Five or more dopamine receptor genes are expressed in brain. However, the pharmacological similarities of the encoded D1-D5 receptors have hindered studies of the localization and functions of the subtypes. To better understand the roles of the individual receptors, antibodies were raised against recombinant D1 and D2 proteins and were shown to bind to the receptor subtypes specifically in Western blot and immunoprecipitation studies. Each antibody reacted selectively with the respective receptor protein expressed both in cells transfected with the cDNAs and in brain. By immunocytochemistry, D1 and D2 had similar regional distributions in rat, monkey, and human brain, with the most intense staining in striatum, olfactory bulb, and substantia nigra. Within each region, however, the precise distributions of each subtype were distinct and often complementary. D1 and D2 were differentially enriched in striatal patch and matrix compartments, in selective layers of the olfactory bulb, and in either substantia nigra pars compacta or reticulata. Electron microscopy demonstrated that D1 and D2 also had highly selective subcellular distributions. In the rat neostriatum, the majority of D1 and D2 immunoreactivity was localized in postsynaptic sites in subsets of spiny dendrites and spine heads in rat neostriatum. Presynaptic D1 and D2 receptors were also observed, indicating both subtypes may regulate neurotransmitter release. D1 was also present in axon terminals in the substantia nigra. These results provide a morphological substrate for understanding the pre- and postsynaptic functions of the genetically defined D1 and D2 receptors in discrete neuronal circuits in mammalian brain.

Atypical antipsychotics (AAPs) are commonly prescribed medications to treat schizophrenia, bipolar disorders and other psychotic disorders. However, they might cause metabolic syndrome (MetS) in terms of weight gain, dyslipidemia, type 2 diabetes (T2D), and high blood pressure, which are responsible for reduced life expectancy and poor adherence. Importantly, there is clear evidence that early metabolic disturbances can precede weight gain, even if the latter still remains the hallmark of AAPs use. In fact, AAPs interfere profoundly with glucose and lipid homeostasis acting mostly on hypothalamus, liver, pancreatic β-cells, adipose tissue, and skeletal muscle. Their actions on hypothalamic centers via dopamine, serotonin, acetylcholine, and histamine receptors affect neuropeptides and 5′AMP-activated protein kinase (AMPK) activity, thus producing a supraphysiological sympathetic outflow augmenting levels of glucagon and hepatic glucose production. In addition, altered insulin secretion, dyslipidemia, fat deposition in the liver and adipose tissues, and insulin resistance become aggravating factors for MetS. In clinical practice, among AAPs, olanzapine and clozapine are associated with the highest risk of MetS, whereas quetiapine, risperidone, asenapine and amisulpride cause moderate alterations. The new AAPs such as ziprasidone, lurasidone and the partial agonist aripiprazole seem more tolerable on the metabolic profile. However, these aspects must be considered together with the differences among AAPs in terms of their efficacy, where clozapine still remains the most effective. Intriguingly, there seems to be a correlation between AAP’s higher clinical efficacy and increase risk of metabolic alterations. Finally, a multidisciplinary approach combining psychoeducation and therapeutic drug monitoring (TDM) is proposed as a first-line strategy to avoid the MetS. In addition, pharmacological treatments are discussed as well.

We have tested the hypothesis that guanine-nucleotide-binding-protein-coupled receptors may be able to interact with each other at a molecular level. To address this question, we have initially created two chimeric receptors, alpha 2/m3 and m3/alpha 2, in which the C-terminal receptor portions (containing transmembrane domains VI and VII) were exchanged between the alpha 2C-adrenergic and the m3 muscarinic receptor. Transfection of COS-7 cells with either of the two chimeric constructs alone did not result in any detectable binding activity for the muscarinic ligand N-[3H]methylscopolamine or the adrenergic ligand [3H]rauwolscine. However, cotransfection with alpha 2/m3 and m3/alpha 2 resulted in the appearance of specific binding sites (30-35 fmol/mg of membrane protein) for both radioligands. These sites displayed ligand binding properties similar to those of the two wild-type receptors. Furthermore, COS-7 cells cotransfected with alpha 2/m3 and m3/alpha 2 were able to mediate a pronounced stimulation of phosphatidylinositol hydrolysis upon stimulation with the muscarinic agonist carbachol (Emax approximately 40-50% of wild-type m3). A mutant m3 receptor (containing 16 amino acids of m2 receptor sequence at the N terminus of the third cytoplasmic loop) that was capable of binding muscarinic ligands but was virtually unable to stimulate phosphatidylinositol hydrolysis was also used in various cotransfection experiments. Coexpression of this chimeric receptor with other functionally impaired mutant muscarinic receptors (e.g., with an m3 receptor containing a Pro-->Ala point mutation in transmembrane region VII) resulted in a considerable stimulation of phosphatidylinositol breakdown after carbachol treatment (Emax approximately 40-50% of wild-type m3). Thus, these data suggest that guanine-nucleotide-binding-protein-coupled receptors can interact with each other at a molecular level. One may speculate that the formation of receptor dimers involving the intermolecular exchange of N- and C-terminal receptor domains (containing transmembrane domains I-V and VI and VII, respectively) may underlie this phenomenon.

Evidence for heterodimerization has recently been provided for dopamine D₁ and adenosine A₁ receptors as well as for dopamine D₂ and somatostatin SSTR₅ receptors. In this paper, we have studied the possibility that D₂ and D₃receptors interact functionally by forming receptor heterodimers. Initially, we split the two receptors at the level of the third cytoplasmic loop into two fragments. The first, containing transmembrane domains (TM) I to V and the N-terminal part of the third cytoplasmic loop, was named D_2trunk or D_3trunk, and the second, containing the C-terminal part of the third cytoplasmic loop, TMVI and TMVII, and the C-terminal tail, was named D_2tail or D_3tail. Then we defined the pharmacological profiles of the homologous (D_2trunk/D_2tail and D_3trunk/D_3tail) as well as of the heterologous (D_2trunk/D_3tail and D_3trunk/D_2tail) cotransfected receptor fragments. The pharmacological profile of the cross-cotransfected fragments was different from that of the native D₂ or D₃ receptors. In most cases, the D_3trunk/D_2tail was the one with the highest affinity for most agonists and antagonists. Moreover, we observed that all of these receptor fragments reduced the expression of the wild type dopamine D₂ and D₃ receptors, suggesting that D₂ and D₃ receptors can form complexes with these fragments and that these complexes bind [³H]nemonapride less efficiently or are not correctly targeted to the membrane. In a second set of experiments, we tested the ability of the split and the wild type receptors to inhibit adenylyl cyclase (AC) types V and VI. All of the native and split receptors inhibited AC-V and AC-VI, with the exception of D₃, which was unable to inhibit AC-VI. We therefore studied the ability of D₂ and D₃ to interact functionally with one another to inhibit AC-VI. We found that with D₂ alone, R-(+)-7-hydroxydypropylaminotetralin hydrobromide inhibited AC-VI with an IC₅₀ of 2.05 ± 0.15 nm, while in the presence of D₂ and D₃ it inhibited AC-VI with an IC₅₀ of 0.083 ± 0.011 nm. Similar results were obtained with a chimeric cyclase made from AC-V and AC-VI. Coimmunoprecipitation experiments indicate that D₂ and D₃ receptors are capable of physical interaction.