Behavioural, pharmacological and biochemical effects of acute and chronic administration of ketamine in the rat

Chronic Pain Impairs Memory Formation via Disruption of Neurogenesis Mediated by Mesohippocampal Brain-Derived Neurotrophic Factor Signaling

BACKGROUND

Chronic pain patients often complain of their poor memory. The mechanisms underlying chronic pain-related memory impairment remain elusive, and there are few clinical therapeutic strategies available for this condition.

METHODS

In a neuropathic pain model induced by chronic constrictive injury of the sciatic nerve in male mice, we used circuit-specific electrophysiological recording, combined with chemogenetic, molecular, and pharmacologic methods, to examine the circuit and molecular mechanisms underlying chronic pain-related memory impairment.

RESULTS

Our current results show that chronic neuropathic pain impaired the acquisition of spatial memory and, meanwhile, reduced adult neurogenesis in the dentate gyrus. Experimentally reducing dentate gyrus neurogenesis mimicked this pain-induced effect on spatial memory formation in naïve mice. Furthermore, pain-associated impairments of both hippocampal neurogenesis and memory formation were rescued or mimicked by chemogenetic activation or deactivation, respectively, of the ventral tegmental area dopaminergic projection, through which ventral tegmental area-released brain-derived neurotrophic factor was required. Importantly, we found that chronic, but not acute, systematic administration of subanesthetic doses of ketamine, while without relieving pain, ameliorated chronic pain-related impairment of spatial memory formation, potentially by rescuing brain-derived neurotrophic factor-mediated dentate gyrus neurogenesis.

CONCLUSIONS

These findings provide a novel, circuit-based mechanistic link between chronic pain and memory formation deficit, and potential new therapeutic options for chronic pain-related learning deficit and memory impairment.

Insights on current and novel antipsychotic mechanisms from the MAM model of schizophrenia

Current antipsychotic drugs (APDs) act on D₂ receptors, and preclinical studies demonstrate that repeated D₂ antagonist administration downregulates spontaneously active DA neurons by producing overexcitation-induced inactivation of firing (depolarization block). Animal models of schizophrenia based on the gestational MAM administration produces offspring with adult phenotypes consistent with schizophrenia, including ventral hippocampal hyperactivity and a DA neuron overactivity. The MAM model reveals that APDs act differently in a hyperdopamineregic system compared to a normal one, including rapid onset of depolarization block in response to acute D₂ antagonist administration and downregulation of DA neuron population activity following acute and repeated D₂ partial agonist administration, none of which are observed in normal rats. Novel target compounds have been developed based on the theory that glutamatergic dysfunction is central to schizophrenia pathology. Despite showing promise in preclinical research, none of the novel drugs succeeded in clinical trials. However, preclinical research is generally performed in normal, drug-naïve rats, whereas models with disease-relevant pathology and prior APD exposure may improve the predictive validity of preclinical research. Indeed, in MAM rats, chronic D₂ antagonist treatment leads to persistent DA supersensitivity that interferes with the response to drugs that target upstream pathology. Moreover, MAM rats revealed that the peri-pubertal period is a stress-sensitive window that can be targeted to prevent the development of MAM pathology in adulthood. Neurodevelopmental models, such as the MAM model, can thus be used to test potential pharmacotherapies that may be able to treat schizophrenia in early stages of the disease. This article is part of the issue entitled 'Special Issue on Antipsychotics'.

The monoaminergic pathways and inhibition of monoamine transporters interfere with the antidepressive-like behavior of ketamine

Ketamine (KET), a NMDA receptor antagonist, has been studied for its rapid and efficacious antidepressant effect, even for the treatment-resistant depression. Although depression is a major cause of disability worldwide, the treatment can be feasible, affordable and cost-effective, decreasing the population health burden. We evaluated the antidepressive-like effects of KET and its actions on monoamine contents (DA and its metabolites, as well as 5-HT) and on tyrosine hydroxylase (TH). In addition DAT and SERT (DA and 5-HT transporters, respectively) were also assessed. Male Swiss mice were divided into Control and KET-treated groups. The animals were acutely treated with KET (2, 5 or 10 mg/kg, i.p.) and subjected to the forced swimming test, for evaluation of the antidepressive-like behavior. Imipramine and fluoxetine were used as references. The results showed that KET decreased dose-dependently the immobility time and shortly after the test, the animals were euthanized for striatal dissections and monoamine determinations. In addition, the brain (striata, hippocampi and prefrontal cortices) was immunohistochemically processed for TH, DAT and SERT. KET at its higher dose increased DA and its metabolites (DOPAC and HVA) and mainly 5-HT contents, in mice striata, effects associated with increases in TH and decreases in DAT immunoreactivities. Furthermore, reductions in SERT immunoreactivities were observed in the striatum and hippocampus. The results indicate that KET antidepressive-like effect probably involves, among other factors, monoaminergic pathways, as suggested by the increased striatal TH immunoreactivity and reduced brain DA (DAT) and 5-HT (SERT) transporters.

Differential effects of antipsychotic and propsychotic drugs on prepulse inhibition and locomotor activity in Roman high- (RHA) and low-avoidance (RLA) rats

RATIONALE

Animal models with predictive and construct validity are necessary for developing novel and efficient therapeutics for psychiatric disorders.

OBJECTIVES

We have carried out a pharmacological characterization of the Roman high- (RHA-I) and low-avoidance (RLA-I) rat strains with different acutely administered propsychotic (DOI, MK-801) and antipsychotic drugs (haloperidol, clozapine), as well as apomorphine, on prepulse inhibition (PPI) of startle and locomotor activity (activity cages).

