A scaling approach to find order parameters quantifying the effects of dopaminergic agents on unconditioned motor activity in rats

iTat transgenic mice exhibit hyper-locomotion in the behavioral pattern monitor after chronic exposure to methamphetamine but are unaffected by Tat expression

Although antiretroviral therapy (ART) has increased the quality of life and lifespan in people living with HIV (PWH), millions continue to suffer from the neurobehavioral effects of the virus. Additionally, the abuse of illicit drugs (methamphetamine in particular) is significantly higher in PWH compared to the general population, which may further impact their neurological functions. The HIV regulatory protein, Tat, has been implicated in the neurobehavioral impacts of HIV and is purported to inhibit dopamine transporter (DAT) function in a way similar to methamphetamine. Thus, we hypothesized that a combination of Tat expression and methamphetamine would exert synergistic deleterious effects on behavior and DAT expression. We examined the impact of chronic methamphetamine exposure on exploration in transgenic mice expressing human Tat (iTat) vs. their wildtype littermates using the behavioral pattern monitor (BPM). During baseline, mice exhibited sex-dependent differences in BPM behavior, which persisted through methamphetamine exposure, and Tat activation with doxycycline. We observed a main effect of methamphetamine, wherein exposure, irrespective of genotype, increased locomotor activity and decreased specific exploration. After doxycycline treatment, mice continued to exhibit drug-dependent alterations in locomotion, with no effect of Tat, or methamphetamine interactions. DAT levels were higher in wildtype, saline-exposed males compared to all other groups. These data support stimulant-induced changes of locomotor activity and exploration, and suggest that viral Tat and methamphetamine do not synergistically interact to alter these behaviors in mice. These findings are important for future studies attempting to disentangle the effect of substances that impact DAT on HAND-relevant behaviors using such transgenic animals.

Ketamine induces immediate and delayed alterations of OCD-like behavior

RATIONALE

Obsessive-compulsive disorder (OCD) is a psychiatric disorder characterized by intrusive obsessive thoughts and/or compulsive behaviors. Currently, serotonin reuptake inhibitors (SRIs) provide the only pharmacological monotherapy for OCD, but response rates are insufficient. Ketamine, a noncompetitive NMDA receptor antagonist, was reported to have rapid, sustained therapeutic effects in OCD patients. However, the mechanisms remain unknown.

OBJECTIVES

Here, we aimed to provide a platform for investigating mechanisms underlying anti-OCD effects of ketamine treatment by assessing whether ketamine pretreatment could alleviate 5-HT1B receptor (5-HT1BR)-induced OCD-like behavior in mice.

METHODS

We assessed whether acute ketamine (0, 3, 10, 30 mg/kg), administered at two pretreatment time points (30 min, 24 h), would modulate 5-HT1BR-induced OCD-like behavior in mice. Behavioral measures were perseverative hyperlocomotion in the open field and deficits in prepulse inhibition (PPI) induced by acute pharmacological 5-HT1BR challenge.

RESULTS

Three milligrams per kilogram of ketamine reduced 5-HT1BR-induced perseverative hyperlocomotion, but not PPI deficits, 24 h postinjection. In contrast, higher doses of ketamine were either ineffective (10 mg/kg) or exacerbated (30 mg/kg) 5-HT1BR-induced perseverative hyperlocomotion 30 min postinjection. At 24 h postinjection, 30 mg/kg ketamine reduced perseverative hyperlocomotion across all groups.

CONCLUSIONS

Our results suggest that the 5-HT1BR-induced model of OCD-like behavior is sensitive to a low dose of ketamine, a potential fast-acting anti-OCD treatment, and may provide a tool for studying mechanisms underlying the rapid therapeutic effects of ketamine in OCD patients.

Blockade of serotonin 5-HT2A receptors potentiates dopamine D2 activation-induced disruption of pup retrieval on an elevated plus maze, but has no effect on D2 blockade-induced one

Appetitive aspect of rat maternal behavior, such as pup retrieval, is motivationally driven and sensitive to dopamine disturbances. Activation or blockade of dopamine D₂ receptors causes a similar disruption of pup retrieval, which may also reflect an increase in maternal anxiety and/or a disruption of executive function. Recent work indicates that serotonin 5-HT_2A receptors also play an important role in rat maternal behavior. Given the well-known modulation of 5-HT_2A on the mesolimbic and mesocortical dopamine functions, the present study examined the extent to which blockade of 5-HT_2A receptors on dopamine D₂-mediated maternal effects using a pup retrieval on the elevated plus maze (EPM) test. Sprague-Dawley postpartum female rats were acutely injected with quinpirole (a D₂ agonist, 0.10 and 0.25 mg/kg, sc), or haloperidol (a D₂ antagonist, 0.1 or 0.2 mg/kg, sc), in combination of MDL100907 (a 5-HT_2A receptor antagonist, 1.0 mg/kg, sc, 30 min before quinpirole or haloperidol injection) or saline and tested at 30, 90 and 240 min after quinpirole or haloperidol injection on postpartum days 3 and 7. Quinpirole and haloperidol decreased the number of pup retrieved (an index of maternal motivation) and sequential retrieval score (an index of executive function), prolonged the pup retrieval latencies, reduced the percentage of time spent on the open arms (an index of maternal anxiety), and decreased the distance travelled on the maze in a dose-dependent and time-dependent fashion. MDL100907 treatment by itself had no effect on pup retrieval, but it exacerbated the quinpirole-induced disruption of pup retrieval, but had no effect on the haloperidol-induced one. These findings suggest a complex interactive effect between 5-HT_2A and D₂ receptors on one or several maternal processes (maternal motivation, anxiety and executive function), and support the idea that one molecular mechanism by which 5-HT_2A receptors mediate maternal behavior is through its modulation of D₂ receptors.

Blockade of dopamine D1-family receptors attenuates the mania-like hyperactive, risk-preferring, and high motivation behavioral profile of mice with low dopamine transporter levels

BACKGROUND

Patients with bipolar disorder mania exhibit poor cognition, impulsivity, risk-taking, and goal-directed activity that negatively impact their quality of life. To date, existing treatments for bipolar disorder do not adequately remediate cognitive dysfunction. Reducing dopamine transporter expression recreates many bipolar disorder mania-relevant behaviors (i.e. hyperactivity and risk-taking). The current study investigated whether dopamine D1-family receptor blockade would attenuate the risk-taking, hypermotivation, and hyperactivity of dopamine transporter knockdown mice.

