Ventral tegmental area cholinergic mechanisms mediate behavioral responses in the forced swim test

L-type calcium channel blockade attenuates the anxiogenic-like effects of cocaine abstinence in female and male rats

Cocaine abstinence and withdrawal are linked to relapse and heightened anxiety. While L-type calcium channels (LTCCs) have been associated with cocaine use disorders in humans and drug-seeking in rodents, their role in mood-related symptoms during cocaine abstinence remains unclear. We addressed this by investigating the ability of LTCC blockade with isradipine to alter the mood-related behavioral phenotypes induced by cocaine abstinence. Female and male Sprague-Dawley rats were trained to self-administer cocaine or saline, followed by a 14-day period of abstinence. Subsequently, behavioral responses were examined using the sucrose preference test (SPT), elevated plus maze (EPM), and forced swim test (FST). Cocaine-abstinent rats showed decreased open arm time in the EPM and increased immobility time in the FST with no changes in the SPT. Isradipine (0.4 mg/kg or 1.2 mg/kg, i.p.), reversed the anxiogenic-like EPM behavior in female cocaine-abstinent rats, whereas only the higher dose (1.2 mg/kg, i.p.) was effective in male cocaine-abstinent rats. In the FST, the lower dose (0.4 mg/kg, i.p.) of isradipine reversed the increased immobility time observed in cocaine-abstinent female and male rats, with no isradipine effect in saline-abstinent rats. In contrast 1.2 mg/kg, i.p. isradipine decreased immobility time in both cocaine and saline abstinent female and male rats. In summary, isradipine administration reversed the anxiogenic and increased the FST immobility time associated with cocaine abstinence in a dose and sex-dependent manner. The data underscore the importance of further investigation of LTCC mechanisms and their therapeutic potential for mood disorders associated with cocaine use disorder.

Medial prefrontal cortex acetylcholine signaling mediates the ability to learn an active avoidance response following learned helplessness training

Increased brain levels of acetylcholine (ACh) have been observed in patients with depression, and increasing ACh levels pharmacologically can precipitate stress-related behaviors in humans and animals. Conversely, optimal ACh levels are required for cognition and memory. We hypothesize that excessive ACh signaling results in strengthening of negative encoding in which memory formation is aberrantly strengthened for stressful events. The medial prefrontal cortex (mPFC) is critical for both top-down control of stress-related circuits, and for encoding of sensory experiences. We therefore evaluated the role of ACh signaling in the mPFC in a learned helplessness task in which mice were exposed to repeated inescapable stressors followed by an active avoidance task. Using fiber photometry with a genetically-encoded ACh sensor, we found that ACh levels in the mPFC during exposure to inescapable stressors were positively correlated with later escape deficits in an active avoidance test in males, but not females. Consistent with these measurements, we found that both pharmacologically- and chemogenetically-induced increases in mPFC ACh levels resulted in escape deficits in both male and female mice, whereas chemogenetic inhibition of ACh neurons projecting to the mPFC improved escape performance in males, but impaired escape performance in females. These results highlight the adaptive role of ACh release in stress response, but also support the idea that sustained elevation of ACh contributes to maladaptive behaviors. Furthermore, mPFC ACh signaling may contribute to stress-based learning differentially in males and females.

Dopaminergic and Glutamatergic Mechanisms in Addiction: Assessing the Therapeutic Promise of Memantine and Galantamine for Maintenance Treatment: A Review

Addiction comes in various forms and can be related to substances like cocaine, opioids, alcohol, cannabis, amphetamine, and nicotine, as well as behaviors like gambling or sex addiction. The impact of addiction places increased economic and medical burdens on society. Currently, the management of addiction is more focused on symptomatic relief rather than targeting the reinforcing mechanisms of dependence on addictive substances and behaviors. The aim of this review is to identify the specific roles of dopamine and glutamate in addiction, which can guide us to treat the cause rather than the symptoms. The synergistic effect of glutamate and dopamine neurotransmitters plays a crucial role in the development of pathological neuroplasticity in the mesolimbic system, causing compulsive consumption of the substance. Utilizing the brain's natural synthesis of substances such as Kynurenic acid (KYNA) derivatives could potentially disrupt the synergistic effect of glutamate and dopamine. By blocking glutamate release and increasing dopamine release, individuals may experience reward or pleasure without the need for addictive substances. Under this pretext, the review article explores the possibility of memantine and galantamine as maintenance treatment for addiction of various forms.

L-type calcium channel regulation of depression, anxiety and anhedonia-related behavioral phenotypes following chronic stress exposure

Exposure to chronic and unpredictable stressors can precipitate mood-related disorders in humans, particularly in individuals with pre-existing mental health challenges. L-type calcium channels (LTCCs) have been implicated in numerous neuropsychiatric disorders, as LTCC encoding genes have been identified as candidate risk factors for neuropsychiatric illnesses. In these sets of experiments, we sought to examine the ability of LTCC blockade to alter depression, anxiety, and anhedonic-related behavioral responses to chronic unpredictable stress (CUS) exposure in female and male rats. Rats first underwent either 21 days of CUS or no exposure to chronic stressors, serving as home cage controls (HCC). Then rats were examined for anhedonia-related behavior, anxiety and depression-like behavioral responses as measured by the sucrose preference test (SPT), elevated plus maze (EPM), and forced swim test (FST). CUS exposed females and males showed anhedonic and anxiogenic-like behavioral responses on the SPT and EPM, respectively, when compared to HCCs. In female and male rats, systemic administration of the LTCC blocker isradipine (0.4 mg/kg and 1.2 mg/kg, I.P.) attenuated the CUS-induced decrease in sucrose preference and reversed the CUS-induced decrease in open arm time. In the FST, systemic isradipine decreased immobility time across all groups, consistent with an antidepressant-like response. However, there were no significant differences in forced swim test immobility time between HCC and CUS exposed animals. Taken together, these data point to a role of LTCCs in the regulation of mood disorder-related behavioral phenotype responses to chronic stress exposure.