RESULTS

RHA-I rats display a consistent deficit of PPI compared with RLA-I rats. The typical antipsychotic haloperidol (dopamine D2 receptor antagonist) reversed the PPI deficit characteristic of RHA-I rats (in particular at 65 and 70 dB prepulse intensities) and reduced locomotion in both strains. The atypical antipsychotic clozapine (serotonin/dopamine receptor antagonist) did not affect PPI in either strain, but decreased locomotion in a dose-dependent manner in both rat strains. The mixed dopamine D1/D2 agonist, apomorphine, at the dose of 0.05 mg/kg, decreased PPI in RHA-I, but not RLA-I rats. The hallucinogen drug DOI (5-HT2A agonist; 0.1-1.0 mg/kg) disrupted PPI in RLA-I rats in a dose-dependent manner at the 70 dB prepulse intensity, while in RHA-I rats, only the 0.5 mg/kg dose impaired PPI at the 80 dB prepulse intensity. DOI slightly decreased locomotion in both strains. Finally, clozapine attenuated the PPI impairment induced by the NMDA receptor antagonist MK-801 only in RLA-I rats.

CONCLUSIONS

These results add experimental evidence to the view that RHA-I rats represent a model with predictive and construct validity of some dopamine and 5-HT2A receptor-related features of schizophrenia.

Effects of Ketamine and Ketamine Metabolites on Evoked Striatal Dopamine Release, Dopamine Receptors, and Monoamine Transporters

Following administration at subanesthetic doses, (R,S)-ketamine (ketamine) induces rapid and robust relief from symptoms of depression in treatment-refractory depressed patients. Previous studies suggest that ketamine's antidepressant properties involve enhancement of dopamine (DA) neurotransmission. Ketamine is rapidly metabolized to (2S,6S)- and (2R,6R)-hydroxynorketamine (HNK), which have antidepressant actions independent of N-methyl-d-aspartate glutamate receptor inhibition. These antidepressant actions of (2S,6S;2R,6R)-HNK, or other metabolites, as well as ketamine's side effects, including abuse potential, may be related to direct effects on components of the dopaminergic (DAergic) system. Here, brain and blood distribution/clearance and pharmacodynamic analyses at DA receptors (D1-D5) and the DA, norepinephrine, and serotonin transporters were assessed for ketamine and its major metabolites (norketamine, dehydronorketamine, and HNKs). Additionally, we measured electrically evoked mesolimbic DA release and decay using fast-scan cyclic voltammetry following acute administration of subanesthetic doses of ketamine (2, 10, and 50 mg/kg, i.p.). Following ketamine injection, ketamine, norketamine, and multiple hydroxynorketamines were detected in the plasma and brain of mice. Dehydronorketamine was detectable in plasma, but concentrations were below detectable limits in the brain. Ketamine did not alter the magnitude or kinetics of evoked DA release in the nucleus accumbens in anesthetized mice. Neither ketamine's enantiomers nor its metabolites had affinity for DA receptors or the DA, noradrenaline, and serotonin transporters (up to 10 μM). These results suggest that neither the side effects nor antidepressant actions of ketamine or ketamine metabolites are associated with direct effects on mesolimbic DAergic neurotransmission. Previously observed in vivo changes in DAergic neurotransmission following ketamine administration are likely indirect.

Ketamine-enhanced immobility in forced swim test: a possible animal model for the negative symptoms of schizophrenia

Schizophrenia is a chronic and highly complex psychiatric disorder characterised by cognitive dysfunctions, negative and positive symptoms. The major challenge in schizophrenia research is lack of suitable animal models that mimic the core behavioural aspects and symptoms of this devastating psychiatric disorder. In this study, we used classical and atypical antipsychotic drugs to examine the predictive validity of ketamine-enhanced immobility in forced swim test (FST) as a possible animal model for the negative symptoms of schizophrenia. We also evaluated the effects of a selective serotonin reuptake inhibitor (SSRI) on the ketamine-enhanced immobility in FST. Repeated administration of a subanaesthetic dose of ketamine (30 mg kg(-1), i.p., daily for 5 days) enhanced the duration of immobility in FST 24 h after the final injection. The effect, which persisted for at least 21 days after withdrawal of the drug, was neither observed by single treatment with ketamine (30 mg kg(-1) i.p.) nor repeated treatment with amphetamine (1 and 2 mg kg(-1) i.p., daily for 5 days). The enhancing effects of ketamine (30 mg kg(-1) day(-1) i.p.) on the duration of immobility in the FST were attenuated by clozapine (1, 5 and 10 mg kg(-1) i.p.), risperidone (0.25 and 0.5 mg kg(-1) i.p.) and paroxetine (1 and 5 mg kg(-1) i.p.). Haloperidol (0.25 and 0.50 mg kg(-1) day(-1) i.p.) failed to attenuate the ketamine-enhanced immobility in the FST. The repeated ketamine administration neither affects locomotor activity nor motor coordination in rats under the same treatment conditions with the FST, suggesting that the effects of ketamine on the duration of immobility in this study was neither due to motor dysfunction nor peripheral neuromuscular blockade. Our results suggest that repeated treatment with subanaesthetic doses of ketamine enhance the duration of immobility in FST, which might be a useful animal model for the negative symptoms (particularly the depressive features) of schizophrenia.

Effect of ketamine on exploratory behaviour in BALB/C and C57BL/6 mice

In this study, we evaluated the effect of ketamine on exploratory locomotion behaviours in the Balb/c and C57BL/6 strains of mice, which differ in their locomotion behaviours. Intraperitoneal administration of ketamine at three different doses (1, 5 or 10 mg/kg, 0.1 ml/10 gr body weight) was performed on adult male Balb/c and C57BL/6 mice. The same volume of saline was applied to the control group. The open-field and elevated plus maze apparatus were used to evaluate exploratory locomotion. In the open-field test, Balb/c mice less spend time in the centre of the field and was decreased locomotor activity compared to C57BL/6 mice (p<0.01). Ketamine treatment of Balb/c mice at 10 mg/kg dose caused an increase in locomotor activity and an increase in the amount of time spent in the centre in the open-field test, compared to the control group (p<0.05). In C57BL/6 mice, ketamine treatment (1 and 10 mg/kg) decreased locomotor activity (p<0.05). In C57BL/6 mice, the three different doses of ketamine application each caused a decrease in the frequency of centre crossing (p<0.001) and the spent time in the centre (p<0.05). In the elevated plus maze, the number of open-arm entries, the percentage of open-arm time and total arm entries were decreased in Balb/c mice compared to C57BL/6 mice (p<0.001). Ketamine treatment of Balb/c mice at 10 mg/kg dose caused an increase in the open-arm activity (p<0.001). Ketamine application (10 mg/kg) decreased the open-arm activity in C57BL/6 mice (p<0.05). A subanaesthetic dose of ketamine increased exploratory locomotion in Balb/c mice. In contrast, a subanaesthetic dose of ketamine decreased exploratory locomotion in C57BL/6 mice. In conclusion, hereditary factors may play an important role in ketamine-induced responses.

