Intramembrane receptor-receptor interactions: integration of signal transduction pathways in the nervous system

Unraveling the Interplay of Dopamine, Carbon Monoxide, and Heme Oxygenase in Neuromodulation and Cognition

The dopaminergic system plays important roles in neuromodulation, including prominent roles in complex neurological functions such as cognition, reward, motivation, and memory. Understandably, the highly complex nature of such physiological functions means that their regulation is intertwined with other signaling pathways, as has been demonstrated by numerous studies. Contrary to its public perception of being poisonous at all concentrations, carbon monoxide (CO) is produced endogenously from heme degradation by heme oxygenase (HO) as part of the physiological process of red blood cell turnover. Physiological concentrations of CO can reach high micromolar ranges in the hemoglobin bound form. Low-dose CO has shown therapeutic effects in numerous animal models, including traumatic brain injury via engaging various hemoprotein targets. As such, the HO-CO axis has been shown to offer beneficial effects in organ protection, anti-inflammation, and neuroprotection, among many others. Further, a large number of publications have shown the interactions among CO, HO, and the dopaminergic system. In this review, we critically examine such experimental evidence in a holistic fashion and in the context of a possible dopamine-HO-CO signaling axis. We hope that this Perspective will stimulate additional investigations into the molecular connectivity related to this possible axis and open doors to the development of novel therapeutics that impact the dopaminergic system.

Impact of an Adenosine A2A Receptor Agonist and Antagonist on Binding of the Dopamine D2 Receptor Ligand [11C]raclopride in the Rodent Striatum

Adenosine A2A and dopamine D2 receptors in the basal ganglia form heterotetrameric structures that are involved in the regulation of motor activity and neuropsychiatric functions. The present study examines the A2A receptor-mediated modulation of D2 receptor binding in vivo using positron emission tomography (PET) with the D2 antagonist tracer [11C]raclopride. Healthy male Wistar rats (n = 8) were scanned (60 min dynamic scan) with [11C]raclopride at baseline and 7 days later following an acute administration of the A2A agonist CGS21680 (1 mg/kg), using a MicroPET Focus-220 camera. Nondisplaceable binding potential (BPND) values were calculated using a simplified reference tissue model (SRTM), with cerebellum as the reference tissue. SRTM analysis did not show any significant changes in [11C]raclopride BPND (p = 0.102) in striatum after CGS21680 administration compared to the baseline. As CGS21680 strongly affects hemodynamics, we also used arterial blood sampling and a metabolite-corrected plasma input function for compartment modeling using the reversible two-tissue compartment model (2TCM) to obtain the BPND from the k3/k4 ratio and from the striatum/cerebellum volume of distribution ratio (DVR) in a second group of animals. These rats underwent dynamic [11C]raclopride scans after pretreatment with a vehicle (n = 5), a single dose of CGS21680 (1 mg/kg, n = 5), or a single dose of the A2A antagonist KW6002 (1 mg/kg, n = 5). The parent fraction in plasma was significantly higher in the CGS21680-treated group (p = 0.0001) compared to the vehicle-treated group. GCS21680 administration significantly reduced the striatal k3/k4 ratio (p < 0.01), but k3 and k4 estimates may be less reliable. The BPND (DVR-1) decreased from 1.963 ± 0.27 in the vehicle-treated group to 1.53 ± 0.55 (p = 0.080) or 1.961 ± 0.11 (p = 0.993) after the administration of CGS21680 or KW6002, respectively. Our study suggests that the A2A agonist CGS21680, but not the antagonist KW6002, may reduce the D2 receptor availability in the striatum.

