Complete conversion of brain D2 dopamine receptors from the high- to the low-affinity state for dopamine agonists, using sodium ions and guanine nucleotide

New Dopamine D2 Receptor Agonist, [3H]MCL-536, for Detecting Dopamine D2high Receptors in Vivo

Increases in the D2 receptor high affinity state are associated with certain neurological disorders. We synthesized and characterized the high-affinity D2high ligand [³H]MCL-536 in competition binding against the D2/3 agonist R-(-)- N- n-propylnorapomorphine (NPA) and the D2/3 antagonist raclopride. The total binding of [³H]MCL-536 (minus that in the presence of 100 nM NPA) was measured by saturation binding in CHO cells expressing human D2long; the data yielded separable, nonsaturable nonspecific, and saturable specific components. The former represents an aporphine site common to NPA and [³H]MCL-536. The latter indicated specific binding to the total D2 receptors (both high and low-affinity states). [³H]MCL-536 had a K_d of 0.8 nM. In competition binding, NPA had a K_i of 0.16 nM, and raclopride had a K_i of 0.9 nM. Co-incubation with guanylylimidodiphosphate abolished binding to D2high. This unique profile makes radiolabeled MCL-536 a versatile tool for diagnostics and therapeutics, and may quantify D2high sites in schizophrenia and PD patients in vivo.

Assessment of in vitro dopamine-neuroblastoma cell interactions with a bioelectric biosensor: perspective for a novel in vitro functional assay for dopamine agonist/antagonist activity

Current receptor-binding assays for dopamine do not measure the in vitro whole cellular response against dopamine or potential agonist/antagonist molecules. We herewith report the development of a novel functional assay concept for studying the in vitro interaction of the neurotransmitter dopamine with neural cells bearing dopamine receptors. The concept is based on the ultra-rapid measurement of changes in the electric properties of cultured N2a mouse neuroblastoma cells (corresponding to cumulative changes of the cell membrane potential). A close relationship between cumulative cell membrane potential and dopamine concentration was observed. Membrane depolarization was observed at nanomolar dopamine concentrations, while hyperpolarization was associated with micromolar ones. Treatment with the dopamine D2-receptor antagonist eticlopride resulted to a concentration-dependent membrane depolarization. Treatment with sodium chloride caused considerable weakening of the dopamine-associated hyperpolarization effect. The observed bioelectric response to dopamine was highly inversely correlated with the pattern of dopamine release-uptake balance by N2a cells, as determined with cyclic voltammetry. The bioelectric approach was also used to evaluate the dopaminergic activity of chaste tree (Vitex agnus-castus) extracts. The novel assay concept offers promising perspectives for the development of advanced companion diagnostics system for the high throughput, fast functional characterization of neurotransmitter agonists and antagonists.

Functional Characterization of a Novel Series of Biased Signaling Dopamine D3 Receptor Agonists

Dopamine receptors play an integral role in controlling brain physiology. Importantly, subtype selective agonists and antagonists of dopamine receptors with biased signaling properties have been successful in treating psychiatric disorders with a low incidence of side effects. To this end, we recently designed and developed SK609, a dopamine D3 receptor (D3R) selective agonist that has atypical signaling properties. SK609 has shown efficacy in reversing akinesia and reducing L-dopa-induced dyskinesia in a hemiparkinsonian rats. In the current study, we demonstrate that SK609 has high selectivity for D3R with no binding affinity on D2R high- or low-affinity state when tested at a concentration of 10 μM. In addition, SK609 and its analogues do not induce desensitization of D3R as determined by repeated agonist treatment response in phosphorylation of ERK1/2 functional assay. Most significantly, SK609 and its analogues preferentially signal through the G-protein-dependent pathway and do not recruit β-arrestin-2, suggesting a functional bias toward the G-protein-dependent pathway. Structure-activity relationship (SAR) studies using analogues of SK609 demonstrate that the molecules bind at the orthosteric site by maintaining the conserved salt bridge interactions with aspartate 110 on transmembrane 3 and aryl interactions with histidine 349 on transmembrane 6, in addition to several hydrophobic interactions with residues from transmembranes 5 and 6. The compounds follow a strict SAR with reference to the three pharmacophore elements: substituted phenyl ring, length of the linker connecting phenyl ring and amine group, and orientation and hydrophobic branching groups at the amine among SK609 analogues for efficacy and functional selectivity. These features of SK609 and the analogues suggest that biased signaling is an inherent property of this series of molecules.

PET radiotracer development for imaging high-affinity state of dopamine D2 and D3 receptors: Binding studies of fluorine-18 labeled aminotetralins in rodents

Imaging the high-affinity, functional state (HA) of dopamine D2 and D3 receptors has been pursued in PET imaging studies of various brain functions. We report further evaluation of ¹⁸ F-5-OH-FPPAT, and the newer ¹⁸ F-5-OH-FHXPAT and ¹⁸ F-7-OH-FHXPAT. Syntheses of ¹⁸ F-5-OH-FHXPAT and ¹⁸ F-7-OH-FHXPAT were improved by modifications of our previously reported procedures. Brain slices and brain homogenates from male Sprague-Dawley rats were used with the 3 radiotracers (74-111 kBq/cc). Competition with dopamine (1-100 nM) and Gpp(NH)p (10-50 µM) were carried out to demonstrate binding to dopamine D2 and D3 HA-states and binding kinetics of ¹⁸ F-5-OH-FPPAT measured. Ex vivo brain slice autoradiography was carried out on rats administered with ¹⁸ F-5-OH-FHXPAT to ascertain HA-state binding. PET/CT imaging in rats and wild type (WT) and D2 knock-out mice were carried out using ¹⁸ F-7-OH-FHXPAT (2-37 MBq). Striatum was clearly visualized by the three radiotracers in brain slices and dopamine displaced more than 80% of binding, with dissociation rate in homogenates of 2.2 × 10^-2 min^-1 for ¹⁸ F-5-OH-FPPAT. Treatment with Gpp(NH)p significantly reduced 50-80% striatal binding with faster dissociation rates (5.0 × 10^-2 min^-1 ), suggesting HA-state binding of ¹⁸ F-5-OH-FPPAT and ¹⁸ F-5-OH-FHXPAT. Striatal binding of ¹⁸ F-5-OH-FHXPAT in ex vivo brain slices were sensitive to Gpp(NH)p, suggesting HA-state binding in vivo. PET binding ratios of ¹⁸ F-7-OH-FHXPAT in rat brain were ventral striatum/cerebellum = 2.09 and dorsal striatum/cerebellum = 1.65; similar binding ratios were found in the D2 WT mice. These results suggest that in vivo PET measures of agonists in the brain at least in part reflect binding to the membrane-bound HA-state of the dopamine receptor.

Use of radiolabeled antagonist assays for assessing agonism at D2 and D3 dopamine receptors: comparison with functional GTPγS assays

BACKGROUND

Cell-based drug screening assays are essential tools for drug discovery and development targeting G protein-coupled receptors, which include dopamine D3 receptors. D3 is notorious for its poor coupling to G protein in most heterologous cell lines, and therefore D3 agonist-stimulated binding of [(35)S]GTPγS to G protein cannot be observed in many "non-functional" D3 expressing cell lines.

NEW METHOD

The present work explores the use of an alternate method for assessing agonist activity, consisting of measuring the difference in agonist competition between [(3)H]spiperone bound to low-affinity states of the receptor and that with radioligand bound to high-affinity states (GTP shift assay).

COMPARISON WITH EXISTING METHOD

The current study describes the determination of GTP shifts in [(3)H]spiperone binding assays for the assessment of agonists' potencies (at D2 and D3) and efficacies (at D3). Compared with GTPγ(35)S binding assays, the new method removes the cumbersome need of functional D3 cell lines and limited project duration due to short half-life of isotope (35)S.

CONCLUSION

The new method allows the estimation of potency (D2 and D3) and efficacy (D3) at the level of receptor and G protein activation in a simple fashion from shifts in monophasic-inhibition curves. Moreover, it does not require [(35)S]GTPγS binding assays with functional D3 cells. This method will have wide applicability for D3-selective agonist screening. It may also be useful for other GPCRs circumventing the need for functional assays and offering the ability to detect agonist activity regardless of the particular signaling pathway.