METHODS

Dopamine transporter knockdown and wild-type littermate mice were tested in mouse versions of the Iowa Gambling Task (risk-taking), Progressive Ratio Breakpoint Test (effortful motivation), and Behavioral Pattern Monitor (activity). Prior to testing, the mice were treated with the dopamine D1-family receptor antagonist SCH 23390 hydrochloride (0.03, 0.1, or 0.3 mg/kg), or vehicle.

RESULTS

Dopamine transporter knockdown mice exhibited hyperactivity and hyperexploration, hypermotivation, and risk-taking preference compared with wild-type littermates. SCH 23390 hydrochloride treatment decreased premature responding in dopamine transporter knockdown mice and attenuated their hypermotivation. SCH 23390 hydrochloride flattened the safe/risk preference, while reducing activity and exploratory levels of both genotypes similarly.

CONCLUSIONS

Dopamine transporter knockdown mice exhibited mania-relevant behavior compared to wild-type mice. Systemic dopamine D1-family receptor antagonism attenuated these behaviors in dopamine transporter knockdown, but not all effects were specific to only the knockdown mice. The normalization of behavior via blockade of dopamine D1-family receptors supports the hypothesis that D1 and/or D5 receptors could contribute to the mania-relevant behaviors of dopamine transporter knockdown mice.

Modafinil improves attentional performance in healthy, non-sleep deprived humans at doses not inducing hyperarousal across species

The wake-promoting drug modafinil is frequently used off-label to improve cognition in psychiatric and academic populations alike. The domain-specific attentional benefits of modafinil have yet to be quantified objectively in healthy human volunteers using tasks validated for comparison across species. Further, given that modafinil is a low-affinity inhibitor for the dopamine and norepinephrine transporters (DAT/NET respectively) it is unclear if any effects are attributable to a non-specific increase in arousal, a feature of many catecholamine reuptake inhibitors (e.g., cocaine, amphetamine). These experiments were designed to test for domain-specific enhancement of attention and cognitive control by modafinil (200 and 400 mg) in healthy volunteers using the 5-choice continuous performance task (5C-CPT) and Wisconsin Card Sort Task (WCST). An additional cross-species assessment of arousal and hyperactivity was performed in this group and in mice (3.2, 10, or 32 mg/kg) using species-specific versions of the behavioral pattern monitor (BPM). Modafinil significantly enhanced attention (d prime) in humans performing the 5C-CPT at doses that did not affect WCST performance or induce hyperactivity in the BPM. In mice, only the highest dose elicited increased activity in the BPM. These results indicate that modafinil produces domain-specific enhancement of attention in humans not driven by hyperarousal, unlike other drugs in this class, and higher equivalent doses were required for hyperarousal in mice. Further, these data support the utility of using the 5C-CPT across species to more precisely determine the mechanism(s) underlying the pro-cognitive effects of modafinil and potentially other pharmacological treatments.

Amphetamine increases activity but not exploration in humans and mice

RATIONALE

Cross-species quantification of physiological behavior enables a better understanding of the biological systems underlying neuropsychiatric diseases such as bipolar disorder (BD). Cardinal symptoms of manic BD include increased motor activity and goal-directed behavior, thought to be related to increased catecholamine activity, potentially selective to dopamine homeostatic dysregulation.

OBJECTIVES

The objective of this study was to test whether acute administration of amphetamine, a norepinephrine/dopamine transporter inhibitor and dopamine releaser, would replicate the profile of activity and exploration observed in both humans with manic BD and mouse models of mania.

METHODS

Healthy volunteers with no psychiatric history were randomized to a one-time dose of placebo (n = 25), 10 mg d-amphetamine (n = 18), or 20 mg amphetamine (n = 23). Eighty mice were administered one of four doses of d-amphetamine or vehicle. Humans and mice were tested in the behavioral pattern monitor (BPM), which quantifies motor activity, exploratory behavior, and spatial patterns of behavior.

RESULTS

In humans, the 20-mg dose of amphetamine increased motor activity as measured by acceleration without marked effects on exploration or spatial patterns of activity. In mice, amphetamine increased activity, decreased specific exploration, and caused straighter, one-dimensional movements in a dose-dependent manner.

CONCLUSIONS

Consistent with mice, amphetamine increased motoric activity in humans without increasing exploration. Given that BD patients exhibit heightened exploration, these data further emphasize the limitation of amphetamine-induced hyperactivity as a suitable model for BD. Further, these studies highlight the utility of cross-species physiological paradigms in validating biological mechanisms of psychiatric diseases.

Inhibitory deficits in euthymic bipolar disorder patients assessed in the human behavioral pattern monitor

BACKGROUND

Bipolar disorder (BD) is associated with inhibitory deficits characterized by a reduced ability to control inappropriate actions or thoughts. While aspects of inhibition such as exaggerated novelty-seeking and perseveration are quantified in rodent exploration of novel environments, similar models are rarely applied in humans. The human Behavioral Pattern Monitor (hBPM), a cross-species exploratory paradigm, has identified a pattern of impaired inhibitory function in manic BD participants, but this phenotype has not been examined across different BD phases. The objective of this study was to determine if euthymic BD individuals demonstrate inhibitory deficits in the hBPM, supporting disinhibition as an endophenotype for the disorder.

METHODS

25 euthymic BD outpatients and 51 healthy comparison subjects were assessed in the hBPM, where activity was recorded by a concealed videocamera and an ambulatory monitoring sensor.

RESULTS

Euthymic BD individuals, similar to manic subjects, demonstrated increased motor activity, greater interaction with novel objects, and more frequent perseverative behavior relative to comparison participants. The quantity of locomotion was also reduced in BD individuals treated with mood stabilizers compared to other patients.

LIMITATIONS

Low sample size for treatment subgroups limits the evaluation of specific medication regimens.

CONCLUSIONS

Our results suggest that BD is distinguished by both trait- and state-dependent inhibitory deficits optimally assessed with sophisticated multivariate measures. These data support the use of the hBPM as a tool to elucidate the effects of BD across various illness states, facilitate the development of BD animal models, and advance our understanding of the neurobiology underlying the disorder.

