Compulsive-like effects of repeated administration of quinpirole on drinking behavior in rats

Contrafreeloading Indicating the Behavioural Need to Forage in Healthy and Feather Damaging Grey Parrots

Contrafreeloading (CFL) is a concept that describes the preference of an animal to work for food even when identical food is freely available, and reflects an intrinsic motivation to engage in foraging-related activities. However, altered brain neurochemistry, which can be induced by chronic exposure to a suboptimal living environment, may affect this intrinsic motivation in animals with abnormal repetitive behaviours (ARBs), including parrots with feather damaging behaviour. To determine whether this was the case, we evaluated CFL activity in healthy (n = 11) and feather damaging (n = 10) Grey parrots (Psittacus erithacus) by offering them a free choice to obtain identical food from a food bowl or from a foraging device. Differences in CFL activity were observed, with feather damaging Grey parrots displaying less CFL (as indicated by shorter foraging times and lower amounts of food consumed from the foraging devices) compared to healthy conspecifics, indicating altered 'motivation' and time allocation, for which the underlying mechanism needs to be clarified further. Nevertheless, despite the variable level, all birds displayed CFL, which, together with a seemingly positive correlation between CFL activity and the plumage condition of the birds, suggests that parrots are intrinsically motivated to forage and highlights the importance of providing foraging opportunities to captive parrots.

Dissociable dopaminergic and pavlovian influences in goal-trackers and sign-trackers on a model of compulsive checking in OCD

RATIONALE

Checking is a functional behaviour that provides information to guide behaviour. However, in obsessive-compulsive disorder (OCD), checking may escalate to dysfunctional levels. The processes underpinning the transition from functional to dysfunctional checking are unclear but may be associated with individual differences that support the development of maladaptive behaviour. We examined one such predisposition, sign-tracking to a pavlovian conditioned stimulus, which we previously found associated with dysfunctional checking. How sign-tracking interacts with another treatment with emerging translational validity for OCD-like checking, chronic administration of the dopamine D₂ receptor agonist quinpirole, is unknown.

OBJECTIVES

We tested how functional and dysfunctional checking in the rat observing response task (ORT) was affected by chronic quinpirole administration in non-autoshaped controls and autoshaped animals classified as sign-trackers or goal-trackers.

METHODS

Sign-trackers or goal-trackers were trained on the ORT before the effects of chronic quinpirole administration on checking were assessed. Subsequently, the effects on checking of different behavioural challenges, including reward omission and the use of unpredictable reinforcement schedules, were tested.

RESULTS

Prior autoshaping increased checking. Sign-trackers and goal-trackers responded differently to quinpirole sensitization, reward omission and reinforcement uncertainty. Sign-trackers showed greater elevations in dysfunctional checking, particularly during uncertainty. By contrast, goal-trackers predominantly increased functional checking responses, possibly in response to reduced discrimination accuracy in the absence of cues signalling which lever was currently active.

CONCLUSIONS

The results are discussed in terms of how pavlovian associations influence behaviour that becomes compulsive in OCD and how this may be dependent on striatal dopamine D₂ receptors.

A Free-Operant Reward-Tracking Paradigm to Study Neural Mechanisms and Neurochemical Modulation of Adaptive Behavior in Rats

The ability to respond flexibly to changing environmental circumstances is a hallmark of goal-directed behavior, and compromised flexibility is associated with a wide range of psychiatric conditions in humans, such as addiction and stress-related disorders. To identify neural circuits and transmitter systems implicated in the provision of cognitive flexibility, suitable animal paradigms are needed. Ideally, such models should be easy to implement, allow for rapid task acquisition, provide multiple behavioral readouts, and permit combination with physiological and pharmacological testing and manipulation. Here, we describe a paradigm meeting these requirements and employ it to investigate the neural substrates and neurochemical modulation of adaptive behavior. Water-restricted rats learned to emit operant responses for positive reinforcement (water reward) within minutes in a free-operant conditioning environment. Without further training, animals were able to track changes in the reward schedule. Given prior evidence that the medial prefrontal cortex (mPFC) and the dopaminergic system are required for flexible behavior, we aimed to assess both in more detail. Silencing of mPFC compromised flexible behavior when avoidance of punishment was required. Systemic injections of the D2-receptor agonist quinpirole and the D2-receptor antagonist eticlopride had complex, differential impacts on reward seeking and adaptive behavior.

