Interactions of agonists with D-2 dopamine receptors: evidence for a single receptor population existing in multiple agonist affinity-states in rat striatal membranes

Measuring amphetamine-induced dopamine release in humans: A comparative meta-analysis of [11 C]-raclopride and [11 C]-(+)-PHNO studies

The radiotracers [¹¹ C]-raclopride and [¹¹ C]-(+)-PHNO are commonly used to measure differences in amphetamine-induced dopamine release between healthy persons and persons with neuropsychiatric diseases. As an agonist radiotracer, [¹¹ C]-(+)-PHNO should theoretically be roughly 2.7 times more sensitive to displacement by endogenous dopamine than [¹¹ C]raclopride. To date, only one study has been published comparing the sensitivity of these two radiotracers to amphetamine-induced dopamine release in healthy persons. Unfortunately, conflicting findings in the literature suggests that the dose of amphetamine they employed (0.3 mg/kg, p.o.) may not reliably reduce [¹¹ C]-raclopride binding in the caudate. Thus, it is unclear whether the preponderance of evidence supports the theory that [¹¹ C]-(+)-PHNO is more sensitive to displacement by amphetamine in humans than [¹¹ C]-raclopride. In order to clarify these issues, we conducted a comparative meta-analysis summarizing the effects of amphetamine on [¹¹ C]-raclopride and [¹¹ C]-(+)-PHNO binding in healthy humans. Our analysis indicates that amphetamine given at 0.3 mg/kg, p.o. does not reliably reduce [¹¹ C]-raclopride binding in the caudate. Second, the greater sensitivity of [¹¹ C]-(+)-PHNO is evidenced at 0.5 mg/kg, p.o., but not at lower doses of amphetamine. Third, our analysis suggests that [¹¹ C]-(+)-PHNO may be roughly 1.5 to 2.5 times more sensitive to displacement by amphetamine than [¹¹ C]-raclopride in healthy persons. We recommend that future displacement studies with these radiotracers employ 0.5 mg/kg, p.o. of amphetamine with a dose, post-scan interval of at least 3 hr. Using this dose of amphetamine, [¹¹ C]-raclopride studies should employ at least n = 34 participants per group, while [¹¹ C]-(+)-PHNO studies should employ at least n = 6 participants per group, in order to be sufficiently powered (80%) to detect changes in radiotracer binding within the caudate.

A Mechanism Linking Two Known Vulnerability Factors for Alcohol Abuse: Heightened Alcohol Stimulation and Low Striatal Dopamine D2 Receptors

Alcohol produces both stimulant and sedative effects in humans and rodents. In humans, alcohol abuse disorder is associated with a higher stimulant and lower sedative responses to alcohol. Here, we show that this association is conserved in mice and demonstrate a causal link with another liability factor: low expression of striatal dopamine D2 receptors (D2Rs). Using transgenic mouse lines, we find that the selective loss of D2Rs on striatal medium spiny neurons enhances sensitivity to ethanol stimulation and generates resilience to ethanol sedation. These mice also display higher preference and escalation of ethanol drinking, which continues despite adverse outcomes. We find that striatal D1R activation is required for ethanol stimulation and that this signaling is enhanced in mice with low striatal D2Rs. These data demonstrate a link between two vulnerability factors for alcohol abuse and offer evidence for a mechanism in which low striatal D2Rs trigger D1R hypersensitivity, ultimately leading to compulsive-like drinking.

What proportion of striatal D2 receptors are occupied by endogenous dopamine at baseline? A meta-analysis with implications for understanding antipsychotic occupancy

Using molecular imaging techniques - positron emission tomography (PET) and single-photon emission computed tomography (SPECT) - in conjunction with an acute dopamine depletion challenge (alpha-methyl-para-tyrosine) it is possible to estimate endogenous dopamine levels occupying striatal dopamine D₂ receptors (D₂R) in humans in vivo. However, it is unclear what proportion of striatal D₂R are occupied by endogenous dopamine under normal conditions. This is important since it has been suggested that in schizophrenia there may be a substantial proportion of striatal D₂R which are occupied by endogenous dopamine and not accessible by therapeutic doses of antipsychotics. In order to clarify these issues, we conducted a meta-analysis of dopamine depletion studies using substituted benzamide radiotracers in healthy persons. This meta-analysis suggests that anywhere from 8 to 21% (weighted average 11%) of striatal D₂R may be occupied by endogenous dopamine at baseline. Using these estimates, we propose an updated occupancy model and tentatively suggest that antipsychotics inhibit a smaller proportion of the total pool of striatal D₂R in vivo than previously acknowledged. This article is part of the issue entitled 'Special Issue on Antipsychotics'.

Estimating the effect of endogenous dopamine on baseline [(11) C]-(+)-PHNO binding in the human brain

Endogenous dopamine (DA) levels at dopamine D2/3 receptors (D2/3 R) have been quantified in the living human brain using the agonist radiotracer [(11) C]-(+)-PHNO. As an agonist radiotracer, [(11) C]-(+)-PHNO is more sensitive to endogenous DA levels than antagonist radiotracers. We sought to determine the proportion of the variance in baseline [(11) C]-(+)-PHNO binding to D2/3 Rs which can be accounted for by variation in endogenous DA levels. This was done by computing the Pearson's coefficient for the correlation between baseline binding potential (BPND ) and the change in BPND after acute DA depletion, using previously published data. All correlations were inverse, and the proportion of the variance in baseline [(11) C]-(+)-PHNO BPND that can be accounted for by variation in endogenous DA levels across the striatal subregions ranged from 42-59%. These results indicate that lower baseline values of [(11) C]-(+)-PHNO BPND reflect greater stimulation by endogenous DA. To further validate this interpretation, we sought to examine whether these data could be used to estimate the dissociation constant (Kd) of DA at D2/3 R. In line with previous in vitro work, we estimated the in vivo Kd of DA to be around 20 nM. In summary, the agonist radiotracer [(11) C]-(+)-PHNO can detect the impact of endogenous DA levels at D2/3 R in the living human brain from a single baseline scan, and may be more sensitive to this impact than other commonly employed radiotracers.

Synthesis and characterization of a novel series of agonist compounds as potential radiopharmaceuticals for imaging dopamine D₂/₃ receptors in their high-affinity state

Imaging of dopamine D2/3 receptors (D2/3R) can shed light on the nature of several neuropsychiatric disorders in which dysregulation of D2/3R signaling is involved. Agonist D2/3 tracers for PET/SPECT imaging are considered to be superior to antagonists because they are more sensitive to dopamine concentrations and may selectively label the high-affinity receptor state. Carbon-11-labeled D2/3R agonists have been developed, but these short-lived tracers can be used only in centers with a cyclotron. Here, we report the development of a series of novel D2R agonist compounds based on the 2-aminomethylchromane (AMC) scaffold that provides ample opportunities for the introduction of longer-lived [(18)F] or [(123)I]. Binding experiments showed that several AMC compounds have a high affinity and selectivity for D2/3R and act as agonists. Two fluorine-containing compounds were [(18)F]-labeled, and both displayed specific binding to striatal D2/3R in rat brain slices in vitro. These findings encourage further in vivo evaluations.

