The novel adenosine A2A antagonist Lu AA47070 reverses the motor and motivational effects produced by dopamine D2 receptor blockade

Effects of the atypical antipsychotic and D3/D2 dopamine partial agonist cariprazine on effort-based choice behavior: implications for modeling avolition

RATIONALE

Cariprazine is an atypical antipsychotic that acts as a D3/D2 receptor partial agonist. In addition to treating positive symptoms of schizophrenia, cariprazine may have utility for treating negative symptoms. Rodent studies have focused on the effects of cariprazine on cognitive functions and behaviors thought to be related to anhedonia. Avolition, which is characterized by reduced initiation and persistence of goal-directed behavior, is another important negative symptom.

OBJECTIVES

Effort-related choice tasks have been used as animal models of avolition. In these studies, cariprazine was assessed for its effects on effort-based choice in both rats and mice. Previous work has shown that D2 antagonists such as haloperidol and eticlopride produce a low-effort bias in rodents tested on effort-based choice tasks.

RESULTS

Low doses of cariprazine produced a low-effort bias in rats tested on the fixed ratio 5/chow feeding choice task, decreasing lever pressing for high carbohydrate pellets but increasing chow intake. Cariprazine did not alter preference or intake of these foods in free-feeding tests. The effort-related effects of cariprazine were reversed by co-administration of the adenosine A2A antagonist istradefylline, and cariprazine failed to reverse the effort-related effects of the dopamine-depleting agent tetrabenazine. In mouse touchscreen choice tests, low doses of cariprazine also produced a low-effort bias, shifting behavior away from panel pressing.

CONCLUSIONS

These results demonstrate that with these rodent models of avolition, cariprazine appears to act like a D2-family antagonist even at very low doses. Furthermore, the pharmacological regulation of avolition may differ from that of other negative symptoms.

How and why the adenosine A2A receptor became a target for Parkinson's disease therapy

Dopaminergic therapy for Parkinson's disease has revolutionised the treatment of the motor symptoms of the illness. However, it does not alleviate all components of the motor deficits and has only limited effects on non-motor symptoms. For this reason, alternative non-dopaminergic approaches to treatment have been sought and the adenosine A2A receptor provided a novel target for symptomatic therapy both within the basal ganglia and elsewhere in the brain. Despite an impressive preclinical profile that would indicate a clear role for adenosine A2A antagonists in the treatment of Parkinson's disease, the road to clinical use has been long and full of difficulties. Some aspects of the drugs preclinical profile have not translated into clinical effectiveness and not all the clinical studies undertaken have had a positive outcome. The reasons for this will be explored and suggestions made for the further development of this drug class in the treatment of Parkinson's disease. However, one adenosine A2A antagonist, namely istradefylline has been introduced successfully for the treatment of late-stage Parkinson's disease in two major areas of the world and has become a commercial success through offering the first non-dopaminergic approach to the treatment of unmet need to be introduced in several decades.

Neural representation and modulation of volitional motivation in response to escalating efforts

Task-dependent volitional control of the selected neural activity in the cortex is critical to neuroprosthetic learning to achieve reliable and robust control of the external device. The volitional control of neural activity is driven by a motivational factor (volitional motivation), which directly reinforces the target neurons via real-time biofeedback. However, in the absence of motor behaviour, how do we evaluate volitional motivation? Here, we defined the criterion (ΔF/F) of the calcium fluorescence signal in a volitionally controlled neural task, then escalated the efforts by progressively increasing the number of reaching the criterion or holding time after reaching the criterion. We devised calcium-based progressive threshold-crossing events (termed 'Calcium PTE') and calcium-based progressive threshold-crossing holding-time (termed 'Calcium PTH') for quantitative assessment of volitional motivation in response to progressively escalating efforts. Furthermore, we used this novel neural representation of volitional motivation to explore the neural circuit and neuromodulator bases for volitional motivation. As with behavioural motivation, chemogenetic activation and pharmacological blockade of the striatopallidal pathway decreased and increased, respectively, the breakpoints of the 'Calcium PTE' and 'Calcium PTH' in response to escalating efforts. Furthermore, volitional and behavioural motivation shared similar dopamine dynamics in the nucleus accumbens in response to trial-by-trial escalating efforts. In general, the development of a neural representation of volitional motivation may open a new avenue for smooth and effective control of brain-machine interface tasks. KEY POINTS: Volitional motivation is quantitatively evaluated by M1 neural activity in response to progressively escalating volitional efforts. The striatopallidal pathway and adenosine A_2A receptor modulate volitional motivation in response to escalating efforts. Dopamine dynamics encode prediction signal for reward in response to repeated escalating efforts during motor and volitional conditioning. Mice learn to modulate neural activity to compensate for repeated escalating efforts in volitional control.

