Non-competitive N-methyl-D-aspartate antagonists are potent activators of ventral tegmental A10 dopamine neurons

Retigabine promotes ketamine's antidepressant effect in the forced swim test in male and female C57BL/6J mice

Ketamine has been increasingly used as a rapid-onset antidepressant in specific clinical settings. However, as a psychedelic reagent, the potential of physical and psychological dependence limits its clinical use. Here, we added retigabine, a KCNQ channel opener, as an adjunctive treatment to observe its effect on ketamine's antidepressant property in a forced swim test in both male and female C57BL/6 J mice. Behavioral data demonstrated that intraperitoneal injection of ketamine exhibited a dose-dependent effect on animals' immobility performance in the forced swim test. Adding retigabine was sufficient to induce a remarkable antidepressant effect in mice treated with a relatively lower dose of ketamine which failed to be antidepressant when administrated separately. When simultaneously gave retigabine, ketamine's antidepressant effect in the forced swim test was significantly enhanced with a prolonged effective duration. Together, these results from both male and female mice indicated that adjunctive treatment with retigabine was an alternative to promote the antidepressant effect of ketamine, thus holding the possibility of encountering its possible physical and psychological dependence.

Noradrenergic modulation of stress resilience

Resilience represents an active adaption process in the face of adversity, trauma, tragedy, threats, or significant sources of stress. Investigations of neurobiological mechanisms of resilience opens an innovative direction for preclinical research and drug development for various stress-related disorders. The locus coeruleus norepinephrine system has been implicated in mediating stress susceptibility versus resilience. It has attracted increasing attention over the past decades with the revolution of modern neuroscience technologies. In this review article, we first briefly go over resilience-related concepts and introduce rodent paradigms for segregation of susceptibility and resilience, then highlight recent literature that identifies the neuronal and molecular substrates of active resilience in the locus coeruleus, and discuss possible future directions for resilience investigations.

The Resilient Phenotype Induced by Prophylactic Ketamine Exposure During Adolescence Is Mediated by the Ventral Tegmental Area-Nucleus Accumbens Pathway

BACKGROUND

Major depressive disorder is prevalent in children and adolescents and is associated with a high degree of morbidity throughout life, with potentially devastating personal consequences and public health impact. The efficacy of ketamine (KET) as an antidepressant has been demonstrated in adolescent rodents; however, the neurobiological mechanisms underlying these effects are unknown. Recent evidence showed that KET reverses stress-induced (i.e., depressive-like) deficits within major mesocorticolimbic regions, such as the prefrontal cortex, nucleus accumbens (NAc), and hippocampus, in adult rodents. However, little is known about KET's effect in the ventral tegmental area (VTA), which provides the majority of dopaminergic input to these brain regions.

METHODS

We characterized behavioral, biochemical, and electrophysiological effects produced by KET treatment in C57BL/6J male mice during adolescence (n = 7-10 per condition) within the VTA and its major projection regions, namely, the NAc and prefrontal cortex. Subsequently, molecular targets within the VTA-NAc projection were identified for viral gene transfer manipulations to recapitulate the effects of stress or KET treatment.

RESULTS

Repeated KET treatment produced a robust proresilient response to chronic social defeat stress. This effect was largely driven by Akt signaling activity within the VTA and NAc, and it could be blocked or recapitulated through direct Akt-viral-mediated manipulation. Additionally, we found that the KET-induced resilient phenotype is dependent on VTA-NAc, but not VTA-prefrontal cortex, pathway activity.

CONCLUSIONS

These findings indicate that KET exposure during adolescence produces a proresilient phenotype mediated by changes in Akt intracellular signaling and altered neuronal activity within the VTA-NAc pathway.

Reproducing the dopamine pathophysiology of schizophrenia and approaches to ameliorate it: a translational imaging study with ketamine

Patients with schizophrenia show increased striatal dopamine synthesis capacity in imaging studies. The mechanism underlying this is unclear but may be due to N-methyl-D-aspartate receptor (NMDAR) hypofunction and parvalbumin (PV) neuronal dysfunction leading to disinhibition of mesostriatal dopamine neurons. Here, we develop a translational mouse model of the dopamine pathophysiology seen in schizophrenia and test approaches to reverse the dopamine changes. Mice were treated with sub-chronic ketamine (30 mg/kg) or saline and then received in vivo positron emission tomography of striatal dopamine synthesis capacity, analogous to measures used in patients. Locomotor activity was measured using the open-field test. In vivo cell-type-specific chemogenetic approaches and pharmacological interventions were used to manipulate neuronal excitability. Immunohistochemistry and RNA sequencing were used to investigate molecular mechanisms. Sub-chronic ketamine increased striatal dopamine synthesis capacity (Cohen's d = 2.5) and locomotor activity. These effects were countered by inhibition of midbrain dopamine neurons, and by activation of PV interneurons in pre-limbic cortex and ventral subiculum of the hippocampus. Sub-chronic ketamine reduced PV expression in these cortical and hippocampal regions. Pharmacological intervention with SEP-363856, a novel psychotropic agent with agonism at trace amine receptor 1 (TAAR1) and 5-HT1A receptors but no appreciable action at dopamine D2 receptors, significantly reduced the ketamine-induced increase in dopamine synthesis capacity. These results show that sub-chronic ketamine treatment in mice mimics the dopaminergic alterations in patients with psychosis, that this requires activation of midbrain dopamine neurons, and can be ameliorated by activating PV interneurons and by a TAAR1/5-HT1A agonist. This identifies novel therapeutic approaches for targeting presynaptic dopamine dysfunction in patients with schizophrenia and effects of ketamine relevant to its therapeutic use for  treating major depression.

MK801-induced locomotor activity in preweanling and adolescent male and female rats: role of the dopamine and serotonin systems

RATIONALE

MK801, like other NMDA receptor open-channel blockers (e.g., ketamine and phencyclidine), increases the locomotor activity of rats and mice. Whether this behavioral effect ultimately relies on monoamine neurotransmission is of dispute.

OBJECTIVE

The purpose of this study was to determine whether these psychopharmacological effects and underlying neural mechanisms vary according to sex and age.

METHODS

Across four experiments, male and female preweanling and adolescent rats were pretreated with vehicle, the monoamine-depleting agent reserpine (1 or 5 mg/kg), the dopamine (DA) synthesis inhibitor ∝-methyl-_DL-p-tyrosine (AMPT), the serotonin (5-HT) synthesis inhibitor 4-chloro-_DL-phenylalanine methyl ester hydrochloride (PCPA), or both AMPT and PCPA. The locomotor activity of preweanling and adolescent rats was then measured after saline or MK801 (0.3 mg/kg) treatment.

RESULTS

As expected, MK801 increased the locomotor activity of all age groups and both sexes, but the stimulatory effects were significantly less pronounced in male adolescent rats. Preweanling rats and adolescent female rats were more sensitive to the effects of DA and 5-HT synthesis inhibitors, as AMPT and PCPA caused only small reductions in the MK801-induced locomotor activity of male adolescent rats. Co-administration of AMPT+PCPA or high-dose reserpine (5 mg/kg) treatment substantially reduced MK801-induced locomotor activity in both age groups and across both sexes.

CONCLUSIONS

These results, when combined with other recent studies, show that NMDA receptor open-channel blockers cause pronounced age-dependent behavioral effects that can vary according to sex. The neural changes underlying these sex and age differences appear to involve monoamine neurotransmission.

Effects of dopamine and serotonin synthesis inhibitors on the ketamine-, d-amphetamine-, and cocaine-induced locomotor activity of preweanling and adolescent rats: sex differences

The pattern of ketamine-induced locomotor activity varies substantially across ontogeny and according to sex. Although ketamine is classified as an NMDA channel blocker, it appears to stimulate the locomotor activity of both male and female rats via a monoaminergic mechanism. To more precisely determine the neural mechanisms underlying ketamine's actions, male and female preweanling and adolescent rats were pretreated with vehicle, the dopamine (DA) synthesis inhibitor ∝-methyl-_DL-p-tyrosine (AMPT), or the serotonin (5-HT) synthesis inhibitor 4-chloro-_DL-phenylalanine methyl ester hydrochloride (PCPA). After completion of the pretreatment regimen, the locomotor activating effects of saline, ketamine, d-amphetamine, and cocaine were assessed during a 2 h test session. In addition, the ability of AMPT and PCPA to reduce dorsal striatal DA and 5-HT content was measured in male and female preweanling, adolescent, and adult rats. Results showed that AMPT and PCPA reduced, but did not fully attenuate, the ketamine-induced locomotor activity of preweanling rats and female adolescent rats. Ketamine (20 and 40 mg/kg) caused a minimal amount of locomotor activity in male adolescent rats, and this effect was not significantly modified by AMPT or PCPA pretreatment. When compared to ketamine, d-amphetamine and cocaine produced different patterns of locomotor activity across ontogeny; moreover, AMPT and PCPA pretreatment affected psychostimulant- and ketamine-induced locomotion differently. When these results are considered together, it appears that both dopaminergic and serotonergic mechanisms mediate the ketamine-induced locomotor activity of preweanling and female adolescent rats. The dichotomous actions of ketamine relative to the psychostimulants in vehicle-, AMPT-, and PCPA-treated rats, suggests that ketamine modulates DA and 5-HT neurotransmission through an indirect mechanism.

