Renal D1 receptors, and not D2, are upregulated after aortic constriction and may be involved in cardiac hypertrophy

1. The characteristics of dopamine (D) receptors were studied in kidney using the radiolabelled receptor assay of [3H]-SCH-23390 for D1 and [3H]-sulpiride for D2 receptors during cardiac hypertrophy. Male Sprague-Dawley rats (175-200 g) underwent abdominal aortic constriction above the renal arteries and were studied 28 days thereafter. Sham operated animals without aortic constriction were used as control. 2. Membranes obtained from kidney cortex showed an increase in the number of binding sites (Bmax) of D1 receptors in the aortic banded group. The apparent affinity for the ligand (Kd) was unchanged with D1 receptors, as compared to sham control. Both Bmax and Kd were unchanged for D2 receptors in the aortic banded group. 3. Autoradiographic data further reinforced the findings, showing an increased number of D1 receptors in the kidney at 28 days after abdominal aortic constriction. These changes were associated with an increase in plasma renin activity in the aortic banded group. Further, Na(+)-K(+)-ATPase as measured by fmol of 32Pi released from [gamma-32P]-ATP, was decreased in the kidney cortex of banded animals. 4. Reversal of hypertrophic parameters was observed in the aortic banded group treated for 14 days with SCH 23390 hydrochloride (0.1 mg kg-1 i.p.), a known D1 receptor antagonist. 5. The present study shows an upregulation of renal D1 receptors following abdominal aortic constriction and it is suggested that upregulation of D receptors may be involved in the development of cardiac hypertrophy.

D2 dopamine receptor protein location: Golgi impregnation-gold toned and ultrastructural analysis of the rat neostriatum

The neostriatal distribution of D2 dopamine receptor protein has been assessed using subtype-selective polyclonal antibodies generated against three unique polypeptide sequences of the receptor. The experimental tissues were processed by peroxidase based immunohistochemical procedures for routine light microscopy, Golgi impregnation-gold toned morphological characterization, and correlative light/electron microscopy. The results demonstrated a regional gradient of D2-like dopamine receptor expression in the neostriatum, where lateral portions in the nucleus exhibited more reactive cell bodies than medial portions. D2-like expression was detected in the three populations of neostriatal neurons, i.e., the medium-sized spiny projection neurons, and the medium- and large-sized aspiny interneuron types. Morphometric measurements of labeled neurons verified that medium and large diameter neurons expressed the D2-like receptor subtype. D2-like immunoreactivity was distributed throughout the cytoplasm in dendritic processes, and in presynaptic terminal boutons. Immunoreactivity for the receptor protein was also detected in small, thinly myelinated axons, suggesting the possibilities of anterograde transport of the receptor from cell bodies in the substantia nigra to their neostriatal terminal fields, as well as from local axon collaterals of neostriatal projections neurons. These findings provide evidence of widespread distribution of the D2-like receptor protein in neostriatal neurons, and showed that the presynaptic D2 receptors contain analogous epitopes to the postsynaptic receptor subtype.

Dopamine D2 receptor expression in hippocampus and parahippocampal cortex of rat, cat, and human in relation to tyrosine hydroxylase-immunoreactive fibers

A detailed study comparing the distribution of D2 receptors and tyrosine hydroxylase-immunoreactive fibers in the hippocampus and parahippocampal cortices of the rat, cat, and human was conducted. The distribution of [125I]epidepride binding to D2 receptors along the transverse and longitudinal axes of the hippocampus and parahippocampus differed among the species. In rat hippocampus, the number of sites was highest in septal portions of lacunosum-moleculare of CA1 and stratum moleculare of the subiculum. Virtually no binding to D2 receptors existed in the temporal hippocampus. For the cat hippocampus, the highest binding existed in the inner one-third of the molecular layer of the dentate gyrus (DG). There were also significant numbers of D2 receptors in strata radiatum and oriens of the CA subfields, with almost undetectable levels in lacunosum moleculare and subiculum. The number of sites was higher in the septal than temporal hippocampus. In the human hippocampus, highest binding was observed in the molecular layer of DG and the subiculum, with lower levels in strata oriens and lacunosum-moleculare of CA3, and very low binding in CA1. The histochemical demonstration of the pattern of mossy fibers revealed an organization complementary to that of D2 receptors in cat and human. In none of the species was there significant expression of D2 receptors in the entorhinal cortex, except in the caudal extreme of this region in the rat. In that region a trilaminar pattern was exhibited that continued into the perirhinal cortex. A trilaminar pattern of D2 receptor expression was observed in the perirhinal cortex of all species, with the highest values in the external and deep laminae and low expression in the middle laminae. The organization of dopamine fibers was assessed by comparing the distribution of tyrosine hydroxylase-positive and dopamine beta-hydroxylase-immunoreactive fibers in these same regions. It revealed consistent mismatches between the pattern of D2 receptor expression and dopaminergic innervation in all three species. The implications for this mismatch are discussed. It is hypothesized that the distribution of D2 receptors, and not of dopamine fibers, determines what neural systems dopamine influences in the hippocampal complex.

Radioiodinated tracers for the evaluation of dopamine receptors in the neonatal rat brain after hypoxic-ischemic injury

In order to evaluate in vivo single-photon emission tomography (SPET) method of assessing cerebral function after hypoxic-ischemic injury in human neonates, we studied D1 and D2 dopamine receptors in a rat model. Seven-day-old rats underwent permanent unilateral common carotid ligation followed by exposure to 8% O2. Two weeks later, in brains with no visible loss of hemispheric volume, striatal dopaminergic receptors were studied, with [125I]TISCH and [125I]IBZM for the D1 and D2 dopamine receptors, respectively. Using [125I]TISCH, we observed no modifications of D1 receptors, but in contrast, ex vivo and in vitro autoradiographic experiments showed a 40% decrease in the striatal binding of [125I]IBZM on both the ipsilateral and the contralateral side to the carotid ligation. These alterations were detected with IBZM, a D2 dopamine receptor ligand usable for SPET imaging. Therefore, exploration of D2 receptors by SPET in human neonates suffering from perinatal hypoxia-ischemia may be valuable for the diagnosis and follow-up of cerebral function damages.