Vigor, Effort-Related Aspects of Motivation and Anhedonia

In this chapter we provide an overview of the pharmacological and circuit mechanisms that determine the willingness to expend effort in pursuit of rewards. A particular focus will be on the role of the mesolimbic dopamine system, as well the contributing roles of limbic and cortical brains areas involved in the evaluation, selection, and invigoration of goal-directed actions. We begin with a review of preclinical studies, which have provided key insights into the brain systems that are necessary and sufficient for effort-based decision-making and have characterized novel compounds that enhance selection of high-effort activities. Next, we summarize translational studies identifying and expanding this circuitry in humans. Finally, we discuss the relevance of this work for understanding common motivational impairments as part of the broader anhedonia symptom domain associated with mental illness, and the identification of new treatment targets within this circuitry to improve motivation and effort-expenditure.

Haloperidol-induced catalepsy as an animal model for parkinsonism: A systematic review of experimental studies

Several useful animal models for parkinsonism have been developed so far. Haloperidol-induced catalepsy is often used as a rodent model for the study of motor impairments observed in Parkinson's disease and related disorders and for the screening of potential antiparkinsonian compounds. The objective of this systematic review is to identify publications that used the haloperidol-induced catalepsy model for parkinsonism and to explore the methodological characteristics and the main questions addressed in these studies. A careful systematic search of the literature was carried out by accessing articles in three different databases: Web of Science, PubMed and SCOPUS. The selection and inclusion of studies were performed based on the abstract and, subsequently, on full-text analysis. Data extraction included the objective of the study, study design and outcome of interest. Two hundred and fifty-five articles were included in the review. Publication years ranged from 1981 to 2020. Most studies used the model to explore the effects of potential treatments for parkinsonism. Although the methodological characteristics used are quite varied, most studies used Wistar rats as experimental subjects. The most frequent dose of haloperidol used was 1.0 mg/kg, and the horizontal bar test was the most used to assess catalepsy. The data presented here provide a framework for an evidence-based approach to the design of preclinical research on parkinsonism using the haloperidol-induced catalepsy model. This model has been used routinely and successfully and is likely to continue to play a critical role in the ongoing search for the next generation of therapeutic interventions for parkinsonism.

Istradefylline - a first generation adenosine A2A antagonist for the treatment of Parkinson's disease

Introduction It is now accepted that Parkinson's disease (PD) is not simply due to dopaminergic dysfunction, and there is interest in developing non-dopaminergic approaches to disease management. Adenosine A_2A receptor antagonists represent a new way forward in the symptomatic treatment of PD.Areas covered In this narrative review, we summarize the literature supporting the utility of adenosine A_2A antagonists in PD with a specific focus on istradefylline, the most studied and only adenosine A_2A antagonist currently in clinical use.Expert opinion: At this time, the use of istradefylline in the treatment of PD is limited to the management of motor fluctuations as supported by the results of randomized clinical trials and evaluation by Japanese and USA regulatory authorities. The relatively complicated clinical development of istradefylline was based on classically designed studies conducted in PD patients with motor fluctuations on an optimized regimen of levodopa plus adjunctive dopaminergic medications. In animal models, there is consensus that a more robust effect of istradefylline in improving motor function is produced when combined with low or threshold doses of levodopa rather than with high doses that produce maximal dopaminergic improvement. Exploration of istradefylline as a 'levodopa sparing' strategy in earlier PD would seem warranted.

