Panos Zanos

A Negative Allosteric Modulator for α5 Subunit-Containing GABA Receptors Exerts a Rapid and Persistent Antidepressant-Like Action without the Side Effects of the NMDA Receptor Antagonist Ketamine in Mice

Abstract New antidepressant pharmacotherapies that provide rapid relief of depressive symptoms are needed. The NMDA receptor antagonist ketamine exerts rapid antidepressant actions in depressed patients but also side effects that complicate its clinical utility. Ketamine promotes excitatory synaptic strength, likely by producing high-frequency correlated activity in mood-relevant regions of the forebrain. Negative allosteric modulators of GABA-A receptors containing α5 subunits (α5 GABA-NAMs) should also promote high-frequency correlated electroencephalogram (EEG) activity and should therefore exert rapid antidepressant responses. Because α5 subunits display a restricted expression in the forebrain, we predicted that α5 GABA-NAMs would produce activation of principle neurons but exert fewer side effects than ketamine. We tested this hypothesis in male mice and observed that the α5 GABA-NAM MRK-016 exerted an antidepressant-like response in the forced swim test at 1 and 24 h after administration and an anti-anhedonic response after chronic stress in the female urine sniffing test (FUST). Like ketamine, MRK-016 produced a transient increase in EEG γ power, and both the increase in γ power and its antidepressant effects in the forced swim test were blocked by prior administration of the AMPA-type glutamate receptor antagonist 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonamide (NBQX). Unlike ketamine, however, MRK-016 produced no impairment of rota-rod performance, no reduction of prepulse inhibition (PPI), no conditioned-place preference (CPP), and no change in locomotion. α5 GABA-NAMs, thus reproduce the rapid antidepressant-like actions of ketamine, perhaps via an AMPA receptor (AMPAR)-dependent increase in coherent neuronal activity, but display fewer potential negative side effects. These compounds thus demonstrate promise as clinically useful fast-acting antidepressants.

Convergent Mechanisms Underlying Rapid Antidepressant Action

Traditional pharmacological treatments for depression have a delayed therapeutic onset, ranging from several weeks to months, and there is a high percentage of individuals who never respond to treatment. In contrast, ketamine produces rapid-onset antidepressant, anti-suicidal, and anti-anhedonic actions following a single administration to patients with depression. Proposed mechanisms of the antidepressant action of ketamine include N-methyl-d-aspartate receptor (NMDAR) modulation, gamma aminobutyric acid (GABA)-ergic interneuron disinhibition, and direct actions of its hydroxynorketamine (HNK) metabolites. Downstream actions include activation of the mechanistic target of rapamycin (mTOR), deactivation of glycogen synthase kinase-3 and eukaryotic elongation factor 2 (eEF2), enhanced brain-derived neurotrophic factor (BDNF) signaling, and activation of α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptors (AMPARs). These putative mechanisms of ketamine action are not mutually exclusive and may complement each other to induce potentiation of excitatory synapses in affective-regulating brain circuits, which results in amelioration of depression symptoms. We review these proposed mechanisms of ketamine action in the context of how such mechanisms are informing the development of novel putative rapid-acting antidepressant drugs. Such drugs that have undergone pre-clinical, and in some cases clinical, testing include the muscarinic acetylcholine receptor antagonist scopolamine, GluN2B-NMDAR antagonists (i.e., CP-101,606, MK-0657), (2R,6R)-HNK, NMDAR glycine site modulators (i.e., 4-chlorokynurenine, pro-drug of the glycineB NMDAR antagonist 7-chlorokynurenic acid), NMDAR agonists [i.e., GLYX-13 (rapastinel)], metabotropic glutamate receptor 2/3 (mGluR2/3) antagonists, GABAA receptor modulators, and drugs acting on various serotonin receptor subtypes. These ongoing studies suggest that the future acute treatment of depression will typically occur within hours, rather than months, of treatment initiation.