Effects of monoamine depletion on the ketamine-induced locomotor activity of preweanling, adolescent, and adult rats: Sex and age differences

Ketamine significantly increases the locomotor activity of rodents, however this effect varies according to the sex and age of the animal being tested. To determine the role monoamine systems play in ketamine's locomotor activating effects: (a) male and female preweanling, adolescent, and adult rats were pretreated with vehicle or the monoamine depleting agent reserpine (1 or 5 mg/kg), and (b) the behavioral actions of ketamine (20 or 40 mg/kg) were then compared to d-amphetamine (2 mg/kg) and cocaine (10 or 15 mg/kg). The ability of reserpine to deplete dorsal striatal dopamine (DA) and serotonin (5-HT) in male and female rats was determined using HPLC. Ketamine caused substantial increases in the locomotion of preweanling rats and older female rats (adolescents and adults), but had only small stimulatory effects on adolescent and adult male rats. When compared to cocaine and d-amphetamine, ketamine was especially sensitive to the locomotor-inhibiting effects of monoamine depletion. Ketamine-induced locomotion is at least partially mediated by monoamine systems, since depleting DA and 5-HT levels by 87-96% significantly attenuated the locomotor activating effects of ketamine in male and female rats from all three age groups. When administered to reserpine-pretreated rats, ketamine produced a different pattern of behavioral effects than either psychostimulant, suggesting that ketamine does not stimulate locomotor activity via actions at the presynaptic terminal. Instead, our results are consistent with the hypothesis that ketamine increases locomotor activity through a down-stream mechanism, possibly involving ascending DA and/or 5-HT projection neurons.

Sex-dependent changes in ketamine-induced locomotor activity and ketamine pharmacokinetics in preweanling, adolescent, and adult rats

Although ketamine has long been known to increase locomotor activity, only recently was it realized that this behavioral effect varies according to both sex and age. The purpose of the present study was threefold: first, to measure the locomotor activating effects of ketamine in male and female rats across early ontogeny and into adulthood; second, to assess ketamine and norketamine pharmacokinetics in the dorsal striatum and hippocampus of the same age groups; and, third, to use curvilinear regression to determine the relationship between locomotor activity and dorsal striatal concentrations of ketamine and norketamine. A high dose of ketamine (80 mg/kg, i.p.) was administered in order to examine the complete cycle of locomotor responsiveness across a 280-min testing session. In separate groups of rats, the dorsal striata and hippocampi were removed at 10 time points (0-360 min) after ketamine administration and samples were assayed for ketamine, norketamine, and dopamine using HPLC. In female rats, ketamine produced high levels of locomotor activity that varied only slightly among age groups. Male preweanling rats responded like females, but adolescent and adult male rats exhibited lesser amounts of ketamine-induced locomotor activity. Ketamine and norketamine pharmacokinetics, especially peak values and area under the curve, generally mirrored age- and sex-dependent differences in locomotor activity. Among male rats and younger female rats, dorsal striatal ketamine and norketamine levels accounted for a large proportion of the variance in locomotor activity. In adult female rats, however, an additional factor, perhaps involving other ketamine and norketamine metabolites, was influencing locomotor activity.

The effects of ketamine on dopaminergic function: meta-analysis and review of the implications for neuropsychiatric disorders

Ketamine is a non-competitive antagonist at the N-methyl-d-aspartate receptor. It has recently been found to have antidepressant effects and is a drug of abuse, suggesting it may have dopaminergic effects. To examine the effect of ketamine on the dopamine systems, we carried out a systematic review and meta-analysis of dopamine measures in the rodent, human and primate brain following acute and chronic ketamine administration relative to a drug-free baseline or control condition. Systematic search of PubMed and PsychInfo electronic databases yielded 40 original peer-reviewed studies. There were sufficient rodent studies of the acute effects of ketamine at sub-anaesthetic doses for meta-analysis. Acute ketamine administration in rodents is associated with significantly increased dopamine levels in the cortex (Hedge's g= 1.33, P<0.01), striatum (Hedge's g=0.57, P<0.05) and the nucleus accumbens (Hedge's g=1.30, P<0.05) compared to control conditions, and 62-180% increases in dopamine neuron population activity. Sub-analysis indicated elevations were more marked in in vivo (g=1.93) than ex vivo (g=0.50) studies. There were not enough studies for meta-analysis in other brain regions studied (hippocampus, ventral pallidum and cerebellum), or of the effects of chronic ketamine administration, although consistent increases in cortical dopamine levels (from 88 to 180%) were reported in the latter studies. In contrast, no study showed an effect of anaesthetic doses (>100 mg kg^-1) of ketamine on dopamine levels ex vivo, although this remains to be tested in vivo. Findings in non-human primates and in human studies using positron emission tomography were not consistent. The studies reviewed here provide evidence that acute ketamine administration leads to dopamine release in the rodent brain. We discuss the inter-species variation in the ketamine induced dopamine release as well as the implications for understanding psychiatric disorders, in particular substance abuse, schizophrenia, and the potential antidepressant properties of ketamine, and comparisons with stimulants and other NMDA antagonists. Finally we identify future research needs.

Effects of ketamine on the unconditioned and conditioned locomotor activity of preadolescent and adolescent rats: impact of age, sex, and drug dose

RATIONALE

Ketamine is used by preadolescent and adolescent humans for licit and illicit purposes.

OBJECTIVE

The goal of the present study was to determine the effects of acute and repeated ketamine treatment on the unconditioned behaviors and conditioned locomotor activity of preadolescent and adolescent rats.

METHODS

To assess unconditioned behaviors, female and male rats were injected with ketamine (5-40 mg/kg), and distance traveled was measured on postnatal day (PD) 21-25 or PD 41-45. To assess conditioned activity, male and female rats were injected with saline or ketamine in either a novel test chamber or the home cage on PD 21-24 or PD 41-44. One day later, rats were injected with saline and conditioned activity was assessed.

RESULTS

Ketamine produced a dose-dependent increase in the locomotor activity of preadolescent and adolescent rats. Preadolescent rats did not exhibit sex differences, but ketamine-induced locomotor activity was substantially stronger in adolescent females than males. Repeated ketamine treatment neither caused a day-dependent increase in locomotor activity nor produced conditioned activity in preadolescent or adolescent rats.

CONCLUSIONS

The activity-enhancing effects of ketamine are consistent with the actions of an indirect dopamine agonist, while the inability of ketamine to induce conditioned activity is unlike what is observed after repeated cocaine or amphetamine treatment. This dichotomy could be due to ketamine's ability to both enhance DA neurotransmission and antagonize N-methyl-D-aspartate (NMDA) receptors. Additional research will be necessary to parse out the relative contributions of DA and NMDA system functioning when assessing the behavioral effects of ketamine during early ontogeny.

The ketamine-like compound methoxetamine substitutes for ketamine in the self-administration paradigm and enhances mesolimbic dopaminergic transmission

RATIONALE

Recently, an increasing number of emergency cases due to a novel ketamine-like drug, methoxetamine (MXE), were reported in several countries. However, very little is known about the neuropsychopharmacological and reinforcing profile of this compound.

OBJECTIVES

Our study aims to investigate the effects of MXE on self-administration (SA) behaviour in comparison to ketamine and on dopaminergic transmission.

METHODS

A SA substitution study was performed in male rats trained to intravenously (IV) self-administer ketamine. At responding stability, rats were exposed to sequential phases of MXE substitution at different dosages (starting from 0.5 and then decreasing to 0.25 and 0.125 mg/kg). Standard electrophysiological techniques were used to record changes in firing activities of ventral tegmental area (VTA) dopamine neurons projecting to the nucleus accumbens (NAc) shell after acute injection of cumulative doses of MXE (0.031-0.5 mg/kg IV). Finally, in vivo microdialysis was performed in freely moving rats to evaluate the effect of acute MXE administration (0.125, 0.25 and 0.5 mg/kg IV) on dopamine release in the NAc shell.

RESULTS

MXE 0.125 and 0.25 mg/kg, but not 0.5 mg/kg, substituted for ketamine SA. MXE also induced a dose-dependent stimulation of firing rate (p < 0.0001) and burst firing (p < 0.05) of NAc-projecting VTA dopamine neurons. Consistently, MXE significantly (p < 0.05) increased dopamine extracellular levels in the NAc shell at 0.5 and 0.25 mg/kg with different time onsets, i.e. at 40 and 100 min, respectively.

CONCLUSIONS

This study, while confirming the reinforcing effects of MXE, highlights an electrophysiological and neurochemical profile predictive of its addictive properties.