Can adenosine A2A receptor antagonists modify motor behavior and dyskinesia in experimental models of Parkinson's disease?

Current treatment of the motor symptoms of Parkinson's disease (PD) focuses on dopamine replacement therapies. While these treatments are initially highly effective, with long-term use and disease progression, the therapeutic response is often limited by the development of motor complications, dopaminergic side effects, and residual unresponsive motor and non-motor symptoms. An alternative or additive treatment approach may be to target non-dopaminergic receptors within the motor control pathways, which function to modulate basal ganglia output. Adenosine A_2A receptors are one potential non-dopaminergic target as they are selectively localized to the basal ganglia and to the indirect output pathway known to modulate the striato-thalamo-cortical loops critical to the expression of the motor symptoms of PD. This paper reviews the preclinical evidence base for the ability of adenosine A_2A receptor blockade to influence motor function and modulate dyskinesia expression. There is consensus that adenosine A_2A receptor antagonists - administered either as a monotherapy or in combination with l-DOPA or dopamine agonists - improve motor function in both rodent and primate models of PD, and should be effective for treating the motor symptoms of PD in humans. Importantly, the improvements in motor function were seen in the absence of dyskinesia. The introduction of a non-dopaminergic approach to modifying basal ganglia function provides a useful addition to the range of available therapies for treating PD, and there is a rational basis for a drug that focuses on modifying basal ganglia output.

Of adenosine and the blues: The adenosinergic system in the pathophysiology and treatment of major depressive disorder

Major depressive disorder (MDD) is the foremost cause of global disability, being responsible for enormous personal, societal, and economical costs. Importantly, existing pharmacological treatments for MDD are partially or totally ineffective in a large segment of patients. As such, the search for novel antidepressant drug targets, anchored on a clear understanding of the etiological and pathophysiological mechanisms underpinning MDD, becomes of the utmost importance. The adenosinergic system, a highly conserved neuromodulatory system, appears as a promising novel target, given both its regulatory actions over many MDD-affected systems and processes. With this goal in mind, we herein review the evidence concerning the role of adenosine as a potential player in pathophysiology and treatment of MDD, combining data from both human and animal studies. Altogether, evidence supports the assertions that the adenosinergic system is altered in both MDD patients and animal models, and that drugs targeting this system have considerable potential as putative antidepressants. Furthermore, evidence also suggests that modifications in adenosine signaling may have a key role in the effects of several pharmacological and non-pharmacological antidepressant treatments with demonstrated efficacy, such as electroconvulsive shock, sleep deprivation, and deep brain stimulation. Lastly, it becomes clear from the available literature that there is yet much to study regarding the role of the adenosinergic system in the pathophysiology and treatment of MDD, and we suggest several avenues of research that are likely to prove fruitful.

Detection of G Protein-Coupled Receptor Complexes in Postmortem Human Brain by Proximity Ligation Assay

Combining immunological and molecular biological methods, the antibody-based proximity ligation assay (PLA) has been used for more than a decade to detect and quantify protein-protein interactions, protein modification, and protein expression in situ, including in brain tissue. However, the transfer of this technology to human brain samples requires a number of precautions due to the nature of the specimens and their specific processing. Here, we used the PLA brightfield detection technique to assess the expression of dopamine D2 receptor and adenosine A2A receptor and their proximity in human postmortem brains, and we developed a systematic random sampling method to help quantify the PLA signals. © 2019 by John Wiley & Sons, Inc. Basic Protocol 1: Sample preparation and sectioning for PLA_BF Basic Protocol 2: PLA_BF staining of brain tissue Basic Protocol 3: Image acquisition and result analysis Support Protocol: Luxol fast blue/cresyl violet staining.