Agonist high- and low-affinity states of dopamine D₂ receptors: methods of detection and clinical implications

Dopamine D(2) receptors, similar to other G-protein-coupled receptors, exist in a high- and low-affinity state for agonists. Based upon a review of the methods for detecting D(2) receptor agonist high-affinity states, we discuss alterations of such states in animal models of disease and the implications of such alterations for their labelling with positron emission tomography (PET) and single-photon emission computed tomography (SPECT) tracers. The classic approach of detecting agonist high-affinity states compares agonist competition for antagonist radioligands, in most cases using [(3)H]-spiperone as the radioligand; alternative approaches and radioligands have been proposed, but their claimed advantages have not been substantiated by other investigators. In view of the advantages and disadvantages of various techniques, we critically have reviewed reported findings on the detection of D(2) receptor agonist high-affinity states in a variety of animal models. These data are compared to the less numerous findings from human in vivo studies based on PET and SPECT tracers; they are interpreted in light of the finding that D(2) receptor agonist high-affinity states under control conditions may differ between rodent and human brain. The potential advantages of agonist ligands in studies of pathophysiology and as diagnostics are being discussed.

Systematic in vivo screening of a series of 1-propyl-4-arylpiperidines against dopaminergic and serotonergic properties in rat brain: a scaffold-jumping approach

A series of 1-propyl-4-arylpiperidines were synthesized and their effects on the dopaminergic and serotonergic systems tested in vivo and in vitro. Scaffold jumping among five- and six-membered bicyclic aryl rings attached to the piperidine ring had a marked impact on these effects. Potent and selective dopamine D(2) receptor antagonists were generated from 3-indoles, 3-benzoisoxazoles, 3-benzimidazol-2-one, and 3-benzothiophenes. In contrast, 3-benzofuran was a potent and selective inhibitor of monoamine oxidase (MAO) A. The effects of the synthesized compounds on 3,4-dihydroxyphenylacetic acid (DOPAC) levels correlated very well with their affinity for dopamine D(2) receptors and MAO A. In the 4-arylpiperidine series, the most promising compound for development was the 6-chloro-3-(1-propyl-4-piperidyl)-1H-benzimidazol-2-one (19), which displayed typical dopamine D(2) receptor antagonist properties in vivo but produced only a partial reduction on spontaneous locomotor activity. This indicates that the compound may have a lower propensity to induce parkinsonism in patients.

The affinity of D2-like dopamine receptor antagonists determines the time to maximal effect on cocaine self-administration

Differences in the time to maximal effect (T(max)) of a series of dopamine receptor antagonists on the self-administration of cocaine are not consistent with their lipophilicity (octanol-water partition coefficients at pH 7.4) and expected rapid entry into the brain after intravenous injection. It was hypothesized that the T(max) reflects the time required for maximal occupancy of receptors, which would occur as equilibrium was approached. If so, the T(max) should be related to the affinity for the relevant receptor population. This hypothesis was tested using a series of nine antagonists having a 2500-fold range of K(i) or K(d) values for D(2)-like dopamine receptors. Rats self-administered cocaine at regular intervals and then were injected intravenously with a dose of antagonist, and the self-administration of cocaine was continued for 6 to 10 h. The level of cocaine at the time of every self-administration (satiety threshold) was calculated throughout the session. The satiety threshold was stable before the injection of antagonist and then increased approximately 3-fold over the baseline value at doses of antagonists selected to produce this approximately equivalent maximal magnitude of effect (maximum increase in the equiactive cocaine concentration, satiety threshold; C(max)). Despite the similar C(max), the mean T(max) varied between 5 and 157 min across this series of antagonists. Furthermore, there was a strong and significant correlation between the in vivo T(max) values for each antagonist and the affinity for D(2)-like dopamine receptors measured in vitro. It is concluded that the cocaine self-administration paradigm offers a reliable and predictive bioassay for measuring the affinity of a competitive antagonist for D(2)-like dopamine receptors.

Binding of [3H]Haloperidol to Dopamine D2 Receptors in the Rat Striatum

The present study was designed to examine the properties of [3H]haloperidol binding to dopamine D2-receptors in rat striatum membranes, displacement potencies of various chemicals and differences between the affinities of various chemicals and two new 5-hydroxytryptamine (5-HT2) receptor antagonists, MCI-9042, (±)-2-(dimethylamino)-1-[[o-(m-methoxyphenetyl)phenoxy]methyl]ethyl hydrogen succinate hydrochloride and one of its metabolites. The plots of specific binding for the striatum membranes obtained from the Scatchard analysis using [3H]haloperidol were monophasic when nonspecific binding was determined with 10 μm chlorpromazine, and the Kd and Bmax values were 7·42±1·03 Nm and 1·58±0·20 pmol (mg protein)−1 (n= 10), respectively. The displacement potencies of D2 receptor, 5- HT2 receptor, histamine H1-receptor, and adrenoceptor antagonists were characterized by [3H]haloperidol binding to D2 receptors. The pKi values of a new antiplatelet agent, MCI-9042, and its metabolite were 5·02 and 5·53, respectively, and these values were lower than those of the D2-receptor antagonists, fluphenazine, spiperone, haloperidol, prochlorperazine, chlorpromazine, thioridazine, and sulpiride. They were also lower than the pKi values of the 5-HT2-receptor antagonists, pirenperone, ketanserin, methysergide, and mianserin. We conclude that the binding site of [3H]haloperidol in the rat striatum is the D2 receptor, that MCI-9042 and its metabolite have lower affinities for D2 receptors than for 5-HT2 receptors, and that this radioreceptor assay is useful for assessing the affinities of various agents.

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.

Dopamine D2 receptor-mediated G protein activation assessed by agonist-stimulated [35S]guanosine 5'-O-(gamma-thiotriphosphate) binding in rat striatal membranes

In order to investigate the functional interaction between the native dopamine receptors and their coupled guanine nucleotide-binding regulatory (G) proteins, dopamine-stimulated [(35)S]guanosine 5'-O-(gamma-thiotriphosphate) ([(35)S]GTPgammaS) binding was pharmacologically characterized in rat striatal membranes. Following optimizing the experimental conditions as to the concentrations of GDP, MgCl(2) and NaCl in the assay medium, the agonist and antagonist properties for a series of dopamine receptor ligands were determined mainly under the standard assay condition. The pharmacological profile of this response clearly indicated the involvement of dopamine D(2)-like receptors, but not of dopamine D(1)-like receptors. Among the types of dopamine D(2)-like receptors, dopamine D(2) receptors most likely appeared to be involved in dopamine-stimulated [(35)S]GTPgammaS binding in rat striatal membranes, because the affinities of agonists and antagonists determined in the present study were significantly correlated with those reported in the previous literature only for dopamine D(2) receptors, but not for dopamine D(3) or D(4) types. Though the concentration-dependent inhibition curves of dopamine-stimulated [(35)S]GTPgammaS binding by spiperone and S(-)-raclopride were apparently biphasic, the origin of the low-affinity minor components was not fully determined. The antiparkinsonian drugs with the properties of dopamine receptor agonism were shown to behave as stimulants with varied affinities and relative efficacies in the current assay system. On the other hand, neither phencyclidine (PCP) nor ketamine stimulated the specific [(35)S]GTPgammaS binding, in contrast with the previous report demonstrating that these two N-methyl-D-aspartic acid (NMDA) receptor antagonists behaved as agonists at human dopamine D(2) receptors expressed in Chinese hamster ovary (CHO) cells. These results provide important information about the functional activation of G proteins coupled with dopamine D(2) receptors as well as agonist actions of various compounds at native dopamine D(2) receptors, which are potentially involved in pathophysiology and pharmacotherapy of neuropsychiatric diseases such as Parkinson's disease, schizophrenia and depression.

