Autoradiographic analysis of regional alterations in brain receptors following chronic administration and withdrawal of typical and atypical neuroleptics in rats

Summary Rats were administered haloperidol, clozapine, raclopride, or no drug for 28 days or 8 months. Following a 3 week withdrawal period, in vitro autoradiography was utilized to examine receptor binding for dopamine D2 ([³H]spiperone and [³H]raclopride), dopamine D1 ([³H]SCH23390), GABA_A ([³H]muscimol), benzodiazepine ([³H]RO15-1788), and muscarinic ACh receptors ([³H]QNB). [³H]spiperone was elevated in striatal subregions only in haloperidol-treated rats, with the largest increases seen in the 8 month duration animals. Striatal [³H]raclopride binding was increased after both short- and long-term treatment in both haloperidol and raclopride, but not clozapinetreated animals. Clozapine-treated rats showed significant increases in [³H]SCH23390 in the nucleus accumbens after 28-day administration; otherwise no changes were seen for this ligand in any other groups. Increases in [³H]muscimol binding in the substantia nigra reticulata were seen in haloperidol-treated rats after 8 month treatment. Binding of [³H]QNB and [³H]RO15-1788 were not significantly different from control for any of the drug-treated groups. These data suggest that persisting alterations in receptor binding are primarily seen in dopamine D2 and GABA receptors after withdrawal from chronic administration of haloperidol but not the atypical neuroleptics, clozapine and raclopride.     

Sequential changes of cholinergic and dopaminergic receptors in brains after 6-hydroxydopamine lesions of the medial forebrain bundle in rats

Summary. We studied sequential changes in muscarinic cholinergic receptors, high-affinity choline uptake sites and dopamine D₂ receptors in the brain after 6-hydroxydopamine lesions of the medial forebrain bundle in rats. The animals were unilaterally lesioned in the medial forebrain bundle and the brains were analyzed at 1, 2, 4 and 8 weeks postlesion. [³H]Quinuclidinylbenzilate (QNB), [³H]hemicholinum-3 (HC-3) and [³H]raclopride were used to label muscarinic cholinergic receptors, high-affinity choline uptake sites and dopamine D₂ receptors, respectively. The degeneration of nigrostriatal pathway produced a transient decrease in [³H]QNB binding in the parietal cortex of both ipislateral and contralateral sides at 2 and 8 weeks postlesion. [³H] QNB binding also showed a mild but insignificant decrease in the ipsilateral striatum throughout the postlesion periods. No significant change was observed in the substantia nigra (SN) of both ipsilateral and contralateral sides throughout the postlesion periods. In contrast, [³H]HC-3 binding showed no significant change in the parietal cortex of both ipsilateral and contralateral sides during the postlesion. However, [³H]HC-3 binding was upregulated in the ipsilateral dorsolateral striatum throughout the postlesion periods. The ventromedial striatum also showed a significant increase in [³H]HC-3 binding at 1 week and 2 weeks postlesion. On the other hand, no significant change in [³H]raclopride binding was found in the parietal cortex of both ipsilateral and contralateral sides during the postlesion. [³H]Raclopride binding showed a conspicuous increase in the ipsilateral striatum (35–52% of the sham-operated values in the lateral part and 39–54% in the medial part) throughout the postlesion periods. In the contralateral side, a mild increase in [³H]raclopride binding was also found in the striatum (10–15% of the sham-operated values in the lateral part and 22% in the medial part) after lesioning. However, a significant decline in [³H]raclopride binding was observed in the ipsilateral SN and ventral tegmental area during the postlesion. The present study indicates that 6-hydroxydopamine injection of medial forebrain bundle in rats can cause functional changes in high-affinity choline uptake site in the striatum, as compared with muscarinic cholinergic receptors. Furthermore, our studies demonstrate an upregulation in dopamine D₂ receptors in the striatum and a decrease in the receptors in the SN and ventral tegmental area after the 6-hydroxydopamine injection. Thus, these findings provide further support for neurodegeneration of the nigrostriatal pathway that occurs in Parkinson's disease. Received April 26, 1999; accepted November 12, 1999     

Alterations in the dopaminergic receptor system after chronic administration of cocaine