Allosteric Interactions between Adenosine A2A and Dopamine D2 Receptors in Heteromeric Complexes: Biochemical and Pharmacological Characteristics, and Opportunities for PET Imaging

Adenosine and dopamine interact antagonistically in living mammals. These interactions are mediated via adenosine A_2A and dopamine D₂ receptors (R). Stimulation of A_2AR inhibits and blockade of A_2AR enhances D₂R-mediated locomotor activation and goal-directed behavior in rodents. In striatal membrane preparations, adenosine decreases both the affinity and the signal transduction of D₂R via its interaction with A_2AR. Reciprocal A_2AR/D₂R interactions occur mainly in striatopallidal GABAergic medium spiny neurons (MSNs) of the indirect pathway that are involved in motor control, and in striatal astrocytes. In the nucleus accumbens, they also take place in MSNs involved in reward-related behavior. A_2AR and D₂R co-aggregate, co-internalize, and co-desensitize. They are at very close distance in biomembranes and form heteromers. Antagonistic interactions between adenosine and dopamine are (at least partially) caused by allosteric receptor–receptor interactions within A_2AR/D₂R heteromeric complexes. Such interactions may be exploited in novel strategies for the treatment of Parkinson’s disease, schizophrenia, substance abuse, and perhaps also attention deficit-hyperactivity disorder. Little is known about shifting A_2AR/D₂R heteromer/homodimer equilibria in the brain. Positron emission tomography with suitable ligands may provide in vivo information about receptor crosstalk in the living organism. Some experimental approaches, and strategies for the design of novel imaging agents (e.g., heterobivalent ligands) are proposed in this review.

Receptor–receptor interactions as studied with microdialysis. Focus on NTR/D2 interactions in the basal ganglia

Using mono and dualprobe(s) microdialysis in the basal ganglia of the freely moving rat evidence has been obtained that neurotensin (NT) in threshold concentrations can counteract the D(2) agonist (intrastriatally perfused) induced inhibition of striatal dopamine (DA) release and of pallidal GABA release from the striato-pallidal GABA pathway, effects that are blocked by a NTR(1) antagonist SR48692. These results indicate the existence of antagonistic intramembrane NTR/D(2) receptor interactions in the striatal DA terminals and in the somato-dendritic regions of the striato-pallidal GABA neurons. By the NT-induced reduction of the D(2) mediated signals at the striatal pre- and postjunctional level DA transmission is switched towards a D(1) mediated transmission leading to increased activity in the striatopallidal and striatonigral GABA pathways. The former action will contribute to the motor inhibition and catalepsy found with NT treatment and underlies the use of NT receptor antagonists as a treatment strategy for Parkinson's disease. Nigral NT by an antagonistic NTR/D(2) receptor interaction in the DA cell body and dendrites may also increase nigral DA release leading to a D(2) mediated inhibition of the nigrothalamic GABA pathway. Such an effect, will instead result in antiparkinsonian actions. Thus, increases in NT transmission will have different consequences for the motor system depending upon where in the basal ganglia the increase takes place.

Striatal dopamine D1 and D2 receptor balance in twins at increased genetic risk for schizophrenia

The dopamine hypothesis of schizophrenia postulates that a dysfunctional dopaminergic system is a major pathophysiological mechanism in the disease. Most studies have focused on striatal dopamine D2 receptors, but a disturbed link between dopamine D1 and D2 receptors has also been proposed. Schizophrenia is highly heritable, and recent evidence suggests that alterations in the dopaminergic system confer susceptibility for schizophrenia instead of being solely related to the to overt expression of the disease. To explore the impact of genetic vulnerability for schizophrenia on the balance of striatal dopamine D1 and D2 receptors, we studied monozygotic (MZ) and dizygotic (DZ) unaffected co-twins from twin pairs discordant for schizophrenia as well as healthy control twins using positron emission tomography (PET). Both [(11)C]SCH 23390 and [(11)C]raclopride were used to quantitate D1 and D2 receptor binding, respectively, in the same individuals during the same day. The association between D1 and D2 receptor binding was analyzed using conventional region of interests as well as voxel-wise D1/D2 ratio maps. All levels of analyses failed to show any differences in D1/D2 ratio between the unaffected MZ or DZ co-twins and control twins. We noted rostrocaudally declining and dorsoventrally increasing gradients in D1/D2 ratio in the striatum, with no differences between groups in these gradients. In this sample, we did not find evidence for an association between increased genetic risk for schizophrenia and altered D1/D2 receptor balance in the striatum.

