Effect of flunarizine hydrochloride on striatal D-2 dopamine receptors

Flunarizine hydrochloride (FZ) is used to improve cerebral circulation and possesses Ca antagonistic effects. In recent years, this drug has been reported to induce parkinsonism and depressive symptoms as side effects, particularly in the elderly. Effects of FZ on dopamine receptors of the rat striatum were studied by radiolabeled receptor assay to clarify the mechanism of onset of parkinsonism in response to FZ. FZ was found to directly and competitively affect D-2 receptors without affecting D-1 receptors. Furthermore, the effect of FZ on D-2 receptors was found to be antagonistic based on the finding that the displacement curve for FZ in the binding of [3H]spiperone to D-2 receptors remained unchanged even after the addition of GppNHp. The effect of FZ on the D-2 receptors in aged rats was more marked than that in young-adult rats. In addition, the tertiary structures of FZ and the anti-schizophrenic agents, pimozide and haloperidol, were examined using computer graphics. FZ was found to have a tertiary structure highly analogous to pimozide and haloperidol, and FZ also had an alkyl structure linking a fluorophenyl group and a nitrogen atom, believed to be particularly necessary for the binding of anti-schizophrenic agents to D-2 receptors. These results may contribute to clarifying the mechanism of onset of parkinsonism in response to FZ, especially in the elderly.     

PET demonstrates different behaviour of striatal dopamine D-1 and D-2 receptors in early Parkinson's disease

Striatal dopamine D-1 receptor binding was investigated in vivo with positron emission tomography (PET) in five patients with early Parkinson's disease using [11C]-SCH 23390. All patients had predominantly unilateral symptoms and showed a significant reduction in the accumulation of [18F]-6-F-DOPA in the striatum contralateral to the symptoms. None of the patients had received any antiparkinsonian medication. The striatal and cerebellar radioactivity was measured and corresponding striatum/cerebellum ratios were counted. The mean striatum/cerebellum ratio of [11C]-SCH 23390 binding was symmetric between the hemispheres. By contrast, the striatum/cerebellum ratio of [11C]raclopride binding, labelling dopamine D-2 receptors, was increased significantly in the hemisphere contralateral to the symptoms as compared with the opposite hemisphere. Thus, the present results show that the behaviour of striatal D-1 and D-2 receptors is different in early Parkinson's disease.     

Distribution of dopamine D-2 receptors in the rat striatal complex and its comparison with acetylcholinesterase

The distribution of D-2 dopamine receptors in the rat striatal complex was studied with autoradiography after specific in vivo labeling with the dopamine agonist [3H]N-n-propylnorapomorphine and subsequent irreversible fixation. This labeling technique allows the visualization of D-2 receptors at the cellular level by light microscopic emulsion autoradiography. During the preparation of emulsion autoradiograms, the recovery of the label was 75%, the specific and the aspecific label being equally affected. The distribution of label before and after the loss of radioactive label occurred, did not show differences. In rat neostriatum, dopamine D-2 receptors are not homogeneously distributed: in the caudate-putamen the density is laterally higher than medially. Moreover, there exists a mosaic-like pattern of receptor density. In the ventral striatum, comprising the fundus striati, nucleus accumbens septi and olfactory tubercle, the receptor density is lower than in the caudate-putamen, except for the core regions in the islands of Calleja and the rim of these islands, which contain high (as high as the lateral caudate-putamen) and a moderate density of receptors, respectively. In caudate-putamen and lateral nucleus accumbens it appeared that the intensity of acetylcholinesterase staining parallels more or less the distribution of dopamine D-2 receptors. In medial nucleus accumbens and in olfactory tubercle, the high intensity of acetylcholinesterase is not paralleled by a high D-2 receptor labeling density. This receptor labeling density does not seem to be matched by differences in densities of medium-sized neuronal cell bodies.     

Dark-induced supersensitivity of dopamine D-1 and D-2 receptors in rat retina.

Light deprivation of rat retina leads to a rapid (within 6 h) development of a state of supersensitivity (upregulation) of dopamine D-1 receptors (positively coupled to adenylate cyclase), which are essentially involved in the modulation by light of the electrical activity and communication between horizontal cells. In contrast, the supersensitivity of D-2 receptors (negatively coupled to cAMP generating system) appears to develop only after 2 days (better after 4 days) of dark adaptation, although these receptors are linked to multiple light-dependent retinal functions. These results suggest the existence of different mechanisms of sensory adaptation for these two subtypes of dopamine receptors.     