Several studies suggest that one of the most important factors contributing to cocaine dependence is an alteration in the actions of the neurotransmitter dopamine in the central nervous system. In order to understand some of the neuroreceptor consequences of cocaine administration, groups of rats were injected with cocaine (2 daily doses of 15 mg/kg) for 1 to 21 days. Binding of [³H]cocaine, [³H]SCH23390, [³H]raclopride, and [³H]BTCP in striatal and cortical tissue from the treated animals was compared to controls. [³H]Cocaine binding was increased by the drug in the striatum and cortex at days 14 and 21, respectively. The binding of [³H]SCH23390 to D₁ dopamine receptors was significantly increased at day 3 of cocaine exposure. In striatal membranes, [³H]BTCP binding to dopamine uptake sites was significantly increased after day 7, whereas binding in cortical membranes was increased from day 1. [³H]Raclopride binding to D₂ dopamine receptors remained unchanged throughout the study in both cortical and striatal tissues. These results indicate that repeated exposure to cocaine produces an upregulation (possible supersensitivity) in cortical D₁, cocaine, and DA-uptake sites which occurs in a time-dependent manner. These increases are coupled with an upregulation in striatal D₁, cocaine, and DA-uptake sites, without simultaneous changes in D₂ receptors. Thus, cocaine's effects are not uniformly distributed across all brain regions, but rather are focused within areas of the dopamine system. © 1993 Wiley-Liss, Inc.     

Autoradiographic comparison of D1-specific binding of [H]SCH39166 and SCH23390 in the primate cerebral cortex

Quantitative autoradiography was used to compare the binding of the novel dopamine D₁ receptor antagonist, [³H]SCH39166, with that of the widely used radioligand, [³H]SCH23390 (in the presence of ritanserin), in the primate cerebral cortex. Specific binding of both radioligands, determined using SCH23390 or cis-flupentixol as displacing agents, had very similar densities and distributions throughout the cortex. However, the specific binding of [³H]SCH39166 obtained with SCH39166 as a blank was significantly higher than that obtained using SCH23390 or cis-flupentixol as displacing agents in some layers of motor, somatosensory and occipital cortices. In addition, the non-specific binding of [³H]SCH39156 obtained in the presence of an excess of SCH23390 of cis-flupentixol displayed a complex laminar pattern very different from that of the specific binding. These observations suggest that [³H]SCH39166 may have a high affinity to more than the D₁ receptor subtype bound by SCH23390 or cis-flupentixol. Also, these additional sites are likely to be different from 5-HT₂ or 5-HT_1C receptors since the latter sites were not displaced by 1 μM SCH23390.     

SDZ GLC 756, a novel octahydrobenzo[g]quinoline derivative exerts opposing effects on dopamine D1 and D2 receptors

Summary SDZ GLC-756, a novel octahydrobenzo[g]quinoline derivative, is equipotent in displacing [³H]SCH23390 from dopamine D₁ receptors and [³H]205–501 from dopamine D₂ receptor binding sites. It blocks dopamine sensitive adenylate cyclase with the same potency as SCH23390, indicating antagonist properties at dopamine D₁ receptors. On the other hand, SDZ GLC 756 inhibits electrically evoked acetylcholine release from rat striatal slices with the same potency as the selective dopamine D₂ receptor agonist bromocriptine. This effect is blocked by spiperone suggesting that it is mediated by dopamine D₂ receptor activation. The opposing action of SDZ GLC 756 on dopamine D₁ and D₂ receptors is also evident in vivo. SDZ GLC 756, like SCH23390, blocks apomorphine-induced rearing in mice. On the other hand, it inhibits prolactin secretion and produces circling in unilateral 6-OHDA-lesioned rats, which is compatible with stimulant properties at dopamine D₂ receptors. This drug might be a new tool to study linkage between dopamine D₁ and D₂ receptors.     

Autoradiographic localization of D1 dopamine receptors in the rat brain with [H]SCH 23390

The regional distribution of D₁ dopamine (DA) receptors in the rat brain has been studied by quantitative autoradiography using the specific D₁ antagonist [³H]SCH 23390 as a ligand. The binding of [³H]SCH 23390 to striatal sections was saturable, stereospecific, reversible and of high affinity (K_d = 2.05nM); it occurred at single population of sites and possessed the pharmacological features of the D₁ DA receptor. The highest densities of [³H]SCH 23390 binding sites were found in the caudate-putamen, olfactory tubercle, nucleus accumbens and substantia nigra (especially in the pars compacta). High densities were also observed in the nucleus interstitialis striae terminalis, the anterior olfactory nucleus, the entopeduncular nucleus, the subthalamic nucleus, the claustrum and the amygdalohippocampal area. An intermediate labelling was found in the anteromedial and suprarhinal DA terminal fields of the cerebral cortex, the basolateral, medial and lateral amygdaloid nuclei, the endopiriform nucleus, the primary olfactory cortex, the globus pallidus, the superior colliculus (especially the superficial layer), the nucleus amygdaloideus corticalis and the dorsal hippocampus (molecular layer of the CA₁ and dentate gyrus). In the anteromedial and suprarhinal cortices, [³H]SCH 23390 binding was more concentrated in layers V and VI. Moderate levels of [³H]SCH 23390 were found in the thalamus, hypothalamus, the habenula, the ventral tegmental area, the posterior cingulate and entorhinal cortices, the supragenual dopamine terminal system and the cerebellum (molecular layer). This regional distribution of [³H]SCH 23390 closely correlated (except for the cerebellum) with the reported distribution of dopaminergic terminals. The topographical distribution of [³H]SCH 23390 has also been studied in detail in striatal subregions. The density of D₁ receptors was much greater in the ventrolateral sector and medial margin of the striatum than in the ventromedial and dorsolateral sectors. A rostrocaudal decrease in the densities of D₁ sites was also found along the rostrocaudal axis of the caudate-putamen. These lateral to medial and anteroposterior gradients overlapped with the density of the dopaminergic afferents.     

