A 'crash' course on psychostimulant withdrawal as a model of depression

Animal models of depression in drug discovery: a historical perspective

Over the course of the last 50 years many models of major depressive disorder have been developed on the basis of theoretical aspects of this disorder. These models and procedures have been crucial in the discovery and development of clinically-effective drugs. Notwithstanding, there is presently great concern about the discrepancy between positive outcomes of new candidate drugs in animal models and apparent lack of efficacy in humans i.e., the predictive validity of animal models. Some reasons for this concern lie in the over-reliance in the face value of behavioural models, design of clinical trials, placebo responses, genetic variations in response to drugs, species differences in bioavailability and toxicology, and not least, disinterest of pharmaceutical sponsors to continue developing certain drugs. Present model development is focusing on endophenotypic aspects of behaviours rather than trying to model whole syndromes. This essay traces the origins and theoretical bases of our animal models of depression or depressed-like behaviours in humans and indicates how they have evolved from behavioural assays used to measure the potency and efficacy of potential candidate drugs to tools by which endophenotypes of depression may be identified and verified pharmacologically. A cautionary note is included though to indicate that the true predictive validity of our models will not be fully assessed until we can determine the attrition rate of molecules discovered from new drug targets translating into clinically-effective drugs.

Behavioral effects of short-term administration of lithium and valproic acid in rats

Lithium and valproic acid are mood-stabilizing agents that are often used to manage the episodes of mania and depression that characterize bipolar disorder. These agents develop clinical efficacy with chronic treatment, but the neurobiological actions that contribute to their therapeutic effects remain unclear. The present work was designed to study and compare various behavioral effects of short-term administration of lithium chloride (LiCl) and valproic acid (VPA) in rats. Specifically, we examined the effects of acute and sub-acute injections of these agents on locomotor activity, behavior in the forced swim test (FST), and intracranial self-stimulation (ICSS) thresholds. Locomotor activity studies were used to identify the range of doses with gross behavioral effects in rats. At doses below those that suppressed activity (total distance traveled, in cm) in 1-h test sessions, LiCl had prodepressant-like effects: it increased immobility in the FST, an effect opposite to that typically seen with standard antidepressants, and it increased ICSS thresholds, an effect similar to that typically seen during withdrawal from drugs of abuse. In contrast, VPA had no effects in the FST or on ICSS thresholds. This work identifies potentially important characteristics that distinguish the drugs at doses below those that produce non-specific behavioral effects, and thus serves as a basis for designing and interpreting studies of long-term treatment.

Severe deficit in brain reward function associated with fentanyl withdrawal in rats

BACKGROUND

During the last decade, there has been a strong increase in the use of the mu-opioid receptor agonist fentanyl. The aim of these studies was to investigate the effects of fentanyl withdrawal on brain reward function and somatic withdrawal signs.

METHODS

Fentanyl and saline were chronically administered via minipumps. An intracranial self-stimulation procedure was used to provide a measure of brain reward function. Somatic signs were recorded from a checklist of opioid abstinence signs.

RESULTS

The opioid receptor antagonist naloxone induced a dose-dependent elevation in brain reward thresholds and somatic withdrawal signs in fentanyl-treated rats. Discontinuation of fentanyl administration resulted in a time-dependent elevation of brain reward thresholds and somatic withdrawal signs.

CONCLUSIONS

These findings indicate that fentanyl withdrawal is associated with affective and somatic withdrawal signs. The severity of the deficit in brain reward function in this animal model suggests that affective fentanyl withdrawal symptoms may be a strong deterrent to abstinence.