The role of serotonin in the NMDA receptor antagonist models of psychosis and cognitive impairment

OBJECTIVE

To review the evidence that agents which preferentially affect serotonin (5-HT) attenuate the ability of N-methyl-D-aspartate (NMDA) receptor non-competitive antagonists (NMDA-RA), e.g., phencyclidine (PCP), dizocilpine (MK-801), and ketamine, to stimulate locomotor activity (LA), and to impair novel object recognition (NOR).

RATIONALE

NMDA-RA-induced increased LA and impairment of NOR are widely used models of the pathophysiology of schizophrenia, the mechanism of action of antipsychotic drugs (APDs), and the identification of novel treatments. Serotonin (5-HT) plays an important role in attenuating these effects of NMDA-RA.

RESULTS

Selective 5-HT(2A) inverse agonists, e.g., M100907 and ACP-103, and atypical APDs, which are more potent 5-HT(2A) than D(2) antagonists, e.g., clozapine and lurasidone, are more effective than selective D(2) receptor antagonists to attenuate NMDA-RA-induced increased LA. 5-HT(2A) inverse agonists alone are not effective to improve NMDA-RA-impaired NOR, but augment the effects of atypical, but not typical APDs, to improve NOR. The 5-HT(1A) receptor partial agonist tandospirone alone and the 5-HT(1A) agonist effects of atypical APDs may substitute for, or contribute to, the effects of D(2) and 5-HT(2A) receptor antagonism to reverse the NMDA-RA impairment in NOR. 5-HT(6) and 5-HT(7) receptor antagonists may also attenuate these NMDA-RA-induced behaviors. 5-HT(2C) receptor inverse agonist, but not neutral antagonists, block NOR in naïve rats and the effects of atypical APDs to restore NOR in PCP-treated rats, suggesting the importance of the constitutive activity of 5-HT(2C) receptors in NOR.

CONCLUSIONS

Multiple 5-HT receptors contribute to effective treatments to reverse adverse effects of NMDA-RA which model psychosis and cognitive impairment.

Lack of persistent effects of ketamine in rodent models of depression

RATIONALE

We investigated the immediate and enduring effects of ketamine in behavioral and neurochemical assays predictive of antidepressant activity.

MATERIALS AND METHODS

One week after a single administration of ketamine (50-160 mg/kg), otherwise experimentally naive rats and mice were tested either in the forced swim test (FST) or the tail suspension test (TST). Other mice were also tested twice in the FST: immediately and 2 weeks after a single dose (1.25-50 mg/kg) of ketamine. In the next series of experiments, rats treated for 2 weeks twice daily with ketamine (50 or 160 mg/kg) or desipramine (10 mg/kg) were challenged with apomorphine and scored for locomotor activity and assayed for the density of cortical beta-adrenoceptors. The latter test was also carried out in rats that had received a single dose of ketamine (50 mg/kg) 1 week before the assay. The antidepressant-like (FST) and locomotor effects of ketamine (50 mg/kg) and desipramine (10 mg/kg) were assessed after their chronic (2 weeks, twice daily) administration as well.

RESULTS

We report the lack of enduring antidepressant-like effect of ketamine in both rats and mice. A 2-week treatment with ketamine neither changed apomorphine-evoked locomotor hyperactivity nor did it decrease the density of cortical beta-adrenoceptors. However, some tolerance to the antidepressant-like effect of ketamine was noted in the FST, but it was accompanied by sensitization to its locomotor stimulatory effects.

CONCLUSIONS

These data indicate that ketamine neither produces enduring antidepressant-like effects in rodents nor does it display antidepressant-like behavioral or neurochemical effects after chronic treatment.

Functional reactivity of the dopaminergic system following acute and chronic ketamine treatments

This study examined the effects of acute (15 mg/kg, i.p.) and chronic subanesthetic (15 mg/kg, i.p., t.i.d, for 6 days) doses of ketamine [a noncompetitive N-methyl-D: -aspartate (NMDA) receptor antagonist] on amphetamine (presynaptic dopamine releasing agent; 10 mg/kg, i.p.) and apomorphine (a D(2) receptor agonist; 1 mg/kg, i.p.)-induced stereotyped behaviors. The effect of acute and chronic ketamine on haloperidol (a D(2) receptor antagonist; 1.6 mg/kg, i.p.)-induced catalepsy was also examined. Acute ketamine and chronic ketamine pretreatment increased amphetamine-induced stereotyped sniffing and locomotion compared with control groups. Acute ketamine significantly increased apomorphine-induced stereotyped sniffing. However, chronic ketamine had no significant effect on apomorphine-induced stereotyped sniffing. Acute, but not chronic ketamine treatment abolished haloperidol-induced catalepsy. The increase in amphetamine-induced stereotyped behaviors and the reversal of haloperidol-induced catalepsy by acute ketamine suggest that blockade of NMDA receptors by ketamine facilitates dopaminergic transmission. The absence of significant effect of chronic ketamine on apomorphine-induced stereotyped sniffing and haloperidol-induced catalepsy suggests that chronic ketamine does not modulate postsynaptic dopaminergic D(2) receptors. It is suggested that chronic ketamine increased amphetamine-induced behaviors by causing hypersensitivity of presynaptic dopamine releasing mechanisms on dopaminergic terminals.

