The C-terminus of the prototypical M2 muscarinic receptor localizes to the mitochondria and regulates cell respiration under stress conditions.

Muscarinic acetylcholine receptors are prototypical G protein-coupled receptors (GPCRs), members of a large family of 7 transmembrane receptors mediating a wide variety of extracellular signals. We show here, in cultured cells and in a murine model, that the carboxyl terminal fragment of the muscarinic M2 receptor, comprising the transmembrane regions 6 and 7 (M2tail), is expressed by virtue of an internal ribosome entry site localized in the third intracellular loop. Single-cell imaging and import in isolated yeast mitochondria reveals that M2tail, whose expression is up-regulated in cells undergoing integrated stress response, does not follow the normal route to the plasma membrane, but is almost exclusively sorted to the mitochondria inner membrane: here, it controls oxygen consumption, cell proliferation, and the formation of reactive oxygen species (ROS) by reducing oxidative phosphorylation. Crispr/Cas9 editing of the key methionine where cap-independent translation begins in human-induced pluripotent stem cells (hiPSCs), reveals the physiological role of this process in influencing cell proliferation and oxygen consumption at the endogenous level. The expression of the C-terminal domain of a GPCR, capable of regulating mitochondrial function, constitutes a hitherto unknown mechanism notably unrelated to its canonical signaling function as a GPCR at the plasma membrane. This work thus highlights a potential novel mechanism that cells may use for controlling their metabolism under variable environmental conditions, notably as a negative regulator of cell respiration.

Eyes as gateways for environmental light to the substantia nigra: relevance in Parkinson's disease.

Recent data indicates that prolonged bright light exposure of rats induces production of neuromelanin and reduction of tyrosine hydroxylase positive neurons in the substantia nigra. This effect was the result of direct light reaching the substantia nigra and not due to alteration of circadian rhythms. Here, we measured the spectrum of light reaching the substantia nigra in rats and analysed the pathway that light may take to reach this deep brain structure in humans. Wavelength range and light intensity, emitted from a fluorescent tube, were measured, using a stereotaxically implanted optical fibre in the rat mesencephalon. The hypothetical path of environmental light from the eye to the substantia nigra in humans was investigated by computed tomography and magnetic resonance imaging. Light with wavelengths greater than 600 nm reached the rat substantia nigra, with a peak at 709 nm. Eyes appear to be the gateway for light to the mesencephalon since covering the eyes with aluminum foil reduced light intensity by half. Using computed tomography and magnetic resonance imaging of a human head, we identified the eye and the superior orbital fissure as possible gateways for environmental light to reach the mesencephalon.

In-vitro Approaches to Investigate the Detrimental Effect of Light on Dopaminergic Neurons.

Our recent study revealed that fluorescent lamp light can penetrate deep into the brain of mice and rats leading to the development of typical histological characteristics associated with Parkinson's disease such as the loss of dopamine neurons in the substantia nigra. Monochromatic LED lights were thus used in this work to deepen our knowledge on the effects of the major wavelength peaks of fluorescent light on mouse and human dopaminergic cells. In particular, we exposed immortalized dopaminergic MN9D neuronal cells, primary cultures of mouse mesencephalic dopaminergic cells and human dopaminergic neurons differentiated from induced pluripotent stem cells (hiPSC) to different LED light wavelengths. We found that chronic exposure to LED light reduced overall undifferentiated MN9D cell number, with the most significant effects observed at wavelengths of 485 nm and 610 nm. Moreover, LED light especially at 610 nm was able to negatively impact on the survival of mouse mesencephalic dopaminergic cells and of human dopaminergic neurons derived from hiPSC. Notably, differentiated MN9D dopaminergic cells, which closely resemble mature dopamine neuronal phenotype, acutely exposed for 3 h at 610 nm, showed a clear increase in ROS production and cytotoxicity compared to controls undifferentiated MN9D cells. These increases were even more pronounced by the co-treatment with the oxidative agent H2O2. Collectively, these findings suggest that specific wavelengths, particularly those capable of penetrating deep into the brain, could potentially pose an environmental hazard in relation to Parkinson's disease.