LTP in the lateral amygdala during cocaine withdrawal

The amygdala plays key roles in several aspects of addiction to drugs of abuse. This brain structure has been implicated in behaviours that reflect drug reward, drug seeking, and the aversive effects of drug withdrawal. Using a model that involves repeated cocaine injections to approximate 'binge' intoxication, we show in rats that during cocaine withdrawal, the impact of rewarding brain stimulation is attenuated, as quantified by alterations in intracranial self-stimulation (ICSS) behaviour. These behavioural signs of withdrawal are accompanied by enhancements of glutamatergic synaptic transmission within the lateral amygdala (LA) that occlude electrically induced long-term potentiation (LTP) in tissue slices. Synaptic enhancements during periods of cocaine withdrawal are mechanistically similar to LTP induced with electrical stimulation in control slices, as both forms of synaptic plasticity involve an increase in glutamate release. These results suggest that mechanisms of LTP within the amygdala are recruited during withdrawal from repeated exposure to cocaine. As such, they raise the possibility that the development and maintenance of addictive behaviours may involve, at least in part, mechanisms of synaptic plasticity within specific amygdala circuits.

The ascent of mouse: advances in modelling human depression and anxiety

Psychiatry has proven to be among the least penetrable clinical disciplines for the development of satisfactory in vivo model systems for evaluating novel treatment approaches. However, mood and anxiety disorders remain poorly understood and inadequately treated. With the explosion in the use of genetically modified mice, enormous research efforts have been focused on developing mouse models of psychiatric disorders. The success of this approach is largely contingent on the usefulness of available behavioural models of depression- and anxiety-related behaviours in mice. Here, we assess the current status of research into developing appropriate tests for assessing such behaviours.

Recent advances in animal models of chronic antidepressant effects: the novelty-induced hypophagia test

Animal models exhibiting sensitivity to chronic, but not acute, antidepressant treatment are greatly needed for studying the neural mechanisms of the antidepressant response. Although several models of acute antidepressant effects provide excellent tools for antidepressant discovery, they do not permit investigation into their therapeutic effects, which require several weeks of treatment to emerge. The inhibition of feeding produced by novelty, termed 'hyponeophagia', provides an anxiety-related measure that is sensitive to the effects of chronic, but not acute or subchronic, antidepressant treatment. This review evaluates the value of hyponeophagia-based tests as tools for investigating the neurobiology of the therapeutic response to antidepressant treatment. Criteria for the development and validation of animal models used to study neurobiological mechanisms of the antidepressant response are presented. Methodological considerations affecting the reliability, specificity, and ease of use of hyponeophagia-based models are also discussed. Lastly, we present a newly revised hyponeophagia paradigm, called the novelty-induced hypophagia (NIH) test, which attempts to maximize the predictive validity and practicality of the test. The NIH paradigm provides a promising new model for investigations into the neurobiology underlying the antidepressant response.

Paroxetine combined with a 5-HT(1A) receptor antagonist reversed reward deficits observed during amphetamine withdrawal in rats

RATIONALE

"Diminished interest or pleasure" in rewarding stimuli is an affective symptom of amphetamine withdrawal and a core symptom of depression. An operational measure of this symptom is elevation of brain stimulation reward thresholds during drug withdrawal. Data indicated that increasing serotonin neurotransmission by co-administration of the selective serotonin reuptake inhibitor (SSRI) fluoxetine and the serotonin-1A receptor antagonist p-MPPI reversed reward deficits observed during drug withdrawal (Harrison et al. 2001).

OBJECTIVES

We tested the hypothesis that increased serotonergic and noradrenergic neurotransmission, using the SSRI paroxetine which also inhibits noradrenaline reuptake, would alleviate affective aspects of amphetamine withdrawal.

METHODS

A discrete-trial, current-threshold, self-stimulation procedure was used to assess brain reward function. The effects of paroxetine and p-MPPI alone and in combination were assessed in non-drug-withdrawing animals. We assessed also the effects of paroxetine and p-MPPI alone and in combination on reward deficits associated with amphetamine withdrawal.

RESULTS

Paroxetine or p-MPPI alone had no effect on thresholds, while the co-administration of p-MPPI (3 mg/kg) and paroxetine (1.25 mg/kg) elevated thresholds in non-withdrawing rats. Amphetamine withdrawal resulted in threshold elevations. The co-administration of p-MPPI and paroxetine reduced the duration of amphetamine-withdrawal-induced reward deficits.