Dopamine displaces [3H]domperidone from high-affinity sites of the dopamine D2 receptor, but not [3H]raclopride or [3H]spiperone in isotonic medium: Implications for human positron emission tomography

Because the high-affinity state of the dopamine D2 receptor, D2High, is the functional state of the receptor, has a role in demarcating typical from atypical antipsychotics, and is markedly elevated in amphetamine-sensitized rats, it is important to have a method for the convenient detection of this state by a ligand. The present data show that, in contrast to [(3)H]spiperone or [(3)H]raclopride, [(3)H]domperidone labels D2High sites in the presence of isotonic NaCl in either striatum or cloned D2Long receptors, yielding a dopamine dissociation constant (1.75 nM) in agreement with that found with [(3)H]dopamine. Increased labeling of D2High sites occurred with [(3)H]domperidone after severe disruption of the cells, suggesting that [(3)H]domperidone has better access to the D2 receptor from the cytoplasmic aspect of the cell membrane. The density of the [(3)H]domperidone-labeled D2 receptors was the same as that of the [(3)H]raclopride-labeled D2 receptors, but twice the density of [(3)H]spiperone sites for human cloned D2Long receptors, compatible with the monomer-dimer concept of the D2 receptor. [(3)H]domperidone readily labels the D2High sites in postmortem human brain homogenates. Although [(3)H]spiperone or [(3)H]raclopride can occupy D2High sites, the inability of 1-10 nM dopamine to displace these ligands under isotonic conditions suggests that these ligands may not be suitable for monitoring the physiological high-affinity state of the dopamine D2 receptor by means of [(11)C]methylspiperone or [(11)C]raclopride in humans.

Assessment of striatal D1 and D2 dopamine receptor-G protein coupling by agonist-induced [35S]GTP gamma S binding

The dopamine receptor-mediated modulation of guanosine 5'-O-(gamma-[35S]thio)triphosphate ([35S]GTP gamma S) binding has been characterized in rat striatal membranes. In optimized experimental conditions, the potency of dopamine was 4.47 microM [3.02-6.61 microM] and a maximal response representing 54.8 +/- 4.5% increase above basal level was observed. Data obtained with different agonists and antagonists clearly revealed that the most important fraction of this response was reflecting D2 receptor activation. Further analysis with specific antagonists also supported evidence for the involvement of D1 dopamine receptors. The potencies of compounds interacting with D1 and D2 receptors were deduced from [35S]GTP gamma S binding experiments and compared with their binding affinities for these receptors measured in similar experimental conditions. A good correlation between these parameters was observed, supporting the applicability of this technique for the study of dopamine receptors in the central nervous system.

Cerebral trauma-induced changes in corpus striatal dopamine receptor subtypes

A device designed specifically for mild to severe concussions was used to produce quantitative experimental blunt brain injury in male Wistar rats. We have examined the effects of varying magnitudes of cerebral trauma on the maximal binding capacity (Bmax) of D1 and D2 dopamine (DA) receptors. The Bmax for each receptor subtype was obtained from Scatchard analyses of [3H]-SCH 23390 and [3H]Spiperone binding to striatal membrane. Anesthetized rats were injured--one, two, or three times--once every 24 h, with either a 68- or 268-g rubber-headed reflex hammer accelerated from a predetermined distance. Uninjured nonanesthetized (NA) and anesthetized (A) rats served as controls. No significant difference in receptor density was observed between NA and A rats for each receptor subtype. Immediately (0 h) following injury from the 68-g hammer weight, the density of D1 receptors decreased (50%), then increased (30%) above control levels by 24 h. The same pattern was observed with the 268-g hammer weight. Analysis of variance (ANOVA) showed that there was no overall effect of number of injuries or treatment on the density of D1 and D2 receptor subtypes. However, there was an interaction of both variables on the D1, but not D2, receptor subtype. Partial ANOVA for receptor densities after rats were injured either one, two, or three times showed that receptor density was altered only after the rats were injured one time. These results suggest that striatal DA D1 receptors are downregulated and then upregulated following isolated injury to the cerebral cortex.

Affinity for dopamine D2, D3, and D4 receptors of 2-aminotetralins. Relevance of D2 agonist binding for determination of receptor subtype selectivity

A series of 2-aminotetralins, substituted with a methoxy or a hydroxy group on the 5- or 7-position, and with varying N-alkyl or N-arylalkyl substituents, were prepared and evaluated in binding assays for human dopamine (DA) D2, D3, and D4 receptors. Some members of this series were prepared in former studies, but were never tested in vitro with single receptor subtypes, and these were examined again. None of the tested 2-aminotetralins showed high affinity for the dopamine D4 receptor. However, a number of the 2-aminotetralins showed high affinity for both the D2 and the D3 DA receptors, as exemplified by compounds 11-15 and 21-26, while some had a reasonable selectivity for the DA D3 receptors. The affinities of the 2-aminotetralins for the D21, receptor depended on the type of radioligand (agonist or antagonist) used. The agonist affinity data, obtained by using the agonist ligand [3H]N-0437, are thought to be more relevant for calculating DA receptor subtype selectivity.

Characterisation of dopamine receptors in insect (Apis mellifera) brain

In vitro binding experiments using the vertebrate D1 dopamine receptor ligand [3H]SCH23390 and the vertebrate D2 dopamine receptor ligand [3H]spiperone were conducted on membrane preparations of honey bee (Apis mellifera) brain. Specific binding of [3H]SCH23390 was saturable and reversible. Analysis of saturation data gave an apparent Kd of 6.3 +/- 1.0 nM and Bmax of 1.9 +/- 0.2 pmol/mg protein for a single class of binding sites. The specificity of high affinity [3H]SCH23390 binding was confirmed in displacement experiments using a range of dopaminergic antagonists and agonists. The rank order of potency for antagonists was: R(+)-SCH23390 > cis-(Z)-flupentixol > or = chlorpromazine > fluphenazine > S(+)-butaclamol > spiperone. R(+/-)-SKF38393 and dopamine were the most effective agonists tested. [3H]SCH23390 labels a site in bee brain that is similar, but not identical to the vertebrate D1 dopamine receptor subtype. [3H]Spiperone also bound with high affinity to bee brain homogenates. Scatchard analysis of [3H]spiperone saturation data revealed a curvilinear plot suggesting binding site heterogeneity. The high affinity site had a apparent Kd of 0.11 +/- 0.02 nM and Bmax of 9.2 +/- 0.5 fmol/mg protein. The calculated values for the low affinity site were a Kd of 19.9 nM and Bmax of 862 fmol/mg protein. Kinetic analyses also indicated that [3H]spiperone recognises a heterogeneous population of sites in bee brain. Furthermore, agonist competition studies revealed a phenolaminergic as well as a dopaminergic component to [3H]spiperone binding in bee brain. The rank order of potency of dopaminergic antagonists in competing for [3H]spiperone binding was: spiperone > fluphenazine > S(+)-butaclamol > domperidone > R(+)-SCH23390 > S(-)-sulpiride.

Labelling of D2-dopaminergic and 5-HT2-serotonergic binding sites in human trophoblastic cells using [3H]-spiperone

We previously reported that dopamine (DA) inhibited the release of human placental lactogen (hPL) from human placental cells. We also demonstrated the presence of D2-dopamine receptors in membrane preparations of human term placenta. The aim of the present study was to characterize these D2 receptors on freshly isolated human trophoblastic cells. The binding of [3H]-spiperone to these cells showed a curvilinear Scatchard plot suggesting the presence of two classes of binding sites (Kd1 = 1.26nM; Kd2 = 44.3nM). Competition experiments showed the following inhibitory binding potencies: serotonin-2 (5-HT2) > or = D2 >>> alpha-adrenergic, beta-adrenergic, D1-dopamine, thus suggesting the presence of 5-HT2 binding sites. We have examined this possibility by blocking [3H]-spiperone binding to 5-HT2 receptors in the presence of 50nM ketanserin, a selective antagonist of 5-HT2 sites. Under this condition, the linear Scatchard plot obtained suggested a single population of homogeneous binding sites for [3H]-spiperone with a Kd of 0.55nM. To further characterize placental D2 receptors we conducted binding experiments with [3H]-raclopride, an more selective D2 antagonist. The linear Scatchard plot obtained with this ligand suggested one class of binding sites for [3H]-raclopride (Kd = 6nM) with the following inhibitory potencies: D2 >>> beta-adrenergic >> 5-HT2, D1, alpha-adrenergic. These results suggest an important paracrine function for DA in human placenta and show for the first time that [3H]-spiperone binds putative 5-HT2 receptors in human placenta.