Sex-dependent modulation of age-related cognitive decline by the L-type calcium channel gene Cacna1c (Cav1.2)

Increased calcium influx through L‐type voltage‐gated calcium channels has been implicated in the neuronal dysfunction underlying age‐related memory declines. The present study aimed to test the specific role of Cacna1c (which encodes Cav1.2) in modulating age‐related memory dysfunction. Short‐term, spatial and contextual/emotional memory was evaluated in young and aged, wild‐type as well as mice with one functional copy of Cacna1c (haploinsufficient), using the novel object recognition, Y‐maze and passive avoidance tasks, respectively. Hippocampal expression of Cacna1c mRNA was measured by quantitative polymerase chain reaction. Ageing was associated with object recognition and contextual/emotional memory deficits, and a significant increase in hippocampal Cacna1c mRNA expression. Cacna1c haploinsufficiency was associated with decreased Cacna1c mRNA expression in both young and old animals. However, haploinsufficient mice did not manifest an age‐related increase in expression of this gene. Behaviourally, Cacna1c haploinsufficiency prevented object recognition deficits during ageing in both male and female mice. A significant correlation between higher Cacna1c levels and decreased object recognition performance was observed in both sexes. Also, a sex‐dependent protective role of decreased Cacna1c levels in contextual/emotional memory loss has been observed, specifically in male mice. These data provide evidence for an association between increased hippocampal Cacna1c expression and age‐related cognitive decline. Additionally, they indicate an interaction between the Cacna1c gene and sex in the modulation of age‐related contextual memory declines.

Cocaine abstinence induces emotional impairment and brain region‐specific upregulation of the oxytocin receptor binding

The key problem in treating cocaine addiction is the maintenance of a drug‐free state as negative emotional symptoms during abstinence often trigger relapse. The mechanisms underpinning the emotional dysregulation during abstinence are currently not well‐understood. There is evidence suggesting a role of the neuropeptide oxytocin in the modulation of drug addiction processes. However, its involvement during long‐term abstinence from cocaine use remains unclear. In this study, we aimed to behaviourally characterize a mouse model of long‐term cocaine withdrawal and assess the effect of chronic cocaine administration and long‐term cocaine abstinence on the central oxytocinergic system and the hypothalamic–pituitary–adrenal axis. Fourteen‐day escalating‐dose cocaine administration (3 × 15–30 mg/kg/day) and 14‐day withdrawal increased plasma corticosterone levels and oxytocin receptor (OTR) binding in piriform cortex, lateral septum and amygdala. A specific cocaine withdrawal‐induced increase in OTR binding was observed in the medial septum. These biochemical alterations occurred concomitantly with the emergence of memory impairment, contextual psychomotor sensitization and an anhedonic and anxiogenic phenotype during withdrawal. Our study established a clear relationship between cocaine abstinence and emotional impairment in a novel translationally relevant model of cocaine withdrawal and demonstrated for the first time brain region‐specific neuroadaptations of the oxytocin system, which may contribute to abstinence‐induced negative emotional state.

Environmental enrichment enhances conditioned place preference to ethanol via an oxytocinergic-dependent mechanism in male mice

ABSTRACT Environmental conditions, such as stress and environmental enrichment (EE), influence predisposition to alcohol use/abuse; however, the underlying mechanisms remain unknown. To assess the effect of environmental conditions on the initial rewarding effects of alcohol, we examined conditioned place‐preference (CPP) to alcohol following exposure to EE in mice. Since social context is a major factor contributing to initial alcohol‐drinking, we also assessed the impact of EE on the levels of the “social neuropeptide” oxytocin (OT) and its receptor, OTR. Finally, we assessed the effect of pharmacological manipulations of the oxytocinergic system on EE‐induced alcohol CPP. While EE increased sociability and reduced anxiety‐like behaviors, it caused a ˜3.5‐fold increase in alcohol reward compared to controls. EE triggered profound neuroadaptations of the oxytocinergic system; it increased hypothalamic OT levels and decreased OTR binding in the prefrontal cortex and olfactory nuclei of the brain. Repeated administration of the OT analogue carbetocin (6.4mg/kg/day) mimicked the behavioral effects of EE on ethanol CPP and induced similar brain region‐specific alterations of OTR binding as those observed following EE. Conversely, repeated administration of the OTR antagonist L,369–899 (5mg/kg/day) during EE exposure, but not during the acquisition of alcohol CPP, reversed the pronounced EE‐induced ethanol rewarding effect. These results demonstrate for the first time, a stimulatory effect of environmental enrichment exposure on alcohol reward via an oxytocinergic‐dependent mechanism, which may predispose to alcohol abuse. This study offers a unique prospective on the neurobiological understanding of the initial stages of alcohol use/misuse driven by complex environmental‐social interplay. HIGHLIGHTSEnvironmental enrichment (EE) and carbetocin enhance ethanol conditioned‐place preference (CPP).EE and carbetocin decrease oxytocin receptor binding (OTR) in the PFC.EE increases oxytocin levels in the hypothalamus.Blockade of OTR during EE prevents EE‐induced enhancement of ethanol CPP.