Behavioral effects of chronic methamphetamine treatment in HIV-1 gp120 transgenic mice

Methamphetamine (METH) dependence is frequently comorbid with HIV infection. Both factors are independently characterized by inhibitory deficits, which may manifest as increased motor activity, inappropriate perseverative behavior, and elevated exploratory responses to novel stimuli, but the effect of combined METH exposure and HIV is not well understood. In this study, we administered a chronic escalation/binge regimen of METH or vehicle treatment to wildtype (WT) or transgenic (tg) mice expressing the HIV-1 gp120 envelope protein and quantified disinhibition during the 7 days following drug withdrawal. We hypothesized that gp120tg mice administered chronic METH would exhibit more pronounced inhibitory deficits compared to vehicle-treated WT or gp120tg animals. Our results showed that METH treatment alone increased novel object interaction while female METH-treated gp120tg mice exhibited the highest level of exploration (holepoking) compared to other female mice. Transgenic mice exhibited fewer rears relative to WT, slightly less locomotion, and also demonstrated a trend toward more perseverative motor patterns. In summary, both METH treatment and gp120 expression may modify inhibition, but such effects are selective and dependent upon variations in age and sex that could impact dopamine and frontostriatal function. These findings illustrate the need to improve our knowledge about the combined effects of HIV and substance use and facilitate improved treatment methods for comorbid disease and drug dependence.

Four factors underlying mouse behavior in an open field

The observation of the locomotor and exploratory behaviors of rodents in an open field is one of the most fundamental methods used in the field of behavioral pharmacology. A variety of behaviors can be recorded automatically and can readily generate a multivariate pattern of pharmacological effects. Nevertheless, the optimal ways to characterize observed behaviors and concomitant drug effects are still under development. The aim of this study was to extract meaningful behavioral factors that could explain variations in the observed variables from mouse exploration. Behavioral data were recorded from male C57BL/6J mice (n=268) using the Behavioral Pattern Monitor (BPM). The BPM data were subjected to the exploratory factor analysis. The factor analysis extracted four factors: activity, sequential organization, diversive exploration, and inspective exploration. The activity factor and the two types of exploration factors correlated positively with one another, while the sequential organization factor negatively correlated with the remaining factors. The extracted factor structure constitutes a behavioral model of mouse exploration. This model will provide a platform on which one can assess the effects of psychoactive drugs and genetic manipulations on mouse exploratory behavior. Further studies are currently underway to examine the factor structure of similar multivariate data sets from humans tested in a human BPM.

Differential contributions of serotonin receptors to the behavioral effects of indoleamine hallucinogens in mice

Psilocin (4-hydroxy-N,N-dimethyltryptamine) is a hallucinogen that acts as an agonist at 5-HT(1A), 5-HT(2A), and 5-HT(2C) receptors. Psilocin is the active metabolite of psilocybin, a hallucinogen that is currently being investigated clinically as a potential therapeutic agent. In the present investigation, we used a combination of genetic and pharmacological approaches to identify the serotonin (5-HT) receptor subtypes responsible for mediating the effects of psilocin on head twitch response (HTR) and the behavioral pattern monitor (BPM) in C57BL/6J mice. We also compared the effects of psilocin with those of the putative 5-HT(2C) receptor-selective agonist 1-methylpsilocin and the hallucinogen and non-selective serotonin receptor agonist 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT). Psilocin, 1-methylpsilocin, and 5-MeO-DMT induced the HTR, effects that were absent in mice lacking the 5-HT(2A) receptor gene. When tested in the BPM, psilocin decreased locomotor activity, holepoking, and time spent in the center of the chamber, effects that were blocked by the selective 5-HT(1A) antagonist WAY-100635 but were not altered by the selective 5-HT(2C) antagonist SB 242,084 or by 5-HT(2A) receptor gene deletion. 5-MeO-DMT produced similar effects when tested in the BPM, and the action of 5-MeO-DMT was significantly attenuated by WAY-100635. Psilocin and 5-MeO-DMT also decreased the linearity of locomotor paths, effects that were mediated by 5-HT(2C) and 5-HT(1A) receptors, respectively. In contrast to psilocin and 5-MeO-DMT, 1-methylpsilocin (0.6-9.6 mg/kg) was completely inactive in the BPM. These findings confirm that psilocin acts as an agonist at 5-HT(1A), 5-HT(2A), and 5-HT(2C) receptors in mice, whereas the behavioral effects of 1-methylpsilocin indicate that this compound is acting at 5-HT(2A) sites but is inactive at the 5-HT(1A) receptor. The fact that 1-methylpsilocin displays greater pharmacological selectivity than psilocin indicates that 1-methylpsilocin represents a potentially useful alternative to psilocybin for development as a potential therapeutic agent.

Increased risk-taking behavior in dopamine transporter knockdown mice: further support for a mouse model of mania

Reduced functioning of the dopamine transporter (DAT) has been linked to bipolar disorder (BD). Mice with reduced DAT functioning (knockdown, KD) exhibit a behavioral profile in the mouse Behavioral Pattern Monitor (BPM) consistent with patients with BD mania in the human BPM. Patients with BD also exhibit increased risk taking, which can be quantified using the Iowa Gambling Task (IGT). We hypothesized that DAT KD mice would exhibit increased risk-taking behavior in a novel mouse version of the IGT. DAT KD and wildtype (WT) littermates were trained in the mouse IGT. In session 1, KD mice initially made riskier choices, but later performed comparably to WT mice. Once trained to stable choice performance, DAT KD mice continued to exhibit a trend to choose the riskier options more than WT mice. Finally, we confirmed that these DAT KD mice also exhibited an exploratory profile in the BPM consistent with patients with BD mania, where risky choice behavior modestly correlated with specific exploration. These data demonstrate that DAT KD mice chose the riskier options more than WT mice, providing further support for the use of DAT KD mice as a model of BD mania.

Dopamine receptor mediation of the exploratory/hyperactivity effects of modafinil

Modafinil (2-((diphenylmethyl)sulfinyl)acetamide) is described as an atypical stimulant and is a putative cognition enhancer for schizophrenia, but the precise mechanisms of action remain unclear. Receptor knockout (KO) mice offer an opportunity to identify receptors that contribute to a drug-induced effect. Here we examined the effects of modafinil on exploration in C57BL/6J mice, in dopamine drd1, drd2, drd3, and drd4 wild-type (WT), heterozygous (HT), and KO mice, and in 129/SJ mice pretreated with the drd1 antagonist SCH23390 using a cross-species test paradigm based on the behavioral pattern monitor. Modafinil increased activity, specific exploration (rearing), and the smoothness of locomotor paths (reduced spatial d) in C57BL/6J and 129/SJ mice (increased holepoking was also observed in these mice). These behavioral profiles are similar to that produced by the dopamine transporter inhibitor GBR12909. Modafinil was ineffective at increasing activity in male drd1 KOs, rearing in female drd1 KOs, or reducing spatial d in all drd1 KOs, but produced similar effects in drd1 WT and HT mice as in C57BL/6J mice. Neither dopamine drd2 nor drd3 mutants attenuated modafinil-induced effects. Drd4 mutants exhibited a genotype dose-dependent attenuation of modafinil-induced increases in specific exploration. Furthermore, the drd1 KO effects were largely supported by the SCH23390 study. Thus, the dopamine drd1 receptor appears to exert a primary role in modafinil-induced effects on spontaneous exploration, whereas the dopamine drd4 receptor appears to be important for specific exploration. The modafinil-induced alterations in exploratory behavior may reflect increased synaptic dopamine and secondary actions mediated by dopamine drd1 and drd4 receptors.