CONCLUSIONS

Increased serotonergic and noradrenergic neurotransmission decreased reward function in non-withdrawing rats, while the same treatment reversed reward deficits associated with amphetamine withdrawal. Considering that paroxetine acts on both the serotonin and noradrenaline transporter, these results indicate that the affective symptoms of amphetamine withdrawal, similar to non-drug-induced depressions, may be, in part, mediated through reduced serotonergic and noradrenergic neurotransmission.

The role of corticotropin-releasing factor-like peptides in cannabis, nicotine, and alcohol dependence

The corticotropin-releasing factor (CRF)-like peptides, which include the mammalian peptides CRF, urocortin 1, urocortin 2, and urocortin 3, play an important role in orchestrating behavioral and physiological responses that may increase an organism's chance of survival when confronted with internal or external stressors. There is, however, evidence that a chronic overactivity of brain CRF systems under basal conditions may play a role in the etiology and maintenance of psychiatric disorders such as depression and anxiety disorders. In addition, there is evidence of a role for CRF-like peptides in acute and protracted drug abstinence syndromes and relapse to drug-taking behavior. This review focuses on the role of CRF-like peptides in the negative affective state associated with acute and protracted withdrawal from three widely abused drugs, cannabis, nicotine, and alcohol. In addition, we discuss the high comorbidity between stress-associated psychiatric disorders and drug dependence. A better understanding of the brain stress systems that may underlie psychiatric disorders, acute and protracted drug withdrawal, and relapse to drug-taking behavior may help in the development of new and improved pharmacotherapies for these widespread psychiatric disorders.

A role for galanin in antidepressant actions with a focus on the dorsal raphe nucleus

Selective serotonin reuptake inhibitors, such as fluoxetine (FLX), are the most commonly used drugs in the treatment of major depression. However, there is a limited understanding of their molecular mechanism of action. Although the acute effect of selective serotonin reuptake inhibitors in elevating synaptic serotonin concentrations is well known, the clinical amelioration of depressive symptoms requires 14-21 days of treatment, suggesting that numerous other rearrangements of function in the CNS must take place. In the present study, we demonstrated that 14 days of FLX treatment up-regulated galanin mRNA levels by 100% and GalR2-binding sites by 50%, in the rat dorsal raphe nucleus, where galanin coexists with serotonin. Furthermore, a galanin receptor antagonist, M40, attenuated the antidepressant-like effect of FLX in the forced swim test, a rodent preclinical screen commonly used to evaluate antidepressant-like efficacy. Direct activation of galanin receptors by a galanin receptor agonist, galnon, was found to produce an antidepressant-like effect in the same task. Two other antidepressant treatments also affected the galaninergic system in the monoaminergic nuclei: Electroconvulsive shock elevated galanin mRNA levels in dorsal raphe nucleus, whereas sleep deprivation increased galanin mRNA levels in the locus coeruleus, further underlining the connection between activation of the galaninergic system and antidepressant action of various clinically proven treatments.

Psychostimulant withdrawal as an inducing condition in animal models of depression

A large body of evidence indicates that the withdrawal from high doses of psychostimulant drugs in humans induces a transient syndrome, with symptoms that appear isomorphic to those of major depressive disorder. Pharmacological treatment strategies for psychostimulant withdrawal in humans have focused mainly on compounds with antidepressant properties. Animal models of psychostimulant withdrawal have been shown to demonstrate a wide range of deficits, including changes in homeostatic, affective and cognitive behaviors, as well as numerous physiological changes. Many of these behavioral and physiological sequelae parallel specific symptoms of major depressive disorder, and have been reversed by treatment with antidepressant drugs. These combined findings provide strong support for the use of psychostimulant withdrawal as an inducing condition in animal models of depression. In the current review we propound that the psychostimulant withdrawal model displays high levels of predictive and construct validity. Recent progress and limitations in the development of this model, as well as future directions for research, are evaluated and discussed.