Septohippocampal cholinergic system at the intersection of stress and cognition: Current trends and translational implications

Deficits in hippocampus-dependent memory processes are common across psychiatric and neurodegenerative disorders such as depression, anxiety and Alzheimer's disease. Moreover, stress is a major environmental risk factor for these pathologies and it exerts detrimental effects on hippocampal functioning via the activation of hypothalamic-pituitary-adrenal (HPA) axis. The medial septum cholinergic neurons extensively innervate the hippocampus. Although, the cholinergic septohippocampal pathway (SHP) has long been implicated in learning and memory, its involvement in mediating the adaptive and maladaptive impact of stress on mnemonic processes remains less clear. Here, we discuss current research highlighting the contributions of cholinergic SHP in modulating memory encoding, consolidation and retrieval. Then, we present evidence supporting the view that neurobiological interactions between HPA axis stress response and cholinergic signalling impact hippocampal computations. Finally, we critically discuss potential challenges and opportunities to target cholinergic SHP as a therapeutic strategy to improve cognitive impairments in stress-related disorders. We argue that such efforts should consider recent conceptualisations on the dynamic nature of cholinergic signalling in modulating distinct subcomponents of memory and its interactions with cellular substrates that regulate the adaptive stress response.

Differential effects of intra-RMTg infusions of pilocarpine or 4-DAMP on regulating depression- and anxiety-like behaviors

Depression and anxiety are associated with dysfunction of the mesolimbic dopamine system. The rostromedial tegmental nucleus (RMTg) is predominantly composed of GABAergic neurons that exhibit dense projections and strongly inhibit mesolimbic dopaminergic neurons, proposed as a major "brake" for the system. Consequently, the RMTg may be a crucial brain region for regulating these emotions. The central cholinergic system, particularly the muscarinic receptors, plays an important regulatory role in depression and anxiety. M3 muscarinic receptors are distributed on GABAergic neurons in the RMTg, but their involvement in the regulation of depression and anxiety remains uncertain. This study aimed to examine the effects of RMTg M3 muscarinic receptors on regulating depression- and anxiety-like behaviors in adult male Wistar rats, as assessed through the forced swim, tail suspension, and elevated plus maze tests. The results showed that intra-RMTg injections of the M1/M3 muscarinic receptors agonist, pilocarpine (3, 10, and 30 μg/side), or the M3 muscarinic receptors antagonist, 4-DAMP (0.5, 1, and 2 μg/side), did not alter the immobility time in the forced swim and tail suspension tests. Additionally, pilocarpine (30 μg/side) decreased time spent in open arms and increased time in closed arms in the elevated plus maze; while 4-DAMP (1 and 2 μg/side) played the opposite role by increasing time spent in open arms and decreasing time in closed arms. These findings suggest that RMTg M3 muscarinic receptors have differential effects on regulating depression- and anxiety-like behaviors. Enhancing or inhibiting these receptors can produce anxiogenic or anxiolytic effects, but have no impact on depression-like behavior. Therefore, RMTg M3 muscarinic receptors are involved in regulating anxiety and may be a potential therapeutic target for anxiolytic drugs.

Ventral Tegmental Area M5 Muscarinic Receptors Mediate Effort-Choice Responding and Nucleus Accumbens Dopamine in a Sex-Specific Manner

Optimization of effort-related choices is impaired in depressive disorders. Acetylcholine (ACh) and dopamine (DA) are linked to depressive disorders, and modulation of ACh tone in the ventral tegmental area (VTA) affects mood-related behavioral responses in rats. However, it is unknown if VTA ACh mediates effort-choice behaviors. Using a task of effort-choice, rats can choose to lever press on a fixed-ratio 5 (FR5) schedule for a more-preferred food or consume freely available, less-preferred food. VTA administration of physostigmine (1 μg and 2 μg/side), a cholinesterase inhibitor, reduced FR5 responding for the more-preferred food while leaving consumption of the less-preferred food intact. VTA infusion of the M5 muscarinic receptor negative allosteric modulator VU6000181 (3 μM, 10 μM, 30 μM/side) did not affect lever pressing or chow consumption. However, VU6000181 (30 μM/side) coadministration with physostigmine (2 μg/side) attenuated physostigmine-induced decrease in lever pressing in female and male rats and significantly elevated lever pressing above vehicle baseline levels in male rats. In in vivo voltammetry experiments, VTA infusion of combined physostigmine and VU6000181 did not significantly alter evoked phasic DA release in the nucleus accumbens core (NAc) in female rats. In male rats, combined VTA infusion of physostigmine and VU6000181 increased phasic evoked DA release in the NAc compared with vehicle, physostigmine, or VU6000181 infusion alone. These data indicate a critical role and potential sex differences of VTA M5 receptors in mediating VTA cholinergic effects on effort choice behavior and regulation of DA release. SIGNIFICANCE STATEMENT: Effort-choice impairments are observed in depressive disorders, which are often treatment resistant to currently available thymoleptics. The role of ventral tegmental area (VTA) acetylcholine muscarinic M5 receptors, in a preclinical model of effort-choice behavior, is examined. Using the selective negative allosteric modulator of the M5 receptor VU6000181, we show the role of VTA M5 receptors on effort-choice and regulation of dopamine release in the nucleus accumbens core. This study supports M5 receptors as therapeutic targets for depression.

