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

Ex vivo [11C]-(+)-PHNO binding is unchanged in animal models displaying increased high-affinity states of the D2 receptor in vitro

Dopamine (DA) D2 receptor supersensitivity has been linked to an increase in the density of the D2 high-affinity state as measured in vitro. The two- affinity-state model of the D2 receptor predicts that the ex vivo specific binding of [11C]-(+)-PHNO, an agonist radiotracer thought to bind selectively to the high-affinity state in vivo, should be increased in animal models that display in vitro increases in the proportion of receptors in the D2 high-affinity state. Here, we test this hypotheses by comparing the ex vivo SBR of [11C]-(+)-PHNO with that of the antagonist radiotracer [3H]-raclopride in three dopaminergically supersensitive rat models-AMPH-sensitized rats, rats withdrawn from chronic ethanol, and unilaterally 6-OHDA-lesioned rats-using ex vivo dual-radiotracer biodistribution studies. We find that in AMPH-sensitized rats and rats withdrawn from chronic ethanol treatment, models that exhibited approximately 4-fold increases in the D2 high-affinity state in vitro, the SBRs of [11C]-(+)-PHNO and [3H]-raclopride are unchanged relative to control rats. In unilaterally 6-OHDA-lesioned rats, we find that the increase in [11C]-(+)-PHNO SBR is no different than that observed for the antagonist radiotracer [3H]-raclopride (54% +/- 16% and 52% +/- 14%, respectively). In addition, the effect of acute AMPH pretreatment (4 mg/kg, i.v.) on the SBRs of [11C]-(+)-PHNO and [3H]-raclopride is equivalent in AMPH-sensitized (-38% +/- 12% and -36% +/- 8%, respectively) and in control rats (-40% +/- 11% and -38% +/- 7%). These data emphasize a significant discrepancy between in vitro and in vivo measures of D2 agonist binding, indicating that the two-affinity-state model of the D2 receptor may not apply veridically to living systems. The potential implications of this discrepancy are discussed.

Positron emission tomography quantification of [11C]-(+)-PHNO binding in the human brain

The kinetic modeling of [11C]-(+)-PHNO binding to the dopamine D2/3 receptors in six human volunteers using positron emission tomography (PET) is described. [11C]-(+)-PHNO is the first agonist radioligand for the D2/3 in humans and as expected showed high uptake in caudate, putamen, globus pallidus (GP) and ventral striatum, and low uptake in cerebellum. A two-tissue compartment model (2CM) with four parameters was necessary to adequately fit time-activity data in all regions. Although a 2CM provided an excellent estimation of total distribution volumes, which were highly correlated with those obtained with the invasive Logan approach, it provided a poor identification of the k3/k4 ratios. Coupling K1/k2 between brain regions (Method C) or fixing K1/k2 to the value obtained in cerebellum (Method D) enabled more stable estimates of k3/k4 as compared with an unconstrained 2CM. The k3/k4 obtained with Method D ranged from 0.12+/-0.03 in cerebellum to 3.93+/-0.77 in GP and were similar to those obtained when coupling K1/k2. Binding potentials (BPs) obtained using the simplified reference tissue model (BP(SRTM)) ranged from 2.08+/-0.34 in caudate to 3.55+/-0.78 in GP and were highly correlated with k3/k4 estimates obtained with Method D (r=0.98). However, BP(SRTM) were 11%+/-5% lower than values obtained with Method D. BPs derived using the noninvasive Logan approach were slightly lower but not significantly different than BP(SRTM). This study demonstrates that [11C]-(+)-PHNO can be used for the quantitative measurement of D2/3 densities and should enable further studies of potential D2/3 dysregulation in several important psychiatric and neurologic illnesses.

High-affinity states of human brain dopamine D2/3 receptors imaged by the agonist [11C]-(+)-PHNO

BACKGROUND

The high-affinity states of dopamine D2-receptors (D2(high)) are postulated to be functionally responsible for signal transduction. At present, no useful in vivo method exists to selectively measure D2(high) in humans, as current D2 radioligands for positron emission tomography (PET) are either not D2-selective or do not differentiate between D2 high- and low-affinity states.

