Regulation of dopamine D2 receptor affinity by cholecystokinin octapeptide in fibroblast cells cotransfected with human CCKB and D2L receptor cDNAs

Oligomeric Receptor Complexes and Their Allosteric Receptor-Receptor Interactions in the Plasma Membrane Represent a New Biological Principle for Integration of Signals in the CNS

G protein-coupled receptors (GPCRs) not only exist as monomers but also as homomers and heteromers in which allosteric receptor-receptor interactions take place, modulating the functions of the participating GPCR protomers. GPCRs can also form heteroreceptor complexes with ionotropic receptors and receptor tyrosine kinases modulating their function. Furthermore, adaptor proteins interact with receptor protomers and modulate their interactions. The state of the art is that the allosteric receptor-receptor interactions are reciprocal, highly dynamic and substantially alter the signaling, trafficking, recognition and pharmacology of the participating protomers. The pattern of changes appears to be unique for each heteromer and can favor antagonistic or facilitatory interactions or switch the G protein coupling from e.g., Gi/o to Gq or to beta-arrestin signaling. It lends a new dimension to molecular integration in the nervous system. Future direction should be aimed at determining the receptor interface involving building models of selected heterodimers. This will make design of interface-interfering peptides that specifically disrupt the heterodimer possible. This will help to determine the functional role of the allosteric receptor-receptor interactions as well as the integration of signals at the plasma membrane by the heteroreceptor complexes, vs. integration of the intracellular signaling pathways. Integration of signals also at the plasma membrane seems crucial in view of the hypothesis that learning and memory at a molecular level takes place by reorganization of homo and heteroreceptor complexes in the postsynaptic membrane. Homo and heteroreceptor complexes are in balance with each other, and their disbalance is linked to disease. Targeting heteroreceptor complexes represents a novel strategy for the treatment of brain disorders.

Neurobiological background for the development of new drugs in schizophrenia

Psychopharmacology of schizophrenia has remained static for many years because the mechanisms explored have been basically monoaminergics, primarily focused toward the modification of dopaminergic function and, later on, serotonergic. In fact, most of the antipsychotics introduced in clinical practice in the last years have been antagonists or selective agonists of these receptors (D(2)/5-HT(2)). The exploration of other receptor pathways, and in particular those additionally involved in the action of the paradigmatic "atypical" antipsychotic clozapine (ie, cholinergic and noradrenergic), has not been very significant. Besides, research in the antipsychotics field has developed also by exploring pathways that are beyond the spectrum of clozapine. Among the most promising mechanisms are those based on the glutamatergic hypothesis of schizophrenia (agonists at the glycine-binding modulatory site of the N-methyl-D-aspartate receptor, glycine transporter inhibitors, modulators of the AMPA [α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid] receptor and selective agonists of the metabotropic receptor Glu(2)). Other less classic pathways are also under study and have led to some agents that are found in very early stages of development such as those acting on sigma receptors, cholecystokinin antagonists, neurotensin agonists, neurokinin receptor antagonists, GABAergic (+-aminobutyric acid [GABA]) enhancers, and cannabinoid(gamma-aminobutiric) receptor modulators.

Integrin triplets of marine sponges in human D2 receptor heteromers

The evidence for the existence of receptor heteromers opens up a new field for a better understanding of neural transmission. Based on our theory, we have discovered main triplets of amino acid residues in cell-adhesion receptors of marine sponges, which appear also as homologies in several dopamine D2 receptor heteromers of human brain. The obtained results probably mean a general molecular mechanism for receptor-receptor interactions in heteromers originated from the lowest animals (marine sponges).

Extrasynaptic neurotransmission in the modulation of brain function. Focus on the striatal neuronal-glial networks

Extrasynaptic neurotransmission is an important short distance form of volume transmission (VT) and describes the extracellular diffusion of transmitters and modulators after synaptic spillover or extrasynaptic release in the local circuit regions binding to and activating mainly extrasynaptic neuronal and glial receptors in the neuroglial networks of the brain. Receptor-receptor interactions in G protein-coupled receptor (GPCR) heteromers play a major role, on dendritic spines and nerve terminals including glutamate synapses, in the integrative processes of the extrasynaptic signaling. Heteromeric complexes between GPCR and ion-channel receptors play a special role in the integration of the synaptic and extrasynaptic signals. Changes in extracellular concentrations of the classical synaptic neurotransmitters glutamate and GABA found with microdialysis is likely an expression of the activity of the neuron-astrocyte unit of the brain and can be used as an index of VT-mediated actions of these two neurotransmitters in the brain. Thus, the activity of neurons may be functionally linked to the activity of astrocytes, which may release glutamate and GABA to the extracellular space where extrasynaptic glutamate and GABA receptors do exist. Wiring transmission (WT) and VT are fundamental properties of all neurons of the CNS but the balance between WT and VT varies from one nerve cell population to the other. The focus is on the striatal cellular networks, and the WT and VT and their integration via receptor heteromers are described in the GABA projection neurons, the glutamate, dopamine, 5-hydroxytryptamine (5-HT) and histamine striatal afferents, the cholinergic interneurons, and different types of GABA interneurons. In addition, the role in these networks of VT signaling of the energy-dependent modulator adenosine and of endocannabinoids mainly formed in the striatal projection neurons will be underlined to understand the communication in the striatal cellular networks.