Pramipexole derivatives as potent and selective dopamine D(3) receptor agonists with improved human microsomal stability

Herein we report the synthesis and evaluation of a series of new pramipexole derivatives as highly potent and selective agonists of the dopamine-3 (D3 ) receptor. A number of these new compounds bind to the D3 receptor with sub-nanomolar affinity and show excellent selectivity (>10,000) for the D3 receptor over the D1 and D2 receptors. For example, compound 23 (N-(cis-3-(2-(((S)-2-amino-4,5,6,7-tetrahydrobenzo[d]thiazol-6-yl)(propyl)amino)ethyl)-3-hydroxycyclobutyl)-3-(5-methyl-1,2,4-oxadiazol-3-yl)benzamide) binds to the D3 receptor with a Ki value of 0.53 nM and shows a selectivity of >20,000 over the D2 and D1 receptors in the binding assays using a rat brain preparation. It has excellent stability in human liver microsomes. Moreover, in vitro functional assays showed it to be a full agonist for the human D3 receptor.

Agonist binding fraction of dopamine D2/3 receptors in rat brain: a quantitative autoradiographic study

There has arisen considerable interest in the study of dopamine D(2/3) agonist binding sites by positron emission tomography (PET), based on the claim that agonist sites represent a functional subset of the total number of sites labeled by more conventional antagonist ligands. To test the basis of this claim, we used quantitative autoradiography to measure the abundance of binding sites of a dopamine D(2/3) agonist ([(3)H]NPA) and an antagonist ([(3)H]raclopride) in cryosections of rat brain. Saturation binding studies revealed that the B(max) for [(3)H]NPA was nearly identical to that of [(3)H]raclopride in dorsal brain regions, but was 25% less in the ventral striatum and 56% less in the olfactory tubercle. We also tested the displacement of the two ligands by the hallucinogen LSD, which is known to have dopamine agonist properties. Whereas displacement of [(3)H]raclopride by increasing LSD concentrations was monophasic, displacement of [(3)H]NPA was biphasic, suggesting an action of LSD via a subset of dopamine D(2/3) agonist binding sites. Addition of the stable GTP analogue Gpp(NH)p to the medium abolished 90% of the [(3)H]NPA binding, and increased [(3)H]raclopride binding by 10%, with a shift to the right in the LSD competition curve, suggesting retention of endogenous dopamine in washed cryostat sections. Thus [(3)H]NPA and [(3)H]raclopride binding sites have nearly identical abundances in rat dorsal striatum, but are distinct in the ventral striatum, and with respect to their displacement by LSD.

[(3)H]4-(Dimethylamino)-N-[4-(4-(2-methoxyphenyl)piperazin- 1-yl)butyl]benzamide, a selective radioligand for dopamine D(3) receptors. I. In vitro characterization

4-(Dimethylamino)-N-(4-(4-(2-methoxyphenyl)piperazin-1-yl)butyl)benzamide (WC-10), a N-phenyl piperazine analog, has been shown to have high affinity and selectivity for dopamine D(3) receptors versus dopamine D(2) receptors (Chu et al. [2005] Bioorg Med Chem 13:77-87). In this study, WC-10 was radiolabeled with tritium (specific activity = 80 Ci/mmol) and [(3)H]WC-10 binding to genetically cloned dopamine D(2L) and D(3) receptors was evaluated in vitro. [(3)H]WC-10 binds with a 66-fold higher affinity to human HEK D(3) than HEK D(2L) receptors, with a dissociation constant (K(d)) of 1.2 nM at HEK D(3) receptors. However, [(3)H]WC-10 binds to rat Sf9 rD(3) receptors with a K(d) of 3.9 nM, a value that is 3-fold lower than binding to human HEK D(3) receptors and 40-fold value higher than binding to rat Sf9 rD(2L) receptors. The K(d) values obtained from saturation binding experiments were consistent with the results determined from kinetic (k(on) and k(off)) studies. The pharmacologic profiles of a series of dopaminergic drugs for inhibiting the binding of [(3)H]WC-10 to D(3) receptors was in agreement with previously reported data. In vitro autoradiography studies of rat and monkey brains show that [(3)H]WC-10 labeled D(3) sites in the striatal region.

Tonic autoinhibition contributes to the heterogeneity of evoked dopamine release in the rat striatum

Electrically evoked dopamine release as measured by voltammetry in the rat striatum is heterogeneous in both amplitude and temporal profile. Previous studies have attributed this heterogeneity to variations in the density of dopamine (DA) terminals at the recording site. We reach the alternate conclusion that the heterogeneity of evoked DA release derives from variations in the extent to which DA terminals are autoinhibited. We demonstrate that low-amplitude, slow evoked DA responses occur even though recording electrodes are close to DA terminals. Moreover, the D(2) agonist and antagonist, quinpirole and raclopride, respectively, affect the slow responses in a manner consistent with the known functions of pre-synaptic D(2) autoreceptors. Recording sites that exhibit autoinhibited responses are prevalent in the dorsal striatum. Autoinhibition preceded electrical stimulation, which is consistent with our prior reports that the striatum contains a tonic pool of extracellular DA at basal concentrations that exceed the affinity of D(2) receptors. We conclude that the striatum contains DA terminals operating on multiple time courses, determined at least in part by the local variation in autoinhibition. Thus, we provide direct, real-time observations of the functional consequence of tonic and phasic DAergic signaling in vivo.

Imaging the dopamine system with in vivo [11C]raclopride displacement studies: understanding the true mechanism

Measuring changes in dopamine (DA) levels in humans using radioligand-displacement studies and positron emission tomography (PET) has provided important empirical findings in diseases and normal neurophysiology. These studies are based on the assumption that DA exerts a competitive inhibition on D(2)-radioligand binding. However, the transfer of this hypothesis to a proven mechanism has not been fully achieved yet and an accumulating number of studies challenge it. In addition, new evidence suggests that DA exerts a noncompetitive inhibition on D(2)-radioligand binding under amphetamine conditions. This article reviews the theoretical basis for the DA competition hypothesis, the in vivo and in vitro evidences supporting a noncompetitive action of DA on D(2)-radioligand binding under amphetamine conditions, and discusses possible mechanisms underlying this noncompetitive interaction. Finally, we propose that such noncompetitive interactions may have important implications for how one interprets findings obtained from radioligand-displacement PET studies in neuropsychiatric diseases, especially in schizophrenia in which a dysregulation of the DA-promoted internalization of D(2) receptors was recently suggested.

Comparison of the binding distribution of agonist and antagonist ligands for histamine H3 receptors in pig brain by quantitative autoradiography

The relationship between the abundances of agonist and antagonist-binding sites for monoamine receptors is poorly established. Therefore, we used quantitative autoradiography to investigate the distribution and concentration of binding sites for histamine H(3) receptor ligands in cryostat sections of pig brain. As in other species, binding of the histamine H(3) receptor agonist [(3)H]N(alpha)-methylhistamine was highly heterogeneous in the pig brain, with highest B(max) in the substantia nigra, followed by the nucleus accumbens and caudate, intermediate binding in frontal cortex, diencephalon, and mesencephalon, and absent specific binding in cerebellum: the affinity of [(3)H]N(alpha)-methylhistamine was close to 1 nM in all regions of pig brain. Thus, the saturation binding parameters for this H(3) receptor agonist in pig brain were similar to the earlier reports in rat, guinea pig, and human. The distribution of histamine H(3) receptors labeled with the receptor antagonist [(125)I]iodophenpropit in adjacent cryostat sections from the same group of pigs was very similar to that of [(3)H]N(alpha)-methylhistamine. However, the B(max) of the receptor antagonist was 40% higher in the basal ganglia than was the B(max) of the receptor agonist. The K(d) for the receptor antagonist ligand was close to 0.9 nM in all regions. These results suggest that histamine H(3) receptor agonist-binding sites, i.e. those linked to intracellular G-protein, comprise a subset of the total receptor antagonist-binding sites in the basal ganglia, as has been reported for dopamine D(2) receptors.