Differential role of GABAergic and cholinergic ventral pallidal neurons in behavioral despair, conditioned fear memory and active coping

The ventral pallidum (VP), a major component of the reward circuit, is well-associated with appetitive behaviors. Recent evidence suggests that this basal forebrain nucleus may have an overarching role in affective processing, including behavioral responses to aversive stimuli. We investigated this by utilizing selective immunotoxin lesions and a series of behavioral tests in adult male Wistar rats. We made bilateral GAT1-Saporin, 192-IgG-Saporin or PBS (vehicle) injections into the VP to respectively eliminate GABAergic and cholinergic neurons, and tested the animals in the forced swim test (FST), open field test (OFT), elevated plus maze (EPM), Morris water maze (MWM) and cued fear conditioning. Both GAT1-Saporin and 192-IgG-Saporin injections reduced behavioral despair without altering general locomotor activity. During the acquisition phase of cued fear conditioning, this antidepressant effect was accompanied by reduced freezing and increased darting in the 192-IgG-Saporin group, and increased jumping in the GAT1-Saporin group. In the extinction phase, cholinergic lesions impaired fear memory irrespective of the context, while GABAergic lesions reduced memory durability only during the early phases of extinction in a novel context. In line with this, selective cholinergic, but not GABAergic, lesions impaired spatial memory in the MWM. We observed no consistent effect in anxiety-like behavior assessed in the OFT and EPM. These findings indicate that both the GABAergic and cholinergic neuronal groups of the VP may contribute to emotion regulation through modulation of behavioral despair and acquired fear by suppressing active coping and promoting species-specific passive behaviors.

Encore: Behavioural animal models of stress, depression and mood disorders

Animal studies over the past two decades have led to extensive advances in our understanding of pathogenesis of depressive and mood disorders. Among these, rodent behavioural models proved to be of highest informative value. Here, we present a comprehensive overview of the most popular behavioural models with respect to physiological, circuit, and molecular biological correlates. Behavioural stress paradigms and behavioural tests are assessed in terms of outcomes, strengths, weaknesses, and translational value, especially in the domain of pharmacological studies.

Electroacupuncture Ameliorates Depressive-Like State and Synaptic Deficits Induced by Hyper-Cholinergic Tone During Chronic Stress in Rats

BACKGROUND Major depressive disorder (MDD) is the leading cause of disability around the world. It is generally agreed that the central cholinergic system plays an important role in emotional regulation. Acetylcholine (ACh) is now a new target for antidepressants. Therefore, the aim of this study was to evaluate the effect of acupuncture on depressive behaviors, cholinergic tones, and synaptic plasticity in the prefrontal cortex (PFC) in chronic unpredictable mild stress (CUMS). MATERIAL AND METHODS We randomly divided 36 male Sprague-Dawley (SD) rats into the Normal group, Stress group, Physostigmine+stress (Phys+stress) group, and Electroacupuncture+physostigmine+stress (EA+Phys+stress) group. Rats underwent CUMS exposure for 42 days. After 28 days of CUMS, rats received physostigmine or EA treatment for 2 weeks. Rats in the Phys+stress and EA+Phys+stress group received an intraperitoneal injection of physostigmine (TOCRIS, UK, 5 mg/kg) daily. Rats in the EA+Phys+stress group also received EA stimulation at GV 20 (Baihui), GV 29 (Yintang), LI 4 (Hegu), and LR 3 (Taichong) daily for 2 weeks. RESULTS We found that EA ameliorated weight loss and the depressive-like behaviors in the sucrose preference test, novelty-suppressed feeding test, and open-field test. There was significantly decreased expression of ACh and increased expression of acetylcholinesterase (AChE) after EA treatment. Consistent with the behavior tests and cholinergic tones, there were increased spine density and expressions of synaptic proteins, including brain-derived neurotrophic factor (BDNF), glutamate receptor 1 (GluR1), glutamate receptor 2 (GluR2), postsynaptic density protein 95 (PSD95), and synapsin I in the PFC. CONCLUSIONS The results suggest that EA can reverse the depressive-like behaviors and synaptic deficits induced by hyper-cholinergic tone during chronic stress via the modulation of hyper-cholinergic tone.

O-GlcNAcylation in Ventral Tegmental Area Dopaminergic Neurons Regulates Motor Learning and the Response to Natural Reward

Protein O-GlcNAcylation is a post-translational modification that links environmental stimuli with changes in intracellular signal pathways, and its disturbance has been found in neurodegenerative diseases and metabolic disorders. However, its role in the mesolimbic dopamine (DA) system, especially in the ventral tegmental area (VTA), needs to be elucidated. Here, we found that injection of Thiamet G, an O-GlcNAcase (OGA) inhibitor, in the VTA and nucleus accumbens (NAc) of mice, facilitated neuronal O-GlcNAcylation and decreased the operant response to sucrose as well as the latency to fall in rotarod test. Mice with DAergic neuron-specific knockout of O-GlcNAc transferase (OGT) displayed severe metabolic abnormalities and died within 4-8 weeks after birth. Furthermore, mice specifically overexpressing OGT in DAergic neurons in the VTA had learning defects in the operant response to sucrose, and impaired motor learning in the rotarod test. Instead, overexpression of OGT in GABAergic neurons in the VTA had no effect on these behaviors. These results suggest that protein O-GlcNAcylation of DAergic neurons in the VTA plays an important role in regulating the response to natural reward and motor learning in mice.