Effects of monoamine uptake inhibitors given early postnatally on monoamines in the brain stem,caudate/putamen and cortex,and on dopamine D1 and D2 receptors in the caudate/putamen

Summary Rats were treated with desipramine 5mg/kg, nomifensine 10mg/kg, zimelidine 25 mg/kg or with 0.9% sodium chloride once a day during the second and third weeks after birth, and brain stem, caudate/putamen and cortical monoamines, and caudate/putamen dopamine D₁ (³[H]SCH 23390) and D₂ (³[H]spiroperidol) receptor binding were measured when rats were at two months of age. In the brain stem, the concentration of 3-methoxy-4-hydroxy-phenyl glycol was increased in nomifensine rats and the ratio of 5-hydroxyindoleacetic acid to 5-hydroxytryptamine was increased in zimelidine rats. In the caudate/putamen, the concentrations of 3,4-dihydroxyphenylacetic acid and homovanillic acid and the ratio of homovanillic acid to dopamine were increased in desipramine rats; neither³[H]SCH 23390 nor³[H]spiroperidol binding were affected by any of the three monoamine uptake inhibiting antidepressants studied. In the cortex, the ratio of 5-hydroxyindoleacetic acid to 5-hydroxytryptamine was increased in desipramine and zimelidine rats. The findings suggest that desipramine but not nomifensine increases the metabolism of dopamine in the caudate/ putamen and nomifensine but not desipramine increases the metabolism of norepinephrine in the brain stem, and furthermore that the metabolism of serotonin is affected by desipramine as well as by zimelidine. It is possible that also treatment of women with these drugs during late pregnancy causes long-lasting changes in the brain of human fetus.     

Autoradiographic studies in animal models of hemi-parkinsonism reveal dopamine D2 but not D1 receptor supersensitivity. I. 6-OHDA lesions of ascending mesencephalic dopaminergic pathways in the rat

The selective dopaminergic antagonist ligands [3H]SCH 23390 and [3H]sulpiride were used to reveal autoradiographically dopamine D1 and D2 receptors, respectively, in brain sections from rats which had received unilateral 6-hydroxydopamine (6-OHDA) injections destroying ascending nigrostriatal neurones. The binding of both ligands to striatal sections was first shown to be saturable, reversible and of high affinity and specificity [( 3H]SCH 23390: Bmax 2.16 pmol/mg protein, Kd 1.4 nM; [3H]sulpiride; Bmax 0.67 pmol/mg protein, Kd 10.7 nM). After unilateral stereotaxic 6-OHDA injections, rats rotated contralaterally when challenged with apomorphine (0.5 mg/kg), or specific D1 or D2 agonists, SKF 38393 (1.0-5.0 mg/kg) and LY 171555 (0.05-0.5 mg/kg), respectively. Loss of forebrain dopaminergic terminals was assessed autoradiographically using [3H]mazindol to label dopamine uptake sites. A loss of approximately 90-95% of uptake sites was reproducibly accompanied by an enhanced density of binding ipsilaterally for the D2 ligand, [3H]sulpiride, in all areas of the striatum, but most markedly in the lateral areas. An increase in the D2 binding site density was also seen in the ipsilateral nucleus accumbens and the olfactory tubercle. In contrast, in the same animals, the striatal D1 receptors were far less affected by dopaminergic denervation, with no consistent changes seen in the binding of [3H]SCH 23390. These results suggest that dopamine D2 receptors are more susceptible than D1 receptors to changes after dopaminergic denervation, which is expressed as an increase in the density of binding sites revealed here with [3H]sulpiride.     