Genetic deletion of trace amine 1 receptors reveals their role in auto-inhibiting the actions of ecstasy (MDMA).

"Ecstasy" [3,4-methylenedioxymetamphetamine (MDMA)] is of considerable interest in light of its prosocial properties and risks associated with widespread recreational use. Recently, it was found to bind trace amine-1 receptors (TA(1)Rs), which modulate dopaminergic transmission. Accordingly, using mice genetically deprived of TA(1)R (TA(1)-KO), we explored their significance to the actions of MDMA, which robustly activated human adenylyl cyclase-coupled TA(1)R transfected into HeLa cells. In wild-type (WT) mice, MDMA elicited a time-, dose-, and ambient temperature-dependent hypothermia and hyperthermia, whereas TA(1)-KO mice displayed hyperthermia only. MDMA-induced increases in dialysate levels of dopamine (DA) in dorsal striatum were amplified in TA(1)-KO mice, despite identical levels of MDMA itself. A similar facilitation of the influence of MDMA upon dopaminergic transmission was acquired in frontal cortex and nucleus accumbens, and induction of locomotion by MDMA was haloperidol-reversibly potentiated in TA(1)-KO versus WT mice. Conversely, genetic deletion of TA(1)R did not affect increases in DA levels evoked by para-chloroamphetamine (PCA), which was inactive at hTA(1) sites. The TA(1)R agonist o-phenyl-3-iodotyramine (o-PIT) blunted the DA-releasing actions of PCA both in vivo (dialysis) and in vitro (synaptosomes) in WT but not TA(1)-KO animals. MDMA-elicited increases in dialysis levels of serotonin (5-HT) were likewise greater in TA(1)-KO versus WT mice, and 5-HT-releasing actions of PCA were blunted in vivo and in vitro by o-PIT in WT mice only. In conclusion, TA(1)Rs exert an inhibitory influence on both dopaminergic and serotonergic transmission, and MDMA auto-inhibits its neurochemical and functional actions by recruitment of TA(1)R. These observations have important implications for the effects of MDMA in humans.

GPCRs in Intracellular Compartments: New Targets for Drug Discovery.

The architecture of eukaryotic cells is defined by extensive membrane-delimited compartments, which entails separate metabolic processes that would otherwise interfere with each other, leading to functional differences between cells. G protein-coupled receptors (GPCRs) are the largest class of cell surface receptors, and their signal transduction is traditionally viewed as a chain of events initiated from the plasma membrane. Furthermore, their intracellular trafficking, internalization, and recycling were considered only to regulate receptor desensitization and cell surface expression. On the contrary, accumulating data strongly suggest that GPCRs also signal from intracellular compartments. GPCRs localize in the membranes of endosomes, nucleus, Golgi and endoplasmic reticulum apparatuses, mitochondria, and cell division compartments. Importantly, from these sites they have shown to orchestrate multiple signals that regulate different cell pathways. In this review, we summarize the current knowledge of this fascinating phenomenon, explaining how GPCRs reach the intracellular sites, are stimulated by the endogenous ligands, and their potential physiological/pathophysiological roles. Finally, we illustrate several mechanisms involved in the modulation of the compartmentalized GPCR signaling by drugs and endogenous ligands. Understanding how GPCR signaling compartmentalization is regulated will provide a unique opportunity to develop novel pharmaceutical approaches to target GPCRs and potentially lead the way towards new therapeutic approaches.

Interaction of the preferential D3 agonist (+)PHNO with dopamine D3-D2 receptor heterodimers and diverse classes of monoamine receptor: relevance for PET imaging.