Dopamine and renal function and blood pressure regulation

Dopamine is an important regulator of systemic blood pressure via multiple mechanisms. It affects fluid and electrolyte balance by its actions on renal hemodynamics and epithelial ion and water transport and by regulation of hormones and humoral agents. The kidney synthesizes dopamine from circulating or filtered L-DOPA independently from innervation. The major determinants of the renal tubular synthesis/release of dopamine are probably sodium intake and intracellular sodium. Dopamine exerts its actions via two families of cell surface receptors, D1-like receptors comprising D1R and D5R, and D2-like receptors comprising D2R, D3R, and D4R, and by interactions with other G protein-coupled receptors. D1-like receptors are linked to vasodilation, while the effect of D2-like receptors on the vasculature is variable and probably dependent upon the state of nerve activity. Dopamine secreted into the tubular lumen acts mainly via D1-like receptors in an autocrine/paracrine manner to regulate ion transport in the proximal and distal nephron. These effects are mediated mainly by tubular mechanisms and augmented by hemodynamic mechanisms. The natriuretic effect of D1-like receptors is caused by inhibition of ion transport in the apical and basolateral membranes. D2-like receptors participate in the inhibition of ion transport during conditions of euvolemia and moderate volume expansion. Dopamine also controls ion transport and blood pressure by regulating the production of reactive oxygen species and the inflammatory response. Essential hypertension is associated with abnormalities in dopamine production, receptor number, and/or posttranslational modification.

Molecular mechanisms and therapeutical implications of intramembrane receptor/receptor interactions among heptahelical receptors with examples from the striatopallidal GABA neurons

The molecular basis for the known intramembrane receptor/receptor interactions among G protein-coupled receptors was postulated to be heteromerization based on receptor subtype-specific interactions between different types of receptor homomers. The discovery of GABAB heterodimers started this field rapidly followed by the discovery of heteromerization among isoreceptors of several G protein-coupled receptors such as delta/kappa opioid receptors. Heteromerization was also discovered among distinct types of G protein-coupled receptors with the initial demonstration of somatostatin SSTR5/dopamine D2 and adenosine A1/dopamine D1 heteromeric receptor complexes. The functional meaning of these heteromeric complexes is to achieve direct or indirect (via adapter proteins) intramembrane receptor/receptor interactions in the complex. G protein-coupled receptors also form heteromeric complexes involving direct interactions with ion channel receptors, the best example being the GABAA/dopamine D5 receptor heteromerization, as well as with receptor tyrosine kinases and with receptor activity modulating proteins. As an example, adenosine, dopamine, and glutamate metabotropic receptor/receptor interactions in the striatopallidal GABA neurons are discussed as well as their relevance for Parkinson's disease, schizophrenia, and drug dependence. The heterodimer is only one type of heteromeric complex, and the evidence is equally compatible with the existence of higher order heteromeric complexes, where also adapter proteins such as homer proteins and scaffolding proteins can exist. These complexes may assist in the process of linking G protein-coupled receptors and ion channel receptors together in a receptor mosaic that may have special integrative value and may constitute the molecular basis for some forms of learning and memory.

Evidence of dopamine D1 receptor mRNA and binding sites in cultured human amniotic epithelial cells

In this study, we employed RT-PCR and radioligand binding studies to evaluate the gene expression and binding characteristics, respectively, of dopamine D(1) receptors in human amniotic epithelial cells (HAEC). The results showed that HAEC natively expressed D(1) receptor mRNA, as measured by RT-PCR, which was identical to that of human brain. Saturation binding studies using [(3)H]SCH 23390 demonstrated the presence of a high affinity D(1) site in HAEC with K(D) and B(max) values of 2.01+/-0.25 nM and 32.5+/-3.7 fmol/mg protein, respectively. Competition studies showed that selective D(1) antagonists were potent displacers of [(3)H]SCH 23390 binding with a potency order consistent with D(1) receptor characteristics. The current results present compelling evidence that HAEC natively express D(1) receptor mRNA and binding sites. The results also establish a primate cell model that can possibly be used for studying D(1) receptor signal transduction and molecular mechanisms and exploring newly developed drugs acting at these receptors.

