N-0434, A very potent and specific new D-2 dopamine receptor agonist

Optical Control of Dopamine Receptors Using a Photoswitchable Tethered Inverse Agonist

Family A G protein-coupled receptors (GPCRs) control diverse biological processes and are of great clinical relevance. Their archetype rhodopsin becomes naturally light sensitive by binding covalently to the photoswitchable tethered ligand (PTL) retinal. Other GPCRs, however, neither bind covalently to ligands nor are light sensitive. We sought to impart the logic of rhodopsin to light-insensitive Family A GPCRs in order to enable their remote control in a receptor-specific, cell-type-specific, and spatiotemporally precise manner. Dopamine receptors (DARs) are of particular interest for their roles in motor coordination, appetitive, and aversive behavior, as well as neuropsychiatric disorders such as Parkinson's disease, schizophrenia, mood disorders, and addiction. Using an azobenzene derivative of the well-known DAR ligand 2-(N-phenethyl-N-propyl)amino-5-hydroxytetralin (PPHT), we were able to rapidly, reversibly, and selectively block dopamine D1 and D2 receptors (D1R and D2R) when the PTL was conjugated to an engineered cysteine near the dopamine binding site. Depending on the site of tethering, the ligand behaved as either a photoswitchable tethered neutral antagonist or inverse agonist. Our results indicate that DARs can be chemically engineered for selective remote control by light and provide a template for precision control of Family A GPCRs.

A new D₂ dopamine receptor agonist allosterically modulates A(2A) adenosine receptor signalling by interacting with the A(2A)/D₂ receptor heteromer

The structural and functional interaction between D₂ dopamine receptor (DR) and A(2A) adenosine receptor (AR) has suggested these two receptors as a pharmacological target in pathologies associated with dopamine dysfunction, such as Parkinson's disease. In transfected cell lines it has been demonstrated the activation of D₂DR induces a significant negative regulation of A(2A)AR-mediated responses, whereas few data are at now available about the regulation of A(2A)AR by D₂DR agonists at receptor recognition site. In this work we confirmed that in A(2A)AR/D₂DR co-transfected cells, these receptors exist as homo- and hetero-dimers. The classical D₂DR agonists were able to negatively modulate both A(2A)AR affinity and functionality. These effects occurred even if any significant changes in A(2A)AR/D₂DR energy transfer interaction could be detected in BRET experiments. Since the development of new molecules able to target A(2A)/D₂ dimers may represent an attractive tool for innovative pharmacological therapy, we also identified a new small molecule, 3-(3,4-dimethylphenyl)-1-(2-piperidin-1-yl)ethyl)piperidine (compound 1), full agonist of D₂DR and modulator of A(2A)-D₂ receptor dimer. This compound was able to negatively modulate A(2A)AR binding properties and functional responsiveness in a manner comparable to classical D₂R agonists. In contrast to classical agonists, compound 1 led to conformational changes in the quaternary structure in D₂DR homomers and heteromers and induced A(2A)AR/D₂DR co-internalization. These results suggest that compound 1 exerts a high control of the function of heteromers and could represent a starting point for the development of new drugs targeting A(2A)AR/D₂ DR heteromers.

Adenosine A2A receptor-antagonist/dopamine D2 receptor-agonist bivalent ligands as pharmacological tools to detect A2A-D2 receptor heteromers

Adenosine A(2A) (A(2A)R) and dopamine D(2) (D(2)R) receptors mediate the antagonism between adenosinergic and dopaminergic transmission in striatopallidal GABAergic neurons and are pharmacological targets for the treatment of Parkinson's disease. Here, a family of heterobivalent ligands containing a D(2)R agonist and an A(2A)R antagonist linked through a spacer of variable size was designed and synthesized to study A(2A)R-D(2)R heteromers. Bivalent ligands with shorter linkers bound to D(2)R or A(2A)R with higher affinity than the corresponding monovalent controls in membranes from brain striatum and from cells coexpressing both receptors. In contrast, no differences in affinity of bivalent versus monovalent ligands were detected in experiments using membranes from cells expressing only one receptor. These findings indicate the existence of A(2A)R-D(2)R heteromers and of a simultaneous interaction of heterobivalent ligands with both receptors. The cooperative effect derived from the simultaneous interaction suggests the occurrence of A(2A)R-D(2)R heteromers in cotransfected cells and in brain striatum. The dopamine/adenosine bivalent action could constitute a novel concept in Parkinson's disease pharmacotherapy.

