Dynamics of behavioral sensitization induced by the dopamine agonist quinpirole and a proposed central energy control mechanism

The Neurobiological Underpinnings of Obsessive-Compulsive Symptoms in Psychosis, Translational Issues for Treatment-Resistant Schizophrenia

Almost 25% of schizophrenia patients suffer from obsessive-compulsive symptoms (OCS) considered a transdiagnostic clinical continuum. The presence of symptoms pertaining to both schizophrenia and obsessive-compulsive disorder (OCD) may complicate pharmacological treatment and could contribute to lack or poor response to the therapy. Despite the clinical relevance, no reviews have been recently published on the possible neurobiological underpinnings of this comorbidity, which is still unclear. An integrative view exploring this topic should take into account the following aspects: (i) the implication for glutamate, dopamine, and serotonin neurotransmission as demonstrated by genetic findings; (ii) the growing neuroimaging evidence of the common brain regions and dysfunctional circuits involved in both diseases; (iii) the pharmacological modulation of dopaminergic, serotoninergic, and glutamatergic systems as current therapeutic strategies in schizophrenia OCS; (iv) the recent discovery of midbrain dopamine neurons and dopamine D1- and D2-like receptors as orchestrating hubs in repetitive and psychotic behaviors; (v) the contribution of N-methyl-D-aspartate receptor subunits to both psychosis and OCD neurobiology. Finally, we discuss the potential role of the postsynaptic density as a structural and functional hub for multiple molecular signaling both in schizophrenia and OCD pathophysiology.

Acute administration of a dopamine D2/D3 receptor agonist alters behavioral and neural parameters in adult zebrafish

The dopaminergic neurotransmitter system is implicated in several brain functions and behavioral processes. Alterations in it are associated with the pathogenesis of several human neurological disorders. Pharmacological agents that interact with the dopaminergic system allow the investigation of dopamine-mediated cellular and molecular responses and may elucidate the biological bases of such disorders. Zebrafish, a translationally relevant biomedical research organism, has been successfully employed in prior psychopharmacology studies. Here, we evaluated the effects of quinpirole (dopamine D2/D3 receptor agonist) in adult zebrafish on behavioral parameters, brain-derived neurotrophic factor (BDNF) and neurotransmitter levels. Zebrafish received intraperitoneal injections of 0.5, 1.0, or 2.0 mg/kg quinpirole or saline (control group) twice with an inter-injection interval of 48 h. All tests were performed 24 h after the second injection. After this acute quinpirole administration, zebrafish exhibited decreased locomotor activity, increased anxiety-like behaviors and memory impairment. However, quinpirole did not affect social and aggressive behavior. Quinpirole-treated fish exhibited stereotypic swimming, characterized by repetitive behavior followed by immobile episodes. Moreover, quinpirole treatment also decreased the number of BDNF-immunoreactive cells in the zebrafish brain. Analysis of neurotransmitter levels demonstrated a significant increase in glutamate and a decrease in serotonin, while no alterations were observed in dopamine. These findings demonstrate that dopaminergic signaling altered by quinpirole administration results in significant behavioral and neuroplastic changes in the central nervous system of zebrafish. Thus, we conclude that the use of quinpirole administration in adult zebrafish may be an appropriate tool for the analysis of mechanisms underlying neurological disorders related to the dopaminergic system.

A virtual library for behavioral performance in standard conditions-rodent spontaneous activity in an open field during repeated testing and after treatment with drugs or brain lesions

Background

Beyond their specific experiment, video records of behavior have future value—for example, as inputs for new experiments or for yet unknown types of analysis of behavior—similar to tissue or blood sample banks in life sciences where clinically derived or otherwise well-described experimental samples are stored to be available for some unknown potential future purpose.

Findings

Research using an animal model of obsessive-compulsive disorder employed a standardized paradigm where the behavior of rats in a large open field was video recorded for 55 minutes on each test. From 43 experiments, there are 19,976 such trials that amount to over 2 years of continuous recording. In addition to videos, there are 2 video-derived raw data objects: XY locomotion coordinates and plots of animal trajectory. To motivate future use, the 3 raw data objects are annotated with a general schema—one that abstracts the data records from their particular experiment while providing, at the same time, a detailed list of independent variables bearing on behavioral performance. The raw data objects are deposited as 43 datasets but constitute, functionally, a library containing 1 large dataset.

Conclusions

Size and annotation schema give the library high reuse potential: in applications using machine learning techniques, statistical evaluation of subtle factors, simulation of new experiments, or as educational resource. Ultimately, the library can serve both as the seed and as the test bed to create a machine-searchable virtual library of linked open datasets for behavioral performance in defined conditions.

Long-lasting behavioral effects of quinpirole exposure on zebrafish

The human brain matures into a complex structure, and to reach its complete development, connections must occur along exact paths. If at any stage, the processes are altered, interrupted, or inhibited, the consequences can be permanent. Dopaminergic signaling participates in the control of physiological functions and behavioral processes, and alterations in this signaling pathway are related to the pathogenesis of several neurological disorders. For this reason, the use of pharmacological agents able to interact with the dopaminergic signaling may elucidate the biological bases of such disorders. We investigated the long-lasting behavioral effects on adult zebrafish after quinpirole (a dopamine D2/D3 receptor agonist) exposure during early life stages of development (24 h exposure at 5 days post-fertilization, dpf) to better understand the mechanisms underlying neurological disorders related to the dopaminergic system. Quinpirole exposure at the early life stages of zebrafish led to late behavioral alterations. When evaluated at 120 dpf, zebrafish presented increased anxiety-like behaviors. At the open tank test, fish remained longer at the bottom of the tank, indicating anxiety-like behavior. Furthermore, quinpirole-treated fish exhibited increased absolute turn angle, likely an indication of elevated erratic movements and a sign of increased fear or anxiety. Quinpirole-treated fish also showed altered swimming patterns, characterized by stereotypic swimming. During the open tank test, exposed zebrafish swims from corner to corner in a repetitive manner at the bottom of the tank. Moreover, quinpirole exposure led to memory impairment compared to control fish. However, quinpirole administration had no effects on social and aggressive behavior. These findings demonstrate that dopaminergic signaling altered by quinpirole administration in the early life stages of development led to late alterations in behavioral parameters of adult zebrafish.

Social spatial cognition: social distance dynamics as an identifier of social interactions

We suggest that socio-spatial behavior, which is an interaction between social and spatial cognition, can be viewed as a set of excursions that originate and end in close proximity to another individual(s). We present an extension of earlier studies that perceived spatial behavior in individual animals as a series of excursions originating from a particular location. We measured here the momentary distance between two individuals (social distance) to differentiate among eight possible types of social excursion originating in a state of proximity between excursion-participants. The defined excursion types are based on whether or not the excursion initiator also concludes the excursion, whether or not the excursion starts and ends at the same location, and the dynamics of the distance between excursion participants. We validated this approach to socio-spatial behavior as a set of excursions using it to analyze the behavior of the two sexes in rodents, of normal vs. stereotyped rats, as well as of different rodent species. Each of these groups displays a prevalent excursion type that reflects a distinct social dynamics. Our approach offers a useful and comprehensive tool for studying socio-spatial cognition, and can also be applied to distinguish among different social situations in rodents and other animals.