Dopamine D1 and D2 receptors in the human ureter and urinary bladder: a radioligand binding and autoradiographic study

OBJECTIVE

To analyse the pharmacological profile and the anatomical localization of the dopamine D1 and D2 receptors in sections of the pelvic part of the ureter and of the fundus of the urinary bladder.

PATIENTS, MATERIALS AND METHODS

Samples of the pelvic part of the ureter and of the fundus of the urinary bladder were taken in men (age range 61 +/- 4 years) who were undergoing lower urinary tract surgery. Biochemical characterization and autoradiographical techniques were used on frozen sections of the ureter or the urinary bladder. [3H]-SCH 23390 was used as a ligand of dopamine D1 receptors and [3H]-spiroperidol as a ligand of dopamine D2 receptors.

RESULTS

[3H]-SCH 23390 and [3H]-spiroperidol were bound by specific sections of the ureter and of the urinary bladder. The pharmacological profile of the binding was consistent with the labelling of D1 and D2 receptors respectively. Light microscope analysis of the localization of D1 and D2 receptors revealed the accumulation of the two radioligands in the tunica muscularis of the ureter or of the urinary bladder.

CONCLUSION

A possible role of the dopaminergic system in the control of urine flow and of some dysfunctions of the lower urinary tract is suggested.

Opposing roles for D-1 and D-2 dopamine receptors in the regulation of lower esophageal sphincter motility in the rat

In the present study we have identified biochemically DA receptors in rat Lower Esophageal Sphincter (LES) and have identified their role in the control of the sphincter motility. Dopamine (DA) both stimulated and inhibited cyclic AMP formation in rat LES; the pharmacological characterization of these effects indicated that they were mediated by D-1 and D-2 receptors, respectively. The results obtained with LES helical strips showed that DA plays both inhibitory and stimulatory effects on the sphincter function; the pharmacological characterization with selective D-1 and D-2 agonists and antagonists strongly suggested that D-1 receptors are involved in LES contraction, while D-2 receptors mediate the relaxation of the sphincter. The same results were obtained by measuring intraluminal LES pressure in anesthetized rats. The selective D-1 agonist fenoldopam (40 micrograms/kg, i.v.) increased the LES pressure; on the other hand bromocriptine (10 micrograms/kg, i.v.), which preferentially interacts with D-2 receptors, induced a decrease of the resting LES pressure.

Alterations of neurotransmitter receptors and neurotransmitter transporters in progressive supranuclear palsy

Neurotransmitter receptors and neurotransmitter transporters were studied postmortem in the brains of 9 PSP patients by receptor autoradiography. Densities of dopamine uptake sites and neurotensin receptors were significantly reduced in striatum and substantia nigra consistent with a localization of these binding sites on degenerating dopaminergic nigrostriatal projection neurons. The densities of dopamine D1 receptors were unchanged. Dopamine D2 receptors were unaltered when labeled by [125I]-Iodosulpride or [3H]-CV 205 502, but appeared to be significantly reduced when labeled by [3H]-spiperone. Levels of D2 mRNA were comparable to control levels, suggesting that only subtypes of Dopamine D2-like receptors may be affected in PSP. Serotonin (5-HT) uptake sites and 5-HT receptors were not altered. The density of muscarinic receptors was reduced in striatum, possibly related to a degeneration of cholinergic striatal interneurons, but increased in internal globus pallidus. GABAA/BZ receptor binding sites were significantly reduced in both segments of globus pallidus, probably as a consequence of severe degeneration of intrinsic pallidal neurons in PSP. Binding of substance P in striatum tended to be decreased but failed to reach statistical significance. Compared to Parkinson's disease, the densities of more neurotransmitter receptors were altered in PSP. With the exception of increased muscarinic receptor binding sites in medial globus pallidus, the alterations seen in PSP seem to reflect cell loss rather than functional changes.

Clinical progressive supranuclear palsy: differential diagnosis by IBZM-SPECT and MRI

In order to in vivo identify subgroups in eight patients with the clinical diagnosis of progressive supranuclear palsy (PSP), we have performed 123I-iodobenzamide single photon emission computed tomography (IBZM-SPECT), a nuclear medicine technique, to visualize dopamine D2 receptors in vivo, and high resolution (TE/TR 2900/20-90) magnetic resonance imaging (MRI) to evaluate morphological CNS changes. All patients exhibited similar clinical features including supranuclear vertical gaze palsy, especially of downward gaze, predominantly axial rigidity especially in the neck, bradykinesia, instability of balance with easy falls, and poor response to dopaminergic drugs. Specific striatal dopamine D2 receptor binding in IBZM-SPECT, as calculated by a basal ganglia to frontal cortex ratio (BG/FC) was reduced in 5 patients, but normal in 3 patients. In MRI, these 3 patients exhibited multiple hyperintense white matter lesions; 2 of them had no midbrain atrophy. In contrast, all 5 patients with reduced IBZM binding lacked multiple white matter lesions in MRI, but 4 of them showed marked midbrain atrophy. This pilot study with IBZM-SPECT for in vivo imaging of striatal dopamine D2 receptors and T2-weighted MRI supports published neuropathological findings that clinical signs of PSP appeared to be due to heterogeneous neuropathology.

(-)-2-(N-propyl-N-2-thienylethylamino)-5-hydroxytetralin (N-0923), a selective D2 dopamine receptor agonist demonstrates the presence of D2 dopamine receptors in the mouse vas deferens but not in the rat vas deferens