D1 dopamine receptor binding and mRNA levels are not altered after neonatal 6-hydroxydopamine treatment: evidence against dopamine-mediated induction of D1 dopamine receptors during postnatal development

The role of dopaminergic innervation on the postnatal developmental expression of D1 dopamine receptors was investigated. Bilateral destruction of dopamine-containing neurons was achieved by treating rats intracisternally with 6-hydroxydopamine (6-OHDA) on postnatal day 3, and rats were killed on day 21. To ensure effective reduction of D1 receptor activation by residual dopamine, a group of 6-OHDA-lesioned rats was given twice daily injections of the D1 receptor antagonist SCH-23390, from day 4 to 20. D1 dopamine receptor binding was assessed in the caudate-putamen, nucleus accumbens, and olfactory tubercle by quantitative autoradiographic analysis of [3H]SCH-23390 binding. In addition, the relative amount of D1A receptor mRNA was assessed by in situ hybridization of a 35S-labeled riboprobe. In the developing rats, neither the amount of [3H]SCH-23390 binding nor the amount of D1A receptor mRNA was altered by 6-OHDA lesioning followed by chronic treatment with SCH-23390. Thus, bilateral destruction of dopamine-containing neurons and treatment with SCH-23390 in neonatal rats did not interfere with the developmental expression of D1 receptors or alter the levels of mRNA that code for this receptor protein. Treatment of intact rats with SCH-23390 from postnatal day 4 to 20 also did not alter [3H]SCH-23390 binding or levels of D1 receptor mRNA. However, adult rats treated chronically with SCH-23390 exhibited increased [3H]SCH-23390 binding but did not show a significant change in D1 receptor mRNA levels.

Dopamine D2 receptors are unevenly distributed in the rat hippocampus and are modulated differently than in striatum

The characteristics of dopamine (DA) D2 receptors were studied in rat hippocampus and for comparison in striatum using the [3H]spiperone radioreceptor assay in vitro. D2 receptors exhibit a bimodal distribution along the length of the hippocampus. Heterogeneity also exists in the transverse axis with high densities in the molecular layers of the subiculum and CA1 plus CA2. As in the striatum, D2 receptors in both dorsal and ventral hippocampus display high and low affinity states for agonists, but all three regions differ with regard to the percentage of receptors in these states. The modulation of these affinity states by GTP, Mg2+ and the iron-chelator, 1,1-O-phenanthroline, differs markedly between dorsal and ventral hippocampus, and between these regions and the striatum. A new model for the modulation of the affinity of D2 receptors for agonists is presented. These data suggest the presence of regional differences in the composition of the D2 receptor-regulatory protein complex.

Expression and characterization of human D4 dopamine receptors in baculovirus-infected insect cells

The human D4 dopamine receptor has been genetically engineered for expression in insect cells using the baculovirus system. A D4 cDNA gene fusion construct [(1991) Nature 350, 610-614] was synthetically modified to remove two introns from the coding region, and expressed in S. frugiperda (Sf9) cells as a fusion with a short sequence from the polyhedrin protein. Binding assays with [3H]spiperone indicated high levels of D4 receptor binding 90 h after infection and a pharmacological profile identical to that reported for D4 receptors expressed in COS-7 cells using the cDNA gene hybrid. We also show that the agonist binding affinity of D4 receptors expressed in Sf9 cells can be shifted by GTP-gamma-S, indicating coupling to G-proteins.

Modulation by estradiol and progesterone of the GTP effect on striatal D-2 dopamine receptors

Agonist binding properties of rat striatal D-2 dopamine (DA) receptors were investigated after in vivo or in vitro estradiol or progesterone exposures in order to elucidate the mechanism of action of steroid hormones on DA receptors. Chronic estradiol treatment of ovariectomized rats (10 micrograms, twice each day, for 2 weeks) increased lateral striatum total receptor density and left unchanged the proportion and affinity of the agonist high- and low-affinity states of this receptor in the striatum. In addition, when GTP was added in DA competition for [3H]spiperone binding experiments, D-2 receptors in the medial part of the striatum from estrogen-treated animals were more sensitive to GTP than those in the lateral part, whereas GTP had equal activity in both parts of the striatum in vehicle-treated rats. With apomorphine, but not with DA competition for [3H]spiperone binding, addition of estradiol (1 nM) to striatal homogenates of intact male rats prevented the expected shift of the high- to the low-affinity state of D-2 receptors, normally induced by GTP (100 microM) under these conditions. This effect of estradiol was not observed in the presence of 4 mM MgCl2, while in vitro progesterone (100 nM) had no effect in either the absence or presence of MgCl2. In addition, in vivo chronic progesterone treatment of ovariectomized rats left striatal [3H]spiperone density and affinity unchanged. Moreover, 1 nM estradiol increased the IC50 of GTP for inhibition of [3H]N-propylnorapomorphine binding to the high-affinity state of striatal D-2 receptors. This effect was also observed but decreased by 2-fold in the presence of MgCl2. Our data suggest that estradiol in vivo and in vitro interferes with the effect of GTP on striatal D-2 DA receptors.

Synthesis and characterization of biotinylated and photoactivatable neuroleptics. Novel bifunctional probes for dopamine receptors

We have synthesized and characterized a series of novel derivatives of established antagonists of the neurotransmitter dopamine, i.e. butyrophenones, hexahydrocarbolines and phenothiazines. All derivatives were biotinylated, some of them carried an additional (photoactivatable) azido group. In the case of butyrophenones, the structural modifications were introduced at the aliphatic keto group and/or the heterocyclic ring system, both modifications resulting in significant decreases in binding affinity to dopamine D2 and dopamine D1 receptor subtypes. Biotinylation of hexahydrocarbolines significantly increased their binding affinity to D1 receptors, with the affinity for D2 receptors increasing only slightly, or remaining approximately the same, as compared to the parent compound. As a consequence, the derivatized hexahydrocarbolines behaved as nonselective antagonists of dopamine. Biotinylation of phenothiazines increased their binding affinity to both main subtypes of dopamine receptors by at least one order of magnitude, resulting in binding affinities in the nM range. These derivatives bound to both D1 and D2 receptor subtypes. In three of the biotinylated derivatives the photoactivatable azido group was introduced. These compounds bound to synaptosomal membranes from bovine caudate nuclei with similar affinity and subtype specificity as the biotinylated derivatives, and photoaffinity labelling was shown to proceed under mild conditions and selectively. These novel bifunctional ligands may become useful tools in the purification and characterization of dopamine receptors including their visualization and localization in the central nervous system and in tissue culture.

