Effects of ketamine on different types of anxiety/fear and related memory in rats with lesions of the median raphe nucleus

Prolonged ketamine therapy differentially rescues psychobehavioural deficits via modulation of nitro-oxidative stress and oxytocin receptors in the gut-brain-axis of chronically-stressed mice

Ketamine is an anaesthetic known to have short but rapid-acting anti-depressant effects; however, the neurobehavioural effects of its prolonged use and its role on the oxytocin system in the gut-brain axis are largely undetermined. Female BALB/c mice were either exposed to the chronic unpredictable mild stress (CUMS) paradigm for 21 days and then treated with ketamine in four doses for 14 days or exposed to CUMS and treated simultaneously in four doses of ketamine during the last two weeks of CUMS exposure. After each dose, the forced swim test was conducted to assess depressive-like behaviour. Before sacrifice, all the mice were subjected to behavioural tests to assess anxiety, memory, and social interaction. Prolonged treatment of depression with ketamine did not rescue depressive-like behaviour. It did, however, improve depression-associated anxiety-like behaviours, short-term memory and social interaction deficits when compared to the stressed untreated mice. Furthermore, ketamine treatment enhanced plasma oxytocin levels, expression of oxytocin receptors; as well as abrogated nitro-oxidative stress biomarkers in the intestinal and hippocampal tissues. Taken together, our findings indicate that while short-term use of ketamine has anti-depressant benefits, its prolonged therapeutic use does not seem to adequately resolve depressive-like behaviour in mice.

Ketamine retards recovery from reward downshift and supports conditioned taste aversion

Ketamine is a noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonist with antidepressant, anxiolytic, and memory effects in clinical and preclinical studies. The present studies investigated the behavioral effects of ketamine in animals exposed to a consummatory successive negative contrast (cSNC) task involving unexpected reward downshift, negative emotion (frustration), and aversive memory. Food-restricted male rats had 5-min access to 32 % sucrose in each of 10 preshift sessions followed by 4 % sucrose in 4 postshift sessions. Unshifted controls had access to 4 % sucrose during all 14 sessions. Ketamine (10 mg/kg, ip) was injected 30 min before sessions 11 and 12 (Experiment 1) or immediately after session 11 (Experiment 3). The results showed that both pre- and postdownshift session injection of ketamine increased consummatory suppression, as Group 32/Ket exhibited lower sucrose intake than Groups 32/Sal, 4/Ket, and 4/Sal. These effects extended beyond the day(s) of injection. Experiments 2 and 4 showed that the same dose, route of administration, and time of injection induced significant conditioned taste aversion to 4 % sucrose, in the absence of reward downshift. These data suggest that ketamine induces an aversive state that may summate with frustration induced by reward downshift in the cSNC task and also support a conditioned taste aversion to 4 % sucrose in the absence of reward downshift. Implications for these and other experiments involving pre- and postsession administration of ketamine are discussed.

Evidence on the impairing effects of Ayahuasca on fear memory reconsolidation

RATIONALE

To uncover whether psychedelic drugs attenuate fear memory responses would advance the development of better psychedelic-based treatments for posttraumatic stress disorder (PTSD). Ayahuasca (AYA), a psychedelic brew containing indolamine N, N-dimethyltryptamine (DMT) and β-carbolines, facilitates fear extinction and improves neural plasticity. Upon retrieval, fear memory undergoes labilization and reconsolidation; however, the effects of AYA on this memory stabilization phase are unknown.

OBJECTIVES

We aimed to investigate the effects of AYA treatment on fear memory reconsolidation.

METHODS

Fear-conditioned Wistar rats received AYA (60, 120, or 240 mg/kg) or H₂O orally via gavage o.g. 20 min before, immediately, or 3 h after a short retrieval session. Analysis of AYA through liquid chromatography-tandem mass spectrometry was used to determine the content of DMT and β-carbolines in AYA.

RESULTS

AYA impaired fear memory reconsolidation when given 20 min before or 3 h after memory retrieval, with the dose of 60 mg/kg being effective at both moments. This dose of AYA was devoid of anxiolytic effect. Importantly, during retrieval, AYA did not change fear expression. The lack of retrieval abolished the reconsolidation impairing effect of AYA. The effects of AYA treatment 20 min before or 3 h after memory retrieval lasted at least 22 days, suggesting no spontaneous recovery of fear memory. Fear memory impairments induced by AYA treatment, at both moments, do not show reinstatement.