Crosstalk Between Kappa Opioid and Dopamine Systems in Compulsive Behaviors

The strength of goal-oriented behaviors is regulated by midbrain dopamine neurons. Dysfunctions of dopaminergic circuits are observed in drug addiction and obsessive-compulsive disorder. Compulsive behavior is a feature that both disorders share, which is associated to a heightened dopamine neurotransmission. The activity of midbrain dopamine neurons is principally regulated by the homeostatic action of dopamine through D2 receptors (D2R) that decrease the firing of neurons as well as dopamine synthesis and release. Dopamine transmission is also regulated by heterologous neurotransmitter systems such as the kappa opioid system, among others. Much of our current knowledge of the kappa opioid system and its influence on dopamine transmission comes from preclinical animal models of brain diseases. In 1988, using cerebral microdialysis, it was shown that the acute activation of the Kappa Opioid Receptors (KOR) decreases synaptic levels of dopamine in the striatum. This inhibitory effect of KOR opposes to the facilitating influence of drugs of abuse on dopamine release, leading to the proposition of the use of KOR agonists as pharmacological therapy for compulsive drug intake. Surprisingly, 30 years later, KOR antagonists are instead proposed to treat drug addiction. What may have happened during these years that generated this drastic change of paradigm? The collected evidence suggested that the effect of KOR on synaptic dopamine levels is complex, depending on the frequency of KOR activation and timing with other incoming stimuli to dopamine neurons, as well as sex and species differences. Conversely to its acute effect, chronic KOR activation seems to facilitate dopamine neurotransmission and dopamine-mediated behaviors. The opposing actions exerted by acute versus chronic KOR activation have been associated with an initial aversive and a delayed rewarding effect, during the exposure to drugs of abuse. Compulsive behaviors induced by repeated activation of D2R are also potentiated by the sustained co-activation of KOR, which correlates with decreased synaptic levels of dopamine and sensitized D2R. Thus, the time-dependent activation of KOR impacts directly on dopamine levels affecting the tuning of motivated behaviors. This review analyzes the contribution of the kappa opioid system to the dopaminergic correlates of compulsive behaviors.

Changes in gut microbiota during development of compulsive checking and locomotor sensitization induced by chronic treatment with the dopamine agonist quinpirole

Long-term treatment of rats with the D2/D3 dopamine agonist quinpirole induces compulsive checking (proposed as animal model of obsessive-compulsive disorder) and locomotor sensitization. The mechanisms by which long-term use of quinpirole produces those behavioral transformations are not known. Here we examined whether changes in gut microbiota play a role in these behavioral phenomena, by monitoring the development of compulsive checking and locomotor sensitization at the same time as measuring the response of gut microbiota to chronic quinpirole injections. Two groups of rats received nine injections of saline (n=16) or quinpirole (n=15; 0.25 mg/kg), at weekly intervals for the first 5 weeks and then two injections per week until the end of treatment. After each injection, rats were placed on a large open field for 55 min, and their behavior was video recorded for subsequent analysis. Fecal matter was collected after each trial and frozen for bacterial community profiling of the 16S rRNA gene, using paired-end reads of the V3 region. The results indicated that the induction of locomotor sensitization and compulsive checking was accompanied by changes in several communities of bacteria belonging to the order Clostridiales (class Clostridia, phylum Firmicutes), and predominantly in Lachnospiraceae and Ruminococcaceae families of bacteria. It is suggested that changes in these microbes may serve to support the energy use requirements of compulsive checking and obsessive-compulsive disorder.

An Adenosine A2A Receptor Antagonist Improves Multiple Symptoms of Repeated Quinpirole-Induced Psychosis

Obsessive-compulsive disorder (OCD) is a neuropsychiatric disorder characterized by the repeated rise of concerns (obsessions) and repetitive unwanted behavior (compulsions). Although selective serotonin reuptake inhibitors (SSRIs) is the first-choice drug, response rates to SSRI treatment vary between symptom dimensions. In this study, to find a therapeutic target for SSRI-resilient OCD symptoms, we evaluated treatment responses of quinpirole (QNP) sensitization-induced OCD-related behaviors in mice. SSRI administration rescued the cognitive inflexibility, as well as hyperactivity in the lateral orbitofrontal cortex (lOFC), while no improvement was observed for the repetitive behavior. D₂ receptor signaling in the central striatum (CS) was involved in SSRI-resistant repetitive behavior. An adenosine A_2A antagonist, istradefylline, which rescued abnormal excitatory synaptic function in the CS indirect pathway medium spiny neurons (MSNs) of sensitized mice, alleviated both of the QNP-induced abnormal behaviors with only short-term administration. These results provide a new insight into therapeutic strategies for SSRI-resistant OCD symptoms and indicate the potential of A_2A antagonists as a rapid-acting anti-OCD drug.

A dose-response study of separate and combined effects of the serotonin agonist 8-OH-DPAT and the dopamine agonist quinpirole on locomotor sensitization, cross-sensitization, and conditioned activity

Chronic treatment with the dopamine D2/D3 agonist, quinpirole, or the serotonin 1A agonist, 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), induces behavioral sensitization. It is not known whether both drugs produce sensitization through a shared mechanism. Here, we examine whether quinpirole and 8-OH-DPAT show cross-sensitization and impact sensitization, as would be expected from shared mechanisms. Male rats (N=208) were assigned randomly to 16 groups formed by crossing four doses of quinpirole (0, 0.03125, 0.0625, or 0.125 mg/kg) with four doses of 8-OH-DPAT (0, 0.03125, 0.625, or 0.125 mg/kg). After a course of 10 drug treatments administered twice per week in locomotor activity chambers, all groups were challenged on separate tests with quinpirole (0.1 mg/kg), 8-OH-DPAT (0.1 mg/kg), or saline, and locomotor activity was evaluated. Challenge tests with quinpirole and 8-OHDPAT showed no cross-sensitization between the drugs. Chronic quinpirole (0.125 mg/kg) administration induced a sensitized quinpirole response that was attenuated dose-dependently by chronic 8-OH-DPAT cotreatment. Cotreatment with quinpirole (0.0625 mg/kg) and 8-OH-DPAT (all doses) induced quinpirole sensitization. Chronic 8-OH-DPAT (0.125 mg/kg) induced a sensitized 8-OHDPAT response that was prevented by chronic cotreatment with the lowest but not the highest dose of quinpirole. Cotreatment with 8-OHDPAT (0.0625) and quinpirole (0.125 mg/kg) induced sensitization to 8-OH-DPAT. The saline challenge test showed elevated locomotor activity in chronic quinpirole (0.125 mg/kg) and 8-OHDPAT (0.0625, 0.125 mg/kg) alone groups, and in seven of nine cotreated groups. The absence of cross-sensitization suggests separate mechanisms of sensitization to quinpirole and 8-OH-DPAT. Cotreatment effects suggest that induction of sensitization can be modulated by serotonin 1A and D2/D3 activity.

Obsessive-compulsive disorder: Insights from animal models

Research with animal models of obsessive-compulsive disorder (OCD) shows the following: (1) Optogenetic studies in mice provide evidence for a plausible cause-effect relation between increased activity in cortico-basal ganglia-thalamo-cortical (CBGTC) circuits and OCD by demonstrating the induction of compulsive behavior with the experimental manipulation of the CBGTC circuit. (2) Parallel use of several animal models is a fruitful paradigm to examine the mechanisms of treatment effects of deep brain stimulation in distinct OCD endophenotypes. (3) Features of spontaneous behavior in deer mice constitute a rich platform to investigate the neurobiology of OCD, social ramifications of a compulsive phenotype, and test novel drugs. (4) Studies in animal models for psychiatric disorders comorbid with OCD suggest comorbidity may involve shared neural circuits controlling expression of compulsive behavior. (5) Analysis of compulsive behavior into its constitutive components provides evidence from an animal model for a motivational perspective on OCD. (6) Methods of behavioral analysis in an animal model translate to dissection of compulsive rituals in OCD patients, leading to diagnostic tests.

