Dopamine D1 and D2 antagonist effects on response likelihood and duration

Endogenous Synthesis of Tetrahydroisoquinoline Derivatives from Dietary Factors: Neurotoxicity Assessment on a 3D Neurosphere Culture

Tetrahydroisoquinoline (THIQ) alkaloids and their derivatives have a structural similarity to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a well-known neurotoxin. THIQs seem to present a broad range of actions in the brain, critically dependent on their catechol moieties and metabolism. These properties make it reasonable to assume that an acute or chronic exposure to some THIQs might lead to neurodegenerative diseases including essential tremor (ET). We developed a method to search for precursor carbonyl compounds produced during the Maillard reaction in overcooked meats to study their reactivity with endogenous amines and identify the reaction products. Then, we predicted in silico their pharmacokinetic and toxicological properties toward the central nervous system. Finally, their possible neurological effects on a novel in vitro 3D neurosphere model were assessed. The obtained data indicate that meat is an alkaloid precursor, and we identified the alkaloid 1-benzyl-1,2,3,4-tetrahydroisoquinoline-6,7-diol (1-benz-6,7-diol THIQ) as the condensation product of phenylacetaldehyde with dopamine; in silico study of 1-benz-6,7-diol-THIQ reveals modulation of dopamine receptor D1 and D2; and in vitro study of 1-benz-6,7-diol-THIQ for cytotoxicity and oxidative stress induction does not show any difference after 24 h contact for all tested concentrations. To conclude, our in vitro data do not support an eventual neurotoxic effect for 1-benz-6,7-diol-THIQ.

Size-Dependent Effects of Polystyrene Nanoparticles (PS-NPs) on Behaviors and Endogenous Neurochemicals in Zebrafish Larvae

Microplastics, small pieces of plastic derived from polystyrene, have recently become an ecological hazard due to their toxicity and widespread occurrence in aquatic ecosystems. In this study, we exposed zebrafish larvae to two types of fluorescent polystyrene nanoparticles (PS-NPs) to identify their size-dependent effects. PS-NPs of 50 nm, unlike 100 nm PS-NPs, were found to circulate in the blood vessels and accumulate in the brains of zebrafish larvae. Behavioral and electroencephalogram (EEG) analysis showed that 50 nm PS-NPs induce abnormal behavioral patterns and changes in EEG power spectral densities in zebrafish larvae. In addition, the quantification of endogenous neurochemicals in zebrafish larvae showed that 50 nm PS-NPs disturb dopaminergic metabolites, whereas 100 nm PS-NPs do not. Finally, we assessed the effect of PS-NPs on the permeability of the blood–brain barrier (BBB) using a microfluidic system. The results revealed that 50 nm PS-NPs have high BBB penetration compared with 100 nm PS-NPs. Taken together, we concluded that small nanoparticles disturb the nervous system, especially dopaminergic metabolites.

Dopaminergic signaling regulates zebrafish larvae's response to electricity

Electrical stimulation of brain or muscle activities has gained attention for studying the molecular and cellular mechanisms involved in electric-induced responses. We recently showed zebrafish's response to electricity. Here, we hypothesized that this response is affected by the dopaminergic signaling pathways. The effects of multiple dopamine agonists and antagonists on the electric response of 6 days-postfertilization zebrafish larvae were investigated using a microfluidic device with enhanced control of experimentation and throughput. All dopamine antagonists decreased locomotor activities, while dopamine agonists did not induce similar behaviors. The D2-selective dopamine agonist quinpirole enhanced the movement. Exposure to nonselective and D1-selective dopamine agonists apomorphine and SKF-81297 caused no significant change in the electric response. Exposing larvae that were pretreated with nonselective and D2-selective dopamine antagonists butaclamol and haloperidol to apomorphine and quinpirole, respectively, restored the electric locomotion. These results reveal a correlation between electric response and dopamine signaling pathway. Furthermore, they demonstrate that electric-induced zebrafish larvae locomotion can be conditioned by modulating dopamine receptor functions. Our electrofluidic assay has profound application potential for fundamental electric-induced response research and brain disorder studies especially those related to the dopamine imbalance and as a chemical screening method when investigating biological pathways and behaviors. This article is protected by copyright. All rights reserved.

