Temporal and environmental effects on quinpirole-induced biphasic locomotion in rats

Early-life risperidone alters locomotor responses to apomorphine and quinpirole in adulthood

An escalating trend of antipsychotic drug use in children with ADHD, disruptive behavior disorder, or mood disorders has raised concerns about the impact of these drugs on brain development. Since antipsychotics chiefly target dopamine receptors, it is important to assay the function of these receptors after early-life antipsychotic administration. Using rats as a model, we examined the effects of early-life risperidone, the most prescribed antipsychotic drug in children, on locomotor responses to the dopamine D1/D2 receptor agonist, apomorphine, and the D2/D3 receptor agonist, quinpirole. Female and male Long-Evans rats received daily subcutaneous injections of risperidone (1.0 and 3.0mg/kg) or vehicle from postnatal day 14-42. Locomotor responses to one of three doses (0.03, 0.1, and 0.3mg/kg) of apomorphine or quinpirole were tested once a week for four weeks beginning on postnatal day 76 and 147 for each respective drug. The locomotor activity elicited by the two lower doses of apomorphine was significantly greater in adult rats, especially females, administered risperidone early in life. Adult rats administered risperidone early in life also showed more locomotor activity after the low dose of quinpirole. Overall, female rats were more sensitive to the locomotor effects of each agonist. In a separate group of rats administered risperidone early in life, autoradiography of forebrain D2 receptors at postnatal day 62 revealed a modest increase in D2 receptor density in the medial caudate. These results provide evidence that early-life risperidone administration can produce long-lasting changes in dopamine receptor function and density.

The impact of dopamine D2-like agonist/antagonist on [18F]VAT PET measurement of VAChT in the brain of nonhuman primates

Vesicular acetylcholine transporter (VAChT) is a promising target for a PET measure of cholinergic deficits which contribute to cognitive impairments. Dopamine D2-like agonists and antagonists are frequently used in the elderly and could alter cholinergic function and VAChT level. Therefore, pretreatment with dopamine D2-like drugs may interfere with PET measures using [18F]VAT, a specific VAChT radioligand. Herein, we investigated the impact of dopaminergic D2-like antagonist/agonist on VAChT level in the brain of macaques using [18F]VAT PET. PET imaging studies were carried out on macaques at baseline or pretreatment conditions. For pretreatment, animals were injected using a VAChT inhibitor (-)-vesamicol, a D2-like antagonist (-)-eticlopride, and a D2-like agonist (-)-quinpirole, separately. (-)-Vesamicol was injected at escalating doses of 0.025, 0.05, 0.125, 0.25 and 0.35 mg/kg; (-)-eticlopride was injected at escalating doses of 0.01, 0.10 and 0.30 mg/kg; (-)-quinpirole was injected at escalating doses of 0.20, 0.30, and 0.50 mg/kg. PET data showed [18F]VAT uptake declined in a dose-dependent manner by (-)-vesamicol pretreatment, demonstrating [18F]VAT uptake is sensitive to reflect the availability of VAChT binding sites. Furthermore, (-)-eticlopride increased [18F]VAT striatal uptake in a dose-dependent manner, while (-)-quinpirole decreased its uptake, suggesting striatal VAChT levels can be regulated by D2-like drug administration. Our findings confirmed [18F]VAT offers a reliable tool to in vivo assess the availability of VAChT binding sites. More importantly, PET with [18F]VAT successfully quantified the impact of dopaminergic D2-like drugs on striatal VAChT level, suggesting [18F]VAT has great potential for investigating the interaction between dopaminergic and cholinergic systems in vivo.

The scalar property during isochronous tapping is disrupted by a D2-like agonist in the nonhuman primate

Dopamine, and specifically the D2 system, has been implicated in timing tasks where the absolute duration of individual time intervals is encoded discretely, yet the role of D2 during beat perception and entrainment remains largely unknown. In this type of timing, a beat is perceived as the pulse that marks equally spaced points in time and, once extracted, produces the tendency in humans to entrain or synchronize their movements to it. Hence, beat-based timing is crucial for musical execution. In this study we investigated the effects of systemic injections of quinpirole (0.005–0.05 mg/kg), a D2-like agonist, on the isochronous rhythmic tapping of rhesus monkeys, a classical task for the study of beat entrainment. We compared the rhythmic timing accuracy, precision, and the asynchronies of the monkeys with or without the effects of quinpirole, as well as their reaction times in a control serial reaction time task (SRTT). The results showed a dose-dependent disruption in the scalar property of rhythmic timing due to quinpirole administration. Specifically, we found similar temporal variabilities as a function of the metronome tempo at the largest dose, instead of the increase in variability across durations that is characteristic of the timing Weber law. Notably, these effects were not due to alterations in the basic sensorimotor mechanism for tapping to a sequence of flashing stimuli, because quinpirole did not change the reaction time of the monkeys during SRTT. These findings support the notion of a key role of the D2 system in the rhythmic timing mechanism, especially in the control of temporal precision.