Increased dopamine D2High receptors in amphetamine-sensitized rats, measured by the agonist [3H](+)PHNO

Repeated injections of amphetamine causes animals to become sensitized and supersensitive to DA. Previous work showed that the striata from such sensitized rats revealed a 3.5-fold increase in the density of D2(High) DA receptors, as measured by the guanine-nucleotide-sensitive component of [(3)H]raclopride binding. The present study was done to confirm these earlier findings by different methods and different ligands. The striata from amphetamine-sensitized rats showed an increase of 2.2-fold in the density of guanine-nucleotide-sensitive D2 receptors labeled by saturation experiments with [(3)H](+)PHNO. The proportion of D2(High) receptors was also found to increase 2.5-fold using the method of competition between DA and [(3)H]domperidone. The overall 2.2-3.5-fold increase of DA D2(High) receptors may explain why amphetamine-sensitized animals are much more sensitive to DA agonists, even though the total density of D2 receptors may apparently be unchanged or even decreased.

Autoradiographic mapping of dopamine-D2/D3 receptor stimulated [35S]GTPgammaS binding in the human brain

Agonist stimulated [35S]guanosine 5'-gamma-thiotriphosphate ([35S]GTPgammaS) binding autoradiography was established for the examination of dopamine-D2/D2 receptors in human brain sections. The distribution of G proteins activated by dopamine-D2/D3 receptors was studied in whole hemisphere cryosections. Dopamine stimulated [35S]GTPgammaS binding in brain regions with high densities of dopamine D2-like receptors, i.e. putamen (23 +/- 2%, mean +/- SEM,% stimulation over basal binding), caudate (20 +/- 0%) and substantia nigra (22 +/- 2%), but also in regions with lower receptor densities such as amygdala (17 +/- 8%), hippocampus (16 +/- 6%), anterior cingulate (13 +/- 3%), and thalamus (12 +/- 2%). Dopamine stimulated [35S]GTPgammaS binding to significantly higher levels in the dorsal than in the ventral part of the striatum. Dopamine caused low or very low stimulation in all cortical areas. Raclopride, a selective D2/D3 receptor antagonist, potently inhibited dopamine stimulated [35S]GTPgammaS binding, whereas R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (SCH23390), a selective D1 antagonist, did not block the [35S]GTPgammaS binding response stimulated by dopamine. Hence, the stimulatory effect of dopamine was primarily mediated by D2/D3 receptors. Quinpirole stimulated [35S]GTPgammaS binding in the same regions as dopamine. The maximal level of stimulation induced by dopamine and quinpirole was not significantly different. The present study demonstrates that agonist stimulated [35S]GTPgammaS binding autoradiography could be a suitable technique for the examination of dopamine-D2/D3 receptors in the human brain. This functional assay could provide useful new information about dopamine receptor/G protein coupling in the postmortem human brain, and reveal possible disease related alterations of the interaction between D2/D3 receptors and G proteins.

Dopamine receptor contribution to the action of PCP, LSD and ketamine psychotomimetics

Although phencyclidine and ketamine are used to model a hypoglutamate theory of schizophrenia, their selectivity for NMDA receptors has been questioned. To determine the affinities of phencyclidine, ketamine, dizocilpine and LSD for the functional high-affinity state of the dopamine D2 receptor, D2High, their dissociation constants (Ki) were obtained on [3H]domperidone binding to human cloned dopamine D2 receptors. Phencyclidine had a high affinity for D2High with a Ki of 2.7 nM, in contrast to its low affinity for the NMDA receptor, with a Ki of 313 nM, as labeled by [3H]dizocilpine on rat striatal tissue. Ketamine also had a high affinity for D2High with a Ki of 55 nM, an affinity higher than its 3100 nM Ki for the NMDA sites. Dizocilpine had a Ki of 0.3 nM at D2High, but a Kd of 1.8 nM at the NMDA receptor. LSD had a Ki of 2 nM at D2High. Because the psychotomimetics had higher potency at D2High than at the NMDA site, the psychotomimetic action of these drugs must have a major contribution from D2 agonism. Because these drugs have a combined action on both dopamine receptors and NMDA receptors, these drugs, when given in vivo, test a combined hyperdopamine and hypoglutamate theory of psychosis.

Measurement of the proportion of D2 receptors configured in state of high affinity for agonists in vivo: a positron emission tomography study using [11C]N-propyl-norapomorphine and [11C]raclopride in baboons

Dopamine D2 receptors are configured in interconvertible states of high (D(2 high)) or low (D(2 low)) affinity for agonists. The in vivo proportion of sites in high-affinity state remains poorly documented. Previous studies have established the D2 agonist [11C]N-propyl-norapomorphine (NPA) as a suitable positron emission tomography radiotracer for imaging D(2 high) in the living brain. To elucidate the proportion of D2 receptors configured in D(2 high) states in vivo, imaging studies were conducted in three baboons with both [11C]NPA and the D2 receptor antagonist [11C]raclopride. These studies were performed under noncarrier- and carrier-added conditions, to compare the Bmax of [11C]NPA and [11C]raclopride in the same animals. [11C]raclopride in vivo KD and Bmax were 1.59 +/- 0.28 nM (n = 3) and 27.3 +/- 3.9 nM (n = 3), respectively. The in vivo KD of [11C]NPA was 0.16 +/- 0.01 nM (n = 3), consistent with its affinity for D(2 high) reported in vitro. The maximal density of sites for [11C]NPA was 21.6 +/- 2.8 nM (n = 3), i.e., 79% of the [11C]raclopride Bmax. This result suggested that 79% of D2 receptors are configured as D(2 high) in vivo. This large proportion of D(2 high) sites might explain the vulnerability of D2 radiotracers to competition by endogenous dopamine, and is consistent with a previous report that the in vivo binding of agonist radiotracer [11C]NPA is more vulnerable to competition by endogenous dopamine than that of antagonist radiotracer [11C]raclopride.

The influence of G protein subtype on agonist action at D2 dopamine receptors

In previous studies, we have shown that agonists influence the ability of D2 dopamine receptors to couple to G proteins and here we extend this work. The human D2Short dopamine receptor and a natural polymorphism of this D(2Short)(Ser311Cys), have been studied by co-expressing the receptors in insect cells with Gbeta1gamma2 and either Galpha(o), Galpha(i1), Galpha(i2) or Galpha(i3) G protein subunits. These preparations have been used to study the G protein coupling profiles of the two receptors and the influence of agonists. Receptor/G protein coupling was analysed in dopamine/[3H]spiperone competition binding experiments and through stimulation of [35S]GTPgammaS binding. Although the Ser311Cys polymorphism itself had no appreciable effect on the G protein coupling specificity of the D2 receptor, agonist stimulation of [35S]GTPgammaS binding, revealed that both dopamine and (+)-3PPP showed a clear preference for Galpha(o) compared to the Galpha(i) subtypes, but quinpirole did not. These results indicate that agonists are able to stabilise different receptor conformations with different abilities to couple to G proteins.

Binding characteristics of high-affinity dopamine D2/D3 receptor agonists, 11C-PPHT and 11C-ZYY-339 in rodents and imaging in non-human primates by PET

We have evaluated the in vitro autoradiographic binding characteristics and in vivo brain distribution of two high-affinity dopamine D2/D3 receptor agonists, (+/-)-2-(N-phenethyl-N-1'-11C-propyl)amino-5-hydroxytetralin (11C-PPHT) and (+/-)-2-(N-cyclohexylethyl-N-1'-11C-propyl)amino-5-hydroxytetralin (11C-ZYY-339) in rodents and in monkeys using positron emission tomography (PET). In vitro autoradiograms in rat brain slices with (11)C-PPHT and 11C-ZYY-339 revealed binding to dopaminergic regions in the striata, which was substantially (>90%) displaced by 10 microM sulpiride. Striatal binding was also removed in the presence of 5-guanylylimidophosphate (Gpp(NH)p), indicative of binding of these radiotracers to the high-affinity (HA) state. The results of in vivo studies in rats exhibited binding of the two radiotracers to the striata (striata/cerebellum approached 2 in 30 min). The regional selectivity to the striata was reduced by preadministration of haloperidol. PET studies in male rhesus monkeys using an ECAT EXACT HR+ scanner indicated localization of 11C-PPHT and 11C-ZYY-339 in the striata and thalamus. Striata to cerebellum and thalamus to cerebellum ratios were low (1.5 and 1.3, respectively, at 30 min postinjection) for both 11C-PPHT and 11C-ZYY-339, apparently due to the slower nonspecific clearance from cerebellum. These findings with 11C-PPHT and 11C-ZYY-339 indicate the possibility of in vivo imaging of high-affinity state of dopamine D2/D3 receptors in both the striata and the thalamus.