The Rapidly Acting Antidepressant Ketamine and the mGlu2/3 Receptor Antagonist LY341495 Rapidly Engage Dopaminergic Mood Circuits

Ketamine is a rapidly acting antidepressant in patients with treatment-resistant depression (TRD). Although the mechanisms underlying these effects are not fully established, inquiry to date has focused on the triggering of synaptogenesis transduction pathways via glutamatergic mechanisms. Preclinical data suggest that blockade of metabotropic glutamate (mGlu2/3) receptors shares many overlapping features and mechanisms with ketamine and may also provide rapid efficacy for TRD patients. Central dopamine circuitry is recognized as an end target for mood regulation and hedonic valuation and yet has been largely neglected in mechanistic studies of antidepressant-relevant effects of ketamine. Herein, we evaluated the changes in dopaminergic neurotransmission after acute administration of ketamine and the mGlu2/3 receptor antagonist LY341495 [(2S)-2-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl) propanoic acid ] in preclinical models using electrophysiologic, neurochemical, and behavioral endpoints. When given acutely, both ketamine and LY341495, but not the selective serotonin reuptake inhibitor (SSRI) citalopram, increased the number of spontaneously active dopamine neurons in the ventral tegmental area (VTA), increased extracellular levels of dopamine in the nucleus accumbens and prefrontal cortex, and enhanced the locomotor stimulatory effects of the dopamine D2/3 receptor agonist quinpirole. Further, both ketamine and LY341495 reduced immobility time in the tail-suspension assay in CD1 mice, which are relatively resistant to SSRI antidepressants. Both the VTA neuronal activation and the antidepressant phenotype induced by ketamine and LY341495 were attenuated by the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo- (9CI)-benzo[f]quinoxaline-7-sulfonamide, indicating AMPA-dependent effects. These findings provide another overlapping mechanism of action of ketamine and mGlu2/3 receptor antagonism that differentiates them from conventional antidepressants and thus support the potential rapidly acting antidepressant actions of mGlu2/3 receptor antagonism in patients.

Examination of clozapine and haloperidol in improving ketamine-induced deficits in an incremental repeated acquisition procedure in BALB/c mice

RATIONALE

Ketamine, an N-methyl-D-aspartate receptor (NMDAR) antagonist, causes locomotor hyperactivity, aberrant prepulse inhibition and impaired reversal learning among other deficits. There are numerous clinical and pre-clinically uses of NMDAR antagonists and a growing need to characterize their neurobehavioral effects.

OBJECTIVES

The present study was designed to characterize 1) ketamine's effect on incremental repeated acquisition (IRA), a procedure that taps multiple neurobehavioral functions and has performance measures correlated with IQ in humans, and 2) the extent to which clozapine (CLZ) and haloperidol (HAL) block ketamine's detrimental effects.

METHODS AND RESULTS

In experiment 1 (Exp. 1), BALB/c mice nose-poked under an IRA procedure for sucrose pellets. Systemic ketamine (1-30 mg/kg) dose-dependently decreased measures of cognitive and motor function. CLZ pretreatment (CLZ 0.1-4.0 mg/kg) dose-dependently attenuated ketamine-induced (30 mg/kg) deficits; the effective dose range of CLZ was 0.3-1.0 mg/kg. HAL pretreatment (0.01-0.1 mg/kg) did not attenuate any ketamine-induced deficits. In experiment 2 (Exp. 2), BALB/c mice lever-pressed under an IRA procedure for sweetened condensed milk. Ketamine (30 mg/kg) produced a global impairment in the IRA procedure and CLZ pretreatment (0.3-1.0 mg/kg) dose-dependently attenuated that impairment; motor-based performance recovered to a greater extent than cognitive performance. When tested alone, these doses of CLZ had little effect on IRA performance.

CONCLUSIONS

These findings support the notion that CLZ is more effective than HAL at blocking ketamine-induced deficits. The IRA procedure may be beneficial for distinguishing the efficacy of drugs that seek to alleviate deficits in complex behavior that result from acute NMDAR antagonism.

Impact of subanesthetic doses of ketamine on AMPA-mediated responses in rats: An in vivo electrophysiological study on monoaminergic and glutamatergic neurons

The rapid antidepressant action of a subanesthetic dose of ketamine in treatment-resistant patients represents the most striking recent breakthrough in the understanding of the antidepressant response. Evidence demonstrates tight interactions between the glutamatergic and monoaminergic systems. It is thus hypothesized that monoamine systems may play a role in the immediate/rapid effects of ketamine. In vivo electrophysiological recordings were carried in male rats following ketamine administration (10 and 25 mg/kg, i.p.) to first assess its effects on monoaminergic neuron firing. In a second series of experiments, the effects of ketamine administration on α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)- and N-methyl-D-aspartate receptor (NMDA)-evoked responses in hippocampus CA3 pyramidal neurons were also investigated using micro-iontophoretic applications. Although acute (~2 hours) ketamine administration did not affect the mean firing activity of dorsal raphe serotonin and ventral tegmental area dopamine neurons, it did increase that of locus coeruleus norepinephrine neurons. In the latter brain region, while ketamine also enhanced bursting activity, it did increase population activity of dopamine neurons in the ventral tegmental area. These effects of ketamine were prevented by the prior administration of the AMPA receptor antagonist 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonamide. An increase in AMPA-evoked response of CA3 pyramidal neurons was also observed 30 minutes following acute ketamine administration. The present findings suggest that acute ketamine administration produces a rapid enhancement of catecholaminergic neurons firing activity through an amplification of AMPA transmission. These effects may play a crucial role in the antidepressant effects of ketamine observed shortly following its infusion in depressed patients.

Restoring mood balance in depression: ketamine reverses deficit in dopamine-dependent synaptic plasticity

BACKGROUND

One of the most novel and exciting findings in major depressive disorder research over the last decade is the discovery of the fast-acting and long-lasting antidepressant effects of ketamine. Indeed, the therapeutic effects of classic antidepressants, such as selective serotonin reuptake inhibitors, require a month or longer to be expressed, with about a third of major depressive disorder patients resistant to treatment. Clinical studies have shown that a low dose of ketamine exhibits fast-acting relatively sustained antidepressant action, even in treatment-resistant patients. However, the mechanisms of ketamine action at a systems level remain unclear.

METHODS

Wistar-Kyoto rats were exposed to inescapable, uncontrollable footshocks. To evaluate learned helplessness behavior, we used an active avoidance task in a shuttle box equipped with an electrical grid floor. After helplessness assessment, we performed in vivo electrophysiological recordings first from ventral tegmental area dopaminergic (DA) neurons and second from accumbens neurons responsive to fimbria stimulation. Ketamine was injected and tested on helpless behavior and electrophysiological recordings.

RESULTS

We show that ketamine is able to restore the integrity of a network by acting on the DA system and restoring synaptic dysfunction observed in stress-induced depression. We show that part of the antidepressant effect of ketamine is via the DA system. Indeed, injection of ketamine restores a decreased dopamine neuron population activity, as well as synaptic plasticity (long-term potentiation) in the hippocampus-accumbens pathway, via, in part, activation of D1 receptors.

CONCLUSIONS

This work provides a unique systems perspective on the mechanisms of ketamine on a disrupted limbic system.

Nicotinic, glutamatergic and dopaminergic synaptic transmission and plasticity in the mesocorticolimbic system: focus on nicotine effects

Cigarette smoking is currently the leading cause of preventable deaths and disability throughout the world, being responsible for about five million premature deaths/year. Unfortunately, fewer than 10% of tobacco users who try to stop smoking actually manage to do so. The main addictive agent delivered by cigarette smoke is nicotine, which induces psychostimulation and reward, and reduces stress and anxiety. The use of new technologies (including optogenetics) and the development of mouse models characterised by cell-specific deletions of receptor subtype genes or the expression of gain-of-function nAChR subunits has greatly increased our understanding of the molecular mechanisms and neural substrates of nicotine addiction first revealed by classic electrophysiological, neurochemical and behavioural approaches. It is now becoming clear that various aspects of nicotine dependence are mediated by close interactions of the glutamatergic, dopaminergic and γ-aminobutyric acidergic systems in the mesocorticolimbic system. This review is divided into two parts. The first provides an updated overview of the circuitry of the ventral tegmental area, ventral striatum and prefrontal cortex, the neurotransmitter receptor subtypes expressed in these areas, and their physiological role in the mesocorticolimbic system. The second will focus on the molecular, functional and behavioural mechanisms involved in the acute and chronic effects of nicotine on the mesocorticolimbic system.