CONCLUSIONS

Our findings support the view that a low dose of AYA treatment impairs early and late stages of memory reconsolidation instead of facilitating fear extinction.

Remimazolam induced cognitive dysfunction in mice via glutamate excitotoxicity

Objective

Several lines of evidence demonstrated the role of anesthetic drugs in cognitive functions. Some anesthetic agents have been confirmed to be associated with long-term spatial memory and learning in aged animal models.

Methods

C57BL/6 mice were divided into four different groups based on different concentrations of remimazolam treatments. Behavioral phenotype was observed by open field, rota rod, Morris water maze, and elevated plus maze test. Western blot was performed to see the expression pattern of different proteins. Confocal microscopy images were taken for neuronal and glial cells to see the effect of remimazolam on CNS cells.

Results

We showed that remimazolam, a new anesthetic drug, impaired cognitive behavior. Repetitive doses of remimazolam have been found to induce neuronal loss with a significant change in morphology. Here, we showed that a higher concentration of remimazolam had a significant effect on CNS cell activation. We showed that remimazolam caused memory dysfunction by inducing neuronal apoptosis via glutamate excitotoxicity. It also exhibited amyloid β plaque in the brain via abnormal phosphorylation of tau protein. Remimazolam-mediated regulation of glial cells in mouse cortex was observed and robust activation of astrocytes and microglial cells was found. Finally, we assessed the behavioral phenotype of mice and found that treatment with remimazolam induced significant behavioral changes and memory dysfunction.

Conclusions

This study provides insight into the mechanism of anesthetic drug-induced memory deficits and may help improve the therapeutic effects of anesthesia agents in clinical applications.

Intra-prefrontal cyclosporine potentiates ketamine-induced fear extinction in rats

Several brain regions, including the medial prefrontal cortex (mPFC), are important in the process of fear extinction learning. Ketamine is a glutamate N-methyl-D-aspartate (NMDA) receptor antagonist, which is shown to play a role in extinction modulation. Ketamine and calcineurin (CN), an intracellular protein phosphatase, have several common targets in the cells. Therefore, in the present study, our aim is to investigate the possible role of calcineurin in the mPFC on the enhancing effects of ketamine in fear extinction. First, different doses of a CN inhibitor, cyclosporine-A (CsA), were micro-injected into the infralimbic (IL) region of the mPFC prior to extinction training in a classical conditioning model in rats. Next, sub-effective doses of CsA (Intra-mPFC) and ketamine (i.p.) were co-administered in another cohort of rats to find their possible interactions. Enzymatic activity of calcineurin was measured in the IL-mPFC following drug administration. We used the elevated plus-maze (EPM) and open field (OF) test for further behavioral assessments. The results showed that CsA can enhance the extinction of conditioned fear and inhibit the enzyme CN at a dose of 20 nM. The combination of sub-effective doses of CsA (5 nM) and ketamine (10 mg/kg) could again enhance the extinction of fear and reduce CN activity in the region. Our results propose that inhibition of CN in the IL-mPFC is involved in the extinction of fear and ketamine enhancement of extinction is probably mediated by reducing CN activity in this part of the brain.

Effects of Ketamine on Rodent Fear Memory

Ketamine, a multimodal anesthetic drug, has become increasingly popular in the treatment of pain following traumatic injury as well as treatment-resistant major depressive disorders. However, the psychological impact of this dissociative medication on the development of stress-related disorders such as post-traumatic stress disorder (PTSD) remains controversial. To address these concerns, preclinical studies have investigated the effects of ketamine administration on fear memory and stress-related behaviors in laboratory animals. Despite a well-documented line of research examining the effects of ketamine on fear memory, there is a lack of literature reviews on this important topic. Therefore, this review article summarizes the current preclinical literature on ketamine and fear memory with a particular emphasis on the route, dose, and timing of ketamine administration in rodent fear conditioning studies. Additionally, this review describes the molecular mechanisms by which ketamine may impact fear memory and stress-related behaviors. Overall, findings from previous studies are inconsistent in that fear memory may be increased, decreased, or unaltered following ketamine administration in rodents. These conflicting results can be explained by factors such as the route, dose, and timing of ketamine administration; the interaction between ketamine and stress; and individual variability in the rodent response to ketamine. This review also recommends that future preclinical studies utilize a clinically relevant route of administration and account for biological sex differences to improve translation between preclinical and clinical investigations.