D1- and D2-like receptors differentially mediate the effects of dopaminergic transmission on cost-benefit evaluation and motivation in monkeys

It has been widely accepted that dopamine (DA) plays a major role in motivation, yet the specific contribution of DA signaling at D₁-like receptor (D₁R) and D₂-like receptor (D₂R) to cost–benefit trade-off remains unclear. Here, by combining pharmacological manipulation of DA receptors (DARs) and positron emission tomography (PET) imaging, we assessed the relationship between the degree of D₁R/D₂R blockade and changes in benefit- and cost-based motivation for goal-directed behavior of macaque monkeys. We found that the degree of blockade of either D₁R or D₂R was associated with a reduction of the positive impact of reward amount and increasing delay discounting. Workload discounting was selectively increased by D₂R antagonism. In addition, blocking both D₁R and D₂R had a synergistic effect on delay discounting but an antagonist effect on workload discounting. These results provide fundamental insight into the distinct mechanisms of DA action in the regulation of the benefit- and cost-based motivation, which have important implications for motivational alterations in both neurological and psychiatric disorders.

Using quantitatively controlled pharmacological manipulations, this study teases apart the role of D1- and D2-like dopamine receptors in motivation and goal-directed behavior in monkeys, revealing complementary roles of two dopamine receptor subtypes in the computation of the cost/benefit trade-off to guide action.

The role of the nucleus accumbens in learned approach behavior diminishes with training

Nucleus accumbens dopamine plays a key role in reward-directed approach. Past findings suggest that dopamine's role in the expression of learned behavior diminishes with extended training. However, little is known about the central substrates that mediate the shift to dopamine-independent reward approach. In the present study, rats approached and inserted the head into a reward compartment in response to a cue signaling food delivery. On days 4 and 5 of 28-trial-per-day sessions, D1 receptor antagonist R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (SCH23390) infused to the NAc core reduced the probability and speed of cued approach. The disruptive effect of D1 receptor blockade was specific to the nucleus accumbens core and not seen with drug infusions to nearby dopamine target regions. In rats that received drug infusions after extended training (days 10 or 11), accumbens core D1 receptor blockade produced little effect on the expression of the same behavior. These results could have been due to a continued accumbens mediation of cued approach even after the behavior had become independent of accumbens D1 receptors. However, accumbens core ionotropic glutamate receptor blockade disrupted cued approach during early but not late stages of training, similar to the effects of D1 antagonist infusions. The results suggest that with extended training, a nucleus accumbens D1-dependent behavior becomes less dependent not only on nucleus accumbens D1 transmission but also on excitatory transmission in the nucleus accumbens. These findings fill an important gap in a growing literature on reorganization of striatal function over the course of training.

What Is the Relationship between Dopamine and Effort?

The trade-off between reward and effort is at the heart of most behavioral theories, from ecology to economics. Compared to reward, however, effort remains poorly understood, both at the behavioral and neurophysiological levels. This is important because unwillingness to overcome effort to gain reward is a common feature of many neuropsychiatric and neurological disorders. A recent surge in interest in the neurobiological basis of effort has led to seemingly conflicting results regarding the role of dopamine. We argue here that, upon closer examination, there is actually striking consensus across studies: dopamine primarily codes for future reward but is less sensitive to anticipated effort cost. This strong association between dopamine and the incentive effects of rewards places dopamine in a key position to promote reward-directed action.

Decreasing Striatopallidal Pathway Function Enhances Motivation by Energizing the Initiation of Goal-Directed Action