METHODS

The D2-agonist (+)-PHNO [(+)4-propyl-9-hydroxynaphthoxazine] was labeled with carbon-11 and studied with PET. Eight [11C]-(+)-PHNO scans were acquired in four healthy volunteers.

RESULTS

We observed greatest [11C]-(+)-PHNO accumulation in caudate, putamen, and globus pallidus [binding potentials (BPs): 3.00 +/- .4, 3.10 +/- .2, and 4.17 +/- 1.2]. Small but detectable binding was identified in the substantia nigra/ventral tegmental area. Preliminary test-retest data in two subjects suggests BP-estimates to be reliable. Pre-treatment with haloperidol reduced BPs in regions showing specific binding with no detectable changes in cerebellum. Parallel imaging with [11C]-raclopride showed substantial differences in the globus pallidus.

CONCLUSIONS

[11C]-(+)-PHNO proved to be a D2/3-receptor agonist-radioligand with good brain uptake and favorable kinetics for PET in humans. [11C]-(+)-PHNO delineated D2/3-receptor rich brain regions with high signal-to-noise ratio. This is the first demonstration of a viable agonist-radioligand for D2 receptors in humans and opens the door for investigating D2(high) in health and disease.

Association of seven polymorphisms of the D2 dopamine receptor gene with brain receptor-binding characteristics

Association of alleles at the Taq1 A, Taq1 B, intron 6, Taq1 D, exon 7, exon 8, and promoter-141C sites of the D2 dopamine receptor gene with D2 dopamine receptor binding characteristics in the caudate nucleus of Caucasian alcoholic and nonalcoholic subjects was determined. For the Taq1 D, exon 7, exon 8, and promoter- 141C sites there were no significant allelic differences in Bmax (number of binding sites) or Kd (binding affinity) of the D2 dopamine receptors. However, subjects having the minor alleles at the Taq1 A, Taq1 B, and intron 6 sites had significantly lower Bmax than subjects not having them. None of these three polymorphisms had any significant effect on Kd. Highly significant linkage disequilibria were observed among the Taq1 A, Taq1 B, and intron 6 polymorphic sites, but linkage disequilibria between these three sites and each of the Taq1 D, exon 7, exon 8, and promoter-141C sites were of lesser or of no significance. Taken together, these findings suggest that the Taq1 A, Taq1 B, and intron 6 polymorphisms, but not the Taq1 D, exon 7, exon 8, and promoter-141C polymorphisms, are in linkage disequilibrium with a functional allelic variant that affects D2 dopamine receptor expression.

Dopamine D2 receptor gene expression in human adenohypophysial adenomas

The inhibitory effects of dopamine on adenohypophysial cells are mediated via dopamine subtype 2 receptor (D2R). Dopamine agonists inhibit hormone release and induce tumor shrinkage in most prolactin-secreting adenomas, whereas in other adenoma types such effects are sporadic. We investigated D2R gene expression by in situ hybridization (ISH) and immunocytochemistry in different types of pituitary adenomas. By ISH, a variable D2R signal was detected in 79 of 89 cases: 4 of 6 densely granulated and 8 of 8 sparsely granulated somatotroph, 4 of 4 mammosomatotroph, 7 of 7 mixed somatotroph-lactotroph, 4 of 4 acidophil stem cell, 16 of 16 sparsely granulated lactotroph, 11 of 16 corticotroph (functioning and silent), 3 of 4 silent subtype 3, 5 of 5 thyrotroph, 5 of 6 gonadotroph, 5 of 6 null cell, and 7 of 7 oncocytic adenomas. By immunocytochemistry, D2R protein was localized in cytoplasm and nuclei of 60 of 62 adenomas. In lactotroph adenomas, long-acting bromocriptine (BEC-LAR) induced a major increase in D2R mRNA, which was not accompanied by increased D2R immunoreactivity, suggesting mRNA stabilization. In conclusion, D2R gene is expressed in the majority of pituitary adenomas representing all tumor types. The significance of nuclear localization of D2R protein remains to be clarified.