Targeting muscarinic receptors to treat schizophrenia

Schizophrenia is a severe neuropsychiatric disorder characterized by a diverse range of symptoms that can have profound impacts on the lives of patients. Currently available antipsychotics target dopamine receptors, and while they are useful for ameliorating the positive symptoms of the disorder, this approach often does not significantly improve negative and cognitive symptoms. Excitingly, preclinical and clinical research suggests that targeting specific muscarinic acetylcholine receptor subtypes could provide more comprehensive symptomatic relief with the potential to ameliorate numerous symptom domains. Mechanistic studies reveal that M1, M4, and M5 receptor subtypes can modulate the specific brain circuits and physiology that are disrupted in schizophrenia and are thought to underlie positive, negative, and cognitive symptoms. Novel therapeutic strategies for targeting these receptors are now advancing in clinical and preclinical development and expand upon the promise of these new treatment strategies to potentially provide more comprehensive relief than currently available antipsychotics.

Multiple cholinesterase inhibitors have antidepressant-like properties in the mouse forced swim test

There is high clinical interest in improving the pharmacological treatment of individuals with Major Depressive Disorder (MDD). This neuropsychiatric disorder continues to cause significant morbidity and mortality worldwide, where existing pharmaceutical treatments such as selective serotonin reuptake inhibitors often have limited efficacy. In a recent publication, we demonstrated an antidepressant-like role for the acetylcholinesterase inhibitor (AChEI) donepezil in the C57BL/6J mouse forced swim test (FST). Those data added to a limited literature in rodents and human subjects which suggests AChEIs have antidepressant properties, but added the novel finding that donepezil only showed antidepressant-like properties at lower doses (0.02, 0.2 mg/kg). At a high dose (2.0 mg/kg), donepezil tended to promote depression-like behavior, suggesting a u-shaped dose-response curve for FST immobility. Here we investigate the effects of three other AChEIs with varying molecular structures: galantamine, physostigmine, and rivastigmine, to test whether they also exhibit antidepressant-like effects in the FST. We find that these drugs do exhibit therapeutic-like effects at low but not high doses, albeit at lower doses for physostigmine. Further, we find that their antidepressant-like effects are not mediated by generalized hyperactivity in the novel open field test, and are also not accompanied by anxiolytic-like properties. These data further support the hypothesis that acetylcholine has a u-shaped dose-response relationship with immobility in the C57BL/6J mouse FST, and provide a rationale for more thoroughly investigating whether reversible AChEIs as a class can be repurposed for the treatment of MDD in human subjects.

Repurposing Cholinesterase Inhibitors as Antidepressants? Dose and Stress-Sensitivity May Be Critical to Opening Possibilities

When stress becomes chronic it can trigger lasting brain and behavioral changes including Major Depressive Disorder (MDD). There is conflicting evidence regarding whether acetylcholinesterase inhibitors (AChEIs) may have antidepressant properties. In a recent publication, we demonstrated a strong dose-dependency of the effect of AChEIs on antidepressant-related behavior in the mouse forced swim test: whereas the AChEI donepezil indeed promotes depression-like behavior at a high dose, it has antidepressant-like properties at lower doses in the same experiment. Our data therefore suggest a Janus-faced dose-response curve for donepezil in depression-related behavior. In this review, we investigate the mood-related properties of AChEIs in greater detail, focusing on both human and rodent studies. In fact, while there have been many studies showing pro-depressant activity by AChEIs and this is a major concept in the field, a variety of other studies in both humans and rodents show antidepressant effects. Our study was one of the first to systematically vary dose to include very low concentrations while measuring behavioral effects, potentially explaining the apparent disparate findings in the field. The possibility of antidepressant roles for AChEIs in rodents may provide hope for new depression treatments. Importantly, MDD is a psychosocial stress-linked disorder, and in rodents, stress is a major experimental manipulation for studying depression mechanisms, so an important future direction will be to determine the extent to which these depression-related effects are stress-sensitive. In sum, gaining a greater understanding of the potentially therapeutic mood-related effects of low dose AChEIs, both in rodent models and in human subjects, should be a prioritized topic in ongoing translational research.

Rhythms, Reward, and Blues: Consequences of Circadian Photoperiod on Affective and Reward Circuit Function

Circadian disruptions, along with altered affective and reward states, are commonly associated with psychiatric disorders. In addition to genetics, the enduring influence of environmental factors in programming neural networks is of increased interest in assessing the underpinnings of mental health. The duration of daylight or photoperiod is known to impact both the serotonin and dopamine systems, which are implicated in mood and reward-based disorders. This review first examines the effects of circadian disruption and photoperiod in the serotonin system in both human and preclinical studies. We next highlight how brain regions crucial for the serotoninergic system (i.e., dorsal raphe nucleus; DRN), and dopaminergic (i.e., nucleus accumbens; NAc and ventral tegmental area; VTA) system are intertwined in overlapping circuitry, and play influential roles in the pathology of mood and reward-based disorders. We then focus on human and animal studies that demonstrate the impact of circadian factors on the dopaminergic system. Lastly, we discuss how environmental factors such as circadian photoperiod can impact the neural circuits that are responsible for regulating affective and reward states, offering novel insights into the biological mechanisms underlying the pathophysiology, systems, and therapeutic treatments necessary for mood and reward-based disorders.

