Effects of dopamine receptor antagonists (D1 and D2) on the demand for smoked cocaine base in rhesus monkeys

Performance and Mechanism of Uranium Adsorption from Seawater to Poly(dopamine)-Inspired Sorbents

Developing facile and robust technologies for effective enrichment of uranium from seawater is of great significance for resource sustainability and environmental safety. By exploiting mussel-inspired polydopamine (PDA) chemistry, diverse types of PDA-functionalized sorbents including magnetic nanoparticle (MNP), ordered mesoporous carbon (OMC), and glass fiber carpet (GFC) were synthesized. The PDA functional layers with abundant catechol and amine/imine groups provided an excellent platform for binding to uranium. Due to the distinctive structure of PDA, the sorbents exhibited multistage kinetics which was simultaneously controlled by chemisorption and intralayer diffusion. Applying the diverse PDA-modified sorbents for enrichment of low concentration (parts per billion) uranium in laboratory-prepared solutions and unpurified seawater was fully evaluated under different scenarios: that is, by batch adsorption for MNP and OMC and by selective filtration for GFC. Moreover, high-resolution X-ray photoelectron spectroscopic and extended X-ray absorption fine structure studies were performed for probing the underlying coordination mechanism between PDA and U(VI). The catechol hydroxyls of PDA were identified as the main bidentate ligands to coordinate U(VI) at the equatorial plane. This study assessed the potential of versatile PDA chemistry for development of efficient uranium sorbents and provided new insights into the interaction mechanism between PDA and uranium.

Therapeutic doses of buspirone block D3 receptors in the living primate brain

Dopamine D3 receptor (D3R) antagonists may be effective medications for multiple substance use disorders (SUDs). However, no selective D3R antagonists are currently available for clinical testing. Buspirone, originally characterized as a 5-HT1A partial agonist and used as an anxiolytic, also binds to D3R and D4R with high affinity, with lower affinity to D2R, and interferes with cocaine reward. Here we used PET with [11C]PHNO (D3R-preferring radioligand), [11C]raclopride (D2R/D3R radioligand) and [11C]NNC-112 (D1R radioligand) to measure occupancy of oral and parenteral buspirone in the primate brain. Intramuscular buspirone (0.19 and 0.5 mg/kg) blocked both [11C]PHNO and [11C]raclopride binding to striatum, exhibiting high occupancy (50-85%) at 15 min and rapid wash-out over 2-6 h. In contrast, oral buspirone (3 mg/kg) significantly blocked [11C]PHNO binding in D3-rich regions (globus pallidum and midbrain) at 3 h, but had minimal effects on [11C]raclopride binding (28-37% at 1 h and 10% at 3 h). Buspirone did not block [11C]NNC-112. Our findings provide evidence that i.m. buspirone blocks D3R and D2R, whereas oral buspirone is more selective towards D3R blockade in vivo, consistent with extensive first pass metabolism and supporting the hypothesis that its metabolites (5- and 6'-hydroxybuspirone) merit evaluation for treating SUDs. They also indicate that for oral buspirone to achieve greater than 80% sustained D3R occupancy, as might be needed to treat addiction, higher doses (at least three-fold) than those used to treat anxiety (maximal 60 mg) will be required. Nonetheless, based on previous clinical studies, these doses would be safe and well tolerated.

Behavioral economic assessment of price and cocaine consumption following self-administration histories that produce escalation of either final ratios or intake

Various self-administration procedures are being developed to model specific aspects of the addiction process. For example, 'increased cocaine intake over time' has been modeled by providing long access (LgA) to cocaine during daily self-administration sessions under a fixed-ratio (FR1) reinforcement schedule. In addition, 'increased time and energy devoted to acquire cocaine' has been modeled by providing access to cocaine during daily self-administration sessions under a progressive-ratio (PR) schedule. To investigate the distinctiveness of these models, the behavioral economics variables of consumption and price were applied to cocaine self-administration data. To assess changes in consumption and price, cocaine self-administration was tested across a descending series of doses (0.237-0.001 mg per injection) under an FR1 reinforcement schedule to measure drug intake in the high dose range and thresholds in the low range. Cocaine consumption remained relatively stable across doses until a threshold was reached, at which maximal responding was observed. It was found that a history of LgA training produced an increase in cocaine consumption; whereas a history of PR training produced an increase in the maximal price (P(max)) expended for cocaine. Importantly, the concepts of consumption and price were found to be dissociable. That is, LgA training produced an increase in consumption but a decrease in P(max), whereas PR training produced an increase in P(max) without increasing consumption. These results suggest that distinct aspects of the addiction process can be parsed using self-administration models, thereby facilitating the investigation of specific neurobiological adaptations that occur through the addiction process.

