Withdrawal from chronic phencyclidine treatment induces long-lasting depression in brain reward function

Persistent Anhedonia After Intermittent Long-Access Nicotine Self-Administration in Rats

Tobacco use disorder is a chronic condition characterized by compulsive nicotine use and withdrawal symptoms after smoking cessation. Smoking is the leading preventable cause of morbidity and mortality worldwide. Smoking cessation leads to anhedonia, which is an inability to experience pleasure from previously enjoyed activities and is caused by dysregulation of the brain's reward and stress systems. It is also a key withdrawal symptom that contributes to relapse to smoking after a period of abstinence. To better understand the development of anhedonia, we investigated its onset and time course in rats that self-administered nicotine. Rats were implanted with intracranial self-stimulation (ICSS) electrodes to assess reward function and intravenous catheters for nicotine self-administration. Elevations in ICSS brain reward thresholds reflect decreased sensitivity to rewarding electrical stimuli, indicating anhedonia. The rats self-administered 0.06 mg/kg of nicotine intermittently, three days per week, for seven weeks. Brain reward thresholds were determined once a week 24 h after nicotine self-administration during weeks 1 to 3, and at 12, 24, and 48 h during weeks 4, 5, and 7. Elevations in brain reward thresholds were not observed during the first four weeks of nicotine self-administration. However, the brain reward thresholds were elevated in both weeks 5 and 7 at least 12 h after nicotine self-administration, indicating that anhedonia emerges gradually and then persists. As withdrawal severity gradually increases, smoking cessation may become more challenging. Therefore, behavioral or pharmacological interventions soon after smoking initiation are critical to prevent the development of a tobacco use disorder.

1-Phenylcyclohexan-1-amine hydrochloride (PCA HCl) alters mesolimbic dopamine system accompanied by neuroplastic changes: A neuropsychopharmacological evaluation in rodents

The recreational use of N-methyl-D-aspartate (NMDA) antagonist phencyclidine (PCP) and ketamine have grown rapidly due to their psychotomimetic properties. These compounds induce both non-fatal and fatal adverse effects and despite the enhanced regulation, they are continuously synthesized and are being sold in the illegal drug market, including 1-phenylcyclohexan-1-amine hydrochloride (PCA). Therefore, we evaluated its abuse potential through the conditioned-place preference (CPP), self-administration, and locomotor sensitization paradigms. Pretreatment with SCH 2 3390 and haloperidol was also performed during a CPP test. We used ELISA to measure dopamine (DA) levels and western blotting to determine effects on the DA-related proteins as well as on phosphorylated CREB, deltaFosB, and brain-derived neurotrophic factor (BDNF) in the ventral tegmental area (VTA) and nucleus accumbens (NAc). Finally, we examined the effects on brain wave activity using electroencephalography (EEG). PCA induced CPP in mice and was self-administered by rats, suggesting that PCA has rewarding and reinforcing properties. PCA increased locomotor of mice on the first treatment and challenge days. SCH 23390 and haloperidol blocked the CPP. PCA altered the DA, tyrosine hydroxylase, dopamine D1 and D2 receptors as well as p-CREB and deltaFosB. Also, PCA altered the delta and gamma waves in the brain, which were then normalized by SCH 2 3390 and haloperidol. The present findings indicate that PCA may induce abuse potential through the dopaminergic system and probably accompanied with alterations in brain wave activity which is similar to that of other psychotomimetic NMDA antagonists. We advocate thorough monitoring of PCP analogs as they pose potential harm to public health.

Candidate Strategies for Development of a Rapid-Acting Antidepressant Class That Does Not Result in Neuropsychiatric Adverse Effects: Prevention of Ketamine-Induced Neuropsychiatric Adverse Reactions

Non-competitive N-methyl-D-aspartate/glutamate receptor (NMDAR) antagonism has been considered to play important roles in the pathophysiology of schizophrenia. In spite of severe neuropsychiatric adverse effects, esketamine (racemic enantiomer of ketamine) has been approved for the treatment of conventional monoaminergic antidepressant-resistant depression. Furthermore, ketamine improves anhedonia, suicidal ideation and bipolar depression, for which conventional monoaminergic antidepressants are not fully effective. Therefore, ketamine has been accepted, with rigorous restrictions, in psychiatry as a new class of antidepressant. Notably, the dosage of ketamine for antidepressive action is comparable to the dose that can generate schizophrenia-like psychotic symptoms. Furthermore, the psychotropic effects of ketamine precede the antidepressant effects. The maintenance of the antidepressive efficacy of ketamine often requires repeated administration; however, repeated ketamine intake leads to abuse and is consistently associated with long-lasting memory-associated deficits. According to the dissociative anaesthetic feature of ketamine, it exerts broad acute influences on cognition/perception. To evaluate the therapeutic validation of ketamine across clinical contexts, including its advantages and disadvantages, psychiatry should systematically assess the safety and efficacy of either short- and long-term ketamine treatments, in terms of both acute and chronic outcomes. Here, we describe the clinical evidence of NMDAR antagonists, and then the temporal mechanisms of schizophrenia-like and antidepressant-like effects of the NMDAR antagonist, ketamine. The underlying pharmacological rodent studies will also be discussed.

Brain NMDA Receptors in Schizophrenia and Depression

N-methyl-D-aspartate (NMDA) receptor antagonists such as phencyclidine (PCP), dizocilpine (MK-801) and ketamine have long been considered a model of schizophrenia, both in animals and humans. However, ketamine has been recently approved for treatment-resistant depression, although with severe restrictions. Interestingly, the dosage in both conditions is similar, and positive symptoms of schizophrenia appear before antidepressant effects emerge. Here, we describe the temporal mechanisms implicated in schizophrenia-like and antidepressant-like effects of NMDA blockade in rats, and postulate that such effects may indicate that NMDA receptor antagonists induce similar mechanistic effects, and only the basal pre-drug state of the organism delimitates the overall outcome. Hence, blockade of NMDA receptors in depressive-like status can lead to amelioration or remission of symptoms, whereas healthy individuals develop psychotic symptoms and schizophrenia patients show an exacerbation of these symptoms after the administration of NMDA receptor antagonists.

Alterations of acoustic features of 50 kHz vocalizations by nicotine and phencyclidine in rats

Ultrasonic vocalizations are widely used to examine affective states in rats, yet relatively few studies explore the acoustic features of vocalizations, especially in relation to drug exposure, and no studies have explored alterations in acoustic features over time. The goal of this study was to examine nicotine- and phencyclidine-induced alterations of bandwidth, duration, and frequency of 50 kHz vocalizations. The minimum and maximum frequency, bandwidth, and duration of calls were examined after 7 days of daily subcutaneous administration of phencyclidine (2.0 mg/kg) and nicotine (0.2 and 0.4 mg/kg) in male Sprague-Dawley rats. Bandwidth was significantly decreased in rats treated with both nicotine (0.2 and 0.4 mg/kg) and phencyclidine. Maximum frequency was lowest on the first day of exposure compared with all other days and was not altered by drug exposure. Call duration was not affected by time or drug exposure. These findings suggest the importance of studying alterations in acoustic features in time, especially those induced by drug exposure.

