Alterations in different populations of striatal dopamine receptors produced by 18 months continuous administration of cis- or trans-flupenthixol to rats

Administration of cis-flupenthixol (0.8-1.2 mg/kg per day) for 18 months enhanced stereotyped behaviour induced by apomorphine, bromocriptine and lergotrile, but not that induced by amphetamine or lisuride. Catalepsy induced by acute administration of haloperidol, trifluoperazine or cis-flupenthixol was reduced by continuous chronic intake of cis-flupenthixol. The number (Bmax) and dissociation constant (KD) of specific [3H]spiperone binding sites on striatal membranes was increased by chronic administration of cis-flupenthixol, but not trans-flupenthixol. In contrast, the Bmax and KD for specific binding of [3H]N,n-propylnorapomorphine were decreased by administration of cis-flupenthixol compared to the effect of the trans-isomer. Specific binding of [3H]piflutixol was unaffected by chronic administration of cis- or trans-flupenthixol, but chronic administration of cis-flupenthixol enhanced stimulation by dopamine of the activity of striatal adenylate cyclase. As a result of chronic continuous administration of cis-flupenthixol dopamine receptors in the striatum appeared to be supersensitive to most dopamine agonists but sub-sensitive to dopamine antagonists. This was reflected by increased numbers of D-2 antagonist receptor sites of decreased affinity, but by a decreased number of agonist sites of higher affinity. The D-1 recognition sites appeared to be unaltered, but activity of adenylate cyclase stimulated by dopamine was enhanced, suggesting post-junctional changes. The D-2 receptors appear to be primarily concerned with altered function of dopamine receptors.     

Enhanced striatal dopamine D(2) receptor-induced [35S]GTPgammaS binding after haloperidol treatment.

Dopamine receptor-G protein coupling and dopamine D(2) receptor density were assessed in rats treated for 3 weeks with either haloperidol (2 mg/kg; i.p.) or vehicle. After 3 days of withdrawal, agonist-induced guanosine 5'-O-(gamma-[35S]thio)triphosphate ([35S]GTPgammaS) and [3H]spiperone binding were determined in striatal homogenates. Maximal [3H]spiperone binding was increased (24.8%, P<0.01) following haloperidol treatment. The efficacy of dopamine and the dopamine D(2) receptor agonist R(-)-10, 11-dihydroxy-N-n-propylnorapomorphine (NPA) to induce [35S]GTPgammaS binding were found to be increased by 24.1% (P<0.01) and 44.6% (P<0. 001), respectively. When measured in the presence of a saturating concentration of a dopamine D(2) receptor antagonist, the response to dopamine was not significantly affected by haloperidol treatment. In addition, the measurement of haloperidol-induced catalepsy confirmed that the efficient dopamine receptor blockade was followed by a progressive development of dopaminergic supersensitivity. Taken together, these results indicate that a functional pool of dopamine D(2) receptors is increased after prolonged haloperidol administration.     

Characteristics of [3H]sultopride binding to rat brain

The binding of [3H]sultopride, a benzamide drug, to rat brain was investigated in vitro. Specific [3H]sultopride binding was observed in dopaminergic regions: striatum, nucleus accumbens, olfactory tubercle, substantia nigra, frontal cortex and anterior pituitary. Specific [3H]sultopride binding to striatum was saturable and had one high affinity binding site with a KD of 5.8 nM and a total density of receptors 25.7 pmol/g. [3H]Sultopride binding was stereoselectively displaced by (-)- and (+)-sultopride. Inhibition studies indicated that all neuroleptic drugs and dopamine were capable of displacing sultopride from its binding sites. A highly significant correlation was observed between IC50 values against [3H]sultopride and those against [3H]spiperone binding. Specific [3H]sultopride binding was highly dependent on the presence of sodium ions. The results suggest that the characteristics of sultopride binding sites seem to be similar to those of the D2-receptor labeled by spiperone and haloperidol. The sultopride binding site was highly dependent on the presence of sodium ions and may thus be characterized as a sodium-dependent D2-receptor.     

