Lack of abuse potential in a highly selective dopamine D3 agonist, PF-592,379, in drug self-administration and drug discrimination in rats

Dopamine D3 receptor: A neglected participant in Parkinson Disease pathogenesis and treatment?

Parkinson disease (PD) is a neurodegenerative disorder characterized by motor and non-motor symptoms which relentlessly and progressively lead to substantial disability and economic burden. Pathologically, these symptoms follow the loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) associated with abnormal α-synuclein (α-Syn) deposition as cytoplasmic inclusions called Lewy bodies in pigmented brainstem nuclei, and in dystrophic neurons in striatal and cortical regions (Lewy neurites). Pharmacotherapy for PD focuses on improving quality of life and primarily targets dopaminergic pathways. Dopamine acts through two families of receptors, dopamine D1-like and dopamine D2-like; dopamine D3 receptors (D3R) belong to dopamine D2 receptor (D2R) family. Although D3R's precise role in the pathophysiology and treatment of PD has not been determined, we present evidence suggesting an important role for D3R in the early development and occurrence of PD. Agonist activation of D3R increases dopamine concentration, decreases α-Syn accumulation, enhances secretion of brain derived neurotrophic factors (BDNF), ameliorates neuroinflammation, alleviates oxidative stress, promotes neurogenesis in the nigrostriatal pathway, interacts with D1R to reduce PD associated motor symptoms and ameliorates side effects of levodopa (L-DOPA) treatment. Furthermore, D3R mutations can predict PD age of onset and prognosis of PD treatment. The role of D3R in PD merits further research. This review elucidates the potential role of D3R in PD pathogenesis and therapy.

The Significance of Chirality in Drug Design and Synthesis of Bitopic Ligands as D3 Receptor (D3R) Selective Agonists

Because of the large degree of homology among dopamine D₂-like receptors, discovering ligands capable of discriminating between the D₂, D₃, and D₄ receptor subtypes remains a significant challenge. Previous work has exemplified the use of bitopic ligands as a powerful strategy in achieving subtype selectivity for agonists and antagonists alike. Inspired by the potential for chemical modification of the D₃ preferential agonists (+)-PD128,907 (1) and PF592,379 (2), we synthesized bitopic structures to further improve their D₃R selectivity. We found that the (2S,5S) conformation of scaffold 2 resulted in a privileged architecture with increased affinity and selectivity for the D₃R. In addition, a cyclopropyl moiety incorporated into the linker and full resolution of the chiral centers resulted in lead compound 53 and eutomer 53a that demonstrate significantly higher D₃R binding selectivities than the reference compounds. Moreover, the favorable metabolic stability in rat liver microsomes supports future studies in in vivo models of dopamine system dysregulation.

Effects of Acute and Chronic Treatments with Dopamine D2 and D3 Receptor Ligands on Cocaine versus Food Choice in Rats

Dopamine D3 receptor ligands are potential medications for psychostimulant addiction. Medication assessment may benefit from preclinical studies that evaluate chronic medication effects on choice between an abused drug and an alternative, nondrug reinforcer. This study compared acute and chronic effects of dopamine D2- and D3-preferring ligands on choice between intravenous cocaine and palatable food in rats. Under baseline conditions, cocaine maintained dose-dependent increases in cocaine choice and reciprocal decreases in food choice. Acutely, the D2 agonist R-(-)-norpropylapomorphine (NPA) and antagonist L-741,626 [3-[[4-(4-chlorophenyl)-4-hydroxypiperidin-l-yl]methyl-1H-indole] produced leftward and rightward shifts in cocaine dose-effect curves, respectively, whereas the partial agonist terguride had no effect. All three drugs dose-dependently decreased food-maintained responding. Chronically, the effects of R-(-)-norpropylapomorphine and L-741,626 on cocaine self-administration showed marked tolerance, whereas suppression of food-reinforced behavior persisted. Acute effects of the D3 ligands were less systematic and most consistent with nonselective decreases in cocaine- and food-maintained responding. Chronically, the D3 agonist PF-592,379 [5-[(2R,5S)-5-methyl-4-propylmorpholin-2-yl]pyridin-2-amine] increased cocaine choice, whereas an intermediate dose of the D3 antagonist PG01037 [N-[(E)-4-[4-(2,3-dichlorophenyl)piperazin-1-yl]but-2-enyl]-4-pyridin-2-ylbenzamide] produced a therapeutically desirable decrease in cocaine choice early in treatment; however, tolerance to this effect developed, and lower and higher doses were ineffective. D3 ligands failed to significantly modify total cocaine intake but caused persistent decreases in food intake. Thus, D2-and D3-preferring ligands showed distinct profiles, consistent with different pharmacological actions. In addition, these results highlight the role of acute versus chronic treatment as a determinant of test drug effects. With the possible exception of the D3 antagonist PG01037, no ligand was promising in terms of cocaine addiction treatment.

Targeting the dopamine D3 receptor: an overview of drug design strategies

INTRODUCTION

Dopamine is a neurotransmitter widely distributed in both the periphery and the central nervous system (CNS). Its physiological effects are mediated by five closely related G protein-coupled receptors (GPCRs) that are divided into two major subclasses: the D1-like (D1, D5) and the D2-like (D2, D3, D4) receptors. D3 receptors (D3Rs) have the highest density in the limbic areas of the brain, which are associated with cognitive and emotional functions. These receptors are therefore attractive targets for therapeutic management.