Altered dopamine D2 receptor (D2R) binding in the striatum has been associated with abnormal motivation in neuropsychiatric disorders, including schizophrenia. Here, we tested whether motivational deficits observed in mice with upregulated D2Rs (D2R-OEdev mice) are reversed by decreasing function of the striatopallidal "no-go" pathway. To this end, we expressed the Gαi-coupled designer receptor hM4D in adult striatopallidal neurons and activated the receptor with clozapine-N-oxide (CNO). Using a head-mounted miniature microscope we confirmed with calcium imaging in awake mice that hM4D activation by CNO inhibits striatopallidal function measured as disinhibited downstream activity in the globus pallidus. Mice were then tested in three operant tasks that address motivated behavior, the progressive ratio task, the progressive hold-down task, and outcome devaluation. Decreasing striatopallidal function in the dorsomedial striatum or nucleus accumbens core enhanced motivation in D2R-OEdev mice and control littermates. This effect was due to increased response initiation but came at the cost of goal-directed efficiency. Moreover, response vigor and the sensitivity to changes in reward value were not altered. Chronic activation of hM4D by administering CNO for 2 weeks in drinking water did not affect motivation due to a tolerance effect. However, the acute effect of CNO on motivation was reinstated after discontinuing chronic treatment for 48 h. Used as a therapeutic approach, striatopallidal inhibition should consider the risk of impairing goal-directed efficiency and behavioral desensitization.

SIGNIFICANCE STATEMENT

Motivation involves a directional component that allows subjects to efficiently select the behavior that will lead to an optimal outcome and an activational component that initiates and maintains the vigor and persistence of actions. Striatal output pathways modulate motivated behavior, but it remains unknown how these pathways regulate specific components of motivation. Here, we found that the indirect pathway controls response initiation without affecting response vigor or the sensitivity to changes in the reward outcome. A specific enhancement in the activational component of motivation, however, can come at the cost of goal-directed efficiency when a sustained response is required to obtain the goal. These data should inform treatment strategies for brain disorders with impaired motivation such as schizophrenia and Parkinson's disease.

Reappraising striatal D1- and D2-neurons in reward and aversion

The striatum has been involved in complex behaviors such as motor control, learning, decision-making, reward and aversion. The striatum is mainly composed of medium spiny neurons (MSNs), typically divided into those expressing dopamine receptor D1, forming the so-called direct pathway, and those expressing D2 receptor (indirect pathway). For decades it has been proposed that these two populations exhibit opposing control over motor output, and recently, the same dichotomy has been proposed for valenced behaviors. Whereas D1-MSNs mediate reinforcement and reward, D2-MSNs have been associated with punishment and aversion. In this review we will discuss pharmacological, genetic and optogenetic studies that indicate that there is still controversy to what concerns the role of striatal D1- and D2-MSNs in this type of behaviors, highlighting the need to reconsider the early view that they mediate solely opposing aspects of valenced behaviour.

Dissociable hippocampal and amygdalar D1-like receptor contribution to discriminated Pavlovian conditioned approach learning

Pavlovian conditioning is an elementary form of reward-related behavioral adaptation. The mesolimbic dopamine system is widely considered to mediate critical aspects of reward-related learning. For example, initial acquisition of positively-reinforced operant behavior requires dopamine (DA) D1 receptor (D1R) activation in the basolateral amygdala (BLA), central nucleus of the amygdala (CeA), and the ventral subiculum (vSUB). However, the role of D1R activation in these areas on appetitive, non-drug-related, Pavlovian learning is not currently known. In separate experiments, microinfusions of the D1-like receptor antagonist SCH-23390 (3.0 nmol/0.5 μL per side) into the amygdala and subiculum preceded discriminated Pavlovian conditioned approach (dPCA) training sessions. D1-like antagonism in all three structures impaired the acquisition of discriminated approach, but had no effect on performance after conditioning was asymptotic. Moreover, dissociable effects of D1-like antagonism in the three structures on components of discriminated responding were obtained. Lastly, the lack of latent inhibition in drug-treated groups may elucidate the role of D1-like in reward-related Pavlovian conditioning. The present data suggest a role for the D1 receptors in the amygdala and hippocampus in learning the significance of conditional stimuli, but not in the expression of conditional responses.