Understanding the neurobiological consequences of early exposure to psychotropic drugs: linking behavior with molecules

Children receive significant exposure to psychotropic drugs. Some psychiatric disorders are diagnosed and treated in children as young as 2 years old, resulting in exposure to prescription stimulants, antidepressants, and mood stabilizers during brain development. Difficulties in diagnoses at such young ages increase the likelihood that children who are not affected by these disorders receive drug exposure inadvertently. Additionally, the increased availability of caffeine-containing beverages in schools has facilitated exposure to this stimulant in children. However, the consequences of exposure to psychotropic drugs during brain development are not understood. When we exposed rats to the prescription stimulant methylphenidate during early adolescence, we discovered long-lasting behavioral and molecular alterations that were consistent with dramatic changes in the function of brain reward systems. In future work, it will be important to determine if other classes of psychotropic drugs cause these same effects, and whether these effects will also occur if drug exposure begins during other periods of development. Moreover, it will be critical to use more powerful behavioral methods that are sensitive to high-level aspects of motivation and cognitive function, and to establish causal links between developmental exposure-related alterations in these complex behaviors and specific alterations in the molecular biology of key brain regions. This approach may identify classes of psychotropic drugs that have high or low propensities to cause behavioral and molecular adaptations that endure into adulthood. It may also identify periods of development during which administration of these agents is particularly safe or risky.

Neurobiological mechanisms in addictive and psychiatric disorders

The studies reviewed indicate that brain stress system play an important role in the acquisition and maintenance of drugs of abuse that target the brain's reward centers. In doing so, they may destabilize these areas, making the perception of pleasure more elusive and difficult to attain. Withdrawal from drugs of abuse leads to the activation of brain CRF systems that may produce the anxiogenic response associated with drug withdrawal. More research, however, is needed to investigate the role of brain stress systems and neuropeptides in other drug withdrawal symptoms such as anhedonia. A better understanding of the brain systems underlying drug withdrawal may help in the development of improved pharmacotherapies that can alleviate drug withdrawal symptoms. The second part of the article indicated that there is a very high comorbidity between depression and drug dependence. The reviewed studies suggest that depressed patients initiate drug-taking behavior to self-medicate the symptoms associated with their psychiatric disorder. Chronic use of drugs of abuse, however, may exacerbate the symptoms of pre-existing mental disorders and subsequently increase drug-taking behavior. Conversely, professional treatment of pre-existing psychiatric disorders may decrease the use of illicit substances.

Effects of chronic fluoxetine in animal models of anxiety and depression

The onset of the therapeutic response to antidepressant treatment exhibits a characteristic delay. Animal models sensitive to chronic, but not acute, antidepressant treatment are greatly needed for studying antidepressant mechanisms. We initially assessed four inbred mouse strains for their behavioral response to chronic treatment with the selective-serotonin reuptake inhibitor fluoxetine (0, 5, 10 mg/kg/day in drinking water), which is used for the treatment of mood and anxiety disorders. Only the highly anxious BALB/c strain exhibited sensitivity to fluoxetine in the forced swim test. Additionally, fluoxetine reduced locomotion in C57BL/6 and 129SvEv, but not BALB/c and DBA/2, strains. We then evaluated the effects of subchronic (approximately 4 days) and chronic (approximately 24 days) fluoxetine treatment (0, 10, 18, 25 mg/kg/day) on measures of anxiety and depression in BALB/c mice. Anxiety measures were obtained using the open field and novelty-induced hypophagia tests. Antidepressant effects were evaluated using the forced swim test. We found 18 mg/kg/day of chronic fluoxetine to be active in all three paradigms; subchronic treatment had no effect. Anxiety-related measures were reduced by 18 mg/kg/day. In the forced swim test, 10 and 18 mg/kg/day increased swimming and reduced immobility. Here we present several novel effects of chronic, but not subchronic, antidepressant treatment.