CRF2 null mutation increases sensitivity to isolation rearing effects on locomotor activity in mice

BACKGROUND

Developmental stressors are consistently reported to increase risk for certain neuropsychiatric disorders including schizophrenia, depression, and post-traumatic stress disorder. Recent clinical evidence supports a "double-hit" hypothesis of genetic vulnerability interacting with developmental challenges to modulate this risk. Early life stressor effects on behavior may be modulated in part by alterations in corticotropin releasing factor (CRF) signaling via two known receptors, CRF(1) and CRF(2). One extant hypothesis is that CRF(2) activation may modulate long-term adaptive responses after homeostatic challenge. As such, loss of CRF(2) activity via genetic variance may increase sensitivity to the long-term effects of developmental stress.

METHODS

We tested the hypothesis that CRF(2) function may mitigate the behavioral effects of isolation rearing, predicting that loss of CRF(2) function increases sensitivity to this developmental challenge. Using the behavioral pattern monitor (BPM), we examined exploratory behavior and locomotor patterns in adult CRF(2) wild-type (WT) and gene knockout (KO) mice reared socially or in isolation.

RESULTS

Isolation housing produced robust increases in the amount of locomotor activity and investigatory holepoking, and altered the temporal distribution of activity in CRF(2) KO but not CRF(2) WT mice. Isolation housing significantly increased rearing behavior and altered spatial patterns of locomotor activity regardless of genotype.

CONCLUSIONS

Loss of CRF(2) function increased sensitivity to the effects of chronic social isolation on exploratory locomotor behavior. Thus, CRF(2) activation appears to mitigate isolation rearing effects on exploratory behavior. Further research assessing the interaction between CRF(2) function and developmental challenges is warranted.

GBR 12909 administration as a mouse model of bipolar disorder mania: mimicking quantitative assessment of manic behavior

RATIONALE

Mania is a core feature of bipolar disorder (BD) that traditionally is assessed using rating scales. Studies using a new human behavioral pattern monitor (BPM) recently demonstrated that manic BD patients exhibit a specific profile of behavior that differs from schizophrenia and is characterized by increased motor activity, increased specific exploration, and perseverative locomotor patterns as assessed by spatial d.

OBJECTIVES

It was hypothesized that disrupting dopaminergic homeostasis by inhibiting dopamine transporter (DAT) function would produce a BD mania-like phenotype in mice as assessed by the mouse BPM.

METHODS

We compared the spontaneous locomotor and exploratory behavior of C57BL/6J mice treated with the catecholamine transporter inhibitor amphetamine or the selective DAT inhibitor GBR 12909 in the mouse BPM. We also assessed the duration of the effect of GBR 12909 by testing mice in the BPM for 3 h and its potential strain dependency by testing 129/SvJ mice.

RESULTS

Amphetamine produced hyperactivity and increased perseverative patterns of locomotion as reflected in reduced spatial d values but reduced exploratory activity in contrast to the increased exploration observed in BD patients. GBR 12909 increased activity and reduced spatial d in combination with increased exploratory behavior, irrespective of inbred strain. These effects persisted for at least 3 h.

CONCLUSIONS

Thus, selectively inhibiting the DAT produced a long-lasting cross-strain behavioral profile in mice that was consistent with that observed in manic BD patients. These findings support the use of selective DAT inhibition in animal models of the impaired dopaminergic homeostasis putatively involved in the pathophysiology of BD mania.

Pharmacological validation of a chronic social stress model of depression in rats: effects of reboxetine, haloperidol and diazepam

Chronic social stress is one of the most important factors responsible for precipitation of depressive disorder in humans. In recent years, the impact of social stress on the development of psychopathologies has been thoroughly investigated in preclinical animal studies. We have shown recently that behavioural effects of chronic social stress in rats can be reversed by citalopram and fluoxetine. This study has been designed for further pharmacological validation of the chronic social stress paradigm as a model of depressive symptoms in rats. For this, rats were subjected to 5 weeks of daily social defeat and were in parallel treatment for a clinically relevant period of 4 weeks with the antidepressant drug reboxetine (40 mg/kg/day) and the neuroleptic drug haloperidol (2 mg/kg/day). The anxiolytic diazepam (1 mg/kg) was administered acutely at the end of the stress period. Stress caused decreased locomotor and exploratory behaviours, decreased sucrose preference and increased immobility in the forced swim test, but did not affect behaviour in the elevated plus maze. Four weeks of oral treatment with reboxetine ameliorated the adverse effects of social stress and normalized behaviours related to motivation and reward sensitivity. The treatment with haloperidol worsened the adverse effects of chronic social stress having effects similar to stress on reward and motivation-related behaviours. Diazepam reduced anxiety-related behaviours as measured in elevated plus maze in control animals having no effects on socially stressed individuals. Neither sucrose preference nor performance in forced swim test was affected by diazepam. The effectiveness and selectivity of the treatment with the antidepressant reboxetine in ameliorating socially induced behavioural disturbances supports the validity of the chronic social stress as a model of depressive-like symptoms in rats.

A reverse-translational approach to bipolar disorder: rodent and human studies in the Behavioral Pattern Monitor

Mania is the defining feature of bipolar disorder (BD). There has been limited progress in understanding the neurobiological underpinnings of BD mania and developing novel therapeutics, in part due to a paucity of relevant animal models with translational potential. Hyperactivity is a cardinal symptom of mania, traditionally measured in humans using observer-rated scales. Multivariate assessment of unconditioned locomotor behavior using the rat Behavioral Pattern Monitor (BPM) developed in our laboratory has shown that hyperactivity includes complex multifaceted behaviors. The BPM has been used to demonstrate differential effects of drugs on locomotor activity and exploratory behavior in rats. Studies of genetically engineered mice in a mouse BPM have confirmed its utility as a cross-species tool. In a "reverse-translational" approach to this work, we developed the human BPM to characterize motor activity in BD patients. Increased activity, object interactions, and altered locomotor patterns provide multi-dimensional phenotypes to model in the rodent BPM. This unique approach to modeling BD provides an opportunity to identify the neurobiology underlying BD mania and test novel antimanic agents.