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.

QSAR studies in substituted 1,2,3,4,6,7,12,12a-octa-hydropyrazino[2',1':6,1]pyrido[3,4-b]indoles--a potent class of neuroleptics

A series of nineteen substituted 1,2,3,4,6,7,12,12a-octahydropyrazino[2',1':6,1]pyrido[3, 4-b]indoles analogues of neuroleptic drug, Centbutindole have been studied using quantitative structure-activity relationship analysis. The derived models display good fits to the experimental data (r>or=0.75) having good predictive power (r(cv)>or=0.688). The best model describes a high correlation between predicted and experimental activity data (r=0.967). Statistical analysis of the equation populations indicates that hydrophobicity (as measured by pi(R), logP(o/w) and SlogP_VSA8), dipole y and structural parameters in terms of indicator variable, (In(1)) and globularity are important variables in describing the variation in the neuroleptic activity in the series.

D1/D2 dopamine receptor interaction in membrane abolished by 6-hydroxydopamine lesion

D1/D2 interaction in rat striatum was investigated by examining the effect of the D2 antagonist spiperone on the binding of [3H]SCH23390 to D1 dopamine (DA) receptors. In the presence of endogenous DA, spiperone blocked D2 receptors, then caused the increase of the binding of [3H]SCH23390 in rat striatal homogenate. After the 6-hydroxydopamine (6-OHDA) lesion, the increase was not found even if in the addition of exogenous DA. The results suggest that the D2 antagonist can modify the interaction between endogenous DA and D1 receptors labeled with [3H]SCH23390, while 6-OHDA lesion may change the state of D1/D2 interaction operating at the receptor level.

Neuropeptide Y shifts equilibrium between alpha- and beta-adrenergic tonus in proximal tubule cells

Renal sympathetic nerves play a central role in the regulation of tubular Na+ reabsorption. Norepinephrine (NE) and neuropeptide Y (NPY) are colocalized in renal sympathetic nerve endings. The purpose of this study is to examine the integrated effects of these neurotransmitters on the regulation of Na+-K+-ATPase, the enzyme responsible for active Na+ reabsorption in renal tubular cells. Studies were performed on proximal tubular segments, which express adrenergic alpha- and beta-receptors, as well as NPY-Y2 receptors. It was found that alpha- and beta-adrenergic agonists had opposing effects on Na+-K+-ATPase activity. beta-Adrenergic agonists induced a dose-dependent inhibition of the Na+-K+-ATPase activity, whereas alpha-adrenergic agonists stimulated the enzyme. NPY abolished beta-agonist-induced deactivation of Na+-K+-ATPase and enhanced alpha-agonist-induced activation of Na+-K+-ATPase. The beta-adrenergic agonist appeared to inhibit Na+-K+-ATPase activity via a cAMP pathway. NPY antagonized beta-agonist-induced accumulation of cAMP. In our preparation, NE alone had no net effect but stimulated the Na+-K+-ATPase activity in the presence of beta-adrenergic antagonists, as well as in the presence of NPY. The results indicate that, in renal tissue, NPY determines the net effect of its colocalized transmitter, NE, by its ability to attenuate the beta- and enhance the alpha-adrenergic effect.