Selective activation of M4 muscarinic acetylcholine receptors reverses MK-801-induced behavioral impairments and enhances associative learning in rodents

Positive allosteric modulators (PAMs) of the M4 muscarinic acetylcholine receptor (mAChR) represent a novel approach for the treatment of psychotic symptoms associated with schizophrenia and other neuropsychiatric disorders. We recently reported that the selective M4 PAM VU0152100 produced an antipsychotic drug-like profile in rodents after amphetamine challenge. Previous studies suggest that enhanced cholinergic activity may also improve cognitive function and reverse deficits observed with reduced signaling through the N-methyl-d-aspartate subtype of the glutamate receptor (NMDAR) in the central nervous system. Prior to this study, the M1 mAChR subtype was viewed as the primary candidate for these actions relative to the other mAChR subtypes. Here we describe the discovery of a novel M4 PAM, VU0467154, with enhanced in vitro potency and improved pharmacokinetic properties relative to other M4 PAMs, enabling a more extensive characterization of M4 actions in rodent models. We used VU0467154 to test the hypothesis that selective potentiation of M4 receptor signaling could ameliorate the behavioral, cognitive, and neurochemical impairments induced by the noncompetitive NMDAR antagonist MK-801. VU0467154 produced a robust dose-dependent reversal of MK-801-induced hyperlocomotion and deficits in preclinical models of associative learning and memory functions, including the touchscreen pairwise visual discrimination task in wild-type mice, but failed to reverse these stimulant-induced deficits in M4 KO mice. VU0467154 also enhanced the acquisition of both contextual and cue-mediated fear conditioning when administered alone in wild-type mice. These novel findings suggest that M4 PAMs may provide a strategy for addressing the more complex affective and cognitive disruptions associated with schizophrenia and other neuropsychiatric disorders.

Sigma-1 receptor agonists provide neuroprotection against gp120 via a change in bcl-2 expression in mouse neuronal cultures

Although combined antiretroviral therapy has significantly improved the prognosis of HIV-1 infected patients and decreased the incidence of HIV-1 associated dementia, the cumulative prevalence of this disease, in particular, mild or asymptomatic neurocognitive impairment, has not decreased. Thus, in addition to active antiretroviral therapy, the search for an effective neuroprotective approach is very important. Sigma-1 receptors are widely distributed in the central nervous system. Sigma-1 receptor agonists are robustly neuroprotective in many neuropathy and neurotoxicity in vivo and in vitro studies. This study aims to investigate possible neuroprotective effects of sigma-1 receptor agonist, 4-phenyl-1-(4-phenylbutyl) piperidine (PPBP) against HIV-1 protein gp120. Primary cortical neuronal cultures were exposed to gp120 in different concentrations; to investigate neuroprotective effects of sigma-1 receptor agonist, cells were pre-treated with PPBP (10μM) in the presence or absence of pre-incubated sigma-1 receptor antagonist rimcazole (5μM). Cell apoptosis was confirmed with calcein/PI uptake test, lactate dehydrogenase (LDH) leakage assay or TUNEL assay and neurite degeneration was evaluated with morphometry via MAP-2 stained immunofluorescence. The mRNA and protein levels of apoptosis associated bax and bcl-2 were determined with real-time qPCR and Western blot. The results showed that gp120 could induce neuronal apoptosis and neurite degeneration in a concentration dependent manner and PPBP could attenuate the neurotoxicity of gp120. Simultaneously, gp120 could induce low expression of bcl-2 and bax, but only low expression of bcl-2 could be reversed by PPBP. The present data suggest that PPBP, at least, in part protects the neuron against gp120 by regulating bcl-2 expression.

Pharmacological properties of ketamine

Ketamine is a dissociative anaesthetic that is being used in non-medical contexts. The effects of ketamine are very similar to those of phencyclidine, another dissociative anaesthetic that has enjoyed considerable popularity as a recreational drug. The effects of ketamine include analgesia, cardiovascular and respiratory stimulation, dissociation, hallucinations and anaesthesia. The potential dangers of uncontrolled ketamine use include psychosis and violence, accidents and marked psychomotor and cognitive impairment. Although studies have shown potential for tolerance to and physical dependence on ketamine, further investigation of these phenomena is needed. Ketamine is thought to produce most of its effects through antagonist activity at the PCP site of the NMDA receptor complex. Ketamine has sympathomimetic properties resulting from enhancement of catecholamine, and particularly dopamine, activity. While opioid receptor activity has been identified, this is relatively weak and the contribution to the effects of ketamine is not clear. Although much is known of the clinical uses and effects of ketamine, as yet little is understood of ketamine as a recreational drug and potential drug of dependence.

The absence of 5-HT(1A) receptors has minor effects on dopamine but not serotonin release evoked by MK-801 in mice prefrontal cortex

RATIONALE

Non-competitive NMDA receptor antagonists markedly increase neuronal activity in medial prefrontal cortex (mPFC), an effect which partly underlies their schizomimetic actions. Projection pyramidal neurons and local GABAergic interneurons in mPFC express 5-HT(1A) receptors, whose activation modulates dopaminergic (DA) and serotonergic (5-HT) activity in midbrain and the cortical release of both monoamines.

OBJECTIVE

To examine whether the presence of 5-HT(1A) receptors can modulate the effect of NMDA receptor blockade with MK-801 (dizocilpine) on DA and 5-HT release in mouse mPFC.

MATERIALS AND METHODS

Brain microdialysis and locomotor activity measures in wild-type and 5-HT(1A) receptor knockout mice.

RESULTS

Systemic MK-801 administration (0.125, 0.25, 0.50, and 1 mg/kg i.p.) induced a dose-dependent increase in mPFC 5-HT output, which was independent of the genotype. MK-801 increased DA output in a dose-dependent manner with a significant effect of genotype on low doses (0.125, 0.25 mg/kg). These differences were not paralleled by differences in gross locomotor activity. Overall, MK-801 increased more markedly DA than 5-HT output in both genotypes. Finally, the local perfusion of MK-801 in mPFC (30, 100, 300 muM) by reverse dialysis did not elevate dialysate DA or 5-HT concentrations in mPFC.

CONCLUSION

5-HT(1A) receptors partly modulate the increase in mPFC DA (but not 5-HT) release produced by NMDA receptor blockade. The lack of effect observed after the local MK-801 application suggests that the change in cortical monoamines is mainly driven by subcortical NMDA receptor blockade, without a significant involvement of PFC 5-HT(1A) receptors.

Effects of antipsychotic treatments and D-serine supplementation on the electrophysiological activation of midbrain dopamine neurons induced by the noncompetitive NMDA antagonist MK 801

The acute administration of the noncompetitive glutamate N-methyl-D-aspartate (NMDA) receptor antagonist dizocilpine (MK 801) is known to increase central dopaminergic activity in rats and to elicit schizophreniform behavior in human. The current study was undertaken to compare the effects of different acute or chronic neuroleptic treatments, on the response of ventral tegmental area dopamine (DA) neurons to MK 801, using the in vivo electrophysiological paradigm in anesthetized preparations. Sprague Dawley male rats were treated, acutely or chronically during 3 weeks, with saline, olanzapine (10 mg/kg), haloperidol (1 mg/kg) or the combination of haloperidol with D-serine (1 mg/kg/300 mg/kg), a gliotransmitter coagonist of the NMDA receptor that has been shown to improve the efficacy of typical neuroleptics. In control animals, the acute administration of MK 801 (0.5 mg/kg, i.v.) increased significantly both the firing and burst activity of DA neurons by 20 and 26%, respectively, the latter effect being partially reversed by the selective 5-HT2A antagonist M 100,907 (0.4 mg/kg, i.v.). The acute preadministration of haloperidol (1 mg/kg, i.p.) and olanzapine (10 mg/kg, i.p.) failed to prevent or reverse the activatory effect of MK 801 on firing activity. On the other hand, MK 801-induced burst activity, was partially prevented by olanzapine, but not by haloperidol pretreatment. All antipsychotic treatments, when administered chronically, prevent the activatory effect of MK 801 on both firing and burst activity, and occasionally convert the response to MK 801 on burst activity to an inhibitory response, the latter occurring more predominantly in rats treated with the combination haloperidol/D-serine. These results suggest that a chronic antipsychotic regime alters the function of the NMDA receptors that tonically control the firing activity of midbrain dopaminergic neurons.

Behavioral and neurochemical characterization of mice deficient in the phosphodiesterase-1B (PDE1B) enzyme

PDE1B is a calcium-dependent cyclic nucleotide phosphodiesterase that is highly expressed in the striatum. In order to investigate the physiological role of PDE1B in the central nervous system, PDE1B knockout mice (C57BL/6N background) were assessed in behavioral tests and their brains were assayed for monoamine content. In a variety of well-characterized behavioral tasks, including the elevated plus maze (anxiety-like behavior), forced swim test (depression-like behavior), hot plate (nociception) and two cognition models (passive avoidance and acquisition of conditioned avoidance responding), PDE1B knockout mice performed similarly to wild-type mice. PDE1B knockout mice showed increased baseline exploratory activity when compared to wild-type mice. When challenged with amphetamine (AMPH) and methamphetamine (METH), male and female PDE1B knockout mice showed an exaggerated locomotor response. Male PDE1B knockout mice also showed increased locomotor responses to higher doses of phencyclidine (PCP) and MK-801; however, this effect was not consistently observed in female knockout mice. In the striatum, increased dopamine turnover (DOPAC/DA and HVA/DA ratios) was found in both male and female PDE1B knockout mice. Striatal serotonin (5-HT) levels were also decreased in PDE1B knockout mice, although levels of the metabolite, 5HIAA, were unchanged. The present studies demonstrate increased striatal dopamine turnover in PDE1B knockout mice associated with increased baseline motor activity and an exaggerated locomotor response to dopaminergic stimulants such as methamphetamine and amphetamine. These data further support a role for PDE1B in striatal function.