Basolateral amygdala input to the medial prefrontal cortex controls obsessive-compulsive disorder-like checking behavior

Obsessive-compulsive disorder (OCD) affects ∼1 to 3% of the world's population. However, the neural mechanisms underlying the excessive checking symptoms in OCD are not fully understood. Using viral neuronal tracing in mice, we found that glutamatergic neurons from the basolateral amygdala (BLA^Glu) project onto both medial prefrontal cortex glutamate (mPFC^Glu) and GABA (mPFC^GABA) neurons that locally innervate mPFC^Glu neurons. Next, we developed an OCD checking mouse model with quinpirole-induced repetitive checking behaviors. This model demonstrated decreased glutamatergic mPFC microcircuit activity regulated by enhanced BLA^Glu inputs. Optical or chemogenetic manipulations of this maladaptive circuitry restored the behavioral response. These findings were verified in a mouse functional magnetic resonance imaging (fMRI) study, in which the BLA-mPFC functional connectivity was increased in OCD mice. Together, these findings define a unique BLA^Glu→mPFC^GABA→Glu circuit that controls the checking symptoms of OCD.

Contrafreeloading in Rats Is Adaptive and Flexible: Support for an Animal Model of Compulsive Checking

Contrafreeloading involves working unnecessarily to obtain a reward that is otherwise freely available. It has been observed in numerous species and can be adaptive when it provides an organism with updated information about available resources. Humans frequently update their knowledge of the environment through checking behaviors. Compulsive checking occurs when such actions are performed with excessive frequency. In a putative animal model of compulsive checking, rats treated chronically with the dopamine agonist quinpirole display exaggerated contrafreeloading for water. Although this effect has been attributed to behavioral rigidity, some evidence suggests the behavior remains somewhat flexible and may be adaptive under certain conditions. We assessed the ability of quinpirole-treated rats with contrafreeloading experience to adapt to changing contingencies by requiring them to alternate between response levers. Rats treated with quinpirole or saline were first trained to obtain water by pressing either of two levers. Next, free water was made available for 8 days, and contrafreeloading was measured. Rates of contrafreeloading were significantly higher in the drug-treated rats than in controls. On the following 5 days, each reward caused the associated lever to become inactive until a reward was earned from the alternate lever. Quinpirole-treated rats learned this new response requirement more quickly than controls. Thus, exaggerated checking behavior induced by chronic quinpirole treatment can be advantageous when environmental contingencies change. These results provide support for this animal model of compulsive checking and hint at the presence of a specialized neural checking module involving the dopamine system.

Inhibition of hippocampal plasticity in rats performing contrafreeloading for water under repeated administrations of pramipexole

RATIONALE

Compulsive symptoms develop in patients exposed to pramipexole (PPX), a dopaminergic agonist with high selectivity for the D3 receptor. Consistently, we demonstrated that PPX produces an exaggerated increase in contrafreeloading (CFL) for water, a repetitive and highly inflexible behavior that models core aspects of compulsive disorders.

OBJECTIVES

Given the role of the hippocampus in behavioral flexibility, motivational control, and visuospatial working memory, we investigated the role of hippocampus in the expression of PPX-induced CFL. To this aim, rats were subjected to CFL under chronic PPX, and then examined for the electrophysiological, structural, and molecular properties of their hippocampus.

METHODS

We measured long-term potentiation (LTP) at CA1 Schaffer collaterals, dendritic spine density in CA1 pyramidal neurons, and then glutamate release and expression of pre and postsynaptic proteins in hippocampal synaptosomes. The effects of PPX on hippocampal-dependent working memory were assessed through the novel object recognition (NOR) test.