Dopamine D1 and D2 agonist effects on prepulse inhibition and locomotion: comparison of Sprague-Dawley rats to Swiss-Webster, 129X1/SvJ, C57BL/6J, and DBA/2J mice

D2 receptors have been studied in relation to therapeutic uses of dopaminergic drugs, and psychomotor stimulant effects [as manifested by decreased prepulse inhibition (PPI) of startle and increased locomotor activity] are hallmark behavioral effects of D2 agonists in rats. Genetic studies with mutant mice might be useful in this line of investigation; however, recent studies suggest that mice differ from rats with respect to D2 agonist effects. Accordingly, we studied a wide range of doses of the D2-like agonist quinelorane (0.0032-5.6 mg/kg) and the D1-like agonist R-6-Br-APB [R(+)-6-bromo-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide] (0.032-5.6 mg/kg) in outbred Sprague-Dawley rats, outbred Swiss-Webster mice, and inbred 129X1/SvJ, C57BL/6J, and DBA/2J mice. Whereas the D2 agonist dose-dependently decreased PPI and increased locomotion in rats, neither of these effects was observed in outbred or inbred mice. In contrast, the D1 agonist reduced PPI and increased locomotion in Sprague-Dawley rats and in Swiss-Webster, 129X1/SvJ, and C57BL/6J mice. Neither agonist decreased PPI in DBA/2J mice, although PPI was increased in this strain by a D2 antagonist. Pretreatment with either the D2 antagonist eticlopride (1 mg/kg) or the D1 antagonist SCH39166 [(-)-trans-6,7,7a,8,9,13b-hexahydro-3-chloro-2-hydroxy-N-methyl-5H-benzo[d]naptho-(2,1-b)azepine] (1 mg/kg) prevented the PPI-disruptive effects of quinelorane in rats and R-6-Br-APB in mice, suggesting receptor interactions in both species. In summary, psychomotor stimulant effects of a D2 agonist that were robustly observed in outbred rats were absent in several outbred and inbred strains of mice. These results may have implications for the study of mutant mice to investigate genes involved in psychomotor function in humans.

The balance between approach and avoidance behaviors in a novel object exploration paradigm in mice

Approach and avoidance are critical components of novelty seeking, which plays an important role in susceptibility to drug abuse and aspects of cognition. This experiment was designed to examine whether brief periods of handling or prior exposure to a novel environment affect various measures of novel object exploration in mice. Forty male C57BL/6J mice were handled by the experimenter or received minimal exposure to human contact. In addition to manipulating the degree of familiarity with the experimenter (handling), we also manipulated the degree of familiarity with the object. All mice were tested over a 3-day period. On day 1, all mice were tested in the open field for 60 min. On day 2, there were two, 30-min sessions. In the first 30-min session, there was no object present. In the second 30-min session, half of the mice were exposed to a novel object. On day 3, all mice were placed in the open field for 30 min followed by a 30-min period in which the object was placed in the center of the open field. Handled mice showed a trend toward more object exploration on day 2 compared to non-handled mice. Mice with prior exposure to the novel object showed more object exploration compared to object-naïve mice on day 3. These results are consistent with the hypothesis that a certain degree of familiarity with the object or with the experimenter decreases avoidance and increases exploration of novel stimuli. In combination, these results show that the approach and avoidance dimensions of novelty seeking can be manipulated experimentally and may be used in subsequent studies to examine the effects of drugs of abuse.

Behavioral response profiles following drug challenge with dopamine receptor subtype agonists and antagonists in developing rat

As part of an investigation into the effects of gestational ethanol (ETOH) exposure on the developing dopamine (DA) system, pregnant Sprague-Dawley rats were exposed to one of three conditions: ETOH, pair-fed (PF) to the ETOH group, or ad libitum lab chow controls (LC). In this paper we report behavioral drug challenge effects for offspring of the two control groups (PF and LC). Male and female pups between postnatal days (PNDs) 21 and 23 in age were exposed to one of three intraperitoneal/subcutaneous doses of one of eight drugs chosen to assess the functional status of the DA D(1), D(2), and D(3) receptor subtype, or a saline control. Agonists were SKF 38393, apomorphine (APO), quinpirole (QUIN), and 7-hydroxy-N,N-di-n-propyl-2-amino-tetralin [7-OH-DPAT (DPAT)]; antagonists were spiperone (SPIP), SCH 23390, and two recently developed D(3) antagonists nafadotride (NAF) and PD 152255. Immediately following drug injection, pups were placed in observation cages, where eight behaviors (square entries, grooming, circling, rearing, sniffing, head and oral movements, and yawning) were scored at 3-min intervals for 30 min. Classic behavioral profiles were generally obtained for the high-dose mixed agonists APO, DPAT, and QUIN, which potently increased square entries, rearing, and sniffing, while reducing grooming and head movements. However, low-dose APO had no effect on behavior. The D(1) agonist, SKF 38393, had a strikingly different behavioral profile; it had no effect on square entries at any dose, while increasing grooming and sniffing at the medium dose. The D(1) antagonist, SCH 23390, profoundly decreased all behaviors except oral and head movements, especially at high doses. In contrast, the effects of the D(2) antagonist, SPIP, were limited to increasing sniffing at the medium dose. The two putative D(3) antagonists, NAF and PD 152255, presented strikingly different profiles. NAF induced a pattern of behavioral suppression that resembled the profile of high-dose SCH, while high-dose PD 152255 stimulated behavior. The failure of low-dose APO to have any effect on behavior suggests that the D(2) autoreceptor is not functional in preweanling rats. This hypothesis is further supported by the lack of behavioral suppression seen with low-dose QUIN and DPAT. Failure of NAF to produce behavioral activation at low doses and the stimulatory effects seen with PD 152255 suggests that either the D(3) autoreceptor, the postsynaptic D(3) receptor, or both are not fully functional at this age as well.