Alterations in striatal and extrastriatal D-1 and D-2 dopamine receptors in the MPTP-treated common marmoset: An autoradiographic study

In adult common marmosets (Callithrix jacchus), MPTP (1-methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine) treatment induced almost total depletion of cells in the substantia nigra pars compacts (SNc) but partial cell loss in the ventral tegmental area (VTA). There was severe depletion of [³H]-mazindol binding to dopamine (DA) uptake sites in the caudate, putamen, and SNc. The loss of [³H]-mazindol binding in the nucleus accumbens (NAc) and olfactory tubercle (OT) was less marked. [³H]-mazindol binding in the body of caudate nucleus showed a small but significant recovery with increasing post-lesion survival times. The specific binding of [³H]-SCH 23390 to D-1 DA receptor sites was increased after MPTP treatment in all subregions of both caudate and putamen but was unaltered in the NAc and OT. Substantia nigra pars reticulata (SNr), frontal cortex, and medial segment of globus pallidus (GPm) all demonstrated moderate levels of [³H]-SCH 23390 binding in control animals, which were unaffected by MPTP treatment. Specific [³H]-spiperone binding to D-2 DA receptor sites was not altered by MPTP treatment in the subregions of caudate-putamen. Moderate levels of [³H]-spiperone binding were observed in control animals in the NAc, OT, SNc, and the lateral segment of globus pallidus (GPl). [³H]-spiperone binding in the SNc and OT was partially decreased in MPTP-treated animals. The changes in specific [³H]-spiperone and [³H]-SCH 23390 binding induced by MPTP-treatment did not alter with post-lesion survival times. These results demonstrate that MPTP treatment causes greater dopaminergic denervation of the caudate-putamen than in NAc/OT. This resulted in an increase in postsynaptic D-1 DA receptor sites in the caudate-putamen but not in the NAc/OT. Also, there appeared to be loss of presynaptic D-2 DA receptic sites in the SNc and OT. In the caudate-putamen, the loss of presynaptic D-2 DA receptor sites may have masked postsynaptic D-2 DA receptor upregulation. © 1993 Wiley-Liss, Inc.     

D-1 dopamine receptors in the rat brain: a quantitative autoradiographic analysis

The distribution of dopamine D-1 receptors has been determined in the rat brain by a quantitative in vitro light-microscopic autoradiographic method. The binding of [N-methyl-3H]-SCH 23390 to slide-mounted tissue sections takes place with characteristics expected of a substance that recognizes D-1 receptors. The binding is saturable, has high affinity, and exhibits an appropriate pharmacology and stereospecificity in several discrete microscopic brain regions as determined by quantitative autoradiography. The highest density of D-1 receptors occurs in the caudate-putamen, accumbens nucleus, olfactory tubercle, and the substantia nigra pars reticulata. High concentrations of D-1 receptors were associated with the intercalated and medial nuclei of the amygdala, entopeduncular nucleus, and major island of Calleja. Furthermore, moderate to low concentrations were observed in several other structures, such as the frontal cortex, subthalamic nucleus, and several thalamic, hypothalamic, and hippocampal areas. The distribution of D-1 receptors correlates very well with projection areas of dopaminergic pathways. This technique furnishes a powerful assay for the accumulation of detailed pharmacologic and anatomical data about D-1 receptors, and the results suggest possible CNS sites of action of D-1 dopamine receptor selective compounds.     

Effect of cocaine self-administration on striatal dopamine D1 receptors in rhesus monkeys

An array of evidence indicates that long-term exposure to cocaine alters several components of the brain dopamine system. Because the release of dopamine in the nucleus accumbens (NAc) has been implicated in mediating the reinforcing effects of cocaine, changes in dopamine function can have profound effects on drug-seeking and drug-taking behavior. The present study examined the effects of the chronic self-administration of cocaine on the D₁ family of dopamine receptors in the rhesus monkey. The brains of three rhesus monkeys that had intravenously self-administered an average of 1.35 mg/kg cocaine per day for 18–22 months were compared to the brains of three cocaine-naive controls. The in vitro quantitative autoradiographic technique was used to quantify binding densities of the D₁ ligand [³H]SCH-23390 on cryostat-cut sections of fresh frozen tissue. In animals that self-administered cocaine, the density of D₁ binding was significantly lower in the regions of the striatum at the level where the nucleus accumbens is most fully developed. The shell of the NAc showed the largest difference with significantly lower D₁ binding also detected in adjacent regions of the caudate nucleus and the putamen. No differences were found in the rostral pole of the NAc or the dorsal striatum at that level. These findings suggest that chronic self-administration of cocaine can modulate the density of dopamine D₁ receptors in specific portions of the primate striatum. Such changes might underlie some of the behavioral consequences, like drug dependence and craving, of long-term cocaine use. Synapse 28:1–9, 1998. © 1998 Wiley-Liss, Inc.     