(+)-4-Propyl-9-hydroxynaphthoxazine ((+)PHNO) is a high affinity, preferential dopamine D3 versus D2 agonist employed in view of its high specificity and excellent signal-to-noise ratio as a radiotracer for positron emission tomography (PET) imaging. Surprisingly, its profile at other classes of monoamine receptor remains undocumented. In addition to hD3 and hD2L receptors, (+)PHNO revealed high affinity at hD4.4 but not hD1 or hD5 receptors. It also revealed significant affinity for several other G protein-coupled monoaminergic receptors, in particular h5-HT1A and h5-HT7. (+)PHNO behaved as a full agonist at hD4.4 and h5-HT1A receptors with potencies comparable to its actions at hD3 and hD2L receptors, and with less potency at 5-HT7 receptors. In binding assays with membranes derived from cells co-expressing hD3 and hD2L receptors and labeled with [3H]Nemonapride or [3H]Spiperone, the proportion of high affinity binding sites recognized by (+)PHNO was higher than an equivalent mixture of membranes from cells expressing hD3or hD2L receptors, suggesting that (+)PHNO promotes formation of hD3-hD2L heterodimers. Further, in cells co-expressing hD3 and hD2L receptors, (+)PHNO showed higher efficacy for inhibiting forskolin stimulated adenylyl cyclase and inducing adenylyl cyclase super-sensitization than in cells transfected with only hD2L receptors. In conclusion, (+)PHNO is a potent agonist at hD4.4, h5-HT1A and h5-HT7 as well as hD3 and hD2L receptors, and it potently activates dopamine hD3-hD2L heterodimers. These interactions should be considered when interpreting PET studies with [11C](+)PHNO and may be relevant to its functional and potential clinical properties in Parkinson's disease and other disorders.

The tetrahydroisoquinoline derivative SB269,652 is an allosteric antagonist at dopamine D3 and D2 receptors.

In view of the therapeutic importance of dopamine D(3) and D(2) receptors, there remains considerable interest in novel ligands. Herein, we show that the tetrahydroisoquinoline 1H-indole-2-carboxylic acid {4-[2-(cyano-3,4-dihydro-1H-isoquinolin-2-yl)-ethyl]-cyclohexyl}-amide (SB269,652) behaves as an atypical, allosteric antagonist at D(3) and D(2) receptors. Accordingly, SB269,652 potently (low nanomolar range) abolished specific binding of [(3)H]nemanopride and [(3)H]spiperone to Chinese hamster ovary-transfected D(3) receptors when radioligands were used at 0.2 and 0.5 nM, respectively. However, even at high concentrations (5 µM), SB269,652 only submaximally inhibited the specific binding of these radioligands when they were employed at 10-fold higher concentrations. By analogy, although SB269,652 potently blocked D(3) receptor-mediated activation of Gα(i3) and phosphorylation of extracellular-signal-regulated kinase (ERK)1/2, when concentrations of dopamine were increased by 10-fold, from 1 µM to 10 µM, SB269,652 only submaximally inhibited dopamine-induced stimulation of Gα(i3). SB269,652 (up to 10 µM) only weakly and partially (by approximately 20-30%) inhibited radioligand binding to D(2) receptors. Likewise, SB269,652 only submaximally suppressed D(2) receptor-mediated stimulation of Gα(i3) and Gα(qi5) (detected with the aequorin assay) and phosphorylation of ERK1/2 and Akt. Furthermore, SB269,652 only partially (35%) inhibited the dopamine-induced recruitment of ß-arrestin2 to D(2) receptors. Finally, Schild analysis using Gα(i3) assays, and studies of radioligand association and dissociation kinetics, supported allosteric actions of SB269,652 at D(3) and D(2) receptors.

Allosteric Modulators of G Protein-Coupled Dopamine and Serotonin Receptors: A New Class of Atypical Antipsychotics.