Selective and stereospecific interactions of R-SK & F 38393 with [3H]piflutixol but not [3H]spiperone binding to striatal D1 and D2 dopamine receptors: comparisons with SCH 23390

Four benzazepine derivatives, racemic SK&F 38393, its resolved R- and S-enantiomers, and SCH 23390 have been investigated for their interactions with striatal D1 and D2 dopamine receptors, as indexed by the binding of [3H] piflutixol and [3H]spiperone respectively. For the agonist SK&F 38393, its R-enantiomer was active in displacing [3H] piflutixol while its S- antipode was 100 fold less potent. In contrast, both enantiomers showed similar and negligible activity to displace [3H]spiperone. SCH 23390, an antagonist analogue with an R-configuration, potently displaced [3H] piflutixol but not [3H]spiperone. R-SK&F 38393 and SCH 23390 may help clarify the structural requirements for, and functional consequences of, selective actions at the D1 dopamine receptor.     

Properties of rat striatal D-2 dopamine receptors solubilized with the zwitterionic detergent CHAPS

The specific binding of [3H]spiperone was determined in membrane preparations of rat striatum and following solubilization treatment with the zwitterionic detergent CHAPS. Membrane protein solubilization was confirmed by ultrafiltration, gel filtration and increased heat sensitivity. Specific binding of [3H]spiperone to the solubilized preparation was saturable and of high affinity, although solubilization led to an approximate 10 fold decrease in receptor affinity for [3H]spiperone. The drug displacement profile of binding to the CHAPS solubilized preparation corresponded to that of the dopamine D-2 receptor; binding was stereoselectively displaced by the isomers of butaclamol. The sodium dependence of sulpiride displacement of specific [3H]spiperone binding was retained in the CHAPS solubilized preparation. GTP (100 microM) only altered the ability of dopamine to displace [3H]spiperone binding to the solubilized preparation in the presence of 120 mM sodium chloride. The GTP effect was small compared with that observed in the membranes. Specific [3H]spiperone binding sites in the solubilized preparation were preferentially retained by a wheat germ agglutinin affinity column and subsequently eluted with N-acetyl-D-glucosamine. Gel filtration of the solubilized preparation using a Sepharose column resulted in two peaks of specific [3H]spiperone binding, the larger component had a Stokes radius of 7.7 nm. CHAPS treatment of rat striatal membranes results in solubilization of the D-2 receptor in an active form. The D-2 site appears to be a glycoprotein of high molecular weight.     

Dopamine D1 receptor and dopamine D2 receptor binding activity changes during chronic administration of nicotine in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated mice

The effect of nicotine on MPTP-induced changes in striatal dopamine receptors binding activity was investigated. Dopamine D1 and D2 receptors were labeled with [3H]SCH-23390 and [3H]spiperone respectively in BALB/cBy mice. With administration of only MPTP, which caused more than an 80% decrease in striatal dopamine level, binding of 0.15 nM [3H]spiperone was increased by 37%; whereas 0.3 nM [3H]SCH-23390 binding was unchanged. With chronic nicotine treatment (0.4 mg/kg twice daily for 7-9 days), [3H]SCH-23390 binding activity was increased by 27% and [3H]spiperone binding activity was unchanged. When nicotine was administered after MPTP, their separate effects could be seen in that both the D1 and D2 dopamine receptor ligand binding activities were increased and that nicotine elevated the ratio of D1/D2 receptor binding activities in MPTP-treated mice.     

