK-801). The present study investigated the effects of prior apparatus experience and manipulation of the testing environment on locomotor activity following peripheral MK-801. Gerbils were given zero or nine sessions of apparatus exposure prior to testing with MK-801 (0.1 mg/kg ip) or saline. Sessions were 10 min in duration and separated by 24 h. As previously reported, naive animals treated with MK-801 were significantly more active relative to controls. Exposure to the apparatus for nine sessions resulted in a significant reduction in MK-801-induced activity, but did not alter the activity levels of control animals. To evaluate the effect of changes to the testing environment, animals previously evaluated in the familiar condition were retested in the identical apparatus relocated to a novel experimental room. MK-801-treated animals exhibited a significant increase in activity when tested in this novel environment while the locomotor activity of control gerbils was not significantly altered. The results illustrate the importance of repetitive testing and environmental changes as moderating variables in studies that evaluate locomotor activity. These data also indicate that the effects of MK-801 on activity are sensitive to prior experience with the apparatus and the novelty of the testing environment.