Dopamine agonists used in the treatment of Parkinson's disease and their selectivity for the D1, D2, and D3 dopamine receptors in human striatum

1. It has been suggested that an ideal antiparkinsonian treatment requires stimulation of both D1 and D2 dopamine receptors. Bromocriptine and lisuride are regarded as pure D2 receptor agonists, whereas pergolide and apomorphine are thought to stimulate both D1 and D2 receptors. 2. The aim of this study was to compare the affinities of bromocriptine, lisuride, pergolide, and apomorphine for the D1, D2, and D3 receptors in postmortem human striatum. The dissociation constants (Ki values) of the dopamine agonists were determined from competition binding experiments with selective radioligands. 3. The Ki values of the orally administered agonists--bromocriptine, pergolide, and lisuride--for the D2 receptors were proportional to their optimal doses against parkinsonism. Ki(D1)/Ki(D2) ratios were 23 for lisuride, 67 for pergolide, 60 for bromocriptine, and 2.6 for apomorphine. Ki(D3)/Ki(D2) ratios were 0.4 for lisuride, 1 for pergolide, 5.4 for bromocriptine, and 21 for apomorphine. 4. The present results support the hypothesis that the antiparkinsonian effect of dopamine agonists is mediated primarily by D2 receptors. Apomorphine is a mixed D1/D2 agonist, but pergolide has no more D1 agonist properties than bromocriptine and lisuride. The role of the D3 receptors is unknown, but their activation might either be associated with the generation of psychiatric side-effects or dyskinesias, or alternatively add to antiparkinsonian activity.

Dopamine D4 receptors and effects of guanine nucleotides on [3H]raclopride binding in postmortem caudate nucleus of subjects with schizophrenia or major depression

The densities of dopamine-D4 receptors were determined in postmortem samples of caudate nucleus from patients with schizophrenia (n = 9) and age-matched controls (n = 10). D4 receptor binding was defined as the difference between binding sites labeled by [3H]YM-09151-2 (D2 + D3 + D4 receptors) and those by [3H]raclopride, in the presence of 5'-guanylylimidodiphosphate (Gpp(NH)p) (D2 + D3 receptors). D4 receptor binding was measurable in all the subjects with schizophrenia (mean = 3.8 pmol/g tissue) but only in 3/10 controls. To determine the specificity of these findings for schizophrenia, D4 receptor binding was also measured in the caudate nucleus of suicide victims with major depression (n = 6) and age-matched controls (n = 6). A small amount of D4 binding was noted in some of the controls + depressed subjects and there was no significant difference between controls and patients with major depression. The addition of 200 microM Gpp(NH)p to the assay significantly increased the amount of specific binding of [3H]raclopride in control tissues, but not in tissues from subjects with schizophrenia, suggesting an abnormality in the G-protein component coupled to the D2 receptor. [3H]Raclopride binding was also significantly increased by Gpp(NH)p in subjects with major depression. These results confirm a previous report of Seeman et al. (1993) and suggest that measurable D4 receptor binding in the caudate nucleus is more frequent in patients with schizophrenia as compared with normal controls and subjects with major depression and that guanine nucleotides do not enhance [3H]raclopride binding in schizophrenia.(ABSTRACT TRUNCATED AT 250 WORDS)

An autoradiographic study of the differential effects of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) on striatal and extrastriatal D-1 and D-2 dopamine receptors in the rat