Ketamine blocks the formation of a gustatory memory trace in rats

N-methyl-D-aspartate (NMDA) receptors appear to play a central role in learning and memory processes, as the administration of antagonistic substances of these receptors hinders learning acquisition by using different behavioral paradigms (e.g., Riedel G, Platt B, Micheau J. Glutamate receptor function in learning and memory. Behavioural Brain Research, 2003;140 (1-2):1-47.). In the specific case of conditioned taste aversion, the administration of ketamine seems to affect the acquisition of conditioning when the drugs are administered before the experimental treatment. In this paper we present three experiments designed to analyze the effect of different ketamine doses (25 mg/kg, 50 mg/kg, 75 mg/kg and 120 mg/kg), administered between exposure to a taste (the conditioned stimulus) and the administration of the unconditioned stimulus, on the acquisition of a taste aversion association. The results reveal that higher ketamine doses (75 mg/kg and 120 mg/kg) have a disruptive effect on conditioned taste aversion by impeding the formation of the gustatory trace.

Increased response to ketamine following treatment at long intervals: implications for intermittent use

BACKGROUND

Ketamine has been used for many years as a dissociative anesthetic; however, there is evidence of increasing abuse, especially at dance clubs and raves. In addition, there is increasing interest in the use of subanesthetic doses of ketamine for the treatment of pain and depression, as well as for clinical research on schizophrenia. Despite growing use, relatively little is known about the consequences of repeated administration of low doses of ketamine.

METHODS

To determine the changes in response to repeated administration, ketamine (20 mg/kg or 50 mg/kg intraperitoneal [IP]) was administered once weekly to laboratory rats and the locomotor response was assessed following each injection.

RESULTS

Repeated administration of ketamine led to an escalation in the stimulant effects of the drug, characteristic of behavioral sensitization. The development of sensitization was greater when ketamine was repeatedly administered in the presence of distinct environmental cues.

CONCLUSIONS

Intermittent administration of ketamine at weekly intervals leads to the development of locomotor sensitization. These results suggest caution in the repeated use of ketamine for recreational or clinical purposes.

Effects of repeated phencyclidine administration on adult hippocampal neurogenesis in the rat

Dysfunctional maturation of neural networks, particularly hippocampus-prefrontal networks, may be of particular interest in determining the pathophysiology of schizophrenia. Phencyclidine (PCP)-induced symptoms in humans appear to offer a more complete model of schizophrenia than do amphetamine-induced symptoms. This study investigated the effects of intermittent i.p. injections of PCP (7.5 mg/kg) on cell proliferation and survival of granule cells in the dentate gyrus of the rat brain using quantitative immunohistochemical techniques for 5-bromo-2'-deoxyuridine (BrdU)-positive cells. After repeated PCP injection for 14 days, mean scores for stereotyped behavior increased with the number of injections, while scores for ataxia and backpedaling as serotonergic behaviors gradually decreased. The number of BrdU-positive cells decreased by 23% in the subgranular zone of the dentate gyrus by 24 h after repeated injections. However, decreased levels of BrdU-positive cells returned to control levels within 1 week. Differentiation of newly formed cells was not influenced. Repeated PCP administration after BrdU injection did not exert any effects on survival of newly generated cells. These findings suggest that transient disturbances of cell proliferation in the dentate gyrus occur under PCP-related behavioral abnormalities. Whether disturbed cell proliferation would thus be closely implicated in the development of behavioral sensitization induced by PCP administration is unclear, but this would possibly result from adaptation to new pharmacological conditions under behavioral sensitization or stressful conditions of PCP-related abnormal behaviors. Further studies are required to elucidate the biological significance of hippocampal neurogenesis in the mechanisms underlying the development of cognitive dysfunctions and the psychosis of schizophrenia.

Transcriptional response to the neuroleptic-like compound Ampullosporin A in the rat ketamine model

Psychotic disorders affecting up to 1% of the human population represent pathological changes to the metabolic homeostasis of the brain. Increasing evidence in the literature suggests complex biochemical and/or transcriptional alterations accompanying schizophrenia-like phenomena. Sub-chronic treatment with sub-anaesthetic doses of ketamine induces schizophrenia-related psychotic alterations that can be used as an animal model in the study of this disorder. Ampullosporin A belongs to a specific group of pore-forming fungal peptides, peptaibols. We focused on the analysis of molecular events occurring in the brain of ketamine-pre-treated rats after administration of Ampullosporin A with neuroleptic-like activity. The complex experimental approach allowed us to correlate the use of low molecular weight substances with a transcriptome fingerprint in the prefrontal cortex. We found 63 genes to be up-regulated and 22 genes suppressed, with transthyretin, syndecan-1 and NeuroD1 showing the highest degree of up-regulation. Our results suggest the possibility that Ampullosporin A belongs to the group of neuroleptic-like compounds, inducing massive changes in neurotransmitter receptor composition, calcium signalling cascades and second messenger systems, and leading to the plastic reorganization of brain tissue, metabolic pathways and synapses.

Ketamine affects memory consolidation: Differential effects in T-maze and passive avoidance paradigms in mice

The effects of ketamine, an N-methyl-D-aspartate (NMDA) antagonist, on memory in animals have been limited to the sub-anesthetic dose given prior to training in previous studies. We evaluated the effects of post-training anesthetic doses of ketamine to selectively manipulate memory consolidation, and the effect of pre-retention sub-anesthetic doses of ketamine on memory retrieval in passive avoidance and T-maze tasks in mice. Repeated post-training anesthetic doses of ketamine impaired the consolidation of memory in the T-maze but not in passive avoidance paradigms. This impairment was not permanent but diminished 1-2 days after ketamine withdrawal. Sub-anesthetic post-training doses of ketamine (5 mg/kg) had no effect on memory consolidation, and larger doses (10, 20 and 50 mg/kg) did not influence the retrieval of memory in the T-maze. The data suggest that repeated anesthetic doses of ketamine block NMDA receptors and affect memory consolidation. Moreover, NMDA mechanisms antagonized by ketamine appear to be selectively involved in spatial (T-maze) memory mechanisms but may not be necessary for non-spatial (passive avoidance) memory consolidation.