The non-selective adenosine antagonist theophylline reverses the effects of dopamine antagonism on tremor, motor activity and effort-based decision-making

Considerable evidence indicates that adenosine and dopamine systems interact in the regulation of basal ganglia function. Nonselective adenosine antagonists such as the methylxanthine caffeine as well as selective adenosine A_2A antagonists have been shown to produce antiparkinsonian and antidepressant effects in animal models. The present studies were conducted to assess if another methylxantine, theophylline, can reverse motor and motivational impairments induced by dopamine antagonism in rats. RESULTS: Theophylline (3.75-30.0 mg/kg, IP) reversed tremulous jaw movements (TJMs), catalepsy, and locomotor suppression induced by the dopamine D2 antagonist pimozide. It also reversed TJMs induced by the muscarinic receptor agonist pilocarpine, which is a well-known tremorogenic agent. Parallel studies assessed the ability of theophylline (5.0-20.0 mg/kg, IP) to reverse the changes in effort-related choice behavior induced by the dopamine D1 antagonist ecopipam (0.2 mg/kg, IP) and the D2 antagonist haloperidol (0.1 mg/kg, IP). Rats were tested on two different operant choice tasks which assess the tendency to work for a preferred reinforcer by lever pressing (for palatable pellets or a high 5% sucrose solution) vs. approaching and consuming a less preferred reinforcer (freely available lab chow or a less concentrated 0.3% sucrose solution). Theophylline restored food and sucrose-reinforced lever pressing in animals treated with the D2 antagonist. However, it was unable to reverse the effects of the D1 antagonist. Overall, the effects of theophylline resembled those previously reported for adenosine A_2A antagonists, and suggest that theophylline could be clinically useful for the treatment of motor and motivational symptoms in humans.

Therapeutic Potential of Agonists and Antagonists of A1, A2a, A2b and A3 Adenosine Receptors

Adenosine is a naturally occurring nucleoside and an essential component of the energy production and utilization systems of the body. Adenosine is formed by the degradation of adenosine-triphosphate (ATP) during energy-consuming processes. Adenosine regulates numerous physiological processes through activation of four subtypes of G-protein coupled membrane receptors viz. A1, A2A, A2B and A3. Its physiological importance depends on the affinity of these receptors and the extracellular concentrations reached. ATP acts as a neurotransmitter in both peripheral and central nervous systems. In the peripheral nervous system, ATP is involved in chemical transmission in sensory and autonomic ganglia, whereas in central nervous system, ATP, released from synaptic terminals, induces fast excitatory postsynaptic currents. ATP provides the energetics for all muscle movements, heart beats, nerve signals and chemical reactions inside the body. Adenosine has been traditionally considered an inhibitor of neuronal activity and a regulator of cerebral blood flow. Since adenosine is neuroprotective against excitotoxic and metabolic dysfunctions observed in neurological and ocular diseases, the search for adenosinerelated drugs regulating adenosine transporters and receptors can be important for advancement of therapeutic strategies against these diseases. This review will summarize the therapeutic potential and recent SAR and pharmacology of adenosine and its receptor agonists and antagonists.

The Psychopharmacology of Effort-Related Decision Making: Dopamine, Adenosine, and Insights into the Neurochemistry of Motivation

Effort-based decision making is studied using tasks that offer choices between high-effort options leading to more highly valued reinforcers versus low-effort/low-reward options. These tasks have been used to study the involvement of neural systems, including mesolimbic dopamine and related circuits, in effort-related aspects of motivation. Moreover, such tasks are useful as animal models of some of the motivational symptoms that are seen in people with depression, schizophrenia, Parkinson's disease, and other disorders. The present review will discuss the pharmacology of effort-related decision making and will focus on the use of these tasks for the development of drug treatments for motivational dysfunction. Research has identified pharmacological conditions that can alter effort-based choice and serve as models for depression-related symptoms (e.g., the vesicular monoamine transport-2 inhibitor tetrabenazine and proinflammatory cytokines). Furthermore, tests of effort-based choice have identified compounds that are particularly useful for stimulating high-effort work output and reversing the deficits induced by tetrabenazine and cytokines. These studies indicate that drugs that act by facilitating dopamine transmission, as well as adenosine A_2A antagonists, are relatively effective at reversing effort-related impairments. Studies of effort-based choice may lead to the identification of drug targets that could be useful for treating motivational treatments that are resistant to commonly used antidepressants such as serotonin transport inhibitors.

