Effect of intermittent subchronic MK-801 administration on dopamine synthesis capacity and responsiveness in the prefrontal cortex.

AIM: The therapeutic potential of N-methyl-D-aspartate glutamate receptor (NMDAR) antagonists, particularly ketamine, in mood disorders, is linked to their modulation of dopamine dynamics in the medial prefrontal cortex (mPFC). However, conflicting effects of distinct NMDAR antagonists, like ketamine and phencyclidine, on mPFC dopamine levels stem from variances in their receptor affinity profiles. This study investigates the impact of intermittent subchronic administration of an NMDAR antagonist on dopamine synthesis capacity and responsiveness within the mPFC, focusing on Dizocilpine (MK-801), a highly selective NMDAR antagonist. METHODS: In vivo microdialysis and high-performance liquid chromatography assessed extracellular dopamine levels in the mPFC following subchronic MK-801 treatment. Locomotor activity was measured using a computed video tracking system. RESULTS: Intermittent subchronic MK-801 administration, followed by a 24-h withdrawal, preserved both dopamine synthesis capacity and responsiveness to MK-801 challenge in the mPFC. However, altered locomotor activity was observed, deviating from previous findings indicating impaired dopamine synthesis and responsiveness in the mPFC with twice-daily subchronic NMDAR antagonist treatment. CONCLUSION: These findings offer crucial biochemical insights into the diverse impacts of NMDAR antagonists on dopamine dynamics and the distinct therapeutic mechanisms associated with ketamine in depression treatment. However, further investigation is imperative to pinpoint potential inconsistencies stemming from variances in drug type, dosage, or administration frequency.

Selective impairment of timing in a NMDA hypofunction animal model of psychosis.

Schizophrenia is severe neuropsychiatric disease, which is commonly accompanied not only by positive or negative symptoms, but also by cognitive impairment. To study neuronal mechanisms underlying cognitive distortions and mechanisms underlying schizophrenia, animal pharmacological models of cognitive symptoms are commonly used. Between various cognitive impairments in schizophrenia patients, disturbed time perception has often been reported. Here, we examined temporal and spatial cognition in a modified Carousel maze task in the animal model of schizophrenia induced by non-competitive NMDA-receptor antagonists MK-801. Male Long-Evans rats (n = 18) first learned to avoid the aversive sector on a rotating arena in both dark and light intervals. We verified that during dark, rats used temporal cues, while during light they relied predominantly on spatial cues. We demonstrated that the timing strategy depends on the stable rotation speed of the arena and on the repositioning clues such as aversive stimuli. During testing (both in light and dark intervals), half of the rats received MK-801 and the control half received saline solution. We observed dose-dependent disruptions of both temporal and spatial cognition. Namely, both doses of MK-801 (0.1 and 0.12 mg/kg) significantly impaired timing strategy in the dark and increased locomotor activity. MK-801 dose 0.1 mg/kg, but not 0.12, also impaired spatial avoidance strategy in light. We found that the timing strategy is more sensitive to NMDA antagonist MK-801 than the spatial strategy. To conclude, a modified version of the Carousel maze is a useful and sensitive tool for detecting timing impairments in the MK-801 induced rodent model of schizophrenia.

The Impact of the Combined Administration of 1MeTIQ and MK-801 on Cell Viability, Oxidative Stress Markers, and Glutamate Release in the Rat Hippocampus.

MK-801, as an N-methyl-D-aspartate (NMDA) receptor inhibitor, causes elevation in glutamate release, which may lead to an increase in excitotoxicity, oxidative stress and, consequently, cell death. 1-Methyl-1,2,3,4-tetrahydroisoquinoline (1MeTIQ) shows antioxidant activity. The aim of the present study was to evaluate the effect of combined treatment with 1MeTIQ and MK-801 on cell viability, antioxidant enzyme activity, and glutamate release in the rat hippocampus. Cytotoxicity was measured using lactate dehydrogenase leakage assay (LDH) and the methyl tetrazolium (MTT) assay; antioxidant enzyme activity (glutathione peroxidase (GPx), glutathione reductase (GR), superoxide dismutase (SOD), and catalase (CAT)) were measured by ELISA kits. The release of glutamate in the rat hippocampus was measured using in vivo microdialysis methodology. An in vitro study showed that MK-801 induced cell death in a concentration-dependent manner and that 1MeTIQ partially reduced this adverse effect of MK-801. An ex vivo study indicated that MK-801 produced an increase in antioxidant enzyme activity (GPx, GR, and SOD), whereas coadministration of MK-801 and 1MeTIQ restored the activity of these enzymes to the control level. An in vivo microdialysis study demonstrated that combined treatment with both drugs decreased the release of glutamate in the rat hippocampus. The above results revealed that 1MeTIQ shows limited neuroprotective activity under conditions of glutamate-induced neurotoxicity.