Effects of single and repeated phencyclidine administration on [3H]flunitrazepam binding in rat brain

Repeated administration of phencyclidine (PCP) induces behavioral sensitization to dopaminergic neural transmission. This phenomenon has been implicated in the pathophysiology of schizophrenia. Recently, GABAergic agonists have been shown to reduce behavioral activity induced by enhanced dopaminergic neural transmission, which is mediated by the GABA(A)/benzodiazepine (BZD) receptor complex. Thus, to investigate the role of BZD receptors during induction and expression of behavioral sensitization in PCP-sensitized animals, the effects of both single and repeated PCP administration on BZD receptors in rat brain were examined using in vitro quantitative autoradiography. Repeated PCP administration failed to significantly alter levels of [3H]flunitrazepam (FNZ) binding in any of the regions examined. However, significant increases in levels of [3H]FNZ binding were found in the nucleus accumbens and ventral pallidum 1 h after single administration of PCP. These results suggest that BZD binding sites may not play important roles in the development of PCP-induced sensitization at several sites of GABA(A)/BZD receptor complex, while changes in GABA function in the nucleus accumbens differ from other areas following single administration of N-methyl-D-aspartate (NMDA) antagonist.

Ketamine-induced changes in rat behaviour: A possible animal model of schizophrenia

It was investigated whether subchronic application of 30 mg/kg ketamine (Ket) induces reliable changes in behaviour and parameters of dopaminergic, glutamatergic, and serotonergic neurotransmissions, which might be the basis of an animal model in schizophrenia research. To test this, rats were injected with 30 mg/kg ip Ket daily for five consecutive days. In response to the first Ket injection, there was a decrease in activity time representing an acute Ket effect. Following the fifth injection, there were no differences between Ket- and saline (sal)-injected control rats in activity time, which might be a tolerance reaction. The following experiments were performed 2 or 4 weeks after Ket treatment. There were no effects on anxiety in either vehicle or Ket-treated rats using either low or high illumination levels in the elevated plus-maze. In the social interaction test, both groups of rats spent comparable times in social contact. The percentage of nonaggressive behaviour was decreased in Ket-treated rats. Two weeks after completion of the treatment, there was no effect on prepulse inhibition (PPI). Four weeks after the final Ket injection, latent inhibition (LI) was disrupted. There was no difference in the animals' activity in reaction to apomorphine (Apo) administration. Ket-treated rats injected with 0.1 mg/kg MK-801 showed an enhancement in locomotor activity. Ket treatment leads to an increase in D2 receptor binding in the hippocampus and a decrease in glutamate receptor binding in the frontal cortex. The authors did not find any changes in D1 receptor binding. The density of dopamine transporters was increased in the striatum. The density of 5-HT transporters was increased in the striatum, the hippocampus, and the frontal cortex. The results suggest that subchronic treatment with subanaesthetic doses of Ket induce schizophrenia-related alterations, which might be a useful animal model in the study of this disease.

Subchronic continuous phencyclidine administration potentiates amphetamine-induced frontal cortex dopamine release

Functional dopaminergic hyperactivity is a key feature of schizophrenia. Etiology of this dopaminergic hyperactivity, however, is unknown. We have recently demonstrated that subchronic phencyclidine (PCP) treatment in rodents induces striatal dopaminergic hyperactivity similar to that observed in schizophrenia. The present study investigates the ability of PCP to potentiate amphetamine-induced dopamine release in prefrontal cortex (PFC) and nucleus accumbens (NAc) shell. Prefrontal dopaminergic hyperactivity is postulated to underlie cognitive dysfunction in schizophrenia. In contrast, the degree of NAc involvement is unknown and recent studies have suggested that PCP-induced hyperactivity in rodents may correlate with PFC, rather than NAc, dopamine levels. Rats were treated with 5-20 mg/kg/day PCP for 3-14 days by osmotic minipump. PFC and NAc dopamine release to amphetamine challenge (1 mg/kg) was monitored by in vivo microdialysis and HPLC-EC. Doses of 10 mg/kg/day and above produced serum PCP concentrations (50-150 ng/ml) most associated with PCP psychosis in humans. PCP-treated rats showed significant, dose-dependent enhancement in amphetamine-induced dopamine release in PFC but not NAc, along with significantly enhanced locomotor activity. Enhanced response was observed following 3-day, as well as 14-day, treatment and resolved within 4 days of PCP treatment withdrawal. These findings support the concept that endogenous NMDA receptor dysfunction could account for the pattern of dopaminergic dysfunction observed in schizophrenia, and suggest that even short duration abuse of PCP-like agents may greatly potentiate behavioral effects of psychostimulants in drug abuse situations. Finally, these studies provide a model system in which to evaluate effects of potential psychotherapeutic agents.

Prolonged effect of an anesthetic dose of ketamine on behavioral despair

The present study investigated the effect of a single, anesthetic dose of ketamine, a noncompetitive N-methyl-D-aspartate (NMDA) glutamate receptor antagonist, on behavioral despair, an animal model of depression. Separate groups of male Wistar rats injected with an anesthetic dose of ketamine (160 mg/kg ip) and tested 3, 7, or 10 days later showed significantly less immobility in the second of two forced-swim tests compared to saline-injected controls. Ketamine- and saline-treated animals did not differ significantly in the swim tests with respect to other behavioral measures, namely diving, jumping, and head shakes. The present findings point to an ameliorative effect of ketamine on behavioral despair and support the view that NMDA antagonists may have a beneficial effect on depression.