Ketamine effects on anxiety and fear-related behaviors: Current literature evidence and new findings

Ketamine, a non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist, presents a rapid and sustained antidepressant effect in clinical and preclinical studies. Regarding ketamine effects on anxiety, there is a widespread discordance among pre-clinical studies. To address this issue, the present study reviewed the literature (electronic database MEDLINE) to summarize the profile of ketamine effects in animal tests of anxiety/fear. We found that ketamine anxiety/fear-related effects may depend on the anxiety paradigm, schedule of ketamine administration and tested species. Moreover, there was no report of ketamine effects in animal tests of fear related to panic disorder (PD). Based on that finding, we evaluated if treatment with ketamine and another NMDA antagonist, MK-801, would induce acute and sustained (24 hours later) anxiolytic and/or panicolytic-like effects in animals exposed to the elevated T-maze (ETM). The ETM evaluates, in the same animal, conflict-evoked and fear behaviors, which are related, respectively, to generalized anxiety disorder and PD. Male Wistar rats were systemically treated with racemic ketamine (10, 30 and 80 mg/kg) or MK-801 (0.05 and 0.1 mg/kg) and tested in the ETM in the same day or 24 hours after their administration. Ketamine did not affect the behavioral tasks performed in the ETM acutely or 24 h later. MK-801 impaired inhibitory avoidance in the ETM only at 45 min post-injection, suggesting a rapid but not sustained anxiolytic-like effect. Altogether our results suggest that ketamine might have mixed effects in anxiety tests while it does not affect panic-related behaviors.

Psilocin and ketamine microdosing: effects of subchronic intermittent microdoses in the elevated plus-maze in male Wistar rats

Short-term moderate doses of serotonergic and dissociative hallucinogens can be useful in the treatment of anxiety. Recently, a trend has developed for long-term intermittent 'microdosing' (usually one-tenth of a 'full' active dose), with reports of long-lasting relief from anxiety and related disorders; however, there is no scientific evidence for the efficacy of therapeutic microdosing nor to show its lasting effects. The objective of this study was to test for lasting effects on anxiety in rats after microdosing with ketamine or psilocin. Over 6 days, Wistar rats (N=40) were administered ketamine (0.5 or 3 mg/kg), psilocin (0.05 or 0.075 mg/kg), or saline on three occasions. A 5-min elevated plus-maze test was conducted 48 h after the final drug treatment (n=8). Dependent variables were entries (frequency), spent time (%), and distance traveled (cm) in each zone, as well as total frequency of rears, stretch-attend postures, and head dips. Statistical analyses of drug effects used separate independent one-way analysis of variance and pair-wise comparisons using independent t-tests. Statistical effects were modest or borderline and were most consistent with a mildly anxiogenic profile, which was significant at lower doses; however, this conclusion remains tentative. The lower doses of ketamine and psilocin produced comparable effects (to one another) across each variable, as did the higher doses. This pattern of effects may suggest a common (e.g. neurotransmitter/receptor) mechanism. We conclude that microdosing with hallucinogens for therapeutic purposes might be counter-productive; however, more research is needed to confirm our findings and to establish their translational relevance to clinical 'psychedelic' therapy.

Effects of Anesthetic Ketamine on Anxiety-Like Behaviour in Rats

There is scarce information regarding the effects of anesthetic doses of the non-competitive N-methyl-D-aspartate receptor antagonist ketamine on anxiety. The current study evaluated the acute effects of intraperitoneally (i.p.) administered anesthetic ketamine (100 mg/kg) i.p. on anxiety in rats. For this purpose, the light/dark and the open field tests were utilized. The effects of anesthetic ketamine on motility were also examined using a motility cage. In the light/dark test, anesthetic ketamine, administered 24 h before testing reduced the number of transitions between the light and dark compartments and the time spent in the light compartment in the rats compared with their control cohorts. In addition, ketamine was found to exert a depressive effect on rats' motility. In the open field test, animals treated with anesthetic ketamine 24 h before testing spent essentially no time in the central area of the apparatus, decreased horizontal ambulatory activity, and preserved to a certain extent their exploratory behaviour compared to their control counterparts. The results suggest that, in spite of its hypokinetic effect, a single anesthetic ketamine administration apparently induces an anxiety-like state, while largely preserving exploratory behaviour in the rat. These effects were time-dependent they since they were extinguished when testing was carried out 48 h after anesthetic ketamine administration.