Effects of kappa-opioid receptor ligands on intracranial self-stimulation in rats

RATIONALE

Elevations in cAMP response element binding protein (CREB) function within the mesolimbic system of rats reduce cocaine reward in place conditioning studies and increase immobility in the forced swim test. Each of these behavioral adaptations can be interpreted as a depressive-like effect (i.e., anhedonia, despair) that may reflect reduced activity of brain reward systems. Furthermore, each effect appears due to increases in CREB-mediated expression of dynorphin, since each is attenuated by intracranial injections of the kappa-opioid receptor antagonist norBNI.

OBJECTIVES

Intracranial self-stimulation (ICSS) studies were conducted in rats to determine whether administration of a kappa-agonist would have depressive-like effects on brain stimulation reward, and whether pretreatment with a kappa-antagonist would attenuate any such effects. Conditions that have depressive effects in people (e.g., drug withdrawal) increase the threshold amounts of stimulation required to sustain ICSS in rats.

METHODS

Sprague-Dawley rats with lateral hypothalamic stimulating electrodes were tested in a "curve-shift" variant of the ICSS procedure after systemic administration of the kappa-agonist U-69593 alone, the novel kappa-antagonist 5'-acetamidinoethylnaltrindole (ANTI) alone, or co-administration of both drugs.

RESULTS

U-69593 dose dependently increased ICSS thresholds, suggesting that activation of kappa-receptors reduced the rewarding impact of the brain stimulation. ANTI had no effects on its own, but it attenuated increases in ICSS thresholds caused by the agonist.

CONCLUSIONS

These data provide further evidence that stimulation of brain kappa-receptors may trigger certain depressive-like signs, and that kappa antagonists may have efficacy as antidepressants without having reward-related actions of their own.

Lack of depression-like effects of saccharin deprivation in rats: forced swim test, differential reinforcement of low rates and intracranial self-stimulation procedures

In humans and laboratory animals, drug withdrawal often is associated with depression-like behaviors. In the present study, rats had unlimited free-choice access to water and a saccharin-containing solution before being subjected to repeated episodes of saccharin deprivation. Saccharin deprivation (1) reduced immobility time in the forced swim test, (2) increased reinforcement rate in rats trained to lever-press under the differential reinforcement of a low-rate (72-sec) schedule of food reinforcement, and (3) lowered intracranial self-stimulation thresholds in a discrete-trial current titration procedure. Taken together, these findings suggest that deprivation from a nondrug reinforcer, saccharin, is not associated with depression-like behaviors. In contrast, saccharin-deprived rats demonstrated improved performance in the behavioral paradigms used here.

Evidence for alterations in α2-adrenergic receptor sensitivity in rats exposed to repeated cocaine administration

It is well established that cocaine stimulates monoamine transmission by blocking reuptake of norepinephrine (NE), dopamine and serotonin into nerve cells, yet few investigations have addressed the effects of chronic cocaine on NE function. In the present study, we examined the effects of repeated cocaine injections on neuroendocrine responses evoked by the alpha2-adrenergic receptor agonist, clonidine. Previous findings show that clonidine increases pituitary growth hormone (GH) secretion by a central mechanism involving postsynaptic alpha2-adrenergic receptors. Male rats previously fitted with indwelling jugular catheters received two daily injections of cocaine (15 mg/kg, i.p.) or saline for 7 days. At 42 h and 8 days after treatment, rats were challenged with clonidine (25 microg/kg, i.v.) or saline, and serial blood samples were withdrawn. Plasma GH and corticosterone levels were measured by radioimmunoassay. Prior cocaine exposure did not affect basal levels of either hormone. However, cocaine-pretreated rats displayed a significant reduction in clonidine-evoked GH secretion at 42 h, and this blunted response was still apparent 8 days later. Corticosterone responses produced by clonidine were similar regardless of pretreatment. The present data suggest that withdrawal from repeated cocaine injections may be accompanied by desensitization of postsynaptic alpha2-adrenoreceptors coupled to GH secretion. Since human patients with depression often exhibit blunted GH responses to clonidine, our findings provide evidence that cocaine withdrawal might produce depressive-like symptoms via dysregulation of NE mechanisms.