MK-801-induced behavioral and dopaminergic responses in the shell part of the nucleus accumbens in adult male rats are disrupted after neonatal blockade of the ventral subiculum.

The present study was conducted in the context of animal modeling of schizophrenia. It investigated in adult male rats, after transient neonatal blockade of the ventral subiculum (VSub), the impact of a very specific non-competitive antagonist of NMDA receptors (MK-801) on locomotor activity and dopaminergic (DAergic) responses in the dorsomedial shell part of the nucleus accumbens (Nacc), a striatal subregion described as the common target region for antipsychotics. The functional neonatal inactivation of the VSub was achieved by local microinjection of tetrodotoxin (TTX) at postnatal day 8 (PND8). Control pups were microinjected with the solvent phosphate buffered saline (PBS). Locomotor responses and DAergic variations in the dorsomedial shell part of the Nacc were measured simultaneously using in vivo voltammetry in awake, freely moving male animals after sc administration of MK-801. The following results were obtained: 1) a dose-dependent increase in locomotor activity in PBS and TTX animals, greater in TTX rats/PBS rats; and 2) divergent DAergic responses for PBS and TTX animals. A decrease in DA levels with a return to around basal values was observed in PBS animals. An increase in DA levels was obtained in TTX animals. The present data suggest that neonatal blockade of the VSub results in disruption in NMDA glutamatergic transmission, causing a disturbance in DA release in the dorsomedial shell in adults male rats. In the context of animal modeling of schizophrenia using the same approach it would be interesting to investigate possible changes in postsynaptic NMDA receptors-related proteins in the dorsomedial shell region in the Nacc.

N-Methyl-D-aspartate Glutamate Receptor Modulates Cardiovascular and Neuroendocrine Responses Evoked by Hemorrhagic Shock in Rats.

Here, we report the participation of N-methyl-D-aspartate (NMDA) glutamate receptor in the mediation of cardiovascular and circulating vasopressin responses evoked by a hemorrhagic stimulus. In addition, once NMDA receptor activation is a prominent mechanism involved in nitric oxide (NO) synthesis in the brain, we investigated whether control of hemorrhagic shock by NMDA glutamate receptor was followed by changes in NO synthesis in brain supramedullary structures involved in cardiovascular and neuroendocrine control. Thus, we observed that intraperitoneal administration of the selective NMDA glutamate receptor antagonist dizocilpine maleate (MK801, 0.3 mg/kg) delayed and reduced the magnitude of hemorrhage-induced hypotension. Besides, hemorrhage induced a tachycardia response in the posthemorrhage period (i.e., recovery period) in control animals, and systemic treatment with MK801 caused a bradycardia response during hemorrhagic shock. Hemorrhagic stimulus increased plasma vasopressin levels during the recovery period and NMDA receptor antagonism increased concentration of this hormone during both the hemorrhage and postbleeding periods in relation to control animals. Moreover, hemorrhagic shock caused a decrease in NOx levels in the paraventricular nucleus of the hypothalamus (PVN), amygdala, bed nucleus of the stria terminalis (BNST), and ventral periaqueductal gray matter (vPAG). Nevertheless, treatment with MK801 did not affect these effects. Taken together, these results indicate that the NMDA glutamate receptor is involved in the hemorrhagic shock by inhibiting circulating vasopressin release. Our data also suggest a role of the NMDA receptor in tachycardia, but not in the decreased NO synthesis in the brain evoked by hemorrhage.

Adult and adolescent C57BL/6J and DBA/2J mice are differentially susceptible to fear learning deficits after acute ethanol or MK-801 treatment.