Behavioural and biochemical studies on 6-methylamino-4,5,6,7-tetrahydrobenzothiazole (14.839JL), a new potent dopaminergic agonist

6-Methylamino-4,5,6,7-tetrahydrobenzothiazole monochlorhydrate (14.839JL) is a new, potent dopaminergic agonist. The stereotypy induced by this drug was greater than that induced by an equivalent dose of apomorphine, was antagonized by pretreatment with sulpiride and counteracted the hypomotility induced by reserpine. Striatal levels of the dopamine metabolites homovanillic acid (HVA), dihydroxyphenylacetic acid (DOPAC) and 3-methoxytyramine (3-MT) were significantly lowered for up to 4-6 h by doses from 0.05 to 1 mg/kg. The drug was also very effective in lowering prolactine secretion. 14.839JL displaced [3H]N-n-propylnorapomorphine [3H]NPA from striatal binding sites with an IC50 similar to dopamine (DA). Conversely, the ability of 14.839JL to displace 3H spiperone from its binding sites was 100 and 10 times lower than that of haloperidol and sulpiride, and similar to that of SCH 23390. Differently from the latter, however, 14.839JL did not modify adenylate cyclase activity. All these data suggest that 14.839JL is a new, potent, long-lasting direct DA agonist, probably acting on D2 receptors.     

Dopamine D2 receptor binding in adrenal medulla: characterization using [3H]spiperone

The possibility that dopamine may function as a neuromodulator or neurotransmitter in the adrenal gland, and not merely serve as a precursor to the catecholamines, has been suggested. If this hypothesis is correct, receptors for dopamine should be identifiable in the adrenal. The present work demonstrates the existence of a high-affinity receptor in adrenal medulla using [3H]spiperone as the radioligand to label the receptors. [3H]Spiperone bound rapidly, reversibly, and with high affinity to bovine adrenal medullary membranes. Scatchard analysis yielded a Kd of 0.09 nM and a Bmax of 51 fmol/mg protein. In competition binding experiments, dopaminergic antagonists were at least 100 times more potent in displacing [3H]spiperone from its binding sites than adrenergic or serotonergic receptor antagonists. Similarly, agonists at the dopamine receptor more readily competed for [3H]spiperone binding than other receptor agonist drugs tested. Furthermore, D2 selective antagonists and agonists were much more potent than D1 receptor ligands. These results suggest that [3H]spiperone may bind to a high-affinity D2 dopamine receptor in adrenal medulla.     

Characterization of dopamine autoreceptor and [3H]spiperone binding sites in vitro with classical and novel dopamine receptor agonists

The specific D2 receptor agonist, LY 141865, but not the specific D1-receptor agonist, SK&F 38393, potently inhibited electrically evoked [3H]dopamine release from slices of the cat caudate. Similarly, LY 141865, but not SK&F 38393, inhibited [3H]spiperone binding to membranes of the cat caudate. The inhibition by dopamine receptor agonists of electrically evoked [3H]dopamine release was antagonized by the specific D2-receptor antagonist S-sulpiride. The inhibition of the electrically evoked release of [3H]dopamine by apomorphine was not, however, antagonized by the specific D1-receptor antagonist, bulbocapnine. Similarly, S-sulpiride but not bulbocapnine potently inhibited [3H]spiperone binding to membranes of the cat caudate. These results suggest that the dopamine autoreceptor modulating the depolarization-evoked release of [3H]dopamine, and the binding site of [3H]spiperone, are valid in vitro models for D2-dopamine receptors. Contrary to some previous reports, DPI was inactive in both in vitro dopamine receptor models. The IC50 values of a series of dopamine receptor agonists correlated very well in the two in vitro dopamine receptor models. One exception to this correlation was bromocriptine, which was more potent at [3H]spiperone binding sites than at the dopamine autoreceptor. With the exception of bromocriptine, all dopamine receptor agonists had one-hundred fold higher potency at the dopamine autoreceptor than at [3H]spiperone binding sites. [3H]Spiperone binding sites are localized primarily postsynaptic to dopamine terminals. Possible differences between the pharmacological properties of pre- and postsynaptic dopamine receptors should become apparent in the comparison of the two in vitro dopamine receptor models. However, the order of potency of dopamine receptor agonists with both in vitro models, dopamine autoreceptor and [3H]spiperone binding, was the same: N-n-propylnorapomorphine greater than TL-99 = 7-HAT greater than M-7 greater than Apomorphine greater than LY 141865.     

