Ketamine and amphetamine both enhance synaptic transmission in the amygdala-nucleus accumbens pathway but with different time-courses

Lower Fractional Anisotropy in the Gray Matter of Amygdala-Hippocampus-Nucleus Accumbens Circuit in Methamphetamine Users: an In Vivo Diffusion Tensor Imaging Study

The basolateral amygdala (BLA), hippocampal ventral subiculum, and nucleus accumbens (NAc) comprise the amygdala-hippocampus-NAc (AHN) circuit, which is implicated in drug seeking and reward. The goal of this study was to evaluate microstructural changes and relevant clinical features of the AHN circuit gray matter (GM) in methamphetamine (MA) users using diffusion tensor imaging (DTI). Thirty MA users and 30 age-matched normal volunteers underwent 3-T MR imaging to obtain structural T1-weighted images and DTI data. Freesurfer software was used to automatically segment the NAc and subiculum. A Jülich probability map was employed to parcellate the BLA. Fractional anisotropy (FA) and mean diffusivity (MD) maps were generated and non-linearly coregistered to structural space. DTI measures of the AHN circuit GM were compared between MA users and controls using repeated measures analysis of variance. Correlation analyses were performed between DTI measures and clinical characteristics. Anatomical correlations between the NAc and BLA/subiculum in both groups were assessed using correlation analyses. The MA group had significant lower FA in the bilateral BLA, subiculum, and NAc. Higher total MA dose corresponded with lower FA in all three structures. Hamilton Anxiety Rating Scale scores negatively correlated with the right subiculum FA. Lower left BLA FA was associated with higher thinking disorder and hostile-suspicion factor scores. Left BLA FA was significantly associated with bilateral NAc FA in MA users. Those findings provided neuroimaging evidence of MA-induced microstructural impairment in the AHN circuit GM. Enhanced anatomical correlations between the left BLA and bilateral NAc may be part of the mechanism of MA intake relapse and for development of psychosis.

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.

Novel glutamatergic agents for major depressive disorder and bipolar disorder

Mood disorders such as major depressive disorder (MDD) and bipolar disorder (BPD) are common, chronic, recurrent mental illnesses that affect the lives and functioning of millions of individuals worldwide. Growing evidence suggests that the glutamatergic system is central to the neurobiology and treatment of these disorders. Here, we review data supporting the involvement of the glutamatergic system in the pathophysiology of mood disorders as well as the efficacy of glutamatergic agents as novel therapeutics.

Intra-accumbal NMDA but not AMPA/kainate receptor antagonist attenuates WIN55,212-2 cannabinoid receptor agonist-induced antinociception in the basolateral amygdala in a rat model of acute pain

Previous studies showed the role of basolateral amygdala (BLA) in cannabinoid-induced antinociception. Several lines of evidence indicated that the nucleus accumbens (NAc) receives excitatory glutamatergic inputs primarily from limbic-related structures, including the hippocampus, BLA, and various thalamic nuclei. Additionally, it has been shown that the NAc plays an important role in mediating the suppression of animal models of pain. In the present study, we examined the role of NMDA and AMPA/kainate receptors within the NAc in antinociception induced by intra-BLA cannabinoid receptor agonist WIN55,212-2 in rats. 126 adult male albino Wistar rats weighing 230-280 g were unilaterally implanted by two separate cannulae into the BLA and NAc. Dose-response antinociceptive effects of different doses of intra-BLA WIN55,212-2 (5, 10 and 15 μg/0.3 μl/rat) were evaluated in this study. Moreover, animals received intra-accumbal infusions of either NMDA receptor antagonist, AP5 (0.5, 2.5 and 5 μg/0.5 μl saline) or AMPA/kainate receptor antagonist, CNQX (0.1, 0.5 and 2.5 μg/0.5 μl DMSO), just 2 min before microinjection of WIN55,212-2 into the BLA. Antinociceptive responses of drugs were obtained by tail-flick analgesiometer and represented as maximal possible effect (%MPE) at 5, 15, 30, 45 and 60 min after their administrations. Results showed that intra-accumbal AP5 dose-dependently prevented antinociception induced by intra-BLA administration of WIN55,212-2 (15 μg/rat) in time set intervals. Nonetheless, administration of AMPA/kainate receptor antagonist, CNQX, could not affect WIN-induced analgesia. Additionally, solely administration of intra-accumbal injection of CNQX (2.5 μg/0.5 μl DMSO), but not AP5 (5 μg/0.5 μl saline), could significantly change the baseline tail-flick latencies in the rats. It seems that NMDA receptors located in the NAc, in part, mediate the antinociceptive responses of cannabinoid within the BLA in acute model of pain.