The effect of in vivo administration of the alkylating agent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) on striatal and extrastriatal D-1 and D-2 dopamine (DA) receptors was investigated in the rat. N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline treatment reduced specific [3H]SCH 23390 (7-chloro-8-hydroxy-2,3,4,5-tetrahydro-3-methyl-1-phenyl-1H-3-benzaze pin e) binding to D-1 DA receptors in the striatum (42-46% of saline-treated controls), entopeduncular nucleus (20%) and substantia nigra pars reticulata (23%). Similarly, specific [3H]spiperone binding to D-2 DA receptors was decreased in the striatum (28-37% of saline-treated controls). However, [3H]spiperone binding in the substantia nigra pars compacta (67%) was much less affected. In vivo pretreatment with the D-1 DA antagonist SCH 23390 selectively and dose dependently protected [3H]SCH 23390 binding against the effects of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline in the striatal/extrastriatal regions. Pretreatment with the D-2 DA antagonist raclopride or the D-2 DA agonist quinpirole selectively protected [3H]spiperone binding. In contrast, pretreatment with the D-1 DA agonist SKF 38393 (7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine) not only protected [3H]-SCH 23390 binding but at very high doses protected striatal [3H]spiperone binding. The differential alkylating effects of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline on striatal vs extrastriatal D-1 and D-2 DA receptors may be related to their post- (striatal DA receptors) and pre-synaptic (extrastriatal DA receptors) localizations, respectively. The present results further demonstrate that in vivo, SCH 23390 and raclopride/quinpirole retain their D-1 and D-2 DA receptor selectivity.

Biphasic displacement of [3H]YM-09151-2 binding in the rat brain by thioridazine, risperidone and clozapine, but not by other antipsychotics

The radioligand [3H]YM-09151-2 ((+/-)-cis-N-(1-benzyl-2- methylpyrrolidin-3-yl)-5-chloro-2-methoxy-4-methylamino benzamide) was used to study the binding of various antipsychotic agents. Saturation experiments showed that [3H]YM-09151-2 labelled a single population of binding sites in both the olfactory tubercle and the striatum (dissociation constants (KD): 36 +/- 3 pM and 26 +/- 2 pM, respectively). The total number of binding sites (Bmax) was greater in the striatum than in the olfactory tubercle (18.1 +/- 1.8 fmol/mg tissue and 5.3 +/- 0.9 fmol/mg tissue respectively). Risperidone and thioridazine displaced [3H]YM-09151-2 in a biphasic manner in both brain regions, and clozapine also produced biphasic displacement curves in the olfactory tubercle but not in the striatum. All other dopamine D2 receptor antagonists tested displaced [3H]YM-09151-2 in a monophasic manner in both brain regions, in agreement with previously published data. Biphasic displacement did not appear to result from interactions with either the dopamine D3, dopamine D4, 5-HT2, 5-HT1C or the 5-HT1A receptor binding sites. It is suggested that thioridazine, risperidone and clozapine might discriminate between different affinity states and/or subtypes of the dopamine D2 receptor which may be different from the recently identified D2short and D2long receptors.

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.

Subtypes and localization of dopamine receptors in human brain

Dopamine receptors in the human brain play an important role in the pathophysiology and treatment of psychosis and movement disorders. Pharmacological and biochemical studies, and more recently gene cloning techniques, have demonstrated that there are multiple receptors for dopamine in the brain. There is confusion in the literature as new subtypes (D3, D4 and D5 receptors) were classified according to unspecified criteria. At present, however, all subtypes of dopamine receptors that have been identified still fit in the traditional D1/D2 dopamine receptor classification scheme. In this article, a more hierarchical system of nomenclature is proposed and our insights into the distribution and localization of the dopamine receptor subtypes in human brain are summarized. Although the current status of the different subtypes of the D1 and D2 receptor families in human brain remains unclear, their discovery has created hope for the development of more efficacious and specific medicines with less side-effects.

Regional Distribution of ?2A-and ?2B-Adrenoceptor Subtypes in Postmortem Human Brain