A2A adenosine receptor functional states characterized by 19F-NMR

The human proteome contains 826 G protein-coupled receptors (GPCR), which control a wide array of key physiological functions, making them important drug targets. GPCR functions are based on allosteric coupling from the extracellular orthosteric drug binding site across the cell membrane to intracellular binding sites for partners such as G proteins and arrestins. This signaling process is related to dynamic equilibria in conformational ensembles that can be observed by NMR in solution. A previous high-resolution NMR study of the A_2A adenosine receptor (A_2AAR) resulted in a qualitative characterization of a network of such local polymorphisms. Here, we used ¹⁹F-NMR experiments with probes at the A_2AAR intracellular surface, which provides the high sensitivity needed for a refined description of different receptor activation states by ensembles of simultaneously populated conformers and the rates of exchange among them. We observed two agonist-stabilized substates that are not measurably populated in apo-A_2AAR and one inactive substate that is not seen in complexes with agonists, suggesting that A_2AAR activation includes both induced fit and conformational selection mechanisms. Comparison of A_2AAR and a constitutively active mutant established relations between the ¹⁹F-NMR spectra and signaling activity, which enabled direct assessment of the difference in basal activity between the native protein and its variant.

Potential Therapeutic Applications of Adenosine A2A Receptor Ligands and Opportunities for A2A Receptor Imaging

Adenosine A_2A receptors (A_2A Rs) are highly expressed in the human striatum, and at lower densities in the cerebral cortex, the hippocampus, and cells of the immune system. Antagonists of these receptors are potentially useful for the treatment of motor fluctuations, epilepsy, postischemic brain damage, or cognitive impairment, and for the control of an immune checkpoint during immunotherapy of cancer. A_2A R agonists may suppress transplant rejection and graft-versus-host disease; be used to treat inflammatory disorders such as asthma, inflammatory bowel disease, and rheumatoid arthritis; be locally applied to promote wound healing and be employed in a strategy for transient opening of the blood-brain barrier (BBB) so that therapeutic drugs and monoclonal antibodies can enter the brain. Increasing A_2A R signaling in adipose tissue is also a potential strategy to combat obesity. Several radioligands for positron emission tomography (PET) imaging of A_2A Rs have been developed in recent years. This review article presents a critical overview of the potential therapeutic applications of A_2A R ligands, the use of A_2A R imaging in drug development, and opportunities and limitations of PET imaging in future research.

Subthalamic and Cortical Local Field Potentials Associated with Pilocarpine-Induced Oral Tremor in the Rat

Tremulous jaw movements (TJMs) are rapid vertical deflections of the lower jaw that resemble chewing but are not directed at any particular stimulus. In rodents, TJMs are induced by neurochemical conditions that parallel those seen in human Parkinsonism, including neurotoxic or pharmacological depletion of striatal dopamine (DA), DA antagonism, and cholinomimetic administration. Moreover, TJMs in rodents can be attenuated by antiparkinsonian agents, including levodopa (L-DOPA), DA agonists, muscarinic antagonists, and adenosine A_2A antagonists. In human Parkinsonian patients, exaggerated physiological synchrony is seen in the beta frequency band in various parts of the cortical/basal ganglia/thalamic circuitry, and activity in the tremor frequency range (3–7 Hz) also has been recorded. The present studies were undertaken to determine if tremor-related local field potential (LFP) activity could be recorded from motor cortex (M1) or subthalamic nucleus (STN) during the TJMs induced by the muscarinic agonist pilocarpine, which is a well-known tremorogenic agent. Pilocarpine induced a robust TJM response that was marked by rhythmic electromyographic (EMG) activity in the temporalis muscle. Compared to periods with no tremor activity, TJM epochs were characterized by increased LFP activity in the tremor frequency range in both neocortex and STN. Tremor activity was not associated with increased synchrony in the beta frequency band. These studies identified tremor-related LFP activity in parts of the cortical/basal ganglia circuitry that are involved in the pathophysiology of Parkinsonism. This research may ultimately lead to identification of the oscillatory neural mechanisms involved in the generation of tremulous activity, and promote development of novel treatments for tremor disorders.