RESULTS

We found that PPX-treated rats showing CFL exhibited a significant decrease in hippocampal LTP and failed to exhibit the expected increase in hippocampal spine density. Glutamate release and PSD-95 expression were decreased, while pSYN expression was increased in hippocampal synaptosomes of PPX-treated rats showing CFL. Despite a general impairment of hippocampal synaptic function, working memory was unaffected by PPX treatment.

CONCLUSIONS

Our findings demonstrate that chronic PPX affects synaptic function in the hippocampus, an area that is critically involved in the expression of flexible, goal-centered behaviors. We suggest that the hippocampus is a promising target in the pharmacotherapy of compulsive disorders.

Animal models of obsessive-compulsive disorder: utility and limitations

Obsessive-compulsive disorder (OCD) is a disabling and common neuropsychiatric condition of poorly known etiology. Many attempts have been made in the last few years to develop animal models of OCD with the aim of clarifying the genetic, neurochemical, and neuroanatomical basis of the disorder, as well as of developing novel pharmacological and neurosurgical treatments that may help to improve the prognosis of the illness. The latter goal is particularly important given that around 40% of patients with OCD do not respond to currently available therapies. This article summarizes strengths and limitations of the leading animal models of OCD including genetic, pharmacologically induced, behavioral manipulation-based, and neurodevelopmental models according to their face, construct, and predictive validity. On the basis of this evaluation, we discuss that currently labeled "animal models of OCD" should be regarded not as models of OCD but, rather, as animal models of different psychopathological processes, such as compulsivity, stereotypy, or perseverance, that are present not only in OCD but also in other psychiatric or neurological disorders. Animal models might constitute a challenging approach to study the neural and genetic mechanism of these phenomena from a trans-diagnostic perspective. Animal models are also of particular interest as tools for developing new therapeutic options for OCD, with the greatest convergence focusing on the glutamatergic system, the role of ovarian and related hormones, and the exploration of new potential targets for deep brain stimulation. Finally, future research on neurocognitive deficits associated with OCD through the use of analogous animal tasks could also provide a genuine opportunity to disentangle the complex etiology of the disorder.

The dopamine D2/D3 receptor agonist quinpirole increases checking-like behaviour in an operant observing response task with uncertain reinforcement: a novel possible model of OCD

Excessive checking is a common, debilitating symptom of obsessive-compulsive disorder (OCD). In an established rodent model of OCD checking behaviour, quinpirole (dopamine D2/3-receptor agonist) increased checking in open-field tests, indicating dopaminergic modulation of checking-like behaviours. We designed a novel operant paradigm for rats (observing response task (ORT)) to further examine cognitive processes underpinning checking behaviour and clarify how and why checking develops. We investigated i) how quinpirole increases checking, ii) dependence of these effects on D2/3 receptor function (following treatment with D2/3 receptor antagonist sulpiride) and iii) effects of reward uncertainty. In the ORT, rats pressed an 'observing' lever for information about the location of an 'active' lever that provided food reinforcement. High- and low-checkers (defined from baseline observing) received quinpirole (0.5mg/kg, 10 treatments) or vehicle. Parametric task manipulations assessed observing/checking under increasing task demands relating to reinforcement uncertainty (variable response requirement and active-lever location switching). Treatment with sulpiride further probed the pharmacological basis of long-term behavioural changes. Quinpirole selectively increased checking, both functional observing lever presses (OLPs) and non-functional extra OLPs (EOLPs). The increase in OLPs and EOLPs was long-lasting, without further quinpirole administration. Quinpirole did not affect the immediate ability to use information from checking. Vehicle and quinpirole-treated rats (VEH and QNP respectively) were selectively sensitive to different forms of uncertainty. Sulpiride reduced non-functional EOLPs in QNP rats but had no effect on functional OLPs. These data have implications for treatment of compulsive checking in OCD, particularly for serotonin-reuptake-inhibitor treatment-refractory cases, where supplementation with dopamine receptor antagonists may be beneficial.

Effects of the 5HT2C antagonist SB242084 on the pramipexole-induced potentiation of water contrafreeloading, a putative animal model of compulsive behavior

RATIONALE

In rats, quinpirole, a dopaminergic D2/D3 receptor agonist, elicits both hyperdipsia and water "contrafreeloading" (CFL), a putative model of compulsivity. The role of D3 receptors in this effect remains unclear. Clomipramine (CIM) was found to contrast both hyperdipsia and CFL, but the role of serotonin in this effect requires further investigation.