Identification of serotonin 5HT2 receptors in bovine pineal gland

The concentration of serotonin in the pineal gland is extremely high, which prompted speculation that in addition to serving as a precursor of melatonin, serotonin may have an independent function of its own. By using [3H]-spiperone as a ligand, and ketanserine as a selective serotonin 5HT2 receptor antagonist, we have identified 5HT2 receptor in the bovine pineal gland, revealing a single population of binding sites with a dissociation equilibrium constant (Kd) value of 1.26 +/- 0.41 nM and a receptor density (Bmax) value of 193 +/- 38.85 fmol/mg protein. In displacement experiments, the concentrations of the drugs required to inhibit 50% of the specific binding of [3H]-spiperone in descending order of potency were methysergide greater than ritanserin greater than pirenperone greater than pipamperone greater than ketanserin greater than cyproheptadine greater than M-trifluoromethylphenyl-piperazine greater than prazosin greater than 5-methoxy-N-N-dimethyltryptamine hydrogen oxalate greater than 1-(3-chlorophenol) piperazine greater than serotonin. In the rat pineal gland, [3H]-spiperone revealed a low affinity serotonin binding site with a Kd value of 25.77 +/- 10.7 nM and a Bmax value of 1244 +/- 472 fmol/mg protein. The results of these studies are interpreted to indicate that the bovine pineal gland possess serotonin 5HT2 receptor. However, the rat pineal gland possess a serotoninergic binding site of unknown nature.

Hallucinogenic and neuroleptic drug interactions with potential neurotransmitter receptors in parasitic nematodes

Receptors potentially involved in neurotransmitting have been characterised in the muscle tissue and in whole worms of the nematodes Ascaris suum and Onchocerca volvulus, respectively. Binding studies revealed a high affinity for LSD with apparent KD values of 94 nM for A. suum and 120 nM for O. volvulus, whereas those of the neuroleptics haloperidol, spiperone and mianserin were found to be in the micromolar range. A variety of neurotransmitter antagonists, known to bind with high affinities either to mammalian D1/2 or to 5-HT1/2 receptors, were tested for their ability to bind to the nematode receptor. Results from these displacement experiments using tritiated LSD, mianserin, spiperone and haloperidol show distinct specificities of the nematode receptors compared to known receptor classes of mammals. With respect to this novel specificity, the nematode receptors seem to be unique and clearly distinct from those of the hosts.

Synaptic concentration of dopamine in the mouse striatum in relationship to the kinetic properties of the dopamine receptors and uptake mechanism

The concentration of dopamine (DA) in the synaptic cleft in the mouse striatum in vivo was estimated from the competition between the synaptic DA and the 3H-labelled DA D2 receptor agonists N-n-propylnorapomorphine (NPA) or N,N-diethyl-N'-[(3 alpha, 4a alpha, 10 beta)-1,2,3,4,4a,5,10,10a-octahydro- 7-hydroxyl-1-propyl-3-benzo (g) quinolinyl]sulfamide (Sandoz 205-501) injected intravenously in tracer doses. Knowing the inhibitor constant for DA in inhibiting the binding of these receptor agonists in vitro, attempts were made to calculate the changes in the synaptic DA concentration from the changes in the in vivo binding of the receptor agonists evoked by various pharmacological agents. Inhibiting the firing of the dopaminergic neurons by gamma-butyrolactone (GBL) increased the binding of the receptor agonists corresponding to a decrease in the synaptic DA concentration of 55 +/- 2 nM in the experiments with [3H]Sandoz 205-501 and 48 +/- 3 nM in the experiments with tracer doses of [3H]NPA. These values may therefore approximate the normal DA concentration in the synaptic cleft in the mouse striatum. With this technique it was also possible to determine the synaptic concentration of NPA by its competition with [3H]Sandoz 205-501 for the DA D2 receptors in the striatum of GBL-treated mice in vivo. To compare the estimated synaptic concentration of DA with the affinity of DA to D1 and D2 receptors and to the DA transporter in the mouse striatum the kinetic parameters were determined at 37 degrees C in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential effects of dopamine agonists upon stimulated limbic and striatal dopamine release: in vivo voltammetric data

1. Fast cyclic voltammetry at carbon fibre microelectrodes was used in rats anaesthetized with chloral hydrate to monitor dopamine release in the caudate and nucleus accumbens evoked by electrical stimulation of the median forebrain bundle. Stimulation trains (50 Hz sinusoidal current, 100 +/- 10 microA r.m.s., 2s duration) were repeated every 5 min throughout the experiment. 2. The actions of the dopamine agonists quinpirole, pergolide, SKF 38393, bromocriptine, (+)-3-(3-hydroxyphenyl)-N-n-propylpiperidine ((+)-3PPP) and (-)-3PPP were compared in the two nuclei. 3. Bromocriptine (10 mg kg-1, i.p.) did not affect release in either nucleus while SKF 38393 caused a fleeting decrease in limbic but not striatal dopamine release at a high dose (20 mg kg-1, i.p.). 4. Quinpirole and pergolide (both 1 mg kg-1, i.p.) decreased stimulated dopamine release in the nucleus accumbens while in the caudate the drugs each caused a transient, though not quite significant, elevation of stimulated dopamine release followed by decrease in release of the same magnitude as that seen in the nucleus accumbens. 5. The (-)-enantiomer of 3PPP (20 mg kg-1, i.p.), a partial agonist at the dopamine autoreceptor, increased stimulated dopamine release in both nuclei although the action in the caudate was larger and more prolonged. (+)-3PPP (20 mg kg-1, i.p.), a full agonist, decreased release in the nucleus accumbens. A small, transient and not significant increase in the caudate was followed by decreased release. 6. The results are interpreted as being evidence for differences in the dopamine autoreceptor in the two nuclei, possibly in the affinity state of the receptor in each nucleus.

Synthesis of some phenothiazine derivatives as potential affinity ligands for the central dopamine receptors

Syntheses of several phenothiazine ligands as potential affinity probes for the D1- and D2-dopamine receptors derived from 4-(3-(10-(2-trifluoromethyl)phenothiazinyl)propyl)-1-(2-aminoethyl )-piperazine hydrochloride are described and their interactions with D1- and D2-dopamine receptors of the bovine caudate nucleus have been characterized. The bromoacetylamido-, maleinimido-, and isothiocyanato-derivatives expressed low selectivity and moderate affinity for both categories of dopamine receptors. 4-(3-(10-(2-Trifluoromethyl)phenothiazinyl)propyl)-1-(2- (isothiocyanatobenzoyl)ethyl)-piperazine hydrochloride did not discriminate among the two subclasses of the dopamine receptors, but showed an extremely strong irreversible binding to the D1-receptors and thus is promising as a highly specific affinity ligand for biochemical and pharmacological studies of the D1-dopamine receptors.

Cloning of ligand-specific cell lines via gene transfer: identification of a D2 dopamine receptor subtype

Using rat genomic DNA, we have established a transfected mouse fibroblast cell line that expresses a spiperone binding site with the pharmacological characteristics of a D2 dopamine receptor. The expressed D2 receptors are the product of a gene that is distinct from that reported by Bunzow et al. [Bunzow, J. R., Van Tol, H. H. M., Granoly, D. K., Albert, P., Salon, J., Christie, M., Machida, C. A., Neve, K. A. & Civelli, O. (1988) Nature (London) 336, 783-787]. Flow cytometry with the Ca2+-sensitive dye indo-1 demonstrated that activation of the expressed D2 sites resulted in increases in intracellular calcium that were dependent on the influx of external Ca2+. These general cloning procedures should be applicable to the production of cell lines expressing a variety of genes for which only functional assays are available.