Antidepressant-like effect of low dose of scopolamine in the H/Rouen genetic mouse model of depression

Rodent models of depression are useful for the investigation of cellular and neuronal mechanisms of antidepressant drugs and for the discovery of potential new targets. In this study, we examined the antidepressant-like effect of scopolamine, a non-selective muscarinic antagonist, in a genetic mouse model of depression obtained through a selective breeding strategy and called H/Rouen. In this model, we observed that scopolamine was active both in males and females at a lower dose (0.03 mg/kg) in the tail suspension test, 30 min following its administration, than observed in CD-1 mice. In addition, we showed this antidepressant-like effect was partly inhibited by an injection of 10 mg/kg of the AMPA receptor antagonist NBQX in both males and females, suggesting the antidepressant-like effect of scopolamine was mainly driven by AMPA receptors in the H/Rouen mouse line. Altogether, our results showed the high sensitivity of the H/Rouen mouse model of depression to study the antidepressant-like effects of pharmacological compounds.

Converging evidence that short-active photoperiod increases acetylcholine signaling in the hippocampus

Seasonal variations in environmental light influence switches between moods in seasonal affective disorder (SAD) and bipolar disorder (BD), with depression arising during short active (SA) winter periods. Light-induced changes in behavior are also seen in healthy animals and are intensified in mice with reduced dopamine transporter expression. Specifically, decreasing the nocturnal active period (SA) of mice increases punishment perseveration and forced swim test (FST) immobility. Elevating acetylcholine with the acetylcholinesterase inhibitor physostigmine induces depression symptoms in people and increases FST immobility in mice. We used SA photoperiods and physostigmine to elevate acetylcholine prior to testing in a probabilistic learning task and the FST, including reversing subsequent deficits with nicotinic and scopolamine antagonists and targeted hippocampal adeno-associated viral administration. We confirmed that physostigmine also increases punishment sensitivity in a probabilistic learning paradigm. In addition, muscarinic and nicotinic receptor blockade attenuated both physostigmine-induced and SA-induced phenotypes. Finally, viral-mediated hippocampal expression of human AChE used to lower ACh levels blocked SA-induced elevation of FST immobility. These results indicate that increased hippocampal acetylcholine neurotransmission is necessary for the expression of SA exposure-induced behaviors. Furthermore, these studies support the potential for cholinergic treatments in depression. Taken together, these results provide evidence for hippocampal cholinergic mechanisms in contributing to seasonally depressed affective states induced by short day lengths.

The cholinesterase inhibitor donepezil has antidepressant-like properties in the mouse forced swim test

Finding new antidepressant agents is of high clinical priority given that many cases of major depressive disorder (MDD) do not respond to conventional monoaminergic antidepressants such as the selective serotonin reuptake inhibitors (SSRIs), tricyclic antidepressants, and monoamine oxidase inhibitors. Recent findings of effective fast-acting antidepressants indicate that there are biological substrates to be taken advantage of for fast relief of depression and that we may find further treatments in this category. In this vein, the cholinergic system may be a relatively overlooked target for antidepressant medications, given its major role in motivation and attention. Furthermore, the classically engaged monoaminergic neurotransmitter systems in depression treatment-serotonin, norepinephrine, and dopamine-interact directly at times with cholinergic signaling. Here we investigate in greater detail how the cholinergic system may impact depression-related behavior, by administering widely ranging doses of the cholinesterase inhibitor drug, donepezil, to C57BL/6J mice in the forced swim test. First, we confirm prior findings that this drug, which is thought to boost synaptic acetylcholine, promotes depression-like behavior at a high dose (2.0 mg/kg, i.p.). But we also find paradoxically that it has an antidepressant-like effect at lower doses (0.02 and 0.2 mg/kg). Further this antidepressant-like effect is not due to generalized hyperactivity, since we did not observe increased locomotor activity in the open field test. These data support a novel antidepressant-like role for donepezil at lower doses as part of an overall u-shaped dose-response curve. This raises the possibility that donepezil could have antidepressant properties in humans suffering from MDD.

Examining the role of muscarinic M5 receptors in VTA cholinergic modulation of depressive-like and anxiety-related behaviors in rats

Acetylcholine is implicated in mood disorders including depression and anxiety. Increased cholinergic tone in humans and rodents produces pro-depressive and anxiogenic-like effects. Cholinergic receptors in the ventral tegmental area (VTA) are known to mediate these responses in male rats, as measured by the sucrose preference test (SPT), elevated plus maze (EPM), and the forced swim test (FST). However, these effects have not been examined in females, and the VTA muscarinic receptor subtype(s) mediating the pro-depressive and anxiogenic-like behavioral effects of increased cholinergic tone are unknown. We first examined the behavioral effects of increased VTA cholinergic tone in male and female rats, and then determined whether VTA muscarinic M5 receptors were mediating these effects. VTA infusion of the acetylcholinesterase inhibitor physostigmine (0.5 μg, 1 μg and 2 μg/side) in males and females produced anhedonic-like, anxiogenic, pro-depressive-like responses on the SPT, EPM, and FST. In females, VTA administration of the muscarinic M5 selective negative allosteric modulator VU6000181 (0.68 ng, 2.3 ng, 6.8 ng/side for a 3 μM, 10 μM, 30 μM/side infusion) did not alter SPT, EPM nor FST behavior. However, in males intra-VTA infusion of VU6000181 alone reduced time spent immobile on the FST. Furthermore, co-infusion of VU6000181 with physostigmine, in male and female rats, attenuated the pro-depressive and anxiogenic-like behavioral responses induced by VTA physostigmine alone, in the SPT, EPM, and FST. Together, these data reveal a critical role of VTA M5 receptors in mediating the anhedonic, anxiogenic, and depressive-like behavioral effects of increased cholinergic tone in the VTA.