The newly available and highly selective radiolabeled antagonist [3H]RX 821002 was used to examine the distribution of alpha 2 adrenoceptors in human brain. High densities of alpha 2 adrenoceptors were found in the hippocampus, frontal cortex, thalamus, amygdala, pons, and medulla oblongata. Intermediate densities were observed in the striatum (nucleus accumbens, nucleus caudatus, and putamen), globus pallidus, and substantia nigra. The KD values for [3H]RX 821002 were similar in all regions (ranging from 2.8 to 7.5 nM). On the basis of their different affinities for prazosin and oxymetazoline, the alpha 2 adrenoceptors have been divided into alpha 2A and alpha 2B subtypes. To examine the alpha 2A/alpha 2B-adrenoceptor ratio in the different brain regions, we performed oxymetazoline and prazosin/[3H]RX 821002 competition binding experiments. In frontal cortex membranes, the competition curves with prazosin were steep, indicating a single class of binding sites, whereas the competition curves with oxymetazoline were shallow and fitted by computer best to a two-site model. However, in the presence of GTP, the high-affinity sites for oxymetazoline were partially converted into low-affinity sites, indicating that this agonist interacts with high- and low-affinity states of the alpha 2 adrenoceptors. This implies that oxymetazoline is not very suitable for discriminating the alpha 2A- and alpha 2B-receptor subtypes in radioligand binding studies. Therefore, prazosin/[3H]RX 821002 competition binding experiments were used to investigate the distribution of the alpha 2-adrenoceptor subtypes in human brain. The alpha 2A-receptor subtype was detected in all brain regions examined. In contrast, alpha 2B receptors were only observed in striatum and globus pallidus.

Dopamine, the dopamine D2 receptor and pituitary tumours

Dopamine plays an important role in the hypothalamic-pituitary axis where its major effects are to inhibit pituitary hormone secretion and cell division. Chronic dopamine deficiency has been postulated as a cause of pituitary tumour formation and several lines of evidence exist to suggest that a functional deficiency may develop as a result of defective dopamine receptor action. The available data suggest that a number of sites in the dopamine-D2 receptor-second messenger pathways may be implicated. These abnormalities are reflected in the variety of responses to dopamine and its agonists which have been observed in pituitary tumours both in the clinical situation and in cultured cells in vitro. Whilst it seems likely that the primary defect in pituitary tumour formation lies within the pituitary itself, the role of hypothalamic factors in facilitating tumour growth remains to be explored. Further studies of the dopamine receptor and its function will be of value not only in pathophysiological studies of human pituitary adenomas, but also in the development of new pharmacological agents to treat patients with these tumours.

A post-mortem study on striatal dopamine receptors in Parkinson's disease

Striatal dopamine D1 and D2 receptors were investigated in 49 patients with Parkinson's disease (PD) and 33 age-matched controls with [3H]SCH 23390 and [3H]spiroperidol as ligands respectively. A full Scatchard analysis giving Bmax and Kd values was performed. In the caudate nucleus, a small but significant decrease in the number of D1 and D2 receptors was seen, whereas in the putamen the number of dopamine receptors was unchanged. Treatment with neuroleptics was found to increase the number of D2 receptors both in the caudate nucleus and putamen. The number of neither D1 nor D2 receptors correlated neither with the duration of disease or levodopa treatment of the patients nor with the post-mortem delay or storage time of the samples. Furthermore, no association was found between either D1 or D2 receptor number and clinical variables of the patients. The activity of choline acetyltransferase (ChAT) was found to be unchanged in the striatum, whereas a marked decline was seen in the hippocampus and cortical areas, indicating that intrinsic striatal cholinergic neurons are not affected in PD. The present results suggest that there is a modest decline in the number of striatal dopamine D2 receptors in advanced patients with PD at the end stage of the disease.

Dopamine receptor changes in untreated and (+)-PHNO-treated MPTP parkinsonian primates