Integrated events in central dopamine transmission as analyzed at multiple levels. Evidence for intramembrane adenosine A2A/dopamine D2 and adenosine A1/dopamine D1 receptor interactions in the basal ganglia

An analysis at the network and membrane level has provided evidence that antagonistic interactions between adenosine A2A/dopamine D2 and adenosine A1/dopamine D1 receptors in the ventral and dorsal striatum are at least in part responsible for the motor stimulant effects of adenosine receptor antagonists like caffeine and for the motor depressant actions of adenosine receptor agonists. The results obtained in stably cotransfected cells also underline the hypothesis that the intramembrane A2A/D2 and A1/D1 receptor interactions represent functionally important mechanisms that may be the major mechanism for the demonstrated antagonistic A2A/D2 and A1/D1 receptor interactions found in vivo in behavioural studies and in studies on in vivo microdialysis of the striopallidal and strioentopeduncular GABAergic pathways. A major mechanism for the direct intramembrane A2A/D2 and A1/D1 receptor interactions may involve formation of A2A/D2 and A1/D1 heterodimers leading to allosteric changes that will alter the affinity as well as the G protein coupling and thus the efficacy to control the target proteins in the membranes. This is the first molecular network to cellular integration in the nerve cell membrane and may be well suited for a number of integrated tasks and can be performed in a short-time scale, in comparison with the very long-time scale observed when receptor heteroregulation involves phosphorylation or receptor resynthesis. Multiple receptor-receptor interactions within the membranes through formation of receptor clusters may lead to the storage of information within the membranes. Such molecular circuits can represent hidden layers within the membranes that substantially increase the computational potential of neuronal networks. These molecular circuits are biased and may therefore represent part of the molecular mechanism for the storage of memory traces (engrams) in the membranes.

Selective adenosine A2A receptor/dopamine D2 receptor interactions in animal models of schizophrenia

In the apomorphine-induced climbing mouse assay, the potencies of the selective adenosine A1 receptor agonist, N6-cyclohexyladenosine (CHA), and the selective A2A adenosine receptor agonist, 2-p-(2-carboxyethyl) phenethylamino 5'-N-ethyl-carboxamidoadenosine (CGS 21680), and various dopamine receptor antagonists were as follows: SCH 23390 = haloperidol > raclopride > CHA = CGS 21680. While in catalepsy, their potencies were SCH 23390 > haloperidol > raclopride > CGS 21680. CHA failed to induce catalepsy due to significant sedation/ataxia. The combined administration of the ED15 dose of CHA failed to potentiate the ED50 value of SCH 23390, raclopride, or haloperidol in the apomorphine-induced climbing mouse assay. However, the combined administration of the ED15 dose of CGS 21680 significantly decreased the ED50 of raclopride by 8.0-fold and haloperidol by 35-fold. The adenosine A2A receptor antagonist, 1,3,7-trimethyl-8-(3-chlorostyryl)xanthine (CSC), significantly decreased catalepsy induced by raclopride and haloperidol, while the adenosine A1 receptor antagonist, 1,3-dimethyl-8-phenylxanthine (8-PT), was ineffective. The present results show that in behavioral assays predictive for antipsychotic activity, adenosine receptor agonists block behaviors in a similar manner to dopamine receptor antagonists.

DRD4 dopamine receptor genotype and CSF monoamine metabolites in Finnish alcoholics and controls

The DRD4 dopamine receptor is thus far unique among neurotransmitter receptors in having a highly polymorphic gene structure that has been reported to produce altered receptor functioning. These allelic variations are caused by a 48-bp segment in exon III of the coding region which may be repeated from 2-10 times. Varying the numbers of repeated segments changes the length, structure, and, possibly, the functional efficiency of the receptor, which makes this gene an intriguing candidate for variations in dopamine-related behaviors, such as alcoholism and drug abuse. Thus far, these DRD4 alleles have been investigated for association with schizophrenia, bipolar disorder, Parkinson's disease, and chronic alcoholism, and all have been largely negative for a direct association. We evaluated the DRD4 genotype in 226 Finish adult males, 113 of whom were alcoholics, many of the early onset type with features of impulsivity and antisocial traits. Genotype frequencies were compared to 113 Finnish controls who were free of alcohol abuse, substance abuse, and major mental illness. In 70 alcoholics and 20 controls, we measured CSF homovanillic acid (HVA), the major metabolite of dopamine, and 5-hydroxyindoleacetic acid (5-HIAA). No association was found between a particular DRD4 dopamine receptor allele and alcoholism. CSF concentrations of the monoamine metabolites showed no significant difference among the DRD4 genotypes. This study of the DRD4 dopamine receptor in alcoholics is the first to be conducted in a clinically and ethnically homogeneous population and to relate the DRD4 genotype to CSF monoamine concentrations. The results indicate that there is no association of the DRD4 receptor with alcoholism.