Effects of systemic glutamatergic manipulations on conditioned eyeblink responses and hyperarousal in a rabbit model of post-traumatic stress disorder

Glutamatergic dysfunction is implicated in many neuropsychiatric conditions, including post-traumatic stress disorder (PTSD). Glutamate antagonists have shown some utility in treating PTSD symptoms, whereas glutamate agonists may facilitate cognitive behavioral therapy outcomes. We have developed an animal model of PTSD, based on conditioning of the rabbit's eyeblink response, that addresses two key features: conditioned responses (CRs) to cues associated with an aversive event and a form of conditioned hyperarousal referred to as conditioning-specific reflex modification (CRM). The optimal treatment to reduce both CRs and CRM is unpaired extinction. The goals of the study were to examine whether treatment with the N-methyl-D-aspartate glutamate receptor antagonist ketamine could reduce CRs and CRM, and whether the N-methyl-D-aspartate agonist D-cycloserine combined with unpaired extinction treatment could enhance the extinction of both. Administration of a single dose of subanesthetic ketamine had no significant immediate or delayed effect on CRs or CRM. Combining D-cycloserine with a single day of unpaired extinction facilitated extinction of CRs in the short term while having no impact on CRM. These results caution that treatments may improve one aspect of the PTSD symptomology while having no significant effects on other symptoms, stressing the importance of a multiple-treatment approach to PTSD and of animal models that address multiple symptoms.

Methoxetamine affects brain processing involved in emotional response in rats

BACKGROUND AND PURPOSE

Methoxetamine (MXE) is a novel psychoactive substance that is emerging on the Internet and induces dissociative effects and acute toxicity. Its pharmacological effects have not yet been adequately investigated.

EXPERIMENTAL APPROACH

We examined a range of behavioural effects induced by acute administration of MXE (0.5-5 mg·kg^-1 ; i.p.) in rats and whether it causes rapid neuroadaptive molecular changes.

KEY RESULTS

MXE (0.5-5 mg·kg^-1 ) affected motor activity in a dose- and time-dependent manner, inducing hypermotility and hypomotility at low and high doses respectively. At low and intermediate doses (0.5 and 1 mg·kg^-1 ), MXE induced anxious and/or obsessive-compulsive traits (marble burying test), did not significantly increase sociability (social interaction test) or induce spatial anxiety (elevated plus maze test). At a high dose (5 mg·kg^-1 ), MXE induced transient analgesia (tail-flick and hot-plate test), decreased social interaction time (social interaction test) and reduced immobility time while increasing swimming activity (forced swim test), suggesting an antidepressant effect. Acute MXE administration did not affect self-grooming behaviour at any dose tested. Immunohistochemical analysis showed that behaviourally active doses of MXE (1 and 5 mg·kg^-1 ) increased phosphorylation of ribosomal protein S6 in the medial prefrontal cortex and hippocampus.

CONCLUSIONS AND IMPLICATIONS

MXE differentially affected motor activity, behaviour and emotional states in rats, depending on the dose tested. As reported for ketamine, phosphorylation of the ribosomal protein S6 was increased in MXE-treated animals, thus providing a 'molecular snapshot' of rapid neuroadaptive molecular changes induced by behaviourally active doses of MXE.

The Combination of Long-term Ketamine and Extinction Training Contributes to Fear Erasure by Bdnf Methylation

A combination of antidepressant drugs and psychotherapy exhibits more promising efficacy in treating fear disorders than either treatment alone, but underlying mechanisms of such treatments remain largely unknown. Here we investigated the role of DNA methylation of the brain-derived neurotrophic factor (Bdnf) gene in the therapeutic effects of ketamine in combination with extinction training in a mouse model of post-traumatic stress disorder (PTSD) induced by inescapable electric foot shocks (IFS). Male mice received ketamine for 22 consecutive days starting 1 h after the IFS (long-term ketamine treatment) or 2 h prior to the extinction training on days 15 and 16 after the IFS (short-term ketamine treatment). The Open Field (OF) and Elevated Plus Maze (EPM) tests were conducted on days 18 and 20. The spontaneous recovery and fear renewal tests were performed on day 23. Mice, subjected to IFS, exhibited anxiety-like behavior and fear relapse, accompanied by the increased levels of DNA methyltransferases, hyper-methylation of Bdnf gene, and decreased BDNF mRNA expression in the medial prefrontal cortex (mPFC) and hippocampus (HIP). Long-term treatment with ketamine combined with extinction training alleviated the IFS-induced abnormalities. These results suggest that long-term ketamine treatment in combination with extinction training may ameliorate fear relapse in the murine model of PTSD, at least in part, by normalizing DNA methylation of Bdnf gene.