Fluoxetine Administration Exacerbates Oral Tremor and Striatal Dopamine Depletion in a Rodent Pharmacological Model of Parkinsonism

The cardinal motor symptoms of Parkinson's disease (PD) include resting tremor, akinesia, bradykinesia, and rigidity, and these motor abnormalities can be modeled in rodents by administration of the VMAT-2 (type-2 vesicular monoamine transporter) inhibitor tetrabenazine (9,10-dimethoxy-3-(2-methylpropyl)-1,3,4,6,7, 11b hexahydrobenzo[a]quinolizin-2-one; TBZ). Depression is also commonly associated with PD, and clinical data indicate that selective serotonin reuptake inhibitors (SSRIs) such as fluoxetine ((±)-N-methyl-γ-[4-(trifluoromethyl)phenoxy]benzenepropanamine hydrochloride; FLX) are frequently used to treat depression in PD patients. The aim of the present study was to characterize the effect of FLX on the motor dysfunctions induced by a low dose of TBZ (0.75 mg/kg), and investigate the neural mechanisms involved. This low dose of TBZ was selected based on studies with rat models of depressive symptoms. In rats, coadministration of FLX (2.5, 5.0, and 10.0 mg/kg) increased TBZ-induced oral tremor (tremulous jaw movements), and decreased locomotor activity compared with administration of TBZ alone. Coadministration of the serotonin 5-HT2A/2C antagonist mianserin (2.5 and 5.0 mg/kg) attenuated the increase in oral tremor induced by coadministration of TBZ (0.75 mg/kg) with FLX (5.0 mg/kg). Consistent with these behavioral data, coadministration of TBZ and FLX decreased DA tissue levels in the rat ventrolateral neostriatum compared with TBZ alone, and coadministration of mianserin with TBZ and FLX attenuated this effect, increasing DA tissue levels compared with the TBZ/FLX condition. These data suggest that SSRI administration in PD patients may result in worsening of motor symptoms, at least in part, by exacerbating existing DA depletions through 5-HT2A/2C-mediated modulation of DA neurotransmission.

History and perspectives of A2A adenosine receptor antagonists as potential therapeutic agents

Growing evidence emphasizes that the purine nucleoside adenosine plays an active role as a local regulator in different pathologies. Adenosine is a ubiquitous nucleoside involved in various physiological and pathological functions by stimulating A1 , A2A , A2B , and A3 adenosine receptors (ARs). At the present time, the role of A2A ARs is well known in physiological conditions and in a variety of pathologies, including inflammatory tissue damage and neurodegenerative disorders. In particular, the use of selective A2A antagonists has been reported to be potentially useful in the treatment of Parkinson's disease (PD). In this review, A2A AR signal transduction pathways, together with an analysis of the structure-activity relationships of A2A antagonists, and their corresponding pharmacological roles and therapeutic potential have been presented. The initial results from an emerging polypharmacological approach are also analyzed. This approach is based on the optimization of the affinity and/or functional activity of the examined compounds toward multiple targets, such as A1 /A2A ARs and monoamine oxidase-B (MAO-B), both closely implicated in the pathogenesis of PD.

Adenosine and its receptors as therapeutic targets: An overview

The main goal of the authors is to present an overview of adenosine and its receptors, which are G-protein coupled receptors. The four known adenosine receptor subtypes are discussed along with the therapeutic potential indicating that these receptors can serve as targets for various dreadful diseases.