A novel pentadecapeptide, BPC 157, blocks the stereotypy produced acutely by amphetamine and the development of haloperidol-induced supersensitivity to amphetamine

BACKGROUND

A novel gastric pentadecapeptide, BPC 157, has been shown to attenuate different lesions (i.e., gastrointestinal tract, liver, pancreas, somatosensory neurons). This suggests an interaction with the dopamine system. When used alone, BPC 157 does not affect gross behavior or induce stereotypy.

METHODS

We first investigated the effect of pentadecapeptide BPC 157 on stereotypy and acoustic startle response in rats, given as either a prophylactic (10 micrograms/kg i.p.) or therapeutic (10 ng/kg i.p.) regimen, with the dopamine indirect agonist amphetamine (10 mg/kg i.p.).

RESULTS

There was a marked attenuation of stereotypic behavior and acoustic startle response. When the medication was given at the time of maximum amphetamine-induced excitability, there was a reversal of this behavior. A further focus was on the effect of this pentadecapeptide on increased climbing behavior in mice pretreated with the dopamine antagonist haloperidol (5.0 mg/kg i.p.), and subsequently treated with amphetamine (20 mg/kg i.p. challenge 1, 2, 4, and 10 days after haloperidol pretreatment). This protocol is usually used for the study of behavioral supersensitivity to the amphetamine stimulating effect.

CONCLUSIONS

An almost complete reversal was noted when pentadecapeptide was coadministered with haloperidol. Together, these data provide compelling evidence for the interaction of pentadecapeptide BPC 157 with the dopamine system.

MK-801 inhibits methamphetamine-induced conditioned place preference and behavioral sensitization to apomorphine in mice

Intraperitoneal administration of MK-801 (0.1 mg/ kg), an N-methyl-D-aspartate (NMDA) receptor antagonist, before and during methamphetamine treatment inhibited methamphetamine-induced conditioned place preference (CPP) in mice. Behavioral sensitization to a dopamine (DA) receptor agonist apomorphine that developed in methamphetamine-induced CPP mice was also inhibited by MK-801. Furthermore, MK-801 inhibited apomorphine-induced postsynaptic dopaminergic action, cage-climbing behavior. Therefore, the present studies suggest that methamphetamine-induced behaviors, such as CPP and behavioral sensitization, may be closely related to the dopaminergic activation mediated via the NMDA receptor. The behavioral sensitization to apomorphine may be a possible underlying mechanism of methamphetamine-induced CPP, because behavioral sensitization developed in methamphetamine-induced CPP mice, as well as apomorphine-induced climbing behavior in mice, were inhibited by MK-801.

Accumbal dopamine overflow after ethanol: localization of the antagonizing effect of mecamylamine

It has been suggested that ethanol exerts its mesolimbic dopamine activating effects and its reinforcing effects via interaction with central nicotinic acetylcholine receptors, thus providing a basis for the often observed covariation between ethanol and nicotine consumption. We have previously demonstrated that the central nicotinic acetylcholine receptor antagonist mecamylamine totally counteracts the ethanol-induced elevation of extracellular dopamine in the nucleus accumbens, as measured by in vivo microdialysis. A contribution of peripheral nicotinic receptor blockade could, however, not be excluded. In the present study, mecamylamine (1.0 mg/kg, i.p.) again totally counteracted the ethanol-induced dopamine overflow, as measured by in vivo microdialysis, while the quarternary nicotinic receptor antagonist hexamethonium (10 mg/kg, i.p.) did not. Furthermore, the increase in accumbal dopamine overflow after systemic ethanol (2.5 g/kg, i.p.) was counteracted by local perfusion of mecamylamine (50 microM) in the ipsilateral ventral tegmental area, but not by mecamylamine perfusion in the nucleus accumbens. Ethanol-induced accumbal dopamine overflow was also counteracted by perfusion of hexamethonium (250 microM) in the ventral tegmental area. These results provide further evidence that ethanol-induced activation of the mesolimbic dopamine system is mediated via stimulation of central nicotinic acetylcholine receptors, and that the receptor population within the ventral tegmental area may be the most important in this regard. It is suggested that antagonists of central nicotinic acetylcholine receptors may be useful in the treatment of alcoholism.

Sensitization of the striatal dopaminergic system induced by chronic administration of a glutamate antagonist in the rat

The aim of the present study was to assess in the rat the pharmacological, biochemical and molecular (including in situ hybridization) consequences in the striatum of a prolonged (50 days) treatment with dizocilpine maleate (MK-801), an N-methyl-D-aspartate (NMDA) antagonist. We observed a sensitization-like effect characterized by a behavioural hyperresponsiveness to an acute injection of haloperidol (0.25 mg/kg), a dopaminergic antagonist. In rats chronically treated with MK-801, this hyperresponsiveness was associated with an increased D2 receptor (D2R) density in the striatum. At the transcriptional level, the D2R mRNA was also enhanced in the striatum. Quantitative in situ hybridization studies revealed that the number of neurons expressing the D2R mRNA was significantly enhanced in treated rats, whereas the mean amount of message per cell was unchanged. These changes could represent the neurobiological substrate of the observed sensitization. These results suggest that the D2R gene is under glutamate control via NMDA receptor in striatal neurons.

