N-methyl-D-aspartate and dopamine receptor involvement in the modulation of locomotor activity and memory processes

Gm527 deficiency in dentate gyrus improves memory through upregulating dopamine D1 receptor pathway

AIMS

Dopamine D1 receptor (D1R) hypofunction is associated with negative and cognitive symptoms in schizophrenia; therefore, the mechanism of D1R function modulation needs further investigation. Gm527 is the rodent homologous of the schizophrenia-related gene C14orf28, encoding a predicated D1R-interacting protein. However, the role of Gm527-D1R interaction in schizophrenia needs to be clarified.

METHODS

Gm527-floxed mice were generated and crossed with D1-Cre mice (D1:Gm527-/-) to knockout Gm527 in D1R-positive neurons. Then behavioral tests were performed to explore the schizophrenia-related phenotypes. Immunofluorescence, fluorescence in situ hybridization, electrophysiological recording, quantitative real-time PCR, and western blotting were conducted to investigate the mechanisms.

RESULTS

Working memory, long-term memories, and adult neurogenesis in the DG were enhanced in D1:Gm527-/- mice. LTP was also increased in the DG in D1:Gm527-/- mice, resulting from the Gm527 knockout-induced D1R expression enhancement on the plasma membrane and subsequently cAMP signaling and NMDA receptor pathways activation. The requirement of Gm527 knockout in the DG was confirmed by reversing Gm527 expression or knockdown Gm527 in the DG D1R-positive neurons through AAV-CAG-FLEX-Gm527-GFP or AAV-CMV-FLEX-EGFP-Gm527-RNAi injection.

CONCLUSIONS

The DG Gm527 knockout induces D1R hyperfunction in improving schizophrenia cognitive symptoms.

Parvalbumin Interneuron Activation-Dependent Adult Hippocampal Neurogenesis Is Required for Treadmill Running to Reverse Schizophrenia-Like Phenotypes

Physical exercise can alleviate some of the schizophrenia symptoms in patients, the mechanisms, however, are still unclear. To investigate whether the GABAergic interneuron involved in the therapeutic effect of treadmill running on schizophrenia, the parvalbumin (PV)-positive GABAergic interneurons in the dentate gyrus (DG) was specifically activated or abolished and the effects were evaluated. In the MK801-induced schizophrenia-like animal model, we found:(1) Treadmill running rescued the schizophrenia-related behavioral phenotypes, promoted the adult hippocampal neurogenesis, and increased the dendrite number and complexity of newborn neurons. (2) Treadmill running increased the number of PV-positive interneurons in the DG; genetic ablation of these interneurons reduced adult neurogenesis and abolished the effect of treadmill running on the schizophrenia-related behaviors. Consistently, chemogenetic activation of these interneurons improved neurogenesis and alleviated the schizophrenia-related behaviors. These results suggest a pivotal role of PV-positive interneuron-mediated adult neurogenesis in exercise. (3) However, schizophrenia-related behavioral phenotypes and adult neurogenesis in the DG could still be reversed by exercise after specifically knocking out the schizophrenia-related gene ErbB4 in PV interneurons, as a means to reduce their GABA release. These results suggest that activation of PV interneurons in the DG is sufficient for treadmill running to reverse schizophrenia-like phenotypes.

The effect of CA1 dopaminergic system on amnesia induced by harmane in mice

In the present study, the effects of bilateral injections of dopaminergic drugs into the hippocampal CA1 regions (intra-CA1) on harmane-induced amnesia were examined in mice. We used a single-trial step-down inhibitory avoidance task for the assessment of memory acquisition in adult male mice. Our data indicated that pre-training intra-peritoneal (i.p.) administration of harmane (12 mg/kg) impaired memory acquisition. Moreover, intra-CA1 administration of dopamine D1 receptor agonist, SKF38393 (0.25 µg/mouse), dopamine D1 receptor antagonist, SCH23390 (0.25 µg/mouse), dopamine D2 receptor agonist, quinpirole (0.125 and 0.25 µg/mouse) and dopamine D2 receptor antagonist, sulpiride (0.2 and 0.4 µg/mouse) decreased the learning of a single-trial inhibitory avoidance task. Furthermore, pre-training intra-CA1 injection of sub-threshold doses of SKF38393 (0.0625 µg/mouse) or sulpiride (0.1 µg/mouse) increased pre-training harmane (4 and 8 mg/kg, i.p.)-induced amnesia. On the other hand, pre-training intra-CA1 injection of a sub-threshold dose of SCH23390 (0.0625 µg/mouse) reversed amnesia induced by an effective dose of harmane (12 mg/kg; i.p.). In addition, Pre-training intra-CA1 injection of quinpirole (0.0625 µg/mouse) had no effect on memory impairment induced by harmane. These findings indicate the involvement of CA1 dopaminergic system on harmane-induced impairment of memory acquisition.

Novelty-related behavior of young and adult dopamine transporter knockout rats: Implication for cognitive and emotional phenotypic patterns

Attention deficit hyperactivity disorder (ADHD) is a neuropsychiatric disorder characterized by a developmentally inappropriate, pervasive and persistent pattern of severe inattention, hyperactivity and impulsivity. Despite onset in early childhood, ADHD may continue into adulthood with substantial impairment in social, academic and occupational functioning. A new animal model of this disorder was developed in rats with genetic deletion of the dopamine transporter (DAT) gene (dopamine transporter knockout rats; DAT-KO rats). We analyzed the behavior of DAT-KO rats for a deeper phenotypical characterization of this model. We first tested rats of the 3 genotypes at different ages (preadolescent, adolescent and adult), in a novelty-seeking test using a black/white box (Experiment 1). After that, we tested adult rats in a novelty-preference test using a 3-chamber apparatus with different shapes (Experiment 2). Experiment 1: as evidenced by analysis of time spent in the novel environment, adult DAT heterozygous (DAT-HET) rats show an increased curiosity-driven exploration compared with wild-type (WT) controls while DAT-KO rats did not recognize novelty. The locomotor activity data show a minimal difference between genotypes at adolescent age while the preadolescent and adult DAT-KO rats have significantly increased activity rate compared with WT and DAT-HET subjects. Experiment 2: in this case, due to more clearly evident spatial differences, time spent in novel environment was not significantly different among genotypes. During first 10 minutes, DAT-KO rats showed a decreased hyperactivity, apparently related to curiosity and attention to the new environments. In conclusion, DAT-KO rats may show some inattention while more novelty-seeking traits appear in DAT-HET rats.

Enhanced LTP in aged rats: Detrimental or compensatory?

Age-dependent memory deterioration has been well documented and yet an increase in rat hippocampal LTP upon aging has been reported. This poses the question of whether the enhanced LTP is a cause or an attempt to compensate the memory deficits described in aged rats. Hippocampal slices from young, adult and aged Wistar rats were pre-incubated, with an NMDA receptor (NMDAR) antagonist, memantine (1 μM, 4 h), and hippocampal LTP was evaluated. The results show that memantine significantly decreases the larger LTP magnitude recorded in hippocampal slices from aged rats without compromising LTP recorded in slices from young and adult animals. To unveil the impact of in vivo administration of memantine, different doses (1, 5 and 10 mg/kg/day) or saline vehicle solution were intraperitoneally administered, for 15-20 days, to both young and aged animals. Memantine did not significantly affect neither the place learning of young animals, evaluated by Morris Water Maze, nor LTP recorded from hippocampal slices from the same group of animals. However, memantine (5 and 10 mg/kg/day) significantly decreased the large LTP recorded in hippocampal slices from aged animals. Moreover, aged animals treated with memantine (10 mg/kg/day) showed a significantly compromised place learning when compared to aged control animals. Overall, these results suggest that the larger LTP observed in aged animals is a compensatory phenomenon, rather than pathological. The finding that age-dependent blockade of LTP by a NMDAR antagonist leads to learning deficits, implies that the increased LTP observed upon aging may be playing an important role in the learning process.