Schizophrenia was first described by Emil Krapelin in the 19th century as one of the major mental illnesses causing disability worldwide. Since the introduction of chlorpromazine in 1952, strategies aimed at modifying the activity of dopamine receptors have played a major role for the treatment of schizophrenia. The introduction of atypical antipsychotics with clozapine broadened the range of potential targets for the treatment of this psychiatric disease, as they also modify the activity of the serotoninergic receptors. Interestingly, all marketed drugs for schizophrenia bind to the orthosteric binding pocket of the receptor as competitive antagonists or partial agonists. In recent years, a strong effort to develop allosteric modulators as potential therapeutic agents for schizophrenia was made, mainly for the several advantages in their use. In particular, the allosteric binding sites are topographically distinct from the orthosteric pockets, and thus drugs targeting these sites have a higher degree of receptor subunit specificity. Moreover, "pure" allosteric modulators maintain the temporal and spatial fidelity of native orthosteric ligand. Furthermore, allosteric modulators have a "ceiling effect", and their modulatory effect is saturated above certain concentrations. In this review, we summarize the progresses made in the identification of allosteric drugs for dopamine and serotonin receptors, which could lead to a new generation of atypical antipsychotics with a better profile, especially in terms of reduced side effects.

A New Threat to Dopamine Neurons: The Downside of Artificial Light.

Growing awareness of adverse impacts of artificial light on human health has led to recognize light pollution as a significant global environmental issue. Despite, a large number of studies in rodent and monkey models of Parkinson's disease have reported that near infrared light has neuroprotective effects on dopaminergic neurons, recent findings have shown that prolonged exposure of rodents and birds to fluorescent artificial light results in an increase of neuromelanin granules in substantia nigra and loss of dopaminergic neurons. The observed detrimental effect seems to be dependent on a direct effect of light on the substantia nigra rather than a secondary effect of the alterations of circadian rhythms. Moreover, inferences from animal models to human studies have shown a positive correlation between the prevalence of Parkinson's disease and light pollution. The present article discusses experimental evidence supporting a potentially deleterious impact of light on dopaminergic neurons and highlights the mechanisms whereby light might damage neuronal tissue. Moreover, it analyses epidemiological evidence that suggests light pollution to be an environmental risk factor for Parkinson's disease.

Neurotoxic and Neuroprotective Role of Exosomes in Parkinson's Disease.

Exosomes are extracellular vesicles produced by eukaryotic cells that are also found in most biological fluids and tissues. While they were initially thought to act as compartments for removal of cellular debris, they are now recognized as important tools for cell-to-cell communication and for the transfer of pathogens between the cells. They have attracted particular interest in neurodegenerative diseases for their potential role in transferring prion-like proteins between neurons, and in Parkinson's disease (PD), they have been shown to spread oligomers of α-synuclein in the brain accelerating the progression of this pathology. A potential neuroprotective role of exosomes has also been equally proposed in PD as they could limit the toxicity of α-synuclein by clearing them out of the cells. Exosomes have also attracted considerable attention for use as drug vehicles. Being nonimmunogenic in nature, they provide an unprecedented opportunity to enhance the delivery of incorporated drugs to target cells. In this review, we discuss current knowledge about the potential neurotoxic and neuroprotective role of exosomes and their potential application as drug delivery systems in PD.

Distinctive binding properties of the negative allosteric modulator, [3H]SB269,652, at recombinant dopamine D3 receptors.

Recently, employing radioligand displacement and functional coupling studies, we demonstrated that SB269,652 (N-[(1r,4r)-4-[2-(7-cyano-1,2,3,4-tetrahydroisoquinolin-2-yl)ethyl]cyclohexyl]-1H-indole-2-carboxamide) interacts in an atypical manner with dopamine D3 receptor displaying a unique profile reminiscent of a negative allosteric ligand. Here, we characterized the binding of radiolabelled [3H]SB269,652 to human dopamine D3 receptor stably expressed in Chinese Hamster Ovary cells. Under saturating conditions, SB269,652 showed a KD value of ≈ 1nM. Consistent with high selectivity for human dopamine D3 receptor, [3H]SB269,652 binding was undetectable in cells expressing human dopamine D1, D2L or D4 receptors and absent in synaptosomes from dopamine D3 receptor knockout vs. wild-type mice. In contrast to saturation binding experiments, the dissociation kinetics of [3H]SB269,652 from human dopamine D3 receptors initiated with an excess of unlabelled ligand were best fitted by a bi-exponential binding model. Supporting the kinetic data, competition experiments with haloperidol, S33084 (a dopamine D3 receptor antagonist) or dopamine, were best described by a two-site model. In co-transfection experiments binding of SB269,652 to dopamine D3 receptor was able to influence the functional coupling of dopamine D2 receptor, supporting the notion that SB269,652 is a negative allosteric modulator across receptor dimers. However, because SB269,652 decreases the rate of [3H]nemonapride dissociation, the present data suggest that SB269,652 behaves as a bitopic antagonist at unoccupied dopamine D3 receptor, binding simultaneously to both orthosteric and allosteric sites, and as a pure negative allosteric modulator when receptors are occupied and it can solely bind to the allosteric site.