Involvement of muscarinic receptor mechanisms in antidepressant drug action

Conventional antidepressants typically require weeks of daily dosing to achieve full antidepressant response in antidepressant responders. A newly evolving group of compounds can engender more rapid response times in depressed patients. These drugs include the newly approved antidepressant (S)-ketamine (esketamine, Spravato). A seminal study by Furey and Drevets in 2006 showed antidepressant response in patients after only a few doses with the antimuscarinic drug scopolamine. Several clinical reports have generally confirmed scopolamine as a rapid-acting antidepressant. The data with scopolamine are consistent with the adrenergic/cholinergic hypothesis of mania/depression derived from clinical reports originating in the 1970s from Janowsky and colleagues. Additional support for a role for muscarinic receptors in mood disorders comes from the greater efficacy of conventional antidepressants that have relatively high levels of muscarinic receptor blocking actions (e.g., the tricyclic antidepressant amitriptyline vs the selective serotonin reuptake inhibitor fluoxetine). There appears to be appreciable overlap in the mechanisms of action of scopolamine and other rapid-acting antidepressants (ketamine) or putative rapid-acting agents (mGlu2/3 receptor antagonists) although gaps exist in the experimental literature. Current hypotheses regarding the mechanisms underlying the rapid antidepressant response to scopolamine posit an M₁ receptor subtype-initiated cascade of biological events that involve the amplification of AMPA receptors. Consequent impact on brain-derived neurotrophic factor and mTor signaling pathways result in the induction of dendritic spines that enable augmented functional connectivity in brain areas regulating mood. Two major goals for research in this area focus on finding ways in which scopolamine might best be utilized for depressed patients and the discovery of alternative compounds that improve upon the efficacy and safety of scopolamine.

The molecular and cellular mechanisms of depression: a focus on reward circuitry

Depression is a complex disorder that takes an enormous toll on individual health. As affected individuals display a wide variation in their clinical symptoms, the precise neural mechanisms underlying the development of depression remain elusive. Although it is impossible to phenocopy every symptom of human depression in rodents, the preclinical field has had great success in modeling some of the core affective and neurovegetative depressive symptoms, including social withdrawal, anhedonia, and weight loss. Adaptations in select cell populations may underlie these individual depressive symptoms and new tools have expanded our ability to monitor and manipulate specific cell types. This review outlines some of the most recent preclinical discoveries on the molecular and neurophysiological mechanisms in reward circuitry that underlie the expression of behavioral constructs relevant to depressive symptoms.

A Mini-Review of the Role of Glutamate Transporter in Drug Addiction

Goals: The development of new treatment for drug abuse requires identification of targetable molecular mechanisms. The pathology of glutamate neurotransmission system in the brain reward circuit is related to the relapse of multiple drugs. Glutamate transporter regulates glutamate signaling by removing excess glutamate from the synapse. And the mechanisms between glutamate transporter and drug addiction are still unclear.

Methods: A systematic review of the literature searched in Pubmed and reporting drug addiction in relation to glutamate transporter. Studies were screened by title, abstract, and full text.

Results: This review is to highlight the effects of drug addiction on glutamate transporter and glutamate uptake, and targeting glutamate transporter as an addictive drug addiction treatment. We focus on the roles of glutamate transporter in different brain regions in drug addiction. More importantly, we suggest the functional roles of glutamate transporter may prove beneficial in the treatment of drug addiction.

Conclusion: Overall, understanding how glutamate transporter impacts central nervous system may provide a new insight for treatment of drug addiction.

Muscarinic receptors in energy homeostasis: Physiology and pharmacology

Despite increased awareness and intensified biomedical research efforts, the prevalence of obesity continues to rise worldwide. This is alarming, because obesity accelerates the progression of several chronic disorders, including type 2 diabetes, cancer and cardiovascular disease. Individuals who experience significant weight loss must combat powerful counter-regulatory energy homeostatic processes, and, typically, most individuals regain the lost weight. Therefore, decoding the neural mechanisms underlying the regulation of energy homeostasis is necessary for developing breakthroughs in obesity management. It has been known for decades that cholinergic neurotransmission both directly and indirectly modulates energy homeostasis and metabolic health. Despite this insight, the molecular details underlying the modulation remain ill-defined, and the potential for targeting cholinergic muscarinic receptors for treating metabolic disease is largely uncharted. In this MiniReview, we scrutinize the literature that has formed our knowledge of muscarinic acetylcholine receptors (mAChRs) in energy homeostasis. The role of mAChRs in canonical appetite-regulating circuits will be discussed as will the more indirect regulation of energy homoeostasis via neurocircuits linked to motivated behaviours and emotional states. Finally, we discuss the therapeutic prospects of targeting mAChRs for the treatment of obesity and type 2 diabetes.