A2a/D2 receptor interactions are not observed in COS-7 cells transiently transfected with dopamine D2 and adenosine A2a receptor cDNA

The rat D2 receptor and the dog A2a receptor subcloned into the pXM vector were transiently transfected into COS-7 cells using the DEAE-dextran method. The transfected cells expressed approx. 200 fmol D2 receptors/mg protein and approx. 5 pmol/mg protein of the A2a receptor as judged by binding experiments with [3H]raclopride [or[3H]-N-propyl-apomorphine (NPA)] and [3H]-CGS 21680, respectively. The high affinity KD values were 0.43 and 19 nM for D2 and A2a receptors, respectively, in agreement with results obtained from other cells and tissues. The non-selective adenosine receptor agonist NECA stimulated cAMP accumulation both in non-transfected and transfected COS-7 cells with only a slight difference in potency, suggesting that most of the stimulation is due to activation of A2b receptors known to be present on virtually every cell. The two A2a selective agonists CGS 21680 and CV-1808 were essentially inactive in transfected COS-7 cells, but were very active in PC-12 cells known to possess functional A2a receptors. Dopamine did not decrease cAMP accumulation in the transfected COS-7 cells. CGS 21680 (30 nM) did not affect the binding characteristics of D2 receptors in the co-transfected COS-7 cells in contrast to the increased KH, KL and RH values found previously in rat striatal membranes after CGS 21680 treatment. The present findings indicate that transiently transfected A2a and D2 receptors in COS-7 cells have normal binding properties, but couple poorly to adenylyl cyclase, despite the presence of Gs protein and adenylyl cyclase in these cells. Our results also demonstrate that the previously reported interactions between A2a receptors and D2 receptors do not occur when only the receptor proteins are expressed in COS-7 cells, suggesting that the two receptor molecules do not interact directly to influence binding characteristics.

Possible mechanisms for the powerful actions of neuropeptides

In order to understand the mechanisms underlying the powerful actions of neuropeptides, the present article has emphasized the unique ability of neuropeptides to act as VT signals, which via high-affinity G-protein coupled receptors can exert long-lasting actions and control synaptic transmission via receptor-receptor interactions. Also of substantial importance is the ability of neuropeptides to act as a set of signals via the formation of different types of active fragments, which can act as negative-feedback or positive-feedback signals to modulate the response elicited by the parent peptide and to give origin to syndromic responses. Also in the actions of the fragments on the neuronal network, receptor-receptor interactions may play an important role both by modulating the parent peptide receptors and by modulating other types of VT and/or WT receptors. Future work will have to evaluate the role of neuropeptides as transcellular signals and as regulators of neuronal excitabilities after the formation of carbamates, but certainly new important developments are within the horizon of today's research.

Receptor-receptor link in membranes revealed by ligand competition: example for dopamine D1 and D2 receptors

An interaction or link between dopamine D1 receptors and dopamine D2 receptors was found by a ligand competition method, using [3H]raclopride to label dopamine D2 receptors and SCH 23390 to block dopamine D1 receptors. In the presence of endogenous or exogenous dopamine, SCH 23390 increased the binding of [3H]raclopride in post-mortem human striata homogenates or in tissue culture cells containing human dopamine D1 and D2 receptors. In order to reveal such intramembrane receptor-receptor interactions in general, therefore, it appears essential to add two agonists, one for each receptor, and then to block one of the receptors when measuring the binding of a ligand to the second receptor.