AREAS COVERED

This review summarizes the functional and pharmacological characteristics of D3Rs, including the design and clinical relevance of full agonists, partial agonists and antagonists, as well as the capacity of these receptors to form active homodimers, heterodimers or higher order receptor complexes as pharmacological targets in several neurological and neurodegenerative disorders.

EXPERT OPINION

The high sequence homology between D3R and the D2-type challenges the development of D3R-selective compounds. The design of new D3R-preferential ligands with improved physicochemical properties should provide a better pharmacokinetic/bioavailability profile and lesser toxicity than is found with existing D3R ligands. It is also essential to optimize D3R affinity and, especially, D3R vs. D2-type binding and functional selectivity ratios. Developing allosteric and bitopic ligands should help to improve the D3R selectivity of these drugs. As most evidence points to the ability of GPCRs to form homomers and heteromers, the most promising therapeutic strategy in the future is likely to involve the application of heteromer-selective drugs. These selective ligands would display different affinities for a given receptor depending on the receptor partners within the heteromer. Therefore, designing novel compounds that specifically target and modulate D1R-D3R heteromers would be an interesting approach for the treatment of levodopa (L-DOPA)-induced dyskinesias.

Differential Roles for Dopamine D1-Like and D2-Like Receptors in Mediating the Reinforcing Effects of Cocaine: Convergent Evidence from Pharmacological and Genetic Studies

A series of studies by Drs. Barak Caine, James Woods, Gregory Collins, Jonathan Katz and Takato Hiranita demonstrated a novel and unique reinforcing effect using dopamine (DA) D2-like receptor [D2-like R: D₂, D₃, and D₄ receptor subtypes (respectively, D₂R, D₃R, and D₄R)] agonists in rats and genetically modified mice. In order to understand how important their findings are, a comparison was made regarding the reinforcing effects of DA D2-like R full agonists with those of DA uptake inhibitors and of a DA D1-like receptor [D1-like R, D₁ and D₅ receptor subtypes (D₁R and D₅R)] full agonist (±)-SKF 82958.

Effects of dopamine D(2)-like receptor agonists in mice trained to discriminate cocaine from saline: influence of feeding condition

In rats, the discriminative stimulus effects of direct- and indirect-acting dopamine receptor agonists are mediated by multiple dopamine receptor subtypes and the relative contribution of dopamine D2 and D3 receptors to these effects varies as a function of feeding condition. In these studies, free-fed and food-restricted mice were trained to discriminate 10.0mg/kg cocaine using a two-lever discrimination procedure in which responding was maintained by food. Both groups of mice acquired the discrimination; however, free-fed mice responded at lower rates than food-restricted mice. Dopamine D3 receptor agonists, pramipexole and quinpirole, increased cocaine-appropriate responding (>85%) in food-restricted, but not in free-fed mice. The dopamine D2 receptor agonist, sumanirole, and the nonselective dopamine receptor agonist, apomorphine, failed to increase cocaine-appropriate responding in either group. Free-fed mice were more sensitive than food-restricted mice to the rate-decreasing effects of dopamine receptor agonists and these effects could not be overcome by increasing the magnitude of reinforcement. Because feeding condition did not alter quinpirole-induced hypothermia, it is unlikely that differences in the discriminative stimulus or rate-decreasing effects of dopamine D2-like receptor agonists were due to differences in the pharmacokinetic properties of the drugs. Although these results suggest that the discriminative stimulus effects of cocaine are mediated by both dopamine D2 and D3 receptors in food-restricted mice, the increased sensitivity of free-fed mice to the rate-decreasing effects of dopamine D2-like receptor agonists limited conclusions about the impact of feeding conditions on the relative contribution of dopamine D2 and D3 receptors to the discriminative stimulus effects of cocaine.

Dopamine D3 and 5-HT1B receptor dysregulation as a result of psychostimulant intake and forced abstinence: Implications for medications development

Addiction to psychostimulants, including cocaine and amphetamine, is associated with dysregulation of dopamine and serotonin (5-HT) neurotransmitter systems. Neuroadaptations in these systems vary depending on the stage of the drug taking-abstinence-relapse cycle. Consequently, the effects of potential treatments that target these systems may vary depending on whether they are given during abstinence or relapse. In this review, we discuss evidence that dopamine D3 receptors (D3Rs) and 5-HT1B receptors (5-HT1BRs) are dysregulated in response to both chronic psychostimulant use and subsequent abstinence. We then review findings from preclinical self-administration models which support targeting D3Rs and 5-HT1BRs as potential medications for psychostimulant dependence. Potential side effects of the treatments are discussed and attention is given to studies reporting positive treatment outcomes that depend on: 1) whether testing occurs during self-administration versus abstinence, 2) whether escalation of drug self-administration has occurred, 3) whether the treatments are given repeatedly, and 4) whether social factors influence treatment outcomes. We conclude that D3/D2 agonists may decrease psychostimulant intake; however, side effects of D3/D2R full agonists may limit their therapeutic potential, whereas D3/D2R partial agonists have fewer undesirable side effects. D3-selective antagonists may not reduce psychostimulant intake during relapse, but nonetheless, may decrease motivation for seeking psychostimulants with relatively few side-effects. 5-HT1BR agonists provide a striking example of treatment outcomes that are dependent on the stage of the addiction cycle. Specifically, these agonists initially increase cocaine's reinforcing effects during maintenance of self-administration, but after a period of abstinence they reduce psychostimulant seeking and the resumption of self-administration. In conclusion, we suggest that factors contributing to dysregulation of monoamine systems, including drug history, abstinence, and social context, should be considered when evaluating potential treatments to better model treatment effects in humans. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.