Dissociation in the effects of the D2/D3 dopaminergic agonist quinpirole on drinking and on vasopressin levels in the rat

The low dopamine hypothesis: A plausible mechanism underpinning residual urine, overactive bladder and nocturia (RON) syndrome in older patients

INTRODUCTION

Aging is associated with a combination of several lower urinary tract (LUT) signs and symptoms, including residual urine, overactive bladder and nocturia. One of the mechanisms of this LUT dysfunction that has not been discussed in dept so far is the role of dopamine (DA).

METHODS

In this narrative review, we explore the dopaminergic hypothesis in the development of this combination of LUT signs and symptoms in older adults.

RESULTS

DA is one of the neurotransmitters whose regulation and production is disrupted in aging. In synucleinopathies, altered DAergic activity is associated with the occurrence of LUTS and sleep disorders. Projections of DAergic neurons are involved in the regulation of sleep, diuresis, and bladder activity. The low dopamine hypothesis could explain the genesis of a set of LUT signs and symptoms commonly seen in this population, including elevated residual urine, Overactive bladder syndrome and Nocturia (discussed as the RON syndrome). This presentation is however also common in older patients without synucleinopathies or neurological disorders and therefore we hypothesise that altered DAergic activity because of pathological aging, and selective destruction of DAergic neurons, could underpin the presentation of this triad of LUT dysfunction in the older population.

CONCLUSION

The concept of RON syndrome helps to better understand this common phenotypic presentation in clinical practice, and therefore serves as a useful platform to diagnose and treat LUTS in older adults. Besides recognizing the synucleinopathy "red flag" symptoms, this set of multi-causal LUT signs and symptoms highlights the inevitable need for combination therapy, a challenge in older people with their comorbidities and concomitant medications.

Fluid intake, what's dopamine got to do with it?

Maintaining fluid balance is critical for life. The central components that control fluid intake are only partly understood. This contribution to the collection of papers highlighting work by members of the Society for the Study of Ingestive Behavior focuses on the role that dopamine has on fluid intake and describes the roles that various bioregulators can have on thirst and sodium appetite by influencing dopamine systems in the brain. The goal of the review is to highlight areas in need of more research and to propose a framework to guide that research. We hope that this framework will inspire researchers in the field to investigate these interesting questions in order to form a more complete understanding of how fluid intake is controlled.

Differences in the structure of drinking, cart expression and dopamine turnover between polydipsic and non polydipsic rats in the quinpirole model of psychotic polydipsia

RATIONALE

Dopaminergic D2/D3 agonist quinpirole (QNP) elicits nonregulatory drinking in rats, a model of psychotic polydipsia. Why only a fraction of QNP-treated rats responds to the treatment becoming polydipsic is still unclear.

OBJECTIVES

To unveil possible factors contributing to such variability, we analyzed drinking microstructure in saline and QNP-treated rats, the hypothalamic expression of the cocaine and amphetamine regulated transcript (CART), and the monoaminergic turnover in selected brain areas.

METHODS

Rats were daily treated with saline or QNP 0.5 mg/kg, and their 5-h water intake was measured for five consecutive days. The number of bouts and episodes of licking, and their duration, were also measured. Brain CART expression was measured by in situ hybridization and monoamines turnover by HPLC analysis of tissue extracts. Based on the amount of water ingested during the 5-h session, QNP-treated rats were post hoc grouped in polydipsic (PD) and in nonpolydipsic (NPD) rats, and the results compared accordingly.

RESULTS

The number of drinking bouts and episodes increased in PD rats, while NPD rats behaved as the controls. CART expression decreased in the arcuate nucleus of the hypothalamus of the PD rats. In contrast, both PD and NPD rats showed a reduction of DA turnover in both ventral tegmental area (VTA) and nucleus accumbens (NAcc). No difference was detected in the turnover of 5HT and NA.

CONCLUSIONS

Microstructure analysis confirms that QNP acts on the appetitive component of drinking behavior, making it compulsive. CART expression reduction in response to dopaminergic hyperstimulation might sustain excessive drinking in PD rats.

Syndrome of inappropriate antidiuretic hormone associated with aripiprazole

PURPOSE

A probable case of aripiprazole-induced syndrome of inappropriate antidiuretic hormone (SIADH) is reported.

SUMMARY

A 65-year-old Caucasian man arrived in the emergency department (ED) with dizziness, headache, abdominal pain, nausea, and vomiting. There had been no recent additions or changes to the patient's medication regimen except for an increase in the daily dose of aripiprazole (from 10 to 20 mg) about two months prior. On admission, the patient's serum sodium concentration was 108 meq/L, prompting discontinuation of aripiprazole use and fluid restrictions. Over the next 72 hours, the serum sodium level increased to a near-normal concentration (127 meq/L), and the man was discharged back to a nursing facility. Three days later, the patient was readmitted to the ED with recurrent symptoms and a serum sodium concentration of 118 meq/L, a serum osmolality of 254 mOsm/kg, a urine osmolality of 575 mOsm/kg, and a urine sodium concentration of 101 meq/L. It was learned that aripiprazole use had been inappropriately resumed at the nursing facility. Aripiprazole was again discontinued, and fluid restrictions were imposed, with subsequent abatement of hyponatremia over four days. Application of the adverse drug reaction probability scale of Naranjo et al. in this case yielded a score of 7, indicating probable aripiprazole-associated SIADH.