Identification and characterization of D1 and D2 dopamine receptors in cultured neuroblastoma and retinoblastoma clonal cell lines

The recent availability of high specific activity radiolabeled dopaminergic antagonists with specificity for dopamine receptor subtypes has allowed us to screen a wide variety of cultured mammalian cell lines for the presence of D1 and D2 dopamine receptors. Specific receptor binding of the D1 selective antagonists [3H]SCH 23390 and [125I]SCH 23982 was detected in membranes prepared from NS20Y cells, a clonal cell line derived from the C1300 murine neuroblastoma. Saturation analysis of [3H]SCH 23390 binding revealed the presence of saturable, high affinity binding sites with a dissociation constant (Kd) of 575 pM and a receptor density of 138 fmol/mg protein (approximately 9000 receptors/cell). Inhibition of [3H]SCH 23390 binding by a series of dopaminergic agonists and antagonists exhibited appropriate stereoselectivity and pharmacological specificity, verifying the D1 nature of this site. Dopamine inhibition of [3H]SCH 23390 binding revealed the presence of high and low affinity agonist binding sites which were converted to a homogeneous low affinity state by the addition of GppNHp. In membranes prepared from the WERI 27 human retinoblastoma cell line, specific receptor binding of the D2 antagonists [3H]methylspiperone and [125I]NAPS was observed. Saturation analysis of [3H]methylspiperone binding revealed the presence of a single class of high affinity, saturable binding sites with a Kd of 140 pM and a Bmax of 223 fmol/mg protein (approximately 2500 receptor sites/cell). Inhibition of [3H]methylspiperone binding by dopaminergic antagonists exhibited a rank order of potency consistent with the identification of a D2 dopamine receptor subtype. In addition, dopamine inhibition of [3H]methylspiperone binding exhibited both high and low affinity agonist binding sites which were converted to low affinity by the addition of GppNHp. These results represent the first direct demonstration of D1 and D2 dopamine receptors in cultured mammalian clonal cell lines. These cells should provide powerful model systems for investigating the molecular mechanisms involved in dopamine receptor/effector coupling and regulation.

D2 dopamine receptors in the human brain: heterogeneity based on differences in guanine nucleotide effect on agonist binding, and their presence on corticostriatal nerve terminals

In human brain, regulation of agonist binding to the D2 dopamine receptors by guanine nucleotides is different between several regions. In membranes of the anterior pituitary lobe, agonist binding is fully sensitive to GTP or Gpp(NH)p, whereas it is resistant in membranes of globus pallidus. Both guanine nucleotide-sensitive (G-S) and -insensitive (G-I) receptors are found in membranes of the striatum. The G-S and G-I type receptors display similar affinities for antipsychotic drugs of different classes, suggesting that they only differ in their effector-coupling system. We investigated the D2 receptors in striatal membranes of postmortem human brains in which corticostriatal pathways were unilaterally destroyed by an infarction. Compared to the intact side, D2 receptor densities in striata from the lesioned side were reduced by 46-65%, whereas the densities of the muscarinic cholinergic receptors, dopamine uptake sites, and D1 dopamine receptors were unchanged. In the deafferented striata all G-I, but also a substantial number of G-S receptors were lost, suggesting that both receptor subtypes are present on corticostriatal nerve terminals.

Mono- and divalent cations modulate the affinities of brain D1 and D2 receptors for dopamine by a mechanism independent of receptor coupling to guanyl nucleotide binding proteins

In order to clarify the question of whether the modulatory effects of cations on dopamine receptor affinities are brought about by shifts in the equilibrium of receptor - G protein - coupling, it was investigated whether mono- and divalent cations were still able to modulate rat striatal D1 and D2 receptor affinities after selective inactivation of the G-proteins linked to the two receptors. The Gs-protein coupled to the D1 receptor was eliminated by mild thermal inactivation, and the Gi- (or Go-) protein associated with the D2 receptor by alkylation with a low concentration of N-ethyl-maleimide. Incubation of striatal membranes at 60 degrees C completely abolished the specific binding of 3H-GTP. Both treatments resulted in an increase of the IC50-values for dopamine as a displacer of 3H-SCH 23390 from D1- and of 3H-spiperone from D2 receptors. Concomitantly, the formerly shallow D1 displacement curves became steeper, with their Hill coefficients increasing. This effect was less evident at D2 receptors. Guanosine triphosphate (GTP), which increased the IC50's of dopamine for both receptors approximately two-fold in control membranes, was without effect in pretreated samples, indicating an effective inactivation of the G-proteins. Na+ ions were still able to lower, and Ca2+ ions to increase the affinities of D1 and D2 receptors for dopamine after such inactivation of the respective G-proteins. It is concluded that the mechanism underlying the regulation of dopamine receptor affinities by mono- and divalent cations is independent of and superimposed upon the coupling of these receptors to guanyl nucleotide binding proteins.

[125]I-spectramide: a novel benzamide displaying potent and selective effects at the D2 dopamine receptor

The new substituted benzamide Spectramide, (N-[2-[4-iodobenzyl-N-methylamino]-2-methoxy-4-ethyl]-5-chloro- methylamine] benzamide) labelled with 125I was used as a potent and highly selective dopamine-D2 receptor antagonist in rat striatal homogenates for in vitro receptor binding. Kinetic experiments demonstrated the reversibility of the binding and the estimated Kd from saturation analysis was 25 pM, with a Bmax of 20 pmol/g of tissue. Competition studies showed that spectramide did not interact potently with the D1 or dopamine-uptake site. Drugs known to interact with other receptor systems were weak competitors of the binding, while binding was potently inhibited by other D2 antagonists, such as spiperone and eticlopride. These data indicate that Spectramide binds selectively and with high affinity to the dopamine D2 receptors, and may prove to be a useful tool for the study of these receptors in vivo using PET or SPECT.

Ontogenetic changes in [3H]-spiroperidol binding sites in posthatch chick brain

The ontogenetic development of [3H]-spiroperidol binding sites was measured in the optic tectum, cerebellum, forebrain base, and forebrain roof of 1-, 4-, and 16-day-old chicks. In the chick optic tectum and cerebellum both the density and the total number of [3H]-spiroperidol binding sites increased from 4- to 16-days-posthatch, but no significant differences were found in either brain area across the initial four posthatch days. In the forebrain base, [3H]-spiroperidol receptor density and total binding increased significantly between 1- and 4-days-posthatch, but at 16-days-posthatch there was a slight decrease in receptor density. Binding sites in the forebrain roof were minimal at all ages. As expected, saturation experiments yielded curvilinear plots indicating the presence of high- and low-affinity binding sites. The high-affinity sites probably reflect dopamine D-2 receptors; whereas, the low-affinity sites may reflect other receptor types, possibly serotonin S-2. These results suggest that large doses of haloperidol, which are normally used in chick behavioral research, may produce behavioral effects by antagonizing multiple receptors.

Difference in catalepsy response in inbred rats during chronic haloperidol treatment is not predictive of the intensity of behavioral hypersensitivity which subsequently develops

An animal model was utilized to explore the observed clinical association between a history of significant neuroleptic-induced parkinsonism and an increased incidence for the subsequent development of tardive dyskinesia. Catalepsy-sensitive Fisher rats and catalepsy-resistant Brown Norway rats were treated for 14 days with haloperidol at a dose of either 1 mg/kg or 5 mg/kg daily. Following a 7-day drug withdrawal period, rats were tested for behavioral hypersensitivity to acute challenge with apomorphine and then striata were assayed for 3H-spiroperidol receptor binding. Despite significant interstrain difference in catalepsy response to either neuroleptic dose, Brown Norway rats treated with 5 mg/kg developed behavioral hypersensitivity and D-2 receptor supersensitivity equivalent to that of the similarly treated Fisher rats. Catalepsy, a possible rat analog for neuroleptic-induced parkinsonism, therefore did not predict the intensity of those subsequent behavioral and receptor changes considered to result from chronic antagonism of striatal dopamine receptors and to possibly underlie tardive dyskinesia. Further studies are required to elucidate the less than obvious relationship between extrapyramidal behavioral effects of chronic neuroleptic treatment.