Fluorescent light induces neurodegeneration in the rodent nigrostriatal system but near infrared LED light does not.

We investigated the effects of continuous artificial light exposure on the mouse substantia nigra (SN). A three month exposure of C57Bl/6J mice to white fluorescent light induced a 30% reduction in dopamine (DA) neurons in SN compared to controls, accompanied by a decrease of DA and its metabolites in the striatum. After six months of exposure, neurodegeneration progressed slightly, but the level of DA returned to the basal level, while the metabolites increased with respect to the control. Three month exposure to near infrared LED light (∼710nm) did not alter DA neurons in SN, nor did it decrease DA and its metabolites in the striatum. Furthermore mesencephalic cell viability, as tested by [3H]DA uptake, did not change. Finally, we observed that 710nm LED light, locally conveyed in the rat SN, could modulate the firing activity of extracellular-recorded DA neurons. These data suggest that light can be detrimental or beneficial to DA neurons in SN, depending on the source and wavelength.

Selective blockade of mGlu5 metabotropic glutamate receptors is protective against methamphetamine neurotoxicity.

Methamphetamine (MA), a widely used drug of abuse, produces oxidative damage of nigrostriatal dopaminergic terminals. We examined the effect of subtype-selective ligands of metabotropic glutamate (mGlu) receptors on MA neurotoxicity in mice. MA (5 mg/kg, i.p.; injected three times, every 2 hr) induced, 5 d later, a substantial degeneration of striatal dopaminergic terminals associated with reactive gliosis. MA toxicity was primarily attenuated by the coinjection of the noncompetitive mGlu5 receptor antagonists 2-methyl-6-(phenylethynyl)pyridine and (E)-2-methyl-6-styrylpyridine both at 10 mg/kg, i.p.). In contrast, the mGlu1 receptor antagonist 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester (10 mg/kg, i.p.), and the mGlu2/3 receptor agonist (-)-2-oxa-4-aminocyclo[3.1.0]hexane-4,6-dicarboxylic acid (1 mg/kg, i.p.), failed to affect MA toxicity. mGlu5 receptor antagonists reduced the production of reactive oxygen species but did not reduce the acute stimulation of dopamine release induced by MA both in striatal synaptosomes and in the striatum of freely moving mice. We conclude that endogenous activation of mGlu5 receptors enables the development of MA neurotoxicity and that mGlu5 receptor antagonists are neuroprotective without interfering with the primary mechanism of action of MA.

The atypical antipsychotic clozapine selectively inhibits interleukin 8 (IL-8)-induced neutrophil chemotaxis.