Ethanol and other drugs of abuse disrupt learning and memory processes, creating problems associated with drug use and addiction. Understanding individual factors that determine susceptibility to drug-induced cognitive deficits, such as genetic background, age, and sex, is important for prevention and treatment. Comparison of adolescent and adult mice of both sexes across inbred mouse strains can reveal age, sex, and genetic contributions to phenotypes. We treated adolescent and adult, male and female, C57BL/6J and DBA/2J inbred mice with ethanol (1 g/kg or 1.5 g/kg) or MK-801 (0.05 mg/kg or 0.1 mg/kg), an NMDA receptor antagonist, prior to fear conditioning training. Contextual and cued fear retention were tested one day and eight or nine days after training. After ethanol exposure, adult C57BL/6J mice experienced greater deficits in contextual learning than adult DBA/2J mice. C57BL/6 J adolescents were less susceptible to ethanol-induced contextual learning disruptions than C57BL/6J adults, and adolescent males of both strains exhibited greater ethanol-induced contextual learning deficits than adolescent females. After MK-801 exposure, adolescent C57BL/6J mice experienced more severe contextual learning deficits than adolescent DBA/2J mice. Both ethanol and MK-801 had greater effects on contextual learning than cued learning. Collectively, we demonstrate that genetic background contributes to contextual and cued learning outcomes after ethanol or MK-801 exposure. Further, we report age-dependent drug sensitivities that are strain-, sex-, and drug-specific, suggesting that age, sex, and genetic background interact to determine contextual and cued learning impairments after ethanol or MK-801 exposure.

Cannabidiol prevents disruptions in sensorimotor gating induced by psychotomimetic drugs that last for 24-h with probable involvement of epigenetic changes in the ventral striatum.

Cannabidiol (CBD), a major non-psychotomimetic component of the Cannabis sativa plant, shows therapeutic potential in several psychiatric disorders, including schizophrenia. The molecular mechanisms underlying the antipsychotic-like effects of CBD are not fully understood. Schizophrenia and antipsychotic treatment can modulate DNA methylation in the blood and brain, resulting in altered expression of diverse genes associated with this complex disorder. However, to date, the possible involvement of DNA methylation in the antipsychotic-like effects of CBD has not been investigated. Therefore, this study aimed at evaluating in mice submitted to the prepulse inhibition (PPI) model: i) the effects of a single injection of CBD or clozapine followed by AMPH or MK-801 on PPI and global DNA methylation changes in the ventral striatum and prefrontal cortex (PFC); and ii). if the acute antipsychotic-like effects of CBD would last for 24-h. AMPH (5 mg/kg) and MK-801 (0.5 mg/kg) impaired PPI. CBD (30 and 60 mg/kg), similar to clozapine (5 mg/kg), attenuated AMPH- and MK801-induced PPI disruption. AMPH, but not MK-801, increased global DNA methylation in the ventral striatum, an effect prevented by CBD. CBD and clozapine increased, by themselves, DNA methylation in the prefrontal cortex. The acute effects of CBD (30 or 60 mg/kg) on the PPI impairment induced by AMPH or MK-801 was also detectable 24 h later. Altogether, the results show that CBD induces acute antipsychotic-like effects that last for 24-h. It also modulates DNA methylation in the ventral striatum, suggesting a new potential mechanism for its antipsychotic-like effects.

Pentadecapeptide BPC 157 counteracts L-NAME-induced catalepsy. BPC 157, L-NAME, L-arginine, NO-relation, in the suited rat acute and chronic models resembling 'positive-like' symptoms of schizophrenia.