NEW & NOTEWORTHY Perceiving and moving to the beat of music is a fundamental trait of musical cognition. We measured the effect of quinpirole, a D2-like agonist, on the precision and accuracy of rhythmic tapping to a metronome in two rhesus monkeys. Quinpirole produced a flattening of the temporal variability as a function of tempo duration, instead of the increase in variability across durations that is characteristic of the scalar property, a hallmark property of timing.

Prosocial effects of prolactin in male rats: Social recognition, social approach and social learning

Prolactin (PRL) and oxytocin (OT) are pituitary hormones essential for lactation, but also promote sexual behavior. OT stimulates social behaviors, such as recognition, approach, and learning, but less is known about PRL in these behaviors. Since PRL and OT have complementary functions in reproduction, we hypothesized that PRL increases social recognition, approach, and learning. Male Long-Evans rats received ovine PRL (oPRL; 0.5, 2.0 or 5.0mg/kg), the PRL antagonist bromocriptine (0.1, 3.0 or 5.0mg/kg) or saline 20 mins before testing for recognition of familiar vs. unfamiliar stimulus males. Saline controls preferred the unfamiliar male (p<0.05), while bromocriptine blocked this preference. oPRL did not increase preference. To measure social approach, we determined if PRL restores approach 2h after defeat by an aggressive male. Defeated rats avoided the aggressive male. 2mg/kg oPRL, before or after defeat, restored approach towards the aggressive male (p<0.05). In non-defeated rats, oPRL or 3mg/kg bromocriptine had no effect. To determine if PRL increases social learning, we tested social transmission of food preference. Rats choose between two unfamiliar flavors, one of which they have previously been exposed to through interaction with a demonstrator rat. Vehicle controls preferred chow with the demonstrated flavor over the novel flavor. oPRL-treated rats were similar. Bromocriptine-treated rats failed to show a preference. When tested one week later, only oPRL-treated rats preferred the demonstrated flavor. The results suggest that PRL is required for social recognition and learning, and that increasing PRL enhances social memory and approach, similar to OT.

The effects of long-term dopaminergic treatment on locomotor behavior in rats

Long-term treatments with dopaminergic agents are associated with adverse effects, including augmentation. Augmentation consists of an exacerbation of restless legs syndrome (a sleep-related movement disorder) symptoms during treatment compared to those experienced during the period before therapy was initiated. The objective of this study was to examine locomotor activity in rats after long-term dopaminergic treatment and its relationship with expression of the D2 receptor, in addition to demonstrating possible evidence of augmentation. The rats were divided into control (CTRL) and drug (Pramipexole—PPX) groups that received daily saline vehicle and PPX treatments, respectively, for 71 days. The locomotor behavior of the animals was evaluated weekly in the Open Field test for 71 days. The expression of the dopamine D2 receptor was evaluated by Western Blot analysis. The animals that received the PPX demonstrated a significant reduction in locomotor activity from day 1 to day 57 and a significant increase in immobility time from day 1 to day 64 relative to baseline values, but these values had returned to baseline levels at 71 days. No changes in the expression of the D2 receptor were demonstrated after treatment with a dopaminergic agonist. This study suggests changes in locomotor activity in rats after long-term PPX treatment that include an immediate reduction of locomotion and an increase in immobilization, and after 64 days, these values returned to baseline levels without evidence of augmentation. In addition, it was not possible to demonstrate a relationship between locomotor activity and the expression of D2 receptors under these conditions.

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.

Repeated exposure to MDMA and amphetamine: sensitization, cross-sensitization, and response to dopamine D₁- and D₂-like agonists

RATIONALE

Acute exposure to (±) 3, 4-methylenedioxymethamphetamine (MDMA) produces hyperlocomotion that is preferentially expressed in the periphery of closed chambers. Following repeated administration, however, a sensitized hyperlocomotor response is preferentially expressed in the center of an activity box, so that the response resembles the more generalized activity that is produced by D-amphetamine (AMPH).

OBJECTIVES

The present study was designed to determine whether common neuroadaptations underlie the acute and sensitized responses to MDMA and AMPH.

METHODS

Rats were pretreated with five daily injections of MDMA (10.0 mg/kg), AMPH (2.0 mg/kg), or saline. Following a 2-day drug-free period, dose-response curves for hyperactivity produced by MDMA (2.5-10.0 mg/kg), AMPH (0.5-2.0 mg/kg), SKF-81297 (1.0-2.0 mg/kg), or quinpirole (0.25-1.0 mg/kg) were obtained.

RESULTS

Effects of MDMA and AMPH were increased by pretreatment with both drugs. The sensitized response following MDMA exposure was preferentially expressed in the center compartment, but, following AMPH pretreatment, the sensitized response was observed in both compartments. Cross-sensitization was unidirectional; AMPH pretreatment failed to sensitize to the effects of MDMA, but MDMA pretreatment sensitized to the effects of AMPH. MDMA and AMPH pretreatment produced marginal increases in the effects of SKF-81297. The response to quinpirole was, however, greater following MDMA, but not AMPH, pretreatment.