Electrophysiological evidence for the existence of both D-1 and D-2 dopamine receptors in the rat nucleus accumbens

Extracellular single-unit recording and microiontophoretic techniques were used to characterize the pharmacological properties of dopamine (DA) receptors within the rat nucleus accumbens (NAc), a forebrain structure that receives a dense innervation from mesolimbic DA-containing neurons (A10 DA neurons) located in the ventral tegmental area (VTA). Of the NAc neurons tested, 75% were inhibited by microiontophoretic administration of the selective D-2 receptor agonist, LY-141865, whereas 38% were inhibited by microiontophoretic administration of the selective D-1 receptor agonist, SKF-38393. Of the 30 NAc neurons that were tested with both of these agonists, nine were inhibited by both agonists, 11 were inhibited only by LY-141865, five were inhibited only by SKF-38393, and five were not affected by either of these compounds. The inhibitory effects of LY-141865 were blocked and reversed by either intravenous or iontophoretic administration of the selective D-2 antagonist (-)-sulpiride, which, however, failed to alter the inhibitory effects of SKF-38393. In contrast, the purportedly selective D-1 antagonist, SCH-23390, selectively blocked and reversed the inhibitory effects of SKF-38393, suggesting that the two agonists were producing their inhibitory effects via distinct DA receptors. Additional experiments indicated that intravenous administration of LY-141865 caused a biphasic increase/decrease in the activity of NAc neurons. The initial rate increase was apparently due to disinhibition since it was also shown that D-2 DA receptors located on A10 DA neurons exhibited a 3-10-fold greater sensitivity to LY-141865 and DA as compared to the NAc D-2 receptors.(ABSTRACT TRUNCATED AT 250 WORDS)     

Behavioral differentiation between effects elicited at dopamine D-1 and D-2 receptors in rats with normosensitive DA receptors

The interaction between D-1 and D-2 receptor agonists was investigated in three behavioral test models. In two models (circling behavior in rats with hemitransection; stereotyped behavior in naive rats) an enabling role of D-1 receptor agonists in expressing the effect of D-2 agonists was found: D-1 agonists alone were ineffective, D-2 agonists had low maximum effects, whereas the combination induced marked activity. Consistent with this, the effect of combined D-1/D-2 receptor stimulation was antagonized either by D-1 or D-2 antagonists. In contrast, the effect of D-1 and D-2 agonists and antagonists were differentiated in drug discrimination studies, using the cue induced by the D-1 agonist SK & F 38393 or by the D-2 agonist (-)-NPA versus saline. The results thus indicate a functional relevance of D-1 receptors and show that the coupling between D-1 and D-2 receptors critically depends on the specific behavior under study.     

Brain dopamine D-1 receptors in senile dementia

Brain dopamine D-1 binding sites were studied by using [3H]flupenthixol in 4 brain regions of 44 senile patients with neuropathologically verified organic dementia and 28 age-matched controls. The D-1 binding sites were decreased in the substantia nigra and nucleus accumbens in patients with Alzheimer's disease, while no change was found in multi-infarct or combined dementia. The striatal D-1 binding sites were unchanged in all groups of patients. Only a few correlations between various clinical and post-mortem variables and the [3H]flupenthixol binding of the dementia patients were found. The findings of this study indicate that there is reduction of brain D-1 binding sites in patients with Alzheimer's disease.     

In vivo labelling of rat brain dopamine D-2 receptors

Summary The stereospecific blockade by raclopride and FLB472 (the R enantiomer of raclopride) of the specific in vivo binding of [³H]-spiperone, [³H]-N,N-propylnorapomorphine (NPA) and [³H]-raclopride was studied in seven brain regions (e.g., caudate nucleus, olfactory tubercle, septum, hippocampus, frontal cortex, substantia nigra, pituitary gland) of the male albino rat. The binding of all three ligands was dose-dependently blocked by raclopride and FLB472. The blockade by FLB 472 occurred at doses 50–100 times higher than that obtained by raclopride. The maximal blockade by raclopride of [³H]-spiperone binding differed between brain areas. Thus, the largest blockade was obtained in the substantia nigra (95%), septum (90%), caudate nucleus (60%) and olfactory tubercle (60%), while the blockade of [³H]-spiperone binding in the frontal cortex and pituitary gland did not exceed 30% and 50%, respectively. In contrast to [³H]-spiperone, the in vivo binding of [³H]-NPA and [³H]-raclopride was prevented by 90–100% in all brain areas examined. Taken together, the present findings indicate that the in vivo binding of three radioactive ligands to a central dopamine D-2 receptor can be stereoselectively blocked by the enantiomers of raclopride. The findings suggest that, under in vivo conditions, [³H]-raclopride and [³H]-NPA may label a closely related receptor site. However only some of the [³H]-spiperone binding sites may be identical to the [³H]-raclopride binding sites. The findings indicate furthermore that the relative overlap of D-2 sites shared by [³H]-spiperone and [³H]-raclopride may vary between brain regions.Raclopride is identical to ()-(S)-3,5,dichloro-N-(1-ethyl-2-pyrrolidinyl)methyl-6-methoxysalicylamide tartrate [FLA870()]. The (R) enantiomer of this compound will in the present paper be referred to as FLB472.     