In the suited rat-models, we focused on the stable pentadecapeptide BPC 157, L-NAME, NOS-inhibitor, and L-arginine, NOS-substrate, relation, the effect on schizophrenia-like symptoms. Medication (mg/kg intraperitoneally) was L-NAME (5), L-arginine (100), BPC 157 (0.01), given alone and/or together, at 5 min before the challenge for the acutely disturbed motor activity (dopamine-indirect/direct agonists (amphetamine (3.0), apomorphine (2.5)), NMDA-receptor non-competitive antagonist (MK-801 (0.2)), or catalepsy, (dopamine-receptor antagonist haloperidol (2.0)). Alternatively, BPC 157 10 µg/kg was given immediately after L-NAME 40 mg/kg intraperitoneally. To induce or prevent sensitization, we used chronic methamphetamine administration, alternating 3 days during the first 3 weeks, and challenge after next 4 weeks, and described medication (L-NAME, L-arginine, BPC 157) at 5 min before the methamphetamine at the second and third week. Given alone, BPC 157 or L-arginine counteracted the amphetamine-, apomorphine-, and MK-801-induced effect, haloperidol-induced catalepsy and chronic methamphetamine-induced sensitization. L-NAME did not affect the apomorphine-, and MK-801-induced effects, haloperidol-induced catalepsy and chronic methamphetamine-induced sensitization, but counteracted the acute amphetamine-induced effect. In combinations (L-NAME + L-arginine), as NO-specific counteraction, L-NAME counteracts L-arginine-induced counteractions in the apomorphine-, MK-801-, haloperidol- and methamphetamine-rats, but not in amphetamine-rats. Unlike L-arginine, BPC 157 maintains its counteracting effect in the presence of the NOS-blockade (L-NAME + BPC 157) or NO-system-over-stimulation (L-arginine + BPC 157). Illustrating the BPC 157-L-arginine relationships, BPC 157 restored the antagonization (L-NAME + L-arginine + BPC 157) when it had been abolished by the co-administration of L-NAME with L-arginine (L-NAME + L-arginine). Finally, BPC 157 directly inhibits the L-NAME high dose-induced catalepsy. Further studies would determine precise BPC 157/dopamine/glutamate/NO-system relationships and clinical application.

Effects of haloperidol inhalation on MK-801- and memantine-induced locomotion in mice.

The administration of therapeutic agents is difficult in many patients, such as patients with post-operative delirium or dementia or patients with schizophrenia, who are upset in an emergency room. Therefore, the development of a new method for administering therapeutic agents to the central nervous system is desired. In this study, we investigated if inhalation was an effective route of administration for haloperidol, a commonly used, strong antipsychotic. Dizocilpine, also known as MK-801, is a noncompetitive antagonist of the N-methyl-D-aspartate receptor. MK-801 or memantine-induced motor hyperactivity was evaluated in mice following either intraperitoneal injection or inhalation of haloperidol or the histamine neuroactivator betahistine. Pretreatment with haloperidol inhalation inhibited the MK-801-induced or memantine-induced increase in locomotor activity. This effect was similar to that of the intraperitoneal administration of haloperidol. However, pretreatment with inhaled betahistine or the intraperitoneal administration of betahistine did not suppress the MK-801-induced or memantine-induced increase in locomotor activity. Thus, haloperidol when inhaled acts on the central nervous system of mice and suppresses the MK-801-induced increase in mouse locomotor activity. Our findings suggest that inhalation may be a novel method for administering haloperidol. ABBREVIATIONS: ANOVA: analysis of variance.

Aberrant Auditory Steady-State Response of Awake Mice After Single Application of the NMDA Receptor Antagonist MK-801 Into the Medial Geniculate Body.

BACKGROUND: Systemic administration of noncompetitive N-methyl-D-aspartate receptor (NMDAR) antagonists such as MK-801 is widely used to model psychosis of schizophrenia (SZ). Acute systemic MK-801 in rodents caused an increase of the auditory steady-state responses (ASSRs), the oscillatory neural responses to periodic auditory stimulation, while most studies in patients with SZ reported a decrease of ASSRs. This inconsistency may be attributable to the comprehensive effects of systemic administration of MK-801. Here, we examined how the ASSR is affected by selectively blocking NMDAR in the thalamus. METHODS: We implanted multiple electrodes in the auditory cortex (AC) and prefrontal cortex to simultaneously record the local field potential and spike activity (SA) of multiple sites from awake mice. Click-trains at a 40-Hz repetition rate were used to evoke the ASSR. We compared the mean trial power and phase-locking factor and the firing rate of SA before and after microinjection of MK-801 (1.5 µg) into the medial geniculate body (MGB). RESULTS: We found that both the AC and prefrontal cortex showed a transient local field potential response at the onset of click-train stimulus, which was less affected by the application of MK-801 in the MGB. Following the onset response, the AC also showed a response continuing throughout the stimulus period, corresponding to the ASSR, which was suppressed by the application of MK-801. CONCLUSION: Our data suggest that the MGB is one of the generators of ASSR, and NMDAR hypofunction in the thalamocortical projection may account for the ASSR deficits in SZ.

Interaction between Mesocortical and Mesothalamic Catecholaminergic Transmissions Associated with NMDA Receptor in the Locus Coeruleus.