Differential effects and potential adverse outcomes of ionic silver and silver nanoparticles in vivo and in vitro

Nanoparticles are of concern because of widespread use, but it is unclear if metal nanoparticles cause effects directly or indirectly. We explored whether polyvinylpyrrolidone-coated silver nanoparticles (PVP-AgNPs) cause effects through intact nanoparticles or dissolved silver. Females of the model species fathead minnow (Pimephales promelas) were exposed to either 4.8 μg/L of AgNO3 or 61.4 μg/L of PVP-AgNPs for 96h. Microarray analyses were used to identify impacted receptors and toxicity pathways in liver and brain tissues that were confirmed using in vitro mammalian assays. AgNO3 and PVP-AgNP exposed fish had common and distinct effects consistent with both intact nanoparticles and dissolved silver causing effects. PVP-AgNPs and AgNO3 both affected pathways involved in Na(+), K(+), and H(+) homeostasis and oxidative stress but different neurotoxicity pathways. In vivo effects were supported by PVP-AgNP activation of five in vitro nuclear receptor assays and inhibition of ligand binding to the dopamine receptor. AgNO3 inhibited ligand binding to adrenergic receptors α1 and α2 and cannabinoid receptor CB1, but had no effect in nuclear receptor assays. PVP-AgNPs have the potential to cause effects both through intact nanoparticles and metal ions, each interacting with different initiating events. Since the in vitro and in vivo assays examined here are commonly used in human and ecological hazard screening, this work suggests that environmental health assessments should consider effects of intact nanoparticles in addition to dissolved metals.

Acute administration of dopaminergic drugs has differential effects on locomotion in larval zebrafish

Altered dopaminergic signaling causes behavioral changes in mammals. In general, dopaminergic receptor agonists increase locomotor activity, while antagonists decrease locomotor activity. In order to determine if zebrafish (a model organism becoming popular in pharmacology and toxicology) respond similarly, the acute effects of drugs known to target dopaminergic receptors in mammals were assessed in zebrafish larvae. Larvae were maintained in 96-well microtiter plates (1 larva/well). Non-lethal concentrations (0.2-50 μM) of dopaminergic agonists (apomorphine, SKF-38393, and quinpirole) and antagonists (butaclamol, SCH-23390, and haloperidol) were administered at 6 days post-fertilization (dpf). An initial experiment identified the time of peak effect of each drug (20-260 min post-dosing, depending on the drug). Locomotor activity was then assessed for 70 min in alternating light and dark at the time of peak effect for each drug to delineate dose-dependent effects. All drugs altered larval locomotion in a dose-dependent manner. Both the D1- and D2-like selective agonists (SKF-38393 and quinpirole, respectively) increased activity, while the selective antagonists (SCH-23390 and haloperidol, respectively) decreased activity. Both selective antagonists also blunted the response of the larvae to changes in lighting conditions at higher doses. The nonselective drugs had biphasic effects on locomotor activity: apomorphine increased activity at the low dose and at high doses, while butaclamol increased activity at low to intermediate doses, and decreased activity at high doses. This study demonstrates that (1) larval zebrafish locomotion can be altered by dopamine receptor agonists and antagonists, (2) receptor agonists and antagonists generally have opposite effects, and (3) drugs that target dopaminergic receptors in mammals appear, in general, to elicit similar locomotor responses in zebrafish larvae.

A model of reward- and effort-based optimal decision making and motor control

Costs (e.g. energetic expenditure) and benefits (e.g. food) are central determinants of behavior. In ecology and economics, they are combined to form a utility function which is maximized to guide choices. This principle is widely used in neuroscience as a normative model of decision and action, but current versions of this model fail to consider how decisions are actually converted into actions (i.e. the formation of trajectories). Here, we describe an approach where decision making and motor control are optimal, iterative processes derived from the maximization of the discounted, weighted difference between expected rewards and foreseeable motor efforts. The model accounts for decision making in cost/benefit situations, and detailed characteristics of control and goal tracking in realistic motor tasks. As a normative construction, the model is relevant to address the neural bases and pathological aspects of decision making and motor control.