Signaling Mechanisms of the D3Dopamine Receptor

A substantial body of evidence shows the capacity of the dopamine D3 receptor to couple functionally to G proteins when expressed in an appropriate milieu in heterologous expression systems. In these systems, activation of D3 receptors inhibits adenylate cyclase, modulates ion flow through potassium and calcium channels, and activates kinases, most notably mitogen-activated protein kinase. Coupling to Gi/Go is implicated in many of these effects, but other G proteins may contribute. Studies with chimeric receptors implicate the third intracellular loop in the mediation of agonist-induced signal transduction. Finally, D3-preferring drugs modulate expression of c-fos in neuronal cultures and brain. Signaling mechanisms of the D3 receptor in brain, however, remain to be definitively determined.

Alcohol-withdrawn animals have a prolonged increase in dopamine D2high receptors, reversed by general anesthesia: relation to relapse?

The biochemical basis for alcohol addiction and relapse is not known. Although ethanol promotes the release of dopamine like other drugs of abuse, many unknown factors remain to be investigated concerning the biochemical abnormalities which persist after ethanol drinking and which contribute to alcohol relapse. Although ethanol withdrawal is associated with enhanced sensitivity to dopamine in animals and humans, only minor changes in the striatal density of dopamine D2 receptors have been found in humans, and animals show a small reduction in striatal D2 receptors. But how can dopamine-related functions be increased in ethanol withdrawal in the face of an unchanged or reduced density of dopamine D2 receptors? Considering that ethanol sensitizes rats to amphetamine, and that the high-affinity state of D2, or D2High, is markedly increased in striata from amphetamine-sensitized rats, we measured the density of D2High in striata from rats withdrawn from ethanol. These sites were elevated by 360% (7.2 pmol/g) for at least 8 days after stopping ethanol and returned to normal levels of 2 pmol/g after 2 weeks of ethanol withdrawal. In addition, 1 h of deep general anesthesia given 5 days into withdrawal resulted in a normal level of D2High within 24 h. Because the D2High states are the functional form of D2, their elevated density in ethanol withdrawal may be related to ethanol relapse in humans. General anesthesia may alleviate aspects of alcohol or amphetamine abuse or psychosis associated with elevated D2High.

Synthesis and biological evaluation of the binding of dopamine D2/D3 receptor agonist, (R,S)-5-hydroxy-2-(N-propyl-N-(5′-18F-fluoropentyl)aminotetralin (18F-5-OH-FPPAT) in rodents and nonhuman primates

We have synthesized a new fluorinated dopamine D2 receptor agonist, (R,S)-2-(N-propyl-N-5'-fluoropentyl)amino-5-hydroxytetralin (5-OH-FPPAT). The radiosynthesis of the fluorine-18 analog, (18)F-5-OH-FPPAT was achieved in decay corrected yields of 10 to 15% in specific activities of approx. 1.5 to 2 Ci/micromol. In vitro binding and autoradiographic studies of this new radiotracer have been investigated. Using rat striatal homogenate binding assay, 5-OH-FPPAT exhibited an affinity of IC(50) = 6.95 nM. The octanol-buffer partition coefficient, Log P was found to be 1.60. In vitro autoradiographs in rat brain slices with (18)F-5-OH-FPPAT revealed selective binding to the dopaminergic regions in the striata that was displaceable by sulpiride. This selective binding to the striata was also removed in the presence of the GTP analog, 5'-guanylylimidodiphosphate, indicative of predominant binding of (18)F-5-OH-FPPAT to the high-affinity state of the D2 receptor. In vivo regional distribution of (18)F-5-OH-FPPAT in rat brains revealed selective localization in the striata with striata/cortex ratio of 1.5 and striata/cerebellum ratio of 1.8 to 2.0. The binding of (18)F-5-OH-FPPAT in the striata was reduced upon pretreatment with the antagonist, risperidone and the agonist, PPHT. A PET study in rhesus monkeys showed selective localization of (18)F-5-OH-FPPAT in the striata and the ratio between striata and cerebellum approached approximately 2 at 40 min post-injection.

In vivo vulnerability to competition by endogenous dopamine: Comparison of the D2 receptor agonist radiotracer (-)-N-[11C]propyl-norapomorphine ([11C]NPA) with the D2 receptor antagonist radiotracer [11C]-raclopride

(-)-N-Propyl-norapomorphine (NPA) is a full dopamine (DA) D2 receptor agonist and [11C]NPA is a suitable radiotracer to image D2 receptors configured in a state of high affinity for agonists with positron emission tomography (PET). In this study the vulnerability of the in vivo binding of [11C]NPA to acute fluctuation in synaptic DA was assessed with PET in baboons and compared to that of the reference D2 receptor antagonist radiotracer [11C]raclopride. Three male baboons were studied with [11C]raclopride and [11C]NPA under baseline conditions and following administration of the potent DA releaser amphetamine (0.3, 0.5, and 1.0 mg kg(-1) i.v.). Kinetic modeling with an arterial input function was used to derive the striatal specific-to-nonspecific equilibrium partition coefficient (V3"). [11C]Raclopride V3" was reduced by 24 +/- 10%, 32 +/- 6%, and 44 +/- 9% following amphetamine doses of 0.3, 0.5, and 1.0 mg kg(-1), respectively. [11C]NPA V3" was reduced by 32 +/- 2%, 45 +/- 3%, and 53 +/- 9% following amphetamine doses of 0.3, 0.5, and 1.0 mg kg(-1), respectively. Thus, endogenous DA was more effective at competing with [11C]NPA binding compared to [11C]raclopride binding, a finding consistent with the pharmacology of these tracers (agonist vs. antagonist). These results also suggest that 71% of D2 receptors are configured in a state of high affinity for agonists in vivo. In conclusion, [11C]NPA might provide a superior radiotracer to probe presynaptic DA function with PET in health and disease.

Link between dopamine D1 and D2 receptors in rat and human striatal tissues

Many psychomotor behaviors depend on an interaction between dopamine D(1) and D(2) receptors. This study tested the hypothesis that agonist stimulation of dopamine D(1) receptors leads to the conversion of D(2) receptors from a state of high affinity for dopamine into a state of low affinity for dopamine. To test this hypothesis, dopamine was competed against [(3)H]raclopride for binding to rat and human striatal homogenates. Although the detection of high-affinity states of the dopamine D(2) receptor in rat or postmortem human striatum is normally difficult because the proportions of such sites are very low in the presence of physiological concentrations of sodium ions, the present work found that in the presence of 100 nM SCH 23390 to block D(1) receptors, a significant proportion of D(2) high-affinity sites were unmasked and readily revealed to be 10-35% of the D(2) population, illustrating the presence of a strong D(1)-D(2) link in both rat and human striata.

Amphetamine-sensitized animals show a marked increase in dopamine D2 high receptors occupied by endogenous dopamine, even in the absence of acute challenges

While a range of dopamine D(2)-related behaviors are exaggerated in amphetamine-sensitized animals, studies of the dopamine D(2) receptor have reported either no change or a decrease in dopamine D(2) receptor density--especially when measured using radioraclopride. We hypothesized that a decrease in D(2) receptors may actually be "apparent" and that these receptors may still be present, but are noncompetitively "occupied" by endogenous dopamine. Animals sensitized to amphetamine (and their saline controls) were examined 4 weeks after their last injection. We first measured the [(3)H]raclopride binding in vivo, and observed that sensitized animals showed a 29% lower level of raclopride binding in vivo, suggesting an apparently lower level of dopamine D(2) receptors. To assess the reason for this we examined the density of receptors (using Scatchard analysis in vitro) measured by [(3)H]raclopride in the presence and absence of guanilylimidodiphosphate. This guanine nucleotide converts the dopamine-bound high-affinity state of D(2) receptors into low-affinity states, thereby making measurable the absolute density of the sites. As previously reported, the amphetamine-sensitized animals showed a 31% lower number of D(2) receptors in conventional binding (B(max) 15.6 vs. 22.7 pmol/g). However, with the addition of guanilylimidodiphosphate there was an equalization of both groups (B(max) 25.9 vs. 25.6 pmol/g), revealing an additional 10.3 pmol/g in the sensitized animals, but only 2.9 pmol/g in saline controls. There were no changes in the dissociation constant of [(3)H]raclopride for the receptors. The nearly four-fold increase of dopamine D(2) receptors in the high-affinity state occupied by dopamine may explain why amphetamine-sensitized animals show almost an order of magnitude greater response to dopamine-releasing challenges or dopamine agonists, even though the absolute receptor number is unchanged and the "apparent" receptor number is decreased.