Study of neurometabolic and behavioral alterations in rodent model of mild traumatic brain injury: a pilot study

Mild traumatic brain injury (mTBI) is the most common form of TBI (70-90%) with consequences of anxiety-like behavioral alterations in approximately 23% of mTBI cases. This study aimed to assess whether mTBI-induced anxiety-like behavior is a consequence of neurometabolic alterations. mTBI was induced using a weight drop model to simulate mild human brain injury in rodents. Based on injury induction and dosage of anesthesia, four animal groups were included in this study: (i) injury with anesthesia (IA); (ii) sham1 (injury only, IO); (iii) sham2 (only anesthesia, OA); and (iv) control rats. After mTBI, proton magnetic resonance spectroscopy (¹ H-MRS) and neurobehavioral analysis were performed in these groups. At day 5, reduced taurine (Tau)/total creatine (tCr, creatine and phosphocreatine) levels in cortex were observed in the IA and IO groups relative to the control. These groups showed mTBI-induced anxiety-like behavior with normal cognition at day 5 post-injury. An anxiogenic effect of repeated dosage of anesthesia in OA rats was observed with normal Tau/tCr levels in rat cortex, which requires further examination. In conclusion, this mTBI model closely mimics human concussion injury with anxiety-like behavior and normal cognition. Reduced cortical Tau levels may provide a putative neurometabolic basis of anxiety-like behavior following mTBI.

The novel ketamine analog methoxetamine produces dissociative-like behavioral effects in rodents

RATIONALE

Methoxetamine (MXE) is a ketamine analog sold online that has been subject to widespread abuse for its dissociative and hallucinogenic effects. Previous studies have shown that MXE has high affinity for the phencyclidine (PCP) binding site located within the channel pore of the NMDA receptor (NMDAR), but little is known about its behavioral effects. Dissociative anesthetics such as ketamine and PCP produce a characteristic behavioral profile in rats that includes locomotor hyperactivity and disruption of prepulse inhibition (PPI) of acoustic startle.

METHODS

The goal of the present investigation was to determine whether MXE produces PCP-like effects in Sprague-Dawley rats using the PPI paradigm and the behavioral pattern monitor (BPM), which enables analyses of patterns of locomotor activity and investigatory behavior. PPI studies were conducted with several other uncompetitive NMDAR antagonists that produce dissociative effects in humans, including PCP, the S-(+) and R-(-) isomers of ketamine, and N-allylnormetazocine (NANM; SKF-10,047).

RESULTS

MXE disrupted PPI when administered at 3 and 10 mg/kg SC. The rank order of potency of MXE and the other test compounds in the PPI paradigm (PCP > MXE > S-(+)-ketamine > NANM > R-(-)-ketamine) parallels their affinities for the PCP binding site reported in the literature. When tested in the BPM, 10 mg/kg MXE induced locomotor hyperactivity, reduced the number of rearings, increased the roughness of locomotor paths, and produced perseverative patterns of locomotion. Administration of PCP (2.25 and 6.75 mg/kg, SC) produced a similar profile of effects in the BPM.

CONCLUSIONS

These results indicate that MXE produces a behavioral profile similar to that of other psychotomimetic uncompetitive NMDAR antagonists. Our findings support the classification of MXE as a dissociative drug and suggest that it likely has effects and abuse potential similar to that of PCP and ketamine.