Fifteen monkeys (Macaca fascicularis) were utilized in this study. Seven naive animals received no treatment and served as controls. Eight animals were rendered parkinsonian with serial injections of MPTP. Three of the parkinsonian monkeys were treated with (+)-4-propyl-9-hydroxynaphthoxasine [(+)-PHNO], a selective dopamine D2 agonist. (+)-PHNO (2-5 micrograms/kg/h) was administered continuously using subcutaneous osmotic pumps. All animals were given weekly scored neurologic examinations throughout the study. Their movement was quantitated in an activity box. The animals were sacrificed 30-120 days after their last MPTP injection by an overdose of sodium pentobarbital. The brains were removed, frozen and cut into 20-microns sections. The density of D1 and D2 receptors was studied in the basal ganglia of these animals at the level of the anterior commissure. For the D2 assay, total binding was determined using various concentrations of [3H]spiperone in buffer containing 300 nm mianserine. For the D1 assay, total binding was determined using various concentrations of [3H]SCH-23390. Tissue isotope concentration was determined from the autoradiographs. The parkinsonian animals demonstrated 90-97% dopamine depletion in the striatum. There was a 75-90% decrease in free movement in the untreated parkinsonian monkeys and their composite clinical score was 8.9 on a scale of 0-16 (zero being normal). Control monkey scores averaged 0.6. The untreated parkinsonian monkeys demonstrated an increase in the number of D2 sites as compared to controls. This increase was greatest at the lateral putamen. The (+)-PHNO-treated monkeys demonstrated increased activity, a neurologic score of 3.4, and a 40-70% decreased in D2 sites in both caudate and putamen. There was no change in the number of D1 binding sites in both the untreated and the (+)-PHNO-treated parkinsonian monkeys as compared to controls.

Mechanisms of action of atypical antipsychotic drugs. Implications for novel therapeutic strategies for schizophrenia

The mechanisms which contribute to the actions of atypical antipsychotic drugs, such as clozapine and the putative atypical agents remoxipride and raclopride, are reviewed. Examination of available preclinical and clinical data leads to two hypotheses concerning the mode of action of atypical antipsychotic drugs. The first hypothesis is that antagonism of the dopamine D2 receptor is both necessary and sufficient for the atypical profile, but that interaction with subtypes of the D2 receptor differentiates typical from atypical antipsychotic drugs. The second hypothesis has been previously advanced, and suggests that a relatively high ratio of serotonin 5-HT2:dopamine D2 receptor antagonism may subserve the atypical profile. It seems likely that the atypical antipsychotic drug profile may be achieved in more than one way.

Dopamine D2 receptors attenuate phosphatidylinositol 4,5-bisphosphate in synaptosomal membranes from rat neostriatum

The effects of dopamine on [32P]ATP-labelled phosphatidylinositol 4-phosphate, phosphatidylinositol 4,5-bisphosphate, and phosphatidic acid were analyzed by TLC in synaptosomal membranes of the rat neostriatum. The incorporation of 32P into these compounds was found to be stable within 1 min and was maintained during the 30 min of incubation. Dopamine (0.1-10 microM) was found to attenuate the levels of phosphatidylinositol 4,5-bisphosphate without affecting the levels of phosphatidylinositol 4-phosphate or phosphatidic acid. The maximal decrease (-35 +/- 4%) was reached at 10 microM of dopamine after 30 min of incubation. The dopamine (0.1 microM)-induced decrease was blocked by the D2 selective antagonist raclopride (1 microM), but not by the D1 selective antagonist SCH 23390 (1 microM). These findings indicate the existence of an intramembrane coupling of dopamine D2 receptors to phosphoinositide turnover and may underlie some of the physiological effects of D2 receptor stimulation.

Pituitary and brain D2 receptor density measured in vitro and in vivo in EEDQ treated male rats

The effect of the alkylating compound N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) (20 mg/kg, 24 h) on dopamine D2 receptor density in rat pituitary and brain was measured using in vitro and in vivo radioligand binding techniques. In the in vitro radioligand binding experiments EEDQ was found to reduce the density (Bmax) of [3H]-spiperone binding sites in the striatum by 86% while in the pituitary the corresponding decrease was only 37%. The affinity (KD) of the remaining striatal and pituitary D2 receptors was not different in EEDQ treated animals as compared to controls. When D2 receptor density was measured in vivo the effect of EEDQ was less pronounced. Thus, in rats given EEDQ the specific binding of either of the two D2 ligands [3H]-raclopride or [3H]-spiperone (administered in a single dose) in striatum and in the limbic forebrain was reduced by 45-62%; moreover, no significant decrease in pituitary D2 receptor density was observed. The data are discussed in relation to the finding (presented in a separate paper) that the same dose of EEDQ that failed to influence pituitary D2 receptor density as measured in vivo effectively antagonizes the prolactin decreasing effect of the partial D2 agonist (-)-3-(3-hydroxyphenyl)-N-n-propyl-piperidine [(-)-3-PPP].