Experiments were carried out using the D2 dopamine receptor-selective agonist (-)-2-(N-propyl-N-2-thienylethylamino)-5-hydroxytetralin (N-0923) in the rat and the mouse isolated vas deferens to determine whether these tissues contained inhibitory D2 receptors in addition to their inhibitory alpha-2 adrenoceptors. In the mouse vas deferens N-0923 and the alpha-2 adrenoceptor agonist clonidine inhibited the electrically evoked twitch responses. The actions of clonidine, but not of N-0923, were antagonized by the alpha-2 antagonist idazoxan (pKb = 7.9), and responses to N-0923 were antagonized by the D2 antagonist sulpiride (pKb = 8.1). In the rat vas deferens, clonidine, but not N-0923, inhibited the twitch responses and these inhibitions were antagonized by idazoxan (pKb = 7.9) but not by sulpiride. Other D2 receptor agonists were tested in the mouse and in the rat vas deferens for their ability to activate D2 and alpha-2 receptors, respectively. At the D2 receptors in the mouse vas deferens (alpha-2 blocked) the potency order was (+)-propyl-9-hydroxy-naphtoxazine > pergolide > N-0923 = apomorphine > bromocriptine > quinpirole > dopamine. At the alpha-2 receptors in the rat vas deferens the potency order was pergolide > bromocriptine > (+)-propyl-9-hydroxynapthoxazine > apomorphine > quinpirole > or = dopamine. N-0923 was inactive but antagonized the responses to clonidine. N-0923 was therefore the most D2 receptor selective agonist tested.(ABSTRACT TRUNCATED AT 250 WORDS)

123I-iodobenzamide-SPECT in 83 patients with de novo parkinsonism

Single photon emission computed tomography (SPECT) with 123I-iodobenzamide (123I-IBZM) was used in a prospective study to investigate 83 patients with parkinsonism (Hoehn and Yahr stages I to III) who had not been previously treated with dopamimetic drugs. All patients had clinical signs that were compatible with Parkinson's disease. An additional 13 patients had clinical signs of another basal ganglia disorder, such as progressive supranuclear palsy or multisystem atrophy. 123I-IBZM-SPECT results were compared with clinical responses to subcutaneous injections of the D1/D2-receptor agonist apomorphine (83 patients) and to long-term oral dopamimetic therapy (62 patients). Results from 123I-IBZM-SPECT predicted a positive or negative response to apomorphine in 69 of 76 patients (apomorphine responses were equivocal in 7 patients) and a response to dopamimetic therapy in 54 of 62 patients. All patients with a clinical diagnosis of progressive supranuclear palsy or multisystem atrophy had reduced 123I-IBZM binding. In six of these patients, the response to apomorphine was negative, and none clearly benefited from long-term oral levodopa therapy. Imaging of dopamine D2 receptors with 123I-IBZM-SPECT appears to distinguish between patients with de novo parkinsonism that is levodopa-responsive (probably Parkinson's disease of Lewy body type) and that which does not respond to levodopa therapy.

Evaluation of 5-[18F]fluoropropylepidepride as a potential PET radioligand for imaging dopamine D2 receptors

This study evaluated the utility of (S)-N-[(1-ethyl-2-pyrrolidinyl)methyl]-5-(3-[18F]fluoropropyl)-2,3 - dimethoxybenzamide ([18F]fluorpropylepidepride), [18F]5-FPrEpid, as a ligand for PET studies of cerebral dopamine D2 receptors. The in vitro affinity for the rat striatal dopamine D2 receptor, KD 138 pM, was determined by Scatchard analysis of in vitro binding to rat striatal homogenate. The apparent lipophilicity, log kw 1.6, was measured with reverse phase HPLC at pH 7.5. The receptor specificity was determined by competitive displacement of [18F]5-FPrEpid by a variety of neurotransmitter ligands. Only dopamine D2 ligands displaced [18F]5-FPrEpid with high affinity. Positron tomographic imaging studies in primates of [18F]5-FPrEpid demonstrated a stable striatal uptake of 0.02% injected dose/ml for up to 5 h after injection. The striatal: cerebellar ratio increased from 2 at 15 min, to 7 at 200 min, and to 10 at 300 min. Striatal uptake was displaceable by haloperidol (1 mg/kg) or raclopride (2.5 mg/kg) to cerebellar levels with a t1/2 of washout of 9 or 15 min. Striatal uptake was mildly susceptible to displacement by d-amphetamine (1-2 mg/kg) released endogenous dopamine; d-amphetamine administration produced a 10% h increase in the rate of striatal washout. Although uptake in the striatum is reversible, an equilibrium between receptor bound [18F]5-FPrEpid in striatum and [18F]5-FPrEpid in plasma is not reached within 5 h postinjection.

Biodistribution and dosimetry of iodine-123-IBF: a potent radioligand for imaging the D2 dopamine receptor

Iodine-123-labeled iodo-benzofuran (IBF) is a potent D2 dopamine receptor antagonist that has been developed as a potential SPECT imaging agent. This report documents its biodistribution and radiation dosimetry in seven healthy humans. Approximately 100 MBq of IBF were administered to each volunteer. Urine was collected to measure the fraction of the activity that was voided by the renal system. Conjugate images were serially acquired over 24 hr to determine the fraction of activity in the other organs. Standard image analysis techniques were used to measure the geometric mean count rates in the brain, GI tract, heart, liver, lungs and thyroid at each time point. Corrections for attenuation were made with 123I transmission scans. Multicompartmental modeling was used to stimulate and predict the biokinetic behavior of 123I-IBF in the rest of the body. The absorbed doses for 24 organs were then estimated with the MIRD formalism. Rapid biological washout minimized the absorbed dose to most tissues. The excretory organs were exposed to the most radiation. The lower large intestine received about 0.13 mGy/MBq (0.48 rad/mCi), and the urinary bladder received 0.11 mGy/MBq. This low radiation burden will allow more than 370 MBq (10 mCi) to be administered to healthy research subjects during each study of the D2 receptor. Since high quality images of the brain can be obtained with half this amount, the findings suggest that 123I-labeled IBF has a large margin of radiation safety in humans. Its stability in vivo and its high target-to-background contrast ratio in the human brain may make it a useful SPECT imaging agent.

A high dose of EEDQ reduces pituitary dopamine D2 receptor density and the prolactin suppressive potency of agonists

In male rats, a high dose of the alkylating compound N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ, 30 mg/kg s.c., 24 h) caused a reduction of pituitary dopamine D2 receptor density by 27% as measured by means of in vivo radioligand binding (using a single dose of the ligand [3H]spiperone). The same dose of EEDQ reduced the potency, but not the maximal response, of the dopamine D2 receptor agonists R-(-)-N-n-propylnorapomorphine (NPA), (+)-3-(3-hydroxyphenyl)-N-n-propylpiperidine ((+)-3-PPP), and 6-allyl-2-amino-5,6,7,8-tetrahydro-4H-thiazolo[4,5]azepine (B-HT 920) with respect to suppression of prolactin release after pretreatment with gamma-butyrolactone. The measured reduction in dopamine D2 receptor density after EEDQ was of the same magnitude as the reduction in receptor number predicted from the EEDQ induced shift in the dose-response curve of the full dopamine D2 receptor agonist NPA. The findings are discussed in relation to our previous observation that a somewhat lower dose of EEDQ (20 mg/kg s.c., 24 h) effectively reduces the efficacy of partial dopamine D2 receptor agonists while affecting neither the prolactin response to full dopamine D2 receptor agonists nor the density of pituitary dopamine D2 receptors.