OBJECTIVES

We studied the effects of the preferential D3 agonist pramipexole (PPX) in both models. Furthermore, we tested the sensitivity of PPX-induced CFL to CIM and to the 5HT2c antagonist SB242084.

METHODS

In experiment 1, drinking was measured at 2 and 5 h after eight daily injections of PPX (0 to 1.0 mg/kg intraperitoneally). In the CFL study, every other third lever press, the rat was reinforced by the delivery of water. On days 1-6, water was only available upon lever pressing. On days 7-15, choice between response-contingent and free access was provided. PPX doses as in the experiment 1 were given. In two further experiments, PPX (0.5 mg/kg) was administered alone or in combination with CIM (5 or 10 mg/kg) or SB242084 (0.3 or 1.0 mg/kg).

RESULTS

PPX did not produce hyperdipsia but enhanced spontaneous CFL. SB242084 attenuated PPX-induced CFL more effectively than CIM, restoring the preference for free access to water.

CONCLUSIONS

CFL, but not polydipsia, was induced by preferential D3 activation, an effect prevented by 5HT2c receptor blockade. Since PPX interferes with decision making and 5HT2c receptor supersensitivity is involved in the expression of compulsive behaviors, this study supports the compulsive nature of dopaminergic-induced CFL.

Increased drinking after intra-striatal injection of the dopamine D2/D3 receptor agonist quinpirole in the rat

RATIONALE

Dopamine D2 receptor hyperactivity has been implicated in the development of psychogenic polydipsia in schizophrenic patients. Repeated treatment with dopamine agonists, including the D2/D3 agonist quinpirole, has been shown to induce hyperdipsia in a number of animal models. Despite these observations, obtained with systemic administrations, little attempt has been made to investigate where in the brain dopamine agonists act to induce hyperdipsia.

OBJECTIVE

The present study investigates the effects of repeated intra-caudate infusions of quinpirole on the intake of water by rats tested under free-drinking conditions.

MATERIALS AND METHODS

Rats with bilateral cannulae placed into the anterior, central or posterior caudate received quinpirole microinfusions (1 μg/side) for five consecutive days in their home cage. Water intake was measured 15 and 60 min after the treatment.

RESULTS

When injected in the central caudate, quinpirole increased water intake, and this effect progressively increased over sessions, indicating the development of sensitization. When injected in the posterior caudate, the dipsogenic effect of quinpirole was less intense and did not undergo sensitization. The infusion of quinpirole in the anterior caudate did not affect drinking.

CONCLUSION

The present study shows that caudate D2/3 receptors play an important role in the development of quinpirole-induced hyperdipsia, an animal model of psychotic polydipsia.

Clomipramine, but not haloperidol or aripiprazole, inhibits quinpirole-induced water contrafreeloading, a putative animal model of compulsive behavior

RATIONALE

Repeated administrations of the D2/D3 agonist quinpirole (QNP) to rats elicit an antieconomical pattern of drinking called "contrafreeloading" (CFL), a putative model of compulsive-like behavior.

OBJECTIVES

We tested the sensitivity of QNP-induced CFL to haloperidol (HAL), aripiprazole (ARI), and clomipramine (CIM), the latter proven effective in the treatment of obsessive-compulsive disorder (OCD).

METHODS

Rats were trained under a schedule of reinforcement (FR3) for water. On days 1-6, water was only available through lever pressing. On days 7-15, a choice between operant and free access was provided. QNP 0.5 mg/kg was administered alone or in combination with HAL (0.1 or 0.2 mg/kg), ARI (0.3 or 1 mg/kg), or CIM (5 or 10 mg/kg).

RESULTS

Acutely QNP suppressed operant behavior and, therefore, water intake; upon repeated administrations, tolerance developed to this suppressant effect on responding but only to a lesser extent to the antidipsic effect. In choice conditions, QNP induced a progressive preference for the operant access (CFL). HAL per se, but not CIM and ARI, significantly reduced both responding and drinking (operant phase). In the choice phase, HAL and CIM inhibited CFL, but only the latter reinstated total water intake. ARI, in combination with QNP, increased responding.