Validity of Quinpirole Sensitization Rat Model of OCD: Linking Evidence from Animal and Clinical Studies

Obsessive-compulsive disorder (OCD) is a neuropsychiatric disorder with 1–3% prevalence. OCD is characterized by recurrent thoughts (obsessions) and repetitive behaviors (compulsions). The pathophysiology of OCD remains unclear, stressing the importance of pre-clinical studies. The aim of this article is to critically review a proposed animal model of OCD that is characterized by the induction of compulsive checking and behavioral sensitization to the D2/D3 dopamine agonist quinpirole. Changes in this model have been reported at the level of brain structures, neurotransmitter systems and other neurophysiological aspects. In this review, we consider these alterations in relation to the clinical manifestations in OCD, with the aim to discuss and evaluate axes of validity of this model. Our analysis shows that some axes of validity of quinpirole sensitization model (QSM) are strongly supported by clinical findings, such as behavioral phenomenology or roles of brain structures. Evidence on predictive validity is contradictory and ambiguous. It is concluded that this model is useful in the context of searching for the underlying pathophysiological basis of the disorder because of the relatively strong biological similarities with OCD.

Is it time to turn our attention toward central mechanisms for post-exertional recovery strategies and performance?

Key Points

Central fatigue is accepted as a contributor to overall athletic performance, yet little research directly investigates post-exercise recovery strategies targeting the brain

Current post-exercise recovery strategies likely impact on the brain through a range of mechanisms, but improvements to these strategies is needed

Research is required to optimize post-exercise recovery with a focus on the brain

Post-exercise recovery has largely focused on peripheral mechanisms of fatigue, but there is growing acceptance that fatigue is also contributed to through central mechanisms which demands that attention should be paid to optimizing recovery of the brain. In this narrative review we assemble evidence for the role that many currently utilized recovery strategies may have on the brain, as well as potential mechanisms for their action. The review provides discussion of how common nutritional strategies as well as physical modalities and methods to reduce mental fatigue are likely to interact with the brain, and offer an opportunity for subsequent improved performance. We aim to highlight the fact that many recovery strategies have been designed with the periphery in mind, and that refinement of current methods are likely to provide improvements in minimizing brain fatigue. Whilst we offer a number of recommendations, it is evident that there are many opportunities for improving the research, and practical guidelines in this area.

Nucleus accumbens core and pathogenesis of compulsive checking

To investigate the role of the nucleus accumbens core (NAc) in the development of quinpirole-induced compulsive checking, rats received an excitotoxic lesion of NAc or sham lesion and were injected with quinpirole (0.5 mg/kg) or saline; development of checking behavior was monitored for 10 biweekly tests. The results showed that even after the NAc lesion, quinpirole still induced compulsive checking, suggesting that the pathogenic effects produced by quinpirole lie outside the NAc. Although the NAc lesion did not prevent the induction of compulsive checking, it altered how quickly it develops, suggesting that the NAc normally contributes toward the induction of compulsive checking. Saline-treated rats with an NAc lesion were hyperactive, but did not develop compulsive checking, indicating that hyperactivity by itself is not sufficient for the pathogenesis of compulsive checking. It is proposed that compulsive checking is the exaggerated output of a security motivation system and that the NAc serves as a neural hub for coordinating the orderly activity of neural modules of this motivational system. Evidence is considered suggesting that the neurobiological condition for the pathogenesis of compulsive checking is two-fold: activation of dopamine D2/D3 receptors without concurrent stimulation of D1-like receptors and long-term plastic changes related to quinpirole-induced sensitization.

Separate mechanisms for development and performance of compulsive checking in the quinpirole sensitization rat model of obsessive-compulsive disorder (OCD)

RATIONALE

Acute administration of serotonergic agonist, meta-chlorophenylpiperazine (mCPP), attenuates performance of compulsive checking in an animal model of obsessive-compulsive disorder (OCD). It is not known whether mCPP has a similar effect on development of compulsive checking.

OBJECTIVES

The objective of the study was to examine whether similar mechanisms mediate the development versus the performance of compulsive checking in the rat model.

METHODS

Four groups of male rats (N = 14/group) were tested: two experimental groups co-injected with D2/D3 dopamine agonist quinpirole (0.25 mg/kg) and mCPP (0.625 mg/kg or 1.25 mg/kg), and two control groups, one co-injected with quinpirole and saline, the other receiving injections of saline. The time course of development of compulsive checking across injections 1 to 10 in quinpirole-treated rats was compared to rats co-injected with quinpirole and mCPP.

RESULTS

Results showed that during the course of chronic treatment, mCPP (1.25 mg/kg) significantly attenuated performance of checking behavior. However, when these rats were retested for expression of compulsive checking (that is, with an injection of quinpirole only), their profile of compulsive checking was no different from the control rats treated throughout with quinpirole only.

CONCLUSIONS

Findings show that mCPP inhibits performance of compulsive checking but does not block quinpirole from inducing the neural substrate underlying this compulsive behavior. Hence, a separate mechanism underlies the induction of compulsive checking and the performance of it. It is suggested that development of the OCD endophenotype reflects neuroplastic changes produced by repeated dopamine D2/D3 receptor stimulation, while stimulation of serotonergic receptors mediates a negative feedback signal that shuts down the motor performance of checking.

Performance of compulsive behavior in rats is not a unitary phenomenon - validation of separate functional components in compulsive checking behavior

A previous analysis of the quinpirole sensitisation rat model of obsessive-compulsive disorder revealed that the behavioral phenotype of compulsive checking consists of three constitutive components – vigor of checking performance, focus on the task of checking, and satiety following a bout of checking. As confirmation of this analysis, the aim of the present study was to reconstitute, without quinpirole treatment, each of the putative components, with the expectation that these would self-assemble into compulsive checking. To reconstitute vigor and satiety, the employed treatment was a bilateral lesion of the nucleus accumbens core (NAc), as this treatment was shown previously to exaggerate these components. To reconstitute focus, the employed treatment was a low dose of the serotonin-1A receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin hydrochloride (DPAT) (0.0625 mg/kg), as high doses of this drug induce compulsive behavior and exacerbate focus. Results showed that injection of DPAT to NAc lesion rats did yield compulsive checking. Neither the drug alone nor the NAc lesion by itself produced compulsive checking. The demonstrated synthesis of compulsive checking by the combined treatment of low-dose DPAT and NAc lesion strengthened the previous fractionation of the model obsessive-compulsive disorder phenotype into three constitutive components, and suggested a role for serotonin-1A receptors outside the NAc in enhanced focus on the task of checking.

Effects of dopamine D2 agonist quinpirole on neuronal activity of anterior cingulate cortex and striatum in rats

RATIONALE

The influence of acute D2 agonist quinpirole on locomotor activity has been effectively characterized. However, few studies have addressed the dynamic changes in neuronal activity of the anterior cingulate cortex (ACC) and striatum (STR), two crucial regions for cognitive and motor functions, after quinpirole administration.

OBJECTIVE

This study was conducted in order to acquire detailed information on the evoked activity of the neurons in the ACC and STR after acute quinpirole administration.

METHODS

Multichannel electrophysiological recording was used for tracking neuronal activity in the ACC and STR of urethane-anesthetized rats after administration of saline or 0.05 or 0.5 mg/kg quinpirole.

RESULTS

In contrast to the responses to saline, quinpirole dose-dependently increased the ratio of neurons, the activity of which was inhibited in the ACC and STR. By examining the ensemble neuronal activities of inhibition-responded neurons, there was no significant activity difference among the "treatments" (saline and low- and high-dose quinpirole), the "periods" (the duration of 0-15 and 16-45 min after i.v. injection), and the interaction between "treatments" and "periods." Regarding activation-responded neurons, however, there was a significant "periods" difference in both ACC and STR, and the activity of 16-45 min was significantly higher than the activity of 0-15 min after high-dose quinpirole administration in ACC (p < 0.05) and STR (p < 0.001).