CONCLUSIONS

These data suggest that repeated MDMA exposure produces sensitization via a unique neurochemical effect.

Quinpirole elicits differential in vivo changes in the pre- and postsynaptic distributions of dopamine D₂ receptors in mouse striatum: relation to cannabinoid-1 (CB₁) receptor targeting

RATIONALE

The nucleus accumbens (Acb) shell and caudate-putamen nucleus (CPu) are respectively implicated in the motivational and motor effects of dopamine, which are mediated in part through dopamine D₂-like receptors (D₂Rs) and modulated by activation of the cannabinoid-1 receptor (CB₁R). The dopamine D(₂/D3) receptor agonist, quinpirole elicits internalization of D₂Rs in isolated cells; however, dendritic and axonal targeting of D₂Rs may be highly influenced by circuit-dependent changes in vivo and potentially influenced by endogenous CB₁R activation.

OBJECTIVE

We sought to determine whether quinpirole alters the surface/cytoplasmic partitioning of D₂Rs in striatal neurons in vivo.

METHODS

To address this question, we examined the electron microscopic immunolabeling of D₂ and CB₁ receptors in the Acb shell and CPu of male mice at 1 h following a single subcutaneous injection of quinpirole (0.5 mg/kg) or saline, a time point when quinpirole reduced locomotor activity.

RESULTS

Many neuronal profiles throughout the striatum of both treatment groups expressed the D₂R and/or CB₁R. As compared with saline, quinpirole-injected mice showed a significant region-specific decrease in the plasmalemmal and increase in the cytoplasmic density of D₂R-immunogold particles in postsynaptic dendrites without CB₁R-immunolabeling in the Acb shell. However, quinpirole produced a significant increase in the plasmalemmal density of D₂R immunogold in CB₁R negative axons in both the Acb shell and CPu.

CONCLUSIONS

Our results provide in vivo evidence for agonist-induced D₂R trafficking that is inversely related to CB₁R distribution in postsynaptic neurons of Acb shell and in presynaptic axons in this region and in the CPu.

Genetic study of BDNF, DRD3, and their interaction in tardive dyskinesia

Tardive dyskinesia (TD) is a neuroleptic-induced movement disorder. Its pathophysiology is unclear. The most consistent genetic findings have shown an association with the Ser9Gly polymorphism of the DRD3 gene. However, only few polymorphisms within DRD3 has been tested, and a comprehensive examination of DRD3 in TD is still lacking. Further, brain-derived neurotrophic factor (BDNF), a neuronal growth and survival peptide, regulates DRD3 expression and may be involved in the neuronal degeneration observed in TD. In the present study, we investigated 15 tag DRD3 polymorphisms and four tag BDNF polymorphisms for association with TD in our sample of Caucasian schizophrenia patients (N=171). While BDNF markers showed no association, a haplotype containing rs3732782, rs905568, and rs7620754 in the 5' region of DRD3 was associated with TD diagnosis (p[10,000 permutations]=0.007). We also found evidence of interaction between BDNF and DRD3 polymorphisms. Further studies are needed to confirm these findings.

Stereotypic behaviour in the deer mouse: pharmacological validation and relevance for obsessive compulsive disorder

Stereotypy is an important manifestation of obsessive compulsive disorder (OCD). OCD involves disturbed serotonin and dopamine pathways, and demonstrates a selective response to serotonin reuptake inhibitors (SRI), with limited to no response to noradrenaline reuptake inhibitors (NRI). Deer mice (Peromyscus maniculatus bairdii) engage in various spontaneous stereotypic behaviours, including somersaulting, jumping and pattern running, and has to date not been explored for possible relevance for OCD. We studied the population diversity of spontaneous stereotypy in these animals, followed by assessing behavioural response to chronic high and low dose SRI (viz. fluoxetine) and NRI (viz. desipramine) treatment (both 10 mg/kg; 20 mg/kg x 21 days). We also studied behavioural responses to the 5-HT(2A/C) agonist, meta-chlorophenylpiperazine (mCPP) and the D2 agonist, quinpirole (2 mg/kg and 5 mg/kg respectively x 4 days). Deer mice showed a distinct separation into high and low stereotypic behaviour populations, with high and low dose fluoxetine, but not desipramine, significantly reducing stereotypic behaviour in both populations. A significant attenuation of stereotypy was also observed in both groups following quinpirole or mCPP challenge. In its response to drug treatment, spontaneous stereotypic behaviour in deer mice demonstrates predictive validity for OCD. States of spontaneous stereotypy are attenuated by 5-HT(2A/C) and dopamine D2 receptor agonists.