CONCLUSIONS

CIM reinstates control patterns of drinking, while HAL and ARI where partially or not effective at all, respectively. As far as CIM is considered a first line treatment in OCD, these results further strengthen the notion that QNP-induced CFL belongs to the realm of dopaminergic drug-induced compulsive behaviors.

Contrafreeloading in grizzly bears: implications for captive foraging enrichment

Although traditional feeding regimens for captive animals were focused on meeting physiological needs to assure good health, more recently emphasis has also been placed on non-nutritive aspects of feeding. The provision of foraging materials to diversify feeding behavior is a common practice in zoos but selective consumption of foraging enrichment items over more balanced "chow" diets could lead to nutrient imbalance. One alternative is to provide balanced diets in a contrafreeloading paradigm. Contrafreeloading occurs when animals choose resources that require effort to exploit when identical resources are freely available. To investigate contrafreeloading and its potential as a theoretical foundation for foraging enrichment, we conducted two experiments with captive grizzly bears (Ursus arctos horribilis). In Experiment 1, bears were presented with five foraging choices simultaneously: apples, apples in ice, salmon, salmon in ice, and plain ice under two levels of food restriction. Two measures of contrafreeloading were considered: weight of earned food consumed and time spent working for earned food. More free than earned food was eaten, with only two bears consuming food extracted from ice, but all bears spent more time manipulating ice containing salmon or apples than plain ice regardless of level of food restriction. In Experiment 2, food-restricted bears were presented with three foraging choices simultaneously: apples, apples inside a box, and an empty box. Although they ate more free than earned food, five bears consumed food from boxes and all spent more time manipulating boxes containing apples than empty boxes. Our findings support the provision of contrafreeloading opportunities as a foraging enrichment strategy for captive wildlife.

Opposite roles of dopamine and orexin in quinpirole-induced excessive drinking: a rat model of psychotic polydipsia

RATIONALE

Repeated administration of the dopamine D2/D3 agonist quinpirole (QNP) progressively increases non-regulatory water intake. This effect may model psychotic polydipsia, a potentially fatal but poorly understood condition.

OBJECTIVES

The growing evidence for a role of orexin in mediating arousal and cognition has linked this peptide to schizophrenia, hence we examined whether manipulations of dopaminergic and orexinergic systems, as well as of setting, would further characterize the model.

METHODS

Water intake was measured in rats sequentially tested in home and then operant conditioning setting, with chronic administration of D2 antagonist haloperidol (Hal) prior to QNP treatment. A group of rats similarly treated was also assessed for orexin A (OxA) expression in the cortex. Finally, the effect of the orexin-1 receptor antagonist SB-334867 on QNP-induced polydipsia was evaluated.

RESULTS

In rats made polydipsic by QNP the amount of water drank during the first 4 h was strongly correlated with the degree of dissociation between appetitive and consummatory components of drinking behavior in the following hour of operant access to water. Hal 0.2 mg/kg prevented both polydipsia and the dissociation, while 0.1 mg/kg only blocked the dissociation. Chronic QNP treatment increased, in a Hal-reversible way, OxA expression in the somatosensory cortex (SI). Moreover, pretreatment with SB-334867 sped up and potentiated QNP-induced polydipsia.

CONCLUSIONS

Results disclose compulsive components in QNP-induced polydipsia that are mediated by dopamine D2 receptors. QNP also regulates OxA expression in the SI, while the block of orexin-1 receptors enhances QNP-induced polydipsia. We suggest that dopamine and OxA play opposite roles in QNP-induced polydipsia.