Two classes of [3H]spiperone binding sites in bovine neurohypophysis: D-2 receptors and putative 5-HT2 receptors

Binding of [3H]spiperone was studied in membranes obtained from bovine neurohypophyses devoid of intermediate lobe tissue. Non-linear Scatchard plot suggested the presence of more than a single class of binding sites. Competition experiments using ketanserin, a ligand selective for 5-HT2 receptors, were carried out to ascertain whether serotonergic, in addition to dopaminergic receptors, might be responsible for the heterogeneity of [3H]spiperone binding. Computer-assisted modeling suggested the presence of two classes of binding sites for ketanserin (Kd = 1.6 +/- 0.2 and 366.7 +/- 20.5 nM, respectively). The Kd value for ketanserin binding to the high-affinity sites, as well as the Kd of [3H]spiperone for these sites suggested by the 2 sites model indicate that they represent serotonin 5-HT2 receptors. The [3H]spiperone Kd at the ketanserin low-affinity sites (65 +/- 7 pM) and the rank order of inhibitory potencies for several antagonists show that the low-affinity sites represent dopamine D-2 receptors.

Cloning and expression of a rat D2 dopamine receptor cDNA

Dopamine receptors are classified into D1 and D2 subtypes on the basis of their pharmacological and biochemical characteristics. The D2 dopamine receptor has been implicated in the pathophysiology and treatment of movement disorders, schizophrenia and drug addiction. The D2 dopamine receptor interacts with guanine nucleotide-binding proteins to induce second messenger systems. Other members of the family of receptors that are coupled to G proteins share a significant similarity in primary amino-acid sequence and exhibit an archetypical topology predicted to consist of seven putative transmembrane domains. We have taken advantage of the expected nucleotide sequence similarities among members of this gene family to isolate genes coding for new receptors. Using the hamster beta 2-adrenergic receptor gene as a hybridization probe we have isolated related genes including a cDNA encoding the rat D2 dopamine receptor. This receptor has been characterized on the basis of three criteria: the deduced amino-acid sequence which reveals that it is a member of the family of G-protein-coupled receptors; the tissue distribution of the mRNA which parallels that of the D2 dopamine receptor; and the pharmacological profile of mouse fibroblast cells transfected with the cDNA.

D2 dopamine receptors in rat striatum are homogeneous as revealed by ligand-binding studies

D2 dopamine receptors in rat striatum have been analysed using the binding of [3H]domperidone. Competition experiments were performed with classical dopamine antagonists and antagonists reported by other workers to discriminate D2 dopamine receptor sub-classes. In all cases competition data conformed to a single binding site interaction so that there is no evidence for heterogeneity of the D2 dopamine receptor.

Reaction time and nigrostriatal dopamine function: the effects of age and practice

Normal aged and Parkinsonian individuals lose the ability to initiate movements rapidly (increased reaction time) in parallel with changes in the nigrostriatal dopamine (DA) system. However, the ability of these individuals to improve their reaction time with practice has not been adequately assessed. We have developed a rodent model of human reaction time in which reaction time performance correlates highly with neurochemical measures of nigrostriatal DA integrity. In the present report, 15 young and 10 old male Sprague-Dawley rats were conditioned in a reaction time task to release a lever quickly in response to external stimuli in order to avoid a mild footshock. In order to examine the effects of practice on this reaction time task, the young animals were tested for 5 days at 3, 6 and 9 months of age and the old animals were tested for 5 days at 18, 21, and 24 months of age. From this well-practiced task, reaction time response latencies were measured and compared to measures of nigrostriatal DA function (steady-state levels of DA and its metabolites, D2DA receptor affinity and density). The old animals were slower in response latencies than the young animals. These age differences in response latencies, however, disappeared after several days of testing at each of the 3 test sessions, so that the old animals were not significantly slower than the young animals on days 4 and 5 of each session. As expected, the old animals showed reduced striatal D2DA receptor density with no age differences in DA receptor affinity.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of permanently uncharged dopamine analogs with the D-2 dopaminergic receptor

The purpose of this study was to determine if structural analogs of dopamine in which the side chain nitrogen has been replaced by a permanently uncharged monomethylsulfide, monomethylselenide or sulfoxide group are capable of binding to the striatal D-2 dopamine receptor and acting as agonists at this receptor. All the permanently uncharged dopamine analogs were found to bind to the D-2 dopamine receptor as evidenced by their abilities to inhibit significantly [3H]spiperone binding to striatal homogenates. However, the inhibition of [3H]spiperone binding by the uncharged dopamine analogs was incomplete and was almost abolished by the addition of NaCl (125 mM) to the incubation medium or by the addition of dopamine or quinpirole at a concentration that that saturates the high-affinity state of the D-2 dopamine receptor. These effects of NaCl, dopamine and quinpirole suggest that the uncharged dopamine analogs bind primarily to the high-affinity state of the D-2 dopamine receptor. Whether the uncharged monomethylsulfide and sulfoxide analogs could function as dopamine agonists at the striatal D-2 dopamine receptor was assessed by determining the abilities of these compounds to inhibit the K+-evoked release of [3H]acetylcholine from striatal slices. Both the monomethylsulfide and sulfoxide analogs inhibited the K+-evoked release of [3H]acetylcholine, but this inhibitory effect does not appear to be due to the activation of the D-2 dopamine receptor since it was not reversed by the selective D-2 dopamine antagonist, sulpiride. Additionally, the uncharged monomethylsulfide and sulfoxide dopamine analogs were found to antagonize the ability of apomorphine to inhibit the K+-evoked release of [3H]acetylcholine, but this antagonistic effect does not appear to be due to the reversible blockade of the D-2 dopamine receptor since it was not reduced by increasing the concentration of apomorphine. Therefore, while the permanently uncharged analogs of dopamine appear to bind to the high-affinity state of the D-2 dopamine receptor, they are not dopamine agonists or antagonists at the striatal D-2 dopamine receptor involved in regulating the release of acetylcholine. These results suggest that a positive charge may be a requirement for the activation of the striatal D-2 dopamine receptor.

Movement initiation characteristics in young adult rats in relation to the high- and low-affinity agonist states of the striatal D2 dopamine receptor

Changes in the speed of movement initiation as a function of age, brain damage, or rat strain are associated with altered characteristics of nigrostriatal dopamine (DA) neurons and of striatal D2 DA receptors. In the present study we investigated the relationship between movement initiation (response parameters: percent of successful responses and response latency) and the agonist binding states of the D2 DA receptor in corpus striatum in 3-month-old Sprague-Dawley rats (n = 51). In contrast to the typical experimental procedure, the variances of the behavioral and receptor binding data were intentionally made as small as possible to provide the most stringent test of putative relationships among variables. Rats were trained to release a lever as rapidly as possible in response to a light/buzzer (CS) combination in order to avoid a mild footshock (UCS). Percent avoidance scores, latencies of the fastest successful trials (successful latencies) and mean latencies for all responses (mean latencies) were collected for 1000-, 500-, 300- and 200-ms CS-UCS intervals. Twenty-four hours following the last behavioral test, animals were euthanized for measurements of the high- and low-affinity binding of DA to D2 receptors in corpus striatum. The standard errors of the mean for both behavioral and receptor binding parameters were, generally, less than 10%. The tightness of the receptor binding data appeared to be related to a lack of biological variance in the animals rather than to an artifact associated with the behavioral testing procedure, since a parallel experiment indicated that different numbers of behavioral shaping sessions had no effects on striatal D2 binding characteristics.(ABSTRACT TRUNCATED AT 250 WORDS)