Novel drug interactions at D(2) dopamine receptors: modulation of [3H]quinpirole binding by monoamine oxidase inhibitors

D(2) dopamine receptors are the principal target of drugs used to treat schizophrenia and Parkinson's disease. Recent findings suggest novel drug interactions at D(2) receptors, specifically interactions of monoamine oxidase inhibitors (MAOIs) at a novel binding site that modulates the binding of [3H]quinpirole to the D(2) receptor. That MAOIs inhibit [3H]quinpirole binding challenges the traditional understanding of ligand interactions at dopamine receptors and may shed light on the mechanism of behavioral sensitization to psychostimulants and the pharmacology and toxicity of MAOIs.

(-)-N-[(11)C]propyl-norapomorphine: a positron-labeled dopamine agonist for PET imaging of D(2) receptors

Imaging neuroreceptors with radiolabeled agonists might provide valuable information on the in vivo agonist affinity states of receptors of interest. We report here the radiosynthesis, biodistribution in rodents, and imaging studies in baboons of [(11)C]-labeled (-)-N-propyl-norapomorphine [(-)-NPA]. (-)-[(11)C]NPA was prepared by reacting norapomorphine with [(11)C]propionyl chloride and a lithium aluminum hydride reduction. [(11)C]Propionyl chloride was prepared by reacting [(11)C]CO(2) with ethylmagnesium bromide, followed by reacting with phthaloyl chloride. The radiochemical yield of (-)-[(11)C]NPA was 2.5% at end of synthesis (EOS), and the synthesis time was 60 min. The specific activity was 1700+/-1900 mCi/micromol ( N=7; ranged 110-5200 mCi/micromol at EOS). Rodent biodistribution studies showed high uptake of [(11)C](-)-NPA in D(2) receptor-rich areas, and the striatum/cerebellum ratios were 1.7, 3.4, and 4.4 at 5 min, 30 min, and 60 min postinjection, respectively. Pretreating the animals with haloperidol (1 mg/kg) decreased the striatum/cerebellum ratio at 30 min postinjection to 1.3. (-)-[(11)C]NPA was also evaluated via baboon positron emission tomography (PET) studies. Under control conditions ( N=4), rapid uptake of the tracer was observed and the striatum/cerebellum ratio reached 2.86+/-0.15 at 45 min postinjection. Following haloperidol pretreatment (0.2 mg/kg IV), the striatum/cerebellum ratio was 1.29 at 45 min postinjection. The result demonstrated the existence of specific binding of this new tracer to the D(2) receptor. To our knowledge, the current finding of a striatum/cerebellum ratio of 2.8 in baboon was the highest reported with a radiolabeled D(2) agonist. (-)-[(11)C]NPA is a promising new D(2) agonist PET tracer for probing D(2) receptors in vivo using PET.

In vitro and in vivo evaluation of the binding of the dopamine D2 receptor agonist (11)C-(R,S)-5-hydroxy-2-(di-n-propylamino)tetralin in rodents and nonhuman primate

The in vitro autoradiographic binding characteristics as well as in vivo imaging characteristics of a dopamine D2 receptor agonist, (R, S)-2-(N-propyl-N-1'-(11)C-propyl)amino-5-hydroxytetralin ((11)C-5-OH-DPAT), were studied. In (3)H-spiperone assays using rat striata, 5-OH-DPAT exhibited an affinity of IC(50) = 2.5 nM. In vitro autoradiographs in rat brain slices with (11)C-5-OH-DPAT revealed selective binding to the dopaminergic regions in the striata which was displaceable by sulpiride. Varying concentrations of dopamine displaced this selective binding of (11)C-5-OH-DPAT to the striata in rat brain slices. This selective binding to the striata was also removed in the presence of the GTP analog, 5'-guanylylimidodiphosphate, indicative of the binding of (11)C-5-OH-DPAT to the high-affinity state of the D2 receptor. Ex vivo autoradiographic study in rats exhibited selective binding of (11)C-5-OH-DPAT to the striata. A PET study in a rhesus monkey showed selective localization of (11)C-5-OH-DPAT in the striata and the ratio between striata and cerebellum approached approximately 2 at 40 min postinjection.

Inhibitory control of the GABAergic transmission in the rat neostriatum by D2 dopamine receptors

The aim of the study was to determine the role of dopamine on the GABAergic input to striatal projection neurons. Accordingly, the effect of the activation of dopamine D2-like receptors on GABA-mediated depolarizing postsynaptic potentials evoked in striatal slices by local stimulation was studied. Conventional intracellular recording techniques were used to record the synaptic responses. The experiments were done in the presence of 6-cyano-7-nitroquinoxaline-2,3-dione (20 microM) and (+)-2-amino-5-phosphonovaleric acid (40 microM) to block the participation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate/kainate and N-methyl-D-aspartate receptors in the synaptic response. The GABAergic nature of the response was assessed by its potentiation by pentobarbital (50 microM) and by its elimination by bicuculline or picrotoxin. At 100 nM, a concentration already maximal, dopamine inhibited by 55% the GABAergic synaptic response. The inhibitory effect was totally blocked by the selective antagonist of D2-like receptors, sulpiride (100 nM). The dopamine inhibition was observed only in one-third of the studied neurons and was concentration dependent (IC50 = 14 nM). The inhibition was not associated with changes in the input resistance or any other membrane property. In addition, dopamine (50 nM) reduced the frequency but not the amplitude of spontaneous, bicuculline-sensitive depolarizing postsynaptic potentials. The D2-like receptor agonist quinpirole also dose-dependently (IC50 = 10 nM) inhibited the GABAergic synaptic response. As with dopamine, the inhibition did not change the membrane properties of the studied neurons. In addition, the quinpirole induced inhibition of the GABA response was accompanied by increased paired-pulse facilitation. The results indicate that D2-like receptors located on intrinsic GABAergic terminals in the rat striatum exert an inhibitory control of the GABAergic input to striatal projection neurons. The dopaminergic effect would be translated in facilitation of the firing of the neurons upon the arrival of the cortical input.

Radiosynthesis and in vitro evaluation of 2-(N-alkyl-N-1'-11C-propyl)amino-5-hydroxytetralin analogs as high affinity agonists for dopamine D-2 receptors

We have developed radiotracers based on agonists that may potentially allow the in vivo assessment of the high affinity (HA) state of the dopamine D-2 receptors. The population of HA state, which is likely the functional state of the receptor, may be altered in certain diseases. We carried out radiosyntheses and evaluated the binding affinities, lipophilicity, and in vitro autoradiographic binding characteristics of three dopamine D-2 receptor agonists: (+/-)-2-(N,N-dipropyl)amino-5-hydroxytetralin (5-OH-DPAT), (+/-)-2-(N-phenethyl-N-propyl)amino-5-hydroxytetralin (PPHT), and (+/-)-2-(N-cyclohexylethyl-N-propyl)amino-5-hydroxytetralin (ZYY-339). In 3H-spiperone assays using rat striata, ZYY-339 exhibited subnanomolar affinity for D-2 receptor sites (IC50 = 0.010 nM), PPHT was somewhat weaker (IC50 = 0.65 nM), and 5-OH-DPAT exhibited the weakest affinity (IC50 = 2.5 nM) of the three compounds. Radiosynthesis of these derivatives, 2-(N-propyl-N-1'-11C-propyl)amino-5-hydroxytetralin (11C-5-OH-DPAT), 2-(N-phenethyl-N-1'-11C-propyl)amino-5-hydroxytetralin (11C-PPHT), and 2-(N-cyclohexylethyl-N-1'-11C-propyl)amino-5-hydroxytetralin (11C-ZYY-339) was achieved by first synthesizing 11C-1-propionyl chloride and subsequent coupling with the appropriate secondary amine precursor to form the respective amide, which was then reduced to provide the desired tertiary amine products. The final products were obtained by reverse-phase high performance liquid chromatography (HPLC) purification in radiochemical yields of 5-10% after 60-75 min from the end of 11CO2 trapping and with specific activities in the range of 250-1,000 Ci/mmol. In vitro autoradiographs in rat brain slices with 11C-5-OH-DPAT, 11C-PPHT, and 11C-ZYY-339 revealed selective binding of the three radiotracers to the dopamine D-2 receptors in the striata.