Dopamine D2-receptor imaging with 123I-iodobenzamide SPECT in migraine patients abusing ergotamine: does ergotamine cross the blood brain barrier?

Two migraine patients were studied by in vivo SPECT using the dopamine D2-receptor specific radioligand 123I-3-iodo-6-methoxybenzamide (123I-IBZM) during ergotamine abuse and after withdrawal. Results were compared with 15 healthy controls. Striatum/cerebellum and striatum/occipital cortex ratios of count rate density were calculated as a semiquantitative measurement for striatal dopamine D2-receptor binding potential. No differences were found in striatal uptake of 123I-IBZM between healthy controls and the patients when on or off ergotamine. Preliminary evidence suggests that ergotamine may not occupy striatal dopamine D2-receptors to a large extent and thus may not cross the blood brain barrier in large quantities.

Localization of D1 and D2 dopamine receptors in brain with subtype-specific antibodies

Five or more dopamine receptor genes are expressed in brain. However, the pharmacological similarities of the encoded D1-D5 receptors have hindered studies of the localization and functions of the subtypes. To better understand the roles of the individual receptors, antibodies were raised against recombinant D1 and D2 proteins and were shown to bind to the receptor subtypes specifically in Western blot and immunoprecipitation studies. Each antibody reacted selectively with the respective receptor protein expressed both in cells transfected with the cDNAs and in brain. By immunocytochemistry, D1 and D2 had similar regional distributions in rat, monkey, and human brain, with the most intense staining in striatum, olfactory bulb, and substantia nigra. Within each region, however, the precise distributions of each subtype were distinct and often complementary. D1 and D2 were differentially enriched in striatal patch and matrix compartments, in selective layers of the olfactory bulb, and in either substantia nigra pars compacta or reticulata. Electron microscopy demonstrated that D1 and D2 also had highly selective subcellular distributions. In the rat neostriatum, the majority of D1 and D2 immunoreactivity was localized in postsynaptic sites in subsets of spiny dendrites and spine heads in rat neostriatum. Presynaptic D1 and D2 receptors were also observed, indicating both subtypes may regulate neurotransmitter release. D1 was also present in axon terminals in the substantia nigra. These results provide a morphological substrate for understanding the pre- and postsynaptic functions of the genetically defined D1 and D2 receptors in discrete neuronal circuits in mammalian brain.

Dopamine D2 receptors in normal human brain: effect of age measured by positron emission tomography (PET) and [11C]-raclopride

Human post-mortem and animal experimental results suggest a decline of the cerebral dopaminergic neuronal system with age. In this study, the radiotracer [11C]raclopride (dopamine D2 antagonist) and positron emission tomography were applied to determine the effect of age on striatal dopamine D2 receptors in 32 healthy volunteer subjects (age range 21-68 years). Subjects were divided in two age groups on the basis of median age (31 years). An index for specific tracer uptake was calculated for caudate nucleus and putamen. Uptake indices in the older group of subjects were reduced on average 26% in putamen and 20% in caudate nucleus. The decline appeared to be steep until 30 years, but slower afterwards. After 30 years of age the decline of specific raclopride binding was found to be 0.6% per year. These results suggest that dopamine D2 receptor binding sites (mainly post-synaptically located) decrease as a consequence of normal aging in parallel with the decline of the pre-synaptic nigrostriatal dopaminergic system.

The use of [18F]4-fluorobenzyl iodide (FBI) in PET radiotracer synthesis: model alkylation studies and its application in the design of dopamine D1 and D2 receptor-based imaging agents

[18F]4-Fluorobenzyl iodide ([18F]FBI) was prepared, and a series of model alkylation studies were conducted to determine its chemical reactivity toward nitrogen and sulfur nucleophiles of varying nucleophilicities. [18F]FBI was found to react rapidly with secondary amines and anilines to give the corresponding N-[18F]4-fluorobenzyl analogue in high yield. Amides and thiol groups required the use of a base catalyst. The utility of [18F]FBI was documented by investigation of dopamine D1 and D2 receptor-based radiotracers.

Reduced density of dopamine D2 receptors in the rat tail artery as a function of age

The present study was designed to investigate the influence of aging on noradrenaline content and the density and pattern of prejunctional dopamine D2 receptors in the tail (ventral caudal) artery of male Sprague-Dawley rats. Tail artery is frequently used as a model for investigating mechanisms of sympathetic vascular control and contains prejunctional dopamine receptor belonging to the D2 subtype. Noradrenaline levels were reduced in rats of 12 months of age in comparison with 3-month-old animals. A further reduction in catecholamine concentration was found in 24-month-old rats. The density of prejunctional D2 receptors, which was measured in frozen sections of the tail artery by using both radioligand binding and autoradiographic techniques, was reduced by about 35% in 12-month rats in comparison with 3-month rats. A decrease by about 55% versus 3-month rats and by about 20% versus 12-month rats was observed in 24-month-old rats. Neither the pharmacological profile nor the anatomical localization of prejunctional D2 receptors was changed in the rat tail artery as a function of age. The parallel decrease in noradrenaline content and in the density of prejunctional dopamine D2 receptor sites in the tail artery of Sprague-Dawley rats of different ages probably accounts for the lack of an age-related change of prejunctional dopamine D2 receptor-linked responses reported in functional studies.