Age-related changes in the human nigrostriatal dopaminergic system

A deficiency of dopamine in the striatum may contribute to the decline in motor function associated with aging. We examined the effect of aging on the densities of the D1 and D2 dopamine receptors, their high-agonist affinity sites, and the dopamine uptake sites in postmortem human putamen (n = 32; age range, 19-88 years). With aging a steady decrease was seen in dopamine uptake sites (p less than 0.001), confirming previous morphometric and biochemical data of an age-related loss of nigrostriatal axons. In contrast, the concentrations of the D1 and D2 dopamine receptors and their high-agonist affinity sites, which are believed to represent the functionally active receptors, were not affected by the aging process. These results may have implications for the pharmacological treatment of the age-related decline in motor function.

The dopamine D2 receptor: two molecular forms generated by alternative splicing

Cloned human dopamine D2 receptor cDNA was isolated from a pituitary cDNA library and found to encode an additional 29 amino acid residues in the predicted intracellular domain between transmembrane regions 5 and 6 relative to a previously described rat brain D2 receptor. Results from polymerase chain reactions as well as in situ hybridization revealed that mRNA encoding both receptor forms is present in pituitary and brain of both rat and man. The larger form was predominant in these tissues and, as shown in the rat, expressed by dopaminergic and dopaminoceptive neurons. Analysis of the human gene showed that the additional peptide sequence is encoded by a separate exon. Hence, the two receptor forms are generated by differential splicing possibly to permit coupling to different G proteins. Both receptors expressed in cultured mammalian cells bind [3H]spiperone with high affinity and inhibit adenylyl cyclase, as expected of the D2 receptor subtype.

In human brain two subtypes of D1 dopamine receptors can be distinguished on the basis of differences in guanine nucleotide effect on agonist binding

D1 dopamine receptors were identified in membranes of human nucleus caudatus, nucleus accumbens, amygdala, and globus pallidus, by the specific binding of [3H](+)-R-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-benzazepine-7 -ol [( 3H]SCH 23390). In these four brain regions, dopamine/[3H]SCH 23390 competition binding curves were computer-analyzed to a two-site model, distinguishing a high- (RH) and low- (RL) affinity site for dopamine. The ability of guanine nucleotides (0.4 mM GTP or 0.1 mM 5'-guanylylimidodiphosphate) to provoke a conversion of RH into RL was different between these brain regions. In amygdala, a complete conversion was seen, whereas there was no guanine nucleotide-effect on RH in globus pallidus. In nucleus caudatus and nucleus accumbens, guanine nucleotides provoked only a partial conversion of RH into RL, suggesting that these brain regions may contain guanine nucleotide-sensitive and -insensitive receptors. Heating of the membranes at 60 degrees C for 5 min had the same effect as guanine nucleotides. The pharmacological profiles of the guanine nucleotide-sensitive and -insensitive D1 receptors were similar, suggesting that D1 receptors in human brain are heterogeneous only with respect to their effector-coupling mechanism: guanine nucleotide-sensitive receptors, which are capable of undergoing functional coupling with Gs, and guanine nucleotide-insensitive receptors, which are not.(ABSTRACT TRUNCATED AT 250 WORDS)

Lack of GTP-insensitive D2 dopamine receptors in Huntington's disease

Previous studies have shown that the mammalian neostriatum contains two subtypes of D2 dopamine receptors, which can be distinguished on the basis of the ability of GTP to convert high (RH) into low (RL) affinity sites for dopamine: GTP-sensitive (GS) and GTP-insensitive (GI) D2 receptors. The GI-D2 receptors in rat and human neostriatum are confined to the corticostriatal terminals. In rats, these receptors mediate the inhibitory effect of dopamine on the release of glutamate in the striatum. Here we report that the putamen in Huntington's disease (HD) lacks GI-D2 receptors. Their absence might be responsible for an inappropriate release of glutamate, which is neurotoxic in high concentrations, and might thus contribute to striatal cell death in HD.