Behavioral effects of dopamine agonists and antagonists: influence of estrous cycle, ovariectomy, and estrogen replacement in rats

The influence of the hormonal condition on the reactivity of central dopamine (DA) receptors was studied in male and in intact and ovariectomized (OVX) female rats. They were injected with selective DA agonists, acting either on D1 (SKF 38393, 2.5 or 10 mg/kg) or D2 receptors (PPHT, 31.3 or 125 microg/kg), or with selective DA antagonists, acting either on D1 (SCH 23390, 6.25 or 25 microg/kg), or D2 receptors (sulpiride, 10 or 40 mg/kg). The acquisition of an avoidance conditioning response (CAR) and the performance of some spontaneous motor behaviors were tested. Both D1 and D2 agonists and antagonists impaired the acquisition of CARs in diestrous, OVX, and male rats. Nevertheless, the effects of these drugs during estrus and in estradiol-primed OVX rats were different according to the drug and the dose injected. Whereas SKF 38393 failed to induce significative changes, PPHT and low doses of SCH 23390 and sulpiride improved the acquisition of CARS in those groups. The effects on conditioning were not accompanied with equivalent changes in spontaneous motor activity. Estradiol level fluctuations that occur in female rats within the estrous cycle or in OVX rats primed with estradiol would be responsive of changes in the response to DA agents. Although the reactivity of central DA systems is differentially affected by the hormonal condition of the rat, the precise mechanism of this modulatory action remains unknown.

Transient loss of dopamine autoreceptor control in the presence of highly potent dopamine agonists

The concentrations of endogenous ligands generally remain in a bounded range around a basal level, a manifestation of control. The dopaminergic system is an excellent example of a control system in which a negative feedback signal is associated with receptor occupancy of a D2-like dopamine autoreceptor. A consequence of the control theory is that autoreceptor occupancy by an agonist results in dopamine levels below the basal, whereas similar stimulation by a dopamine competitive antagonist results in an increase of dopamine to levels above the basal. These consequences of control theory were tested and verified in the rat striatum by infusing graded doses of either the agonist, quinpirole, or the antagonist, sulpiride, into the rat striatum via a microdialysis probe and sampling dopamine and metabolite levels at various times after the start of infusion. Control was maintained even at the very highest doses of these compounds, i.e., striatal dopamine concentration rose in response to the antagonist and fell in response to the agonist. In contrast, administration of each of two high affinity dopamine agonists, 7-OH-DPAT and PPHT showed dose-dependent control only up to certain doses. Above these doses the dopamine concentration actually increased to levels well above basal, an indication of loss of control. These findings suggest that the control of this endogenous ligand does not extend to the very highest levels of autoreceptor occupancy.

Biphasic effects of dopamine agonist N-0434 on locomotor behaviors in rats

The effects of a dopamine agonist, (+/-)-2-(N-penylethyl-N-propyl)amino-5- hydroxytetralin (N-0434) (SC doses of 0.00, 0.01, 0.1, and 1.0 mg/kg) were tested in rats for 120 min in an activity monitor. The durations in seconds of horizontal locomotor time, rearing time, stereotypy time, and margin time (thigmotaxis) were measured during 12 10-min time blocks. N-0434 (0.1 and 1.0 mg/kg) resulted in biphasic effects (initial inhibition followed by potentiation) of linear locomotor time and an attenuation of thigmotaxis. The 0.1- and 1.0-mg/kg doses initially suppressed rearing time but had mixed potentiation effects. The 0.01- to 1.0-mg/kg doses suppressed stereotypy time. The differential behavioral profiles were discussed in reference to the functions of dopamine receptors.

Adenylate cyclase from sea urchin eggs is positively and negatively regulated by D-1 and D-2 dopamine receptors

The adenylate cyclase present in membranes prepared from sea urchin eggs is sensitive to dopamine stimulation. The receptor sites coupled to sea urchin adenylate cyclase were characterized by means of specific agonists and antagonists. The D-1 dopamine agonist SKF-38393 was able to stimulate enzyme activity, while the two D-1 dopamine antagonists, SCH-23390 and SKF-83566, suppressed the stimulatory effect of dopamine. In addition, the D-2 dopamine agonists, PPHT and metergoline, brought about a dose-dependent inhibition of dopamine-stimulated adenylate cyclase activity. These data show that: (i) in sea urchin eggs adenylate cyclase is regulated by dopamine receptors; (ii) these receptors share characteristics with D-1 and D-2 dopamine receptors present in the mammalian brain.

Glucose concentrations in brain and blood: regulation by dopamine receptor subtypes

The stimulation of D1 and D2 dopamine (DA) receptors by selective agonists produced large increases in brain glucose concentrations. D2 receptor stimulation also produced large increases in blood glucose. The D1-induced increases were somewhat variable in normal animals, but were more reliably observed and greatly increased in animals with brain DA depletions. Blockade of D1 receptors prevented these increases. Likewise, centrally acting D2 antagonists, but not the peripheral D2 antagonist domperidone, prevented D2 agonist-induced increases in brain and blood glucose. These observations point to an important role for DA in the regulation of glucose homeostasis.

Affinity chromatography with the immobilized agonist N-0434 yields an active and highly purified preparation of the dopamine D-2 receptor from bovine striatum

Partial purification of the dopamine D-2 receptor from bovine striatum, solubilized in the presence of 1% digitonin, was obtained by chromatography on wheat germ lectin agarose. The preparation was purified approximately 10-fold. The stability of the receptor preparation was considerably improved and non-specific protein absorption on the affinity gel used later was decreased. Further purification was achieved on a column containing a D-2-selective agonist, N-0434. Approximately 90% of the receptor activity was bound to the gel and 20-40% of the activity could be eluted by pH shock. The total purification factor after one affinity chromatography step was estimated to be at least 1500. An active preparation of at least 20% purity was obtained after a second cycle of affinity chromatography. This corresponds to an enrichment of more than 5000 times compared to the solubilized receptor preparation.