125I]Spiperone is not a useful ligand for studying the CHAPS solubilized dopamine D-2 receptor from rat striatum

The interaction of [125I]spiperone and [3H]spiperone with CHAPS solubilized preparations of rat striatal tissue has been compared. Only about 15% of [125I]spiperone binding was displaced by sulpiride compared with about 67% of [3H]spiperone binding. In the presence of (+)-butaclamol the displacement of [125I]spiperone was twice that found with sulpiride suggesting an interaction with sites other than D-2 receptors. The specific binding of [125I]spiperone was not saturable within the maximum concentration range that could be employed and its affinity for soluble preparations was far lower than that of [3H]spiperone. Despite its very high specific activity [125I]spiperone offers no advantage over [3H]spiperone in the identification of dopamine receptors in soluble tissue preparations.     

Competitive inhibition of [3H]spiperone binding to D-2 dopamine receptors in striatal homogenates by organic calcium channel antagonists and polyvalent cations

The specific binding of [3H]spiperone (30 pM) to D-2 dopamine receptors in homogenates of the rat corpus striatum, as defined by the D-2 antagonist haloperidol (100 nM), was displaced by organic calcium channel antagonists and by polyvalent cations. Both classes of agents were able to totally displace [3H]spiperone binding by an apparently competitive mechanism in that the dissociation constant was increased while the density of binding sites was unchanged. The rank order of inhibition potency for the cations was Zn2+ greater than Cd2+ greater than La3+ greater than Cu2+ greater than Ni2+ greater than Co2+ greater than Mn2+ greater than Mg2+, Ca2+ greater than Ba2+. Of the organic calcium channel antagonists, D600 and verapamil were the most potent displacers of [3H]spiperone binding (IC50 values both 2.0 microM), while diltiazem possessed an IC50 of 33 microM. Nicardipine (IC50 6.0 microM) was the only 1,4-dihydropyridine to inhibit [3H]spiperone binding. The results suggest that sites labelled by [3H]spiperone also bind organic calcium channel antagonists and polyvalent cations.     

Agonist high- and low-affinity states of the D2-dopamine receptor in calf brain. Partial conversion by guanine nucleotide

In order to determine whether D2-dopaminergic receptors in brain exist in different affinity states for agonists and whether these receptors could be completely converted from their agonist high-affinity state to their agonist low-affinity state, we examined the effect of a guanine nucleotide on the competition between [3H]spiperone and dopamine agonists for binding to homogenates of calf caudate nucleus. [3H]spiperone labeled sites having different affinities for agonists as well as antagonists. Agonists recognized three components of [3H]spiperone binding. Two of these components were related to the D2-dopaminergic receptor. These two sites appeared to represent interconvertible states, each having different affinities for agonists. This was supported by the observation of an apparent guanine nucleotide-induced "conversion" of sites with high affinity to those having low affinity for the agonist. This effect of the guanine nucleotide was incomplete, such that a significant proportion of the high-affinity sites (21%) remained in the presence of an excess of the nucleotide. These high-affinity, guanine nucleotide-insensitive sites may represent a distinct class of binding sites having high affinity for both agonists and antagonists or may be the result at equilibrium of an agonist-independent interaction of the receptor and the guanine nucleotide.     

Differential in vivo inhibition of [3H]nemonapride binding by atypical antipsychotics in rat striatum, olfactory lobes, and frontal cortex