Immediate ketamine treatment does not prevent posttraumatic stress responses in an animal model for PTSD

Clinical studies suggest that administration of ketamine hydrochloride-an antagonist at the N-methyl-d-aspartate ionophore-provides short-term amelioration for depressive symptoms. The effects of a brief course of ketamine given immediately following exposure to psychogenic stress on the behavioral stress responses were assessed in an animal model of posttraumatic stress disorder. Animals exposed to stress were treated 1h later with ketamine (0.5, 5, and 15 mg/kg) or vehicle for three days (N = 107). Outcome measures included behavior in the elevated plus maze (EPM) and acoustic startle response (ASR) tests 30 days after initial exposure and freezing behavior upon exposure to a trauma-cue on day 31. Pre-set cut-off behavioral criteria classified exposed animals according to their EPM and ASR response-patterns into "extreme," "minimal," or "partial" behavioral response for analysis of prevalence rates of "PTSD-like behavior." Circulating corticosterone levels were assessed 20 min after injection of ketamine in exposed and unexposed animals (N = 62). The dexamethasone suppression test was used to assess negative feedback inhibition of the HPA axis. Prevalence rates of extremely-, partially-, or minimally-disrupted behavior demonstrated that ketamine administered immediately following stress exposure was ineffective in alleviating "PTSD-like behavior" at day 30 after exposure. Administration of ketamine was associated with increase in freezing behavior after exposure to a trauma-cue on day 31. Corticosterone levels were significantly suppressed by ketamine only in the exposed animals. Administration of ketamine immediately following trauma-exposure may not only be ineffective but actually detrimental in the long term. A disruption of the post-stress HPA-response has been raised as a contributing factor.

Guanosine produces an antidepressant-like effect through the modulation of NMDA receptors, nitric oxide-cGMP and PI3K/mTOR pathways

Guanosine is an extracellular signaling molecule implicated in the modulation of glutamatergic transmission and neuroprotection. The present study evaluated the antidepressant-like effect of guanosine in the forced swimming test (FST) and in the tail suspension test (TST) in mice. The contribution of NMDA receptors as well as l-arginine-NO-cGMP and PI3K-mTOR pathways to this effect was also investigated. Guanosine administered orally produced an antidepressant-like effect in the FST (0.5-5 mg/kg) and TST (0.05-0.5 mg/kg). The anti-immobility effect of guanosine in the TST was prevented by the treatment of mice with NMDA (0.1 pmol/site, i.c.v.), d-serine (30 μg/site, i.c.v., a co-agonist of NMDA receptors), l-arginine (750 mg/kg, i.p., a substrate for nitric oxide synthase), sildenafil (5 mg/kg, i.p., a phosphodiesterase 5 inhibitor), LY294002 (10 μg/site, i.c.v., a reversible PI3K inhibitor), wortmannin (0.1 μg/site, i.c.v., an irreversible PI3K inhibitor) or rapamycin (0.2 nmol/site, i.c.v., a selective mTOR inhibitor). In addition, the administration of ketamine (0.1 mg/kg, i.p., a NMDA receptor antagonist), MK-801 (0.001 mg/kg, i.p., another NMDA receptor antagonist), 7-nitroindazole (50 mg/kg, i.p., a neuronal nitric oxide synthase inhibitor) or ODQ (30 pmol/site i.c.v., a soluble guanylate cyclase inhibitor) in combination with a sub-effective dose of guanosine (0.01 mg/kg, p.o.) reduced the immobility time in the TST when compared with either drug alone. None of the treatments affected locomotor activity. Altogether, results firstly indicate that guanosine exerts an antidepressant-like effect that seems to be mediated through an interaction with NMDA receptors, l-arginine-NO-cGMP and PI3K-mTOR pathways.

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.

Behavioral alterations and pro-oxidant effect of a single ketamine administration to mice

A growing body of evidence has pointed to the ionotropic glutamate N-methyl-d-aspartate receptor (NMDA) as an important player in the etiology of psychopathologies, including anxiety and major depression. Clinical findings suggest that ketamine may be used for the treatment of major depression. There is evidence that reactive oxygen species also play an important role in the pathogenesis of many diseases, particularly those which are neurological and psychiatric in nature. This study examined the behavioral and oxidative stress alterations after a single administration of ketamine (5, 10 and 20mg/kg i.p.) in mice. Ketamine presented a significant anxiogenic effect in the elevated plus-maze model of anxiety, also increasing locomotor activity. In the forced swimming and tail suspension tests, a significant decrease in immobility time after ketamine administration was observed. In addition to the behavioral changes induced by ketamine, this drug also increased lipid peroxidation, nitrite content and catalase activity, while decreased GSH levels in mice prefrontal cortex. In conclusion, our results confirm the antidepressant effects of ketamine, also showing a pro-oxidant effect of this drug.