Polyamines as Snake Toxins and Their Probable Pharmacological Functions in Envenomation

While decades of research have focused on snake venom proteins, far less attention has been paid to small organic venom constituents. Using mostly pooled samples, we surveyed 31 venoms (six elapid, six viperid, and 19 crotalid) for spermine, spermidine, putrescine, and cadaverine. Most venoms contained all four polyamines, although some in essentially trace quantities. Spermine is a potentially significant component of many viperid and crotalid venoms (≤0.16% by mass, or 7.9 µmol/g); however, it is almost completely absent from elapid venoms assayed. All elapid venoms contained larger molar quantities of putrescine and cadaverine than spermine, but still at levels that are likely to be biologically insignificant. As with venom purines, polyamines impact numerous physiological targets in ways that are consistent with the objectives of prey envenomation, prey immobilization via hypotension and paralysis. Most venoms probably do not contain sufficient quantities of polyamines to induce systemic effects in prey; however, local effects seem probable. A review of the pharmacological literature suggests that spermine could contribute to prey hypotension and paralysis by interacting with N-methyl-d-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, nicotinic and muscarinic acetylcholine receptors, γ-Aminobutyric acid (GABA) receptors, blood platelets, ryanodine receptors, and Ca²⁺-ATPase. It also blocks many types of cation-permeable channels by interacting with negatively charged amino acid residues in the channel mouths. The site of envenomation probably determines which physiological targets assume the greatest importance; however, venom-induced liberation of endogenous, intracellular stores of polyamines could potentially have systemic implications and may contribute significantly to envenomation sequelae.

Manganese-Disrupted Interaction of Dopamine D1 and NMDAR in the Striatum to Injury Learning and Memory Ability of Mice

Manganese (Mn) is widely regarded as a neurotoxic heavy metal that causes learning and memory deficits. Recently, it has been proved that the striatum is related to memory and learning ability. However, no previous study focused on the effect of Mn-induced learning and memory deficits on the striatum. This study aims to investigate the probable interaction of dopamine D1 receptor (DR1) and N-methyl-D-aspartate receptor (NMDAR), two cognition-related receptors in the striatum during Mn exposure. Mice are randomly divided into four groups, including control group, 12.5 mg/kg MnCl₂ group, 25 mg/kg MnCl₂ group, and 50 mg/kg MnCl₂ group. The mice receive intraperitoneal injections of 0, 12.5, 25, and 50 mg/kg MnCl₂ once daily for 2 weeks. Then, learning and memory ability, pathological changes, expression, and interaction of DR1 and NMDAR are determined. It has been found that Mn disrupted spatial learning and memory ability of mice by Morris water maze test and the passive avoidance test. Pathological and ultrastructure were injured. Mn decreased the immunohistochemical activities, protein levels, and messenger RNA (mRNA) expression of DR1, NR1, and NR2A. Mn exposure inhibited interaction between DR1 and NMDAR in striatum by double immunofluorescent staining and co-immunoprecipitation. In conclusion, our study illustrated that Mn caused learning and memory dysfunction via injury of striatum and inhibition of interaction between DR1 and NMDAR in striatum.

Harmaline-induced amnesia: Possible role of the amygdala dopaminergic system

In this study, we examined the effect of bilateral intra-basolateral amygdala (intra-BLA) microinjections of dopamine receptor agents on amnesia induced by a β-carboline alkaloid, harmaline in mice. We used a step-down method to assess memory and then, hole-board method to assess exploratory behaviors. The results showed that pre-training intra-BLA injections of dopamine D1 receptor antagonist and agonist (SCH23390 (0.5μg/mouse) and SKF38393 (0.5μg/mouse), respectively) impaired memory acquisition. In contrast, pre-training intra-BLA injections of dopamine D2 receptor antagonist and agonist (sulpiride and quinpirole, respectively) have no significant effect on memory acquisition. Pre-training intra-peritoneal (i.p.) injection of harmaline (1mg/kg) decreased memory acquisition. However, co-administration of SCH 23390 (0.01μg/mouse) with different doses of harmaline did not alter amnesia. Conversely, pre-training intra-BLA injection of SKF38393 (0.1μg/mouse), sulpiride (0.25μg/mouse) or quinpirole (0.1μg/mouse) reversed harmaline (1mg/kg, i.p.)-induced amnesia. Furthermore, all above doses of drugs had no effect on locomotor activity. In conclusion, the dopamine D1 and D2 receptors of the BLA may be involved in the impairment of memory acquisition induced by harmaline.

Local N-methyl-d-aspartate receptor antagonism in the prefrontal cortex attenuates spatial cognitive deficits induced by gonadectomy in adult male rats

Gonadectomy in adult male rats significantly impairs spatial working memory, behavioral flexibility and other functions associated with the prefrontal cortex (PFC). However, the mechanisms through which this occurs are largely unknown. In this study, intracortical drug challenge with the selective N-methyl-d-aspartate receptor (NMDAR) antagonist D(-)-2-amino-5-phosphonopentanoic acid (APV) was combined with Barnes maze testing, gonadectomy (GDX) and hormone replacement (17β-estradiol, testosterone propionate) to explore the contributions of NMDAR-mediated activity within the PFC to hormone effects on spatial cognition in adult male rats. Previous studies have shown that Barnes maze testing reveals significant estrogen-dependent, GDX-induced deficits in spatial working memory and androgen-sensitive, GDX-induced deficits in spatial search strategy. Here we found that bilateral infusion of APV into the medial PFC prior to testing significantly improved both sets of behaviors in gonadectomized rats and significantly worsened performance measures in gonadally intact controls. In hormone-replaced cohorts, we further found that behaviors that are normally similar to controls were significantly disrupted by APV, and those that are normally similar to gonadectomized rats were rescued by intracortical APV infusion. There were, however, no residual effects of APV on retention testing conducted 24h later. Together these findings suggest that hormone regulation of NMDAR-mediated activity specifically within the PFC may be fundamental to the effects of gonadal steroids on spatial cognition in males. Our findings further identify NMDAR antagonists as potentially novel, non-steroidal means of attenuating the cognitive deficits that can accompany gonadal hormone decline in human males in aging, clinical cases of hypogonadalism and in certain neurologic and psychiatric illnesses. Accordingly, it may be important to obtain in males the kind of detailed knowledge concerning hormone effects on, for example, the channel and electrophysiological properties of NMDAR that currently exists for the female brain.

LMTK3 deficiency causes pronounced locomotor hyperactivity and impairs endocytic trafficking

LMTK3 belongs to the LMTK family of protein kinases that are predominantly expressed in the brain. Physiological functions of LMTK3 and other members of the LMTK family in the CNS remain unknown. In this study, we performed a battery of behavioral analyses using Lmtk3(-/-) mice and showed that these mice exhibit abnormal behaviors, including pronounced locomotor hyperactivity, reduced anxiety behavior, and decreased depression-like behavior. Concurrently, the dopamine metabolite levels and dopamine turnover rate are increased in the striata of Lmtk3(-/-) mice compared with wild-type controls. In addition, using cultured primary neurons from Lmtk3(-/-) mice, we found that LMTK3 is involved in the endocytic trafficking of N-methyl-d-aspartate receptors, a type of ionotropic glutamate receptor. Altered membrane traffic of the receptor in Lmtk3(-/-) neurons may underlie behavioral abnormalities in the mutant animals. Together, our data suggest that LMTK3 plays an important role in regulating locomotor behavior in mice.