CONCLUSION

Dose-dependent ACC and STR neuronal responses to quinpirole may offer a possible mechanism for understanding the locomotor responses to quinpirole in behaving rats. The late excitatory effect of high-dose quinpirole in the STR further suggests that this region would be critical for the activation of locomotor activity.

Cabergoline, a dopamine receptor agonist, has an antidepressant-like property and enhances brain-derived neurotrophic factor signaling

RATIONALE

Dopamine agonists have been implicated in the treatment of depression. Cabergoline is an ergot derivative with a high affinity to dopamine D(2)-like receptors; however, there have been few preclinical studies on its antidepressant-like effects.

MATERIALS AND METHODS

Behavioral effects of cabergoline were examined in rats using forced swimming (FST), novelty-suppressed feeding (NST), open field (OFT), and elevated-plus maze (EPT) tests. In a single treatment paradigm, behaviors of rats were analyzed 4 h after single injection of cabergoline (s.c., 0-4 micromol/kg). In a repeated-treatment paradigm, OFT, EPT, and FST were conducted on days 11, 12, and 13-14, respectively, during daily cabergoline injections (s.c., 0.5 micromol/kg), and then hippocampus was removed 24 h after the last injection. NST was conducted in a separate experiment at day 14. Western blotting was used for the analysis of the protein levels of brain-derived neurotrophic factor (BDNF) and the activation of intracellular signaling molecules.

RESULTS

Single injection of cabergoline demonstrated decreased immobility in FST and distance traveled during 0-10 min in OFT, while time spent and entry into open arms were increased at 4 micromol/kg. When cabergoline was repeatedly administered, immobility in FST and the latency of feeding in NSF were significantly reduced, while vertical movement was increased in OFT. The time in closed arms was tended to be decreased in EPT. Expression of BDNF and activation of extracellular signal-regulated kinase 1 were up-regulated after the chronic administration of cabergoline.

CONCLUSIONS

Cabergoline exerts antidepressant- and anxiolytic-like effects, which may be mediated by potentiation of intracellular signaling of BDNF.

High-frequency stimulation of the nucleus accumbens core and shell reduces quinpirole-induced compulsive checking in rats

Electrical deep brain stimulation (DBS) is currently studied in the treatment of therapy-refractory obsessive compulsive disorders (OCDs). The variety of targeted brain areas and the inconsistency in demonstrating anti-compulsive effects, however, highlight the need for better mapping of brain regions in which stimulation may produce beneficial effects in OCD. Such a goal may be advanced by the assessment of DBS in appropriate animal models of OCD. Currently available data on DBS of the nucleus accumbens (NAc) on OCD-like behavior in rat models of OCD are contradictory and partly in contrast to clinical data and theoretical hypotheses about how the NAc might be pathophysiologically involved in the manifestation of OCD. Consequently, the present study investigates the effects of DBS of the NAc core and shell in a quinpirole rat model of OCD. The study demonstrates that electrical modulation of NAc core and shell activity via DBS reduces quinpirole-induced compulsive checking behavior in rats. We therefore conclude that both, the NAc core and shell constitute potential target structures in the treatment of OCD.

Effects of salvinorin A on locomotor sensitization to D2/D3 dopamine agonist quinpirole

Locomotor sensitization induced by the dopamine agonist quinpirole can be potentiated by co-treatment with the synthetic kappa opioid agonist U69593. The identification of salvinorin A, an active component of the psychotropic sage Salvia divinorum, as a structurally different agonist of kappa-opioid receptors raised the question of whether this compound would similarly potentiate sensitization to quinpirole. Rats were co-treated with 0.5 mg/kg quinpirole and either salvinorin A (0.04, 0.4 or 2.0 mg/kg) or U69593 (0.3 mg/kg). Control groups were co-treated with vehicle and saline, vehicle and quinpirole (0.5 mg/kg), or saline and salvinorin A (0.4 mg/kg). Rats were injected biweekly for a total of 10 injections and locomotor activity measured after each treatment. Results showed that the highest dose of salvinorin A potentiated sensitization to quinpirole as did U69593, the middle salvinorin A dose had no effect on quinpirole sensitization, and the lowest dose of salvinorin A attenuated sensitization to quinpirole. These findings indicate that structural differences between salvinorin A and U69593 do not affect the potentiation of quinpirole sensitization. Moreover, the opposite effects of high and low salvinorin A doses suggest that salvinorin A can produce bidirectional modulation of sensitization to dopamine agonists.

Mental fatigue: costs and benefits

A framework for mental fatigue is proposed, that involves an integrated evaluation of both expected rewards and energetical costs associated with continued performance. Adequate evaluation of predicted rewards and potential risks of actions is essential for successful adaptive behaviour. However, while both rewards and punishments can motivate to engage in activities, both types of motivated behaviour are associated with energetical costs. We will review findings that suggest that the nucleus accumbens, orbitofrontal cortex, amygdala, insula and anterior cingulate cortex are involved evaluating both the potential rewards associated with performing a task, as well as assessing the energetical demands involved in task performance. Behaviour will only proceed if this evaluation turns out favourably towards spending (additional) energy. We propose that this evaluation of predicted rewards and energetical costs is central to the phenomenon of mental fatigue: people will no longer be motivated to engage in task performance when energetical costs are perceived to outweigh predicted rewards.

The psychobiology of burnout: are there two different syndromes?

BACKGROUND

Plasma prolactin levels are sensitive to dopamine and serotonin function, and fatigue. Low cortisol, dopamine and/or serotonin may be involved in burnout and detachment.

METHODS

In this double-blind within-subject study, we treated 9 female burnout subjects and 9 controls with 35 mg cortisol and placebo orally. We measured state affect and plasma prolactin, oxytocin, cortisol and adrenocorticotropic hormone levels, and administered an attachment questionnaire.

RESULTS

The burnout subjects displayed an extreme distribution of basal prolactin levels, displaying higher or lower levels compared to the controls. The low prolactin burnouts had profoundly low attachment scores and tended to have low oxytocin levels. The high prolactin burnout subjects tended to show cortisol-induced decreased prolactin and fatigue, and increased vigor.

CONCLUSION

Results are consistent with the hypothesis that burnout subjects are either characterized by low serotonergic function or by low dopaminergic function, and that the latter group benefits from cortisol replacement. These preliminary results suggest that differentiating between two syndromes may resolve inconsistencies in research on burnout, and be necessary for selecting the right treatment strategy.

High frequency stimulation and temporary inactivation of the subthalamic nucleus reduce quinpirole-induced compulsive checking behavior in rats

Obsessive-compulsive disorder (OCD) represents a highly prevalent and impairing psychiatric disorder. Functional and structural imaging studies implicate the involvement of basal ganglia-thalamo-cortical circuits in the pathophysiology of this disorder. In patients remaining resistant to pharmaco- and behavioral therapy, modulation of these circuits may consequently reverse clinical symptoms. High frequency stimulation (HFS) of the subthalamic nucleus (STN), an important station of the basal ganglia-thalamo-cortical circuits, has been reported to reduce obsessive-compulsive symptoms in a few Parkinson's disease patients with comorbid OCD. The present study tested the effects of bilateral HFS of the STN and of bilateral pharmacological inactivation of the STN (via intracranial administration of the GABA agonist muscimol) on checking behavior in the quinpirole rat model of OCD. We demonstrate that both HFS and pharmacological inactivation of the STN reduce quinpirole-induced compulsive checking behavior. We conclude that functional inhibition of the STN can alleviate compulsive checking, and suggest the STN as a potential target structure for HFS in the treatment of OCD.