Effects of age and sex on ketamine-induced hyperactivity in rats

Investigators tested the hypothesis that administration of the NMDA antagonist ketamine would result in differential effects on activity levels in rats depending upon the age and the sex of the animal. Twenty-two-, 35-, and 50-day-old rats were given doses of ketamine (0.0 or 10.0 mg/kg) and tested for open-field activity and frequencies of reverse locomotion, rearing, turning, and head weaving. Results indicated that ketamine produced hyperactivity in both males and females at 22 days of age but only in females at 35 days of age. There was no effect of ketamine on locomotor activity in 50-day-old rats, regardless of sex. Effects of ketamine on turning, reverse locomotion, and head weaving were similar with administration, in general, causing increments in these behaviors in both sexes at 22 days and in females at all ages tested. Ketamine resulted in reductions in rearing in both sexes regardless of age.

Sigma receptor agonists provide neuroprotection in vitro by preserving bcl-2

BACKGROUND

Sigma (sigma)-receptor agonists attenuate brain injury after experimental focal cerebral ischemia in several species. We tested the hypothesis that the potent, prototypical sigma(1)-receptor agonist, 4-phenyl-1-(4-phenylbutyl) piperidine (PPBP), protects neurons by a mechanism involving the antiapoptotic protein bcl-2.

METHODS

Primary cortical neuronal cultures were exposed to either 2 h of oxygen-glucose deprivation (OGD) or glutamate (100 microM). PPBP treatment was initiated either 15 min prior to the insult or at 15 min postinsult then continued for 24 h. In another set of experiments, cultured neurons were preincubated for 2 h prior to PPBP treatment with sigma1-receptor antagonist, rimcazole, in a dose-dependent manner. Alive and dead cells were detected with calcein-AM and propidium iodide respectively. Bcl-2 and bax expression were determined by quantitative real time reverse transcription polymerase chain reaction and western blotting, and DNA damage was detected by TUNEL staining.

RESULTS

PPBP pretreatment attenuated neuronal injury induced by OGD or glutamate (50 or 100 microM). This protection was reversed with rimcazole (cell death: OGD 48 +/- 2%, OGD plus PPBP 31 +/- 3%, OGD plus PPBP with rimcazole 46 +/- 2%). PPBP treatment increased bcl-2 but not bax mRNA levels. PPBP's ability to preserve bcl-2 protein after OGD by PPBP was fully abolished by rimcazole. Lastly, PPBP reduced the number of TUNEL-positive cells after OGD, suggesting fewer cells with overt DNA damage.

CONCLUSIONS

These data demonstrate that PPBP reduces cell death in vitro by a mechanism involving receptor-dependent preservation of protective genes such as bcl-2.

Ketamine-related expression of glutamatergic postsynaptic density genes: Possible implications in psychosis

Systemic administration of ketamine, a non-competitive antagonist of the N-methyl-d-aspartate receptor (NMDA-R), produces a condition of NMDA-R hypofunction, which is considered one of the putative molecular mechanisms involved in psychosis. In this study, we evaluated the effect of ketamine on glutamatergic markers of the postsynaptic density (PSD), a pivotal site for dopamine-glutamate interaction. We assessed gene expression of Homer1a, alpha and betaCaMKII, and dopamine transporter (DAT) by two different doses of ketamine. These genes were chosen because of their impact on signal transduction and dopamine-glutamate interplay in postsynaptic density. Moreover, Homer1a is modulated by antipsychotics and represents a candidate gene for schizophrenia. Male Sprague-Dawley rats were injected with saline, 12mg/kg ketamine or 50mg/kg ketamine, and sacrificed 90 minutes after injections. In situ hybridization histochemistry was used to quantitate the rate of gene expression in rat forebrain. Homer1a was induced by 50mg/kg ketamine in ventral striatum and by both 50 and 12mg/kg ketamine in nucleus accumbens, whereas gene expression was not affected in dorsal striatum. alphaCaMKII was increased by 12mg/kg ketamine against saline in almost all subregions assessed. betaCaMKII was not affected by ketamine. DAT was increased by both doses of ketamine in the ventro-tegmental area and substantia nigra pars compacta. We suggest that these changes may represent molecular adaptations to the perturbation in glutamatergic transmission induced by ketamine blockade of NMDA receptors and may be implicated in molecular alterations occurring in schizophrenia.

The Differential Effects of Nitrous Oxide and Xenon on Extracellular Dopamine Levels in the Rat Nucleus Accumbens: A Microdialysis Study

Dopamine release in the nucleus accumbens (NAC) plays a crucial role in the action of various psychotropic and addictive drugs, such as antagonists of the N-methyl-D-aspartate subtype of the glutamate. Although both nitrous oxide and xenon are N-methyl-D-aspartate receptor antagonists, they differ in their potential for producing neuropsychological toxicity; therefore, we decided to examine their effects on both spontaneous and ketamine-induced extracellular dopamine levels in the NAC. A microdialysis probe was implanted into the NAC in each of 35 rats, which were randomly assigned to one of six groups: exposure to 40% O2, exposure to 60% nitrous oxide (0.27 MAC), exposure to 43% xenon (0.27 MAC) for 60 min, and three groups exposed to either 40% O2, 60% nitrous oxide, or 43% xenon for 70 min and 80 mg/kg ketamine was given i.p. 10 min after the initiation of gas exposure. Perfusate samples were collected every 20 min, and the dopamine levels were measured using a high-performance liquid chromatography system. Nitrous oxide, but not xenon, significantly increased the dopamine level. Ketamine significantly increased the dopamine level, and this was significantly inhibited by xenon, but not by nitrous oxide. These data suggest that the difference in neuropsychological activity between nitrous oxide and xenon is partly due to their differential effects on the mesolimbic dopamine system.

Ketamine induces dopamine-dependent depression of evoked hippocampal activity in the nucleus accumbens in freely moving rats

Noncompetitive NMDA receptor antagonists, such as ketamine, induce a transient schizophrenia-like state in healthy individuals and exacerbate psychosis in schizophrenic patients. In rodents, noncompetitive NMDA receptor antagonists induce a behavioral syndrome that represents an experimentally valid model of schizophrenia. Current experimental evidence has implicated the nucleus accumbens in the pathophysiology of schizophrenia and the psychomimetic actions of ketamine. In this study, we have demonstrated that acute systemic administration of ketamine, at a dose known to produce hyperlocomotion and stereotypy, depressed the amplitude of the monosynaptic component of fimbria-evoked field potentials recorded in the nucleus accumbens. A similar effect was observed using the more selective antagonist dizocilpine maleate, indicating the depression was NMDA receptor dependent. Paired-pulse facilitation was enhanced concomitantly with, and in proportion to, ketamine-induced depressed synaptic efficacy, indicative of a presynaptic mechanism of action. Notably, the depression of field potentials recorded in the nucleus accumbens was markedly reduced after a focal 6-hydroxydopamine lesioning procedure in the nucleus accumbens. More specifically, pretreatment with the D2/D4 antagonist haloperidol, but not the D1 antagonist SCH23390 blocked ketamine-induced depression of nucleus accumbens responses. Our findings provide supporting evidence for the contemporary theory of schizophrenia as aberrant excitatory neurotransmission at the level of the nucleus accumbens.

Effects of phencyclidine on schedule-controlled responding following neurotoxic lesions of the striatum

The effects of phencyclidine on an operant task were evaluated prior to and after neurotoxic lesions of the striatum in rats. Subjects were trained to respond on a fixed-interval 90-second schedule for water presentation. The degree to which phencyclidine disrupted responding was first evaluated (dose range 1.0-4.0 mg/kg). The subjects were then divided into three matched groups and received bilateral intraventricular injections of 6-hydroxydopamine (6-OHDA) (100 microg), kainic acid (0.25 microg), or vehicle delivered stereotaxically. 6-OHDA was used to destroy the presynaptic neurons of the nigro-striatal pathway and kainic acid was employed to destroy the postsynaptic neurons whose cell bodies are located in the striatum. Following recovery, the phencyclidine dose-response curve was repeated in the fixed-interval paradigm. It was observed that 6-OHDA-induced damage resulted in a rightward shift of the dose-response curve indicating tolerance to phencyclidine and caused a significant depletion of striatal dopamine and gamma-aminobutyric acid (GABA). Kainic acid-induced damage resulted in a leftward shift in the dose-response curve indicating sensitivity to the schedule-disruptive effects of phencyclidine and produced a significant GABA depletion. The vehicle-treated rats exhibited no shift in their sensitivity to phencyclidine. These observations indicate that the effects of phencyclidine are mediated, at least in part, by striatal dopaminergic neurons.