The role of the tripartite glutamatergic synapse in the pathophysiology and therapeutics of mood disorders

Bipolar disorder and major depressive disorder are common, chronic, and recurrent mood disorders that affect the lives of millions of individuals worldwide. Growing evidence suggests that glutamatergic system dysfunction is directly involved in mood disorders. This article describes the role of the "tripartite glutamatergic synapse," comprising presynaptic and postsynaptic neurons and glial cells, in the pathophysiology and therapeutics of mood disorders. Glutamatergic neurons and glia directly control synaptic and extrasynaptic glutamate levels/ release through integrative effects that target glutamate excitatory amino acid transporters, postsynaptic density proteins, ionotropic receptors (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid [AMPA], N-methyl-D-aspartate [NMDA], and kainate), and metabotropic receptors. This article also explores the glutamatergic modulators riluzole and ketamine, which are considered valuable proof-of-concept agents for developing the next generation of antidepressants and mood stabilizers. In therapeutically relevant paradigms, ketamine preferentially targets postsynaptic AMPA/NMDA receptors, and riluzole preferentially targets presynaptic voltage-operated channels and glia.

Ketamine and the next generation of antidepressants with a rapid onset of action

Existing treatments for major depressive disorder (MDD) usually take weeks to months to achieve their antidepressant effects, and a significant number of patients do not have adequate improvement even after months of treatment. In addition, increased risk of suicide attempts is a major public health concern during the first month of standard antidepressant therapy. Thus, improved therapeutics that can exert their antidepressant effects within hours or a few days of their administration are urgently needed, as is a better understanding of the presumed mechanisms associated with these rapid antidepressant effects. In this context, the N-methyl-D-aspartate (NMDA) antagonist ketamine has consistently shown antidepressant effects within a few hours of its administration. This makes it a valuable research tool to identify biomarkers of response in order to develop the next generation of fast-acting antidepressants. In this review, we describe clinical, electrophysiological, biochemical, and imaging correlates as relevant targets in the study of the antidepressant response associated with ketamine, and their implications for the development of novel, fast-acting antidepressants. We also review evidence that alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) to NMDA throughput may represent a convergent mechanism for the rapid antidepressant actions of ketamine. Overall, understanding the molecular basis of this work will likely lead to the ultimate development of improved therapeutics for MDD.

Memory and psychostimulants: modulation of Pavlovian fear conditioning by amphetamine in C57BL/6 mice

RATIONALE AND OBJECTIVES

With the use of prescription stimulants on the rise, it is important to examine the cognitive effects of low and moderate doses of stimulants rather than only those typical of addicts.

MATERIALS AND METHODS

The present study examined the effects a range of doses (0.005-8 mg/kg) of D: -amphetamine sulfate on cued and contextual Pavlovian fear conditioning in mice.

RESULTS

In agreement with previous research, subjects administered with a moderately high dose of amphetamine (8 mg/kg) pre-training, typical of what addicts might take, displayed impaired conditioned freezing when tested off-drug. Alternately, subjects injected with a very low dose of amphetamine (0.005, 0.025, or 0.05 mg/kg) pre-training, similar to the therapeutic doses for attention deficit hyperactivity disorder, displayed enhanced memory when tested off-drug. A control study showed that these effects were not due to state-dependent learning.

CONCLUSIONS

Thus, dose is a critical determinant of the cognitive effects of psychostimulants.

Amphetamine exposure enhances accumbal responses to reward-predictive stimuli in a pavlovian conditioned approach task

Acute and repeated exposure to psychostimulants such as amphetamine enhances the effects of pavlovian conditioned stimuli on conditioned behavior. It is hypothesized that amphetamine facilitates conditioned stimulus (CS) effects by selectively enhancing accumbal neuronal responses to stimuli. To test this hypothesis, rats were trained to discriminate between two pavlovian stimuli. One stimulus (i.e., CS+) was paired with sucrose delivery [i.e., unconditioned stimulus (US)], and the other stimulus (i.e., CS-) was paired with the absence of sucrose. Animals developed a conditioned approach response that occurred during the CS+ but not during the CS-. We tested the effect of different doses of amphetamine (0, 0.25, 0.5, or 1.0 mg/kg) on this conditioned approach behavior as well as on accumbal neuronal responses time locked to the CS+, the CS-, and the US. Acute amphetamine exposure increased conditioned approach behavior during the CS+, but not during the CS-. This change in behavior was associated with a selective increase in the magnitude of accumbal responses during the CS+. Repeated amphetamine administration followed by a drug-free period and reexposure did not affect the conditioned behavior, but increased accumbal responses to the CS+. These findings support the hypothesis that amphetamine exposure enhances behavioral responses to pavlovian conditioned stimuli by amplifying accumbal responses to those stimuli.

Modulation of high-frequency oscillations associated with NMDA receptor hypofunction in the rodent nucleus accumbens by lamotrigine

We reported recently that ketamine can increase the power of high-frequency oscillations (HFO) in the rodent nucleus accumbens (NAc), a region implicated in the pathophysiology of schizophrenia. Lamotrigine is known to reduce several of the abnormal behaviors induced by NMDA receptor antagonists in humans and rodents. This prompted us to examine whether lamotrigine would disrupt ketamine-enhanced HFO. Local field potentials (LFPs) and locomotor activity were recorded from male Wistar rats chronically implanted with electrodes in the NAc. Rats were pretreated with either saline or lamotrigine for 60min followed by injection of ketamine (25mg/kg). A separate group received a unilateral intra-NAc infusion of lamotrigine immediately followed by systemic injection of ketamine. We found systemic injection of a high dose of lamotrigine (20.1mg/kg) reduced the power and frequency of ketamine-enhanced HFO. This dose of lamotrigine was also associated with a decrease in both spontaneous HFO and locomotor activity, but did not significantly reduce locomotor activity induced by ketamine. In contrast, a low dose of lamotrigine (2.0mg/kg) produced a small, but significant increase of both ketamine-enhanced HFO and locomotor activity. Local infusion of lamotrigine into the NAc did not significantly affect ketamine-induced HFO, suggesting lamotrigine produces its effect on structures afferent to the NAc, and effects on HFO most likely result from modulating excitatory transmission to the NAc.