Compulsive use of dopamine replacement therapy in Parkinson's disease: reward systems gone awry?

Dopamine replacement therapy (DRT) is the most effective treatment for Parkinson's disease (PD); it provides substantial benefit for most patients, extends independence, and increases survival. A few patients with PD, however, take increasing quantities of medication far beyond those required to treat their motor disabilities. These patients demand rapid drug escalation and continue to request more DRT despite the emergence of increasingly severe drug-induced motor complications and harmful behavioural consequences. In this article we detail the features of compulsive DRT-seeking and intake in PD, in relation to theories of compulsive drug use.

Phospholipase Cgamma in distinct regions of the ventral tegmental area differentially modulates mood-related behaviors

Neurotrophic factor signaling pathways modulate cellular and behavioral responses to drugs of abuse. In addition, chronic exposure to morphine increases expression of phospholipase Cgamma1 (PLCgamma1) (a protein involved in neurotrophic signaling) in the ventral tegmental area (VTA), a neural substrate for many drugs of abuse. Using viral-mediated gene transfer to locally alter the activity of PLCgamma1, we show that overexpression of PLCgamma1 in rostral portions of the VTA (R-VTA) results in increased morphine place preference, whereas PLCgamma1 overexpression in the caudal VTA (C-VTA) results in avoidance of morphine-paired compartments. In addition, overexpression of PLCgamma1 in R-VTA causes increased preference for sucrose and increased anxiety-like behavior but does not affect responses to stress or nociceptive stimuli. In contrast, overexpression of PLCgamma1 in C-VTA decreases preference for sucrose and increases sensitivity to stress and nociceptive stimuli, although there was a tendency for increased anxiety-like behavior as seen for the R-VTA. These results show that levels of PLCgamma1 in the VTA regulate responsiveness to drugs of abuse, natural rewards, and aversive stimuli and point to the possibility that distinct topographical regions within the VTA mediate generally positive versus negative responses to emotional stimuli. Moreover, these data also support a role for drug-induced elevations in PLCgamma1 expression in the VTA in mediating long-term adaptations to drugs of abuse and aversive stimuli.

Withdrawal from chronic amphetamine induces depressive-like behavioral effects in rodents

BACKGROUND

Amphetamine withdrawal and major depression share many behavioral commonalities in humans. Therefore, the examination of the behavioral effects of amphetamine withdrawal in rodents may provide insights into the neurobiological mechanisms underlying both disorders and aid in the development of animal models of depression that are sensitive to antidepressant agents.

METHODS

We examined the behavioral effects of withdrawal from chronic continuous infusion of amphetamine (via minipump) in three behavioral paradigms: the intracranial self-stimulation (ICSS) procedure in rats, the modified forced swim test in rats, and the tail suspension test in mice.

RESULTS

Amphetamine withdrawal resulted in a prolonged (5 day) deficit in brain reward function as assessed by elevations in ICSS thresholds. Using a similar regimen of amphetamine administration, we examined the behavioral effects of withdrawal in a modified rat forced swim test. Animals that were treated with the highest dose of amphetamine (10 mg/kg/day) exhibited increased climbing behavior and decreased immobility 24 hours after withdrawal; by the 48-hour testing time point, this effect had dissipated. In contrast, animals that had been pretreated with 5 mg/kg/day amphetamine exhibited a pronounced increase in immobility indicative of an increase in "depressive-like" behavior, coupled with decreases in swimming and climbing. In the mouse tail suspension test, both regimens of amphetamine pretreatment induced increases in immobility scores, also indicative of "depressive-like" behavior, 24 hours following withdrawal.

CONCLUSIONS

Withdrawal from chronic amphetamine administration results in behavioral changes that may be analogous to some aspects of depression in humans, such as reward deficits (i.e., elevations in brain reward thresholds) and behaviors opposite to those seen after treatment with antidepressant drugs, such as decreased immobility in the forced swim test and the tail suspension test.