Cotreatment with the kappa opioid agonist U69593 enhances locomotor sensitization to the D2/D3 dopamine agonist quinpirole and alters dopamine D2 receptor and prodynorphin mRNA expression in rats

RATIONALE

The repeated coadministration of the kappa opioid receptor agonist U69593 with the D2/D3 dopamine (DA) agonist quinpirole (QNP) potentiates locomotor sensitization induced by QNP. Behavioral evidence has implicated both pre- and postsynaptic changes as being involved in this augmentation.

OBJECTIVES

The objectives of this study were to obtain supporting molecular evidence of pre- and/or postsynaptic alterations in the DA system with U69593/QNP cotreatment and to examine the relationship of such changes to locomotor sensitization.

MATERIALS AND METHODS

Gene expression of D1 and D2 receptors (D1R and D2R), the DA transporter, as well as the endogenous opioid prodynorphin (DYN), in the basal ganglia was examined by in situ hybridization in rats after one or ten drug injections.

RESULTS

After one injection, changes that were specific to U69593/QNP cotreatment were decreased D1R and D2R messenger RNA (mRNA) in the nucleus accumbens (Acb) shell and increased DYN mRNA in the dorsal striatum (STR). After ten injections, U69593/QNP-specific changes were decreased D2R mRNA in substantia nigra (SN) and increased DYN mRNA in STR and Acb core. Only in U69593/QNP rats was the sensitized locomotor performance on injection ten positively correlated with DYN mRNA levels in Acb and STR.

CONCLUSIONS

Distinct alterations of D2R and DYN mRNA levels in SN and Acb/STR, respectively, strengthen the evidence implicating pre- and postsynaptic changes in augmented locomotor sensitization to U69593/QNP cotreatment. It is suggested that repeated U69593/QNP cotreatment may augment locomotor sensitization to QNP by activating D1R-expressing DYN neurons and attenuating presynaptic D2R function.

Neonatal immune system activation with lipopolysaccharide enhances behavioural sensitization to the dopamine agonist, quinpirole, in adult female but not male rats

Administration of the bacterial cell wall component, lipopolysaccharide (LPS), stimulates the immune and endocrine systems inducing an acute phase of sickness and stress responses in adult and neonatal rats. Neonatal LPS exposure has been shown to alter a variety of behavioural and physiological processes in the adult animal. Early developmental stress, such as maternal separation, causes similar acute as well as long-term behavioural changes in adults, including altered sensitivity to drugs of abuse. Moreover, results of studies have shown evidence of a direct link between immune activation and sensitivity to dopamine-based drugs of abuse. The current study examined the effects of neonatal LPS treatment on subsequent locomotor sensitization to the dopamine (D(2)/D(3)) agonist, quinpirole, in adult rats as an index of drug sensitivity. Male and female Long-Evans rats were treated systemically with either LPS (50microg/kg) or saline (0.9%) on postnatal days 3 and 5. Locomotor sensitization was then examined in the adult rats (postnatal day 70). Animals were injected with quinpirole (0.5mg/kg, s.c.) or saline every other day for a total of 10 injections and locomotor activity was assessed for 60min immediately following injections 1, 2, 4, 6, and 10. Animals also received a 'challenge' injection of 0.5mg/kg quinpirole 28 days after injection 10, to assess persistence of behavioural sensitization. Locomotor activity progressively increased with repeated administration of quinpirole, indicating locomotor sensitization in all of the drug-treated groups. There was an overall sex difference, with females showing significantly greater sensitization than males. Moreover, neonatal LPS treatment potentiated both the level and the rate of development of locomotor sensitization to quinpirole administration in females, but not in males. Thus, the current study revealed that neonatal exposure to bacterial infection increases dopamine (D(2)/D(3)) agonist sensitivity in a sex-specific manner. These findings have important implications for the sexually dimorphic development of addictions to both natural and artificial rewards.

Continuous dopaminergic stimulation: Is it the answer to the motor complications of Levodopa?

Continuous dopaminergic stimulation (CDS) is a treatment strategy hypothesized to avoid or reduce the motor complications of long-term levodopa therapy, motor fluctuations, and dyskinesia, by preventing or reversing sensitization induced by pulsatile dopaminergic stimulation. The CDS hypothesis is itself based on several hypotheses. First, tonic dopaminergic stimulation is physiological. Second, sensitization is undesirable and should be reversed. Third, reduction of off time and dyskinesia can be induced simultaneously. Finally, clinical studies substantiate the CDS hypothesis. The evidence for these hypotheses is reviewed, and the need for randomized clinical trials that rigorously test the CDS hypothesis is emphasized.

Modulation of quinpirole-induced compulsive-like behavior in rats by environmental changes: implications for OCD rituals and for exploration and navigation

Background

Rats treated chronically with the D_2–3 dopamine agonist quinpirole were previously proposed as an animal model of obsessive-compulsive disorder (OCD) since their behavior is based on repeated, compulsive-like persistent traveling between a few places in the open field. The aim of the present study was to determine properties of the physical environment that shape such behavior. For this, quinpirole-treated rats were first exposed to an arena with an array of objects (landmarks) and after the development of compulsive-like behavior, the arena was manipulated by multiplying the number of objects, changing their spacing, relocating object array, or removing the objects.

Results

When the number of objects was retained but they were spaced further apart, rat routes converged at 1–2 of the objects and at the corner at which the rats had been introduced into the arena (start corner). When object spacing was retained but their number was increased, the rats traveled between the objects with the routes converging only at the start corner. Finally, when object array was relocated to different places within the arena, the rats extended their routes from the start corner to the object array, regardless of array location.

Conclusion

Quinpirole-treated rats organized and updated their progression primarily according to the proximal layout of landmarks, but did so with excessive repetitions compared with saline-treated rats. The behavior of quinpirole-treated rats paralleled human OCD rituals that are linked to the immediate physical environment, featuring an excessive rate of performance. Finally, when only a few objects were present, they were perceived by the rats as positional cues (beacons) that routes converged at them. In contrast, in the presence of many objects, the routes passed between the objects as if using them as directional cues.

Differential modulation of a 40 kDa catecholamine regulated protein in the core and shell subcompartments of the nucleus accumbens following chronic quinpirole and haloperidol administration in the rat

Past reports have shown dopamine (DA) D2/D3 receptor agonist quinpirole (QNP) and the DA D2 receptor antagonist, haloperidol (HAL) display a significant increase in expression of catecholamine regulated protein (CRP40) in the nucleus accumbens (NAcc) and the striatum, respectively. The present study investigated the in vivo effects of QNP and HAL on CRP40 protein levels within the core and shell subcompartments of the NAcc. As significant homology exists between CRP40 and Hsp70/Hsc70, parallel studies with inducible Hsp70 and constitutive Hsc70 were conducted to establish the specificity with respect to QNP on Hsp70 and CRP40. Results demonstrated that CRP40 protein was significantly expressed in the shell relative to the core region of NAcc following chronic QNP (+16.28%+/-0.42%, P<0.05) and CRP40 protein was significantly expressed in the core vs. the shell following chronic HAL (+36.02%+/-0.75%, P<0.05). There was no significant change in Hsp70 protein levels following chronic QNP or HAL administration. The results demonstrated selective modulation of CRP40 within NAcc by QNP and HAL treatment, without affecting Hsp70.