Dopamine D2 receptor blockade is thought to be mandatory for antipsychotic action because most of the currently used antipsychotics have high affinity at these receptors. Here, we examined the in vivo binding characteristics of the D2-like receptor antagonist [3H]nemonapride in rat brain areas including the striatum, olfactory lobes and frontal cortex and its inhibition by a series of D2 antagonist antipsychotics. In vivo affinity of [3H]nemonapride was similar (apparent Kd value: 0.05 micromol/kg) in all brain regions examined. The estimated number of binding sites was higher in the striatum (66 fmol/mg wet weight) than in the olfactory lobes (28 fmol/mg wet weight) and the frontal cortex (21 fmol/mg wet weight). In the striatum, [3H]nemonapride binding was inhibited in a dose-dependent manner with the following order of potency (ED50, mg/kg): nemonapride (0.04), raclopride (0.13), spiperone and risperidone (0.14), haloperidol (0.21), clozapine (7.2) and thioridazine (9.4); in the olfactory lobes: nemonapride (0.03), raclopride and spiperone (0.09), haloperidol (0.10), risperidone (0.15), thioridazine and clozapine (11); in the frontal cortex, only the high affinity dopamine D2 antagonist compounds nemonapride (0.05), haloperidol (0.09), and raclopride (0.12) significantly decreased the binding of [3H]nemonapride. The present data suggest that conventional and atypical antipsychotics may be distinguished by their differential occupancy of striatal versus frontocortical D2-like receptors in vivo.     

3H] A-369508 ([2-[4-(2-cyanophenyl)-1-piperazinyl]-N-(3-methylphenyl) acetamide): an agonist radioligand selective for the dopamine D4 receptor

Tritiation of the dopamine D(4) receptor selective agonist A-369508 ([2-[4-(2-cyanophenyl)-1-piperazinyl]-N-(3-methylphenyl) acetamide) has provided a radioligand for the characterization of dopamine D(4) receptors. [(3)H] A-369508 binds with high affinity to the major human dopamine D(4) receptor variants D(4.2), D(4.4) and D(4.7) (K(d)=1.7, 4, and 1.2 nM, respectively). It also binds to the rat dopamine D(4) receptor, (K(d)=4.4 nM), implying similar binding affinity across human and rat receptors. A-369508 shows >400-fold selectivity over D(2L), >350-fold selectivity over 5-HT(1A) and >700-1,000-fold selectivity over all other receptors tested. Agonist activity determined by inhibition of forskolin-induced cAMP in Chinese hamster ovary cells transfected with the human dopamine D(4.4) receptor (EC(50)=7.5 nM, intrinsic activity=0.71) indicates that A-369508 is a potent agonist at the human dopamine D(4) receptor. Similar data was observed in other functional assays. [(3)H] A-369508 binds to a single, high affinity site on membranes containing the human dopamine D(4.4) receptor. When compared to the D(2)-like antagonist [(3)H] spiperone, competition binding for agonists like dopamine and apomorphine were 2-10-fold more potent with [(3)H] A-369508, while the antagonists clozapine, haloperidol and L-745870 bind with similar affinity to both ligands. Binding to rat brain regions demonstrated that the most abundant area was cerebral cortex (51.2 fmol/mg protein) followed by hypothalamus, hippocampus, striatum and cerebellum. [(3)H] A-369508 is a useful tool to define the localization and physiological role of dopamine D(4) receptors in central nervous system and can facilitate measuring accurate affinities (K(i)) for structure/activity relationship studies designed to identify dopamine D(4) receptor selective agonists.     

Definition of the in-vivo binding of [3H]spiperone in rat brain using substituted benzamide drugs

The ability of some substituted benzamide drugs to define in-vivo the binding of [3H]spiperone to brain dopamine receptors in rats was assessed using behaviourally effective doses in comparison with haloperidol. As judged using haloperidol, [3H]spiperone identified dopamine receptors in the substantia nigra, striatum, tuberculum olfactorium and hypothalamus, but not in frontal cortex or nucleus accumbens. The substituted benzamide compounds alizapride, metoclopramide, clebopride and YM 09151-2 prevented the accumulation of [3H]spiperone in the substantia nigra, striatum, tuberculum olfactorium and hypothalamus. However, YM 09151-2 also caused displacement of [3H]spiperone accumulation in the nucleus accumbens and frontal cortex. (+/- )-Sulpiride, (+/- )-sultopride, amisulpiride and prosulpride all prevented the accumulation of [3H]spiperone in the hypothalamus but were ineffective in one or more of the other regions containing dopamine receptors defined by [3H]spiperone. The isomers of sulpiride and sultopride stereoselectively defined the accumulation of [3H]spiperone in dopamine containing brain regions. The (-)-isomers of both drugs prevented the accumulation of [3H]spiperone in the substantia nigra, striatum, tuberculum olfactorium and hypothalamus. In contrast, (+)-sulpiride and (+)-sultopride were ineffective. Selected substituted benzamide drugs can be used to define the interaction of ligands with dopamine receptors in-vivo. These substances may be useful in PET studies in man. The isomers of some substituted benzamine drugs may be used to define dopamine receptors in-vivo by enantiomeric selectivity.     