The Dopamine Allosteric Agent, PAOPA, Demonstrates Therapeutic Potential in the Phencyclidine NMDA Pre-clinical Rat Model of Schizophrenia

PAOPA, a potent analog of prolyl-leucyl-glycinamide, has shown therapeutic potential at the preclinical stage for dopaminergic related illnesses, including animal models of schizophrenia, Parkinson’s disease and haloperidol-induced extrapyramidal movement disorders. PAOPA’s unique allosteric mechanism and dopamine D2 receptor specificity provide a unique composition of properties for the development of potential therapeutics for neuropsychiatric illnesses. We sought to investigate PAOPA’s therapeutic prospects across the spectrum of schizophrenia-like symptoms represented in the established phencyclidine-induced rat model of schizophrenia, (5 mg/kg PCP twice daily for 7 days, followed by 7 days of drug withdrawal). PAOPA was assessed for its effect on brain metabolic activity and across a battery of behavioral tests including, hyperlocomotion, social withdrawal, sensorimotor gating, and novel object recognition. PAOPA showed therapeutic efficacy in behavioral paradigms representing the negative (social withdrawal) and cognitive-like (novel object recognition) symptoms of schizophrenia. Interestingly, some behavioral indices associated with the positive symptoms of schizophrenia that were ameliorated in PAOPA’s prior examination in the amphetamine-sensitized model of schizophrenia were not ameliorated in the PCP model; suggesting that the deficits induced by amphetamine and PCP—while phenotypically similar—are mechanistically different and that PAOPA’s effects are restricted to certain mechanisms and systems. These studies provide insight on the potential use of PAOPA for the safe and effective treatment of schizophrenia.

DARK Classics in Chemical Neuroscience: Phencyclidine (PCP)

Phencyclidine (PCP, "angel dust", an arylcyclohexylamine) was the first non-natural, man-made illicit drug of abuse, and was coined 'the most dangerous drug in America" in the late 1970s (amidst sensational horror stories of the drug's effects); however, few other illicit drugs have had such a significant and broad impact on society-both good and bad. Originally developed as a new class of anesthetic, PCP-derived psychosis gave way to the PCP hypothesis of schizophrenia (later coined the NMDA receptor hypofunction hypothesis or the glutamate hypothesis of schizophrenia), which continues to drive therapeutic discovery for schizophrenia today. PCP also led to the discovery of ketamine (and a new paradigm for the treatment of major depression), as well as other illicit, designer drugs, such as methoxetamine (MXE) and a new wave of Internet commerce for illicit drugs (sold as research chemicals, or RCs). Furthermore, PCP is a significant contaminant/additive of many illegal drugs sold today, due to its ease of preparation by clandestine chemists. Here, we will review the history, importance, synthesis (both legal and clandestine), pharmacology, drug metabolism, and folklore of PCP, a true DARK classic in chemical neuroscience.

Animal Models of Addiction and Neuropsychiatric Disorders and Their Role in Drug Discovery: Honoring the Legacy of Athina Markou

Each of the co-authors worked with Athina Markou, at different stages of our careers and in different capacities, to develop, optimize, and use animal models of drug addiction and, more generally, mental health disorders such as anxiety, depression, and schizophrenia. Here, we briefly summarize some of our work with Athina, primarily involving the use of the intracranial self-stimulation and intravenous drug self-administration procedures. This work established that excessive consumption of addictive drugs can induce profound dysfunction in brain reward circuits. Such drug-induced reward deficits are likely to play a key role in precipitating the emergence of compulsive drug-seeking behaviors. We also summarize findings suggesting that perturbations in glutamatergic transmission contribute to brain reward deficits in drug-dependent animals and that metabotropic glutamate receptors are potential targets for the development of novel medications to facilitate long-term drug abstinence and prevention of relapse.

Phencyclidine increased while isolation rearing did not affect progressive ratio responding in rats: Investigating potential models of amotivation in schizophrenia

BACKGROUND

Schizophrenia is a debilitating neurodevelopmental disorder affecting 1% of the global population with heterogeneous symptoms including positive, negative, and cognitive. While treatment for positive symptoms exists, none have been developed to treat negative symptoms. Animal models of schizophrenia are required to test targeted treatments and since patients exhibit reduced effort (breakpoints) for reward in a progressive ratio (PR) task, we examined the PR breakpoints of rats treated with the NMDA receptor antagonist phencyclidine or those reared in isolation - two common manipulations used to induce schizophrenia-relevant behaviors in rodents.

METHODS

In two cohorts, the PR breakpoint for a palatable food reward was examined in Long Evans rats after: 1) a repeated phencyclidine regimen; 2) A subchronic phencyclidine regimen followed by drug washout; and 3) post-weaning social isolation.

RESULTS

Rats treated with repeated phencyclidine and those following washout from phencyclidine exhibited higher PR breakpoints than vehicle-treated rats. The breakpoint of isolation reared rats did not differ from those socially reared, despite abnormalities of these rats in other schizophrenia-relevant behaviors.

CONCLUSION

Despite their common use for modeling other schizophrenia-relevant behaviors neither phencyclidine treatment nor isolation rearing recreated the motivational deficits observed in patients with schizophrenia, as measured by PR breakpoint. Other manipulations, and negative symptom-relevant behaviors, require investigation prior to testing putative therapeutics.

PCP-based mice models of schizophrenia: differential behavioral, neurochemical and cellular effects of acute and subchronic treatments

RATIONALE

N-methyl-D-aspartate receptor (NMDA-R) hypofunction has been proposed to account for the pathophysiology of schizophrenia. Thus, NMDA-R blockade has been used to model schizophrenia in experimental animals. Acute and repeated treatments have been successfully tested; however, long-term exposure to NMDA-R antagonists more likely resembles the core symptoms of the illness.

OBJECTIVES

To explore whether schizophrenia-related behaviors are differentially induced by acute and subchronic phencyclidine (PCP) treatment in mice and to examine the neurobiological bases of these differences.

RESULTS

Subchronic PCP induced a sensitization of acute locomotor effects. Spontaneous alternation in a T-maze and novel object recognition performance were impaired after subchronic but not acute PCP, suggesting a deficit in working memory. On the contrary, reversal learning and immobility in the tail suspension test were unaffected. Subchronic PCP significantly reduced basal dopamine but not serotonin output in medial prefrontal cortex (mPFC) and markedly decreased the expression of tyrosine hydroxylase in the ventral tegmental area. Finally, acute and subchronic PCP treatments evoked a different pattern of c-fos expression. At 1 h post-treatment, acute PCP increased c-fos expression in many cortical regions, striatum, thalamus, hippocampus, and dorsal raphe. However, the increased c-fos expression produced by subchronic PCP was restricted to the retrosplenial cortex, thalamus, hippocampus, and supramammillary nucleus. Four days after the last PCP injection, c-fos expression was still increased in the hippocampus of subchronic PCP-treated mice.