Maternal immune activation leads to behavioral and pharmacological changes in the adult offspring

Maternal exposure to viral infection has been associated with an increased risk of schizophrenia in the offspring, and it has been suggested that the maternal immune response may interfere with normal fetal brain development. Although studies in rodents have shown that perinatal viral infections can lead to neuropathological and behavioral abnormalities considered relevant to schizophrenia, it is not clear whether these consequences are due to the infection itself or to the maternal immune response to infection. We show that an induction of maternal immune stimulation without exposure to a virus by injecting pregnant dams with the synthetic cytokine releaser polyriboinosinic-polyribocytidilic acid (poly I:C) leads to abnormal behavioral and pharmacological responses in the adult offspring. As in schizophrenia, these offspring displayed excessive behavioral switching, manifested in the loss of latent inhibition and in rapid reversal learning. Consistent with the clinical pharmacology of schizophrenia, both deficits were alleviated by antipsychotic treatment. In addition, these offspring displayed increased sensitivity to the locomotor-stimulating effects of MK-801, pointing to developmental alterations of the dopaminergic and/or glutamatergic systems. Prenatal poly I:C administration did not produce learning deficits in classical fear conditioning, active avoidance, discrimination learning and water maze. These results show that the maternal immune response is sufficient to cause behavioral and pharmacological alterations relevant to schizophrenia in the adult offspring.

Differential conditioned place preference responses to endomorphin-1 and endomorphin-2 microinjected into the posterior nucleus accumbens shell and ventral tegmental area in the rat

An unbiased conditioned place preference (CPP) paradigm was used to evaluate the reward effects of endogenous mu-opioid receptor ligands endomorphin-1 (EM-1) and endomorphin-2 (EM-2) from the mesolimbic posterior nucleus accumbens (Acb) shell and the ventral tegmental area (VTA) in CD rats. EM-1 (1.6-8.1 nmol) microinjected into posterior Acb shell produced CPP, whereas EM-2 (8.7-17.5 nmol) given into the same Acb shell produced conditioned place aversion (CPA). EM-1 (1.6-16.3 nmol) microinjected into the VTA produced CPP, whereas EM-2 (8.7 and 17.5 nmol) given into the same VTA site did not produce any effect, but at a high dose (35 nmol) produced CPP. EM-1 (3.3 nmol) or EM-2 (17.5 nmol) microinjected into the nigrostriatal substantia nigra was not significantly different from vehicle-injected groups. D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH(2) (CTOP) at 94.13 pmol or 3-methoxynaltrexone at 0.64 pmol microinjected into the posterior Acb shell blocked EM-1-induced CPP and EM-2-induced CPA. At a higher dose, CTOP (941.3 pmol) and 3-methoxynaltrexone (6.4 pmol) produced CPA and CPP, respectively. Coadministration with antiserum against dynorphin A(1-17) (Dyn) (10 microg) microinjected into the posterior Acb shell blocked EM-2-induced CPA. However, it did not affect EM-1-induced CPP. It is concluded that EM-1 and EM-2 produce site-dependent CPP and CPA, respectively, by stimulation of different subtypes of mu-opioid-receptors; stimulation of one subtype of mu-opioid-receptor at the posterior Acb shell and VTA by EM-1 induces CPP, whereas stimulation of another subtype of mu-opioid receptor at the posterior Acb shell, but not the VTA, by EM-2 induces the release of Dyn to produce CPA.

Long-term potentiation in visual cortical projections to the medial prefrontal cortex of the rat

In order to investigate neural mechanisms by which the prefrontal cortex adaptively modifies its activities based on past experience, we examined whether or not sensory cortical projections to the medial prefrontal cortex support long-term potentiation (LTP) in rats. Monosynaptic projections from the secondary visual cortex, mediomedial area (V2MM) to the infralimbic cortex were confirmed by orthodromic as well as antidromic activation of single units. High-frequency stimulation (50 Hz, 2 s) induced LTP (approximately 45% increase over the baseline) in the V2MM projection to the infralimbic cortex. LTP induction in this pathway was completely blocked by an injection (i.p.) of CPP, an N-methyl-D-aspartate receptor antagonist. LTP was also induced in the ventral hippocampal projection to the infralimbic cortex by the same high-frequency stimulation. The present results suggest that modification of synaptic weights of afferent sensory cortical projections is one mechanism underlying learning-induced changes in prefrontal cortical neural activities.

The "two-headed" latent inhibition model of schizophrenia: modeling positive and negative symptoms and their treatment

RATIONALE

Latent inhibition (LI), namely, poorer performance on a learning task involving a previously pre-exposed non-reinforced stimulus, is disrupted in the rat by the dopamine (DA) releaser amphetamine which produces and exacerbates psychotic (positive) symptoms, and this is reversed by treatment with typical and atypical antipsychotic drugs (APDs) which on their own potentiate LI. These phenomena are paralleled by disrupted LI in normal amphetamine-treated humans, in high schizotypal humans, and in schizophrenia patients in the acute stages of the disorder, as well as by potentiated LI in normal humans treated with APDs. Consequently, disrupted LI is considered to provide an animal model of positive symptoms of schizophrenia with face, construct and predictive validity.

OBJECTIVES

To review most of the rodent data on the neural substrates of LI as well as on the effects of APDs on this phenomenon with an attempt to interpret and integrate these data within the framework of the switching model of LI; to show that there are two distinct LI models, disrupted and abnormally persistent LI; to relate these findings to the clinical condition.

RESULTS

The nucleus accumbens (NAC) and its DA innervation form a crucial component of the neural circuitry of LI, and are involved at the conditioning stage. There is a clear functional differentiation between the NAC shell and core subregions whereby damage to the shell disrupts LI and damage to the core renders LI abnormally persistent under conditions that disrupt LI in normal rats. The effects of shell and core lesions parallel those produced by lesions to the major sources of input to the NAC: entorhinal cortex lesion, like shell lesion, disrupts LI, whereas hippocampal lesion, like core lesion, produces persistent LI with changes in context, and basolateral amygdala (BLA) lesion, like core lesion, produces persistent LI with extended conditioning. Systemically induced blockade of glutamatergic as well as DA transmission produce persistent LI via effects exerted at the conditioning stage, whereas enhancement of DA transmission disrupts LI via effects at the conditioning stage. Serotonergic manipulations can disrupt or potentiate LI via effects at the pre-exposure stage. Both typical and atypical APDs potentiate LI via effects at conditioning whereas atypical APDs in addition disrupt LI via effects at pre-exposure. Schizophrenia patients can exhibit disrupted or normal LI as a function of the state of the disorder (acute versus chronic), as well as persistent LI.

CONCLUSIONS

Different drug and lesion manipulations produce two poles of abnormality in LI, namely, disrupted LI under conditions which lead to LI in normal rats, and abnormally persistent LI under conditions which disrupt it in normal rats. Disrupted and persistent LI are differentially responsive to APDs, with the former reversed by both typical and atypical APDs and the latter selectively reversed by atypical APDs. It is suggested that this "two-headed LI model" mimics two extremes of deficient cognitive switching seen in schizophrenia, excessive and retarded switching between associations, mediated by dysfunction of different brain circuitries, and can serve to model positive symptoms of schizophrenia and typical antipsychotic action, as well as negative symptoms of schizophrenia and atypical antipsychotic action.

Potent sigma 1-receptor ligand 4-phenyl-1-(4-phenylbutyl) piperidine provides ischemic neuroprotection without altering dopamine accumulation in vivo in rats

The in vivo signaling of ischemic neuroprotection provided by sigma-receptor ligands remains unclear. Catecholamines have been implicated in the propagation of ischemic neuronal injury, and previous in vitro studies suggest that sigma ligands modulate dopaminergic neurotransmission. In this study, we tested the hypothesis that the potent sigma(1)-receptor ligand 4-phenyl-1-(4-phenylbutyl) piperidine (PPBP) attenuates the increase of extracellular dopamine in ischemic striatum. Under controlled physiological conditions, a microdialysis probe was implanted in right caudoputamen (CP) complex of adult male Wistar rats. Rats were subjected to 2 h of transient middle cerebral artery occlusion (MCAO) by the intraluminal suture technique. In a blinded, randomized fashion, rats were divided into five treatment groups: Group 1 (n = 8; saline-saline) continuous i.v. infusion of saline vehicle 30 min before MCAO followed by saline at reperfusion until the end of the experiment; Group 2 (n = 8; PPBP-PPBP) i.v. PPBP 30 min before MCAO followed by 1 micromol x kg(-1) x h(-1) of PPBP; Group 3 (n = 8; saline-PPBP) i.v. saline before MCAO followed by PPBP; Group 4 (n = 4) surgical shams (saline-saline); and Group 5 (n = 4) surgical shams (PPBP-PPBP). Infarction volume at 22 h of reperfusion in the CP complex (percentage of ipsilateral structure) was significantly attenuated in rats treated with PPBP-PPBP (27.3% +/- 9.1%) and saline-PPBP (27.8% +/- 12.7%) compared with saline-saline (59.3% +/- 7.3%) treatment. There was a three- to fourfold increase in dopamine concentrations in the microdialysates within 40 min of the onset of MCAO. Dopamine and its metabolites dihydroxy phenylacetic acid and homovallinic acid levels were similar among the three groups subjected to MCAO. Therefore, PPBP provides significant ischemic neuroprotection in the CP complex without altering the acute accumulation of dopamine in vivo during transient focal ischemia in the rat.