In vitro autoradiography of GTPgamma[35S] binding at activated NPY receptor subtypes in adult rat brain

Guanylyl 5'-[gamma[35S]thio]-triphosphate (GTPgamma[35S]) binding to NPY receptor-activated G-proteins was measured in adult rat brain sections in order to determine the neuroanatomical distribution of NPY receptor subtypes. Using the pharmacological specificity of the NPY receptor subtypes, differential stimulation of GTPgamma[] binding by subtype-specific agonists was used to demonstrate the differential distribution of these subtypes in rat brain. Treatment of rat brain slices with selective agonists for the NPY receptor subtypes in the presence of 2000 microM GDP was used to discriminate populations of NPY receptor subtypes. Activation of a NPY Y1 receptor subtype by human [Leu31Pro34]NPY stimulated GTPgamma[35S] binding in the rank order: frontal cortex>dentate gyrus>inferior colliculus>/=thalamus>hypothalamus. In contrast, NPY Y2/Y5 peptide agonist, human PYY(3-36), stimulated GTPgamma[35S] binding in the rank order: hypothalamus>substantia nigra>hippocampus>frontal cortex>/=inferior colliculus. Stimulation of NPY Y5 receptor subtypes by a NPY Y5 selective agonist, rat/human D-Trp, was shown to stimulate GTPgamma[35S] binding in the hypothalamus and discrete nuclei of the thalamus. Little GTPgamma[35S] binding in the dentate gyrus, frontal cortex, or inferior colliculus was measured following stimulation with D-Trp. Stimulation of GTPgamma[35S] binding by [Leu31Pro34]NPY, but not by the other NPY receptor agonists, was blocked by the selective NPY Y1 receptor antagonist, BIBP 3226. In conclusion, functional coupling at NPY receptor subtypes can be shown in rat brain and populations of NPY receptor subtypes can be anatomically discriminated by NPY agonist stimulation of GTPgamma[35S] binding in rat brain.

Modulation of [3H]quinpirole binding to dopaminergic receptors by adenosine A2A receptors

Alteration of ligand binding to dopamine D2 receptors through activation of adenosine A2A receptors in rat striatal membranes has been studied by means of kinetic analysis. The binding of dopaminergic agonist [3H]quinpirole to rat striatal membranes was characterized by the constants Kd = 1.50+/-0.09 nM and Bmax = 115+/-2 fmol/mg of protein. The kinetic analyses revealed that the binding had at least two consecutive and kinetically distinguishable steps, the fast equilibrium of complex formation between receptor and agonist (KA = 5.9+/-1.7 nM), followed by a slow isomerization equilibrium (Ki = 0.06). Activation of adenosine A2A receptors by CGS 21680 caused enhancement of the rate [3H]quinpirole binding, altering mainly the formation of the receptor-ligand complexes (KA) as well as the isomerization rate of this complexes (ki), while the deisomerization rate (k[-i]) and the apparent dissociation rate remained unchanged.

Interaction of the novel antipsychotic drug amperozide and its metabolite FG5620 with central nervous system receptors and monoamine uptake sites: relation to behavioral and clinical effects

Behavioral, biochemical, and electrophysiological studies suggest that amperozide affects mesolimbic and mesocortical dopamine neurotransmission. The receptor binding profile of amperozide is discussed and related to behavioral and clinical, i.e., antipsychotic, effects of the drug. As previously reported, amperozide displayed high affinity for serotonin 5-HT2A receptors (Ki = 16 nmol/L), and moderate affinity for striatal dopamine D2 (Ki = 540 nmol/L) and cortical alpha 1-adrenergic receptors (Ki = 172 nmol/L). In the present study amperozide displayed low affinity for several serotonin receptor subtypes as well as for the dopamine D4 receptor transfected in COS7 cells (Ki D4.2 = 769 nmol/L and Ki D4.4 = 384 nmol/L). Amperozide was very weak or did not interact with several other receptor species including adrenergic, histaminergic, muscarinic, benzodiazepine, gamma-aminobutyric acid, amino acid, opiate, and Ca channels; however, amperozide was found to compete for [3H]paroxetine binding for the serotonin transporter in the nanomolar range (Ki = 49 nmol/L). In vitro and in vivo binding potency of amperozide correlates best with behavioral effects, indicating 5-HT2A antagonism, although serotonin uptake inhibition may contribute to the effects of amperozide on dopamine neurotransmission. The metabolite of amperozide, FG5620, displayed 5-10 times lower pharmacologic activity than amperozide. These properties of amperozide may suggest that the antipsychotic effects of amperozide are mediated by 5-HT2A receptors, although 5-HT uptake inhibition and alpha 1-adrenergic receptor-mediated effects may be considered, particularly at higher doses.

Sensitization of the striatal dopaminergic system induced by chronic administration of a glutamate antagonist in the rat

The aim of the present study was to assess in the rat the pharmacological, biochemical and molecular (including in situ hybridization) consequences in the striatum of a prolonged (50 days) treatment with dizocilpine maleate (MK-801), an N-methyl-D-aspartate (NMDA) antagonist. We observed a sensitization-like effect characterized by a behavioural hyperresponsiveness to an acute injection of haloperidol (0.25 mg/kg), a dopaminergic antagonist. In rats chronically treated with MK-801, this hyperresponsiveness was associated with an increased D2 receptor (D2R) density in the striatum. At the transcriptional level, the D2R mRNA was also enhanced in the striatum. Quantitative in situ hybridization studies revealed that the number of neurons expressing the D2R mRNA was significantly enhanced in treated rats, whereas the mean amount of message per cell was unchanged. These changes could represent the neurobiological substrate of the observed sensitization. These results suggest that the D2R gene is under glutamate control via NMDA receptor in striatal neurons.

Characterization of [125I]S(-)5-OH-PIPAT binding to dopamine D2-like receptors expressed in cell lines

Recently, [125I]S(-)5-OH-PIPAT [5-hydroxy-2-(N-n-propyl-N-3'-iodo-2'-propenyl)aminotetralin], a derivative of S(-)5-OH-DPAT (5-hydroxy-N,N-di-n-propyl-aminotetralin), was reported to be a better radioiodinated dopamine D2-like receptor ligand than the previously reported iodinated ligand, [125I]R(+)7-OH-PIPAT. Therefore, in the present study, the binding profile of [125I]S(-)5-OH-PIPAT to D2-like receptors expressed in cell lines was established. High binding affinity (Kd = 0.3-0.4 nM) and NaCl sensitivity were displayed with this ligand in membranes of human embryonic kidney (HEK293) cells expressing either human D2 or rat D3 receptors and in Chinese hamster ovary (CHO) cells expressing human dopamine D4 receptors. Specific binding to D2 and D4 receptors was significantly increased in the presence of 2 mM MgCl2 and decreased in the presence of 100 microM 5'-guanylyl-imidodiphosphate (GMP-PNP). This finding is consistent with reports that 2-aminotetralin compounds display agonist properties. The specific binding to D3 receptors however, was not affected by either MgCl2 or GMP-PNP. This lack of GMP-PNP sensitivity for D3 receptors may result from inadequate G protein-receptor coupling in this cell line. The rank order of potency for inhibition of [125I]S(-)5-OH-PIPAT binding with various dopamine agents was consistent with reported values for D2, D3 and D4 receptors. In membranes prepared from Spodoptera frugiperda (Sf9) cells infected with baculovirus that contains DNA encoding D3 receptors, [125I]S(-)5-OH-PIPAT recognized only 70% of the receptor population labeled by [125I]NCQ298. This new ligand offers several unique advantages, including high specific activity, high binding affinity and selectivity for D2-like receptors, that make it an excellent probe for the investigation and the characterization of dopamine D2-like receptors.