Update on dopamine agonists in Parkinson's disease: "beyond bromocriptine"

Since the initiation of bromocriptine therapy for Parkinson's disease several newer dopamine agonists have been developed. Pergolide has reached the stage of Phase 3 clinical trials and will probably be available for general use sometime in the foreseeable future. Lisuride shows most promise in its parenteral form for infusion therapy of patients with severe fluctuations. Mesulergine, another ergot-derivative and ciladopa, a new non-ergot agonist, have been withdrawn from further clinical use due to tumorogenesis in rats. It is questionable how applicable these findings are to the use of the drugs in elderly humans with parkinsonism. Recently a small number of drugs have been found to have postsynaptic dopamine agonist properties only in the setting of denervated supersensitive dopamine receptors. These agents may be particularly effective in the early treatment of patients with Parkinson's disease. This paper will review a number of the dopamine agonists which have been developed since the introduction of bromocriptine therapy. Several of these have shown beneficial effects in early clinical trials while others show promise in preclinical studies of animal models of parkinsonism.

Pharmacological profiles of three new, potent and selective dopamine receptor agonists: N-0434, N-0437 and N-0734

A series of new dopamine (DA) receptor agonists, of the 2-aminotetralin group, i.e. N-0434, N-0437 and N-0734 were investigated in both in vivo and in vitro pharmacological test systems. In vivo, the reversal of the gamma-butyrolactone-induced increase in rat central DOPA biosynthesis rate was taken as a measure of presynaptic activity. In addition, the homovanillic acid (HVA) decrease, after intraperitoneal and after oral administration of the drugs was also taken as a measure of presynaptic activity. Postsynaptic activity was measured in two behavioural models, i.e. reserpine reversal and stereotypy induction. The effects of these drugs on noradrenaline and dopamine turnover (alpha-MpT method) were studied in addition. The displacement of [3H]N,N-dipropyl-5,6-dihydroxy-2-aminotetralin [( 3H]DP-5,6-ADTN) binding to rat striatal homogenates was studied in vitro. The results indicate that all three compounds are potent and selective DA agonists that lack significant alpha 2 activity. Because of its long duration of action and high oral activity, N-0437 seems to be a most promising candidate for further evaluation for possible therapeutic use.

Synthesis and radioreceptor binding activity of N-0437, a new, extremely potent and selective D2 dopamine receptor agonist

The synthesis of a new, potent and selective D2 dopamine receptor agonist, N-0437, of the 2-aminotetralin group is described. The results of a radioreceptor binding assay using a homogenate of porcine anterior pituitary as a tissue source for D2 dopamine receptors and 3H-spiperone as radioligand demonstrate that this compound is one of the most potent compounds so far evaluated in this test system.

Two dopamine receptors: biochemistry, physiology and pharmacology

In 1979, two categories of dopamine (DA) receptors (designated as D-1 and D-2) were identified on the basis of the ability of a limited number of agonists and antagonists to discriminate between these two entities. In the past 5 years agonists and antagonists selective for each category of receptor have been identified. Using these selective drugs it has been possible to attribute the effects of DA upon physiological and biochemical processes to the stimulation of either a D-1 or a D-2 receptor. Thus, DA-induced enhancement of both hormone release from bovine parathyroid gland and firing of neurosecretory cells in the CNS of Lymnaea stagnalis has been attributed to stimulation of a D-1 receptor. Likewise, the DA-induced inhibition of the release of prolactin and alpha-MSH from the pituitary gland, as well as of acetylcholine, DA and beta-endorphin from brain, the DA-induced inhibition of chemo-sensory discharge in rabbit carotid body and the DA-induced hyperpolarization of neurosecretory cells in the CNS of Lymnaea stagnalis have been attributed to stimulation of a D-2 receptor. Independently two categories of DA receptors (designated as DA-1 and DA-2) were identified in the cardiovascular system. Stimulation of a DA-1 receptor increases the vascular cyclic AMP content and causes a relaxation of vascular smooth muscle in renal blood vessels, whereas stimulation of a DA-2 receptor inhibits the release of norepinephrine from certain postganglionic sympathetic neurons. Recent studies with the newly developed drugs discriminating between D-1 and D-2 receptors suggest however that the independently developed schemata for classification of dopamine receptors in either the central nervous and endocrine systems or the cardiovascular system are similar although maybe not completely identical.

N,N-disubstituted 2-aminotetralins are potent D-2 dopamine receptor agonists

Mono- and di-N-substituted 2-amino-5-hydroxytetralins stimulate the D-2 dopamine receptor, 2-(N-n-propyl-N-phenylethylamino)-5-hydroxytetralin being the most potent D-2 agonist encountered to date. In contrast, 2-amino-5-hydroxytetralins only marginally stimulate the D-1 receptor; however, 2-(di-N-n-propylamino)-5, 6-dihydroxytetralin is equipotent with dopamine as a D-1 agonist. The results are discussed within the context of the two dopamine receptor hypothesis.