Dopa resistance in multiple-system atrophy: loss of postsynaptic D2 receptors

Postmortem autoradiography was used to explore the mechanisms underlying L-dopa resistance in 2 patients with multiple-system atrophy. Indices of striatal presynaptic dopamine terminal loss and dopamine (D1 and D2) receptors were provided by 3H-mazindol, 3H-SCH 23390, and 125I-sulpiride binding. Neuronal loss, gliosis, and loss of postsynaptic D2 receptors preferentially involved the middle and posterior of the putamen, that region of the striatum most intimately involved in motor function. Loss of D1 receptors in the same area occurred in only 1 patient. These findings suggest that in multiple-system atrophy, resistance to L-dopa is due to a loss of putamental D2 receptors. The differential effects on D1 and D2 receptors in 1 patient implies that different subpopulations of striatal neurons were selectively involved.

Validation of quantitative brain dopamine D2 receptor imaging with a conventional single-head SPET camera

Phantom measurements were performed with a conventional single-head single-photon emission tomography (SPET) camera in order to validate the relevance of the basal ganglia/frontal cortex iodine-123 iodobenzamide (IBZM) uptake ratios measured in patients. Inside a cylindrical phantom (diameter 22 cm), two cylinders with a diameter of 3.3 cm were inserted. The activity concentrations of the cylinders ranged from 6.0 to 22.6 kBq/ml and the cylinder/background activity ratios varied from 1.4 to 3.8. From reconstructed SPET images the cylinder/background activity ratios were calculated using three different regions of interest (ROIs). A linear relationship between the measured activity ratio and the true activity ratio was obtained. In patient studies, basal ganglia/frontal cortex IBZM uptake ratios determined from the reconstructed slices using attenuation correction prior to reconstruction were 1.30 +/- 0.03 in idiopathic Parkinson's disease (n = 9), 1.33 +/- 0.09 in infantile and juvenile neuronal ceroid lipofuscinosis (n = 7) and 1.34 +/- 0.05 in narcolepsy (n = 8). Patients with Huntington's disease had significantly lower ratios (1.09 +/- 0.04, n = 5). The corrected basal ganglia/frontal cortex ratios, determined using linear regression, were about 80% higher. The use of dural-window scatter correction increased the measured ratios by about 10%. Although comprehensive correction methods can further improve the resolution in SPET images, the resolution of the SPET system used by us (1.5-2 cm) will determine what is achievable in basal ganglia D2 receptor imaging.

Localization of D2 dopamine receptors in vertebrate retinae with anti-peptide antibodies

Dopamine plays an important role in modulating various aspects of retinal signal processing. The morphology of dopaminergic neurons and its physiological effects are well characterized. Two classes of receptor molecules (D1 and D2) were shown pharmacologically to mediate specific actions, with differences between individual groups of vertebrates. In an attempt to better understand dopaminergic mechanisms at the cellular level, we used antisera against D2 receptors and investigated the localization of the dopamine D2 receptor in the retinae of rat, rabbit, cow, chick, turtle, frog, and two fish species with immunofluorescence techniques. Antisera were raised in rabbits to two oligopeptides predicted from rat D2 receptor cDNA; one specific for the splice-variant insertion in the third cytoplasmic loop and the other directed towards the extracellular amino terminal region shared by both short and long isoforms. Preadsorption with the synthetic peptide resulted in a significant reduction of label, indicating the presence of specific binding in all species except turtle and goldfish. The pattern of labelling produced by the two antisera was essentially identical; however, the staining obtained with antiserum to the extracellular motif was always more intense. Specific staining was present in photoreceptor inner and outer segments, and in the outer and inner plexiform layers of all species. In mammals and chick, strongly fluorescent perikarya were observed in the ganglion cell layer and at the proximal margin of the inner nuclear layer. Label may be present in the pigment epithelium but could not be established beyond doubt. This pattern of labelling is in accordance with previous observations on D2 receptor localization by means of radioactive ligand binding and in situ hybridization techniques. It suggests that retinal dopamine acts as a neuromodulator as well as a transmitter. In the distal retina, it may reach its targets via diffusion over considerable distances, even crossing the outer limiting membrane; in the inner and outer plexiform layers, conventional synaptic transmission seems to coexist with paracrine addressing of more distant targets, and D2 receptors are expressed by both amacrine and ganglion cells.

D2-like dopamine receptors in amphibian retina: localization with fluorescent ligands

Dopamine induces several light adaptive changes in amphibian retina via receptors with D2-like pharmacology, but the identity of the primary target cells has not been determined. Using a fluorescent probe consisting of a selective D2 antagonist, N-(p-aminophenethyl)-spiperone (NAPS), derivatized with the fluorophore Bodipy (NAPS-Bodipy), we identified the distribution of dopamine binding sites in the retina of two amphibians, post-metamorphic Xenopus laevis and larval Ambystoma tigrinum. Specific labeling was defined as staining that was displaced by D2 selective ligands (eticlopride or sulpiride), but insensitive to D1 selective drugs (SCH 23390), adrenergic catecholamines (epinephrine or norepinephrine), or serotoninergic analogues (ketanserin). Both rod and cone cells showed specific dopamine D2-like binding sites arranged in clustered arrays on discrete membrane domains of the inner segment. Labeling of photoreceptor outer segments was continuous and was not displaced by competition with D2 selective ligands; this labeling was considered nonspecific. In addition, in both species, clustered binding of the D2-probe was found on Müller cells and on a subset of inner retinal cells with the morphology of amacrine/interplexiform cells. Our data provide direct evidence for D2 receptors on both rods and cones, and suggest that the receptors may be clustered into patches within a discrete cellular domain, the inner segment.