Nucleus accumbens neurotransmission and effort-related choice behavior in food motivation: effects of drugs acting on dopamine, adenosine, and muscarinic acetylcholine receptors

Mesolimbic dopamine (DA) is a critical component of the brain circuitry regulating behavioral activation and effort-related processes. Although nucleus accumbens (NAc) DA depletions or antagonism leave aspects of appetite and primary food motivation intact, rats with impaired DA transmission reallocate their instrumental behavior away from food-reinforced tasks with high response requirements, and instead select less effortful food-seeking behaviors. Previous work showed that adenosine A2A antagonists can reverse the effects of DA D2 antagonists on effort-related choice, and that stimulation of adenosine A2A receptors produces behavioral effects that are similar to those induced by DA antagonism. The present review summarizes the literature on the role of NAc DA and adenosine in effort-related processes, and also presents original data on the effects of local stimulation of muscarinic acetylcholine receptors in NAc core. Local injections of the muscarinic agonist pilocarpine directly into NAc core produces shifts in effort-related choice behavior similar to those induced by DA antagonism or A2A receptor stimulation, decreasing lever pressing but increasing chow intake in rats responding on a concurrent fixed ratio/chow feeding choice task. In contrast, injections into a neostriatal control site dorsal to the NAc were ineffective. The actions of pilocarpine on this task were attenuated by co-administration of the muscarinic antagonist scopolamine. Thus, drugs that act on DA, adenosine A2A, and muscarinic receptors regulate effort-related choice behavior, which may have implications for the treatment of psychiatric symptoms such as psychomotor slowing, fatigue or anergia that can be observed in depression and other disorders.

The role of dopaminergic transmission through D1-like and D2-like receptors in amphetamine-induced rat ultrasonic vocalizations

RATIONALE

Systemic amphetamine (AMPH) administration increases the rate of 50-kHz ultrasonic vocalizations (USVs) in adult rats and preferentially enhances the 'trill' subtype; these effects of AMPH critically depend on noradrenergic transmission, but the possible contributions of dopamine are unclear.

OBJECTIVE

To assess the role of dopamine in 50-kHz USVs emitted drug-free and following systemic AMPH administration.

METHODS

Adult male Long-Evans rats pre-selected for high AMPH-induced calling rates were tested with AMPH (1 mg/kg, intraperitoneal (IP)) and saline following pretreatment with the following dopamine receptor antagonists: SCH 23390 (0.005-0.02 mg/kg, subcutaneous (SC)), SCH 39166 (0.03-0.3 mg/kg, SC), haloperidol (0.1, 0.2 mg/kg, IP), sulpiride (20-80 mg/kg, SC), raclopride (0.1-0.5 mg/kg, SC), clozapine (4 mg/kg, SC), risperidone (0.5 mg/kg, SC), and pimozide (1 mg/kg, IP). The dopamine and noradrenaline reuptake inhibitors (GBR 12909 and nisoxetine, respectively) were also tested, alone and in combination.

RESULTS

SCH 23390, SCH 39166, haloperidol, and raclopride dose-dependently inhibited vocalizations under AMPH and suppressed the proportion of trill calls. Sulpiride, however, had no discernable effect on call rate or profile, even at a high dose that reduced locomotor activity. Single doses of clozapine, risperidone, and pimozide all markedly decreased calling under saline and AMPH. Finally, GBR 12909 and nisoxetine failed to promote 50-kHz USVs detectably or alter the subtype profile, when tested alone or in combination.

CONCLUSIONS

The rate of 50-kHz USVs and the call subtype profile following systemic AMPH administration depends on dopaminergic neurotransmission through D1-like and D2-like receptors. However, inhibiting dopamine and/or noradrenaline reuptake appears insufficient to induce calling.

Stabilised G protein-coupled receptors in structure-based drug design: a case study with adenosine A2A receptor

The first example of structure-based drug design with stabilised GPCRs has enabled the identification of a preclinical candidate for the treatment of Parkinson's disease.

