Behavioral responses to cocaine and amphetamine administration in mice lacking the dopamine D1 receptor

Contrasting effects of dopamine antagonists and frequency reduction on Fos expression induced by lateral hypothalamic stimulation

To help further identify the reward-relevant regions activated by electrical stimulation of the lateral hypothalamus, Fos expression was quantified in 23 brain regions in naïve, awake rats following non-contingent stimulation with a frequency that supports self-stimulation (100 Hz), a frequency that supports only minimal responding (50 Hz) and a frequency that does not support self-stimulation (25 Hz). Fos expression was also examined in stimulated and unstimulated rats pretreated with SCH 23390 (a dopamine D1 antagonist) or spiperone (a D2-like antagonist), at doses known to greatly inhibit responding for self-stimulation. Lowering the stimulation frequency from 100 to 50 Hz reduced Fos labelling in all areas, except for a few cells immediately surrounding the electrode tip. No differences were observed between unstimulated rats and those receiving 25 Hz stimulation. This suggests that a critical threshold of stimulation is required before other reward-relevant regions in the midbrain and forebrain are recruited with higher frequency stimulation. Pretreatment with SCH 23390 (0.1 mg/kg) inhibited stimulation-induced Fos expression in some key dopamine terminal areas, such as the nucleus accumbens (core and shell) and medial caudate-putamen, but not in directly driven neurons near the stimulation site. In contrast, spiperone (0.1 mg/kg) did not affect the pattern of stimulation-induced Fos expression, but induced immunolabelling in the dorsolateral caudate-putamen, an area associated with the extrapyramidal side-effects of antipsychotic drugs. These results reveal the utility of Fos immunohistochemistry to show how different treatments that alter the rewarding impact of electrical brain stimulation achieve their effects at the neural level.

Role of dopamine D2-like receptors in cocaine self-administration: studies with D2 receptor mutant mice and novel D2 receptor antagonists

Dopamine receptor subtypes have been classified generally as D1-like (e.g., D1, D5) or D2-like (D2, D3, D4), and converging evidence suggests that D2-like receptors may be especially important in mediating the abuse-related effects of cocaine. However, it has been difficult to differentiate the roles of the D2-like receptor subtypes in the behavioral effects of cocaine because of the relatively low selectivity of drugs for D2, D3, and D4 receptors in vivo. The goal of the present series of studies was to investigate the contributions of D2-like receptor subtypes in the reinforcing effects of cocaine using new genetic and pharmacological tools. First, we evaluated cocaine self-administration behavior, and related effects of nonselective D2-like drugs, in mutant mice that lack the D2 receptor but express D3 and D4 receptors. When high doses of cocaine on the descending limb of the cocaine dose-effect function were available, D2 mutant mice self-administered at higher rates than their heterozygous or wild-type littermates, but the ascending limb of the cocaine dose-effect function did not differ between genotypes. Elevated rates of drug intake were not attributable to nonspecific increases in response rate, because response rates maintained by presentation of a range of food concentrations were significantly lower in D2 mutant mice than in wild-type mice. In wild-type mice, pretreatment with the D2-like antagonist eticlopride increased rates of self-administration of high doses of cocaine, and the D2-like agonist quinelorane served as a positive reinforcer when substituted for cocaine. However, these effects of eticlopride and quinelorane were not observed in mice that lacked the D2 receptor. Next, we compared the effects of novel antagonists selective for different D2 receptor subtypes on cocaine self-administration behavior in outbred rats. In rats, a D2 selective antagonist increased rates of self-administration of high doses of cocaine and also combinations of cocaine and the D2-like agonist quinelorane, whereas D3/D4 antagonists were ineffective. Collectively, these findings suggest that the D2 receptor is not necessary for cocaine self-administration, but this receptor subtype is involved in mechanisms that limit rates of high-dose cocaine self-administration. Our results also suggest that D3 and D4 receptors do not play major roles in the modulation of cocaine self-administration by D2-like drugs.