Identification of extrastriatal dopamine D2 receptors in post mortem human brain with [125I]epidepride

The regional distribution of striatal and extrastriatal dopamine D2 receptors in human brain was studied in vitro with (S)-N-[(1-ethyl-2- pyrrolidinyl)methyl]-5-[125I]iodo-2,3-dimethoxybenzamide, [125I]epidepride, using post mortem brain specimens from six subjects. Scatchard analysis of the saturation equilibrium binding in twenty-three regions of post mortem brain revealed highest levels of binding in the caudate (16.5 pmol/g tissue) and putamen (16.6 pmol/g tissue) with lower levels seen in the globus pallidus (7.0 pmol/g tissue), nucleus accumbens (7.2 pmol/g tissue), hypothalamus (1.8 pmol/g tissue), pituitary (1.3 pmol/g tissue), substantia innominata (1.0 pmol/g tissue), and amygdala (0.87 pmol/g tissue). Of note was the presence of dopamine D2 receptors in the four thalamic nuclei studied, i.e. anterior nucleus (1.0 pmol/g tissue), dorsomedial nucleus (0.96 pmol/g tissue), ventral nuclei (0.72 pmol/g tissue), and pulvinar (0.86 pmol/g tissue), at levels comparable to the amygdala (0.87 pmol/g tissue) and considerably higher than levels seen in anterior cingulate (0.26 pmol/g tissue) or anterior hippocampus (0.36 pmol/g tissue). The frontal cortex had very low levels of dopamine D2 receptors (0.17-0.20 pmol/g tissue) while the inferior and medial temporal cortex had relatively higher levels (0.31-0.46 pmol/g tissue). Inhibition of [125I]epidepride binding by a variety of neurotransmitter ligands to striatal, ventral thalamic and inferior temporal cortical homogenates demonstrated that [125I]epidepride binding was potently inhibited only by dopamine D2 ligands. The present study demonstrates that dopamine D2 receptors are present in basal ganglia, many limbic regions, cortex and in the thalamus. The density of thalamic D2 receptors is comparable to many limbic regions and is considerably higher than in cortex.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic clozapine treatment decreases 5-hydroxytryptamine1C receptor density in the rat choroid plexus: comparison with haloperidol

We studied the effects of chronic treatment (14 days) with clozapine (10 and 25 mg/kg/day s.c.) and haloperidol (0.5 mg/kg/day s.c.) on 5-hydroxytryptamine (5-HT)1C receptor characteristics in the rat choroid plexus. In addition, we measured the effects of these treatments on dopamine D2 receptor characteristics in the rat striatum and determined the brain clozapine concentrations. Finally, the functional role of clozapine at the 5-HT1C receptors was evaluated by using phosphoinositide hydrolysis assay. Chronic administration of clozapine decreased, in a dose-related manner, 5-HT1C receptor density in the choroid plexus (by 49 and 70% with 10- and 25-mg/kg/day dose regimens of clozapine, respectively). The affinity of 5-HT1C receptors was not significantly affected, although there was a tendency toward a higher 5-HT1C receptor KD value in the group of rats treated with the 25-mg/kg/day dose regimen of clozapine. However, no detectable levels of residual clozapine were found in the cortices of rats treated with either clozapine dose regimen. Clozapine did not affect striatal D2 receptor characteristics. In turn, haloperidol, in a dose of 0.5 mg/kg/day that caused dopamine D2 receptor upregulation in the striatum, had no effects on 5-HT1C receptor characteristics in the choroid plexus. Phosphoinositide hydrolysis assays showed that clozapine is a 5-HT1C receptor antagonist. In conclusion, an atypical antipsychotic, clozapine, induced a marked downregulation of 5-HT1C receptors after chronic treatment, whereas a classical antipsychotic, haloperidol, did not. Therefore, we suggest that the decrease in the 5-HT1C receptor density after chronic clozapine treatment may contribute to some of its atypical properties.

[Changes in the properties of the brain opiate, DA2, GABA, and 5-HT1 receptors in the progeny of female rats consuming ethanol for a long time during pregnancy and lactation]

The study was undertaken to examine two-week and two-month rats whose mothers had been given alcohol for a long period of time during pregnancy and lactation. Profound changes were found in the properties of all types of the receptors under study. The changes were found in the receptors of both group animals. Substantial changes were encountered in the properties of GABA and DA2 receptors in the brain of two-month rats after acute alcoholization. The changes were differentiated from those seen in GABA and DA2 receptors in the brain of the rats whose mothers had not been given ethanol.

Heterogeneous distribution of dopamine D1 and D2 receptors in the human ventral striatum

The distribution of dopamine D1 and D2 receptors was examined in the human ventral striatum using in vitro quantitative autoradiography. Both D1 and D2 receptors have a markedly heterogeneous distribution, that includes regional differences in binding for the D1 receptor and a pattern of smaller heterogeneities for both receptors. The latter heterogeneities in D1 and D2 binding are interrelated and, in addition, appear to be related to inhomogeneities in the acetylcholinesterase histochemistry and cellular density of the ventral striatum. The present data indicate that in the human ventral striatum the ratio between D1 and D2 receptors varies widely from one area to another.

[3H]quinpirole binding to putative D2 and D3 dopamine receptors in rat brain and pituitary gland: a quantitative autoradiographic study

The putative D2 dopamine receptor agonist quinpirole (LY 171,555) has been extensively used in a variety of in vivo and in vitro studies of D2 receptor-mediated effects and may have even higher affinity for the recently described D3 dopamine receptor. In the present study, conditions for autoradiographic visualization of [3H]quinpirole-labeled D2-like dopamine receptors were optimized and binding to slide-mounted sections was characterized with respect to pharmacology, guanine nucleotide sensitivity and regional distribution. The pharmacological profile of [3H]quinpirole binding in slide-mounted brain sections was: (+/-)-2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene > or = quinpirole > dopamine for putative dopamine agonists; spiperone > (+)-butaclamol > (-)-sulpiride > SCH 23390 >> cinanserin > (-)-butaclamol for antagonists. [3H]Quinpirole binding was decreased in the presence of guanine nucleotides in most brain regions except in the islands of Calleja and the molecular layer of cerebellar lobules 9 and 10. The regional distribution of [3H]quinpirole binding sites roughly paralleled the distribution of [3H]-(-)-sulpiride binding sites, with greatest densities present in the olfactory bulb glomerular layer, islands of Calleja, pituitary intermediate lobe, caudate/putamen, olfactory tubercles and nucleus accumbens. However, significantly greater densities of [3H]quinpirole binding than [3H]-(-)-sulpiride binding were observed in the molecular layer of cerebellar lobules 9 and 10, the islands of Calleja and olfactory bulb glomerular layer in concordance with the recently reported distribution of D3 receptor mRNA in these brain regions. Higher concentrations of [3H]quinpirole binding were also observed in the dorsomedial caudate and pituitary intermediate lobe. These data indicate the utility of [3H]quinpirole to label D3 as well as D2 dopamine receptors.