The intrahippocampal infusion of crotamine from Crotalus durissus terrificus venom enhances memory persistence in rats

Previous research has shown that crotamine, a toxin isolated from the venom of Crotalus durissus terrificus, induces the release of acetylcholine and dopamine in the central nervous system of rats. Particularly, these neurotransmitters are important modulators of memory processes. Therefore, in this study we investigated the effects of crotamine infusion on persistence of memory in rats. We verified that the intrahippocampal infusion of crotamine (1 μg/μl; 1 μl/side) improved the persistence of object recognition and aversive memory. By other side, the intrahippocampal infusion of the toxin did not alter locomotor and exploratory activities, anxiety or pain threshold. These results demonstrate a future prospect of using crotamine as potential pharmacological tool to treat diseases involving memory impairment, although it is still necessary more researches to better elucidate the crotamine effects on hippocampus and memory.

Dopamine and glutamate receptor genes interactively influence episodic memory in old age

Both the dopaminergic and glutamatergic systems modulate episodic memory consolidation. Evidence from animal studies suggests that these two neurotransmitters may interact in influencing memory performance. Given that individual differences in episodic memory are heritable, we investigated whether variations of the dopamine D2 receptor gene (rs6277, C957T) and the N-methyl-D-aspartate 3A (NR3A) gene, coding for the N-methyl-D-aspartate 3A subunit of the glutamate N-methyl-D-aspartate receptor (rs10989591, Val362Met), interactively modulate episodic memory in large samples of younger (20-31 years; n = 670) and older (59-71 years; n = 832) adults. We found a reliable gene-gene interaction, which was observed in older adults only: older individuals carrying genotypes associated with greater D2 and N-methyl-D-aspartate receptor efficacy showed better episodic performance. These results are in line with findings showing magnification of genetic effects on memory in old age, presumably as a consequence of reduced brain resources. Our findings underscore the need for investigating interactive effects of multiple genes to understand individual difference in episodic memory.

Antipsychotic compounds differentially modulate high-frequency oscillations in the rat nucleus accumbens: a comparison of first- and second-generation drugs

Improved understanding of the actions of antipsychotic compounds is critical for a better treatment of schizophrenia. Abnormal oscillatory activity has been found in schizophrenia and in rat models of the disease. N-Methyl-D-aspartic acid receptor (NMDAR) antagonists, used to model certain features of schizophrenia, increase the frequency and power of high-frequency oscillations (HFO, 130-180 Hz) in the rat nucleus accumbens, a brain region implicated in schizophrenia pathology. Antipsychotics can be classified as first- and second-generation drugs, the latter often reported to have wider benefit in humans and experimental models. This prompted the authors to examine the pre- and post-treatment effects of clozapine, risperidone (second-generation drugs) and sulpiride and haloperidol (first-generation drugs) on ketamine and MK801-enhanced accumbal HFO. Both NMDAR antagonists increased HFO frequency. In contrast, clozapine and risperidone markedly and dose-dependently reduced the frequency of spontaneous and NMDAR-antagonist-enhanced HFO, whilst a moderate effect was found for sulpiride and a much weaker effect for haloperidol. Unexpectedly, we found reductions in HFO frequency were associated with an increase in its power. These findings indicate that modulation of accumbal HFO frequency may be a fundamental effect produced by antipsychotic compounds. Of the drugs investigated, first- and second-generation compounds could be dissociated by their potency on this measure. This effect may partially explain the differences in the clinical profile of these drugs.

Chronic blockade of CB(1) receptors reverses startle gating deficits and associated neurochemical alterations in rats reared in isolation

BACKGROUND AND PURPOSE

Pharmacological interventions aimed at restoring the endocannabinoid system functionality have been proposed as potential tools in the treatment of schizophrenia. Based on our previous results suggesting a potential antipsychotic-like profile of the CB(1) receptor inverse agonist/antagonist, AM251, here we further investigated the effect of chronic AM251 administration on the alteration of the sensorimotor gating functions and endocannabinoid levels induced by isolation rearing in rats.

EXPERIMENTAL APPROACH

Using the post-weaning social isolation rearing model, we studied its influence on sensorimotor gating functions through the PPI paradigm. The presence of alterations in the endocannabinoid levels as well as in dopamine and glutamate receptor densities was explored in specific brain regions following isolation rearing. The effect of chronic AM251 administration on PPI response and the associated biochemical alterations was assessed.

KEY RESULTS

The disrupted PPI response in isolation-reared rats was paralleled by significant alterations in 2-AG content and dopamine and glutamate receptor densities in specific brain regions. Chronic AM251 completely restored normal PPI response in isolated rats. This behavioural recovery was paralleled by the normalization of 2-AG levels in all the brain areas analysed. Furthermore, AM251 partially antagonized isolation-induced changes in dopamine and glutamate receptors.

CONCLUSIONS AND IMPLICATIONS

These results demonstrate the efficacy of chronic AM251 treatment in the recovery of isolation-induced disruption of PPI. Moreover, AM251 counteracted the imbalances in the endocannabinoid content, specifically 2-AG levels, and partially reversed the alterations in dopamine and glutamate systems associated with the disrupted behaviour. Together, these findings support the potential antipsychotic-like activity of CB(1) receptor blockade.

LINKED ARTICLES

This article is part of a themed section on Cannabinoids. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.167.issue-8.

Behavioral screening for neuroactive drugs in zebrafish

The larval zebrafish has emerged asa vertebrate model system amenable to small molecule screens for probing diverse biological pathways. Two large-scale small molecule screens examined the effects of thousands of drugs on larval zebrafish sleep/wake and photomotor response behaviors. Both screens identified hundreds of molecules that altered zebrafish behavior in distinct ways. The behavioral profiles induced by these small molecules enabled the clustering of compounds according to shared phenotypes. This approach identified regulators of sleep/wake behavior and revealed the biological targets for poorly characterized compounds. Behavioral screening for neuroactive small molecules in zebrafish is an attractive complement to in vitro screening efforts, because the complex interactions in the vertebrate brain can only be revealed in vivo.

Dopamine-glutamate interplay in the ventral striatum modulates spatial learning in a receptor subtype-dependent manner

The ventral striatum (VS) is characterized by a distinctive neural architecture in which multiple corticolimbic glutamatergic (GLUergic) and mesolimbic dopaminergic (DAergic) afferents converge on the same output cell type (the medium-sized spiny neuron, MSN). However, despite the gateway function attributed to VS and its involvement in action selection and spatial navigation, as well as the evidence of physical and functional receptor-receptor interaction between different members of ionotropic GLUergic and DAergic receptors, there is no available knowledge that such reciprocal interaction may be critical in shaping the ability to learn novel spatial and non-spatial arrangement of stimuli. In this study, it was evaluated whether intra-VS bilateral infusion of either N-methyl-D-aspartate (NMDA) or α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-selective antagonists may suppress the ability to detect spatial or non-spatial novelty in a non-associative behavioral task. In a second set of experiments, we further examined the hypothesis that VS-mediated spatial information processing may be subserved by some preferential receptor-receptor interactions among specific GLUergic and DAergic receptor subtypes. This was assessed by concomitant intra-VS infusion of the combination between subthreshold doses of either NMDA or AMPA receptor antagonists with individual D1 or D2 receptor blockade. The results of this study highlighted the fact that NMDA or AMPA receptors are differentially involved in processing of spatial and non-spatial novelty, and showed for the first time that preferential NMDA/D1 and AMPA/D2 receptor-receptor functional communication, but not NMDA/D2 and AMPA/D1, is required for enabling learning of novel spatial information in the VS.