Dopamine D2/D3 receptors play a specific role in the reversal of a learned visual discrimination in monkeys

Converging evidence supports a role for mesocorticolimbic dopaminergic systems in a subject's ability to shift behavior in response to changing stimulus-reward contingencies. To characterize the dopaminergic mechanisms involved in this function, we quantified the effects of subtype-specific dopamine (DA) receptor antagonists on acquisition, retention, and reversal of a visual discrimination task in non-human primates (Chlorocebus aethiops sabaeus). We used a modified Wisconsin General Test Apparatus that was equipped with three food boxes, each fitted with a lid bearing a unique visual cue; one of the cues concealed a food reward, whereas the other two concealed an empty box. The monkeys were trained first to acquire a novel discrimination (eg A(+), B(-), C(-)) in a single session, before experiencing either a reversal of the discrimination (eg A(-), B(+), C(-)) or the acquisition of a completely new discrimination (eg D(+), E(-), F(-)), on the following day. Systemic administration of the D(2)/D(3) receptor antagonist raclopride (0.001-0.03 mg/kg) failed to significantly affect the performance of reversal learning when reversal sessions were run without a retention session. But, raclopride (0.03 mg/kg) significantly impaired performance under the reversal condition when reversal sessions were run right after a retention session; however, it did not affect acquisition of a novel visual discrimination. Specifically, raclopride significantly increased the number of reversal errors made before reaching the performance criterion in the reversal, but not in new learning sessions. In contrast, the D(1)/D(5) receptor antagonist SCH 23390 did not significantly modulate acquisition of a novel discrimination or reversal learning at doses (0.001-0.03 mg/kg, i.m.) that did not suppress behavior generally. In addition, none of the drug treatments affected retention of a previously learned discrimination. The results strongly suggest that D(2)/D(3) receptors, but not D(1)/D(5) receptors, selectively mediate reversal learning, without affecting the capacity to learn a new stimulus-reward association. These data support the hypothesis that phasic DA release, acting through D(2)-like receptors, mediates behavioral flexibility.

Differential contributions of dopaminergic D1- and D2-like receptors to cognitive function in rhesus monkeys

RATIONALE

Dopaminergic neurotransmission is critically involved in many aspects of complex behavior and cognition beyond reward/reinforcement and motor function. Mental and behavioral disorders associated with major disruptions of dopamine neurotransmission, including schizophrenia, attention deficit/hyperactivity disorder, Parkinson's disease, Huntington's disease, and substance abuse produce constellations of neuropsychological deficits in learning, memory, and attention in addition to other defining symptoms.

OBJECTIVE

To delineate the role dopaminergic D1- and D2-like receptor subtypes play in complex brain functions.

MATERIALS AND METHODS

Monkeys (N = 6) were trained on cognitive tests adapted from a human neuropsychological assessment battery (CAmbridge Neuropsychological Test Automated Battery). The battery included tests of spatial working memory (self-ordered spatial search task), visuo-spatial associative memory and learning (visuo-spatial paired associates learning task, vsPAL) and motivation (progressive ratio task, PR). Tests of motor function (bimanual motor skill task, BMS; rotating turntable task, RTT) were also included. The effects of the dopamine D2-like antagonist raclopride (10-56 microg/kg, i.m.) and the D1-like antagonist SCH23390 (SCH, 3.2-56 microg/kg, i.m.) on cognitive performance were then determined.

RESULTS

Deficits on PR, RTT, and BMS performance were observed after both raclopride and SCH23390. Spatial working memory accuracy was reduced to a greater extent by raclopride than by SCH, which was unexpected, given prior reports on the involvement of D1 signaling for spatial working memory in monkeys. Deficits were observed on vsPAL performance after raclopride, but not after SCH23390.

CONCLUSIONS

The intriguing results suggest a greater contribution of D2- over D1-like receptors to both spatial working memory and object-location associative memory.