Differential effects of single and repeated ketamine administration on dopamine, serotonin and GABA transmission in rat medial prefrontal cortex

Cognitive functions regulated by the prefrontal cortex are sensitive to changes in dopaminergic and serotoninergic transmission. The non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist ketamine influences dopaminergic transmission and induces psychotic symptoms in normal and schizophrenic individuals. This study examined the effect of single and repeated ketamine (25 mg/kg, i.p.) administration on extracellular levels of dopamine, GABA and the serotonin metabolite 5-hydroxyindoleacetic (5-HIAA) acid in the medial prefrontal cortex using in vivo microdialysis in conscious rat. In line with earlier studies, we observed a transient five-fold increase in dopamine release following single ketamine administration in drug naive animals. However, we also observed a two-fold increase in basal dopamine levels and an almost complete attenuation of the ketamine-induced increase in dopamine release in animals pre-treated with ketamine once daily for 7 days. Extracellular 5-HIAA levels were increased by ketamine in both drug naive and even more enhanced in ketamine-pre-treated animals but without a change in basal 5-HIAA levels. GABA levels were unaffected by either single or repeated ketamine administration. We demonstrate evidence for a differential effect of single and repeated ketamine administration on dopamine, serotonin and GABA transmission in the medial prefrontal cortex. We provide new evidence for a complex adaptation of neurotransmission following repeated NMDA receptor blockade whereby in the presence of increased basal dopamine levels the ketamine-induced increase in dopamine is attenuated and the increase in 5-HIAA is enhanced. It appears from our results that ketamine pre-treatment reduces the dynamics of dopaminergic transmission in the prefrontal cortex and may possibly alter the balance between dopamine and serotonin transmission.

Effect of reserpine-induced depletion of synaptic dopamine on [11C]raclopride binding to D2-dopamine receptors in the monkey brain

Positron emission tomography was used to examine the in vivo binding of [11C]raclopride to D2-dopamine (DA) receptors in the striatum of two Cynomolgus monkeys after a single dose of reserpine (1 mg/kg, i.v.). A Scatchard procedure was repeated five times to follow D2 receptor density and apparent affinity for 7 weeks after reserpine. Reserpine-induced depletion of DA lead to a marked increase in [11C]raclopride binding, which was still detectable 20 days after treatment. Scatchard analyses indicated that the measured increase in [11C]raclopride binding reflected an increase in receptor affinity but no evident change in receptor density (Bmax). Thus, the increase in [11C]raclopride binding after reserpine should correspond to a reduced competition with endogenous DA for binding to D2 receptors. The results were used to estimate the DA-induced D2 occupancy to be about 40% at physiological conditions.

The effects of repeated administration of diazepam, MK-801 and CGP 37849 on rat behavior in two models of anxiety

The effects of repeated administration of diazepam, MK-801 and CGP 37849 on rat behavior in the Vogel conflict test, and in the open field test of neophobia, were studied in rats. The drugs were given at doses active acutely, for 5 days, the last dose was administered 30 or 60 min prior to testing. It appeared that diazepam and MK-801 treated animals showed clear-cut signs of behavioral tolerance and motor sensitization, respectively. CGP 37849 was characterized by the best pharmacological profile, in that on repeated administration the drug not only retained its anxiolytic-like potency in the Vogel test, but even enhanced rat exploratory behavior in a new environment, independently of changes in animal motor activity. Repeated injections of the examined agents did not cause any significant modifications in monoamine levels and their turnover rates, in the striatum and limbic forebrain. It is concluded that the new class of competitive NMDA receptor antagonists, exemplified by CGP 37849, is the most promising candidate for clinical trials in anxiety disorders.

Inhibition by noncompetitive NMDA receptor antagonists of apomorphine-induced climbing behavior in mice

The N-methyl-D-aspartate (NMDA) subtype of glutamate receptors is an important mediator of several forms of neural and behavioral plasticity. In the present study, we examined the potential role of NMDA receptors in the glutamatergic modulation of dopaminergic function at the postsynaptic dopamine receptor by determining the effects of NMDA antagonists on apomorphine-induced climbing behavior in mice. The noncompetitive NMDA receptor antagonists, MK-801, ketamine, dextrorphan, and dextromethorphan attenuated the apomorphine-induced climbing behavior at does well below those that produce untoward side effects. These results suggest that the NMDA receptors play important roles in the glutamatergic modulation of dopaminergic function at the postsynaptic dopamine receptors that mediate the apomorphine-induced climbing behavior in mice.

Cortical stimulation induces Fos expression in striatal neurons via NMDA glutamate and dopamine receptors

Cortical electrical stimulation has been shown to induce dense and widespread Fos expression throughout the ipsilateral and contralateral striatum. This raises interest for studying the mechanisms underlying the regulation of striatal neuron activity by cortical afferents, and for elucidating the interactions with other systems. However, the receptors mediating cortical-stimulation-induced expression of Fos in striatal neurons have not been identified. This was studied in the work reported here by stimulating the cortex after administration of glutamate or dopamine receptor antagonists, or after 6-hydroxydopamine (6-OHDA) lesion of the nigrostriatal dopaminergic system. Pretreatment with the non-competitive N-methyl-D-aspartate (NMDA) glutamate receptor antagonist MK-801 led to a marked reduction in the stimulation-induced density of Fos-immunoreactive nuclei in both the medial (about 80% reduction) and lateral (about 50-60% reduction) striatum. Preadministration of the D1-selective dopamine antagonist SCH-23390 alone or in combination with the D2-selective dopamine antagonist eticlopride led to a reduction in the stimulation-induced density of Fos-positive nuclei of about 60-65% in the lateral striatum, but no significant change in the medial region. The effects of 6-OHDA lesion were less pronounced, and the stimulation-induced density of Fos-immunoreactive nuclei decreased by only about 25% in the lateral region. These results indicate that both dopamine and NMDA glutamate receptors are involved in the induction of Fos by cortical stimulation, and support the hypothesis that cortex-dopamine interactions in the lateral striatum may be functionally different from those in the medial striatum.