Dopamine agonists increase perseverative instrumental responses but do not restore habit formation in a rat model of Parkinsonism

Dopamine (DA) deafferentation of the dorsolateral striatum has been shown to prevent habit development, leaving instrumental behavior under action-outcome control that is persistently sensitive to modification of the motivational value of the reward. The present experiment further explored the basis of this dysfunction by examining the ability of intrastriatal DA agonist injections (D1 SKF 38393 or D2/D3 Quinpirole) during overtraining of a signaled instrumental task to restore habit formation in rats subjected to bilateral 6-hydroxydopamine (6-OHDA) lesions of the nigrostriatal dopaminergic pathway. Overtraining was followed by a test of goal sensitivity by satiety-specific devaluation of the reward. The results confirmed the impaired shift in performance from action to habit in control lesioned rats. However, lesioned rats repeatedly injected with quinpirole D2/D3 agonist showed an increase in non-rewarded instrumental responses (intertrials periods) during overtraining, suggesting the development of perseverative behavior. Following the procedure of devaluation, quinpirole D2/D3 agonist treatment, and to a lesser extent SKF 38393 D1 agonist, caused the persistence of sensitivity to reward devaluation, indicating clear goal-directed behavior despite extended training. This absence of restoration of habit formation by DA agonist treatment is discussed in the light of DA agonist effects in Parkinson patients.

Influence of conditioned reinforcement on the response-maintaining effects of quinpirole in rats

D2-like agonists, such as quinpirole, maintain responding in monkeys, rats, and mice when they are substituted for cocaine. This study examined the influence of operant history and cocaine-paired stimuli (CS) on quinpirole-maintained responding in rats trained to nose poke for cocaine. Upon acquisition of responding for cocaine, substitutions were performed in the presence or absence of injection-CS pairings. Although cocaine maintained responding regardless of whether injections were accompanied by CS, quinpirole maintained responding only when CS were paired with injections. To assess the influence of operant history, injections of cocaine, quinpirole, remifentanil, nicotine, or saline were made available on a previously inactive lever, while nose pokes continued to result in CS presentation. Although responding was reallocated from the nose poke to the lever when cocaine or remifentanil was available, lever presses remained low, and nose poking persisted when quinpirole or nicotine was made contingent upon lever presses. Finally, quinpirole pretreatments resulted in high rates of nose poking when nose pokes resulted in CS presentation alone, but failed to maintain nose poking when the CS was omitted. Taken together, these results suggest that the response-maintaining effects of quinpirole are primarily mediated by an enhancement of the conditioned reinforcing effects of earlier CS, rather than by a reinforcing effect of quinpirole.

Narrowing in on compulsions: dopamine receptor functions

A series of experiments in rats explored the possibility that D3/D2 dopamine receptors are involved in behaviors that might be related to compulsion. A series of D3/D2 agonists and antagonists were shown to elicit yawning (D3-receptor mediated) and its inhibition (D2-receptor mediated). In rats with histories of cocaine exposure, D3-agonist-elicited yawning was enhanced, and quinpirole led to persistent operant responding only if conditioned stimuli associated with cocaine were presented for responding. Finally, a more selective D3 partial agonist was reported that had a novel profile of activity that could have relevance to the suppression of dopamine-related compulsions.

Haloperidol both prevents and reverses quinpirole-induced nonregulatory water intake, a putative animal model of psychogenic polydipsia

RATIONALE

Polydipsia is a severe complication of long-term schizophrenia and, despite its unknown pathogenesis, is empirically treated with typical or atypical antipsychotics. In the rat, nonregulatory water intake is induced by repeated administration of amphetamine-like compounds or by the D2/3 agonist, quinpirole.

OBJECTIVE

This study is aimed at determining the potential activity of antipsychotic compounds with different affinities for D2 receptors in preventing and/or reversing quinpirole-induced polydipsia.

MATERIALS AND METHODS

Male Sprague-Dawley rats were treated with five injections of quinpirole (0.5 mg/kg i.p.) to induce polydipsia. The oral effects of haloperidol, olanzapine, clozapine, and ST2472 on QNP-induced polydipsia were analyzed in the following two schedules. In the preventive schedule, haloperidol (0.2, 0.4, and 0.8 mg/kg), olanzapine (1.5, 3, and 6 mg/kg), ST2472 (1 and 2 mg/kg), and clomipramine (5, 10, and 20 mg/kg) were given in combination with quinpirole from day 1 to day 5. In the reversal schedule, rats showing quinpirole-induced polydipsia on the third day received haloperidol (0.4 mg/kg), olanzapine (1.5 and 3 mg/kg), clozapine (10, 20, and 40 mg/kg), ST2472 (1, 2, 5, and 10 mg/kg), and clomipramine (5, 10, and 20 mg/kg) before quinpirole on days 4 and 5.