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.

D3 dopamine receptor, behavioral sensitization, and psychosis

Behavioral sensitization is a progressive, enduring enhancement of behaviors that develops following repeated stimulant administration. It is mediated in part by dopaminergic pathways that also modulate a number of psychiatric conditions including the development of psychosis. We propose that down-regulation of D3 dopamine receptor function in critical brain regions contributes to sensitization. Rodent locomotion, a sensitizable behavior, is regulated by the opposing influence of dopamine receptor subtypes, with D3 stimulation opposing concurrent D1 and D2 receptor activation. The D3 dopamine receptor has a 70-fold greater affinity for dopamine than D1 or D2 dopamine receptors. This imbalance in ligand affinity dictates greater occupancy for D3 than D1 or D2 receptors at typical dopamine concentrations following stimulant drug administration, resulting in differences in the relative tolerance at D3 vs D1 and D2 receptors. Sensitization may therefore result in part from accommodation of the inhibitory D3 receptor 'brake' on D1/D2 mediated behaviors, leading to a progressive locomotion increase following repeated stimulant exposure. The requirement for differential tolerance at D3 vs D1 and D2 receptors may explain the observed development of sensitization following application of cocaine, but not amphetamine, directly into nucleus accumbens. If correct, the 'D3 Dopamine Receptor Hypothesis' suggests D3 antagonists could prevent sensitization, and may interrupt the development of psychosis when administered during the prodromal phase of psychotic illness. Additional study is needed to clarify the role of the D3 dopamine receptor in sensitization and psychosis.

The role of the brain reward system in depression

The goal of this review is to familiarize the reader about the potential involvement of the brain reward system (BRS) in symptoms of Major Depressive Disorder (MDD). The authors introduce a novel approach to study the pathophysiology of MDD that includes pharmacological probing of BRS pathways (e.g. d-amphetamine, hydromorphone) together with an elicited and measurable behavioral component (e.g. pleasant effects, increased energy, altered cognition). To this date, the major focus of MDD pathophysiology studies has been to characterize biological differences between healthy subjects and depressed patients such as alteration in the monoaminergic and endocrine systems. The relative importance of the various biological changes has not been elucidated, that is, linking these with specific behavioral manifestations in MDD have rarely been attempted. One core symptom of MDD is a decreased experience of pleasure or interest in previously enjoyed activities (i.e. anhedonia) such as work or hobbies, and is accompanied by decreased motivation or drive. The BRS consists of the neural pathways involved in eliciting rewarding experiences in animals and humans. The hypothesis is that altered BRS function may be an underlying brain mechanism of the loss of pleasure/interest experienced in MDD, and will be manifested through an altered response to a BRS probe. The authors have examined BRS function in MDD by introducing a pharmacological probe (i.e. d-amphetamine/d-amph). Amphetamine is defined as a probe due to its ability to release dopamine within major components of the BRS (i.e. the mesocorticolimbic dopamine system.) In addition to the objective pharmacological effects (e.g. altered heart rate), BRS probes like d-amph elicit reliable and measurable behavior, that is, the hedonic effects. A review of the neurobiology of MDD, the BRS, the rationale for implicating the BRS in depressive symptoms, and preliminary data, are presented in this article.

The psychopharmacology-molecular biology interface: exploring the behavioural roles of dopamine receptor subtypes using targeted gene deletion ('knockout')

In the absence of selective agonists and antagonists able to discriminate between individual members of the D1-like and D2-like families of dopamine receptor subtypes, functional parcellation has remained problematic. 'Knockout' of these subtypes by targeted gene deletion offers a new approach to evaluating their roles in the regulation of behaviour. Like any new technique, 'knockout' has associated with it a number of methodological limitations that are now being addressed in a systematic manner. Studies on the phenotype of D1(A/1), D(1B/5), D2, D3 and D4 'knockouts' at the level of spontaneous and agonist/antagonist-induced behaviour are reviewed, in terms of methodological issues, neuronal implications and potential clinical relevance. Dopamine receptor subtype 'knockout' is a nascent technology that is now beginning to fulfil its potential. It is being complemented by more systematic phenotypic characterisation at the level of behaviour and additional, molecular biologically-based approaches.

