A synergistic approach towards understanding the functional significance of dopamine receptor interactions

Dopaminergic Modulation of Forced Running Performance in Adolescent Rats: Role of Striatal D1 and Extra-striatal D2 Dopamine Receptors

Improving exercise capacity during adolescence impacts positively on cognitive and motor functions. However, the neural mechanisms contributing to enhance physical performance during this sensitive period remain poorly understood. Such knowledge could help to optimize exercise programs and promote a healthy physical and cognitive development in youth athletes. The central dopamine system is of great interest because of its role in regulating motor behavior through the activation of D1 and D2 receptors. Thus, the aim of the present study is to determine whether D1 or D2 receptor signaling contributes to modulate the exercise capacity during adolescence and if this modulation takes place through the striatum. To test this, we used a rodent model of forced running wheel that we implemented recently to assess the exercise capacity. Briefly, rats were exposed to an 8-day period of habituation in the running wheel before assessing their locomotor performance in response to an incremental exercise test, in which the speed was gradually increased until exhaustion. We found that systemic administration of D1-like (SCH23390) and/or D2-like (raclopride) receptor antagonists prior to the incremental test reduced the duration of forced running in a dose-dependent manner. Similarly, locomotor activity in the open field was decreased by the dopamine antagonists. Interestingly, this was not the case following intrastriatal infusion of an effective dose of SCH23390, which decreased motor performance during the incremental test without disrupting the behavioral response in the open field. Surprisingly, intrastriatal delivery of raclopride failed to impact the duration of forced running. Altogether, these results indicate that the level of locomotor response to incremental loads of forced running in adolescent rats is dopamine dependent and mechanistically linked to the activation of striatal D1 and extra-striatal D2 receptors.

The Deep Roots of Addiction: A Comparative Perspective

Addiction is a debilitating condition that extracts enormous social and economic tolls. Despite several decades of research, our knowledge of its etiology, preventive measures, and treatments is limited. A relatively recent research field with the potential to provide a more holistic understanding, and subsequently treatments, takes a phylogenetic view of addiction. This perspective is based on deep homologies at the genetic, proteomic, and behavioral levels, which are shared across all metazoan life; particularly those organisms faced with plant secondary metabolites as defensive compounds against insect herbivory. These addictive alkaloids, such as nicotine, cocaine, or cathinone, are commonly referred to as "human drugs of abuse" even though humans had little to no role in the co-evolutionary processes that determined their initial emergence or continued selection. This commentary discusses the overwhelming homologies of addictive alkaloid effects on neural systems across a wide range of taxa, as we aim to develop a broader comparative view of the "addicted brain." Taking nicotine as an example, homologous physiological responses to this compound identify common underlying cellular and molecular mechanisms that advocate for the adoption of a phylogenetic view of addiction.

Haloperidol Interactions with the dop-3 Receptor in Caenorhabditis elegans

Haloperidol is a typical antipsychotic drug commonly used to treat a broad range of psychiatric disorders related to dysregulations in the neurotransmitter dopamine (DA). DA modulates important physiologic functions and perturbations in Caenorhabditis elegans (C. elegans) and, its signaling have been associated with alterations in behavioral, molecular, and morphologic properties in C. elegans. Here, we evaluated the possible involvement of dopaminergic receptors in the onset of these alterations followed by haloperidol exposure. Haloperidol increased lifespan and decreased locomotor behavior (basal slowing response, BSR, and locomotion speed via forward speed) of the worms. Moreover, locomotion speed recovered to basal conditions upon haloperidol withdrawal. Haloperidol also decreased DA levels, but it did not alter neither dop-1, dop-2, and dop-3 gene expression, nor CEP dopaminergic neurons' morphology. These effects are likely due to haloperidol's antagonism of the D2-type DA receptor, dop-3. Furthermore, this antagonism appears to affect mechanistic pathways involved in the modulation and signaling of neurotransmitters such as octopamine, acetylcholine, and GABA, which may underlie at least in part haloperidol's effects. These pathways are conserved in vertebrates and have been implicated in a range of disorders. Our novel findings demonstrate that the dop-3 receptor plays an important role in the effects of haloperidol.

Crayfish Learning: Addiction and the Ganglionic Brain

Recognizing addiction as a phenomenon with deep evolutionary roots grants valuable new perspectives into understanding its behavioral features, as well as its underlying neural mechanisms and genetic architecture. Although now generally misbranded as "human drugs of abuse," addictive plant alkaloids originally arose as potent chemical defenses against insect herbivory. The products of this evolutionary arms race, compounds such as nicotine, cathinone, or morphine, target essential biological mechanisms for motivation and learning and act as weaponized disruptors. Human vulnerabilities to these addictive drugs may thus represent little more than collateral damage arising from deep homology, i.e., shared biological implementation of behavioral functions with taxa that trace back to the early divergence of bilateral metazoans. Consistent with such a view, invertebrate preparations exhibit a rich spectrum of behavioral and neural consequences in response to drug exposure. Although there is certainly evidence for addiction-like phenomena in many invertebrate lineages, the present review focuses attention primarily on our recent work in crayfish. Using this decapod crustacean model, we have characterized a range of amphetamines, cathinones, and opioids for evidence of unconditioned intoxication, sympathomimetic properties, psychostimulant sensitization, conditioned cue learning, and operant self-administration. Overall, our findings on drug-sensitive reward in crayfish bear striking similarities to equivalent phenomena illustrated in mammals. Experimentally tractable invertebrate models may thus provide fundamental insights into the homo- and paralogous mechanisms mediating responses to addictive drugs, while illuminating the limits of such contrasts.

Pramipexole protects dopaminergic neurons through paraplegin against 6-hydroxydopamine

The neurotransmitter dopamine (DA) regulates various physiological and psychological functions, such as movement, motivation, behavior, and learning. DA exerts its function through DA receptors and a series of studies have reported the role of DAergic receptors in preventing DAergic neuronal degeneration. Here, we studied the DA receptor-mediated neuroprotective effect of the D2-like receptor agonists against 6-hydroxydopamine (6-OHDA)-induced DAergic neurodegeneration. D2-like receptor agonists were administered in the substantia nigra in vivo and to primary cultured neurons. Treatment of 6-OHDA decreased tyrosine hydroxylase (TH) and paraplegin (mitochondrial regulation protein) immunoreactivity, whereas pretreatment with quinpirole (a full D2-like receptor agonist) preserved TH and paraplegin reactivity. This led us to test which DA receptors were necessary for the neuroprotective effect and whether paraplegin can be regulated by D2 or D3 receptor agonists. Pretreatment with the D2 receptor selective agonist, sumanirole, did not preserve TH and paraplegin reactivity from 6-OHDA. However, the D3 receptor agonist, pramipexole, protected TH reactivity and restored paraplegin expression to the control level in the presence of 6-OHDA. Interestingly, pretreatment with the D3 receptor antagonist GR103691 reduced TH and paraplegin expression levels. These results suggest that the D3 receptor agonist may protect DA neurons from the effect of 6-OHDA through the modulation of the mitochondrial regulation protein paraplegin.