Noncompetitive N-methyl-D-aspartate/glutamate receptor (NMDAR) antagonists contribute to the pathophysiology of schizophrenia and mood disorders but improve monoaminergic antidepressant-resistant mood disorder and suicidal ideation. The mechanisms of the double-edged sword clinical action of NMDAR antagonists remained to be clarified. The present study determined the interaction between the NMDAR antagonist (MK801), α1 adrenoceptor antagonist (prazosin), and α2A adrenoceptor agonist (guanfacine) on mesocortical and mesothalamic catecholaminergic transmission, and thalamocortical glutamatergic transmission using multiprobe microdialysis. The inhibition of NMDAR in the locus coeruleus (LC) by local MK801 administration enhanced both the mesocortical noradrenergic and catecholaminergic coreleasing (norepinephrine and dopamine) transmissions. The mesothalamic noradrenergic transmission was also enhanced by local MK801 administration in the LC. These mesocortical and mesothalamic transmissions were activated by intra-LC disinhibition of transmission of γ-aminobutyric acid (GABA) via NMDAR inhibition. Contrastingly, activated mesothalamic noradrenergic transmission by MK801 enhanced intrathalamic GABAergic inhibition via the α1 adrenoceptor, resulting in the suppression of thalamocortical glutamatergic transmission. The thalamocortical glutamatergic terminal stimulated the presynaptically mesocortical catecholaminergic coreleasing terminal in the superficial cortical layers, but did not have contact with the mesocortical selective noradrenergic terminal (which projected terminals to deeper cortical layers). Furthermore, the α2A adrenoceptor suppressed the mesocortical and mesothalamic noradrenergic transmissions somatodendritically in the LC and presynaptically/somatodendritically in the reticular thalamic nucleus (RTN). These discrepancies between the noradrenergic and catecholaminergic transmissions in the mesocortical and mesothalamic pathways probably constitute the double-edged sword clinical action of noncompetitive NMDAR antagonists.

Prolonged Use of NMDAR Antagonist Develops Analgesic Tolerance in Neuropathic Pain via Nitric Oxide Reduction-Induced GABAergic Disinhibition.

Neuropathic pain is usually persistent due to maladaptive neuroplasticity-induced central sensitization and, therefore, necessitates long-term treatment. N-methyl-D-aspartate receptor (NMDAR)-mediated hypersensitivity in the spinal dorsal horn represents key mechanisms of central sensitization. Short-term use of NMDAR antagonists produces antinociceptive efficacy in animal pain models and in clinical practice by reducing central sensitization. However, how prolonged use of NMDAR antagonists affects central sensitization remains unknown. Surprisingly, we find that prolonged blockage of NMDARs does not prevent but aggravate nerve injury-induced central sensitization and produce analgesic tolerance, mainly due to reduced synaptic inhibition. The disinhibition that results from the continuous decrease in the production of nitric oxide from neuronal nitric oxide synthase, downstream signal of NMDARs, leads to the reduction of GABAergic inhibitory synaptic transmission by upregulating brain-derived neurotrophic factor expression and inhibiting the expression and function of potassium-chloride cotransporter. Together, our findings suggest that chronic blockage of NMDARs develops analgesic tolerance through the neuronal nitric oxide synthase-brain-derived neurotrophic factor-potassium-chloride cotransporter pathway. Thus, preventing the GABAergic disinhibition induced by nitric oxide reduction may be necessary for the long-term maintenance of the analgesic effect of NMDAR antagonists.

Memory reconsolidation impairments in sign-tracking to an audiovisual compound stimulus.

Studies of memory reconsolidation of pavlovian memories have typically employed unimodal conditioned stimuli, despite the use of multimodal compound stimuli in other settings. Here we studied sign-tracking behaviour to a compound audiovisual stimulus. First, we observed not unexpectedly that sign-tracking was poorer to the audiovisual compound than to unimodal visual stimuli. Then, we showed that, depending on the parameters of compound stimulus re-exposure at memory reactivation, systemic MK-801 treatment either impaired extinction to improve sign-tracking at test, or disrupted reconsolidation to impair test behaviour. When memory reactivation consisted of re-exposure to only the auditory component of the compound stimulus, we observed sign-tracking impairments following MK-801 treatment, but only under certain test conditions. This was in contrast to the consistent impairment following reactivation with the full audiovisual compound. Moreover, the parameters of auditory stimulus presentation to enable MK-801-induced impairment at test varied depending on whether the stimulus was presented within or outside the training context. These findings suggest that behaviour under the control of appetitive pavlovian compound stimuli can be modulated by targeting both extinction and reconsolidation, and that it is not necessary to re-expose to the full compound stimulus in order to achieve a degree of modulation of behaviour.