Kappa-opioid agonist U69593 potentiates locomotor sensitization to the D2/D3 agonist quinpirole: pre- and postsynaptic mechanisms

To assess whether the development and expression of behavioral sensitization to the dopamine D2/D3 agonist quinpirole (QNP) is influenced by coadministration of the kappa opioid receptor agonist U69593, rats received every 3-4 days for a total of 10 treatments an injection of U69593 (0.3 mg/kg) together with an injection of either a postsynaptic (0.5 mg/kg) or a presynaptic dose of QNP (0.05 mg/kg); locomotor activity was measured after each treatment. Control rats were injected as appropriate with QNP, U69593, and vehicle/saline. Following chronic treatment, dose-response profiles to QNP were obtained to assess the expression of sensitization; the effect of U69593 on locomotor activity in animals already sensitized to QNP was also assessed. Results showed that cotreatment of U69593 with a postsynaptic dose of QNP doubled the speed and magnitude of sensitization to QNP, while U69593 cotreatment with a presynaptic dose of QNP switched the effects of QNP from locomotor depression to locomotor sensitization. However, U69593 cotreatment with a presynaptic dose of QNP changed a different set of measures of sensitization than did cotreatment with a postsynaptic dose of the dopamine agonist. Together, findings suggest that sensitization to QNP is not a unitary phenomenon but has components that are relatively independent, mediated by distinct pre- and postsynaptic mechanisms and modulated by kappa receptor activity.

Differential effects of clorgyline on sensitization to quinpirole in rats tested in small and large environments

RATIONALE

Cotreatment with clorgyline shifts the development of sensitization to the D2/D3 dopamine receptor agonist quinpirole from locomotion to mouthing, an effect apparently unrelated to the monoamine oxidase inhibition property of clorgyline. This phenomenon was demonstrated in rats examined in small activity chambers. However, like with other psychostimulant drugs, sensitization to quinpirole is modulated by environmental context. It is not known whether the clorgyline cotreatment effect is likewise influenced by the environment.

OBJECTIVE

To determine the generality of the clorgyline effect on behavioral sensitization by evaluating the effects of clorgyline cotreatment on sensitization to quinpirole in two different environments: a small activity chamber and a large open field.

METHODS

Male rats received eight injections of quinpirole (0.5 mg/kg, twice weekly) in an open field or activity chamber; one group in each environment received a constant infusion of clorgyline (1 mg/kg/day via osmotic minipumps) while the other group served as the sham surgery control. For quinpirole injection 7 or 8, rats were tested in the alternate environment.

RESULTS

In activity chambers, clorgyline cotreatment switched sensitization to quinpirole from locomotion to mouthing. In the open field, clorgyline cotreatment increased mouthing and expanded the explored space without a change in path stereotypy or the amount of locomotion compared to treatment with quinpirole alone.

CONCLUSIONS

Structure of the environment can modulate the clorgyline cotreatment effect on behavioral sensitization to quinpirole. The behavioral profiles produced by clorgyline cotreatment in the two environments resembled the behavioral effects observed with quinpirole and D1 agonist cotreatment. It is suggested that clorgyline cotreatment produces a behavioral profile characteristic of enhanced dopamine D1 and D2 receptor costimulation.

Quinpirole-induced behavioral sensitization is enhanced by prior scheduled exposure to sucrose: A multi-variable examination of locomotor activity

Sensitization of dopaminergic neural reward circuits has been hypothesized to be involved in the development of drug addiction. Highly palatable foods activate these same brain areas, specifically the nucleus accumbens. In this study, the effects of a highly palatable food (sucrose) on these circuits were investigated using the dopamine D(2)/D(3) receptor agonist quinpirole. Male Long-Evans rats received 30 min daily access to 0.3 M sucrose solution or water over nine consecutive days, followed by nine daily injections of quinpirole (0.5 mg/kg, s.c.) or saline. Locomotor activity was assessed using an automated open-field system. Locomotor sensitization developed, as quinpirole-treated rats traveled significantly more, and exhibited a greater number of movements than saline controls. A characteristic pattern of an initial suppression of locomotor activity, followed by excitation of activity was observed in quinpirole-treated rats. Pre-exposure to sucrose attenuated the initial suppression of activity, and facilitated excitation of activity. Rats that were pre-exposed to sucrose exhibited a reduced suppression of activity as compared to rats pre-exposed to water. Rats receiving sucrose and quinpirole also displayed a significantly greater enhancement of locomotor activity as compared to rats receiving water and quinpirole. These results support the hypothesis that highly palatable foods can alter the same neural reward circuits as drugs of abuse, and may facilitate sensitization-related addiction. This may aid in further understanding the neural basis of eating disorders.

Rituals, stereotypy and compulsive behavior in animals and humans

From a survey of the behavior of animals in the wild, in captivity, under the influence of psychoactive drugs and in a model of obsessive-compulsive disorder (OCD), we identify that the behavioral repertoire invariably includes motor rituals, and that such rituals are performed at a few specific locations/objects in the environment with an orderly transition amongst locations/objects. The concept and parameters of this stable organization of rituals in time and space were used to analyze rituals of OCD patients, compared with control individuals performing the same actions (e.g. car locking). It was found that human rituals also converged to a few places/objects where repetitive acts were performed in a regular order, with the acts in OCD patients overlapping with those of control individuals. Across a very diverse range of animals and conditions, motor rituals are thus characterized by their close linkage to a few environmental locations and the repeated performance of relatively few acts. Such similarity in form may reflect a similarity in the mechanisms that control motor rituals in both animals and humans.

Mental fatigue, motivation and action monitoring

In this study we examined whether the effects of mental fatigue on behaviour are due to reduced action monitoring as indexed by the error related negativity (Ne/ERN), N2 and contingent negative variation (CNV) event-related potential (ERP) components. Therefore, we had subjects perform a task, which required a high degree of action monitoring, continuously for 2h. In addition we tried to relate the observed behavioural and electrophysiological changes to motivational processes and individual differences. Changes in task performance due to fatigue were accompanied by a decrease in Ne/ERN and N2 amplitude, reflecting impaired action monitoring, as well as a decrease in CNV amplitude which reflects reduced response preparation with increasing fatigue. Increasing the motivational level of our subjects resulted in changes in behaviour and brain activity that were different for individual subjects. Subjects that increased their performance accuracy displayed an increase in Ne/ERN amplitude, while subjects that increased their response speed displayed an increase in CNV amplitude. We will discuss the effects prolonged task performance on the behavioural and physiological indices of action monitoring, as well as the relationship between fatigue, motivation and individual differences.

Psychostimulant-induced behavior as an animal model of obsessive-compulsive disorder: an ethological approach to the form of compulsive rituals

Rats treated chronically with the D2/D3 dopamine receptor agonist quinpirole show a pattern of behavior that meets a set of ethologically derived criteria of compulsive behavior in obsessive-compulsive disorder (OCD). Moreover, in both quinpirole-treated rats and OCD patients, the structure of compulsive rituals appear similar in being composed of relatively few motor acts that are organized in a flexible yet recurrent manner. In addition, the development of compulsive behavior in quinpirole-treated rats is attenuated by the OCD pharmacotherapeutic drug clomipramine. These similarities support the validity of quinpirole-treated rats as a psychostimulant-induced animal model of OCD. Considering that the induction of compulsive behavior in the rat model involves chronic hyperstimulation of dopamine receptors, this raises the possibility that dopaminergic mechanisms may play a role in OCD, at least in some subtypes of this disorder.

Effects of quinpirole on operant conditioning: perseveration of behavioral components

Quinpirole (QNP) is reported to elicit repetitive spontaneous behaviors as well as reduce extinction of operant responses. To determine whether these effects represent perseveration of learned behaviors, behavioral components were examined during the acquisition and extinction of operant responses. Rats, receiving either 0, 0.08, or 0.60 mg/kg QNP were trained to nose poke to receive water. The lower dose interfered with acquisition, but once learned, behavioral characteristics were normal. The higher dose produced excessive time in the drinking well when water was delivered. When water was withheld, the control and 0.08 mg/kg dose groups altered their behavior by initially increasing nose poke duration, followed by a progressive extinction of the operant response. The higher dose group, however, did not modify the characteristics of their behaviors, but continued to perform the behavioral sequence in the absence of reward. These effects are not ascribable to generalized locomotor activation in that response rates during reinforced responses, as well as at the beginning of the extinction phase, did not differ significantly across treatment groups. These results indicate that perseveration effects of QNP are not accountable by general behavioral arousal, nor are specific to extinction. Instead, these effects appear to reflect reduced adaptability of learned behavioral patterns to changes in reinforcement contingencies.