Cholinergic Receptor Blockade in the VTA Attenuates Cue-Induced Cocaine-Seeking and Reverses the Anxiogenic Effects of Forced Abstinence

Drug relapse after periods of abstinence is a common feature of substance abuse. Moreover, anxiety and other mood disorders are often co-morbid with substance abuse. Cholinergic receptors in the ventral tegmental area (VTA) are known to mediate drug-seeking and anxiety-related behavior in rodent models. However, it is unclear if overlapping VTA cholinergic mechanisms mediate drug relapse and anxiety-related behaviors associated with drug abstinence. We examined the effects of VTA cholinergic receptor blockade on cue-induced cocaine seeking and anxiety during cocaine abstinence. Male Sprague-Dawley rats were trained to self-administer intravenous cocaine (~0.5 mg/kg/infusion, FR1 schedule) for 10 days, followed by 14 days of forced abstinence. VTA infusion of the non-selective nicotinic acetylcholine receptor antagonist mecamylamine (0, 10, and 30 μg/side) or the non-selective muscarinic receptor antagonist scopolamine (0, 2.4 and 24 μg /side) significantly decreased cue-induced cocaine seeking. In cocaine naïve rats, VTA mecamylamine or scopolamine also led to dose-dependent increases in open arm time in the elevated plus maze (EPM). In contrast, rats that received I.V. cocaine, compared to received I.V. saline rats, displayed an anxiogenic response on day 14 of abstinence as reflected by decreased open arm time in the EPM. Furthermore, low doses of VTA mecamylamine (10 μg /side) or scopolamine (2.4 μg /side), that did not alter EPM behavior in cocaine naive rats, were sufficient to reverse the anxiogenic effects of cocaine abstinence. Together, these data point to an overlapping role of VTA cholinergic mechanisms to regulate relapse and mood disorder-related responses during cocaine abstinence.

Cholinergic regulation of mood: from basic and clinical studies to emerging therapeutics

Mood disorders are highly prevalent and are the leading cause of disability worldwide. The neurobiological mechanisms underlying depression remain poorly understood, although theories regarding dysfunction within various neurotransmitter systems have been postulated. Over 50 years ago, clinical studies suggested that increases in central acetylcholine could lead to depressed mood. Evidence has continued to accumulate suggesting that the cholinergic system has a important role in mood regulation. In particular, the finding that the antimuscarinic agent, scopolamine, exerts fast-onset and sustained antidepressant effects in depressed humans has led to a renewal of interest in the cholinergic system as an important player in the neurochemistry of major depression and bipolar disorder. Here, we synthesize current knowledge regarding the modulation of mood by the central cholinergic system, drawing upon studies from human postmortem brain, neuroimaging, and drug challenge investigations, as well as animal model studies. First, we describe an illustrative series of early discoveries which suggest a role for acetylcholine in the pathophysiology of mood disorders. Then, we discuss more recent studies conducted in humans and/or animals which have identified roles for both acetylcholinergic muscarinic and nicotinic receptors in different mood states, and as targets for novel therapies.

Enhanced cholinergic-tone during the stress induce a depressive-like state in mice

Major depressive disorder has a heterogeneous etiology, since it arises from the interaction of multiple factors and different pathophysiological mechanisms are involved in the symptomatology. This study aimed to investigate the role of the cholinergic system in the susceptibility to stress and, consequently, in the depression-like behavior. C57BL/6 mice were treated with Physostigmine (PHYS), an acetylcholinesterase (AChE) inhibitor, and were submitted to the social defeat stress. For the behavioral evaluation of the locomotor activity, anxiety-like and depression-like behaviors the open field, elevated plus maze, sucrose preference, social interaction and forced swim were used. Hippocampus and prefrontal cortex samples were collected for evaluation of AChE activity, as well as blood samples for analysis of serum cortisol levels. Our results showed that 15 min after the injection of PHYS there was a significant inhibition of AChE activity in the hippocampus and in the prefrontal cortex. On the other hand, in the end of the experimental design, day 12, there was no difference in AChE activity levels. Inhibition of AChE and exposure to the stress led to an increase in cortisol levels. Animals that received PHYS and were exposed to stress showed less social interaction and greater learned helplessness, anhedonia and anxious-like behavior. Taken together, our findings suggest that increasing the cholinergic tone shortly before stress induction impacts on the ability to cope with upcoming stressful situations, leading to a depressive-like state.

Blockade of muscarinic acetylcholine receptors in the ventral tegmental area blocks the acquisition of reward-related learning

In the present study we investigated whether stimulation of muscarinic acetylcholine (mACh) receptors in the ventral tegmental area (VTA) plays a role in the acquisition of food-based conditioned approach learning. Rats were exposed to 3 (in Experiment 1) or 7 (in Experiment 2) conditioning sessions in which 30, randomly presented light (CS) presentations were paired with delivery of food pellets (US), followed by one session with no light or food and finally one CS-only test session with only light stimulus presentations. Bilateral microinjections of scopolamine (a mACh receptor antagonist) were made either prior to each conditioning session (Experiment 1; to test effects on acquisition) or prior to the CS-only test (Experiment 2; to test effects on performance of the learned response). Scopolamine produced a dose-related significant reduction in the acquisition of conditioned approach but had no effect on its performance. These results suggest that mACh receptor stimulation in the VTA plays a necessary role in the acquisition of reward-related learning.