CONCLUSIONS

Acute and subchronic PCP administration differently affects neuronal activity in brain regions relevant to schizophrenia, which could account for their different behavioral effects.

Repeated administration of aripiprazole produces a sensitization effect in the suppression of avoidance responding and phencyclidine-induced hyperlocomotion and increases D2 receptor-mediated behavioral function

The present study investigated how repeated administration of aripiprazole (a novel antipsychotic drug) alters its behavioral effects in two behavioral tests of antipsychotic activity and whether this alteration is correlated with an increase in dopamine D2 receptor function. Male adult Sprague-Dawley rats were first repeatedly tested with aripiprazole (3, 10 and 30 mg/kg, subcutaneously (sc)) or vehicle in a conditioned avoidance response (CAR) test or a phencyclidine (PCP) (3.20 mg/kg, sc)-induced hyperlocomotion test daily for five consecutive days. After 2-3 days of drug-free retraining or resting, all rats were then challenged with aripiprazole (1.5 or 3.0 mg/kg, sc). Repeated administration of aripiprazole progressively increased its inhibition of avoidance responding and PCP-induced hyperlocomotion. More importantly, rats previously treated with aripiprazole showed significantly lower avoidance response and lower PCP-induced hyperlocomotion than those previously treated with vehicle in the challenge tests. An increased sensitivity to quinpirole (a selective D2/3 agonist) in prior aripiprazole-treated rats was also found in the quinpirole-induced hyperlocomotion test, suggesting an enhanced D2/3-mediated function. These findings suggest that aripiprazole, despite its distinct receptor mechanisms of action, induces a sensitization effect similar to those induced by other antipsychotic drugs and this effect may be partially mediated by brain plasticity involving D2/3 receptor systems.

Attention to pleasant stimuli in early adolescence predicts alcohol-related problems in mid-adolescence

Attenuated responses to natural rewards have been found to predict subsequent substance use among dependent populations, suggesting that this may be a premorbid risk factor for later problematic substance use. However, research on adolescent risk-taking suggests that exaggerated, rather than blunted, reward responsiveness predicts later substance abuse. Acoustic startle-induced event-related potentials (ERP) were recorded in a sample of 11-13 year-olds while they viewed affective pictures, and participants were reassessed four years later regarding alcohol use and experience of alcohol-related problems. Increased attenuation of the amplitude of the P300 component of the ERP during viewing of pleasant pictures, relative to amplitude during neutral pictures (an indicator of increased attention to pleasant pictures), predicted increased likelihood of alcohol-related problems at follow-up. These findings further support research indicating that increased reward responsiveness predicts risky behaviours in adolescence, with anhedonia primarily a consequence of substance dependence.

Comparison of antidepressant-like and abuse-related effects of phencyclidine in rats

Preclinical Research N-methyl-D-aspartate (NMDA) receptor antagonists, such as ketamine, have emerged as novel candidate treatments for major depressive disorder, but abuse potential of these agents is a concern. The NMDA antagonist phencyclidine has known abuse liability but undefined efficacy as an antidepressant. To further evaluate the relationship between antidepressant-like and abuse-related effects of NMDA antagonists, this study evaluated the effects of phencyclidine (1.0-10.0 mg/kg) in male Sprague-Dawley rats responding under two procedures that have been used to assess antidepressant-like effects (differential-reinforcement-of-low-rate [DRL] 72 s schedule of food reinforcement; n = 9) and abuse-related drug effects (intracranial self-stimulation [ICSS]; n = 6). Under the DRL 72 s schedule, phencyclidine (10.0 mg/kg) increased reinforcers and decreased responses without shifting the peak location of the interresponse time (IRT) distribution. Ketamine (10.0 mg/kg) also increased reinforcers and decreased responses, but unlike phencyclidine, it produced a rightward shift in the peak location of the IRT distribution. The 10.0 mg/kg phencyclidine dose that decreased DRL 72 s responding also decreased rates of ICSS for 50 min after its administration; however, abuse-related ICSS facilitation was observed at later times (100-300 min) or after a lower phencyclidine dose (3.2 mg/kg). These results suggest that phencyclidine produces weaker antidepressant-like effects, but stronger abuse-related effects than ketamine in these procedures.

Anhedonia, avolition, and anticipatory deficits: assessments in animals with relevance to the negative symptoms of schizophrenia

Schizophrenia represents a complex, heterogeneous disorder characterized by several symptomatic domains that include positive and negative symptoms and cognitive deficits. Negative symptoms reflect a cluster of symptoms that remains therapeutically unresponsive to currently available medications. Therefore, the development of animal models that may contribute to the discovery of novel and efficacious treatment strategies is essential. An animal model consists of both an inducing condition or manipulation (i.e., independent variable) and an observable measure(s) (i.e., dependent variables) that are used to assess the construct(s) under investigation. The objective of this review is to describe currently available experimental procedures that can be used to characterize constructs relevant to the negative symptoms of schizophrenia in experimental animals. While negative symptoms can encompass aspects of social withdrawal and emotional blunting, this review focuses on the assessment of reward deficits that result in anhedonia, avolition, and abnormal reward anticipation. The development and utilization of animal procedures that accurately assess reward-based constructs related to negative symptomatology in schizophrenia will provide an improved understanding of the neural substrates involved in these processes.

Acute and chronic effects of NMDA receptor antagonists in rodents, relevance to negative symptoms of schizophrenia: a translational link to humans

Negative symptoms of schizophrenia remain an unmet clinical need as they are common, persistent, respond poorly to existing treatments and lead to disability. Blunted affect, alogia, asociality, anhedonia and avolition are regarded as key negative symptoms despite DSM-IV-TR specifying a more limited range. The key to development of improved therapies is improved animal models that mimic the human condition in terms of behaviour and pathology and that predict efficacy of novel treatments in patients. Accumulating evidence shows that NMDA receptor (NMDAR) antagonists mimic cognitive deficits of relevance to schizophrenia in animals, along with associated pathological changes. This review examines evidence for the ability of NMDAR antagonists to mimic anhedonia and asociality, two negative symptoms of schizophrenia, in animals. The use of various species, paradigms and treatment regimens are reviewed. We conclude that sub-chronic treatment with NMDAR antagonists, typically PCP, induces social withdrawal in animals but not anhedonia. NMDAR antagonists have further effects in paradigms such as motivational salience that may be useful for mimicking other aspects of negative symptoms but these require further development. Sub-chronic treatment regimens of NMDAR antagonists also have some neurobiological effects of relevance to negative symptoms. It is our view that a sub-chronic treatment regime with NMDAR antagonists, particularly PCP, with animals tested following a wash-out period and in a battery of tests to assess certain behaviours of relevance to negative symptoms and social withdrawal (the animal equivalent of asociality) is valuable. This will enhance our understanding of the psycho and neuropathology of specific negative symptom domains and allow early detection of novel pharmacological targets.