Basal local cerebral glucose utilization is not altered after behavioral sensitization to quinpirole

Sensitization to psychostimulants results in a behavioral response of a greater magnitude than that produced by a given single dose. Previously, we have shown that sensitization to the D(2)/D(3) dopamine receptor agonist quinpirole produces alterations in quinpirole-stimulated local cerebral glucose utilization (LCGU) in ventral striatal and limbic cortical regions. To determine whether basal neuronal activity is altered in the sensitized animal, this study examined the effects of a sensitizing course of quinpirole on basal neuronal activity using the [(14)C]-2-deoxyglucose (2-DG) method in rats with verified sensitization. Adult, male Long-Evans rats (n = 7 or 10/group) were subjected to 10 injections of quinpirole (0.5 mg/kg, s.c.) or saline administered every 3rd day. Sensitization was verified on the basis of locomotor activity. The 2-DG procedure was performed in freely moving rats 3 days after the last quinpirole injection. LCGU was determined by quantitative autoradiography. No alterations in basal LCGU were detected in quinpirole-sensitized rats compared to those treated with saline. The present finding suggests that either the basal activity of very discrete populations of neurons is affected by sensitization to quinpirole that are not likely to be detected by the 2-DG method, or that the neurobiological changes that result in the sensitized behavioral response affect only stimulated, but not basal, neuronal activity.

Reduction of corticostriatal glutamatergic fibers in basic fibroblast growth factor deficient mice is associated with hyperactivity and enhanced dopaminergic transmission

BACKGROUND

Basic fibroblast growth factor (FGF2) plays a crucial role during the development of the cerebral cortex. Mice with a knockout of the FGF2 gene have a reduced number of glutamatergic neurons within the deep layers of the cerebral cortex.

METHODS

We used molecular and behavioral analyses to investigate possible alterations in corticostriatal function in FGF2 -/- mice.

RESULTS

We found that FGF2 deficiency leads to decreased expression of presynaptic markers of integrity for glutamatergic fibers in the striatum, namely the membrane excitatory amino acid transporter 3 (EAAT3) and the vesicular glutamate transporter 1 (VGLUT1). The reduction of corticostriatal glutamatergic function in FGF2 -/- mice is associated with enhanced locomotor activity in a novel environment and increased responsiveness to dopaminergic drugs, such as cocaine or amphetamine. The behavioral alterations of FGF2 -/- can be normalized by injection of a low dose of the dopaminergic agonist apomorphine (.1 mg/kg) that reduces dopamine release by acting on presynaptic receptors.

CONCLUSIONS

Our data demonstrate that FGF2 -/- mice have an increased tone and responsiveness of the dopaminergic system and suggest that these animals might represent a model to study disorders that are characterized by an imbalance between glutamatergic and dopaminergic neurotransmission.

Clorgyline-induced modification of behavioral sensitization to quinpirole: effects on local cerebral glucose utilization

Sensitization refers to augmented behavioral responses produced by repeated, intermittent injections of dopaminergic psychostimulants. The locomotor manifestations observed after a sensitizing course of quinpirole, a D(2)/D(3) dopamine agonist, can be modified by the MAO(A) inhibitor clorgyline, by a mechanism apparently unrelated to its actions on MAO(A). Alterations in regional neuronal activity produced by quinpirole in quinpirole-sensitized rats with or without clorgyline pretreatment were assessed based on LCGU using the [(14)C]-2-deoxyglucose (2-DG) method. Adult, male Long-Evans rats (180-200 g, n=9-10/group) were subjected to an injection of either clorgyline (1.0 mg/kg, s.c.) or saline 90 min prior to an injection of quinpirole (0.5 mg/kg, s.c.) or saline, 1 set of injections administered every 3rd day for 10 sets. The 2-DG procedure was initiated 60 min after an 11th set of injections in freely moving rats. LCGU was determined by quantitative autoradiography. LCGU was decreased in a number of limbic (nucleus accumbens and ventral pallidum) and cortical (medial/ventral orbital and infralimbic) regions and in the raphe magnus nucleus in quinpirole-sensitized rats (P<0.05 vs. saline-saline). Quinpirole-sensitized rats pretreated with clorgyline had similar alterations in LCGU, but LCGU was higher in the locus coeruleus compared to quinpirole alone (P<0.05), was not decreased in the raphe magnus nucleus, and was decreased in the piriform cortex and septum. This implicates altered activity of the noradrenergic, serotonergic, olfactory, and limbic systems in the modified behavioral response to quinpirole with clorgyline pretreatment.