[Advanced findings on the molecular mechanisms for behavioral sensitization to psychostimulants]

Repeated administration of psychostimulants like methamphetamine and cocaine induce behavioral sensitization, which is recognized as an animal model for dependence and psychoses. Many previous studies have proved two major cascades play a crucial roles for molecular mechanisms underling sensitization. The first one is activation of D1 dopamine receptors by robust increase of dopamine release, followed by activation of adenylyl cyclase, increase of cyclic AMP, activation of protein kinase A (PKA) and phosphorylation of proteins by PKA. The second one is activation of NMDA receptor by enhanced release of glutamine, followed by increased intracellular Ca2+ concentration, formation of Ca2+/calmodulin complex, and phosphorylation of several proteins such as calcineurin, CaM-K II and nitric oxide synthase. Recent advanced findings on sensitization mechanisms were reviewed from three different aspects: 1) Studies using knockout mice offered quite amazing findings that D1DA-receptor-lacking mice or dopamine-transporter-lacking mice can develop sensitization and dependence, which were not consistent with the previously established hypotheses based on behavioral pharmacology. In addition, those data showed the important roles of vesicular monoamine transporter 2, 5HT1B receptors and delta FosB. 2) Research on neural-plasticity-related sensitization revealed the involvement of several molecules such as tissue plasminogen activator, arc (activity-regulated, cytoskeleton-associated), synaptophysin and stathmin. Increased expression of these genes may participate in the rearrangement of neural networks with synaptogenesis and expansion of dendrites 3) Trials to discover novel-genes-involved sensitization phenomenon using differential display or subtraction cloning found some candidate genes, mrt-1, NAC-1 and CART. Although in these areas are still in progress, accumulating findings will elucidate the details of the molecular mechanism of behavioral sensitization and dependence.

Dopamine receptors: from structure to behavior

The responses obtained with drugs that act at dopamine receptors depend on the spectrum of receptors stimulated, the pattern of stimulation and the neuronal signal-transduction pathways that are activated. In the absence of drugs that reliably discriminate between the various cloned receptors, elucidating the role of these receptors has largely relied on molecular genetic approaches that include expression of genes for receptors in cell lines and manipulation of this expression in animal models. Connecting molecular events that occur consequent to receptor stimulation with the resulting physiological effects entails bridging a complex network of interactions. This article reviews the current understanding of the molecular, cellular and systemic consequences of activation of the different dopamine receptors.

Probing the role of the dopamine D1 receptor in psychostimulant addiction

Pharmacological studies have shown that blockade of the dopamine D1 receptor attenuates locomotor behaviors and prevents sensitization to psychostimulants. However, due to possible cross-reactivities of the D1 receptor antagonists, the exact role of the D1 receptor in response to psychostimulants is still not definitive. To address this issue, we used D1 receptor mutant mice and tested locomotor responses of the mutant mice and wild-type control mice after cocaine and amphetamine treatments. We found that the D1 receptor mutant mice exhibit significantly reduced locomotor responses to repeated cocaine administration compared to wild-type mice. Moreover, D1 receptor mutant mice were less sensitive than the wild-type mice to acute amphetamine administration over a dose range, although they exhibited apparently similar behavioral responses to those of the wild-type mice after repeated amphetamine administration at the 5 mg/kg dose. These studies suggest that the D1 receptor plays an essential role in mediating cocaine-induced locomotor responses in mice. In addition, the D1 receptor also participates in behavioral responses induced by amphetamine administration. Further molecular studies are in progress to address the intracellular signaling mechanisms in response to D1 receptor activation by psychostimulants.