Modeling combined schizophrenia-related behavioral and metabolic phenotypes in rodents

Schizophrenia is a chronic, debilitating disorder with a complex behavioral and cognitive phenotype underlined by a similarly complex etiology involving an interaction between susceptibility genes and environmental factors during early development. Limited progress has been made in developing novel pharmacotherapy, partly due to a lack of valid animal models. The recent recognition of the potentially causal role of central and peripheral energy metabolism in the pathophysiology of schizophrenia raises the need of research on animal models that combine both behavioral and metabolic phenotypic domains, similar to what have been identified in humans. In this review we focus on selected genetic (DBA/2J mice, leptin receptor mutants, and PSD-93 knockout mice), early neurodevelopmental (maternal protein deprivation) and pharmacological (acute phencyclidine) animal models that capture the combined behavioral and metabolic abnormalities shown by schizophrenic patients. In reviewing behavioral phenotypes relevant to schizophrenia we apply the principles established by the Research Domain Criteria (RDoC) for better translation. We demonstrate that etiologically diverse manipulations such as specific breeding, deletion of genes that are primarily involved in metabolic regulation and in synaptic plasticity, as well as early metabolic deprivation and adult pharmacological challenge of the glutamate system can lead to schizophrenia-related behavioral and metabolic phenotypes, which suggest that these pathways might be interlinked. We propose that using animal models that combine different domains of schizophrenia can be used as a translationally valid approach to capture the system-level complex interplay between peripheral and central processes in the development of psychopathology.

Anhedonia in substance use disorders: a systematic review of its nature, course and clinical correlates

OBJECTIVE

There is growing evidence that anhedonia is a commonly experienced symptom among substance-using populations. This systematic review synthesises findings across a range of substances to address questions regarding the time course of anhedonia, how anhedonia relates to other symptoms of substance dependence and whether it is similarly prevalent across all addictive drugs.

METHOD

A literature search was conducted on PubMed, PsycINFO and MEDLINE, yielding 32 studies that used self-report measures of anhedonia among participants with a history of a substance abuse, dependence or long-term daily use of addictive substances.

RESULTS

Findings from these studies indicate that anhedonia (1) is elevated in samples dependent on a range of substances; (2) typically appears as a consequence of substance abuse or dependence, and diminishes with abstinence; and (3) predicts increased drug cravings and the likelihood of relapse in those attempting abstinence.

CONCLUSIONS

The common experience of anhedonia in substance-dependent populations, and its relationship to relapse, emphasises the importance of developing therapeutic interventions that specifically target anhedonia in the treatment of all substance use disorders.

The effects of phencyclidine (PCP) on anxiety-like behavior in the elevated plus maze and the light-dark exploration test are age dependent, sexually dimorphic, and task dependent

Previous research in our laboratory revealed sexually dimorphic effects of prior exposure to phencyclidine (PCP) on elevated plus maze behavior. In an attempt to examine the developmental time course of this effect and determine the extent to which it generalizes to other anxiety paradigms, young adult (61-64 days old) and adult (96-107 days old) male and female rats were treated with PCP (15 mg/kg) or saline. Following a two week withdrawal period, animals were tested in either the elevated plus maze (EPM) or a light-dark exploration (LD) test. In adults, both tests revealed a sexually dimorphic effect driven by PCP-induced decreases in anxiety in females as indicated by increased time spent in the open arms of the EPM and in the lit compartment of the LD test and increased anxiety in males as indicated by decreased time spent in the lit compartment of the LD. In young animals, PCP pretreatment decreased open arm exploration in the elevated plus maze, indicating increased anxiety. However, PCP increased time spent in the light compartment in the light-dark exploration test, indicating decreased anxiety. Corticosterone levels measured 15 min after the onset of the EPM failed to reveal an association between the behavioral effects of PCP and corticosterone levels. The results in adults substantiate the previously observed sexually dimorphic effect of PCP on elevated plus maze behavior in adults and indicate that the effect generalizes to another anxiety paradigm. The results in the younger animals suggest an age dependent effect of PCP on anxiety in general and indicate that behaviors in the elevated plus maze and the light-dark exploration test reflect dissociable psychobiological states.

Analysis of licking microstructure provides no evidence for a reduction in reward value following acute or sub-chronic phencyclidine administration

RATIONALE

The N-methyl D-aspartate antagonist phencyclidine (PCP) is purported to mimic the negative, cognitive and positive symptoms of schizophrenia. Thus, acute and sub-chronic PCP treatment in rodents might produce anhedonia, a decrease in the pleasure produced by rewards.

OBJECTIVES

Experiment 1 investigated whether acute PCP treatment changes the value of sucrose. A comparison was made to (+)MK-801, a drug often used interchangeably with PCP in preclinical studies. Experiment 2 assessed the effects of withdrawal from sub-chronic PCP treatment on the value of sucrose.

METHODS

Experiment 1 examined the dose-response effects of PCP and (+)MK-801 on licking microstructure during sucrose consumption. Experiment 2 assessed the effects of withdrawal from sub-chronic PCP treatment (5 mg/kg twice daily for 7 days), on licking microstructure during sucrose consumption. Locomotor activity testing was carried out in experiment 2 to confirm the sensitisation effect of the PCP regimen on amphetamine-induced hyperlocomotion.

RESULTS

Low to moderate acute doses of PCP and (+)MK-801 increased the amount of sucrose consumed. Higher doses decreased consumption and the number of licks per cluster (cluster size) but also increased the average inter-lick interval, which may indicate motor impairment. There was no evidence that withdrawal from sub-chronic PCP treatment produced decreases in consumption or lick cluster size.

CONCLUSIONS

Following acute PCP treatment, we found no evidence of reduced reward value without the presence of confounding motor deficits. Sub-chronic PCP withdrawal also produced no decrease in reward value. Therefore, the current results indicate that neither acute PCP treatment nor sub-chronic PCP withdrawal produce consummatory anhedonia.

Depression research: where are we now?

Extensive studies have led to a variety of hypotheses for the molecular basis of depression and related mood disorders, but a definite pathogenic mechanism has yet to be defined. The monoamine hypothesis, in conjunction with the efficacy of antidepressants targeting monoamine systems, has long been the central topic of depression research. While it is widely embraced that the initiation of antidepressant efficacy may involve acute changes in monoamine systems, apparently, the focus of current research is moving toward molecular mechanisms that underlie long-lasting downstream changes in the brain after chronic antidepressant treatment, thereby reaching for a detailed view of the pathophysiology of depression and related mood disorders. In this minireview, we briefly summarize major themes in current approaches to understanding mood disorders focusing on molecular views of depression and antidepressant action.