Effects of cortical and striatal dopamine D1 receptor blockade on cued versus noncued behavioral responses

Various lines of evidence suggest that disruptions in brain dopamine (DA) transmission produce behavioral impairments that can be overcome by salient response-eliciting environmental stimuli. We examined here whether D1 receptor blockade within striatal or frontal cortical DA target regions would differentially affect head entry responses elicited by an auditory cue compared with those occurring during noncued intertrial intervals. Rats received 2 drug-free 28-trial daily sessions in which an auditory cue was immediately followed by food delivery. On the following day, separate groups of rats received bilateral infusions of D1 antagonist SCH23390 to the dorsomedial striatum (DMS), nucleus accumbens (NAcc) core, or the medial prefrontal cortex (mPFC). SCH23390 infused into the DMS and NAcc core suppressed noncued head entries but had no effect on head entries in response to the auditory cue. SCH23390 infused to the mPFC did not reduce either cued or noncued approach responses. Systemic administration of the drug, in contrast, reduced the frequency of both cued and noncued approaches. The results are consistent with the notion that has emerged from the Parkinson's literature that reduced DA transmission produces behavioral suppression that can be overcome by salient environmental response elicitors, and extends this notion by showing that D1 receptor transmission within the striatum strongly suppresses noncued responses while leaving the identical behavior intact when cued by an environmental stimulus.

D1 and D2 dopamine receptor antagonists decrease behavioral bout duration, without altering the bout's repeated behavioral components, in a naturalistic model of repetitive and compulsive behavior

Nest building behavior in the pregnant female rabbit (Oryctolagus cuniculus) is a model for compulsive behavior in Obsessive Compulsive Disorder (OCD). This behavior comprises a cycle of repeated, stereotyped components (collecting straw, entering nest box and depositing the straw there, returning to collect more straw), which itself is repeated 80+ times in a single bout that lasts approximately 50min. The bout, in turn, is repeated if necessary, according to the rabbit's perception of whether or not the nest is finished. We administered SCH23390 (5-100μg/kg; D1/D5 antagonist) or raclopride (0.05-1.0mg/kg; D2/D3 antagonist), subcutaneously to day 28 pregnant female rabbits, 30 or 60min before placing straw inside their home cage. At doses that minimally affected ambulatory behavior in open field (5-12.5μg/kg SCH23390, 0.5-1.0mg/kg raclopride), both antagonists dramatically reduced bout duration while not significantly affecting the initiation of straw carrying behavior, the sequential performance of the individual cycle components, maximum cycle frequency, or the total number of bouts performed. These results point to an important role for dopamine neurotransmission for the prolonged expression of a normal, repetitive and compulsive-like behavior. Moreover, the finding that dopamine receptor antagonists decrease the time spent engaged in repetitive behavior (without significantly altering the form of the repetitive behavior itself) suggests a possible explanation for why neuroleptics can be clinically effective for treating OCD.

Effects of systemic or nucleus accumbens-directed dopamine D1 receptor antagonism on sucrose seeking in rats

RATIONALE

Conditioned cues can elicit relapse to drug- and food-seeking behavior over prolonged periods of abstinence. If seeking behavior depends on mesolimbic dopamine D1 receptors, blocking these receptors should reduce seeking behavior.

OBJECTIVES

We examined the effects of either systemic or intra-nucleus accumbens administration of the D1 antagonist SCH 23390 on extinction responding (sucrose seeking) by rats either 1 or 30 days into forced abstinence.

MATERIALS AND METHODS

Rats self-administered 10% sucrose paired with a tone + light cue for 10 days. After either 1 or 30 days of forced abstinence, rats received systemic (0, 1, 5, or 25 μg/kg IP) or bilateral nucleus accumbens core or shell (0.3 or 0.6 μg/site) injections of SCH 23390 prior to extinction testing.

RESULTS

Saline-treated rats responded more during extinction following 30 vs. 1 day of forced abstinence ("incubation of craving"). Systemic SCH 23390 reduced sucrose seeking after 1 day of forced abstinence, significantly reducing responding following pretreatment with 1, 5, and 25 μg/kg SCH 23390, but only 25 μg/kg significantly reduced sucrose seeking after 30 days of forced abstinence. SCH 23390 (0.3 or 0.6 μg/site) in the core or shell of the nucleus accumbens reduced sucrose seeking in all groups.

CONCLUSION

Nucleus accumbens D1 receptors are involved in sucrose seeking, but it is not clear if they are involved in the incubation of craving. The fact that D1 antagonism reduced sucrose seeking across an extended period of abstinence may be of use for development of treatment strategies for relapse.