Sensitivity to cognitive effort mediates psychostimulant effects on a novel rodent cost/benefit decision-making task

Amotivational states and insufficient recruitment of mental effort have been observed in a variety of clinical populations, including depression, traumatic brain injury, post-traumatic stress disorder, and attention deficit hyperactivity disorder. Previous rodent models of effort-based decision making have utilized physical costs whereas human studies of effort are primarily cognitive in nature, and it is unclear whether the two types of effortful decision making are underpinned by the same neurobiological processes. We therefore designed a novel rat cognitive effort task (rCET) based on the 5-choice serial reaction time task, a well-validated measure of attention and impulsivity. Within each trial of the rCET, rats are given the choice between an easy or hard visuospatial discrimination, and successful hard trials are rewarded with double the number of sugar pellets. Similar to previous human studies, stable individual variation in choice behavior was observed, with 'workers' choosing hard trials significantly more than their 'slacker' counterparts. Whereas workers 'slacked off' in response to administration of amphetamine and caffeine, slackers 'worked harder' under amphetamine, but not caffeine. Conversely, these stimulants increased motor impulsivity in all animals. Ethanol did not affect animals' choice but invigorated behavior. In sum, we have shown for the first time that rats are differentially sensitive to cognitive effort when making decisions, independent of other processes such as impulsivity, and these baseline differences can influence the cognitive response to psychostimulants. Such findings could inform our understanding of impairments in effort-based decision making and contribute to treatment development.

The novel adenosine A(2A) antagonist prodrug MSX-4 is effective in animal models related to motivational and motor functions

Adenosine A(2A) and dopamine D2 receptors interact to regulate diverse aspects of ventral and dorsal striatal functions related to motivational and motor processes, and it has been suggested that adenosine A(2A) antagonists could be useful for the treatment of depression, parkinsonism and other disorders. The present experiments were performed to characterize the effects of MSX-4, which is an amino acid ester prodrug of the potent and selective adenosine A(2A) receptor antagonist MSX-2, by assessing its ability to reverse pharmacologically induced motivational and motor impairments. In the first group of studies, MSX-4 reversed the effects of the D2 antagonist eticlopride on a concurrent lever pressing/chow feeding task that is used as a measure of effort-related choice behavior. MSX-4 was less potent after intraperitoneal administration than the comparison compound, MSX-3, though both were equally efficacious. With this task, MSX-4 was orally active in the same dose range as MSX-3. MSX-4 also reversed the locomotor suppression induced by eticlopride in the open field, but did not induce anxiogenic effects as measured by the relative amount of interior activity. Behaviorally active doses of MSX-4 also attenuated the increase in c-Fos and pDARPP-32(Thr34) expression in nucleus accumbens core that was induced by injections of eticlopride. In addition, MSX-4 suppressed the oral tremor induced by the anticholinesterase galantamine, which is consistent with an antiparkinsonian profile. These actions of MSX-4 indicate that this compound could have potential utility as a treatment for parkinsonism, as well as some of the motivational symptoms of depression and other disorders.

Pharmacological and physiological characterization of the tremulous jaw movement model of parkinsonian tremor: potential insights into the pathophysiology of tremor

Tremor is a cardinal symptom of parkinsonism, occurring early on in the disease course and affecting more than 70% of patients. Parkinsonian resting tremor occurs in a frequency range of 3-7 Hz and can be resistant to available pharmacotherapy. Despite its prevalence, and the significant decrease in quality of life associated with it, the pathophysiology of parkinsonian tremor is poorly understood. The tremulous jaw movement (TJM) model is an extensively validated rodent model of tremor. TJMs are induced by conditions that also lead to parkinsonism in humans (i.e., striatal DA depletion, DA antagonism, and cholinomimetic activity) and reversed by several antiparkinsonian drugs (i.e., DA precursors, DA agonists, anticholinergics, and adenosine A(2A) antagonists). TJMs occur in the same 3-7 Hz frequency range seen in parkinsonian resting tremor, a range distinct from that of dyskinesia (1-2 Hz), and postural tremor (8-14 Hz). Overall, these drug-induced TJMs share many characteristics with human parkinsonian tremor, but do not closely resemble tardive dyskinesia. The current review discusses recent advances in the validation of the TJM model, and illustrates how this model is being used to develop novel therapeutic strategies, both surgical and pharmacological, for the treatment of parkinsonian resting tremor.