MK-801 produces a deficit in sucrose preference that is reversed by clozapine, D-serine, and the metabotropic glutamate 5 receptor positive allosteric modulator CDPPB: relevance to negative symptoms associated with schizophrenia?

Currently prescribed antipsychotics attenuate the positive symptoms of schizophrenia but fail or only mildly improve negative symptoms. The present study aimed to establish an animal model of negative symptoms by examining the effects of the NMDA receptor antagonist MK-801 on sucrose preference. We sought to validate the model by examining the effects of clozapine and D-serine, for which there are positive clinical data regarding their effects on negative symptoms, and haloperidol which is clinically ineffective. We extended our analysis by examining CDPPB, an mGlu5 receptor positive allosteric modulator. Acute MK-801 produced effects indicative of a shift in the hedonic experience of sucrose not confounded by disruptions in motor abilities or taste as revealed by: 1) a decrease in sucrose intake at low concentrations (0.8% or 1.2%), but no effect on water, 2) an increase in consumption for higher (7%) sucrose concentrations, reflecting a shift to the right in the concentration-consumption curve, and 3) no effect on quinine intake. Sub-chronic clozapine and acute d-serine attenuated the MK-801-induced deficit in 1.2% sucrose consumption, whereas sub-chronic haloperidol (0.02 mg/kg) did not. Finally, acute treatment with CDPPB also attenuated this deficit. These data suggest that this model may be useful for identifying novel agents that improve negative symptoms, and that compounds which enhance NMDA receptor function, such as mGlu5 receptor PAMs, may have clinical utility in this regard.

Effect of subchronic phencyclidine administration on sucrose preference and hippocampal parvalbumin immunoreactivity in the rat

Persistent blockade of NMDA receptor function by repeated phencyclidine dosing produces pathophysiological changes that model deficits observed in schizophrenia. The present study investigates the effects of subchronic phencyclidine administration (PCP; 2 or 5mg/kg bi-daily for 7 days followed by a drug-free period) on sucrose choice, a measure of anhedonia. Sucrose preference in a two-bottle sucrose-water choice test was assessed 1 and 2 weeks after PCP. Results showed no differences in sucrose intake between PCP rats and controls, nor a difference in water intake or total volume of liquid consumed at either time-point. Six weeks post-PCP, analysis of brains showed a reduction in expression of parvalbumin immunoreactive neurons in the hippocampus with significant reductions localised to the CA1 and CA2/3 regions. These results demonstrate that while subchronic PCP may not be a valid model for the negative symptom of anhedonia observed in schizophrenia, it induces pathology in the brain in hippocampal subregions that are reminiscent of changes observed in schizophrenia.

Preclinical behavioral models for predicting antipsychotic activity

Schizophrenia is a major psychiatric disease that is characterized by three distinct symptom domains: positive symptoms, negative symptoms, and cognitive impairment. Additionally, treatment with classical antipsychotic medication can be accompanied by important side effects that involve extrapyramidal symptoms (EPS). The discovery of clozapine in the 1970s, which is efficacious in all three symptom domains and has a reduced propensity to induce EPS, has driven research for new antipsychotic agents with a wider spectrum of activity and a lower propensity to induce EPS. The following chapter reviews existing behavioral procedures in animals for their ability to predict compound efficacy against schizophrenia symptoms and liability to induce EPS. Rodent models of positive symptoms include procedures related to hyperfunction in central dopamine and serotonin (5-hydroxytryptamine) systems and hypofunction of central glutamatergic (N-methyl-d-aspartate) neurotransmission. Procedures for evaluating negative symptoms include rodent models of anhedonia, affective flattening, and diminished social interaction. Cognitive deficits can be assessed in rodent models of attention (prepulse inhibition (PPI), latent inhibition) and of learning and memory (passive avoidance, object and social recognition, Morris water maze, and operant-delayed alternation). The relevance of the conditioned avoidance response (CAR) is also discussed. A final section reviews animal procedures for assessing EPS liability, in particular parkinsonism (catalepsy), acute dystonia (purposeless chewing in rodents, dystonia in monkeys), akathisia (defecation in rodents), and tardive dyskinesia (long-term antipsychotic treatment in rodents and monkeys).

Clozapine attenuates disruptions in response inhibition and task efficiency induced by repeated phencyclidine administration in the intracranial self-stimulation procedure

Currently available antipsychotic medications lack satisfactory effectiveness against several symptom clusters of schizophrenia, including affective symptoms (e.g., anhedonia) and cognitive deficits (e.g., impulsivity). Translational animal models analogous to these symptoms are necessary to provide insights into the neurobiological events underlying these impairments and allow the development of improved schizophrenia treatments. We investigated the effects of repeated administration of the psychotomimetic phencyclidine (PCP), a noncompetitive N-methyl-D-aspartate receptor antagonist, on performance in the intracranial self-stimulation (ICSS) procedure, a test of reward function. We also explored how chronic treatment with clozapine, an atypical antipsychotic with limited effectiveness on affective and cognitive schizophrenia symptoms, would affect PCP-induced disruptions of ICSS performance. A single injection of 2 mg/kg PCP elevated ICSS thresholds, suggesting a reward deficit. Repeated PCP administration (2 mg/kg once daily for 2 consecutive days followed by a 10-day drug free period, and then 5 consecutive days of 2 mg/kg PCP daily, s.c., 30 min pretreatment) resulted in a small, but significant, lowering of ICSS reward thresholds, indicating increased reward function. Chronic clozapine did not alter the effects of repeated PCP on ICSS thresholds. Repeated PCP also increased the number of extra and timeout responses performed during the ICSS procedure, reflecting disinhibition of inappropriate responding and decreased task efficiency. Chronic clozapine attenuated the increase in extra responses induced by repeated PCP and tended to reduce the PCP-induced increase in timeout responses. These results suggest that repeated PCP administration does not produce an anhedonia-like state resembling that seen in schizophrenia. However, the increased impulsivity and reduced task efficiency seen with repeated PCP administration, and the sensitivity of these effects to attenuation with an atypical antipsychotic, suggest that repeated PCP administration may be a useful inducing condition for eliciting cognitive deficits with relevance to schizophrenia.

Subchronic and chronic PCP treatment produces temporally distinct deficits in attentional set shifting and prepulse inhibition in rats

RATIONALE

We have previously demonstrated that subchronic (five daily administrations of 2.6 mg/kg PCP) and chronic intermittent administration of 2.6 mg/kg PCP to rats produces hypofrontality and other neurochemical changes akin to schizophrenia pathology (Cochran et al., Neuropsychopharmacology, 28:265-275, 2003).