Effects of exposure to amphetamine derivatives on passive avoidance performance and the central levels of monoamines and their metabolites in mice: correlations between behavior and neurochemistry

RATIONALE

Considerable evidence indicates that amphetamine derivatives can deplete brain monoaminergic neurotransmitters. However, the behavioral and cognitive consequences of neurochemical depletions induced by amphetamines are not well established.

OBJECTIVES

In this study, mice were exposed to dosing regimens of 3,4-methylenedioxymethamphetamine (MDMA), methamphetamine (METH), or parachloroamphetamine (PCA) known to deplete the monoamine neurotransmitters dopamine and serotonin, and the effects of these dosing regimens on learning and memory were assessed.

METHODS

In the same animals, we determined deficits in learning and memory via passive avoidance (PA) behavior and changes in tissue content of monoamine neurotransmitters and their primary metabolites in the striatum, frontal cortex, cingulate, hippocampus, and amygdala via ex vivo high-pressure liquid chromatography.

RESULTS

Exposure to METH and PCA impaired PA performance and resulted in significant depletions of dopamine, serotonin, and their metabolites in several brain regions. Multiple linear regression analysis revealed that the tissue concentration of dopamine in the anterior striatum was the strongest predictor of PA performance, with an additional significant contribution by the tissue concentration of the serotonin metabolite 5-hydroxyindoleacetic acid in the cingulate. In contrast to the effects of METH and PCA, exposure to MDMA did not deplete anterior striatal dopamine levels or cingulate levels of 5-hydroxyindoleacetic acid, and it did not impair PA performance.

CONCLUSIONS

These studies demonstrate that certain amphetamines impair PA performance in mice and that these impairments may be attributable to specific neurochemical depletions.

Identification of transmitter systems and learning tag molecules involved in behavioral tagging during memory formation

Long-term memory (LTM) consolidation requires the synthesis of plasticity-related proteins (PRPs). In addition, we have shown recently that LTM formation also requires the setting of a "learning tag" able to capture those PRPs. Weak training, which results only in short-term memory, can set a tag to use PRPs derived from a temporal-spatial closely related event to promote LTM formation. Here, we studied the involvement of glutamatergic, dopaminergic, and noradrenergic inputs on the setting of an inhibitory avoidance (IA) learning tag and the synthesis of PRPs. Rats explored an open field (PRP donor) followed by weak (tag inducer) or strong (tag inducer plus PRP donor) IA training. Throughout pharmacological interventions around open-field and/or IA sessions, we found that hippocampal dopamine D1/D5- and β-adrenergic receptors are specifically required to induce PRP synthesis. Moreover, activation of the glutamatergic NMDA receptors is required for setting the learning tags, and this machinery further required α-Ca(2+)/calmodulin-dependent protein kinase II and PKA but not ERK1/2 activity. Together, the present findings emphasize an essential role of the induction of PRPs and learning tags for LTM formation. The existence of only the PRP or the tag was insufficient for stabilization of the mnemonic trace.

Effect of antipsychotics on spontaneous hyperactivity and hypersensitivity to MK-801-induced hyperactivity in rats prenatally exposed to methylazoxymethanol

Exposure to methylazoxymethanol (MAM) at embryonic day 17 (E17) in the rat has been proposed to be a promising model for schizophrenia that mimics behavioural abnormalities and deficits in prefrontal cortex (PFC) networks. In this study, we investigated for the first time the effects of antipsychotics on abnormal behaviours observed in prenatally MAM-exposed rats. We first examined spontaneous and MK-801-induced locomotor activity in an open field in adult E17 MAM- or saline-exposed rats. Then, the effect of single injections of haloperidol, clozapine and risperidone was investigated in MAM- or sham-exposed rats on spontaneous and MK-801 (0.05 mg/kg)-induced hyperactivity. Risperidone more selectively counteracted the spontaneous hyperactivity in MAM than in sham rats, while haloperidol and clozapine induced similar effects on spontaneous locomotion in both groups. The main result of this study is that all the tested antipsychotics were more effective in attenuating the MK-801-induced hyperlocomotion in MAM than in sham rats. These findings further support the validity of E17 MAM exposure as a model for schizophrenia and add to its heuristic value in screening therapies for schizophrenia.

Involvement of dopamine D1/D2 receptors on harmane-induced amnesia in the step-down passive avoidance test

Ingestion of harmane and other alkaloids derived from plant Peganum harmala has been shown to elicit profound behavioural and toxic effects in humans, including hallucinations, excitation, feelings of elation, and euphoria. These alkaloids in the high doses can cause a toxic syndrome characterized by tremors and convulsions. Harmane has also been shown to act on a variety of receptor systems in the mammalian brain, including those for serotonin, dopamine and benzodiazepines. In animals, it has been reported to affect short and long term memory. In the present study, effects of dopamine D1 and D2 receptor antagonists on the harmane (HA)-induced amnesia and exploratory behaviors were examined in mice. One-trial step-down and hole-board paradigms were used for the assessment of memory retention and exploratory behaviors in adult male NMRI mice respectively. Intraperitoneal (i.p.) administration of HA (5 and 10 mg/kg) immediately after training decreased memory consolidation, while had no effect on anxiety-like behavior. Memory retrieval was not altered by 15- or 30 min pre-testing administration of the D1 (SCH23390, 0.025, 0.05 and 0.1 mg/kg) or D2 (sulpiride 12.5, 25 and 50 mg/kg) receptor antagonists, respectively. In contrast, SCH23390 (0.05 and 0.1 mg/kg) or sulpiride (25 and 50 mg/kg) pre-test administration fully reversed HA-induced impairment of memory consolidation. Finally, neither D1 nor D2 receptor blockade affected exploratory behaviors in the hole-board paradigm. Altogether, these findings strongly suggest an involvement of D1 and D2 receptors modulation in the HA-induced impairment of memory consolidation.

Intrinsic neuronal plasticity in the juxtacapsular nucleus of the bed nuclei of the stria terminalis (jcBNST)

The juxtacapsular nucleus of the anterior division of the BNST (jcBNST) receives robust glutamatergic projections from the basolateral nucleus of the amygdala (BLA), the postpiriform transition area, and the insular cortex as well as dopamine (DA) inputs from the midbrain. In turn the jcBNST sends GABAergic projections to the medial division of the central nucleus of the amygdala (CEAm) as well as other brain regions. We recently described a form of long-term potentiation of the intrinsic excitability (LTP-IE) of neurons of the juxtacapsular nucleus of BNST (jcBNST) in response to high-frequency stimulation (HFS) of the stria terminalis that was impaired during protracted withdrawal from alcohol, cocaine, and heroin and in rats chronically treated with corticotropin-releasing factor (CRF) intracerebroventricularly. Here we show that DAergic neurotransmission is required for the induction of LTP-IE of jcBNST neurons through dopamine (DA) D1 receptors. Thus, activation of the central CRF stress system and altered DAergic neurotransmission during protracted withdrawal from alcohol and drugs of abuse may contribute to the disruption of LTP-IE in the jcBNST. Impairment of this form of intrinsic neuronal plasticity in the jcBNST could result in inadequate neuronal integration and reduced inhibition of the CEA, contributing to the negative affective state that characterizes protracted abstinence in post-dependent individuals. These results provide a novel neurobiological target for vulnerability to alcohol and drug dependence.