The facilitating effect of MK-801 on inhibitory avoidance memory via mTOR signaling in the mouse hippocampus.

Despite the widespread belief that MK-801 induces memory deficits associated with dementia and schizophrenia in animal models, data regarding the impairing effect of MK-801 on aversive memory have been inconclusive. In this study, we investigated the effect of MK-801 on multiple memory stages of the inhibitory avoidance task, as well as its underlying signaling mechanism in the mouse hippocampus. We successfully replicated a previous finding suggesting that systemic injection of MK-801 impaired memory acquisition, but we observed that an intrahippocampal infusion of MK-801 facilitated the same memory process. We also found that both systemic and intrahippocampal administration of MK-801 facilitated memory consolidation and memory retrieval of the inhibitory avoidance task. We demonstrated that MK-801-induced increases in shock sensitivity and locomotor activity in the pre-training regimen confounded the detrimental effect of MK-801 on memory acquisition, thereby reconciling the inconsistent results in previous studies. In addition, the memory-facilitating effect of MK-801 was found to be dependent on drug dose and shock intensity. We next showed that MK-801 induced a fast-onset increase in the extent of mammalian target of rapamycin (mTOR) phosphorylation in the hippocampus. Finally, we observed that rapamycin, an mTOR inhibitor, blocked both the MK-801-induced increases in phosphorylated mTOR and the facilitating effect of MK-801 on memory consolidation. These results indicate that hippocampal mTOR signaling mediates the facilitating effect of MK-801 on memory consolidation of the inhibitory avoidance task. These findings further imply that MK-801 indeed functions as a memory enhancer and that mTOR signaling serves as a therapeutic target for memory disorders.

Development, characterization, comparative pharmacokinetic and pharmacodynamic studies of iloperidone solid SMEDDS and liquisolid compact.

Iloperidone (ILO) is an anti-psychotic, used in schizophrenia. It has low bioavailability (36%) due to low solubility and first pass effect. Oral solid self microemulsifying drug delivery system (SMEDDS) and liquisolid compact (LSC) of ILO were developed. The hypothesis is to test in vivo performance (PK and PD effects) of these delivery systems, as both systems improve dissolution. Based on solubility Capmul MCM, Labrafac WL 1349 were selected as oils, Lauroglycol 90 and PEG 600 were selected as surfactant and cosurfactant. Syloid XDP was optimized for adsorption of liquid SMEDDS. Syloid XDP and Aerosil 200 were optimized as carrier and coating material in the ratio of 15:1 w/w for liquisolid formulation. SEM and PXRD studies indicated no specific crystallinity due to bulkiness in both formulations, which showed similar flow and release behavior. Pharmacokinetic studies were performed for ILO Coarse suspension (CS), Tablet suspension (TS), optimized solid SMEDDS (A1X) and liquisolid compact (S3) in wistar rats. About 3.80 and 2.19-fold improvements in relative bioavailabilty were found for A1X and S3, respectively, when compared to CS. In comparison to TS, 2.61 and 1.51 fold improvements in bioavailability were found for A1X and S3, respectively. Further, Pharmacodynamic activity was studied by reversal of MK-801 induced hyperlocomotion in rats. A1X and S3 formulations showed maximum reversal after 15 min when compared to CS and found to have similar performance. Thus, in comparison to S3, A1X showed significant difference in pharmacokinetic effects but similar pharmacodynamic effects.

Hypothermia but not NMDA receptor antagonism protects against stroke induced by distal middle cerebral arterial occlusion in mice.