OBJECTIVES

We sought to determine whether behavioral alterations related to discrete aspects of schizophrenia are also induced by these PCP treatment regimes.

MATERIALS AND METHODS

Following administration of vehicle or PCP according to the protocols described above, rats were assessed for attentional set shifting ability, prepulse inhibition (PPI), or social interaction and the locomotor response to a challenge dose of amphetamine.

RESULTS

Ability to shift attentional set was impaired 72 h after the last PCP administration following the subchronic and chronic intermittent treatment regimes. PPI was disrupted after each acute administration of PCP in animals under the subchronic treatment regime. However, PPI deficits were not sustained 72 h after the last of five daily administrations. In subchronic and chronic PCP treated animals, no change was found in social interaction behavior, and there was little change in baseline or amphetamine-stimulated locomotor activity, employed as an indicator of dopaminergic hyperfunction.

CONCLUSIONS

The temporally distinct behavioral effects of these PCP treatment regimes suggest that PPI deficits relate directly to acute NMDA receptor antagonism, whereas the more enduring set shifting deficits relate to the longer term consequences of NMDA receptor blockade. Therefore, these subchronic and chronic PCP treatment regimes produce hypofrontality (Cochran et al., Neuropsychopharmacology, 28:265-275, 2003) and associated prefrontal cortex-dependent deficits in behavioral flexibility which mirror core deficits in schizophrenia.

Comparison between intraperitoneal and subcutaneous phencyclidine administration in Sprague-Dawley rats: a locomotor activity and gene induction study

In a putative model of acute phencyclidine (PCP)-induced psychosis we evaluated effects of the drug on locomotor activity (LMA) and immediate early gene (IEG) induction in the rat using two routes of drug administration, intraperitoneal (i.p.) and subcutaneous (s.c.). Adult male rats received saline or PCP (1.0-5.0 mg/kg) either i.p or s.c. and were assessed for LMA for 60 min. At the end of the LMA testing animals were culled and blood and brain samples were collected for PCP concentration analysis. Separate cohorts of animals received 5.0 mg/kg PCP (i.p. or s.c.) and were used to investigate (1) the pharmacokinetics of PCP or (2) induction of IEG (Arc, c-fos, BDNF, junB, Krox-20, sgk-1, NURR1, fra-2, Krox-24, and egr-3) mRNA expression in the prefrontal cortex (PFC). Administration of PCP resulted in locomotor hyperactivity which was more robust and longer-lasting in animals dosed s.c. compared to i.p.-treated-animals. Differences in hyperlocomotion were paralleled by higher concentrations of PCP in the blood and in the brain of s.c.-treated animals compared to i.p.-treated animals. The differences in the concentration of PCP between the two routes of administration were detected 30 min after dosing and persisted for up to 4 h. Administration of PCP via the s.c. route resulted in induction of more IEGs and consistently larger magnitudes of induction than that via the i.p. route. Therefore, we have outlined the dosing conditions to induce rapid and robust effect of acute PCP on behaviour, gene induction, and pharmacokinetic profile, to allow investigation of this as a potential animal model of acute psychosis.

Subsequent exposure to the choline uptake enhancer MKC-231 antagonizes phencyclidine-induced behavioral deficits and reduction in septal cholinergic neurons in rats

This study examined the effects of subsequent, subchronic, treatment with choline uptake enhancer MKC-231 on the behavioral and cellular deficits induced by repeated PCP exposure in rats. Prior subchronic PCP exposure resulted in increased locomotion following an acute PCP or cocaine challenge, but resulted in decreased locomotor activity in response to a carbachol-challenge. MKC-231 significantly antagonized the alterations in the locomotor responses to cocaine and carbachol, but not to PCP. In the novel object recognition test, repeated PCP exposure caused cognitive deficits in rats, and the PCP-induced cognitive deficits were antagonized by MKC-231. In contrast, no effects of PCP exposure were shown in the repeated passive avoidance test. Furthermore, repeated PCP exposure decreased a number of choline acetyltransferase (ChAT)-positive cells in the medial septum and increased dynorphin A expression in the ventral striatum. Moreover, MKC-231 significantly antagonized the changes in septal ChAT-positive cells, but not the changes in ventrostriatal dynorphin A expression. These results suggest that MKC-231 could be a therapeutic drug for the treatment of schizophrenia.

Behavioral processes mediating phencyclidine-induced decreases in voluntary sucrose consumption

Prior exposure to phencyclidine (PCP) has been shown to decrease voluntary sucrose consumption in rats, which may indicate reduced reward function. To further characterize the effects of PCP on sucrose consumption, we examined the dose-response relationship between PCP and sucrose consumption, the longevity of the effect, the effects of repeated injections of PCP, variation of the PCP effect across sucrose concentrations, and the effects of PCP on gustatory hedonic responses. A single injection of PCP (2.5-20 mg/kg) dose-dependently suppressed sucrose consumption 20 h post-injection, with significant decreases after 15 and 20 mg/kg PCP. These decreases were sustained three days following withdrawal from PCP. Repeated injections of PCP (7.5 mg/kg bid for 7 days) decreased sucrose consumption 20 h after withdrawal, which returned to baseline on the second day. A single injection of PCP (15 mg/kg) suppressed 0.15 M sucrose more than 1 M sucrose consumption, with no effect on 0.3 M sucrose, suggesting that PCP suppressed intake of moderately rewarding taste stimuli. Finally, a single injection of PCP (15 mg/kg) suppressed brief access (20 s) licking for the majority of concentrations of sucrose solutions offered (0.031 M, 0.062 M, 0.125 M, 0.25 M, 0.5 M, and 1.0 M), while it had no effect on licking for 0.016 M sucrose, water, or for bitter quinine hydrochloride solutions (range: 0.94 mM-30 mM), suggesting that the PCP effect is specific to palatable taste stimuli without disruption of sensitivity to taste quality or intensity. We conclude that PCP produces moderate anhedonia as reflected through a specific decrease in the sustained consumption of moderately palatable sucrose solutions.