Components of lemon essential oil attenuate dementia induced by scopolamine

The anti-dementia effects of s-limonene and s-perillyl alcohol were observed using the passive avoidance test (PA) and the open field habituation test (OFH). These lemon essential oils showed strong ability to improve memory impaired by scopolamine; however, s-perillyl alcohol relieved the deficit of associative memory in PA only, and did not improve non-associative memory significantly in OFH. Analysis of neurotransmitter concentration in some brain regions on the test day showed that dopamine concentration of the vehicle/scopolamine group was significantly lower than that of the vehicle/vehicle group, but this phenomenon was reversed when s-limonene or s-perillyl alcohol were administered before the injection of scopolamine. Simultaneously, we found that these two lemon essential oil components could inhibit acetylcholinesterase activity in vitro using the Ellman method.

Neuroregulation of nonexercise activity thermogenesis and obesity resistance

High levels of spontaneous physical activity in lean people and the nonexercise activity thermogenesis (NEAT) derived from that activity appear to protect lean people from obesity during caloric challenge, while obesity in humans is characterized by dramatically reduced spontaneous physical activity. We have similarly demonstrated that obesity-resistant rats have significantly greater spontaneous physical activity than obesity-prone rats, and that spontaneous physical activity predicts body weight gain. Although the energetic cost of activity varies between types of activity and may be regulated, individual level of spontaneous physical activity is important in determining propensity for obesity. We review the current status of knowledge about the brain mechanisms involved in controlling the level of spontaneous physical activity and the NEAT so generated. Focus is on potential neural mediators of spontaneous physical activity and NEAT, including orexin A (also known as hypocretin 1), agouti-related protein, ghrelin, and neuromedin U, in addition to brief mention of neuropeptide Y, corticotrophin releasing hormone, cholecystokinin, estrogen, leptin, and dopamine effects on spontaneous physical activity. We further review evidence that strain differences in orexin stimulation pathways for spontaneous physical activity and NEAT appear to track with the body weight phenotype, thus providing a potential mechanistic explanation for reduced activity and weight gain.

Neuropeptidergic mediators of spontaneous physical activity and non-exercise activity thermogenesis

Lean individuals have high levels of spontaneous physical activity (SPA) and the energy expenditure derived from that activity, termed non-exercise activity thermogenesis or NEAT, appears to protect them from obesity. Conversely, obesity in different human populations is characterized by low levels of SPA and NEAT. Like in humans, elevated SPA in rats appears to protect against obesity: obesity-resistant rats have significantly greater SPA and NEAT than obesity-prone rats. We review the literature on brain mechanisms important in mediating SPA and NEAT. The focus is on neuropeptides, including cholecystokinin, corticotropin-releasing hormone (also known as corticotropin-releasing factor), neuromedin U, neuropeptide Y, leptin, agouti-related protein, orexin-A (also known as hypocretin-1), and ghrelin. We also review information regarding interactions between these neuropeptides and dopamine, a neurotransmitter important in mediating motor function. Finally, we present evidence that elevated signaling of pathways mediating SPA and NEAT may protect against weight gain and obesity.

Age-related differential sensitivity to MK-801-induced locomotion and stereotypy in C57BL/6 mice

Psychomotor effects elicited by systemic administration of the noncompetitive NMDA (N-methyl-D-aspartate) receptor antagonist MK-801 (dizocilpine maleate) represent perturbation of glutamatergic pathways, providing an animal model for psychotic symptoms of schizophrenia. Hyperlocomotion and stereotypy are the two main psychomotor behaviors induced by MK-801. This study compared MK-801-induced hyperlocomotion and stereotypy in young (1-month old) and aged mice (12-month old), in order to determine how the aging process may influence these behaviors. The tested MK-801 doses ranged from 0.015 to 1 mg/kg. The data indicated that MK-801 impacted the aged mice more pronouncedly than the young mice, as both hyperlocomotion and stereotypy were increased significantly more in the aged mice relative to the young mice. These results suggest an age-related increase in MK-801 sensitivity in mice.

NMDA receptors of dorsal hippocampus are involved in the acquisition, but not in the expression of morphine-induced place preference

In the present study, involvement of the N-methyl-d-aspartate (NMDA) receptors of the CA1 region of dorsal hippocampus (intra-CA1) in the acquisition or expression of morphine-induced conditioned place preference in rats was studied. Male Wistar rats were used in these experiments. NMDA-receptor agonist (NMDA) and antagonist (MK-801) were injected into the CA1 region of the dorsal hippocampus (intra-CA1) and morphine was injected subcutaneously. An unbiased conditioned place preference paradigm was used to study the effect of these agents. In the first set of experiments, the drugs were used during the development of conditioned place preference by morphine or they were used alone in order to see if they induce conditioned place preference or conditioned place aversion. Our data showed that subcutaneous (s.c.) injection of morphine sulphate (2.5-10 mg/kg) induced conditioned place preference in rat. NMDA (0.1-1 microg/rat) or MK-801 (1-4 microg/rat) did not induce conditioned place preference or conditioned place aversion. Intra-CA1 administration of different doses of NMDA (0.1-1 microg/rat) increased, while MK-801 (1-4 microg/rat) decreased morphine-induced place preference. MK-801 reversed the effect of NMDA on morphine response. In the second set of experiments, when the drugs were used before testing on Day 5, in order to test their effects on the expression of morphine (7.5 mg/kg)-induced place preference, intra-CA1 administration of NMDA or MK-801 did not alter the morphine response. None of the drugs influenced locomotion. It is concluded that NMDA receptor of the CA1 region of hippocampus are involved in the acquisition but not expression of morphine-induced place preference.

Effects of activation and blockade of NMDA receptors on the extinction of a conditioned passive avoidance response in mice with different levels of anxiety

The effects of an agonist (D-cycloserine) and an antagonist (dizocilpine) of N-methyl-D-aspartate (NMDA) receptors on the learning and extinction of a conditioned passive avoidance response were studied in mice with low, intermediate, and high levels of anxiety. In intermediate-anxiety mice, D-cycloserine (30 mg/kg) had no effect on learning but accelerated extinction, while dizocilpine (0.15 mg/kg) degraded acquisition of the reflex but delayed extinction. In high-anxiety mice, with good learning and no extinction, D-cycloserine had no effect, while dizocilpine decreased learning and facilitated retention of performance of the memory trace at the ongoing level in conditions promoting extinction. In low-anxiety mice, D-cycloserine degraded learning and accelerated extinction, while dizocilpine completely blocked learning and the retention of the passive avoidance response.

Nootropic alpha7 nicotinic receptor allosteric modulator derived from GABAA receptor modulators

Activation of brain alpha7 nicotinic acetylcholine receptors (alpha7 nAChRs) has broad therapeutic potential in CNS diseases related to cognitive dysfunction, including Alzheimer's disease and schizophrenia. In contrast to direct agonist activation, positive allosteric modulation of alpha7 nAChRs would deliver the clinically validated benefits of allosterism to these indications. We have generated a selective alpha7 nAChR-positive allosteric modulator (PAM) from a library of GABAA receptor PAMs. Compound 6 (N-(4-chlorophenyl)-alpha-[[(4-chloro-phenyl)amino]methylene]-3-methyl-5-isoxazoleacet-amide) evokes robust positive modulation of agonist-induced currents at alpha7 nAChRs, while preserving the rapid native characteristics of desensitization, and has little to no efficacy at other ligand-gated ion channels. In rodent models, it corrects sensory-gating deficits and improves working memory, effects consistent with cognitive enhancement. Compound 6 represents a chemotype for allosteric activation of alpha7 nAChRs, with therapeutic potential in CNS diseases with cognitive dysfunction.