Memantine improves memory impairment and depressive-like behavior induced by amphetamine withdrawal in rats

Amphetamine (AMPH) induces deficits in cognition, and depressive-like behavior following withdrawal. The aim of the present study was to investigate whether pre-treatment with memantine (5mg/kg, i.p.), a noncompetitive N-methyl-d-aspartate (NMDA) receptor antagonist, attenuates memory impairment induced by withdrawal from a 1 day binge regimen of AMPH (2mg/kg, four times every 2h, i.p.), in the novel object recognition test in rats. Herein, the influence of scopolamine (0.1mg/kg), an antagonist of the muscarinic cholinergic receptors, and the impact of MK-801 (0.1mg/kg), an antagonist of the NMDA receptors, on the memantine effect, were ascertained. Furthermore, the impact of memantine (5; 10; 20mg/kg, i.p.) was measured on depression-like effects of abstinence, 14 days after the last AMPH treatment (2mg/kg×1×14 days), in the forced swim test. In this test, the efficacy of memantine was compared to that of tricyclic antidepressant imipramine (10; 20; 30mg/kg, i.p.). Our study indicated that withdrawal from a binge regimen of AMPH impaired recognition memory. This effect was attenuated by administration of memantine at both 72h and 7 days of withdrawal. Moreover, prior administration of scopolamine, but not MK-801, decreased the memantine-induced recognition memory improvement. In addition, memantine reversed the AMPH-induced depressive-like behavior in the forced swim test in rats. The antidepressant-like effects of memantine were stronger than those of imipramine. Our study indicates that memantine constitutes a useful approach towards preventing cognitive deficits induced by withdrawal from an AMPH binge regimen and by depressive-like behavior during AMPH abstinence.

Antidepressant-like effect of low dose ketamine and scopolamine co-treatment in mice

Current medications for depression typically require weeks of treatment before significant clinical improvement is observed, and are only effective in a relatively small subset of patients. Recent human clinical studies have demonstrated that ketamine, an NMDA receptor antagonist, and scopolamine, a muscarinic acetylcholine receptor antagonist, produce rapid antidepressant responses within hours of administration, and are effective in treatment-resistant patients. We hypothesize that efficacy and tolerability may be improved by combining lower doses of both drugs in the treatment of depression. We therefore conducted a preclinical study in mice to assess whether co-treatment of low doses of scopolamine and ketamine that alone are ineffective has antidepressant-like effects in the forced swim test (FST), an assay with predictive validity for antidepressant drugs. Whereas single administration of ketamine (3mg/kg intraperitoneal [i.p.]) or scopolamine (0.1mg/kg i.p.) did not reduce immobility time in the FST, co-administration of both drugs at these doses significantly reduced immobility time by 45% compared to vehicle treated controls. These results suggest that the combination of subeffective doses of ketamine and scopolamine may prove efficacious for the treatment of depression and should be evaluated in human clinical trials.

Ventral tegmental area muscarinic receptors modulate depression and anxiety-related behaviors in rats

Cholinergic and dopaminergic mechanisms within the mesolimbic dopamine system are suggested to play a role in the manifestation of depression and anxiety-related disorders. However, despite the fact that cholinergic mechanisms in the ventral tegmental area (VTA) highly regulate dopamine activity, the role of VTA cholinergic mechanisms in depression-related behaviors is relatively unknown. Here we sought to determine whether enhancing cholinergic tone in the VTA would alter depression and anxiety-related behavior in the forced swim test (FST), elevated plus maze (EPM) and sucrose preference test (SPT). Adult Sprague Dawley male rats received VTA infusion of the acetylcholinesterase inhibitor, physostigmine (0, 1, 2μg/side), immediately prior to the FST, EPM, or SPT. Physostigmine administration increased immobility time in the FST, decreased time spent on open arms in the EPM, and decreased sucrose preference. We also examined whether activation of VTA muscarinic receptors was sufficient to alter behavior in the FST and EPM. Similar to physostigmine, VTA infusion of the muscarinic receptor agonist, pilocarpine (0, 3, 30μg/side), increased immobility time in the FST and decreased time spent on open arms in the EPM. These data suggest that enhanced VTA cholinergic tone promotes pro-depressive and anxiogenic-like effects and demonstrate that specific activation of VTA muscarinic receptors is also sufficient to induce pro-depressive and anxiogenic responses. Together, these findings reveal a novel role of VTA cholinergic, and specifically muscarinic receptor, mechanisms in mediating responses to stress and anxiety.

Muscarinic, but not nicotinic, acetylcholine receptor blockade in the ventral tegmental area attenuates cue-induced sucrose-seeking

The mesolimbic dopamine (DA) system is known to play a role in cue-mediated reward-seeking for natural rewards and drugs of abuse. Specifically, cholinergic and glutamatergic receptors in the ventral tegmental area (VTA) have been shown to regulate cue-induced drug-seeking. However, the potential role of these VTA receptors in regulating cue-induced reward seeking for natural rewards is unknown. Here, we examined whether blockade of VTA acetylcholine receptors (AChRs) and N-methyl-d-aspartate receptors (NMDARs) would alter cue-induced sucrose seeking in male Sprague-Dawley rats. Subjects underwent 10 days of sucrose self-administration training (fixed ratio 1 schedule) followed by 7 days of forced abstinence. On withdrawal day 7, rats received bilateral VTA infusion of vehicle, the muscarinic AChR antagonist scopolamine (2.4 or 24 μg/side), the nicotinic AChR antagonist mecamylamine (3 or 30 μg/side), or the NMDAR antagonist AP-5 (0.1 or 1 μg/side) immediately prior to examination of cue-induced sucrose-seeking. Scopolamine infusion led to robust attenuation, but did not completely block, sucrose-seeking behavior. In contrast, VTA administration of mecamylamine or AP-5 did not alter cue-induced sucrose-seeking. Together, the data suggest that VTA muscarinic AChRs, but not nicotinic AChRs nor NMDARs, facilitate the ability of food-associated cues to drive seeking behavior for a food reward.