Animal models and treatments for addiction and depression co-morbidity

The high rates of co-morbidity of drug addiction with depression may be attributable to shared neurobiology. Here, we discuss shared neurobiological substrates in drug withdrawal and depression, with an emphasis on changes in brain reward circuitry that may underlie anhedonia, a core symptom of depression and drug withdrawal. We explored experimentally whether clinical antidepressant medications or other treatments would reverse the anhedonia observed in rats undergoing spontaneous nicotine or amphetamine withdrawal, defined operationally as elevated brain reward thresholds. The co-administration of selective serotonin reuptake inhibitors with a serotonin-1A receptor antagonist, or the tricyclic antidepressant desipramine, or the atypical antidepressant bupropion ameliorated nicotine or amphetamine withdrawal in rats. Thus, increases in monoaminergic neurotransmission, or neuroadaptations induced by increased monoaminergic neurotransmission, ameliorated depression-like aspects of drug withdrawal. Further, chronic pretreatment with the atypical antipsychotic clozapine, that has some efficacy in the treatment of the depression-like symptoms of schizophrenia, attenuated nicotine and amphetamine withdrawal. Finally, a metabotropic glutamate 2/3 receptor antagonist reversed threshold elevations associated with nicotine withdrawal. The effects of these pharmacological manipulations are consistent with the altered neurobiology observed in drug withdrawal and depression. Thus, these data support the hypothesis of common substrates mediating the depressive symptoms of drug withdrawal and those seen in psychiatric patients. Accordingly, the anhedonic state associated with drug withdrawal can be used to study the neurobiology of anhedonia, and thus contribute to the identification of novel targets for the treatment of depression-like symptoms seen in various psychiatric and neurological disorders.

Differential effects of acute and subchronic clozapine and haloperidol on phencyclidine-induced decreases in voluntary sucrose consumption in rats

Prior exposure to the psychotomimetic drug phencyclidine (PCP) decreases voluntary sucrose consumption in rats. This may be indicative of reduced reward function, a phenomenon associated with negative schizophrenic symptomatology. Given that atypical antipsychotics have been shown to ameliorate negative symptoms of schizophrenia more effectively than typical neuroleptics, this effect should be reversed by clozapine but not haloperidol. PCP (15 mg/kg) or saline was administered 20 h prior to testing for voluntary sucrose consumption in non-deprived rats. In the acute experiments, rats were treated with clozapine (5 mg/kg), haloperidol (0.2 mg/kg), or vehicle 45 min prior to testing. In the subchronic experiments, rats were treated with clozapine (3 mg/kg, bid), haloperidol (0.5 mg/kg, bid), or vehicle for 10 days prior to PCP administration. Acute clozapine exacerbated the PCP-induced decrease in sucrose consumption without altering water consumption. Acute haloperidol produced an overall decrease in sucrose consumption in both PCP-pretreated and control groups. Subchronic treatment with clozapine, but not haloperidol, reversed PCP-induced decreases in sucrose consumption. The synergistic effect of acute clozapine and PCP may reflect a PCP-induced increase in the reward-reducing properties of CLZ, normally seen only at higher doses. The observation that subchronic clozapine, but not haloperidol, reversed PCP-induced decreases in sucrose consumption supports the hypothesis that this effect of PCP represents a plausible animal model for negative schizophrenic symptomatology.

Transient hypolocomotion in rats repeatedly exposed to phencyclidine: an appraisal of motor function and motivation

Spontaneous locomotion in rats is reduced about 20 h following phencyclidine (PCP) exposure. Our primary purpose was to test if locomotor irregularities would outlast drug withdrawal and be modulated by a periodic auditory stimulation during or after repeated injections. To circumscribe plausible interpretations, motor coordination and motivation were assessed as well. 10 mg/kg of PCP decreased spontaneous locomotion during the injection period whether or not a sound was presented. Those effects did not persist after a 7-day washout. Motor coordination and motivation to perform a variety of goal-driven conducts in the presence or absence of reinforcement were all unaffected by PCP treatment. The generalized hypolocomotor effects induced by PCP likely resulted from an inability to self-generate behaviors. These deficiencies were apparent in contexts where response production was not clearly specified nor supported, regardless of reward contingency.

The reelin receptors VLDLR and ApoER2 regulate sensorimotor gating in mice

Postmortem brain loss of reelin is noted in schizophrenia patients. Accordingly, heterozygous reeler mutant mice have been proposed as a putative model of this disorder. Little is known, however, about the involvement of the two receptors for reelin, Very-Low-Density Lipoprotein Receptor (VLDLR) and Apolipoprotein E Receptor 2 (ApoER2), on pre-cognitive processes of relevance to deficits seen in schizophrenia. Thus, we evaluated sensorimotor gating in mutant mice heterozygous or homozygous for the two reelin receptors. Mutant mice lacking one of these reelin receptors were tested for prepulse inhibition (PPI) of the acoustic startle reflex prior to and following puberty, and on a crossmodal PPI task, involving the presentation of acoustic and tactile stimuli. Furthermore, because schizophrenia patients show increased sensitivity to N-methyl-d-aspartate (NMDA) receptor blockade, we assessed the sensitivity of these mice to the PPI-disruptive effects of the NMDA receptor antagonist phencyclidine. The results demonstrated that acoustic PPI did not differ between mutant and wildtype mice. However, VLDLR homozygous mice displayed significant deficits in crossmodal PPI, while ApoER2 heterozygous and homozygous mice displayed significantly increased crossmodal PPI. Both ApoER2 and VLDLR heterozygous and homozygous mice exhibited greater sensitivity to the PPI-disruptive effects of phencyclidine than wildtype mice. These results indicate that partial or complete loss of either one of the reelin receptors results in a complex pattern of alterations in PPI function that includes alterations in crossmodal, but not acoustic, PPI and increased sensitivity to NMDA receptor blockade. Thus, reelin receptor function appears to be critically involved in crossmodal PPI and the modulation of the PPI response by NMDA receptors. These findings have relevance to a range of neuropsychiatric disorders that involve sensorimotor gating deficits, including schizophrenia.

Behavioural effects of neonatal lesions of the medial prefrontal cortex and subchronic pubertal treatment with phencyclidine of adult rats

According to the neurodevelopmental hypothesis of schizophrenia, early brain damage renders the brain vulnerable to adverse effects during puberty, which precipitate the disease in young adults. Animal models can be used to test this hypothesis. We investigated the potentially independent or interactive effects of neonatal (postnatal day 7) excitotoxic lesions of the rat medial prefrontal cortex (mPFC) and subchronic pubertal phencyclidine (PCP)-treatment on adult rat behaviour. Sham-lesioned (vehicle-injection) and naive (unoperated) rats served as controls. On postnatal days 42-48 rats were systemically injected with 5 mg/kg PCP or vehicle twice daily. Behavioural testing started at postnatal day 70. Rats were tested for locomotor activity (open field), anxiety (elevated plus maze), social behaviour (conditioned place preference for cage-mates), reward-related operant behaviour [progressive ratio (PR)] and spatial learning (four-arm baited eight-arm radial maze task). Nissl-stained sections revealed considerable regeneration of much of the lesioned tissue in the mPFC, however, with disturbed cytoarchitecture. Locomotor activity was increased by neonatal lesions but reduced after pubertal PCP-treatment. Neonatal lesions alone increased operant behaviour in the PR-test and reduced anxiety in the elevated plus maze. In contrast, PCP-treatment disturbed social behaviour while neonatal lesions had no effect. Different aspects of leaning and memory in the radial maze task were independently disturbed after neonatal lesions and PCP-treatment. Neonatal lesions and pubertal PCP-treatment differentially affected adult rat behaviour and no interactions were found.