Insights into the Role of Dopamine Receptor Systems in Learning and Memory

It is well established that learning and memory are complex processes involving and recruiting different brain modulatory neurotransmitter systems. Considerable evidence points to the involvement of dopamine in various aspects of cognition, and interest has been focused on investigating the clinical relevance of dopamine systems to age-related cognitive decline and manifestations of cognitive impairment in schizophrenia, Alzheimer's disease, Parkinson's disease and other neurodegenerative diseases. In the past decade or so, in spite of the molecular cloning of the five dopamine receptor subtypes, their specific roles in brain function remained inconclusive due to the lack of completely selective ligands that could distinguish between the members of the D1-like and D2-like dopamine receptor families. One of the most important advances in the field of dopamine research has been the generation of mutant mouse models permitting evaluation of the dopaminergic system using gene targeting technologies. These mouse models represent an important approach to explore the functional roles of closely related receptor subtypes. In this review, we present and discuss evidence on the role of dopamine receptors in different aspects of learning and memory at the cellular, molecular and behavioral levels. We compare evidence using conventional pharmacological, lesion or electrophysiological studies with results from mice with targeted deletions of different subtypes of dopamine receptor genes. We particularly focus on dopamine D1 and D2 receptors in an effort to delineate their specific roles in various aspects of cognitive function. We provide strong evidence, from our own recent work as well as others, that dopamine is part of the network that plays a very important role in cognitive function, and that although multiple dopamine receptor subtypes contribute to different aspects of learning and memory, the D1 receptor seems to play a more prominent role in mediating plasticity and specific aspects of cognitive function, including spatial learning and memory processes, reversal learning, extinction learning, and incentive learning.

Dorsal hippocampal dopamine receptors are involved in mediating ethanol state-dependent memory

In the present study, the effects of bilateral injections of dopaminergic agents into the hippocampal CA1 regions (intra-CA1) on ethanol (EtOH) state-dependent memory were examined in mice. A single-trial step-down passive avoidance task was used for the assessment of memory retention in adult male NMRI mice. Pre-training intra-peritoneal (i.p.) administration of EtOH (0.25, 0.5 and 1 g/kg) dose dependently induced impairment of memory retention. Pre-test administration of EtOH (0.5 g/kg)-induced state-dependent retrieval of the memory acquired under pre-training EtOH (0.5 g/kg) influence. Intra-CA1 administration of the dopamine D(1) receptor agonist, SKF 38393 (0.5, 1 and 2 g/mouse) or the dopamine D(2) receptor agonist, quinpirole (0.25, 0.5 and 1 microg/mouse) alone cannot affect memory retention. While, pre-test intra-CA1 injection of SKF 38393 (2 microg/mouse, intra-CA1) or quinpirole (0.25, 0.5 and 1 microg/mouse, intra-CA1) improved pre-training EtOH (0.5 g/kg)-induced retrieval impairment. Moreover, pre-test administration of SKF 38393 (0.5, 1 and 2 microg/mouse, intra-CA1) or quinpirole (0.5 and 1 microg/mouse, intra-CA1) with an ineffective dose of EtOH (0.25 g/kg) significantly restored the retrieval and induced EtOH state-dependent memory. Furthermore, pre-training injection of the dopamine D(1) receptor antagonist, SCH 23390 (4 microg/mouse), but not the dopamine D(2) receptor antagonist, sulpiride, into the CA1 regions suppressed the learning of a single-trial passive avoidance task. Pre-test intra-CA1 injection of SCH 23390 (2 and 4 microg/mouse, intra-CA1) or sulpiride (2.5 and 5 microg/mouse, intra-CA1) 5 min before the administration of EtOH (0.5 g/kg, i.p.) dose dependently inhibited EtOH state-dependent memory. These findings implicate the involvement of a dorsal hippocampal dopaminergic mechanism in EtOH state-dependent memory and also it can be concluded that there may be a cross-state dependency between EtOH and dopamine.

Dopamine D1 receptors co-distribute with N-methyl-D-aspartic acid type-1 subunits and modulate synaptically-evoked N-methyl-D-aspartic acid currents in rat basolateral amygdala

Activation of dopamine D1 or glutamate, N-methyl-d-aspartic acid (NMDA) receptors in the basolateral amygdala (BLA) can potently influence affective behaviors and associative learning. Physical protein-protein interactions also can occur between C-terminal peptides of D1 receptors and the NMDA-receptor subunit-1 (NR1), suggesting intracellular associations of direct relevance to dopaminergic modulation of NMDA currents. We examined this possibility by combining electron microscopic immunolabeling of the D1 and NR1 C-terminal peptides with in vitro patch-clamp recording in the rat BLA. In the in vivo preparations, D1 and NR1 were localized to the surface or endomembranes of many of the same somata and dendrites as well as a few axon terminals, including those forming asymmetric, excitatory-type synapses. In vitro analysis of physiologically characterized projection neurons revealed an excitatory response to bath application of either dopamine or the preferential D1 receptor agonist, dihydrexidine. In these neurons, dopamine also selectively reduced stimulation-evoked isolated NMDA receptor-mediated currents, but not isolated non-NMDA receptor-mediated currents or the response to exogenous NMDA application. The selective reduction of the NMDA receptor-mediated currents suggests that this effect occurs at a postsynaptic locus. Moreover, both D1 and NR1 were localized to postsynaptic surfaces of biocytin-filled and physiologically characterized projection neurons. Our results provide ultrastructural evidence for D1/NR1 endomembrane associations that may dynamically contribute to the attenuation of NMDA receptor-mediated currents following prior activation of D1 receptors in BLA projection neurons. The potential for postsynaptic cross-talk between D1 and NMDA receptors in BLA projection neurons as well as a similar interaction in presynaptic terminals could have important implications for the formation and extinction of affective memories.

Influence of nitric oxide on morphine-induced amnesia and interactions with dopaminergic receptor agents

The interactions of dopaminergic receptors and nitric oxide (NO) with morphine-induced memory of passive avoidance have been investigated in mice. Pre-training administration of morphine (1, 3 and 5 mg/kg, s.c.) dose-dependently decreased the learning of a one-trial passive avoidance task. Pre-training administration of L-arginine, a nitric oxide precursor (50, 100 and 200 mg/kg, i.p.), alone did not affect memory formation. The drug (100 and 200 mg/kg) decreased significantly amnesia induced by pre-training morphine (5 mg/kg). Pre-training administration of L-NAME (N(G)-nitro-L-arginine methyl ester), a nitric oxide synthase (NOS) inhibitor (20 and 30 mg/kg, i.p.), dose-dependently impaired memory formation. In addition, co-pretreatment of different doses of L-NAME (10, 20 and 30 mg/kg) with lower dose of morphine (1 mg/kg), which did not induce amnesia by itself, caused inhibition of memory formation. Pre-training administration of apomorphine, a dopaminergic receptor agonist (0.25, 0.5 and 1 mg/kg, i.p.), alone also did not affect memory formation, but morphine-induced amnesia was significantly inhibited by pretreatment with apomorphine (0.5 and 1 mg/kg, 5 min, i.p.). On the other hand, the inhibition of morphine-induced amnesia by L-arginine (200 mg/kg, i.p.) was significantly decreased by pretreatment with different doses of dopamine D1 receptor antagonist, SCH 23390 (0.001, 0.01 and 0.1 mg/kg, i.p.) or D2 receptor antagonist, sulpiride (12.5, 25, 50 and 100 mg/kg, i.p.). However, the dopamine receptor antagonists could not affect memory formation by themselves. It may be concluded that the morphine-induced impairment of memory formation can be prevented by nitric oxide donor and, in this effect, dopaminergic mechanism is involved.

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.