Subthalamic 5-HT(1A) and 5-HT(1B) receptor modulation of RU 24969-induced behavioral profile in rats

The effects of systemic administration of the serotonin (5-HT)(1A/1B) agonist 5-methoxy-3-(1,2,3,6-tetrahydropyridin-4-yl)1H-indole (RU 24969) on locomotor and investigatory behavior in rats have been well characterized using the behavioral pattern monitor (BPM). To elucidate the neural circuitry underlying this behavioral profile, intracerebral dose--response studies were conducted at two sites with high densities of 5-HT(1B) receptors, the subthalamic nucleus (STN) and substantia nigra. Infusion of RU 24969 into the STN produced systemic RU 24969-like changes in locomotor activity and patterns but an uncharacteristic increase in investigatory holepokes. Intra-STN administration of the selective 5-HT(1A) receptor agonist 8-hydroxy-2-(di-N-propylamino)tetralin (8-OH-DPAT) produced RU 24969-like changes in locomotor patterns only, while the 5-HT(1B) receptor agonist 3(1,2,5,6-tetrahydropyrid-4-yl)pyrrolo[3,2-b]pyrid-5-one dihydrochloride (CP-93,129) increased locomotor activity, produced no change in locomotor patterns and nonsignificantly increased holepokes. Intranigral infusion of RU 24969 produced systemic and intra-STN RU 24969-like increases in locomotor activity. Intranigral RU 24969, however, failed to produce any changes in locomotor patterns or investigatory holepokes. Intranigral infusions of CP-93,129 or 8-OH-DPAT had no effects on locomotor activity, locomotor patterns or investigatory holepokes. These results provide evidence for multiple-site mediation of the locomotor-activating effects of RU 24969 and for a dissociation of the neural substrates underlying locomotor and investigatory components of the RU 24969-induced behavioral profile.

Prepulse inhibition deficits and perseverative motor patterns in dopamine transporter knock-out mice: differential effects of D1 and D2 receptor antagonists

Dopamine is known to regulate several behavioral phenomena, including sensorimotor gating and aspects of motor activity. The roles of dopamine D1 and D2 receptors in these behaviors have been documented in the rat literature, but few reports exist on their role in mice. We used dopamine transporter (DAT) (-/-) mice to examine the behavioral consequences of a chronically hyperdopaminergic state, challenging them with the preferential dopamine D2 receptor antagonist raclopride and D1 receptor antagonist SCH23390. At baseline, DAT (-/-) mice exhibited deficient sensorimotor gating as measured by prepulse inhibition (PPI) of the startle response, exhibited nonfocal preservative patterns of locomotion, and were hyperactive in a novel environment. Pretreatment with raclopride significantly increased PPI in the DAT (-/-) mice, whereas SCH23390 had no significant effect. Blockade of D2 receptors did not affect the predominantly straight patterns of motor behavior produced by the DAT (-/-) mice, but antagonism of D1 receptors significantly attenuated the preservative patterns, producing more of a meandering behavior seen in the DAT (+/+) control mice. Both D1 and D2 receptor antagonists decreased the hyperactivity seen in the DAT (-/-) mice. These findings support the role of the D2, but not the D1, receptor in the modulation of PPI in mice. Furthermore, D1 receptor activation appears to be the critical substrate for the expression of preservative patterns of motor behavior, whereas both D1 and D2 receptors appear to regulate the amount of motor activity.

Chronic oral haloperidol and clozapine in rats: A behavioral evaluation

The present study evaluated chronic oral treatment of rats with haloperidol or clozapine. Drugs were given in the drinking water for a 23-day period. Rat behavior was analyzed once a week in an open field. Rats ingested either 1.7 mg/kg haloperidol or 40 mg/kg clozapine daily. Blood serum analysis revealed concentrations of 6 ng/ml for haloperidol and 22 ng/ml for clozapine at the end of the treatment. Haloperidol decreased overall activity from the onset of treatment. Clozapine showed similar effects only on the last test day. Control animals showed a slight habituation in exploration-related parameters. In conclusion, these results indicate that oral drug administration through the drinking water is a suitable mode of noninvasive chronic treatment that led to sufficiently high drug levels to induce specific pharmacological effects in rats.

Animal models of acute drug-induced akathisia - a review

Akathisia is a complex neurobehavioural side effect of neuroleptics and some other drugs which is characterised by subjective report and objective manifestations of restlessness. Its pathophysiology is poorly understood and there are many limitations to its investigation in humans. This paper reviews the various attempts that have been made in modelling acute akathisia in animals. Homologous as well as isomorphic models have been attempted, but most models are partial as they reproduce either the subjective or the objective features of the syndrome. None of the available models has been fully validated. Neuroleptic-induced defecation in the rat, even though constrained by a lack of symptom similarity and thereby face validity, has been most studied as a model of subjective akathisia. Rat models of restlessness, in particular those involving the use of serotonergic drugs or lesions of the ventral tegmentum or medial prefrontal cortex, are interesting partial models that should be further investigated. Neuroleptic-induced akathisia is observed in primates and has been modelled in dogs, and these should be studied further for their validation. It is also necessary to consider the subtypes of akathisia in the attempts to develop these models.

The genetic liability to stress and postweaning isolation have a competitive influence on behavioral organization in rats

Rats housed in social isolation postweaning (isolates) show profound behavioral and neurobiological differences when compared to socially housed rats (socials). Fischer rats (F344) relative to Lewis rats are hyperresponsive and significantly more susceptible to stressful stimuli. This investigation tested the hypothesis that the behavioral effects of postweaning isolation are more pronounced in a strain of rats with high susceptibility to stress compared to a strain with low susceptibility to stress. Seventy male Sprague-Dawley, Lewis, and F344 rats were housed individually or in groups at weaning on Day 21 and tested on Day 85 in the Behavioral Pattern Monitor. There was no interaction between strain and postweaning isolation for measures of locomotor activity and exploratory behavior (holepoking). However, the postweaning isolation-induced increase in the frequency of repetitive straight movements, a measure of behavioral organization, was more pronounced in Lewis isolates compared to Sprague-Dawley and F344 isolates. These results do not support the hypothesis that rats with a higher susceptibility to stress show more pronounced changes in behavior following postweaning isolation; instead, increased susceptibility to stress may counteract the repetitive movement patterns induced by social isolation.

Differential movement patterns but not amount of activity in unconditioned motor behavior of Fischer, Lewis, and Sprague-Dawley rats

Histocompatible Fischer 344 and Lewis rats have been shown to differ on a wide variety of behavioral, chemical, and molecular measures. This investigation aimed to clarify strain differences in unconditioned motor behavior with respect to the amount and patterns of movements. Twenty female Fischer 344, Lewis, and Sprague-Dawley were tested in the Behavioral Pattern Monitor for 30 min. The locomotor activity and movement patterns, quantified by counts of photobeam breaks and the spatial scaling exponent, d, were assessed. The level of locomotor activity did not differ significantly between Fischer, Lewis, and Sprague-Dawley rats. In contrast, movement patterns differed significantly between the strains. Specifically, Sprague-Dawley rats exhibited significantly more straight movements than both Fischer and Lewis rats. Moreover, Lewis rats showed significantly more straight movements compared to Fischer rats during the first 10 min in the enclosures. Differences in movement patterns across strains may provide an important behavioral variable to further explore the genetic and developmental aspects of behavior.