Biphasic modulation of evoked [3H]D-aspartate release by D-2 dopamine receptors in rat striatal slices

It has been hypothesized that dopamine (DA) inhibits glutamate release from corticostriatal fibers via presynaptically located D-2 DA receptors although the evidence presented in the literature has not been conclusive. In the present experiments, the effect of D-2 receptor ligands on K+-stimulated tritium release from rat striatal slices preloaded with the nonmetabolizable glutamate analog [3H]D-aspartate ([3H]ASP was measured. The D-2 receptor antagonist S-sulpiride increased stimulated [3H]ASP release by 75% (EC50 value = 240 nM) and the biologically less-active isomer R-sulpiride, although equally effective, was tenfold less potent. The D-2 receptor agonists pergolide and (+)-4-propyl-9-hydroxynapthoxazine (+PHNO) inhibited [3H]ASP release at nM concentrations; however, this effect was small (20%). This low efficacy of the exogenous agonists was apparently due to competition by high concentrations of endogenous DA since the effect of pergolide was increased in rats whose striatal DA levels were decreased by 97%. These data support the hypothesis that D-2 DA receptors modulate [3H]ASP release in an inhibitory fashion. However, when the agonists were tested at lower concentrations, [3H]ASP release was increased significantly by 20% in control rats and 60% in DA-depleted rats. Both the facilitory and inhibitory effects of pergolide were blocked by 10 microM S-sulpiride, suggesting D-2 receptor mediation. In addition, the facilitory effect of pergolide was blocked by tetrodotoxin (TTX) and by the GABAA antagonist bicuculline, implying mediation of this D-2 effect by an inhibitory GABAergic interneuron. The inhibitory effect of pergolide was decreased by the muscarinic antagonist atropine.(ABSTRACT TRUNCATED AT 250 WORDS)     

Adenosine A1 receptor-mediated inhibition of dopamine release from rat striatal slices is modulated by D1 dopamine receptors

Dopamine release is regulated by presynaptic dopamine receptors and interactions between adenosine and dopamine receptors have been well documented. In the present study, dopamine release from isolated striatal slices from Wistar rats was measured using fast cyclic voltammetry. Single-pulse stimulation (0.1 ms, 10 V) was applied every 5 min over a 2-h period. Superfusion with the adenosine (A)(1) receptor agonist N(6)-cyclopentyladenosine (CPA), but not the A(2) receptor agonist 3-[4-[2-[[6-amino-9-[(2R,3R,4S,5S)-5-(ethylcarbamoyl)-3,4-dihydroxy-oxolan-2-yl]purin-2-yl]amino]ethyl] phenyl]propanoic acid (CGS 21680), inhibited dopamine release in a concentration-dependent manner (IC(50) 3.80 x 10(-7) m; n = 10). The dose-response curve to CPA was shifted to the right (IC(50) 6.57 x 10(-6) m; n = 6, P < 0.05 vs. control) by the A(1) receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). Neither the D(1) agonist 6-chloro-APB nor the D(1) antagonist R-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3- benzazepine-7-ol (SCH 23390) altered dopamine release on their own. However, SCH 23390 (3 microm) significantly attenuated the response to CPA (IC(50) 1.44 x 10(-5) m; n = 6, P < 0.01 vs. control). Furthermore, the inhibitory effect of CPA was significantly increased in the presence of 6-chloro-APB (1 microm). In radioligand binding experiments, CPA interacted with high- and low-affinity states of [(3)H]DPCPX-lableled A(1) receptors. The high-affinity agonist binding to A(1) receptors was inhibited by the stable guanosine triphosphate analogue Gpp(NH)p. In contrast, neither the proportion nor the affinity of high-affinity A(1) receptors was altered by dopamine or SCH 23390. These results provide evidence that the inhibition of dopamine release by adenosine A(1) receptors is dependent, at least in part, on the simultaneous activation of D(1) dopamine receptors. While the mechanism underlying this interaction remains to be determined, it does not appear to involve an intramembrane interaction between A(1) and D(1) receptors.     