Effects of ceruletide on the dopamine receptor-adenylate cyclase system in striatum and frontal cortex of rats chronically treated with haloperidol

Chronic treatment of rats with haloperidol decanoate (30 mg/kg and 100 mg/kg IM every 4 weeks for 52 weeks) increased [³H] SCH 23390 binding in striatal membranes by 25% and 50% and in frontal cortical membranes by 56% and 125% in 30 and 100 mg/kg haloperidol treatment groups, respectively. These increases in [³H] SCH 23390 binding to the membranes were restored to control levels after ceruletide treatment (100 µg/kg IP twice a day for 5 days). [³H] Spiperone binding to the rat striatal and cortical membranes also increased after chronic haloperidol treatment (by 66% and 99% in striatal membranes and by 27% and 62% in cortical membranes in the 30 and 100 mg/kg haloperidol treatment groups, respectively). Administration of ceruletide to haloperidol-treated rats reduced the increased [³H] spiperone binding to the cortical membranes toward the control level, but ceruletide was not effective in reducing the haloperidol-induced increase of [³H] spiperone binding to the striatal membranes. Activation of adenylate cyclase by dopamine (1 µM or 100 µM) or Gpp(NH)p (1 µM) was reduced in striatal and cortical membranes from haloperidol-treated rats. Ceruletide restored the lowered level of dopamine-stimulated or Gpp(NH)p-stimulated adenylate cyclase activity in the membranes from haloperidol-treated rats to control levels. These findings indicate that systemically administered ceruletide is capable of reversing alterations in D₁ dopamine receptor/D₁ dopamine receptor coupling to adenylate cyclase in striatum and frontal cortex induced by chronic treatment of rats with haloperidol decanoate.     

In vitro characterisation of dopamine receptors in the superior colliculus of the rat

In membrane preparations of superior colliculus of the rat, the binding of [3H]spiperone (0.15 nM) was displaced by the incorporation of (+)-butaclamol, haloperidol, apomorphine and (+/-)-sulpiride, but not by (-)-butaclamol, prazosin, propranolol, ketanserin or cinanserin. The Ki values for the displacement of [3H]spiperone by (+/-)-sulpiride, (+)-butaclamol and haloperidol were similar in tissue preparations from superior colliculus and striatum. Equilibrium analysis of the specific binding of [3H]spiperone (0.03-1.0 nM), defined by 10(-5) M (+/-)-sulpiride, to membrane preparations of the superior colliculus, showed the interaction to be saturable and of high affinity. However, the Bmax was only approximately 10% of that found in preparations of striatum; the apparent dissociation constant (KD) was the same in both preparations of the superior colliculus and striatum. Uptake of [3H]dopamine into synaptosomal preparations of the superior colliculus was approximately 20% of that found in synaptosomes from the striatum. In preparations of striatum nomifensine, but not desipramine or fluoxetine, inhibited the uptake of [3H]dopamine. However, in preparations from the superior colliculus, nomifensine, desipramine and fluoxetine were without effect on the uptake of [3H]dopamine. Dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and 3-methoxytyramine (3-MT) were present in small concentrations in the superior colliculus. Homovanillic acid (HVA) was present in larger concentrations and the HVA plus DOPAC/dopamine ratios were greater in the superior colliculus than in the striatum. The superior colliculus contained only small amounts of noradrenaline but 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were present in larger amounts.(ABSTRACT TRUNCATED AT 250 WORDS)     