Chronical haloperidol and clozapine treatment in rats: differential RNA display analysis, behavioral studies and serum level determination

1. Adult, female rats were treated orally for 23 days with 1.6 mg/kg haloperidol or 36 mg/kg clozapine per day, to study chronic effects of the two neuroleptics. 2. At five time points during the neuroleptic treatment, animal behavior was recorded in an open field and locomotive activity was analysed. At the end of the experiment, rats were decapitated, blood samples were collected and serum concentrations of haloperidol and clozapine were determined by a radioreceptor or HPLC assay, respectively. RNA was isolated from each brain, without cerebellum, and subjected to differential RNA display. 3. Mean serum concentrations were 8 ng/ml for haloperidol and 21 ng/ml for clozapine. Analysis of open field behavior showed that haloperidol and clozapine decreased the total distance moved and the velocity as measures of the overall activity, whereas the number of rearings and the number of entries into the center, reflecting risk assessment behavior, were differentially affected. Three neuroleptic-regulated gene fragment bands were identified in differential RNA display experiments. Two gene fragments of 281 bp and 266 bp were sequenced. 4. We conclude that our study design that used behavioral, pharmacokinetic and molecular analysis increase the likelihood of finding relevant molecular events underlying the pharmacotherapeutic effects of neuroleptics in animal models.

Keeping the Body Straight in the Unconstrained Locomotion of Normal and Dopamine-Stimulant-Treated Rats

During unconstrained locomotor behavior, rats move in and out of a straight posture of the body (including the head). In the present study, the stability of maintaining a straight body was examined in untreated rats and in rats treated with saline (SAL) or with 1 of 3 dopamine stimulants (n = 4 rats per group). The stability of maintaining a straight body can range from very high (with 0.5 mg/kg quinpirole [QUIN]), to high (first half-session with 5 mg/kg (+)-amphetamine [AMPH]), to very low (second half-session with 5 mg/kg AMPH), or can be maintained at a level similar to that observed in untreated rats (with 1.25mg/kg apomorphine [APO]). Stability was assessed by videotaping the rats and, then, by using frame-by-frame analysis, scoring the cumulative proportion of time spent in a straight posture, the frequency of transitions from one hemisphere to the other without being trapped in the midline plane, and the degree of lateral bending during turning and during walking on a curved path. The present study is one in a series identifying key variables that constrain as many degrees of freedom as possible in rat locomotor behavior. The uncovering of such variables is an indispensible step that precedes dynamic systems stability analysis and provides candidates for key variables for the modeling of motor coordination.

Development of an affordable hi-resolution activity monitor system for laboratory animals

We describe a hardware and software system for recording and analyzing the spatial and temporal pattern of locomotor activity of laboratory animals. The system offers maximal spatial resolution 500-fold greater than existing light beam monitors. An infrared motion analysis systems (MacReflex, Qualysis) simultaneously tracks the location of up to 20 subjects (identified by reflective markers) to within 0.04 mm at a rate of up to 50 Hz. Macintosh software provides measures of distance traveled, amount of area traversed, number of position changes (microevents), average time between movements, number of left and right turns, number of forward movements and reversals, as well as temporal and spatial scaling exponents. This system was validated by comparing these parameters to direct observer scoring of video tapes and other commercially available activity monitors. Our findings show that applying reflective markers to the subjects does not significantly alter activity levels. The effect of pharmacological manipulation with d-amphetamine is provided to show the value of the different activity parameters. The main advantages of this system are very high spatial resolution, capacity to monitoring up to 20 animals simultaneously at reasonable cost, and lack of sensitivity of the system to ambient lighting. The main limitation is the need to apply reflective markers.

Development of an animal model of fluoxetine akathisia

1. Akathisia describes the pattern of intense inner restlessness often associated with neuroleptic and antidepressant treatment. 2. The authors postulated that drug-induced akathisia would be characterized by more position changes and less time spent immobile, in the absence of significant increase in ambulation. In contrast, a psychomotor stimulant would produce both activation and ambulation. 3. Procedures and instruments were developed to test this hypothesis. Adult rats were habituated for 72 hours to the testing environment, and their precise pattern of movements was tracked and recorded (10 reading per second; resolution 0.04 mm) by an infrared motion analysis system. Activity was recorded for a 90 min period after a single injection of sub-stereotypic doses of d-amphetamine (0, 0.3, 1.0 mg/kg) or racemic fluoxetine (0, 3.0, 10.0, 20.0, or 30.0 mg/kg, s.c.). 4. Amphetamine produced both activation and ambulation. Activation was indicated by a decrease in time spent immobile, and an increase in the temporal scaling exponent, which reflects the degree the animal is "acting' in its environment, and the number of position changes. Enhanced locomotion was inferred from marked increases in both the total distance traversed and the ratio of forward movements-to-reversals and a decrease in the spatial scaling exponent, indicative of a less complex and more linear movement pattern. 5. Fluoxetine caused animals to spend more time active, but exerted little effect on locomotion. Activation was indicated by a decrease in time spent immobile and an increase in the temporal scaling exponent and number of position changes. Fluoxetine failed to significantly effect either the ratio of forward movements-to-reversals or the spatial scaling exponent. 6. These findings provide an operational definition and methodology that can be used to differentiate between psychostimulant effects and akathisic effects. This approach may have utility for screening drugs for akathisic potential, for exploring underlying mechanisms, and for developing novel treatments.

A phase plane representation of rat exploratory behavior

Rat spontaneous spatial behavior is considered to be stochastic and is therefore commonly analyzed in terms of cumulative measures. Here, we suggest a method which generates a moment-to-moment representation of this behavior. It has been proposed earlier that rat spatial behavior can be partitioned into natural units termed excursions (round trips) performed from a reference place termed the rat's home base. We offer a phase plane representation of excursions (plotting the rat's momentary location against its momentary velocity). The results reveal a geometrical pattern, typical of young age and early exposure. It consists of low velocity and intermittent progression while moving away from the home base (upstream segment), and high velocity while moving back to it (downstream segment). The asymmetry between the two segments defines a field of significance in the rat's operational world. This field undergoes regular transformations, revealing thereby the rat's strategy of occupancy of the environment. The presented dynamics could provide a framework for the interpretation of concurrent neural events associated with navigation and spatial memory.