Isoflurane but Not Halothane Prevents and Reverses Helpless Behavior: A Role for EEG Burst Suppression?

Abstract Background The volatile anesthetic isoflurane may exert a rapid and long-lasting antidepressant effect in patients with medication-resistant depression. The mechanism underlying the putative therapeutic actions of the anesthetic have been attributed to its ability to elicit cortical burst suppression, a distinct EEG pattern with features resembling the characteristic changes that occur following electroconvulsive therapy. It is currently unknown whether the antidepressant actions of isoflurane are shared by anesthetics that do not elicit cortical burst suppression. Methods In vivo electrophysiological techniques were used to determine the effects of isoflurane and halothane, 2 structurally unrelated volatile anesthetics, on cortical EEG. The effects of anesthesia with either halothane or isoflurane were also compared on stress-induced learned helplessness behavior in rats and mice. Results Isoflurane, but not halothane, anesthesia elicited a dose-dependent cortical burst suppression EEG in rats and mice. Two hours of isoflurane, but not halothane, anesthesia reduced the incidence of learned helplessness in rats evaluated 2 weeks following exposure. In mice exhibiting a learned helplessness phenotype, a 1-hour exposure to isoflurane, but not halothane, reversed escape failures 24 hours following burst suppression anesthesia. Conclusions These results are consistent with a role for cortical burst suppression in mediating the antidepressant effects of isoflurane. They provide rationale for additional mechanistic studies in relevant animal models as well as a properly controlled clinical evaluation of the therapeutic benefits associated with isoflurane anesthesia in major depressive disorder.

Wheel running during chronic nicotine exposure is protective against mecamylamine‐precipitated withdrawal and up‐regulates hippocampal α7 nACh receptors in mice

Evidence suggests that exercise decreases nicotine withdrawal symptoms in humans; however, the mechanisms mediating this effect are unclear. We investigated, in a mouse model, the effect of exercise intensity during chronic nicotine exposure on nicotine withdrawal severity, binding of α4β2*, α7 nicotinic acetylcholine (nAChR), μ‐opioid (μ receptors) and D2 dopamine receptors and on brain‐derived neurotrophic factor (BDNF) and plasma corticosterone levels.

Chronic nicotine administration restores brain region specific up-regulation of oxytocin receptor binding levels in a G72 mouse model of schizophrenia.

Nicotine dependence and schizophrenia are two mental health disorders with remarkably high comorbidity. Cigarette smoking is particularly prevalent amongst schizophrenic patients and it is hypothesised to comprise a form of self‐medication for relieving cognitive deficits in these patients. Emerging evidence suggests a role of the neurohypophysial peptide oxytocin in the modulation of drug addiction, as well as schizophrenia symptomology; however, the underlying mechanism remains unclear. Therefore, we sought to investigate the effects of chronic nicotine administration on oxytocin receptor (OTR) binding in the brain of a transgenic mouse model of schizophrenia that carries a bacterial artificial chromosome of the human G72/G30 locus (G72Tg). Female wild‐type (WT) and heterozygous G72 transgenic CD‐1 mice were treated with a chronic nicotine regimen (24 mg/kg/day, osmotic minipumps for 14 days) and quantitative autoradiographic mapping of oxytocin receptors was carried out in brains of these animals. OTR binding levels were higher in the cingulate cortex (CgCx), nucleus accumbens (Acb), and central amygdala (CeA) of saline treated G72Tg mice compared to WT control mice. Chronic nicotine administration reversed this upregulation in the CgCx and CeA. Interestingly, chronic nicotine administration induced an increase in OTR binding in the CeA of solely WT mice. These results indicate that nicotine administration normalises the dysregulated central oxytocinergic system of this mouse model of schizophrenia and may contribute towards nicotines ability to modulate cognitive deficits which are common symptoms of schizophrenia.