Less is more: pathophysiology of dopaminergic-therapy-related augmentation in restless legs syndrome

Therapy-related augmentation of the symptoms of restless legs syndrome (RLS) is an important clinical problem reported in up to 60% of patients treated with levodopa and, to a lesser extent, with dopamine agonists. The efficacy of low-dose dopaminergic drugs for RLS has been established, but the mode of action is unknown. Here, we review the existing data and conclude that augmentation is a syndrome characterised by a severely increased dopamine concentration in the CNS; overstimulation of the dopamine D1 receptors compared with D2 receptors in the spinal cord may lead to D1-related pain and generate periodic limb movements; iron deficiency may be a main predisposing factor of augmentation, probably caused by a reduced function of the dopamine transporter; therapy with levodopa or dopamine agonists should remain at low doses and; iron supplementation and opiates are the therapy of choice to counter augmentation.

Sex differences in neurochemical effects of dopaminergic drugs in rat striatum

Previous data indicate that dopamine neurotransmission is differently regulated in male and female rats. The purpose of the present study was to investigate the dopamine transporter and autoreceptor as potential loci responsible for this sex difference. Fast cyclic voltammetry at carbon-fiber microelectrodes was used to monitor changes in electrically evoked levels of extracellular dopamine in the striata of anesthetized male and female rats before and after administration of an uptake inhibitor, a dopamine D2 antagonist, or a D3/D2 agonist. Administration of 40 mg/kg cocaine ip increased electrically-evoked extracellular dopamine concentrations in both sexes, but to a significantly greater extent in female striatum at the higher stimulation frequencies. The typical antipsychotic, haloperidol, increased dopamine efflux in both sexes but the effect was twice as large in the female striatum. The D3/D2 agonist quinpirole induced an unexpected, transient increase in dopamine efflux following high-frequency stimulation only in females, and evoked dopamine was higher in females across this entire time course. More detailed analysis of cocaine effects revealed no fundamental sex differences in the interaction of cocaine with DAT in vivo or in synaptosomes. These results indicate that nigrostriatal dopamine neurotransmission in the female rat is more tightly regulated by autoreceptor and transporter mechanisms, perhaps related by greater autoreceptor control of DAT activity. Thus, baseline sex differences in striatal dopamine regulation induce different pharmacologic responses. These results contribute to understanding sex differences in stimulant-induced locomotor activity in rats and may have broader implications for neurologic disorders and their pharmacotherapies in humans.

Locomotor activity and cocaine-seeking behavior during acquisition and reinstatement of operant self-administration behavior in rats

Recent studies indicate that administration of dopamine D2-like receptor agonists reinstates drug-seeking behavior in rodents, whereas dopamine D1-like receptor agonists do not. These effects have been related to the ability of these agonists to facilitate the expression of sensitized locomotor activity. Presently, we describe experiments in which locomotor activity was assessed concomitantly with operant performance during acquisition, extinction and reinstatement. We report that locomotor activity was inversely related to drug-seeking behavior during acquisition of cocaine self-administration under a Fixed Ratio (FR) 1 schedule of reinforcement. During a single trial extinction session, animals that had acquired cocaine self-administration exhibited a conditioned increase in drug-seeking behavior, but there was no evidence of a conditioned locomotor response. During reinstatement, cocaine (20 mg/kg) significantly increased both locomotor activity and drug-seeking behavior. The dopamine D2-like receptor agonist quinpirole (0.5 mg/kg) increased drug-seeking behavior, but did not significantly increase locomotor activity. In contrast, the dopamine D1-like receptor agonist SKF 81297 (0.5 mg/kg) failed to reinstate drug-seeking behavior, but produced significant locomotor activation. To determine whether the inability of SKF 81297 to promote reinstatement is related to the strength of operant conditioning, additional rats were trained to self-administer cocaine using an FR-3 schedule of reinforcement. Despite achieving response rates during training almost four times higher compared to the FR-1 condition, administration of SKF 81297 again failed to significantly increase drug-seeking behavior during reinstatement testing. These results extend previous findings, confirming the important role of D2-like, but not D1-like receptor activation in the reinstatement of drug-seeking behavior. An understanding of the mechanisms by which D1- and D2-like agonists differentially influence locomotor activation and drug-seeking behavior in cocaine-experienced rodents may prove critical to the development of increasingly effective pharmacotherapies for substance abuse.

Altered quinpirole-induced local cerebral glucose utilization in anterior cortical regions in rats after sensitization to quinpirole