Age differences in an animal model of obsessive–compulsive disorder: participation of dopamine

The putative age difference in the effect of chronically administered quinpirole (0.125 and 0.5 mg/kg, 11 injections) on alternation in a T maze was studied. Male juvenile (43 days old) and adult (around 90 days old) rats exhibited similar control values of alternation. In adults, quinpirole (0.5 mg/kg) produced a drastic perseveration after 10 and 11 injections (mean number of repetitive choices of 3.4 and 3.1, respectively); conversely, in juvenile, such treatment produced a less marked perseveration (mean number of repetitive choices of 1.7 and 2.1, for the 10th and 11th injection, respectively). We also studied the age difference in the protective actions of clomipramine subchronically administered (15 mg/kg, three times) on the quinpirole-induced perseveration. Clearly, as previously demonstrated, in adult animals, this tricyclic antidepressant completely prevented the drug-induced perseveration (mean number of repetitive choices of 1.7); while in juvenile, animals only produced a weak action (mean number of repetitive choices of 1.8). Data agreed with basic research showing a hyposensitivity of juvenile animals to dopaminergic agonists and with clinical findings suggesting a weaker effect of clomipramine treatment in youth. These results reinforce perseveration in a T maze as a useful animal model for studying age differences in obsessive-compulsive disorder (OCD).

Presynaptic stimulation and development of locomotor sensitization to the dopamine agonist quinpirole

To assess whether locomotor sensitization induced by repeated injections of the dopamine agonist quinpirole reflects tolerance of the drug's presynaptic depressive effects on locomotion, independent groups of rats were treated chronically (every 3 daysx10) with either a presynaptic dose of quinpirole (0.025 mg/kg; n=27), a postsynaptic dose (0.5 mg/kg; n=27), or saline (n=26). Following chronic treatment, a full dose-response profile was determined to assess the presence of sensitization. Results indicated that treatment with the postsynaptic, but not the presynaptic, dose of quinpirole induced locomotor sensitization. Moreover, chronic treatment with low-dose quinpirole did not yield tolerance of the drug's depressive effects. It is suggested that presynaptic dopamine receptors may require extensive spatial and/or temporal summation to become tolerant.

Hypophysectomy does not block sensitization to the dopamine agonist quinpirole or its modulation by the MAOI clorgyline

Repeated administration of the dopamine agonist quinpirole induces behavioral sensitization in rats that is characterized by a four- to eight-fold increase in the amount of locomotion compared to an acute dose of quinpirole, in the absence of any increases in mouthing behavior. The monoamine oxidase (MAO) inhibitor, clorgyline, switches behavioral sensitization to quinpirole from that of locomotion to self-directed mouthing. The mechanism by which clorgyline produces this switch in behavioral sensitization is unknown, but is independent of the known effects of clorgyline, namely, inhibition of MAO, inhibition of striatal dopamine uptake, or stimulation of sigma and I(2) receptors. Because clorgyline also inhibits hypothalamo-pituitary-adrenal (HPA) axis function, and increased HPA activity facilitates the behavioral effects of psychostimulant drugs, the effects of clorgyline on quinpirole sensitization are possibly due to an inhibition of HPA function. Therefore, the present study examined whether HPA activity is required for sensitization to quinpirole, and whether clorgyline exerts its effects on quinpirole sensitization via inhibition of HPA function. Control and hypophysectomized rats were administered clorgyline (1 mg/kg, s.c.) or vehicle 90 min before each injection of quinpirole (0.5 mg/kg x 8, twice weekly) or saline. To assess the level of sensitization reached by control and hypophysectomized rats, test injections of quinpirole (0.0, 0.07, and 0.2 mg/kg) were administered. Chronic quinpirole administration produced equivalent levels of locomotor sensitization in control and hypophysectomized rats. Clorgyline was equally effective in blocking the development of locomotor sensitization in control and hypophysectomized rats, and in sensitizing self-directed mouthing. The present study suggests that (1). HPA function is not necessary for the development of quinpirole sensitization and, (2). clorgyline does not produce its effects on behavioral sensitization to quinpirole via an inhibition of HPA activity. Moreover, the observation that quinpirole sensitization develops normally in the absence of any pituitary endocrine function suggests that pituitary-gonadal and pituitary-thyroid axes activity are also not necessary for quinpirole sensitization to occur.

Effects of quinpirole on behavioral extinction

Behavioral effects of quinpirole (QNP), a dopamine D(2) receptor agonist, suggest it impacts neural mechanisms mediating goal-directed behaviors, as well as behavioral extinction following removal of a primary reinforcer. The present study investigated the effect of QNP on behavioral extinction following the omission of contingent reinforcement, and whether this effect is related to acquisition or processes specific to extinction. Rats were trained on a continuous reinforcement schedule to nose-poke for water reward. Using a free-operant procedure, rats completed approximately 70 responses for each of four consecutive days. On the fifth day reward was withheld. Rats were assigned to one of five groups in which they received 0.3 mg/kg QNP ip either during the first day (acquisition phase), the second 2 days (maintenance phase), the last day (extinction phase), or during all days. A fifth group received vehicle injections. Rats receiving QNP during the acquisition and maintenance phase did not differ significantly from the control group during the extinction phase, although they demonstrated reduced response rates on days they received QNP. However, rats treated during the extinction phase or during all phases demonstrated a significant reduction in the rate of extinction. This effect cannot be attributed to an increase in general behavioral arousal because response rates for reinforced responses did not differ significantly among groups following acquisition of the behavior. The reduced extinction effect does not appear to be related to abnormalities in the initial behavior-reward association, but instead may result from enhanced engagement of learned behavioral patterns, or from interference of signals associated with removal of predicted reinforcement.

Caffeine, fatigue, and cognition

Effects of caffeine and fatigue are discussed with special attention to adenosine-dopamine interactions. Effects of caffeine on human cognition are diverse. Behavioural measurements indicate a general improvement in the efficiency of information processing after caffeine, while the EEG data support the general belief that caffeine acts as a stimulant. Studies using ERP measures indicate that caffeine has an effect on attention, which is independent of specific stimulus characteristics. Behavioural effects on response related processes turned out to be mainly related to more peripheral motor processes. Recent insights in adenosine and dopamine physiology and functionality and their relationships with fatigue point to a possible modulation by caffeine of mechanisms involved in the regulation of behavioural energy expenditure.