Pentobarbital inhibits ketamine-induced dopamine release in the rat nucleus accumbens: a microdialysis study

Dopamine release in the nucleus accumbens (NAC) plays a crucial role in the actions of various psychotropic and addictive drugs. Ketamine and barbiturates have psychotropic effects and addictive properties, but barbiturates prevent ketamine's psychotomimetic effects. We investigated the effects of ketamine and pentobarbital on dopamine release in the NAC. A microdialysis probe was implanted in the NAC in 35 rats, which were randomly assigned to seven groups: a normal saline intraperitoneal injection (ip) group, 50 and 100 mg/kg of ketamine ip groups, 25 and 50 mg/kg of pentobarbital ip groups, and a normal saline or 25 mg/kg of pentobarbital ip followed by 50 mg/kg of ketamine ip groups. Perfusate samples were collected every 20 min, and dopamine concentration was measured by high-performance liquid chromatography. Ketamine at doses of 50 mg/kg and 100 mg/kg significantly increased dopamine release in the NAC. Conversely, pentobarbital significantly decreased dopamine release in the NAC and inhibited the ketamine-induced dopamine release. These data suggest that the dopamine release in the NAC may be involved in ketamine-induced, but not barbiturate-induced, psychotropic effects and addiction. Inhibition of ketamine-induced dopamine release by barbiturates may be a mechanism by which they prevent ketamine emergence reactions.

IMPLICATIONS

Ketamine increased dopamine release in the nucleus accumbens, which was inhibited by pentobarbital. The mesolimbic dopamine system may be involved in the psychotomimetic effects of ketamine, and the suppression of ketamine emergence reactions by barbiturates may be because of the inhibition of ketamine-induced dopamine release in the nucleus accumbens.

MK-801- and ethanol-induced activity in inbred long-sleep and short-sleep mice: dopamine and serotonin systems

Low doses of (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801; dizocilpine) or ethanol induce less locomotor activation in inbred long-sleep (ILS) than short-sleep (ISS) mice. These differences may involve altered dopamine and/or 5-hydroxytryptamine (serotonin; 5-HT) neurotransmission. To address this possibility, the dopaminergic and serotonergic mechanisms underlying the locomotor-stimulant effects of MK-801 and ethanol in ILS and ISS mice were studied. Dopamine D1, D2 and 5-HT(2A) receptor antagonists reduced MK-801-stimulated activity in ILS mice without having any effect in ISS mice. The 5-HT reuptake inhibitor fluoxetine potentiated MK-801-stimulated activity selectively in ILS mice. Strain differences in 5-HT transporters do not explain this selective effect of fluoxetine in ILS mice since [3H]citalopram binding and [3H]5-HT uptake studies found no differences in the affinity, number or function of 5-HT transporters between ILS and ISS mice. Ethanol-induced activity in ISS mice was depressed by dopamine D2 and 5-HT(2C) receptor antagonists and enhanced by a 5-HT(1A) receptor antagonist. These results suggest that in ILS mice the locomotor-stimulant effects of MK-801 require increased dopamine and/or 5-HT neurotransmission. Conversely, in ISS mice, the effects of MK-801 appear to be monoamine-independent. Thus, even though both MK-801 and ethanol inhibit N-methyl-D-aspartate receptors, their stimulant effects appear to involve different neuronal systems.

Sigma(1) (sigma(1)) receptor antagonists represent a new strategy against cocaine addiction and toxicity

Cocaine is a highly addictive substance abused worldwide. Its mechanism of action involves initially inhibition of neuronal monoamine transporters in precise brain structures and primarily the dopamine reuptake system located on mesolimbic neurons. Cocaine rapidly increases the dopaminergic neurotransmission and triggers adaptive changes in numerous neuronal circuits underlying reinforcement, reward, sensitization and the high addictive potential of cocaine. Current therapeutic strategies focus on counteracting the cocaine effects directly on the dopamine transporter, through post-synaptic D(1), D(2) or D(3) receptors or through the glutamatergic, serotoninergic, opioid or corticotropin-releasing hormone systems. However, cocaine administration also results in the activation of numerous particular targets. Among them, the sigma(1) (sigma(1)) receptor is involved in several acute or chronic effects of cocaine. The present review will first bring concise overviews of the present strategies followed to alleviate cocaine addiction and animal models developed to analyze the pharmacology of cocaine addiction. Evidence involving activation of the sigma(1) receptor in the different aspects of cocaine abuse, will then be detailed, following acute, repeated, or overdose administration. The therapeutic potentials and neuropharmacological perspectives opened by the use of selective sigma(1) receptor antagonists in cocaine addiction will finally be discussed.

Involvement of dopaminergic system in the nucleus accumbens in the discriminative stimulus effects of phencyclidine

The effects of microinjection of phencyclidine (PCP) and dizocilpine, non-competitive NMDA receptor antagonists, and dopamine into the nucleus accumbens were examined in rats trained to discriminate PCP (1.5 mg/kg i.p.) from saline under a two-lever fixed ratio 20 schedule of food reinforcement. Microinjection of PCP (2-40 microg) and dizocilpine (2-12 microg) into the bilateral nucleus accumbens produced a dose-dependent increase in PCP-appropriate responding and fully substituted for systemically administered PCP, whereas microinjection of dopamine (1-4 microg) did not produce PCP-like discriminative stimulus effects. The performance of PCP discrimination was assessed after bilateral destruction of the dopaminergic nerve neurons in the nucleus accumbens with dopaminergic neurotoxin, 6-hydroxydopamine (6-OHDA, 4 microg/1 microl/side). The destruction of dopaminergic nerve neurons in the nucleus accumbens failed to prevent the performance of PCP discrimination. There was no difference in the average percentages of PCP-appropriate responding between vehicle and 6-OHDA-treated rats in the dose-response tests. These results suggest that the dopaminergic system in the nucleus accumbens does not play a critical role in the discriminative stimulus effects of PCP.

Intracerebroventricular administration of NMDA-R1 antisense oligodeoxynucleotide significantly alters the activity of ventral tegmental area dopamine neurons: an electrophysiological study

In this study, we determined the activity of midbrain dopamine (DA) neurons in male albino rats following the intracerebroventricular (i.c.v.) administration of antisense oligodeoxynucleotide (aODN) against the mRNA for the NR1 subunit of the NMDA receptor. In addition, the effect of aODN on the specific binding of the NMDA receptor ligand [(3)H]MK-801 was also examined in various brain areas, including the midbrain. Antisense ODN against the NR1 mRNA, the corresponding sense ODN (sODN) or saline was continuously administered into the right ventricle of rats by osmotic minipumps for 7 days (20 nmol/day). Autoradiographic binding studies indicated that aODN significantly reduced the density of [(3)H]MK-801 binding by an average of 20-30% in several forebrain regions, including the anterior cingulate cortex, caudate putamen, and nucleus accumbens. However, [(3)H]MK-801 binding was not significantly altered in the ventral tegmental area (VTA) or substantia nigra pars compacta (SNC). Subsequently, using the technique of extracellular single-unit recording, the number, as well as the firing pattern, of spontaneously active DA neurons was determined in the VTA and SNC. The administration of aODN did not significantly alter the number of spontaneously active VTA and SNC DA neurons compared to saline- of sODN-treated animals. Furthermore, the firing pattern of spontaneously active SNC DA neurons was not significantly altered. However, for spontaneously active VTA DA neurons, the administration of aODN significantly decreased the percent events in bursts, number of bursts, and percentage of DA neurons exhibiting a bursting pattern compared to saline- and sODN-treated animals, i.e., neurons show less bursting activity. The present results suggest that subchronic aODN treatment against the mRNA for the NR1 subunit of the NMDA receptors can reduce NMDA receptor number and can result in an altered activity of spontaneously active VTA DA neurons in anesthetized rats.

Phencyclidine-induced potentiation of brain stimulation reward: acute effects are not altered by repeated administration

Phencyclidine (PCP; 2.5 or 5.0 mg/kg) potentiated the effects of rewarding hypothalamic brain stimulation, causing parallel leftward shifts of the functions that relate rate of responding to stimulation frequency. Thus, like a number of other drugs of abuse, PCP lowered the "dose" of stimulation required to maintain responding at a given criterion. No progressive changes in the reward-potentiating effects of PCP were evident when the rats were tested once per week for 8 weeks; there was neither tolerance nor sensitization to the initial rewarding properties of PCP. However, in subsequent locomotor tests rats appeared to be already sensitized to PCP; this raises the possibility that the electrical stimulation of the lateral hypothalamus itself maximally sensitized the animals to the stimulant effects of the drug.