RESULTS

Haloperidol both prevented and reversed quinpirole-induced polydipsia, whereas olanzapine and ST2472 only reversed it. Clomipramine prevented but did not reverse quinpirole-induced polydipsia, and clozapine did not reverse it either.

CONCLUSIONS

We suggest that, once developed, polydipsia is governed by dopaminergic D2 mechanisms. In contrast, either an increase in the serotoninergic tone or an inhibition of D2 receptors can modulate the development of quinpirole-induced excessive drinking.

The influence of cost manipulation on water contrafreeloading induced by repeated exposure to quinpirole in the rat

RATIONALE

Quinpirole (QNP), a D2/D3 dopaminergic receptor agonist, was found to elicit an apparently antieconomical drinking behavior called contrafreeloading (CFL). The perseverative operant responding observed may represent a compulsive-like behavior prompted by sensitization to the effects of QNP.

OBJECTIVES

In the present study, we investigated the effect of different response costs on instrumental behavior and CFL in rats repeatedly treated with QNP (0.5 mg/kg i.p.). Moreover, we studied the consummatory components of ingestive behavior in no-choice paradigms and the role of learned operant conditioning in free drinking.

MATERIALS AND METHODS

In experiment 1, rats were trained to perform under three different fixed ratio schedules of reinforcement (FR1, FR3, and FR10) and were given a choice between operant and free access to water. In experiment 2, rats were divided into four groups, each one resembling experiment 1 in one or more features, with no choice available and water consumption measured at an interval of 0-60 min.

RESULTS

(a) Increasing FR significantly reduced CFL % in saline -- but not in QNP-injected groups; (b) under free-drinking conditions, QNP caused a progressive hypodipsic effect which was, however, contrasted by maintaining cues formerly contingent on operant access to water; and (c) under CFL conditions QNP-treated rats drank more than under free access conditions.

CONCLUSIONS

QNP confers rigidity in responding for water, impeding adaptation to different contingencies for access to the resource. In QNP-treated rats, CFL behavior appears adaptive as far as it allows animals to partially circumvent the hypodipsic effect of the drug.

Drug and reinforcement history as determinants of the response-maintaining effects of quinpirole in the rat

The present study examined the effect of drug and reinforcement history on quinpirole-maintained responding in rats. Quinpirole (0.01, 0.032, or 0.1 mg/kg per injection) was assessed as a reinforcer in experimentally naive rats, as well as in rats trained to self-administer cocaine, remifentanil, ketamine, or food under a fixed ratio 1 schedule of reinforcement. Quinpirole failed to maintain responding in experimentally naive rats, or in ketamine- or food-trained rats. However, robust responding was maintained in rats with a history of cocaine reinforcement, and modest levels of responding were observed in rats with a history of responding for remifentanil. In a second set of studies, the effects of protracted drug histories on quinpirole-maintained responding in food-trained rats were assessed. Rats were maintained with food reinforcement, and different groups of rats were then allowed to respond for saline, quinpirole, and response-contingent cocaine or were administered noncontingent cocaine; all rats were subsequently allowed to respond for quinpirole. Only rats that previously responded for cocaine showed quinpirole-maintained responding; all other conditions failed to establish quinpirole-maintained responding. Although the high levels of quinpirole-maintained responding observed when quinpirole was substituted for cocaine are suggestive of positive reinforcing effects, these response-maintaining effects were highly dependent upon both drug and reinforcement history, suggesting that quinpirole may only function as a reinforcer under very specific conditions. The behavioral effects of quinpirole under these situations represent a novel constellation of actions relative to other drug reinforcers, and they suggest that the direct effects of self-administered quinpirole may be important in establishing the response-maintaining effects.