Ketamine, at a dose that disrupts motor behavior and latent inhibition, enhances prefrontal cortex synaptic efficacy and glutamate release in the nucleus accumbens

Noncompetitive N-methyl-D-aspartate (NMDA) antagonists such as ketamine represent useful pharmacological tools to model, in both healthy humans and rodents, behavioral and cerebral abnormalities of schizophrenia. These compounds are thought to exert some of their disruptive effects by impairing glutamatergic transmission in corticolimbic circuits including the nucleus accumbens (NAc). In this study, we investigated in freely moving rats behavioral changes as well as electrophysiological and neurochemical alterations in the NAc following acute systemic injection of a subanesthetic dose (25 mg/kg) of ketamine. We found that ketamine induced an immediate behavioral activation, characterized by hyperlocomotion, stereotypies and ataxia, and abolished latent inhibition in a conditioned-fear paradigm when injected at the pre-exposure stage. We also observed that during expression of motor effects which are thought to be related to the positive symptoms of schizophrenia, ketamine potentiated synaptic efficacy in the prefrontal-accumbens pathway and increased the extracellular levels of glutamate in the NAc. These results, taken together with previous findings, suggest that the psychotic-like effects of noncompetitive NMDA antagonists may be, in part, mediated by an increase in glutamate release in the NAc associated with synaptic changes in accumbens glutamatergic inputs including enhancement of synaptic efficacy in the prefrontal input.

Ketamine dose-dependently induces high-frequency oscillations in the nucleus accumbens in freely moving rats

BACKGROUND

In humans, subanesthetic doses of ketamine and recovery from ketamine anesthesia are associated with psychotic-like behavior. In rodents, ketamine produces hyperactivity, stereotypies, and abnormal social interaction used to model certain features of schizophrenia. Increasing evidence has implicated aberrant activity in the nucleus accumbens (NAc) with the pathophysiology of schizophrenia.

METHODS

Here, we examined the effect of an IP injection of ketamine (10, 25, 50, and 200 mg/kg) and d-amphetamine (3 mg/kg) on local field potentials in the rodent NAc. Locomotor activity was recorded simultaneously.

RESULTS

Spontaneous high-frequency oscillations (HFO) (140-180 Hz) were present in local field potentials recorded from the NAc. Ketamine dose-dependently induced rapid and substantial increases in HFO that correlated with behavioral hyperactivity. Similarly, large increases in HFO occurred during recovery from ketamine anesthesia. In contrast, d-amphetamine, which induced locomotor activity, produced only small increases in HFO.

CONCLUSIONS

We propose that ketamine-induced abnormal increases in HFO form part of the complex neurological changes in this model of schizophrenia. Ketamine-induced increases in HFO, although sharing similar temporal dynamics to hyperactivity, may not be functionally related to increased movement.

Regulation of psychostimulant-induced preprodynorphin, c-fos and zif/268 messenger RNA expression in the rat dorsal striatum by mu opioid receptor blockade

Several studies have shown that psychostimulants can induce differential immediate early gene and neuropeptide expression in the patch versus matrix compartments of dorsal striatum. The patch compartment contains a high density of mu opioid receptors and activation of these receptors may contribute to psychostimulant-induced gene expression in the patch versus matrix compartments of dorsal striatum. However, the contribution of mu opioid receptor activation to psychostimulant-induced changes in gene expression in the patch compartment of dorsal striatum has not been examined. The current study examined the role of mu opioid receptors in psychostimulant induction of preprodynorphin, c-fos and zif/268 messenger RNA expression in the patch versus matrix compartments of dorsal striatum. Male Sprague-Dawley rats were treated with the mu opioid receptor antagonist, clocinnamox (1 mg/kg, s.c.), 24 h prior to treatment with cocaine (30 mg/kg, i.p.) or methamphetamine (15 mg/kg, s.c.) and sacrificed 45 min or 3 h later. Mu opioid receptor antagonism blocked psychostimulant-induced preprodynorphin messenger RNA expression only in the rostral patch compartment, whereas psychostimulant-induced zif/268 messenger RNA expression in the patch and matrix compartments was attenuated throughout the dorsal striatum. Clocinnamox pretreatment had no effect on stimulant-induced increases in c-fos expression. These data suggest that mu opioid receptor activation plays a specific role in psychostimulant-induced preprodynorphin messenger RNA expression in the rostral patch compartment and zif/268 messenger RNA expression throughout dorsal striatum.