Thermodynamic analysis of antagonist and agonist interactions with dopamine receptors

The binding of [3H]spiperone to dopamine D-2 receptors and its inhibition by antagonists and agonists were examined in microsomes derived from the sheep caudate nucleus, at temperatures between 37 and 1 degree C, and the thermodynamic parameters of the binding were evaluated. The affinity of the receptor for the antagonists, spiperone and (+)-butaclamol, decreased as the incubation temperature decreased; the affinity for haloperidol did not further decrease at temperatures below 15 degrees C. The binding of the antagonists was associated with very large increases in entropy, as expected for hydrophobic interactions. The enthalpy and entropy changes associated with haloperidol binding were dependent on temperature, in contrast to those associated with spiperone and (+)-butaclamol. The magnitude of the entropy increase associated with the specific binding of the antagonists did not correlate with the degree of lipophilicity of these drugs. The data suggest that, in addition to hydrophobic forces, other forces are also involved in the antagonist-dopamine receptor interactions, and that a conformational change of the receptor could occur when the antagonist binds. Agonist binding data are consistent with a two-state model of the receptor, a high-affinity state (RH) and a low-affinity state (RL). The affinity of dopamine binding to the RH decreased with decreasing temperatures below 20 degrees C, whereas the affinity for the RL increased at low temperatures. In contrast, the affinity of apomorphine for both states of receptor decreased as the temperature decreased from 30 to 8 degrees C. A clear distinction between the energetics of high-affinity and low-affinity agonist binding was observed. The formation of the high-affinity complex was associated with larger increases in enthalpy and entropy than the interaction with the low-affinity state was. The results suggest that the interaction of the receptor with the G-proteins, induced or stabilized by the binding of agonist, leads to an increase in entropy and to negative heat capacity changes in the system.

Correlation of activity of chlorpromazine and respective hydroxy, dimethoxy and sulphoxide analogues on dopamine, muscarinic, histamine and calmodulin sites of action

1. Chlorpromazine (CPZ) is a unique molecule which has many potential sites of action, as well as a propensity to be transformed into a host of metabolites possessing varying degrees of pharmacological and/or toxic reactions. This investigation examined the rank order of potency of CPZ and eight metabolic derivatives with respect to displacement of 3H-spiperone at central dopamine-2 (DA-2) receptors, 3H-pirenzepine at central muscarinic-1 (M-1) receptors, and inhibition of calmodulin-induced activation of cyclic AMP-dependent phosphodiesterase. 2. The most potent CPZ analogues to displace labelled spiperone from DA-2 receptors in rat striatum were: 3-hydroxy-CPZ, CPZ, 3,7-dihydroxy-CPZ, and 7-hydroxy-CPZ. Intermediate potency was observed with 8-hydroxy-CPZ, 3,7,8-trihydroxy-CPZ, and 7,8-dihydroxy-CPZ. Chlorpromazine sulphoxide and 7,8-dimethoxy-CPZ displayed the least activity at DA-2 receptors. 3. Displacement of labelled pirenzepine from M-1 receptors in rat frontal cortex occurred to the greatest extent with CPZ which was one to two orders of magnitude more potent than noted for 3-hydroxy-CPZ greater than 7-hydroxy-CPZ greater than CPZ-sulphoxide greater than 8-hydroxy-CPZ greater than 7,8-dimethoxy-CPZ. The least potent agents were 3,7-and 7,8-dihydroxy-CPZs and 3,7,8-trihydroxy-CPZ. 4. A partially purified calmodulin-sensitive preparation of cyclic AMP-dependent phosphodiesterase from guinea pig heart was most sensitive to inhibition by 7,8-dihydroxy-CPZ, 7,8-dimethoxy-CPZ, 3-hydroxy-CPZ, 7-hydroxy-CPZ, 8-hydroxy-CPZ and CPZ. Least inhibition occurred with 3,7-dihydroxy-CPZ, 3,7,8-trihydroxy-CPZ and CPZ-sulphoxide. 5. The DA-2 receptors were more sensitive to the active CPZ analogues than were the M-1 receptors while calmodulin-activated phosphodiesterase was the least sensitive preparation. 6. Comparisons of data were made with existing information from other laboratories and in general CPZ, 7-hydroxy-CPZ and 3-hydroxy-CPZ were the most potent compounds across different test conditions.

Interaction of L-prolyl-L-leucyl glycinamide with dopamine D2 receptor: evidence for modulation of agonist affinity states in bovine striatal membranes

The role of the hypothalamic tripeptide L-prolyl-L-leucyl-glycinamide (PLG) in modulating the agonist binding to bovine striatal dopamine D2 receptor was investigated using a selective high-affinity agonist, n-propylnorapomorphine (NPA). PLG caused an enhancement in [3H]NPA binding in striatal membranes in a dose-dependent manner, the maximum effect being observed at 10(-7)-10(-6) M concentration of the tripeptide. The Scatchard analysis of [3H]NPA binding to membranes preincubated with 10(-6) M PLG revealed a significant increase in the affinity of the agonist binding sites. In contrast, there was no effect of PLG on the binding pattern of the antagonist [3H]spiroperidol. The antagonist versus agonist competition curves analyzed for agonist high- and low-affinity states of the receptor displayed an increase in the population and affinity of the high-affinity form of the receptor with PLG treatment. The low-affinity sites concomitantly decreased with relatively small change in the affinity for the agonists. Almost similar results were obtained when either NPA or apomorphine was used in the competition experiments. A partial antagonistic effect of PLG on 5'-guanylylimidodiphosphate [Gpp(NH)p]-induced inhibition of high-affinity agonist binding was also observed, as the ratio of high- to low-affinity forms of the receptor was significantly higher in the PLG-treated membranes compared to the controls. Direct [3H]NPA binding experiments demonstrated that PLG attenuated the Gpp(NH)p-induced inhibition of agonist binding by increasing the EC50 of the nucleotide (concentration that inhibits 50% of the specific binding). No effect of PLG on high-affinity [3H]NPA binding, however, could be observed when the striatal membranes were preincubated with Gpp(NH)p.(ABSTRACT TRUNCATED AT 250 WORDS)

Perinatal exposure to cannabinoids alters neurochemical development in rat brain

Adult female rats received daily oral doses of delta 9-tetrahydrocannabinol (delta 9-THC), delta 8-THC and cannabidiol (CBD) throughout gestation and lactation. The offspring were sacrificed at various ages and tissue samples of cerebral cortex and striatum were assayed for alpha 1-adrenergic and D2-dopaminergic receptors, respectively. In addition, tyrosine hydroxylase activity was determined in the striatum. The Kd for ligand binding to alpha 1 receptors in the cerebral cortex was significantly increased in 10-day-old offspring exposed to CBD. Significant increases in the Bmax of these receptors occurred at 20 days of age following perinatal exposure to delta 9-THC or delta 8-THC. Exposure to CBD increased the Kd of D2 receptors in the striatum of 10 and 20-day-old offspring compared to control. There were no significant treatment effects on the Bmax of D2 receptors in the striatum at any age. Tyrosine hydroxylase activity was significantly decreased only at 60 days of age in offspring exposed to delta 8-THC or CBD. These results differ from those previously reported with a crude marihuana extract, suggesting that changes in the development of brain catecholamine mechanisms resulting from perinatal exposure to marihuana extracts may be due to an additional constituent of the extract, interactions between specific cannabinoids or other unknown factors.