Asymmetric modulation of a catecholamine-regulated protein in the rat brain, following quinpirole administration

We previously reported a brain-specific 40 kDa catecholamine-regulated protein (CRP40) that binds dopamine (DA) and related catecholamines. CPR40 shares significant sequence homology with human heat shock protein (Hsp70), GRP78/BIP, and human #BQ24193 protein. Recent studies with the DA D(2) receptor antagonist, haloperidol, demonstrated a significant increase in expression of CRP40 in the striatum (STR). The objective of the present study was to investigate CRP40 expression in various brain regions following treatments with the DA D(2)/D(3) receptor agonist quinpirole (QNP) in rats and examine possible relationships between neurochemical parameters and locomotor activity. Rats received injections of either QNP (0.5 mg/kg, for 27 days every third day) or saline (SAL) and their locomotor activities were measured for 90 min after each injection. At injection 9, QNP-treated rats showed locomotor activity that was significantly greater than SAL controls (F(2,28) = 3.88, P < 0.05, Duncan's multiple range test, P < 0.05). Neurochemically, acute QNP-treated rats demonstrated significant differential expression of CRP40 in the left/right prefrontal cortex (PFC) relative to SAL-treated rats (-17.76 +/- 2.10%, -10.35 +/- 1.23%, P < 0.001). Chronic QNP significantly decreased CRP40 expression in the STR, ventral tegmental area (VTA), and left/right PFC (-24.85+/- 2.10%, -18.15 +/- 5.64%, -49.13 +/- 7.05%, -25 +/- 3.63%, P < 0.001). Finally, chronic QNP treatment resulted in a significant increase in CRP40 levels in the nucleus accumbens (NA) (+39.32 +/- 7.00%, P < 0.001). Heat shock protein (i.e., Hsp70 or Hsc70) expression remained unaltered following QNP treatment. Since QNP is a DA D(2)/D(3) agonist, alterations in CRP40 expression following QNP treatment suggest the protein's function in dopaminergic neurotransmission.

Open-field behavior withstands drastic changes in arena size

Individual voles were tested on successive days under increasing, decreasing, or randomly changing arena size. Locomotor behavior was adjusted to arena size by (i) preserving the same level of activity, (ii) taking longer but less frequent trips in smaller arenas in contrast to taking more frequent yet shorter trips in the larger arenas, and (iii) moving in the entire space available for exploration in the smaller arenas in contrast to remaining along the walls of the open field in the larger arenas. The effect of testing order was minimal, probably being related to increased novelty under increasing arena size, as opposed to habituation under decreasing arena size, when parts of the same area were re-explored. These behavioral changes averaged up to a two-fold difference compared with the larger six-fold change in the perimeter and 44-fold change in the area of the open field. The modest change in open-field behavior indicates that it has a solid spatio-temporal structure that withstands extensive environmental changes. This behavioral stability and consistency further validates studies of pharmacological, neurological, and genetic preparations that use behavior in a small open field as representation of the general pattern of behavior.

Clorgyline-induced switch from locomotion to mouthing in sensitization to the dopamine D2/D3 agonist quinpirole in rats: role of sigma and imidazoline I2 receptors

RATIONALE

The monoamine oxidase inhibitor (MAOI) clorgyline, blocks locomotor sensitization to the D(2)/D(3) dopamine agonist quinpirole and sensitizes self-directed mouthing behavior in rats by a mechanism independent of MAO inhibition. Clorgyline has a high affinity for imidazoline I(2) and sigma receptors, which could account for its effects on quinpirole sensitization.

OBJECTIVES

To examine whether the effect of clorgyline on quinpirole sensitization is attributed to stimulation of either I(2) or sigma receptors.

METHODS

In one experiment, rats received injections of the I(2) receptor agonist 2-BFI (0.2 mg/kg, IP) or vehicle, 90 min prior to each injection of quinpirole (0.5 mg/kg, SC, x 8, twice weekly) or saline. A similar protocol was used to examine the effects of the MAOI Ro 41-1049 (10 mg/kg, SC) on quinpirole sensitization. Unlike clorgyline, Ro 41-1049 has no affinity for sigma or I(2) sites. An initial experiment demonstrated that intermittent injections of clorgyline (1 mg/kg, SC) are as effective as a continuous clorgyline administration (1 mg/kg per day via osmotic mini-pump) on quinpirole sensitization.

RESULTS

Like clorgyline, Ro 41-1049, but not 2-BFI, blocked the development of quinpirole-induced locomotor sensitization and induced instead sensitization of self-directed mouthing.

CONCLUSIONS

Because Ro 41-1049 produced the same effects as clorgyline, and 2-BFI had no effects on quinpirole sensitization, it is unlikely that clorgyline exerts its effects via an action at sigma or I(2) receptors. Our results are consistent with the suggestion that clorgyline and Ro 41-1049 affect the behavioral response to quinpirole via the MAOI-displaceable quinpirole binding (MQB) site, and the hypothesis that the MQB site selects what motor output becomes sensitized to repeated injections of quinpirole.

Motivational views of reinforcement: implications for understanding the behavioral functions of nucleus accumbens dopamine

Although the Skinnerian 'Empirical Law of Effect' does not directly consider the fundamental properties of stimuli that enable them to act as reinforcers, such considerations are critical for determining if nucleus accumbens dopamine systems mediate reinforcement processes. Researchers who have attempted to identify the critical characteristics of reinforcing stimuli or activities have generally arrived at an emphasis upon motivational factors. A thorough review of the behavioral literature indicates that, across several different investigators offering a multitude of theoretical approaches, motivation is seen by many as being fundamental to the process of reinforcement. The reinforcer has been described as a goal, a commodity, an incentive, or a stimulus that is being approached, self-administered, attained or preserved. Reinforcers also have been described as activities that are preferred, deprived or in some way being regulated. It is evident that this 'motivational' or 'regulatory' view of reinforcement has had enormous influence over the hypothesis that DA directly mediates 'reward' or 'reinforcement' processes. Indeed, proponents of the DA/reward hypothesis regularly cite motivational theorists and employ their language. Nevertheless, considerable evidence indicates that low/moderate doses of DA antagonists, and depletions of DA in nucleus accumbens, can suppress instrumental responding for food while, at the same time, these conditions leave fundamental aspects of reinforcement (i.e. primary or unconditioned reinforcement; primary motivation or primary incentive properties of natural reinforcers) intact. Several complex features of the literature on dopaminergic involvement in reinforcement are examined below, and it is argued that the assertions that DA mediates 'reward' or 'reinforcement' are inaccurate and grossly oversimplified. Thus, it appears as though it is no longer tenable to assert that drugs of abuse are simply turning on the brain's natural 'reward system'. In relation to the hypothesis that DA systems are involved in 'wanting', but not 'liking', it is suggested in the present review that 'wanting' has both directional aspects (e.g. appetite to consume food) and activational aspects (e.g. activation for initiating and sustaining instrumental actions; tendency to work for food). The present paper reviews findings in support of the hypothesis that low doses of DA antagonists and accumbens DA depletions do not impair appetite to consume food, but do impair activational aspects of motivation. This suggestion is consistent with the studies showing that low doses of DA antagonists and accumbens DA depletions alter the relative allocation of instrumental responses, making the animals less likely to engage in instrumental responses that have a high degree of work-related response costs. In addition, this observation is consistent with studies demonstrating that accumbens DA depletions make rats highly sensitive to ratio requirements on operant schedules. Although accumbens DA is not seen as directly mediating appetite to consume food, principles of behavioral economics indicate that accumbens DA could be involved in the elasticity of demand for food in terms of the tendency to pay work-related response costs. Future research must focus upon how specific aspects of task requirements (i.e. ratio requirements, intermittence of reinforcement, temporal features of response requirements, dependence upon conditioned stimuli) interact with the effects of accumbens DA depletions, and which particular factors determine sensitivity to the effects of DA antagonism or depletion.

Novel drug interactions at D(2) dopamine receptors: modulation of [3H]quinpirole binding by monoamine oxidase inhibitors

D(2) dopamine receptors are the principal target of drugs used to treat schizophrenia and Parkinson's disease. Recent findings suggest novel drug interactions at D(2) receptors, specifically interactions of monoamine oxidase inhibitors (MAOIs) at a novel binding site that modulates the binding of [3H]quinpirole to the D(2) receptor. That MAOIs inhibit [3H]quinpirole binding challenges the traditional understanding of ligand interactions at dopamine receptors and may shed light on the mechanism of behavioral sensitization to psychostimulants and the pharmacology and toxicity of MAOIs.