Clozapine is the most effective antipsychotic to date, but its benefits are counterbalanced by the risk of severe hematological effects. In this study, we analyzed whether clozapine inhibits polymorphonuclear (PMN) leukocyte chemotaxis. We found that clozapine, within the therapeutic concentration range, potently and selectively inhibits PMN chemotaxis induced by interleukin 8 (IL-8), a chemokine inducing neutrophil migration. The effect was not due to its action at dopamine, serotonin and muscarinic receptors, or to a direct antagonism to IL-8 receptors. Furthermore, clozapine did not inhibit PMN chemotaxis by its presumed toxic mechanism. In fact, after an overnight incubation in cell culture, the drug did not increase the physiological PMN apoptosis. An interference of clozapine with the autocrine release of leukotriene B4 (LTB4), a secondary chemoattractant secreted by neutrophils in response to the primary chemoattractant IL-8, was hypothesized. In agreement with this hypothesis, clozapine attenuated the IL-8-induced release of LTB4 in PMNs. A series of experiments with an antagonist of the LTB4 receptor, U75302, and an inhibitor of LTB4 synthesis, zileuton, provided support to this conjecture. Intriguingly MK-571, an inhibitor of the multi-drug resistance protein MRP4, playing a pivotal role in effluxing LTB4, completely blocked PMN chemotaxis induced by IL-8, but gave conflicting results when tested for its ability to reduce LTB4 release, increasing LTB4 efflux by itself but reducing the release when in combination with IL-8. The reduction of PMN chemotaxis due to clozapine could predispose patients to infections. Whether this effect is a prelude to clozapine agranulocytosis requires further investigation.

Nitric oxide synthase inhibition reverts muscarinic receptor down-regulation induced by pilocarpine- and kainic acid-evoked seizures in rat fronto-parietal cortex.

We investigated how nitric oxide (NO) synthase inhibitor modulates muscarinic receptor expression in epileptic rats. We found that subchronic treatment (4 days) with Nω-nitro-l-arginine reduced the down-regulation of muscarinic receptors induced by pilocarpine and kainic acid in rat fronto-parietal cortex, notwithstanding the dramatic potentiation of seizures induced by both convulsants. Furthermore, functional experiments in fronto-parietal cortex slices, showed that Nω-nitro-l-arginine reduces the down-regulating effect of pilocarpine on carbachol-induced phosphoinositol hydrolysis. Finally, Nω-nitro-l-arginine greatly potentiated the induction of basic fibroblast growth factor (FGF2) by pilocarpine. These data suggest a potential role of NO in a regulatory feedback loop to control muscarinic receptor signal during seizures. The dramatic potentiation of convulsions by NO synthase inhibitors in some animal models of seizures could derive from preventing this feedback loop.

Bright light exposure reduces TH-positive dopamine neurons: implications of light pollution in Parkinson's disease epidemiology.

This study explores the effect of continuous exposure to bright light on neuromelanin formation and dopamine neuron survival in the substantia nigra. Twenty-one days after birth, Sprague-Dawley albino rats were divided into groups and raised under different conditions of light exposure. At the end of the irradiation period, rats were sacrificed and assayed for neuromelanin formation and number of tyrosine hydroxylase (TH)-positive neurons in the substantia nigra. The rats exposed to bright light for 20 days or 90 days showed a relatively greater number of neuromelanin-positive neurons. Surprisingly, TH-positive neurons decreased progressively in the substantia nigra reaching a significant 29% reduction after 90 days of continuous bright light exposure. This decrease was paralleled by a diminution of dopamine and its metabolite in the striatum. Remarkably, in preliminary analysis that accounted for population density, the age and race adjusted Parkinson's disease prevalence significantly correlated with average satellite-observed sky light pollution.

Differential induction of adenylyl cyclase supersensitivity by antiparkinson drugs acting as agonists at dopamine D1/D2/D3 receptors vs D2/D3 receptors only: parallel observations from co-transfected human and native cerebral receptors.