Stimulation of D1- or D2-receptors in drug-naive rats with different degrees of unilateral nigro-striatal dopamine lesions

We had previously found that in animals with moderate nigro-striatal dopamine (DA) lesions (i.e. 45-65% residual neostriatal DA) the mixed D1/D2-agonist apomorphine induced ipsiversive rather than the usual contraversive turning found after more radical DA lesions. Since this result promised to provide a behavioral animal model for pre-clinical Parkinson's disease, we hoped to delineate the responsible receptor by challenging with selective D1- and D2-agonists. Thus, in the present study, the behavioral effects of the D1-agonist SKF38393 (5.0 mg/kg) and the D2-agonist LY171555 (0.5 mg/kg) were tested in drug-naive rats with unilateral 6-hydroxydopamine lesions of the nigro-striatal DA system. This analysis was performed dependent on the degree of the lesion, classified post-mortem with respect to the level of residual DA in the neostriatum: < 20%, 20-45%, 45-65%, and > 65% (as percentage of the intact hemisphere). The measures of turning, thigmotactic scanning and locomotion did not yield differences between animals treated with the D1-agonist and vehicle-treated rats. For example, animals with severe lesions (residual DA < 20%) showed ipsiversive asymmetries in turning and scanning, which were similar after vehicle or the D1-agonist, both with respect to degree and time-course. However, the analysis of grooming behavior, which was performed in a subset of animals with moderate lesions yielded differences between vehicle and the D1-agonist, since the duration of grooming was increased after SKF38393. In contrast to the D1-agonist, behavioral effects after the D2-agonist LY17155 were evident in all behavioral measures. The general response to this agonist could be characterized by a rapid decrease of behavioral activity including turning, scanning, locomotion and grooming. Although we failed to find significant behavioral asymmetries with either agonist, a micro-analysis showed evidence for selective effects after the D2-agonist, since a contraversive asymmetry in turning (and scanning) became apparent between 45 and 60 min after injection in animals with severe lesions (residual DA of about 10% or less), and since there was a weak ipsiversive turning asymmetry in animals with residual DA levels of 45-65%. Such asymmetries were not observed after vehicle or the D1-agonist. The possible physiological mechanisms of these effects, i.e. DA receptor mechanisms and DA availability, are discussed in the context of results from previous experiments using lesioned or intact animals.

Stopping behavior: constraints on exploration in rats (Rattus norvegicus)

In the absence of an obvious reference place, rat locomotor behavior in a novel environment appears haphazard. In previous work, one or two places termed home bases, were shown to stand out from all the other places in the environment in terms of the behaviors performed in them and in terms of their behavioral stability. We use home base location as a reference place for rat movement in locale space, by defining an excursion as a trip starting at a home base and ending at the next stop at a home base. We then establish the uniform distribution as an appropriate model for the number of stops per excursion. This way we show that there is an intrinsic upper bound on the number of times a rat stops during an excursion. As a rat leaves the home base, home base attraction increases with every additional stop performed by it, first slowly and then fast. This cumulative process of attraction may be concluded after each stop, as long as the number of stops does not exceed an intrinsic upper bound; once the upper bound is reached, the rat concludes that excursion and returns to base. The session's upper bound does not increase with the size of the explored area.

Quantitative assessment of the microstructure of rat behavior: I, f(d), the extension of the scaling hypothesis

Previous studies demonstrated that drug effects on the movement sequences of rats in unconditioned motor activity paradigms can be quantified by scaling measures that describe the average relationship between a variable of interest and an experimental parameter. However, rats engage in a wide variety of geometrically distinct movements that can be influenced differentially by drugs. In this investigation, the extended scaling approach is presented to capture quantitatively the relative contributions of geometrically distinct movement sequences to the overall path structure. The calculation of the spectrum of local spatial scaling exponents, f(d), is based on ensemble methods used in statistical physics. Results of the f(d) analysis confirm that the amount of motor activity is not correlated with the geometrical structure of movement sequences. Changes in the average spatial scaling exponent, d, correspond to shifting the entire f(d) function, and indicate overall changes in path structure. With the extended scaling approach, straight movement sequences are assessed independently from highly circumscribed movements. Thus, the f(d) function identifies drug effects on particular ranges of movement sequences as defined by the geometrical structure of movements. More generally, the f(d) function quantifies the relationship between microscopically recorded variables, in this paradigm consecutive (x,y) locations, and the macroscopic behavioral patterns that constitute the animal's response topography.

Quantitative assessment of the microstructure of rat behavior: II. Distinctive effects of dopamine releasers and uptake inhibitors

The effects of four indirect dopamine agonists, d-amphetamine (0.25-4.0 mg/kg), cocaine (2.5-40.0 mg/kg), GBR 12909 (10.0-30.0 mg/kg), and nomifensine (5.0-20.0 mg/kg), on the behavioral organization of movements in an unconditioned motor paradigm were investigated in rats. The extended scaling hypothesis using the fluctuation spectrum of local spatial scaling exponents was used to quantify the geometrical characteristics of movements. The results reveal a qualitatively similar disruption of behavioral organization by lower doses of these drugs. Specifically, rats treated with d-amphetamine (< 2.0 mg/kg), cocaine (< 20.0 mg/kg), GBR 12909 (< 20.0 mg/kg), or nomifensine (< 10.0 mg/kg) exhibited a reduced range in the fluctuation spectrum, reflecting a predominance of meandering movements with local spatial scaling exponents between 1.3 and 1.7. This reduction was accompanied dynamically by a reduced predictability of movement sequences as measured by the dynamical entropy, h. By contrast, higher doses of these drugs produced distinctly different changes in behavioral organization. In particular, 4.0 mg/kg d-amphetamine and 40.0 mg/kg cocaine increased the fluctuation range, reflecting relative increases in both straight and circumscribed movements that are interpreted as a combination of spatially extended and local perseveration. In contrast, high doses of 30.0 mg/kg GBR 12909 and 20.0 mg/kg nomifensine induced only local perseveration. High doses of d-amphetamine, cocaine, GBR 12909 and nomifensine reduced the dynamical entropy, h, indicating an increased predictability of the movement sequences. These results suggest that the generic behavioral change induced by low doses of dopamine agonists is characterized by a reduced variety of path patterns coupled with an increased variability in sequential movement sequences. The differential effects of higher doses of these drugs may be due to their influences on other neurotransmitter systems or differential affinities for different dopamine subsystems.