Effects of dopamine D-1 and D-2 receptors on intraocular pressure in conscious rabbits

Summary This investigation was designed as a randomized, placebocontrolled, double-masked, crossover study in NZW rabbits with normal intraocular pressure (IOP) to investigate dopaminergic effects on IOP. SKF 38393, a selective D1-receptor agonist, increased, and SDZ PSD-958, a selective D1-receptor antagonist, decreased IOP, respectively. The selective D2-receptor agonist quinpirole decreased IOP, whereas the selective D2 receptor antagonist metoclopramide had no significant effect. Combinations of quinpirole with SDZ PSD-958 decreased IOP in an additive manner. SDZ GLC-756, a mixed D1-receptor antagonist/D2-receptor agonist, decreased IOP in a dose-dependent manner with a maximum effect greater than the maximum effects produced either by the D1-receptor antagonist SDZ PSD-958 and the D2-receptor agonist quinpirole. The effect of SDZ GLC-756 could only be partially blocked by the selective D2-receptor antagonist metoclopramide suggesting that both D1-receptor blockade and D2-receptor stimulation participate in its IOP-lowering effect. Tonography suggests that SDZ GLC-756 has no significant effect on outflow facility. Furthermore, the results suggest that both D1 and D2 receptors each play an independent role in the regulation of IOP in rabbits. Thus, simultaneous blockade of D1 receptors and stimulation of D2 receptors may provide a new pharmacological approach for the treatment of ocular hypertension frequently associated with glaucoma.     

Kappa opioid receptors in rat spinal cord vary across the estrous cycle

Kappa opioid receptors (KORs) were immunocytochemically localized in the lumbosacral spinal cord of female rats in different stages of the estrous cycle to examine the influence of hormonal status on receptor density. KOR labeling was primarily in fine processes and a few neuronal cell bodies in the superficial dorsal horn and the dorsolateral funiculus. Quantitative light microscopic densitometry of the superficial dorsal horn revealed that rats in diestrus had significantly lower KOR densities than those in proestrus or estrus. This suggests that female reproductive hormones regulate spinal KOR levels, which may contribute to variations in analgesic effectiveness of KOR agonists across the estrous cycle.     

Effects of GTP and sodium on rat striatal dopamine receptors labeled with lisuride

[³H]Lisuride binding to rat striatal membranes appeared to be stereospecifically displaced by the dopamine antagonist butaclamol.Sodium increased the number of [³H]lisuride binding sites (B_max) without changing the dissciation constant (K_d). GTP did not affect [³H]lisuride binding characteristics, either with or without sodium.These results suggest that dopamine receptor sites labeled by lisuride are at least in part sodium-dependent, possibly the D₂-receptors not involved in adenylate cyclase stimulation.     

Chronic ACTH treatment increases striatal dopamine D-2 receptor binding in developing rat brain

ACTH has been reported to decrease elevated levels of dopamine metabolites in the CSF of patients with infantile spasms who respond clinically to ACTH therapy. To study the possible role of dopamine receptors in the effect of ACTH, we treated rat pups for thirty days with 40 IU/kg subcutaneously of porcine ACTH or with normal saline. Using 10 nM 3H-spiperone and sulpiride to determine nonspecific binding, specific binding of D-2 receptors increased significantly (46%) in the striata of ACTH-treated rats when compared to controls. No significant difference in specific binding was found in the nucleus accumbens. Protein concentration was significantly decreased by ACTH treatment. Saturation studies will be necessary to determine if the increase in dopamine receptor binding induced by a high dose of ACTH represents a change in receptor density or affinity. The effect of lower clinical doses of ACTH on dopamine receptors warrants study.     

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

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

Hyperpolarizing and depolarizing actions of dopamine via D-1 and D-2 receptors on nucleus accumbens neurons

The effect of dopamine (DA) on the nucleus accumbens neurons in guinea-pig brain slices was studied by intracellular recordings. DA caused a hyperpolarization in 28% of the neurons tested, a depolarization in 11%, and a hyperpolarization followed by a depolarization in 53%. The remaining neurons were unaffected. Analyses of the responses revealed that the DA hyperpolarization was produced by activation of the D-1 receptor and associated with an increase in potassium conductance, whereas the DA depolarization was generated by activation of the D-2 receptor and accompanied by a decrease in potassium conductance. DA uptake inhibitors augmented both the hyperpolarizing and depolarizing responses, while cyclic adenosine monophosphate selectively enhanced the former.