Though there is evidence that sustained exposure of dopamine (DA) receptors to agonists can elicit a supersensitivity of adenylyl cyclase (AC), little is known about the pharmacological characteristics of this phenomenon, and possible interrelationships amongst DA receptor subtypes have not been examined. In cells co-transfected with D(1) plus D(2), or D(1) plus D(3), receptors, which are known to physically and functionally interact, long-term exposure to quinpirole, pramipexole and ropinirole (which possess negligible affinities for D(1) sites) elicited supersensitivity of D(1) receptor-activated AC. By contrast, D(2)/D(3) receptor agonists that also act as D(1) receptor agonists, bromocriptine, lisuride, cabergoline, apomorphine and DA itself, did not elicit supersensitivity. Interestingly, AC supersensitivity was also observed in the nucleus accumbens of mice pretreated with twice-daily pramipexole and quinpirole, whereas no change was seen either with lisuride or with the DA precursor, L-DOPA. Thus, AC supersensitivity is elicited by the sustained exposure of cloned human and native mouse populations of dopaminergic receptors, to D(2)/D(3) but not D(1)/D(2)/D(3) agonists. These observations may be related to the exacerbation of gambling in Parkinson's disease that is provoked by antiparkinson agents acting as selective D(2)/D(3) receptor agonists, notably pramipexole.

Heterodimerization of dopamine receptors: new insights into functional and therapeutic significance.

G-protein-coupled receptors (GPCRs) exist both as monomers and also as dimers or higher-order oligomers, representing assemblies either with their peers or with other classes of GPCR ("heterodimers"). The pharmacological profiles of heterodimers often differ from the corresponding monomers or homodimers. Heterodimerization of dopamine receptors has been shown for both the D1/D5 and D2/D3/D4 receptor families, which couple positively and negatively, respectively, to adenylyl cyclase. Notably, heterodimers are formed by: D1 and adenosine A1 receptors; D2 or D3 and adenosine A2 receptors; and D2 and somatostatin SST5 receptors. Further, D1, D2 and D3 receptors physically assemble into functional D1/D2, D1/D3 and D2/D3 heterodimers possessing binding and coupling profiles distinct from the respective monomers. This article reviews data on dopamine D3/D2 and D3/D1 heterodimers, including observations that some antiparkinsonian agents--such as the preferential high-efficacy D3 versus D2 receptor agonists, pramipexole and ropinirole--show amplified potency at D3/D2 heterodimers versus constituent monomers, and others in contrast, such as the D3/D2 receptor agonist pergolide, show no difference. This article also discusses allosteric modulation amongst heterodimeric dopamine receptors, whereby agonist actions at one member of a heterodimer influence functional coupling at the other protomer. Finally, it presents data showing that, in cells co-transfected with D3 and D1 receptors, long-term exposure to pramipexole and ropinirole (which possess negligible affinities for D1 sites) elicits supersensitivity of D1 receptor-activated adenylyl cyclase, and conversely, D3/D2 receptor agonists such as apomorphine and bromocriptine (which also act as D1 receptor agonists) do not. A hypothetical relationship between these observations and the exacerbation of gambling in Parkinson's disease by antiparkinsonian agents is discussed.

Presence of a putative steroidal allosteric site on glycoprotein hormone receptors.

In a previous work we found that the insecticide 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT), inhibits the accumulation of cAMP as induced by the bovine thyroid stimulating hormone (bTSH) in cells transfected with the TSH receptor. In this work, we demonstrate that the DDT molecular analogues, diethylstilbestrol and quercetine, are more potent inhibitors of the TSH receptor activity than DDT itself. The notion that all these compounds interfere with nuclear estrogen receptors, as either agonists (DDT and diethylstilbestrol) or antagonists (quercetin), prompted us to test the ability of the steroid hormone 17-beta-estradiol to inhibit the TSH receptor activity. We found that estrogen exposure causes a modest but significant inhibition of the bTSH induced cAMP accumulation both in transfected CHO-TSH receptor and Fischer Rat Thyroid Low Serum 5% (FRTL-5) cells. When applied to CHO cells transfected with the luteinizing hormone receptor, 17-beta-estradiol proved capable of inhibiting the hCG induced cAMP accumulation at a concentration as low as 10nM, though the effect was not greater than 35%. The effect of 17-beta-estradiol was not estrogen receptors mediated, as co-transfection of the estrogen receptor alpha and beta subunits with LH receptor caused cAMP to increase above the level attained by the sole hCG stimulation, and not to decrease it as expected. These data suggest the presence of a steroidal-like allosteric binding site on glycoprotein hormone receptors.