Excitotoxicity mediated by the N-methyl-D-aspartate receptor (NMDAR) is believed to be a primary mechanism of neuronal injury following stroke. Thus, many drugs and therapeutic peptides were developed to inhibit either the NMDAR at the cell surface or its downstream intracellular death-signaling cascades. Nevertheless, the majority of focal ischemia studies concerning NMDAR antagonism were performed using the intraluminal suture-induced middle cerebral arterial occlusion (MCAO) model, which produces a large cortical and subcortical infarct leading to hypothalamic damage and fever in experimental animals. Here, we investigated whether NMDAR antagonism by drugs and therapeutic peptides was neuroprotective in a mouse model of distal MCAO (dMCAO), which produces a small cortical infarct sparing the hypothalamus and other subcortical structures. For establishment of this model, mice were subjected to dMCAO under normothermic conditions or body-temperature manipulations, and in the former case, their brains were collected at 3-72 h post-ischemia to follow the infarct development. These mice developed cortical infarction 6 h post-ischemia, which matured by 24-48 h post-ischemia. Consistent with the hypothesis that the delayed infarction in this model can be alleviated by neuroprotective interventions, hypothermia strongly protected the mouse brain against cerebral infarction in this model. To evaluate the therapeutic efficacy of NMDAR antagonism in this model, we treated the mice with MK801, Tat-NR2B9c, and L-JNKI-1 at doses that were neuroprotective in the MCAO model, and 30 min later, they were subjected to 120 min of dMCAO either in the awake state or under anesthesia with normothermic controls. Nevertheless, NMDAR antagonism, despite exerting pharmacological effects on mouse behavior, repeatedly failed to show neuroprotection against cerebral infarction in this model. The lack of efficacy of these treatments is reminiscent of the recurrent failure of NMDAR antagonism in clinical trials. While our data do not exclude the possibility that these treatments could be effective at a different dose or treatment regimen, they emphasize the need to test drug efficacy in different stroke models before optimal doses and treatment regimens can be selected for clinical trials.

The D2-family receptor agonist bromocriptine but, not nicotine, reverses NMDA receptor antagonist-induced working memory deficits in the radial arm maze in mice.

Hypofunction of the NMDA receptor (NMDAr) may underlie cognitive deficits associated with schizophrenia and other psychiatric conditions including working memory (WM) impairments. Given that these deficits link closely to functional outcome, treatments remediating such deficits require identification. NMDAr hypofunction can be modeled via treatment with the antagonist MK-801. Hence, the present study determined whether cholinergic or dopaminergic agonists attenuate MK-801-induced WM deficits in mice. WM was assessed in male C57BL/6 mice trained on an automated 12-arm radial arm maze (RAM) paradigm, wherein rewards were delivered after the first but, not after subsequent entries into WM arms (8/12) and never delivered for entries into reference memory (RM) arms (4/12). Mice were then treated with MK-801 (vehicle or 0.3 mg/kg) and nicotine (vehicle, 0.03 or 0.30 mg/kg) in a cross-over design. After a 2-week washout, mice were then retested with MK-801 and the dopamine D2-family receptor agonist bromocriptine (vehicle, 3 or 10 mg/kg). In both experiments, MK-801 reduced WM span and increased RM and WM error rates. Nicotine did not attenuate these deficits. In contrast, a bromocriptine/MK-801 interaction was observed on WM error rate, where bromocriptine attenuated MK-801 induced deficits without affecting MK-801-induced RM errors. Additionally, bromocriptine produced the main effect of slowing latency to collect rewards. Hence, while NMDAr hypofunction-induced deficits in WM was unaffected by nicotine, it was remediated by treatment with the dopamine D2-family agonist bromocriptine. Future studies should determine whether selective activation of dopamine D2, D3, or D4 receptors remediate this NMDAr hypofunction-induced WM deficit.

Acquisition and expression of sucrose conditioned flavor preferences following dopamine D1, opioid and NMDA receptor antagonism in C57BL/6 mice.