Jun, Fos and Krox in the hippocampus after noxious stimulation: simultaneous-input-dependent expression and nuclear speckling

Stimulation of sensory C-fibres produces extensive expression of the Fos, Jun and Krox families of inducible transcription factors (ITFs) in many nociceptive CNS areas [28]. In the hippocampus, however, c-Fos is only weakly induced by such stimulation, and expression of the other ITFs has not been studied. Here we examine the effects of single, repeated and simultaneous C-fibre inputs on ITF expressions in the rat hippocampus. A brief, strong electrical stimulation of sciatic nerve C-fibres induced little or no expression of c-Fos or Krox-20. In contrast, FosB was induced and continued to rise in all areas, whereas the basal expressions of c-Jun and Krox-24 were initially reduced but then returned during the subsequent 36 h. A weak noxious cutaneous stimulus applied to one hindpaw induced only weak expressions of the ITFs. However, if the sciatic stimulation was applied contralaterally and 6 h beforehand, this weak stimulus strongly induced Krox-24, but not other ITFs, i.e. there was a potentiation of Krox-24 expression. When these two stimuli were applied simultaneously a few c-Fos labelled cells did appear, and there was and an increased Krox-24 expression. There was also a strong potentiation of FosB and a strong reduction in c-Jun expression. This simultaneous stimulation was the only type of stimulation to induce expression of Krox-20. Also after simultaneous stimulation the majority of the nuclear labelling for FosB, but not of the other ITFs, had a speckled appearance. MK-801 blocked these changes in ITF expressions, but it could also cause the C-fibre stimulations to induce c-Fos and c-Jun in specific areas of the hippocampus. Thus C-fibre stimulation does affect transcription factor activity in the hippocampus; and the strong responses of some ITFs to simultaneous inputs points to their having a role as 'genetic coincidence detectors' in the hippocampus.

Delayed onset of enhanced MK-801-induced motor hyperactivity after neonatal lesions of the rat ventral hippocampus

BACKGROUND

Abnormalities in the glutamatergic system, glutamate/dopamine/gamma-aminobutyric acid interactions, and cortical development are implicated in schizophrenia. Moreover, patients with schizophrenia show symptom exacerbation in response to N-methyl-D-aspartate (NMDA) antagonist drugs. Using an animal model of schizophrenia, we compared the impact of neonatal and adult hippocampal lesions on behavioral responses to MK-801, a noncompetitive NMDA antagonist.

METHODS

Neonatal rats were lesioned on postnatal day 7. Their motor activity in response to MK-801 was tested at a juvenile age, in adolescence, and in adulthood. We also measured binding of [(3)H]MK-801 and the expression of NR1 messenger RNA (mRNA) in the medial prefrontal cortex and nucleus accumbens. Adult rats received similar lesions and were tested 4 and 8 weeks after the lesion.

RESULTS

As juveniles, neonatally lesioned rats did not differ from control rats in responsiveness to MK-801, whereas in adolescence and adulthood they showed more pronounced hyperactivity than control rats. The adult lesion did not alter behaviors elicited by MK-801. Neonatally lesioned rats showed no apparent changes in [(3)H]MK-801 binding or expression of the NR1 mRNA.

CONCLUSIONS

These results suggest that an early lesion of the ventral hippocampus affects development of neural systems involved in MK-801 action without changes at the NMDA receptor level, and they show that the behavioral changes manifest first in early adulthood.

Pharmacology and behavioral pharmacology of the mesocortical dopamine system

The prefrontal cortex (PFC) has long been known to be involved in the mediation of complex behavioral responses. Considerable research efforts are directed towards refining the knowledge about the function of this brain area and the role it plays in cognitive performance and behavioral output. In the first part, this review provides, from a pharmacological perspective, an overview of anatomical, electrophysiological and neurochemical aspects of the function of the PFC, with an emphasis on the mesocortical dopamine system. Anatomy of the mesocortical system, basic physiological and pharmacological properties of neurotransmission within the PFC, and interactions between dopamine and glutamate as well as other transmitters within the mesocorticolimbic circuit are included. The coverage of these data is largely restricted to what is relevant for the second part of the review which focuses on behavioral studies that have examined the role of the PFC in a variety of phenomena, behaviors and paradigms. These include reward and addiction, locomotor activity and sensitization, learning, cognition, and schizophrenia. Although the focus of this review is on the mesocortical dopamine system, given the intricate interactions of dopamine with other transmitter systems within the PFC and the importance of the PFC as a source of glutamate in subcortical areas, these aspects are also covered in some detail where appropriate. Naturally, a topic as complex as this cannot be covered comprehensively in its entirety. Therefore this review is largely limited to data derived from studies using rats, and it is also specifically restricted to data concerning the medial PFC (mPFC). Since in several fields of research the findings concerning the function or role of the mPFC are relatively inconsistent, the question is addressed whether these inconsistencies might, at least in part, be related to the anatomical and functional heterogeneity of this brain area.

Modulation of amphetamine-induced striatal dopamine release by ketamine in humans: implications for schizophrenia

BACKGROUND

Recent brain imaging studies have indicated that schizophrenia is associated with increased amphetamine-induced dopamine release in the striatum. It has long been hypothesized that dysregulation of subcortical dopamine systems in schizophrenia might result from a failure of the prefrontal cortex (PFC) to adequately control subcortical dopaminergic function. The activity of midbrain dopaminergic neurons is regulated, in part, by glutamatergic projections from the PFC acting via glutamatergic N-methyl-D-aspartate (NMDA) receptors. The goal of this study was to test the hypothesis that a pharmacologically induced disruption of NMDA transmission leads to an increase in amphetamine-induced dopamine release in humans.

METHODS

In eight healthy volunteers, we compared striatal amphetamine-induced (0.25 mg/kg) dopamine release under control conditions and under sustained disruption of NMDA transmission induced by infusion of the noncompetitive NMDA antagonist ketamine (0.2 mg/kg intravenous bolus followed by 0.4 mg/kg/hour intravenous infusion for 4 hours). Amphetamine-induced dopamine release was determined with single photon emission computed tomography, as the reduction in the binding potential (BP) of the radiolabeled D(2) receptor antagonist [(123)I]IBZM.

RESULTS

Ketamine significantly enhanced the amphetamine-induced decrease in [(123)I]IBZM BP, from -5.5% +/- 3.5% under control conditions to -12. 8% +/- 8.8% under ketamine pretreatment (repeated-measures analysis of variance, p =.023).

CONCLUSIONS

The increase in amphetamine-induced dopamine release induced by ketamine (greater than twofold) was comparable in magnitude to the exaggerated response seen in patients with schizophrenia. These data are consistent with the hypothesis that the alteration of dopamine release revealed by amphetamine challenge in schizophrenia results from a disruption of glutamatergic neuronal systems regulating dopaminergic cell activity.

The dopamine antagonist sch 23390 reverses dizocilpine-induced blockade of cocaine sensitization

The present work examined the effects of pre-treatment with Sch 23390, a selective D(1) receptor antagonist, on the dizocilpine-induced blockade of sensitization to the locomotor-stimulating effect of cocaine. Rats were given either cocaine [15mgkg(-1)day(-1), intraperitoneally (i.p.)] from day 1 to day 5 (cocaine-experienced rats) or vehicle (cocaine-naïve rats). From day 6 to day 15, animals remained drug-free in their home cages. On day 16 rats received a challenge injection of cocaine (15mgkg(-1)) or vehicle, and were tested for sensitization to the locomotor-stimulating effect of cocaine. In cocaine-naïve rats the acute effect of cocaine was a 1.5 times increase in locomotor activity. In cocaine-experienced rats, the acute effects of cocaine were considerably more pronounced than in cocaine-naïve rats; the stimulating effect of cocaine in these animals was a doubling in locomotor activity. In cocaine-naïve rats, pre-treatment with dizocilpine (100microgkg(-1)), Sch 23390 (100microgkg(-1)) or a combination of the two drugs from day 1 to day 5 changed neither spontaneous locomotor activity nor cocaine stimulant activity. By contrast, cocaine-experienced animals that had been given 100microgkg(-1) dizocilpine from day 1 to day 5 failed to show the increase in locomotor activity when challenged with cocaine on day 16. Pre-treatment with Sch 23390 (100microgkg(-1)day(-1), i.p.) from day 1 to day 5 was found to prevent completely the cocaine anti-sensitization properties of 100microgkg(-1) dizocilpine, but failed to prevent cocaine sensitization. On the other hand, horizontal activity in cocaine-experienced rats that had been given dizocilpine (100microgkg(-1)) 15min before cocaine challenge on day 16 was higher than in corresponding controls. It is concluded that prevention of cocaine sensitization by dizocilpine may be related to the events set into motion by the NMDA antagonist at the level of dopaminergic transmission involving D(1) receptors.