Dopaminergic psychostimulants produce behavioral responses of greater magnitude with repeated, intermittent administration, than a single, acute dose, a phenomenon known as "sensitization." Most studies of sensitization have focused on the "motive circuit"; however, some additional anterior cortical regions also appear to be affected. In this study, alterations in regional neuronal activity in anterior cortical brain areas produced by quinpirole, a D(2)/D(3) agonist, were assessed on the basis of local cerebral glucose utilization (LCGU) using the [(14)C]-2-deoxyglucose (2-DG) method. Adult, male Long-Evans rats (180-200 g, n = 7-9/group) were subjected to ten injections of quinpirole (0.5 mg/kg, s.c.) administered every third day; controls and drug-naive rats received saline. Locomotor activity was quantitated after injections one and ten to confirm sensitization. The 2-DG procedure was initiated 60 min after an 11th injection in freely moving rats. LCGU was determined in 11 anterior cortical brain regions by quantitative autoradiography. In drug-naive rats, quinpirole decreased LCGU in the cingulate cortex-area 3 (-16%) and infralimbic cortex (-16%). In sensitized rats, quinpirole decreased LCGU in the cingulate cortex-area 1 (-19%), frontal cortex-area 3 (-19%), lateral orbital cortex (-18%), medial/ventral orbital cortex (-17%), and parietal cortex (-17%) as well as in the cingulate cortex-area 3 (-19%) and infralimbic cortex (-20%); (all P < 0.05 v. control). This suggests that decreased neuronal activity in the cingulate cortex-area 1, frontal cortex-area 3, lateral orbital cortex, medial/ventral orbital cortex, and parietal cortex, in addition to altered activity in the motive circuit, may underlie the augmented behavioral response to quinpirole in sensitized animals.

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.

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.

Dopaminergic supersensitivity in G protein-coupled receptor kinase 6-deficient mice

Brain dopaminergic transmission is a critical component in numerous vital functions, and its dysfunction is involved in several disorders, including addiction and Parkinson's disease. Responses to dopamine are mediated via G protein-coupled dopamine receptors (D1-D5). Desensitization of G protein-coupled receptors is mediated via phosphorylation by members of the family of G protein-coupled receptor kinases (GRK1-GRK7). Here we show that GRK6-deficient mice are supersensitive to the locomotor-stimulating effect of psychostimulants, including cocaine and amphetamine. In addition, these mice demonstrate an enhanced coupling of striatal D2-like dopamine receptors to G proteins and augmented locomotor response to direct dopamine agonists both in intact and in dopamine-depleted animals. The present study indicates that postsynaptic D2-like dopamine receptors are physiological targets for GRK6 and suggests that this regulatory mechanism contributes to central dopaminergic supersensitivity.

Differences in quinpirole-induced local cerebral glucose utilization between naive and sensitized rats

Dopaminergic psychostimulants produce behavioral responses of greater magnitude with repeated, intermittent administration, than a single, acute dose, a phenomenon known as 'sensitization'. Alterations in regional neuronal activity produced by quinpirole, a D(2)/D(3) agonist, in quinpirole-naive and quinpirole-sensitized rats were assessed on the basis of local cerebral glucose utilization (LCGU) using the [14C]2-deoxyglucose (2-DG) method. Adult, male Long-Evans rats (180-200 g, n=7-9/group) were subjected to ten injections of quinpirole (0.5 mg/kg, s.c.) administered every 3rd day; controls and quinpirole-naive rats received saline. Locomotor activity was quantitated after injections one and ten to confirm sensitization. The 2-DG procedure was initiated 60 min after an 11th injection in freely moving rats. LCGU was determined in 43 brain regions by quantitative autoradiography. In quinpirole-naive rats, quinpirole decreased LCGU in the caudate/putamen (84% of control), lateral habenula (80% of control), and motor cortex (79% of control). In sensitized rats, quinpirole decreased LCGU in the nucleus accumbens core and shell (77 and 83% of control, respectively) and ventral pallidum (82% of control) as well as in the caudate/putamen (86% of control), lateral habenula (77% of control), and motor cortex (79% of control). This suggests that decreased neuronal activity in the nucleus accumbens and ventral pallidum may underlie the augmented behavioral response to quinpirole in sensitized animals.

Behavioral sensitization to quinpirole is not associated with increased nucleus accumbens dopamine overflow

This study assessed the relationship between extracellular nucleus accumbens (NAc) dopamine (DA) concentrations and sensitized locomotor activation following repeated administration of the DA D2-like receptor agonist quinpirole. Locomotor activity measures and nucleus accumbens microdialysis samples were collected concurrently in response to the first (acute) and tenth (repeated) quinpirole injection (0.5 mg/kg s.c., every other day). Results indicate that acute quinpirole produced locomotor activation and that repeated quinpirole resulted in locomotor sensitization. Acute quinpirole significantly decreased the detection of extracellular concentrations of DA and the DA metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the NAc. Following repeated quinpirole, basal NAc DA levels were decreased, whereas basal DOPAC levels were increased. Nevertheless, quinpirole challenge elicited a significant decrease in DA, DOPAC and HVA following repeated treatment. In addition, although acute quinpirole did not affect NAc levels of the serotonin metabolite 5-hydroxyindolacetic acid (5-HIAA), quinpirole challenge produced a significant increase in 5-HIAA levels following repeated treatment. Taken together, these data indicate that functional DA autoreceptor subsensitivity is not a necessary condition for the expression of behavioral sensitization to quinpirole. Instead, it appears that behavioral sensitization to quinpirole occurs predominantly as a consequence of neuroadaptations that are post-synaptic to DA release.