The study of inbred mouse strains is a useful animal model to assess differences in ingestive behavior responses, including conditioned flavor preferences (CFP). C57BL/6, BALB/c and SWR inbred mice display differential sensitivity to dopamine (DA) D1, opioid and muscarinic cholinergic receptor antagonism of sucrose or saccharin intake as well as to muscarinic cholinergic antagonism of acquisition (learning) of sucrose-CFP. Given that DA D1, opioid and N-methyl-D-aspartate (NMDA) receptor antagonists differentially alter sucrose-CFP in BALB/c and SWR inbred mice, the present study examined whether systemic administration of naltrexone, SCH23390 or MK-801 altered acquisition and expression of sucrose-CFP in C57BL/6 mice. In acquisition experiments, male food-restricted C57BL/6 mice were treated with vehicle, naltrexone (1, 5 mg/kg), SCH23390 (50, 200 nmol/kg) or MK-801 (100, 200 µg/kg) 30 min prior to each of ten daily sessions in which they alternately consumed a flavored (CS+, e.g. cherry) 16% sucrose solution and a differently-flavored (CS-, e.g. grape) 0.05% saccharin solution followed by six two-bottle CS choice tests mixed in 0.2% saccharin without injections. SCH23390 and MK-801, but not naltrexone eliminated sucrose-CFP acquisition in food-restricted C57BL/6 mice. In expression experiments, food-restricted C57BL/6 mice underwent the ten training sessions without injections followed by two-bottle CS choice tests 30 min following vehicle, naltrexone (1, 5 mg/kg), SCH23390 (200, 800 nmol/kg) or MK-801 (100, 200 µg/kg). SCH23390 more effectively reduced the magnitude of sucrose-CFP expression than naltrexone or MK-801 in food-restricted C57BL/6 mice. Thus, dopamine D1 and NMDA receptor signaling is essential for learning of sucrose-CFP in C57BL/6 mice.

Subchronic MK-801 treatment during adolescence induces long-term, not permanent, excitatory-inhibitory imbalance in the rat hippocampus.

Adolescence is a critical neurodevelopmental period for both excitatory and inhibitory (E/I) neurotransmission and often witnesses the typical onsets of schizophrenia. One possibility is that disruptions in adolescent neurodevelopmental processes may produce schizophrenia-like behavioral and neurobiological abnormalities. We previously reported that subchronic treatment of adolescent animals with the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 induced cognitive deficits and reduced interneuron densities in rat medial prefrontal cortex, and these changes persisted one week after MK-801 exposure. However, it remains unclear how this treatment may affect E/I balance in hippocampus, which has long been associated with the pathophysiology of schizophrenia. Here, we examined hippocampal E/I biomarkers in adolescent rats treated with MK-801 (0.2 mg/kg, i.p., 14 days) and found increases in the ratio of the expression levels of vesicular glutamate transporter-1 (VGluT1) and vesicular gamma-aminobutyric acid (GABA) transporter (VGAT) 24 h and 7 days after MK-801 exposure. Interestingly, the increased VGluT1/VGAT ratio at the two time points was driven by upregulated VGluT1 expression and downregulated VGAT expression, respectively. The decrease in VGAT expression persisted 14 days after MK-801 exposure and recovered two weeks later. No alterations in hippocampal interneuron densities were observed. Behaviorally, the treatment decreased prepulse inhibition at 24 h but not 14 days, after MK-801 exposure. Taken together, these results demonstrate that subchronic NMDA receptor blockade during adolescence induces long-term, but not permanent, E/I imbalance in the rat hippocampus, which could be attributed to the dysregulation of glutamatergic transmission in the short term and of GABAergic transmission in the long term.

Endothelin-1-Induced Ischemic Damage and Functional Impairment Is Mediated Primarily by NR2B-Containing NMDA Receptors.

Ischemic stroke accounts for 70-80% of stroke cases worldwide and survivors are frequently left with compromising sensorimotor deficits localized to one or more body regions. Most animal models of stroke involve transient or permanent occlusion of one or more major vessels such as the middle cerebral artery and are characterized by widespread damage to cortical and subcortical structures that result in deficits that can confound studies of neuroprotection and neurorehabilitation. Localized microinjections of the vasoconstricting peptide endothelin-1 (ET-1) into specific brain regions are becoming increasingly popular for such studies, but the pharmacology of endothelin-induced ischemic damage is poorly understood. To test the hypothesis that NMDA receptors, and particularly those containing the NR2B subunit, are involved in ET-1-mediated excitotoxicity and functional impairment, male CD1 rats (N = 32) were pre-treated with either the non-competitive NMDA antagonist MK-801 or the NR2B-selective antagonist Ro25-6981 (or vehicle) prior to unilateral microinjections of endothelin-1 into the somatosensory cortex and striatum. Rats were then tested using 4 established tests of sensory and/or motor function over 14 days. Lesion volumes were quantified post-mortem using standard histology and image analysis. Results confirmed reproducible lesions and significant deficits in all tests in vehicle-treated rats that were significantly reduced in both drug groups but were not different between drugs, providing evidence that endothelin-induced ischemic damage is mediated almost exclusively by NR2B-containing NMDA receptors.