The cloned dopamine D2 receptor reveals different densities for dopamine receptor antagonist ligands. Implications for human brain positron emission tomography

Since [3H]emonapride ([3H]YM-09151-2), a benzamide neuroleptic, consistently detects more dopamine D2 receptors than [3H]spiperone in the same tissue, we tested whether this property was inherent in the cloned dopamine D2 receptor. We found that the density of dopamine D2 receptors labelled by [3H]emonapride was 1.5-fold to 2-fold (mean of 1.8-fold) higher than the density of dopamine D2 receptors labelled by [3H]spiperone in cells expressing cloned dopamine D2 receptors (either the short form (from rat) or the long form (from human)), matching similar findings in anterior pituitary tissue (rat or pig) or in post-mortem human caudate nucleus tissue. The situation was similar for another benzamide, [3H]raclopride, which revealed 1.3-fold to 1.8-fold (mean of 1.5-fold) more binding sites than that for [3H]spiperone in cell membranes containing cloned dopamine D2 receptors. The apparently different dopamine D2 receptor densities revealed by these two types of 3H-ligands (i.e. [3H]spiperone and the [3H]benzamides), therefore, arise from an inherent property of the dopamine D2 receptor protein. These findings for the cloned dopamine D2 receptor, therefore, partly explain the higher dopamine D2 receptor density measured in human brain (by positron emission tomography) when using radioactive raclopride compared to results using radioactive methylspiperone.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacological characterization and autoradiographic localization of dopamine receptors in human epicardial arteries

The pharmacological properties and the anatomical localization of dopamine (DA) D1 and D2 receptor sites were studied in normal samples of the human right coronary and anterior interventricular arteries by assessing the effect of DA on the cyclic AMP generating system and by using combined radioreceptor binding and autoradiographic techniques. DA caused a concentration-dependent accumulation of cyclic AMP in membranes of right and anterior interventricular coronary arteries. This effect was antagonized by the selective D1 receptor antagonist SCH 23390 and by other DA receptor antagonists. D2 receptor responses negatively coupled to cyclic AMP generation were obtained by incubating membranes of coronary arteries with DA together with SCH 23390 or with D2 receptor agonists. This D2 effect was abolished by the selective D2 receptor antagonist (-)-sulpiride. [3H]SCH 23390 was bound to sections of the coronary arteries in a manner consistent with the labeling of D1 sites. Light microscope autoradiography revealed the localization of D1 sites in the medial layer of the coronary arteries. [3H]Spiroperidol, in the presence of ketanserin, was bound to sections of the coronary arteries in a manner consistent with the labeling of D2 sites. D2 receptor sites were located within the adventitia and the adventitial-medial border of the two arteries, and are probably prejunctional in nature. These findings indicate the existence of both D1 and D2 receptor sites in human right and anterior interventricular arteries. Moreover, they suggest that coronary vasodilation induced by DA or DA receptor agonists may be the result of a direct coronary vasodilatory activity.

Studies on the location of catecholamine receptors in canine sympathetic ganglia

Receptors mediating catecholamine-induced inhibition were studied in cardiac ganglia of pentobarbital-anesthetized dogs. Using selective agonists and antagonists the presence of three receptor subtypes was verified: alpha 1- and alpha 2-adrenoceptors and dopamine D2 receptors. Activation of alpha 1-adrenoceptors or dopamine D2 receptors reduced the response to preganglionic nerve stimulation but not to direct stimulation of the nicotinic acetylcholine receptors of the principal ganglion cells: response to both types of stimulation were reduced by activation of ganglionic alpha 2-adrenoceptors. These results suggested that two inhibitory systems were present in canine sympathetic ganglia and mediated the effects of exogenous catecholamines. One system involved alpha 1-adrenoceptors and dopamine D2 receptors located proximal to the synapse of the pre- and postganglionic neurons and the other involved alpha 2-adrenoceptors located distal to the intraganglionic synapse.

Dopamine receptor subtypes colocalize in rat striatonigral neurons

Dopaminergic neurons of the substantia nigra provide one of the major neuromodulatory inputs to the neostriatum. Recent in situ hybridization experiments have suggested that postsynaptic dopamine receptors are segregated in striatonigral and striatopallidal neurons. We have tested this hypothesis in acutely isolated, retrogradely labeled striatonigral neurons by examining the neuromodulatory effects of selective dopaminergic agonists on Na currents and by probing single-cell antisense RNA populations with dopamine receptor cDNAs. In most of the neurons examined (20/31), the application of the D1 dopamine receptor agonist SKF 38393 reduced evoked whole-cell Na+ current. The D2 agonists quinpirole and bromocriptine had mixed effects; in most neurons (23/42), whole-cell Na+ currents were reduced, but in others (8/42), currents were increased. In cell-attached patch recordings, bath application of SKF 38393 decreased currents as in whole-cell recordings, whereas quinpirole consistently (6/10) enhanced currents--suggesting that D2-like receptors could act through membrane delimited and non-delimited pathways. Changes in evoked current were produced by modulation of peak conductance and modest shifts in the voltage dependence of steady-state inactivation. Antisense RNA probes of dopamine receptor cDNA Southern blots consistently (5/5) revealed the presence of D1, D2, and D3 receptor mRNA in single striatonigral neurons. These findings argue that, contrary to a strict receptor segregation hypothesis, many striatonigral neurons colocalize functional D1, D2, and D3 receptors.

Dopamine D1 and D2 receptors in progressive supranuclear palsy: an autoradiographic study

Dopamine D1 and D2 receptors were studied in brain tissue sections from a typical patient with progressive supranuclear palsy and in 7 age-matched brains. The density of D1 receptors in the caudate-putamen and frontal cortex of the patient was within control limits. By contrast, the density of nigral D1 receptors and striatal D2 receptors was dramatically reduced in the patient as compared to the control brains. This work shows again that the loss of striatal D2 receptors is the most plausible explanation for the poor response to dopaminergic drugs in patients with progressive supranuclear palsy. While the loss of nigral D1 receptors can be explained by the loss of nigral neurons, it seems that neurons bearing striatal D1 receptors are spared in progressive supranuclear palsy. The clinical effects of selective D1 agonists are worth testing in this devastating disorder.