Effects of bremazocine on self-administration of smoked cocaine base and orally delivered ethanol, phencyclidine, saccharin, and food in rhesus monkeys: a behavioral economic analysis

There is increasing evidence that kappa-opioid receptor agonists modulate cocaine-maintained behavior, and limited findings implicate the involvement of kappa-opioid receptors in ethanol-maintained behaviors. The purpose of the present study was to investigate the effects of bremazocine, a kappa-opioid agonist, on the self-administration of smoked cocaine base and oral ethanol in rhesus monkeys (Macaca mulatta). To determine the selectivity of bremazocine, the effects of bremazocine pretreatment on the oral self-administration of phencyclidine (PCP), saccharin, and food were also examined. Adult male rhesus monkeys were trained to self-administer oral ethanol, PCP, saccharin (n = 8), food (n = 6), or smoked cocaine base (n = 6) and water during daily sessions. Bremazocine (0.00032-, 0.001-, and 0.0025-mg/kg i.m.) injections were given 15 min before session. The 4 days of stable behavior before pretreatment served as baseline. Demand curves (consumption x fixed ratio; FR) were obtained for smoked cocaine base, ethanol, and PCP by varying the cost (FR) of drug deliveries and measuring consumption (deliveries). Bremazocine (0.001 mg/kg) was administered at each FR value in nonsystematic order. Results indicate that bremazocine dose dependently reduced cocaine, ethanol, PCP, and saccharin intake. Food intake was affected less by bremazocine than the other substances in five of the six monkeys. Generally, bremazocine treatment reduced the demand for cocaine, ethanol, and PCP as well as other measures of response strength. These results extend the findings that kappa-agonists reduce the self-administration of drug and nondrug reinforcers to smoked cocaine base and oral ethanol, PCP, and saccharin in rhesus monkeys.

D1 dopamine receptor: a putative neurochemical and behavioral link to cocaine action

Cocaine is one of the most abused psychostimulants known to man and as such, researchers have been steadfast in their attempts to understand the neurobiological mechanisms responsible for its abuse. Cocaine undoubtedly wreaks havoc on a number of mammalian neuronal neurotransmitter systems, and it is maintained that this dysregulatory effect supports cocaine abuse. Cocaine's mechanism of action has been well described. Cocaine binds differentially to the dopamine, serotonin, and norepinephrine transport proteins and directly prevents the re-uptake of dopamine, serotonin, and norepinephrine into pre-synaptic neurons (Heikkila et al., 1975, Biochem Pharmacol 24(8):847-852; Reith et al., 1986, Biochem Pharmacol 35(7):1123-1129; Ritz et al., 1987, Science 237:1219-1223). Inhibition of re-uptake subsequently elevates the synaptic concentrations of each of these neurotransmitters. In addition to this direct effect, cocaine also produces a number of indirect actions, which alter other neuromodulatory systems (i.e., opioidergic, glutamatergic, and GABAergic systems). Many of these effects are just beginning to be elucidated, but nonetheless contribute to this agent's diverse pharmacological profile. Interestingly, it is the indirect actions of this mellifluous molecule, which mediate most of its sought and unsought effects. The intricacy with which cocaine produces neuronal alterations beyond its direct effects on neurotransmitter re-uptake appear to be most relevant to cocaine abuse, and hence the phenomenon of addiction. In light of cocaine's multifarious effects on numerous neuronal systems, its effect on dopaminergic neurotransmission has attracted the most attention, particularly because of the implicated role of dopamine in brain reward. Pharmacologically, the psychostimulant effects of cocaine appear to be mediated by its ability to enhance dopaminergic activity within the mesocorticolimbic circuit (Roberts et al., 1977, Pharmacol Biochem Behav 6(6):615-620). Additionally, it is the intensity with which cocaine produces alterations in dopaminergic circuitry that have enabled this drug to prevail as one of the most addictive substances known to man. This review will summarize findings relevant to cocaine-induced alterations in dopamine-mediated signal transduction. Specifically, it will concentrate on the D1 dopamine receptor and intracellular signaling mediated by this receptor subtype. It will describe cocaine-induced cellular and behavioral alterations relevant to this pathway and how these changes potentially effect gene transcription and protein expression. This article too will review a common behavioral manifestation associated with repeated cocaine exposure, sensitization, and why the D1 dopamine receptor and its associated signaling pathway have been implicated in this phenomenon. Lastly, this article will discuss how targeting the D1 dopamine receptor and its signaling pathway may offer some insight into understanding cocaine addiction, a somewhat elusive brain disease.