Chronic treatment with dizocilpine maleate increases the number of striatal neurons expressing the D2 receptor gene

N-methyl-D-aspartate antagonists have been proposed as potential therapeutic agents in different neurological diseases, including Parkinson's disease. The effects of gene expression of a chronic treatment with the non-competitive N-methyl-D-aspartate antagonist, dizocilpine maleate (0.8 mg/kg day, per os for 50 days) were analysed in rat striata. Using quantitative in situ hybridization, we measured the messenger RNA expression of the genes encoding D1, D2 dopamine receptors, N-methyl-D-aspartate receptor 1 subunit of N-methyl-D-aspartate receptor, preproenkephalin A and substance P. Chronic treatment with dizocilpine maleate induced a moderate but significant increase in messenger RNA of the N-methyl-D-aspartate receptor 1 subunit in the striatum and the adjacent cortex, suggesting an action of dizocilpine maleate in these two regions. This treatment did not induce any change in D1 receptor, preproenkephalin A or substance P messenger RNA content in the striatum, whereas D2 receptor messenger RNA was increased in the striatum of treated rats. Microscopic analysis revealed that it was the number of medium-sized neurons expressing D2 receptor messenger RNA that was significantly enhanced, while the mean amount of message per cell remained unchanged. These results demonstrate that glutamate via N-methyl-D-aspartate receptors, regulates the D2 receptor gene in striatal neurons. A chronic treatment with dizocilpine maleate increases the number of striatal neurons expressing the D2 receptor gene, suggesting a recruiting phenomenon.

Chronic neonatal blockade of N-methyl-D-aspartate receptor by CGP 39551 increases dopaminergic function in adult rat

Following chronic neonatal treatment with the competitive N-methyl-D-aspartate antagonist CGP 39551, adult rats showed increased behavioral responses to the D2 dopamine receptor stimulation. In nucleus accumbens and in n. striatum of similarly treated rats increases in D2 dopamine receptor number were observed. CGP 39551 was administered daily to neonatal rats with increasing doses from postnatal day 1 to 22. At postnatal days 70-82, the rats were observed for hyperactivity induced by the selective D2 dopamine receptor agonist LY 171555, the grooming behavior elicited by the specific D1 dopamine receptor stimulating agent SKF 38393 and the stereotypies induced by the mixed D1/D2 receptor agonist apomorphine. [3H]Spiroperidol and [3H]SCH 23390 specific binding to membranes of nucleus accumbens, nucleus striatum and frontal cortex of similarly treated rats was measured. The hypermotility and the stereotyped behavior induced by LY 171555 and apomorphine, respectively, were augmented, whereas grooming behavior elicited by SKF 38393 was unaffected, in CGP 39551-treated rats. Consistently, both in nucleus accumbens and in n. striatum an increase in [3H]Spiroperidol specific binding was observed, while [3H]SCH 23390 specific binding did not change. The study demonstrates that chronic blockade of N-methyl-D-aspartate receptor during the critical period of brain maturation results in long-lasting dopaminergic functional changes.

Antidepressant activity of non-competitive and competitive NMDA receptor antagonists in a chronic mild stress model of depression

The antidepressant properties of the non-competitive NMDA receptor antagonist, MK-801 (dizocilpine), and the competitive NMDA receptor antagonist, CGP 37849 (DL-(E)-2-amino-4-methyl-5-phosphono-3-pentonoic acid) and its (R)-enantiomer CGP 40116, were studied in a chronic mild stress model of depression. In this model, animals subjected to a variety of mild stressors for a prolonged period of time show a substantial decrease in the consumption of palatable sucrose solution (anhedonia). It was previously demonstrated that the chronic mild stress-induced anhedonia can be reversed by chronic treatment with various antidepressant drugs. In this study we found that the stress-induced deficit in sucrose intake was gradually reversed by chronic (4-5 weeks) treatment with MK-801 (0.3 mg/kg i.p.), CGP 37849 (5 mg/kg i.p.) and CGP (25 mg/kg p.o.). The magnitude of this effect and its time course were comparable to those observed following similar administration of imipramine (10 mg/kg i.p. or p.o.). The increase in sucrose intake following chronic administration of imipramine and NMDA receptor antagonists was specific to stressed animals; the behaviour of non-stressed controls was unchanged by any of the drugs tested. These results confirm those of previous studies, carried out on 'normal' animals, suggesting that antagonists of NMDA receptors may have antidepressant properties.

NMDA receptor antagonists inhibit catalepsy induced by either dopamine D1 or D2 receptor antagonists

In the present study, we investigated the ability of NMDA receptor antagonists to inhibit catalepsy induced by haloperidol, or SCH23390 and clebopride, selective dopamine D1 and D2 receptor antagonists respectively. Catalepsy was measured by recording the time the animal remained with its forepaws placed over a rod 6 cm above the bench. Pretreatment with either the non-competitive NMDA receptor antagonist, MK-801 (0.25-0.5 mg/kg i.p.) or the competitive antagonist, LY274614 (10-20 mg/kg i.p.) reduced the cataleptic response produced by haloperidol (10 mg/kg), SCH23390 (2.5-10 mg/kp i.p.) or clebopride (5-20 mg/kg i.p.). This demonstrates that NMDA receptor antagonists will reduce both dopamine D1 and D2 receptor antagonist-induced catalepsy. Muscle relaxant doses of chlordiazepoxide (10 mg/kg i.p.) failed to reduce the catalepsy induced by haloperidol, suggesting that the anticataleptic effect of the NMDA receptor antagonists was not due to a non-specific action. These results support the hypothesis that NMDA receptor antagonists may have beneficial effects in disorders involving reduced dopaminergic function, such as Parkinson's disease.

Chronic administration of NMDA antagonists induces D2 receptor synthesis in rat striatum

Dopamine D2 receptor gene expression was examined in rat striatum after chronic treatment with N-methyl-D-aspartate (NMDA) receptor antagonists (ketamine at 15 mg/kg/day or MK-801 at 0.1, 0.2 and 0.4 mg/kg/day per os, for 50 days). The long-isoform mRNA, as well as the total D2 mRNA expression were induced. No change was noticed in striatal dopamine release or turnover. D2 binding studies carried out in MK-801 chronically treated (0.3 mg/kg/day per os, for 50 days) and control rats revealed an increased receptor density in treated animals without a significant change in receptor affinity. These results suggest that the synthesis of both striatal D2 receptor isoforms is postsynaptically regulated at the transcriptional level, by events triggered by glutamate through the NMDA-type receptor.