Anxiolytic- and antidepressant-like properties of ketamine in behavioral and neurophysiological animal models

Ketamine, a dissociative anesthetic agent, appears to have rapid antidepressant effects at sub-anesthetic doses in clinically depressed patients. Although promising, these results need to be replicated in double-blind placebo-controlled studies, a strategy thwarted by the psychoactive effects of ketamine, which are obvious to both patients and clinicians. Alternatively, demonstrations of the psychotherapeutic effects of ketamine in animal models are also complicated by ketamine's side-effects on general activity, which have not been routinely measured or taken into account in experimental studies. In this study we found that ketamine decreased "behavioral despair" in the forced swim test, a widely used rats model of antidepressant drug action. This effect was not confounded by side-effects on general activity, and was comparable to that of a standard antidepressant drug, fluoxetine. Interestingly, ketamine also produced anxiolytic-like effects in the elevated-plus-maze. Importantly, the effective dose of ketamine in the plus-maze did not affect general locomotion measures, in either the plus-maze or in the open field test. While the selective N-methyl-d-aspartic acid (NMDA) receptor antagonist MK-801 also produced antidepressant-like and anxiolytic-like effects, these were mostly confounded by changes in general activity. Finally, in a neurophysiological model of anxiolytic drug action, ketamine reduced the frequency of reticularly-activated theta oscillations in the hippocampus, similar to the proven anxiolytic drug diazepam. This particular neurophysiological signature is common to all known classes of anxiolytic drugs (i.e. benzodiazepines, 5-HT1A agonists, antidepressants) and provides strong converging evidence for the anxiolytic-like effects of ketamine. Further studies are needed to understand the underlying pharmacological mechanisms of ketamine's effects in these experiments, since it is not clear they were mimicked by the selective NMDA antagonist MK-801.

Behavioral effects of ketamine and toxic interactions with psychostimulants

Background

The anesthetic drug ketamine (KT) has been reported to be an abused drug and fatal cases have been observed in polydrug users. In the present study, considering the possibility of KT-enhanced toxic effects of other drugs, and KT-induced promotion of an overdose without making the subject aware of the danger due to the attenuation of several painful subjective symptoms, the intraperitoneal (i.p.) KT-induced alterations in behaviors and toxic interactions with popular co-abused drugs, the psychostimulants cocaine (COC) and methamphetamine (MA), were examined in ICR mice.

Results

A single dose of KT caused hyperlocomotion in a low (30 mg/kg, i.p.) dose group, and hypolocomotion followed by hyperlocomotion in a high (100 mg/kg, i.p.) dose group. However, no behavioral alterations derived from enhanced stress-related depression or anxiety were observed in the forced swimming or the elevated plus-maze test. A single non-fatal dose of COC (30 mg/kg, i.p.) or MA (4 mg/kg, i.p.) caused hyperlocomotion, stress-related depression in swimming behaviors in the forced swimming test, and anxiety-related behavioral changes (preference for closed arms) in the elevated plus-maze test. For the COC (30 mg/kg) or MA (4 mg/kg) groups of mice simultaneously co-treated with KT, the psychostimulant-induced hyperlocomotion was suppressed by the high dose KT, and the psychostimulant-induced behavioral alterations in the above tests were reversed by both low and high doses of KT. For the toxic dose COC (70 mg/kg, i.p.)- or MA (15 mg/kg, i.p.)-only group, mortality and severe seizures were observed in some animals. In the toxic dose psychostimulant-KT groups, KT attenuated the severity of seizures dose-dependently. Nevertheless, the mortality rate was significantly increased by co-treatment with the high dose KT.

Conclusion

Our results demonstrated that, in spite of the absence of stress-related depressive and anxiety-related behavioral alterations following a single dose of KT treatment, and in spite of the KT-induced anticonvulsant effects and attenuation of stress- and anxiety-related behaviors caused by COC or MA, the lethal effects of these psychostimulants were increased by KT.