Dynamic SPECT imaging of dopamine D2 receptors in human subjects with iodine-123-IBZM

We studied the uptake, distribution, metabolism and washout of the dopamine D2 receptor ligand [123I]IBZM in healthy subjects (n = 12) with dynamic brain SPECT. The highest radioactivity level was detected in the striatum. Operationally-defined striatal "specific" uptake peaked at 69 min postinjection of radioligand and showed a gradual decline of 15% per hour thereafter. "Specific" uptake at maximal counts represented 53% of the total striatal radioactivity. Two subjects received haloperidol (20 micrograms/kg i.v.) 80 min postinjection of radioligand. Haloperidol caused a 2.6-fold increase in the rate of washout of specific striatal activity in comparison to that in the 10 control subjects and was consistent with drug-induced displacement of radioligand from the dopamine D2 receptor. Two classes of metabolites were detected in plasma and urine: a polar fraction, not extracted by ethyl acetate, and a nonpolar, extractable fraction consisting of parent compound and two compounds having shorter retention times on reversed-phase HPLC. Greater than half the plasma parent was metabolized within 10-15 min after administration. The volume of distribution, estimated from the peak arterial plasma concentration at 50-75 sec, was 7.7-10.2 l; the free (nonprotein bound) fraction of [123I]IBZM after in vitro incubation with blood or plasma was 4.4% +/- 0.4%. These results suggest that [123I]IBZM exhibits uptake in brain regions with high D2 receptor density and shows a relatively stable washout during which drugs affecting dopaminergic transmission may be administered.

Quantitation of carbon-11-labeled raclopride in rat striatum using positron emission tomography

Using conventional autoradiographic and tissue counting techniques, the experimental quantitation of in vivo kinetics of prospective or established radioligands for PET is animal and labour intensive. The present study tested the feasibility of using PET itself to quantitate the specific binding of [11C]raclopride to rat striatum and to study the effects of experimental manipulation of endogenous dopamine on binding parameters. Carbon-11-labeled raclopride was given i.v. to anaesthetised rats, positioned in a PET camera and dynamic emission scans acquired over 60 min. Time-activity curves were generated for selected regions of interest, representing striatum and cerebellum and the striatal data fitted to a compartmental model, using cerebellum as the input function, thus circumventing the need for individual metabolite-corrected plasma curves. In control rats, the binding potential (BP), defined as the ratio of the rate constants for transfer from "free to bound" and "bound to free" compartments, was of the order of 0.6. This was reduced threefold by predosing with nonradioactive raclopride. Increasing extracellular dopamine levels by predosing with d-amphetamine resulted in a significant decrease in BP whereas reducing extracellular dopamine by predosing with gamma-butyrolactone caused a significant increase. Thus, despite the limitation in spatial resolution of PET, specific binding of raclopride could be assessed from regional time-activity curves from individual rats. The system was sufficiently sensitive that changes in BP could be detected following modulation of endogenous dopamine levels, a finding of potential relevance to the interpretation of clinical PET data.

Adaptive changes of dopamine-D2 receptors in rat brain following ethanol withdrawal: a quantitative autoradiographic investigation

The effect of subchronic treatment with two doses of ethanol (5 and 10 vol% drinking fluid) on the density of dopamine-D2 receptors was investigated at two different phases of withdrawal, namely 24 h and 5 days after the cessation of the ethanol application. The number of dopamine-D2 receptors was affected in regions receiving projections from both the substantia nigra as well as the ventral tegmentum. Twenty-four hours after the replacement of the ethanol solution by water, a dose-dependent decrease of D2 receptors was found in all regions (N. caudatus dorsalis, medialis and ventralis, N. accumbens lateralis and medialis, tuberculum olfactorium) and most of the analyzed planes [interaural 7.7-10.2 according to the atlas of Paxinos and Watson (35)]. At day 5 of withdrawal, the number of dopamine-D2 receptors of the animals treated with 5 vol% ethanol reached the level of water controls in most planes. In contrast, two- to three-fold higher numbers were detected in animals treated with the higher dose. Only in the most caudal parts of the investigated regions, was the number of receptors decreased with the higher dose. The mesocorticolimbic system seems to be less sensitive to the effects of ethanol than the nigrostriatal neurones. The findings of the present study suggest an increased activity of dopaminergic neurons with an adaptive reduction of dopamine-D2 receptors during the subchronic treatment with ethanol during the first day(s) of withdrawal. This phase is followed by a reduced turnover rate for up to 7 days (21). The reduced activity of dopaminergic neurones induces a compensatory increase of the number of receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Autoradiographic study of striatal D1 and D2 dopamine receptors in 6-OHDA-lesioned rats receiving foetal ventral mesencephalic grafts and chronic treatment with L-dopa and carbidopa

Foetal dopamine cell suspensions or sham preparations were implanted into the denervated striatum of rats with a unilateral 6-hydroxy-dopamine (6-OHDA) lesion of the medial forebrain bundle. Some animals were also treated with L-DOPA (200 mg/kg/24 h) and carbidopa (25 mg/kg/24 h) in the drinking water for 5 weeks, followed by a 3-week drug-free period. Rotational responses to apomorphine and (+)-amphetamine were assessed, and the density of D1 and D2 dopamine receptors was evaluated autoradiographically in striatal slices exposed to [3H]SCH 23390 or [3H]spiperone. Foetal grafts reduces apomorphine-induced contralateral rotation and prevented the development of apomorphine-induced stereotypy. Foetal grafts abolished (+)-amphetamine-induced ipsilateral rotation. These effects of the grafts were not altered by treatment with L-DOPA. A unilateral 6-OHDA lesion of the nigrostriatal pathway resulted in an ipsilateral increase in D2 receptor density most marked in the lateral and dorsomedial quadrants of the striatum compared with the contralateral side. Foetal ventral mesencephalic grafts implanted into the lesioned striatum decreased D2 receptor density to levels found in the contralateral intact striatum. Chronic L-DOPA and carbidopa treatment did not alter the effect of the grafts. A 6-OHDA lesion resulted in a reduction of D1 receptor density in the lateral areas of the lesioned striatum at Level 2. The presence of a foetal ventral mesencephalic graft either alone or together with L-DOPA treatment did not alter the lesion-induced changes in D1 binding density.