Characterization of dopamine D2 receptor gene expression and binding sites in human placenta amniotic epithelial cells

This study was to investigate the presence of dopamine (DA) D(2)receptors mRNA and binding sites in human amniotic epithelial cells (HAEC). RT-PCR revealed that HAEC express DA D(2)receptor mRNA that is having 100 per cent homology with human DA D(2)receptors. Radioligand saturation binding studies showed a [3H]YM-09151-2 high affinity binding site with a K(D)and B(max)values of 0.53+/-0.09 nM and 119.6+/-8.5 fmol/mg protein, respectively. Competition experiments demonstrated that selective D(2)antagonists such as spiroperidol, domperidone and eticlopride potently competed with [3H]YM-09151-2 binding, whereas selective D(1)antagonists like SCH 23390 displayed weaker competition for the binding sites. The rank order of potency of these compounds in competing with [3H]YM-09151-2 for the binding sites was consistent with the pharmacology of the DA D(2)receptors. All competition curves were better fitted to a one-site model with a Hill coefficient around unity, indicating that [3H]YM-09151-2 is labelling a single population of receptors. These results provide evidence that HAEC natively express DA D(2)receptor mRNA and binding sites. Although the physiological function of D2 receptors in HAEC is currently unclear, the present results suggest that these cells could represent a source of human DA D(2)receptors without transformation or cloning procedures.

Substance P and cholecystokinin regulate neurochemical responses to cocaine and methamphetamine in the striatum

The mechanism of action of drugs of abuse like cocaine and amphetamines has been studied extensively in the dopamine terminal field areas of the caudate-putamen (CPu) and the nucleus accumbens (NAc) of the rodent brain. These brain regions contain several neuropeptides that must play important roles in the normal physiological functions of these brain regions. The study of neuropeptide physiology in the context of the neurobiological responses to drugs of abuse may shed some light on the intrinsic mechanism of action of neuropeptides of the CPu and the NAc. The neuropeptides substance P (SP) and cholecystokinin (CCK) are present in the striatum where they could play an important role regulating the effects of psychostimulants like cocaine and amphetamines (methamphetamine [METH] is a long acting derivative of d-amphetamine). These highly addictive agents induce the release of dopamine (DA) (and other catecholamines) from dopaminergic terminals of the striatum. The excessive release of DA in the striatum and the NAc has been implicated in the habit-forming properties of these drugs. In order to study the contribution of SP and CCK in the striatum during psychostimulant treatment, we employed selective non-peptide neurokinin-1 (NK-1) and cholecystokinin-2 (CCK-2) receptor antagonists that readily cross the blood brain barrier. We infused the neurokinin-1 receptor (NK-1R) antagonist, L-733,060, into the striatum of freely moving rats via a microdialysis probe in order to assess the effects of SP on cocaine-induced DA overflow in the striatum. Infusion of the NK-1R antagonist prior to a systemic injection of cocaine (10 mg/kg i.p.) significantly attenuated DA overflow in the striatum. Conversely, infusion of a CCK-2 receptor (CCK-2R) antagonist, L-369,293, through the microdialysis probe evoked DA overflow in the striatum in the absence of cocaine and potentiated DA overflow after a single injection of cocaine (10 mg/kg i.p.). Exposure to METH (10 mg/kg 4x at two-hour intervals) produced deficits of dopamine transporters (DAT) in mice striatum that are detectable three days after the treatment and are long lasting. Pre-treatment (i.p. injections) with the NK-1R antagonist, WIN-51,708 30 minutes before the 1st and 4th injections of METH prevented the loss of DAT in the striatum. Moreover, pre-treatment with the NK-1R antagonist prevents METH-induced cell death. Taken together, these results demonstrate that the NK-1R and the CCK-2R are important modulators of the actions of the psychostimulants cocaine and METH. Neuropeptide receptors represent an important control point mediating the effects of the neurotransmitter DA in the striatum of the rodent brain.