Isolation rearing-induced deficits in prepulse inhibition and locomotor habituation are not potentiated by water deprivation

Prepulse inhibition (PPI) of the acoustic startle reflex is an operational measure of sensorimotor gating and is reduced in neuropsychiatric disorders such as schizophrenia. Isolation rearing of rats is a developmentally specific, nonpharmacological manipulation that leads to deficits in sensorimotor gating that mimic those observed in schizophrenia patients. This study examined the effects of an added stressor (water deprivation) on the magnitude of the isolation rearing effect on PPI and locomotor activity. At the time of weaning, male (n = 80) and female (n = 80) rats were assigned to either social housing or isolation housing and were subsequently assigned to the water-deprived or non-water-deprived groups. Rats were tested for acoustic startle and PPI at 3, 5 and 7 weeks postweaning. Isolated rats showed a significant decrease in PPI that was apparent at all 3 weeks. Water deprivation did not significantly affect PPI, nor was there a significant interaction between housing and water treatment or between sex and housing. When tested in the Behavior Pattern Monitor to assess locomotor activity, isolated rats displayed decreased habituation across the 1-h test session. Water deprivation did not affect locomotor activity in any significant, independent manner, nor did it potentiate the effects of isolation rearing on locomotor habituation. In these studies, both male and female Long-Evans rats were sensitive to the PPI-disruptive and locomotor-activating effects of social isolation. Isolation rearing significantly disrupts PPI and locomotor habituation independent of any effects of water deprivation.

Tacrolimus, a specific inhibitor of calcineurin, modifies the locomotor activity of quinpirole, but not that of SKF82958, in male rats

In the present study, we examined the effect of tacrolimus, a specific inhibitor of calcineurin, on the locomotor activity elicited by the selective dopamine D(1) receptor agonist (+/-) 6-chloro-7,8-dyhydroxy-3allyl-1-phenyl-2,3,4,5-tetra-hydro-1H-benzazepine (SKF82958) and the dopamine D(2)/D(3) receptor agonist quinpirole, in male Wistar rats. Tacrolimus (0.5, 1, 2 or 5 mg/kg, i.p.) alone had no significant effect on basal locomotor activity. The dose-related increase in locomotor activity produced by the administration of SKF82958 (0.1, 1 or 5 mg/kg, i.p.) was not significantly altered by 2 mg/kg of tacrolimus. In addition, the increase in locomotor activity produced by 1 mg/kg of SKF82958 was not significantly altered by tacrolimus (0.5, 1, 2 or 5 mg/kg, i.p.). The administration of quinpirole (0.1, 0.25, 0.5, 1 or 3 mg/kg, i.p.) produced a biphasic response, with the minimum and maximal increase in locomotor activity occurring at 0.1 and 1 mg/kg, respectively. The pretreatment of 2 mg/kg of tacrolimus, compared to vehicle-treated animals, significantly lowered the dose of quinpirole that produce a maximal effect on locomotor activity from 1 to 0.5 mg/kg but did not significantly alter the minimum response. The increase in locomotor activity produced by 0.5 mg/kg of quinpirole was significantly potentiated by 0.5, 1, 2 or 5 mg/kg of tacrolimus compared to vehicle-treated animals. Our results suggest that calcineurin may play a role in the alteration of locomotor activity produced by dopamine D(2)/D(3) receptors, but not dopamine D(1) receptors.

Hyperactivity and impaired response habituation in hyperdopaminergic mice

Abnormal dopaminergic transmission is implicated in schizophrenia, attention deficit hyperactivity disorder, and drug addiction. In an attempt to model aspects of these disorders, we have generated hyperdopaminergic mutant mice by reducing expression of the dopamine transporter (DAT) to 10% of wild-type levels (DAT knockdown). Fast-scan cyclic voltammetry and in vivo microdialysis revealed that released dopamine was cleared at a slow rate in knockdown mice, which resulted in a higher extracellular dopamine concentration. Unlike the DAT knockout mice, the DAT knockdown mice do not display a growth retardation phenotype. They have normal home cage activity but display hyperactivity and impaired response habituation in novel environments. In addition, we show that both the indirect dopamine receptor agonist amphetamine and the direct agonists apomorphine and quinpirole inhibit locomotor activity in the DAT knockdown mice, leading to the hypothesis that a shift in the balance between dopamine auto and heteroreceptor function may contribute to the therapeutic effect of psychostimulants in attention deficit hyperactivity disorder.