The neurobiology and control of anxious states

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

The Vogel conflict test: procedural aspects, gamma-aminobutyric acid, glutamate and monoamines

A multitude of mechanisms are involved in the control of emotion and in the response to stress. These incorporate mediators/targets as diverse as gamma-aminobutyric acid (GABA), excitatory amino acids, monoamines, hormones, neurotrophins and various neuropeptides. Behavioural models are indispensable for characterization of the neuronal substrates underlying their implication in the etiology of anxiety, and of their potential therapeutic pertinence to its management. Of considerable significance in this regard are conflict paradigms in which the influence of drugs upon conditioned (trained) behaviours is examined. For example, the Vogel conflict test, which was introduced some 30 years ago, measures the ability of drugs to release the drinking behaviour of water-deprived rats exposed to a mild aversive stimulus ("punishment"). This model, of which numerous procedural variants are discussed herein, has been widely used in the evaluation of potential anxiolytic agents. In particular, it has been exploited in the characterization of drugs interacting with GABAergic, glutamatergic and monoaminergic networks, the actions of which in the Vogel conflict test are summarized in this article. More recently, the effects of drugs acting at neuropeptide receptors have been examined with this model. It is concluded that the Vogel conflict test is of considerable utility for rapid exploration of the actions of anxiolytic (and anxiogenic) drugs. Indeed, in view of its clinical relevance, broader exploitation of the Vogel conflict test in the identification of novel classes of anxiolytic agents, and in the determination of their mechanisms of action, would prove instructive.

Effects of excitotoxic median raphe lesions on scopolamine-induced working memory deficits in inhibitory avoidance

The aim of the present study was to investigate the effects of excitotoxic damage of the serotonergic cell bodies in the median raphe nucleus (MRN) on the scopolamine-induced working memory deficits in a single-trial light/dark inhibitory avoidance task. Rats were given 1 mg/kg of scopolamine hydrobromide (intraperitonal, i.p.) or saline before the inhibitory avoidance training, in which initial preference to the dark compartment (escape latency) was used to measure nonmnemonic behaviors, and response latency to enter the dark compartment immediately after the shock was used to measure working memory. It was found that scopolamine significantly reduced escape latencies in sham-lesioned rats, whereas it had no effect in the rats with MRN lesions. Although MRN lesion per se did not alter response latency, it prevented scopolamine-induced decrease in this parameter. These results suggest that the antagonistic interactive processes between serotonergic projections of the MRN and the muscarinic cholinergic system modulate nonmnemonic attentional component of working memory formation in the inhibitory avoidance.

Excitotoxic median raphe lesions aggravate working memory storage performance deficits caused by scopolamine infusion into the dentate gyrus of the hippocampus in the inhibitory avoidance task in rats

The interactions between the median raphe nucleus (MRN) serotonergic system and the septohippocampal muscarinic cholinergic system in the modulation of immediate working memory storage performance were investigated. Rats with sham or ibotenic acid lesions of the MRN were bilaterally implanted with cannulae in the dentate gyrus of the hippocampus and tested in a light/dark step-through inhibitory avoidance task in which response latency to enter the dark compartment immediately after the shock served as a measure of immediate working memory storage. MRN lesion per se did not alter response latency. Post-training intrahippocampal scopolamine infusion (2 and 4 microg/side) produced a more marked reduction in response latencies in the lesioned animals compared to the sham-lesioned rats. Results suggest that the immediate working memory storage performance is modulated by synergistic interactions between serotonergic projections of the MRN and the muscarinic cholinergic system of the hippocampus.

Effects of excitotoxic median raphe lesion on working memory deficits produced by the dorsal hippocampal muscarinic receptor blockade in the inhibitory avoidance in rats

The experiments investigated the interactions between median raphe nucleus (MRN) serotonergic and septo-hippocampal muscarinic cholinergic systems in the modulation of forming and storing performances of working memory. Rats with ibotenic acid-induced MRN-lesion bilaterally received scopolamine (2-4 microg/each side) infusion into the dentate gyrus of the dorsal hippocampus and were tested in a single trial step-through inhibitory avoidance. Initial preference to the dark compartment (escape latency) was taken as the measure of non-mnemonic behaviours and response latency to enter the dark compartment immediately after the foot-shock was used to measure working memory. The high-dose scopolamine infusion 10 min before the training decreased escape latencies in the sham-lesioned rats, whereas had no effect in the MRN-lesioned rats. Although MRN lesion per se did not alter response latency, it alleviated pre-training scopolamine-induced decrease, but aggravated post-training scopolamine-induced reduction in this parameter. These results suggest that the antagonistic interactive processes between MRN-serotonergic and hippocampal cholinergic systems modulate non-mnemonic component of working memory formation, whereas the storing performance of working memory is modulated by the synergistic interactions between these systems in the hippocampus, mainly in the dentate gyrus.