Overlapping intracellular and differential synaptic distributions of dopamine D1 and glutamate N-methyl-D-aspartate receptors in rat nucleus accumbens

The dopamine D1 receptor (D1R) in the nucleus accumbens (Acb) shell is highly implicated in psychostimulant-evoked locomotor activity and reward, whereas the D1R in the Acb core is more crucial for appetitive instrumental learning. These behavioral effects depend in part on interactions involving glutamatergic N-methyl-D-aspartate (NMDA) receptors, whose essential NR1 subunit has physical associations with the D1R. To determine the relevant sites for D1R activation and interactions involving NMDA receptors, we examined the electron microscopic immunolabeling of D1R and NR1 C-terminal peptides in rat Acb shell and core. In each Acb subdivision, the D1Rs were located principally on extrasynaptic plasma membranes of dendritic shafts and spines and more rarely were associated with cytoplasmic endomembranes. Many D1R-labeled somata and dendrites also contained NR1 immunoreactivity. In comparison with D1R, NR1 immunoreactivity was more often seen in the cytoplasm and near asymmetric synapses on somatodendritic profiles. In these profiles, notable overlapping distributions of D1R and NR1 occurred near endomembranes. The exclusively D1R- or D1R- and NR1-containing dendrites were most prevalent in the Acb shell, but were also present in the Acb core. In each region, NR1 was also detected in axon terminals without D1R, which formed excitatory-type synapses with D1R-labeled dendrites. These results provide ultrastructural evidence that D1Rs in the Acb have subcellular distributions supporting, 1) intracellular cotrafficking with NR1 and 2) modulation of the postsynaptic excitability in spiny neurons affected by presynaptic NMDA receptor activation. The region-specific differences in receptor distributions suggest a major, but not exclusive, involvement of Acb D1R in reward-related processing.

Brain serotonin depletion by lesions of the median raphe nucleus enhances the psychotomimetic action of phencyclidine, but not dizocilpine (MK-801), in rats

We have previously shown that brain serotonin depletion by lesions of the median raphe nucleus (MRN) causes enhancement of phencyclidine-induced locomotor hyperactivity [S. Kusljic, D.L. Copolov, M. van den Buuse, Differential role of serotonergic projections arising from the dorsal and median raphe nuclei in locomotor hyperactivity and prepulse inhibition, Neuropsychopharmacology 28 (2003) 2138-2147]. In this study, we extend our previous work by (1) comparing the effect of phencyclidine with that of another NMDA receptor antagonist, dizocilpine (MK-801); (2) investigate behavioral changes in more detail; (3) assess in detail the effect of raphe lesions on regional serotonin levels in the brain. Male Sprague-Dawley rats received microinjection of the serotonergic neurotoxin 5,7-dihydroxytryptamine into the MRN or dorsal raphe nucleus (DRN). The effects of treatment with saline, phencyclidine and MK-801 on locomotor activity were determined 2 weeks after the surgery. MRN lesions caused serotonin depletion in the dorsal hippocampus, whereas DRN lesions caused serotonin depletion in the frontal cortex, striatum and ventral hippocampus. There was a significant increase in phencyclidine-induced locomotor hyperactivity in the MRN-lesioned group compared to sham-operated controls. Further analysis of behavior showed that phencyclidine-induced hyperambulation, but not stereotypy or rearing, was significantly higher in MRN-lesioned rats compared to controls. In contrast, there was no significant effect of the lesions on the psychotomimetic effect of MK-801. These results indicate that a hyposerotonergic state induced by destruction of projections from the MRN leads to altered brain circuitry that is responsible for the regulation of phencyclidine-but not MK-801-induced locomotor hyperactivity. Thus, MRN projections may play an inhibitory role in mechanisms involved in symptoms of schizophrenia.

Dissociation of the effect of spatial behaviors on the phosphorylation of cAMP-response element binding protein (CREB) within the nucleus accumbens

Several studies have reported a role for the nucleus accumbens (NAcc) in learning and synaptic plasticity. Many of them suggest that the NAcc is involved in translating cortico-limbic information to the motor system mediating spatial learning and memory processes. Previous studies from our laboratory have shown that protein kinase C is activated following training in a food search spatial learning task. The present study further characterizes the molecular substrates associated with NAcc-dependent spatial behavior. The cyclic AMP-response element binding protein (CREB), a transcription factor implicated in the formation of long-term memory, was studied in the NAcc following spatial training in a food search spatial learning task. Western blots were performed to detect phosphorylated (activated) and total CREB protein levels. Our results show that CREB is significantly phosphorylated in the NAcc 48 h after habituation and at 5 min and 1 h after the first spatial training session in comparison with the naive animals that remained in their home cages. Since published data show that NAcc plays a role in novelty detection and reactivity, we conducted further experiments in order to dissociate the effect on CREB phosphorylation and expression of spatial novelty (single exposure), exploration, and spatial learning in the food search apparatus. Results show that CREB phosphorylation is significantly increased 48 h after exposure to a novel environment. The present study suggests that CREB phosphorylation observed in the NAcc during habituation and spatial training may be mainly triggered by detection of spatial novelty.

Systemic administration of MK-801, a non-competitive NMDA-receptor antagonist, elicits a behavioural deficit of rats in the Active Allothetic Place Avoidance (AAPA) task irrespectively of their intact spatial pretraining

Spatial orientation is considered to be an animal model of human cognitive functions. Efficient navigation is believed to require a brain representation of the environment. The role of NMDA-receptor-dependent neurotransmission in encoding spatial representations has been intensively studied; however, its involvement in organizing spatial information into neural representations is poorly understood. We tested the effect of NMDA-receptor blockade on the performance of rats in Active Allothetic Place Avoidance (AAPA), a cognitive task which requires rats to separate two conflicting spatial reference frames (room and arena frames), when only the room frame is relevant for solution of the task. The NMDA-receptor blockade was achieved by systemic administration of either 0.15 mg/kg or 0.2 mg/kg of dizocilpine (MK-801), a non-competitive NMDA-receptor antagonist. Two distinct AAPA setups located in different rooms were used. The rooms had different layouts of robust extramaze landmarks. Intact rats were trained in the upstairs arena and, subsequently, they received saline or MK-801 and their performance was tested (reinforced retention). Re-acquisition of AAPA task in the downstairs room was then tested under saline or MK-801. MK-801 at the dose of 0.2 mg/kg was found to disrupt the reinforced retention of the AAPA. Re-acquisition of AAPA in the novel environment was impaired by both doses of MK-801. Taken together, administration of MK-801 causes an orientation deficit in the AAPA task irrespectively of the rats' prior intact pretraining. These results support the hypothesis that NMDA-receptors play a pivotal role in cognitive processes.

In vivo CREB phosphorylation mediated by dopamine and NMDA receptor activation in mouse hippocampus and caudate nucleus

The pattern of CREB phosphorylation was investigated in the caudate nucleus and hippocampus 10 min or 3 h after i.p. injection of dopamine or NMDA receptor agonists alone, or in combination with antagonists. Ten minutes after C57BL/6 J mice were injected with either the dopamine D1 receptor agonist SKF-38393 hydrobromide or NMDA, immunoreactivity of phosphorylated CREB (pCREB) was significantly increased in all parts of the caudate nucleus but not in hippocampal regions. However, 3 h after the injection of SKF-38393, pCREB levels in the caudate nucleus did not differ significantly from the pCREB levels in control animals, whereas pCREB levels were still elevated 3 h after NMDA injection. Except for the D1 receptor antagonist SCH-23390, which induced CREB phosphorylation in the caudate nucleus, dopamine and NMDA receptor antagonists had little effect on pCREB levels by themselves. However, the NMDA receptor antagonist CGS-19755 injected i.p. blocked both the NMDA- and SKF-38393-induced rise of pCREB levels in the caudate nucleus. Similarly, the D1 receptor antagonist SCH-23390 inhibited the effects produced by SKF-38393 or NMDA. Interestingly, the D2 receptor antagonist sulpiride also blocked the SKF-38393-triggered rise of pCREB. The results demonstrated that NMDA and dopamine receptors modulate pCREB levels in the caudate nucleus and suggest mutual permissive roles for both receptors.