CONCLUSION

A 65-year-old man developed severe hyponatremia after an aripiprazole dosage increase. Hyponatremia resolved promptly with the discontinuation of aripiprazole. After discharge from the hospital, the patient inadvertently received aripiprazole again and was subsequently readmitted with another episode of severe hyponatremia.

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.

Increased schedule-induced polydipsia in the rat following subchronic treatment with MK-801

Primary polydipsia, defined as excessive fluid intake not explained by medical causes, has been reported to occur in over 20% of chronically ill psychiatric inpatients and is especially common in schizophrenic populations. We tested the hypothesis that in an animal model of schizophrenia-like symptoms (subchronic injections of MK-801, 0.5 mg/kg twice daily for 7 days) an increase in the acquisition of schedule-induced polydipsia (SIP) will occur. Young adult, male rats acquired SIP when food-restricted and placed on a non-contingent fixed-time 1-min food schedule. In comparison with saline-treated control animals, subchronic MK-801 treatment significantly increased SIP. These findings suggest an animal model of polydipsia associated with schizophrenia in humans.

The effects of clozapine on quinpirole-induced non-regulatory drinking and prepulse inhibition disruption in rats

RATIONALE

The biological underpinnings of schizophrenic polydipsia are poorly understood.

OBJECTIVES

This study is aimed at fulfilling the requisites of an experimental model of this syndrome through the quinpirole (QNP) induction of non-regulatory drinking in rats.

METHODS

In a first experiment, clozapine (10 and 40 mg/kg p.o.) was substituted for haloperidol during the last 5 days of 10 days QNP (0.5 mg/kg i.p.) administration and water intake measured at 5 h. In a second experiment, animals treated with QNP alone or in combination with clozapine were assessed for water intake and prepulse inhibition (PPI). Expression of genes coding for the dopaminergic D2 receptor, as well as for the early genes BDNF (brain-derived neurotrophic factor) and c-Fos in prefrontal cortex, hippocampus, and striatum was also evaluated.

RESULTS

Clozapine prevented QNP-induced drinking at 10 and 40 mg/kg, but only at 40 mg/kg when it was substituted for haloperidol. In the second experiment, QNP-treated rats showed both non-regulatory drinking and PPI disruption. Both these effects were prevented by clozapine 40 mg/kg. QNP-reduced BDNF expression in the hippocampus and increased c-Fos in the prefrontal cortex. This effect was prevented by clozapine. Given by itself, clozapine reduced the expression of both D2 receptors and BDNF in the prefrontal cortex and striatum.

CONCLUSIONS

The present study lends further support to the hypothesis that non-regulatory drinking induced by QNP in rats is a robust and reliable pharmacological effect that might model psychotic polydipsia also in its sensitivity to clozapine.

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.

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.

The structural evolution of dopamine D3 receptor ligands: structure-activity relationships and selected neuropharmacological aspects

"Evolution consists largely of molecular tinkering."-Following the famous concept of the molecular geneticist and medicine Nobel laureate François Jacob, in this review we describe the structural evolution of dopamine D3 receptor ligands from the natural agonist dopamine (DA) to highly potent and subtype selective new agents by bioisosteric tinkering with well-established and privileged or novel and fancy chemical functionalities and scaffolds. Some of the more than 200 ligands presented herein have already achieved therapeutic or scientific value up to now, some will most likely achieve it in the future. Hence, great importance is not only attached to the relationship between structure and activity of the ligands, but also to their utility as pharmacological tools in animal models or as therapeutics in patients with neurological diseases or other disorders.

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

We have previously reported that repeated administrations of quinpirole, a D2/D3 dopamine receptor agonist, facilitate instrumental behavior in rats given the choice between operant and free access to water (contrafreeloading: CFL). The goal of the present study was to investigate the effects of repeated daily administrations of quinpirole (0.5 mg/kg i.p.) on the appetitive versus the consummatory component of water-reinforced behavior, under two experimental conditions. Under one condition, the rats were given access to tap water according to an FR3 schedule of reinforcement. Under the second condition, the rats were given the choice between operant and free access to water. Five major findings were obtained. First, acutely quinpirole suppressed operant behavior and, therefore, water intake for at least 1h. Second, upon repeated administrations tolerance developed to the suppressant effect of quinpirole on instrumental behavior but only to a lesser extent to the antidipsic effect, dissociating the appetitive from the consummatory components of water-reinforced behavior. Third, in CFL conditions quinpirole induced a progressively larger preference for the operant access. Fourth, even when the rats were given the choice between free access to highly palatable saccharine (0.05 or 0.01%) solutions and operant access to tap water, quinpirole shifted the animals towards the operant access. Fifth, repeated quinpirole produced lasting consequences on drinking behavior, since after rehydration and under drug-free conditions quinpirole-pretreated rats ingested larger amounts of water than control rats. In conclusion, the repeated activation of D2/D3 receptors appears to induce the rats to perseverate in performing needless instrumental behavior.