Oestrogen modulates cholecystokinin: opioid interactions in the nervous system

Responses of the nervous system to introceptive and extroceptive inputs depend upon the state of the brain. Oestrogen has the ability to modulate brain state and dramatically alter interactions among neural circuits to influence an organism's responses to given stimuli. Cholecystokinin (CCK) and endogenous opioid peptides (EOP) have a wide and parallel distribution in the nervous system. Their reciprocal interactions regulate a diverse physiology including reproduction, cortical function and nociception. The actions of CCK and EOP are diametrically opposed, in many regions. For example, when opioids inhibit reproductive behaviour or nociception, CCK facilitates. Because oestrogen is a powerful regulator of the expression of CCK and EOP, we examined whether oestrogen-state also modulated the interactions of these neuropeptides. In this paper we present new data and review previous work that demonstrates oestrogen modulation of functional CCK-opioid interactions that regulate reproductive behaviour, cortical function and nociception.

Reversible and irreversible components of [(3)H]-N-propylnorapomorphine interaction with rat striatal membranes

The kinetics of L-(-)-[N-propyl-(3)H(N)]-norapomorphine ([(3)H]NPA) interactions with rat striatal membranes were studied. The analysis revealed that in addition to specific dopaminergic binding a substantial part of the radioligand was bound irreversibly to heterogeneous populations of non-specific binding sites of these membranes. The specific binding of [(3)H]NPA with dopamine receptors, determined from the differences of kinetic curves of total and non-specific binding, was fast, reversible, and revealed high affinity. The irreversible component was heterogeneous and seems to be related to oxidative degradation of the radioligand, as the rate of this process was substantially reduced by antioxidants like ascorbic acid and dithiothreitol.

Cholecystokinin/dopamine/GABA interactions in the nucleus accumbens: biochemical and functional correlates

The present article reviews our recent biochemical and microdialysis studies showing the evidence for an antagonistic CCK(B)/D(2) receptor interaction in the regulation of dopaminergic transmission in the nucleus accumbens and GABAergic transmission in the ipsilateral ventral pallidum. Since the nucleus accumbens plays a crucial role in regulating the output from the limbic system and consequently motivation, it may be speculated that a dysregulation of this receptor interaction may have consequences in a wide range of central nervous system disorders.

Detection of dopamine D2 receptor mRNA and binding sites in monkey amniotic epithelial cells

Previous results from our laboratory showed that monkey amniotic epithelial cells (MAEC) possess the catecholamine synthesizing enzymes and have the capacity to synthesize and release CA. Recently, we also reported that these cells express dopamine D1 receptor mRNA and binding sites. This study was designed to investigate the presence of dopamine D2 receptors in MAEC. Using RT-PCR, we found that MAEC express dopamine D2 receptor mRNA that is having 98% homology with human dopamine D2 receptors. Radioligand saturation binding studies showed a 3H-YM-09151-2 high-affinity binding site with a K(D) of 0.293+/-0.06 nM and Bmax of 180.69+/-11.61 fmol/mg protein. Competition experiments with a variety of displacing drugs demonstrated that D2 antagonists potently compete with 3H-YM-09151-2 binding, whereas D1 antagonists displayed a weaker competition for the binding sites. The rank order of potency of these compounds in competing with 3H-YM-09151-2 for binding sites was consistent with the pharmacology of the dopamine D2 receptors. All competition curves were better fitted to a one-site model with a Hill coefficient around unity, indicating that 3H-YM-09151-2 is labeling a single population of receptors. These results provide, for the first time, a compelling evidence that MAEC natively express dopamine D2 receptor mRNA and binding sites, and they suggest that monkey amniotic epithelial cells (MAEC) could represent a source of primate dopamine receptors without the need for transformation or cloning procedures using nonprimate cells, as generally happens.

Evidence for the presence of dopamine D1 receptor mRNA and binding sites in monkey amniotic epithelial cells

In this study we examined the presence of dopamine D1 receptors in monkey amniotic epithelial cells (MAEC) using RT-PCR and radioligand binding experiments. We found that MAEC express D1 receptor mRNA that is having 99% homology with human dopamine D1 receptors. Saturation binding studies using [3H]SCH-23390 showed a high affinity D1 site with K(D) and Bmax values of 0.82 +/- 0.12 nM and 20.77 +/- 4.22 fmol/mg protein, respectively. Competition experiments showed that selective D1, but not D2, antagonists are potent displacers of [3H]SCH 23390 binding with a rank order of potency that is consistent with the pharmacology of the dopaminergic D1 site. These data provide, for the first time, compelling evidence that MAEC natively express D1 mRNA and binding sites and suggest that it may be a potential primate cell model to study D1 receptors and to explore new selective drugs active at these receptors.