Bimodal effects of MK-801 on locomotion and stereotypy in C57BL/6 mice

RATIONALE

Systemic injection of the non-competitive NMDA (N-methyl-D-aspartate) receptor antagonist MK-801 (dizocilpine maleate) causes both increased locomotion in rodents and various stereotypic behaviors that are proposed to model certain aspects of schizophrenic symptoms in humans.

OBJECTIVES

This study presents a comprehensive characterization of the bimodal effects of MK-801 on locomotion and stereotypy in the C57BL/6 mouse strain, a strain commonly used for genetically modified mice.

RESULTS

We found that it is important to analyze both locomotion and stereotypy in parallel, as MK-801-induced stereotypy results in abnormal movements that are recorded as locomotion by automated beam detection systems. Furthermore, it is important to analyze the bimodal effects of MK-801 over an extended time span, rather than the commonly used narrower time window, as at higher doses (e.g., above 0.3 mg/kg) the hyperlocomotion phase develops only after the stereotypic phase subsides. We also observed that the apparent dose-response curve is very sensitive to the particular time window chosen for analysis because MK-801 affects both the time course and maximum value of stimulated locomotion. We show that analyzing the absolute peak value of locomotion induced for each animal, rather than group-averaged time courses, provides a measure that is sensitive over a wider range of MK-801 doses. Interestingly, MK-801 even at a very low dose of 0.02 mg/kg suppressed rather than enhanced rearing behavior, differing in this regard from amphetamine.

CONCLUSIONS

The non-competitive NMDA receptor antagonist MK-801 induces a complex pattern of behavioral modification in mice with respect to both the time course and the dose-response relationship of behavioral changes. The results of this study provide a foundation and frame of reference for the growing interest in studying MK-801-induced behavior in mice.

Sustained extracellular signal-regulated kinase 1/2 phosphorylation in neonate 6-hydroxydopamine-lesioned rats after repeated D1-dopamine receptor agonist administration: implications for NMDA receptor involvement

Extracellular signal-regulated kinase (ERK) 1/2, a well known regulator of gene expression, is likely to contribute to signaling events underlying enduring neural adaptations. Phosphorylated (phospho)-ERK was examined immunohistochemically after both single and repeated (i.e., sensitizing) doses of the partial D1-dopamine (DA) receptor agonist SKF-38393 (2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benazepine HCl) to adult rats lesioned as neonates (neonate lesioned) with 6-hydroxydopamine. Remarkably, prolonged phospho-ERK accumulated primarily in layers II-III of medial prefrontal cortex (MPC), where it declined gradually yet remained significantly elevated for at least 36 d after repeated doses of SKF-38393. Sustained (> or =7 d) phospho-ERK was observed for shorter periods in various other cortical regions but was not detectable in striatum or nucleus accumbens. At 36 d, an additional injection of SKF-38393 to sensitized rats restored phospho-ERK to maximal levels only in MPC when examined 7 d later. Phosphorylated cAMP response element-binding protein (CREB), examined 7 d after the sensitizing regimen, was observed exclusively in MPC, where it was abundant throughout all layers. Systemic injections of SL327 (alpha-[amino[(4-aminophenyl)thio]methylene]-2-(trifluoromethyl)benzeneacetonitrile), an inhibitor of the upstream ERK activator mitogen ERK kinase, attenuated both ERK and CREB phosphorylation in layers II-III of MPC. Pretreatment with the D1 antagonist SCH-23390 ((R)-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepine-7-OL maleate) inhibited the prolonged increase in MPC phospho-ERK, whereas the 5-HT2 receptor antagonist ketanserin (3-[2-[4-(4-fluorobenzoyl)-1-piperidinyl]ethyl]-2,4(1H,3H)-quinazolinedione tartrate) was ineffective. Competitive and noncompetitive NMDA receptor antagonists also blocked sustained ERK phosphorylation. Collectively, the present results demonstrate coupling of D1 and NMDA receptor function reflected in sustained activation of the ERK signaling pathway in MPC of SKF-38393-sensitized neonate-lesioned rats. Ultimately, long-lasting phosphorylation of ERK and CREB in MPC may play a pivotal role in any permanent adaptive change(s) in these animals.

Application of a novel Active Allothetic Place Avoidance task (AAPA) in testing a pharmacological model of psychosis in rats: comparison with the Morris Water Maze

Administration of a non-competitive NMDA antagonist dizocilpine (MK-801) was proposed to be an animal model of psychosis. NMDA-receptor blockade is accompanied by increased locomotion, behavioral deficits, and other changes resembling psychotic symptoms. However, the role of NMDA-receptors in organizing brain representations is not understood yet. We tested the effect of NMDA-receptor blockade by systemic administration of dizocilpine at two different doses (0.1 or 0.2 mg/kg) in a recently designed Active Allothetic Place Avoidance (AAPA), a task which requires rats to separate spatial stimuli from two continuously dissociated subsets. The effect of dizocilpine on learning in the AAPA task was compared with its effect on acquisition of the reference memory version of the Morris Water Maze task. Both doses impaired performance in the Morris Water Maze task, whereas only the higher dose impaired performance in the AAPA task. The Morris Water Maze appears to be more sensitive to dizocilpine-induced behavioral deficit than the AAPA task. These findings support the notion that these two tasks are differentially dependent on the NMDA-receptor function.

Effect of the mGluR5 antagonist 6-methyl-2-(phenylethynyl)pyridine (MPEP) on the acute locomotor stimulant properties of cocaine, D-amphetamine, and the dopamine reuptake inhibitor GBR12909 in mice

RATIONALE

Recent evidence suggests that, in addition to ascending monoaminergic systems, glutamate systems also play a role in psychostimulant-induced locomotor activity. The present study was conducted to examine the effects of the selective type-5 metabotropic glutamate receptor (mGluR5) antagonist 6-methyl-2-(phenylethynyl)pyridine (MPEP) on the acute locomotor stimulant effects of cocaine, D-amphetamine, and the dopamine reuptake inhibitor GBR12909.

METHODS

Male DBA/2J mice were treated with saline or MPEP (1, 5, 20 or 30 mg/kg i.p.) 10 min prior to the administration of cocaine (15 mg/kg or 30 mg/kg i.p.), D-amphetamine (3 mg/kg or 5 mg/kg i.p.) or GBR12909 (10 mg/kg or 20 mg/kg i.p.). Locomotor activity was then monitored in an open-field environment for 30 min. The effects of MPEP alone (1, 5, 20 and 30 mg/kg i.p.) on locomotor activity were also examined.

RESULTS

MPEP dose dependently inhibited the acute locomotor stimulant effects of cocaine, D-amphetamine, and the 10-mg/kg dose of GBR12909. However, MPEP had no effect on the locomotor stimulant effects of the higher (20 mg/kg) dose of GBR12909. When tested alone, MPEP increased locomotor activity at doses of 5 mg/kg and 20 mg/kg.

CONCLUSIONS

Our data suggest that mGluR5 receptors not only mediate spontaneous locomotor activity in DBA/2J mice but also the acute locomotor stimulant effects of cocaine, D-amphetamine and lower doses of GBR12909. However, the fact that MPEP did not attenuate the locomotor stimulant effects of the high (20 mg/kg) dose of GBR12909 suggests complex interactions between metabotropic glutamate receptors, dopamine transporters and possibly other monoamines in the regulation of psychostimulant-induced locomotor activity.