DAD1- and DAD2-like agonist effects on motor activity of C57 mice: differences compared to rats

Studies on rats indicate that DAD1- and DAD2-like agonists produce a biphasic action on motor activity, with low doses reducing activity below control levels, and higher doses initially reducing, then elevating, activity for a prolonged period. Although some of the reported effects of DAD1- and DAD2-like receptor agonists on motor activity of mice are consistent with their effects on rats, the possibility of species differences is also apparent. In the current study the effects of DAD1- and DAD2-like agonists on motor activity of C57BL/6 (C57) mice were determined to establish species consistencies and differences with respect to their effects on rats. The partial DAD1-like agonist SKF 38393 reduced the activity of C57 mice at low doses and elevated activity above control levels at higher doses, if the mice were thoroughly habituated to the test chamber. The full DAD1 agonist SKF 82958 also increased the activity of C57 mice, and along with the SKF 38393 results indicates a response to DAD1 receptor stimulation similar to that reported for rats. In contrast to the species similarity in response to DAD1 stimulation, the DAD2-like agonist quinpirole produced only a dose-responsive monotonic reduction in the activity of C57 mice, whether the animals were nonhabituated or well-habituated to the testing environment, male or female, young or mid-aged, injected intraperitoneally (i.p.) or subcutaneously (s.c.), and with either low or high doses. This apparent species difference in response to quinpirole might reflect distinguishable functional properties of the DA subreceptor systems.

3-[[(4-Aryl-1-piperazinyl)alkyl]cyclohexyl]-1H-indoles as dopamine D2 partial agonists and autoreceptor agonists

A series of arylpiperazines and tetrahydropyridines joined to indoles by semirigid cycloalkyl spacers were prepared. Target compounds were studied for their ability to bind to the DA D2 receptor in vitro and to inhibit dopamine synthesis and spontaneous locomotor activity in rats. Effects of tether length and relative stereochemistry were assessed for a series of 2-pyridylpiperazines. The cyclohexylethyl spacer was found to be the most active in the series. Further studies explored effects of changes in the arylpiperazine and indole portions of the molecule. From these studies trans-2-[[4-(1H-3-indolyl)cyclohexyl]ethyl]-4- (2-pyridinyl)piperazine (30a) was selected for further evaluation. It was characterized as a partial agonist of DA D2 receptors in vitro and caused decreases in dopamine synthesis and release as well as dopamine neuronal firing. Compound 30a was shown to be devoid of behavioral effects associated with postsynaptic DA D2 receptor activation. Furthermore, compound 30a was shown both to decrease DA synthesis and to inhibit Sidman avoidance responding in squirrel monkeys. These findings suggest that DA D2 partial agonists with the appropriate level of intrinsic activity can act to decrease dopamine synthesis and release and may have potential utility as antipsychotic agents.

In vivo reconstitution of dopamine D2S receptor-mediated G protein activation in baculovirus-infected insect cells: preferred coupling to Gi1 versus Gi2

Agonist binding of the human D2S receptor overexpressed in baculovirus-infected Sf9 insect cells was of low affinity and GppNHp-insensitive, yet, dopaminergic agonists were able to partly inhibit forskolin-stimulated cAMP accumulation. In order to prove full functionality of the receptor, we used an "in vivo" reconstitution system, which is based on coinfection of Sf9 cells with the appropriate receptor and G protein encoding baculoviruses. In cells coexpressing the D2S receptor and either Gi1 or Gi2, the dopaminergic agonist apomorphine effectively stimulated [35S]GTP gamma S binding and GTPase activity. Agonist-stimulated [35S]GTP gamma S binding was dependent on the ratio of G protein to receptor. Expression levels of receptor and G protein influenced each other reciprocally. G protein activation could be optimized by varying the multiplicity of infection of the receptor and G protein encoding baculoviruses. Coexpression of either Gi1 or Gi2 led to the appearance of GppNHp-sensitive high-affinity agonist binding. Detailed agonist competition binding analysis revealed that the percentage of high-affinity agonist binding sites was significantly higher in D2S receptor-expressing cells coinfected with Gi1 viruses than when coinfected with Gi2 viruses. Moreover, the coexpressed Gi proteins seemed to modulate the affinity of agonists for the high-affinity form of the receptor. In cells coexpressing Gi1, agonist high affinity was 2-4-fold higher than in cells coexpressing Gi2. Na+ increased the dissociation constant of apomorphine for the high-affinity site by 2-4-fold without affecting the percentage of high-affinity sites or the preference for Gi1. In some dopamine competition experiments with coinfected cells, displacement data were best fit assuming three noninteracting classes of sites in the absence and two independent classes of sites in the presence of GppNHp. Dopamine competition curves with cells highly overexpressing the D2S receptor or with membranes from such cells were best fit assuming two independent classes of sites which were insensitive to GppNHp and might reflect abnormal compartimentalization and/or different states of aggregation.

Aryl 1-but-3-ynyl-4-phenyl-1,2,3,6-tetrahydropyridines as potential antipsychotic agents: synthesis and structure-activity relationships

A novel series of aryl 1-but-3-ynyl-4-phenyl-1,2,3,6-tetrahydropyridines with dopaminergic activity is described. The structure-activity relationships of this series were studied by synthesis of analogs and evaluation of their affinities for the dopamine (DA) D2 receptor and inhibition of locomotor activity (LMA) in rodents. The basic amine, alkyne chain length, and aryl groups were varied. Compounds having a 4-phenyl-1,2,3,6-tetrahydropyridine and an aryl group with hydrogen-bonding substituents separated by a butynyl chain were found to have the most potent dopaminergic activity. Several compounds that were found to have exceptional in vivo activity in LMA inhibition in rodents were evaluated for additional pharmacological activity including binding affinities for other DA receptor subtypes as well as effects on brain DA synthesis, DA neuronal firing, and conditioned avoidance responding in squirrel monkeys.

Characterization of binding of [3H]PD 128907, a selective dopamine D3 receptor agonist ligand, to CHO-K1 cells

PD 128907 [4a R, 10 b R-(+)-trans-3, 4, 4a, 10 b - tetrahydro - 4- n -propyl2 H,5H-[1]benzop-yrano[4,3-b]1,4-oxazin-9-ol.], a selective dopamine (DA) D3 receptor agonist ligand exhibits about a 1000-fold selectivity for human D3 receptors (Ki, 1 nM) versus human D2 receptors (Ki, 1183 nM) and a 10000-fold selectivity versus human D4 receptors (Ki, 7000 nM) using [3H]spiperone as the radioligand in CHO-K1-cells. Studies with [3H]PD 128907, showed saturable, high affinity binding to human D3 receptors expressed in CHO-K1 cells (CHO-K1-D3) with an equilibrium dissociation constant (Kd) of 0.99 nM and a binding density (Bmax) of 475 fmol/mg protein. Under the same conditions, there was no significant specific binding in CHO-K1-cells expressing human D2 receptors (CHO-K1-D2). The rank order of potency for inhibition of [3H]PD 128907 binding with reference DA agents was consistent with reported values for D3 receptors. These results indicate that [3H]PD 128907 is a new, highly selective D3 receptor ligand with high specific activity, high specific binding and low non-specific binding and therefore should be useful for further characterizing the DA D3 receptors.

Induction of c-fos, jun B and egr-1 expression by haloperidol in PC12 cells: involvement of calcium

Acute injection of haloperidol, a dopamine D2 receptor antagonist, is known to increase immediate early gene expression of the fos and jun families in rodent striatal neurons. A set of gene induction, including c-fos, jun B and TIS8/egr-1, was found when haloperidol was added to PC12 cells in culture. Electrophoretic mobility-shift assays show that haloperidol-evoked gene induction was accompanied by a transient and dose-dependent increase in AP1 and EGR-1 binding activities in these cells. Gene expression is tentatively explained by the rapid and transient increase in cytosolic free Ca2+ concentration observed upon haloperidol addition. The cytosolic calcium rise and AP1 binding activation elicited by haloperidol were dependent on extracellular Ca2+, suggesting that haloperidol exerted its effects by promoting Ca2+ entry into PC12 cells. The haloperidol-induced increase in AP1 binding activity and intracellular Ca2+ was not reproduced by two other dopamine D2 receptor antagonists, sulpiride and (+)-butaclamol.