Sharing of the home base: a social test in rats

The open field provides abundant opportunities for a pair of rats to express social interactions. Rats demonstrate social proximity while exploring the open field and also during simultaneous occupancy of a home base (HB). The HB is defined as a place in the field for which rats show a long-term preference, both in terms of occupancy and as a starting and ending point of exploratory excursions. In the present study, the social proximity of pairs of rats treated with phencyclidine (PCP; 2 mg/kg) or saline (Sal), and rats treated with clozapine (CLZ; 1.3 mg/kg) alone or in combination with PCP (CLZ-PCP), was videotaped and analysed algorithmically. PCP was tested for its disruptive effects on social interactions, and CLZ was tested for its ability to reverse some forms of the disruptive effects of PCP. The results showed that PCP reduced the rate of pairs establishing a common HB and shortened social HB occupancy, but had no effect on episodes of social exploration in the field. These findings demonstrate that the antisocial effect of PCP cannot be generalized across the entire spectrum of behavioural states in the open field. CLZ further decreased rather than increased social HB occupancy. This effect was derived from the reduction in rate of pairs establishing a common HB.

Decreased sensitivity to the effects of dopamine D1-like, but not D2-like, receptor antagonism in the posterior hypothalamic region/anterior ventral tegmental area on brain reward function during chronic exposure to nicotine in rats

Chronic administration of nicotine induces adaptations in central nervous system function to counteract nicotine's acute effects. When nicotine administration ceases, these adaptations remain unopposed and may lead to drug withdrawal. The present studies were conducted to assess the effects of chronic nicotine administration on dopamine D1- and D2-like receptor activity in the posterior hypothalamus/anterior ventral tegmental area (VTA). An intracranial self-stimulation discrete trial procedure that provides current intensity thresholds was used to provide a measure of brain reward function in rats. Previous studies showed that systemic administration of dopamine D1- or D2-like receptor antagonists induced elevations in brain reward thresholds in drug-free rats, indicative of a decrease in brain reward function. We show here that injections of the D1-like receptor antagonist SCH 23390 (1-4 microg total bilateral dose) into the posterior hypothalamus/anterior VTA differentially elevated brain reward thresholds in rats chronically treated with nicotine (9 mg/kg/day, salt) versus saline-treated rats. The nicotine-treated rats were less sensitive to the threshold elevating effects of D1-like receptor antagonism. By contrast, the D2-like receptor antagonist eticlopride (1-4 microg total bilateral dose) injected into the posterior hypothalamus/anterior VTA significantly elevated brain reward thresholds in saline- and nicotine-treated rats. No differential effect of eticlopride on brain reward thresholds in saline- and nicotine-treated rats was observed. Decreased sensitivity to D1-like receptor antagonism in the posterior hypothalamus/anterior VTA may partly mediate the development of tolerance to the reinforcing effects of nicotine and the manifestation of negative affective signs associated with cessation of nicotine administration.

Personality measures in former heroin users receiving methadone or in protracted abstinence from opiates

OBJECTIVE

Methadone Maintenance Therapy (MMT) and detoxification to abstinence are among the most common treatment options for opiate-dependent patients. This paper compares personality traits in detoxified former heroin users and those on MMT in order to assess their relevance to treatment selection.

METHOD

Twenty-six formerly heroin-dependent subjects receiving MMT (MM), 33 formerly heroin-dependent subjects withdrawn from MMT (MW), and 43 healthy controls were compared on the Millon Clinical Multiaxial Inventory-II (MCMI-II) and the Temperament and Character Inventory (TCI).

RESULTS

On the TCI, MM patients had higher novelty seeking and lower self-directedness scores than controls. Both MM and MW subjects scored higher than controls on multiple MCMI-II scales. MW but not MM subjects scored higher than controls on two Cluster A Scales and the delusional disorder scale.

CONCLUSION

Schizophrenia-spectrum pathology in former opiate users may be greater than previously recognized and could potentially be relevant to treatment selection.

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.

Early social and physical deprivation leads to reduced social motivation in adulthood in Wistar rats

Behavioural abnormalities in adulthood may have their origin in a disturbed interaction with the environment during postnatal development. We tested the consequences for adult social motivation of early deprivation (ED) of rat pups from mothers and littermates relative to nonhandled (NH) pups. Early deprivation was performed at room or warm ambient temperatures, cold-ED and warm-ED, respectively, and during either the dark or light phase of the daily cycle. In adulthood, rats that were unrelated and unfamiliar but of the same treatment group were introduced in pairs to an open field for a 30-min test. Social behaviour in home base and exploration modes was assessed using algorithmic analysis of the XY locations of the two rats. Findings revealed that Cold-ED induced a preference for a separate home base, which limited significantly the episodes of social interactions, in comparison to NH. Warm-ED had no comparable effect on the rats' social behaviour. These findings indicate that ED under ambient conditions that constitute severe thermal stress for rat pups leads to development of reduced social motivation in adulthood.

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.

Progressive ratio performance following challenge with antipsychotics, amphetamine, or NMDA antagonists in adult rats treated perinatally with phencyclidine

RATIONALE

Previous research has shown that rats exposed perinatally to phencyclidine (PCP) exhibited neuroanatomical abnormalities and altered cognition. In addition to cognitive deficits, schizophrenic patients may also exhibit negative symptoms such as lack of motivation.

OBJECTIVES

In this study, we used a progressive ratio (PR) schedule of food reinforcement to assess motivation following early exposure to PCP.

METHODS

Male rat pups were injected SC with 10 mg/kg PCP on postnatal days (PN) 7, 9, and 11. On PN 120, they began training in a PR 5 schedule of food reinforcement.

RESULTS

Significant PCP effects on acquisition and baseline performance were not noted. After acquisition of the task, challenges with PCP, dizocilpine, amphetamine, haloperidol, and clozapine resulted in dose-dependent decreases in response rates of similar magnitudes in both groups. In rats that continued to respond at higher doses, PCP, dizocilpine, and clozapine failed to alter breakpoints. In contrast, a 5.6 mg/kg dose of amphetamine selectively increased breakpoints in PCP-treated rats, although very few rats responded at this dose. Haloperidol decreased breakpoints in most rats at non-sedating doses.

CONCLUSIONS

These results suggest that a regimen of perinatal PCP administration sufficient to disrupt cognition did not alter motivation for food reinforcement, regardless of whether rats also received challenges with NMDA antagonists or antipsychotics. Interpretation of amphetamine's high dose effects on breakpoints was complicated by the failure of many rats to respond at this dose. Further research is needed to determine whether negative symptoms such as social withdrawal may be modeled within this neurodevelopmental approach to schizophrenia.

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.