trans-10,11-dihydroxy-5,6,6a,7,8,12b-hexahydrobenzo[a]phenanthridine: a highly potent selective dopamine D1 full agonist

trans-10,11-Dihydroxy-5,6,6a,7,8,12b-hexahydrobenzo[a]phenan thridine (4a, dihydrexidine) has been found to be a highly potent and selective agonist of the dopamine D1 receptor in rat brain. Dihydrexidine had an EC50 of approximately 70 nM in activating dopamine-sensitive rat striatal adenylate cyclase and a maximal stimulation equal to or slightly greater than that produced by dopamine. Dihydrexidine had an IC50 of 12 nM in competing for [3H]SCH23390 (1a) binding sites in rat striatal homogenate, and of 120 nM versus [3H]spiperone. These data demonstrate that dihydroxidine has about ten-fold selectivity for D1/D2 receptors. More importantly, however, is the fact that dihydrexidine is a full agonist. Previously available agents, such as SKF38393 (1b), while being somewhat more selective for the D1 receptor, are only partial agonists. The isomeric cis-dihydroxybenzo[a]-phenanthridine neither stimulated cAMP synthesis nor inhibited the cAMP synthesis induced by dopamine. The cis isomer also lacked appreciable affinity for [3H]-1a binding sites. N-Methylation of the title compound decreased affinity for D1 sites about 7-8-fold and markedly decreased ability to stimulate adenylate cyclase. Addition of an N-n-propyl group reduced affinity for D1 sites by about 50-fold and essentially abolished the ability to stimulate adenylate cyclase. However, this latter derivative had twice the affinity of the D2-selective agonist quinpirole for the D2 receptor. The results are discussed in the context of a conceptual model for the agonist state of the D1 receptor.     

Dopamine receptor binding sites in the rat superior colliculus

Following intravenous administration of [3H]spiperone or [3H]N,n-propylnorapomorphine (NPA) to rats, radioactivity derived from the ligands accumulated in the striatum and superior colliculus when compared with cerebellar levels. The accumulation of [3H]spiperone in both areas was prevented by intraperitoneal administration of (+)-butaclamol, haloperidol and sulpiride but not by (-)-butaclamol, cinanserin, propranolol or prazosin. The accumulation of [3H]NPA was prevented by administration of (+)-butaclamol, haloperidol and apomorphine but not by (-)-butaclamol. In in-vitro experiments, membrane preparations from the superior colliculus showed a small number of specific binding sites for both [3H]spiperone and [3H]NPA. The dissociation constant (KD) for [3H]NPA was not different from that for striatal preparations but that for [3H]spiperone was 10-fold higher. We conclude that dopamine receptors may be present within the superior colliculus.     

Are dopamine receptors present on human lymphocytes?

The characteristics of [3H] spiperone binding to human lymphocytes have been examined. The haloperidol displaceable component of [3H] spiperone binding to human lymphocytes was not saturable, and stereoselective displacement by the isomers of butaclamol was not observed. These was no correlation between the ability of known dopamine active drug to cause displacement of the ligand and their rank order of potency. Chloroquine, a drug with no dopaminergic action, was the most potent displacing agent examined. Fragmentation of the cells caused a marked decrease in the haloperidol displaceable component of [3H] spiperone binding, and saturable binding to the lymphocyte membrane fragments was not observed. lysis of cells after equilibrium incubation with [3H] spiperone caused a marked reduction in the haloperidol displaceable component of ligand binding. Association of [3H] spiperone with lymphocytes was unaffected by the metabolic inhibitor iodoacetate or by replacement of sodium ions by lithium ions in the incubation medium, suggesting that such association did no involve an active process. We cannot confirm the existence of dopamine receptors on the surface of human lymphocytes. We suggest that the apparent association of [3H] spiperone with lymphocytes is due to some passive uptake process causing accumulation of the ligand within the cells.