Differential effects of [3H]nemonapride and [3H]spiperone binding on human dopamine D4 receptors

We compared some binding parameters of [3H]nemonapride and [3H]spiperone in human dopamine D4 (hD4) receptors with three different numbers of tandem repeat units. Although both of the radioligands showed similar affinity constants for each hD4 receptor variant, the maximal number of binding sites labeled by [3H]nemonapride was approximately 1.35-fold higher than that by [3H]-spiperone for all variants. Estimated Ki values for the inhibition of [3H]nemonapride binding by a series of dopaminergic ligands were highly correlated to respective values obtained for the inhibition of [3H]spiperone binding to each hD4 receptor. These results suggest that the hD4 receptor, as shown for the D2 receptor, may exist in multiple molecular forms as a monomer-dimer equilibria, and that [3H]spiperone may discriminate in the multiple molecular forms.

Differential effects of guanine nucleotides on kainic acid binding and on adenylate cyclase activity in chick optic tectum

In G protein-coupled receptors, neurotransmitter-induced binding of GTP to G proteins triggers the activation of effector systems while simultaneously decreasing the affinity of the transmitter for its specific binding site within the receptor-G protein complex. In the present study we show that, in the chick optic tectum, guanine nucleotides inhibit the binding of the glutamate analog, kainate, and activate adenylate cyclase by different mechanisms and acting on different sites. GMP-PNP, a non-hydrolyzable analog of GTP, binds tightly to G proteins so that the binding is stable even after exhaustive washing. By use of this property, we have prepared membrane samples in which G protein GTP-binding sites are pre-saturated with GMP-PNP. Experiments carried out with these membranes show that GMP-PNP, GDP-S and GMP inhibit the binding of [3H]kainate by interacting with site(s) unrelated to G proteins, whereas GMP-PNP activates adenylate cyclase activity by binding to G proteins.

Preclinical pharmacology of FG5893: a potential anxiolytic drug with high affinity for both 5-HT1A and 5-HT2A receptors

The effects of FG5893 were evaluated by several different methods; rats were used as experimental animals. Receptor binding studies revealed that FG5893 (2-(4-(4,4-bis(4-fluorophenyl)butyl)-1-piperazinyl)-3-pyridinecarboxy lic acid methyl ester) binds with high affinity to both 5-HT1A (Ki = 0.7 nM) and 5-HT2A receptors (Ki = 4.0 nM) but has only low affinity for the 5-HT2C receptor (Ki = 170 nM). FG5893 dose dependently reduced body temperature, and this effect was inhibited by pretreatment with (+/-)-pindolol. FG5893 (0.1 mg/kg) significantly inhibited head twitch behaviour induced by DOI (1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane) and FG5893 was also a potent inhibitor of ultrasound vocalization in rat pups (0.3 mg/kg) and of a passive avoidance response (0.1 mg/kg) in mature animals. FG5893 inhibited the cage-leaving response and induced part of the 5-HT behavioural syndrome, but only at very high doses (5 and 10 mg/kg, respectively). At increased doses (1 mg/kg), FG5893 also elicited corticosterone release and reduced the immobility time in the forced-swim test (1 mg/kg). Together, these data indicate that the mixed 5-HT1A receptor agonist/5-HT2A receptor antagonist FG5983 is a potent stimulator of presynaptic 5-HT1A receptors but is less active at the postsynaptic site. FG5893 had potent anxiolytic-like effects both on separation-induced ultrasound vocalization in rat pups and on a passive avoidance response. At increased doses, FG5893 possessed an antidepressant-like property.

Dopamine D2 receptors mapped in rat brain with [3H](+)PHNO

Previous work with membrane preparations had demonstrated that the agonist (+)-4-propyl-9-hydroxynaphthoxazine (PHNO) labels the high-affinity state of dopamine D2 receptors with 67-fold selectivity over D1 sites. In this study, quantitative autoradiography was used to examine the binding of [3H](+)PHNO to rat brain sections. Highest binding densities were found in caudate-putamen, accumbens, and olfactory tubercles, as expected, and also in specific layers of the olfactory bulb. In addition, a second group of brain regions, including lateral septum, entorhinal cortex, molecular layer of hippocampus, and several brainstem structures showed low but consistent levels of binding. In all brain regions [3H](+)PHNO binding (2 nM) was completely displaced by 10 microM sulpiride (> 99%). Addition of 150 microM guanilylimidodiphosphate, which normally converts D2 receptors from high to low affinity states, abolished [3H](+)PHNO binding in all brain regions (> 99%), except for the islands of Callejas. This is likely to reflect binding to D3 sites in this area. Omission of preincubation in binding assays decreased [3H](+)PHNO binding in a regionally dependent manner, with strongest effects (22%) seen in high-density areas. These preincubation results confirm that (+)PHNO may have limitations for in vivo imaging studies. On the other hand, [3H](+)PHNO's negligible levels of non-specific binding compared to other agonists and overall selectivity would make it an excellent tool for in vitro autoradiographic monitoring of the high affinity state of D2 receptors.

Antisense strategy unravels a dopamine receptor distinct from the D2 subtype, uncoupled with adenylyl cyclase, inhibiting prolactin release from rat pituitary cells

The antisense strategy was used to unravel the functional contribution of the mRNAs encoding dopamine (DA) receptors to the multiple transduction mechanisms operated by DA in rat pituitary cells. An antisense oligonucleotide was designed to recognize seven nucleotides upstream and 11 nucleotides downstream from the initiation translation codon of the mRNA that encodes the DA D2 receptor. Addition of the antisense oligonucleotide for 7 days to primary culture of rat pituitary cells resulted in a decreased expression of DA D2 receptor as shown by (a) the virtual disappearance of [3H]spiroperidol binding sites and (b) the marked reduction in the levels of both the long and the short splice variant of the D2 receptor mRNAs. After this treatment, the DA D2 receptor agonist bromocriptine lost its capability both to inhibit adenylyl cyclase activity and to reduce prolactin mRNA levels. On the contrary, the inhibition of prolactin release induced by bromocriptine was affected minimally by the antisense oligonucleotide treatment. These data indicate that (a) translation of the mRNA encoding DA D2 receptors results in receptors that are negatively coupled with adenylyl cyclase and functionally linked to inhibition of prolactin synthesis; and (b) the release of prolactin might be regulated, at least in part, by a DA receptor that is encoded by mRNA species distinct from those encoding the D2 receptor.

The limbic functional selectivity of amperozide is not mediated by dopamine D2 receptors as assessed by in vitro and in vivo binding

Behavioural, biochemical and electrophysiological studies suggest that amperozide affects mesolimbic dopamine neurotransmission. Amperozide is a potent 5-HT2, receptor antagonist with only a moderate affinity for rat brain dopamine D2 receptors. The brain regional dopamine D2 receptor binding properties of amperozide were investigated by using in vitro and in vivo radioligand binding techniques. Amperozide displaced [3H]spiroperidol binding from rat striatal and limbic dopamine D2 receptors with moderate affinity (Ki = 540 +/- 118 nM and Ki = 403 +/- 84 nM, respectively). The dopamine D2 receptor antagonist l-sulpiride and the agonist dopamine did not show different affinity in the two brain regions. Amperozide potently displaced in vivo [3H]spiroperidol binding in rat frontal cortex (ID50 = 1.4 mg/kg s.c.) but was devoid of effect in striatum, olfactory tubercle and nucleus accumbens (ID50 > 100 mg/kg s.c.). Chronic administration of amperozide (5 mg/kg p.o.) for three weeks did not result in any change of maximal dopamine D2 receptor number in either striatal or limbic tissue. The effects of amperozide on dopamine neurotransmission are thus not likely to occur by a direct interaction with dopamine D2 receptors in either striatal or limbic tissue. The functional limbic selectivity might rather be mediated by serotoninergic pathways.