Mapping quantitative trait loci that regulate sensitivity and tolerance to quinpirole, a dopamine mimetic selective for D(2)/D(3) receptors

Acute sensitivity and tolerance to quinpirole (a dopamine mimetic with selectivity for D(2)/D(3) dopamine receptors) were evaluated in the C57BL/6J and DBA/2J inbred strains of mice, 24 of their BXD recombinant inbred strains, and 233 F(2) mice. Baseline locomotor activity, locomotor activity following 0.03 mg/kg quinpirole (and 0. 01 mg/kg in BXD mice), body temperature following 1 mg/kg quinpirole, and hypothermic tolerance following 2 or 3 days of quinpirole administration were evaluated. Quantitative trait locus (QTL) analysis was employed to identify genetic determinants of baseline locomotor activity and five behavioral responses to quinpirole. We examined correlated allelic variation in genetic markers of known chromosomal location with variation in each of these phenotypes. We definitively mapped a QTL on Chromosome (Chr) 9 linked to the D(2) dopamine receptor gene, Drd2, for hypothermic sensitivity to quinpirole, and identify a suggestive QTL in the same chromosomal region for tolerance to quinpirole after repeated treatments. Suggestive QTLs were also identified on Chr 19 for sensitivity and tolerance to quinpirole-induced hypothermia and for baseline locomotor activity; on Chr 15 for locomotor sensitivity to quinpirole; and on Chr 13 and 5 for baseline locomotor activity. Our results indicate that genetic differences in quinpirole sensitivity and tolerance are associated with QTLs near Drd2, and that baseline locomotor activity is associated with a suggestive QTL in proximity to the dopamine transporter gene Dat1. These data suggest that the genes influencing locomotor activity, dopamine mimetic sensitivity, and tolerance do not overlap completely.

Diazepam-induced hyperphagia in mice is sensitive to quinpirole

The present trial examined the possibility that diazepam (DZP, 1 mg/kg) induces hyperphagia by acting on the dopaminergic system. Quinpirole (QP), dopamine D-2 receptor agonist, was used for this purpose. Mice fasted for 24 hr were treated with QP 1 (QP-1) or 2 (QP-2) mg/kg 30 min prior to termination of the starvation. DZP was given to untreated mice and half of the QP-1 and QP-2 treated mice 10 min before the termination of the starvation. Food consumed during six 30 min intervals (30 min-feeding), food consumed for 3 hr (total feeding), time required to enter the room containing food by passing through a maze with four multiple routes (time to banquet), latent period to commencement of eating food after entering the banquet room (latent period), and feeding frequency for the 30 min intervals as well as for 3 hr were measured. DZP stimulated feeding, shortened the latent period without affecting the time to banquet and increased the feeding frequency. The hyperphagic effect was restricted to the first 30 min interval only. Both QP-1 and QP-2 first reduced, then progressively stimulated, and finally reduced feeding without modifying total feeding, thus making a bell-shaped profile. They also prolonged both the time to banquet and the latent period, and reduced the feeding frequency of the first 30 min interval but not that for 3 hr. Both QP-1 and QP-2 canceled all the effects of DZP. These results imply that dopamine D2 receptor is involved in the induction of hyperphagia by DZP.

Locomotion elicited by MK801 in developing and adult rats: temporal, environmental, and gender effects

The effects of environmental novelty on locomotion elicited by an N-methyl-D-aspartate (NMDA) receptor antagonist, (+)MK-801 hydrogen maleate [(5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5, 10-imine], were investigated. Male and female rats aged 10, 20, 30 or 54-68 days were injected s.c. with MK801 and placed in activity monitors either immediately (no-delay) or after a 60 min delay (delay). In the no-delay condition, MK801 induced an inverse U-shaped dose-response effect on locomotion; peak activation occurred with 0.1 mg/kg and ataxia occurred with higher doses. The introduction of a novel environment 60 min after drug injection shifted the dose-effect function of MK801 to the left; i.e., in rats 20 days of age and older, the activity induced by 0.1 mg/kg MK801 was potentiated in the delay condition. For the 0.5 mg/kg dose, 20-day-olds showed activation in the no-delay condition but ataxia in the delay condition. This dose induced ataxia followed by activation in 30-day-olds and adult males or ataxia in adult females, regardless of delay condition. Age-, gender-, and novelty-dependent variations in MK801-induced locomotion may reflect differences in limbic-motor circuitry.

The locomotor effects of MK801 in the nucleus accumbens of developing and adult rats

This developmental study was an investigation of locomotion induced by the NMDA receptor antagonist, (+)MK-801 hydrogen maleate [(5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5, 10-imine], at doses of 0, 3 or 10 microg injected bilaterally into the nucleus accumbens of rats at 11, 21, 31, or 61-66 days of age. During a 2-h test session, only a few 11-day-old pups responded to either dose of MK801; they displayed short bouts of obstinate progression. In contrast, 21- and 31-day-olds were not affected by 3 microg MK801 but exhibited robust activation after 10 microg MK801. The activation was greatest in 21-day-olds and also occurred after mid-striatal injections in 21- but not 31-day-old rats. Adult rats injected with MK801 were not robustly activated, but they maintained their initial level of activity throughout the test session, instead of habituating to the test monitor, as controls did. Ontological changes in MK801-induced activity are likely to reflect maturation of glutamate transmission in the nucleus accumbens.