Characterisation of dopamine receptors in insect (Apis mellifera) brain

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

Pharmacological characterization and autoradiographic localization of dopamine D1-like receptors in the thymus

The present study was designed to identify the pharmacological profile and the anatomical localisation of dopamine D₁-like receptor sites in the rat thymus using [³H]SCH 23390 as a ligand. [³H]SCH 23390 was specifically bound to sections of the thymus. Binding was time, temperature and concentration-dependent belonging in the range of concentrations of radioligand used to a single class of high affinity sites. The dissociation constant was 1.6 nM and the maximal density of binding sites averaged to 170 fmol/mg tissue. The pharmacological profile of [³H]SCH 23390 binding to sections of the rat thymus is consistent with the labelling of dopamine D₁-like sites. Dopamine was able to compete with [³H]SCH 23390 binding to sections of rat thymus in the range of nanomolar concentrations. This suggests the labelling of dopamine D₅ receptor sites. Light microscope autoradiography revealed the localisation of [³H]SCH 23390 binding sites primarity in the cortex of the thymus and in lesser amounts at the level of thymic corpuscles. The possible functional significance of dopamine D₁-like receptors in the rat thymus is discussed.     

D1 dopamine receptors in neurite regions of embryonic and differentiated retina are highly coupled to adenylate cyclase in the embryonic but not in the mature tissue

[³H]SCH 23390 binds stereospecifically and with high affinity to D₁ dopaminergic receptors in the developing chick retina. Autoradiographic experiments revealed that in retinas from 3-day-old chicken and embryos with 12, 14 and 16 days of development, specific labeling of [³H]SCH 23390 was mainly observed over the plexiform layers of the tissue, showing that dopaminergic D₁ receptors are localized in retina cell neurites since the initial stages of neurite formation. The total number of [³H]SCH 23390 binding sites increased 5-fold during the differentiation of the retina, while the dopamine-dependent cyclic adenosine monophosphate (AMP) accumulation was significantly decreased. Consequently, the ratio between dopamine-dependent cyclic AMP accumulation and [³H]SCH 23390 binding sites decreased 10-fold as retina differentiated, indicating that a significant portion of D₁ receptors in retinas from adult chicken are not effectively coupled to adenylate cyclase molecules.     

Differential labeling of dopamine and sigma sites by [H]nemonapride and [H]raclopride in postmortem human brains

The difference between the binding of [³H]nemonapride and [³H]raclopride has been used to quantify dopamine D₄ receptors in postmortem schizophrenic brain studies. Recent work, however, has suggested that at least part of the differential between [³H]nemonapride and [³H]raclopride binding may represent σ rather than D₄ receptor sites. We applied the nemonapride-raclopride subtraction method to postmortem, non-schizophrenic human striatum to examine the variation in dopaminergic receptor binding labeled by these ligands. Variation in σ receptor binding labeled by [³H]nemonapride was studied in frontal cortex, striatum and cerebellum. Specific binding was defined by sulpiride (dopamine receptor ligand), PPAP (σ receptor ligand) and haloperidol (mixed dopaminergic/σ agent), respectively. Haloperidol defined a combination of sites, which were approximately the sum of the dopaminergic and σ components defined by sulpiride and PPAP, respectively. Significant inter-individual variation in the amount of specific binding for dopaminergic and σ receptor sites was observed. However, no significant nor consistent observation of striatal dopamine D₄ receptors or D₄-like binding sites was observed in the striatum even though two independent sets of tissues, with different dissections were used. The inconsistencies in some previous postmortem studies appear to be at least partially explained by the inclusion of both σ and dopaminergic components in [³H]nemonapride binding and the inherent high inter-individual variability of the different components.     

Endogenously released dopamine inhibits the binding of dopaminergic PET and SPECT ligands in superfused rat striatal slices

Pharmacologically induced changes in synaptic levels of dopamine (DA) have been found, in some studies, to affect the in vivo binding of dopaminergic radioligands. In the present study we used a superfused brain slice preparation to examine the effect of synaptically released dopamine on the binding of some commonly used PET and SPECT radioligands under more controlled conditions than those present in vivo. The release of DA was evoked by electrical stimulation of striatal slices and the sensitivity of binding of the D₁ receptor ligand, [³H]SCH 23390, the D₂ receptor ligands [³H]raclopride and [¹²³I]epidepride, and the DA uptake transporter ligands, [³H]WIN 35,428 and [¹²³I]RTI-55, to the frequency of stimulation examined. Most affected by stimulation was the specific binding of [³H]SCH 23390, which was fully inhibited at 2.5 Hz. This was followed by [³H]raclopride and [¹²³I]epidepride, respectively, the binding of the latter showing only a 50% reduction at the highest frequency of 10 Hz. [³H]WIN 35,428 and [¹²³I]RTI-55 binding was unaffected by stimulation. The effects of stimulation on [³H]raclopride binding were prevented by reserpine pretreatment of the rat, when combined with inclusion of the dopamine synthesis inhibitor, α-methyl-p-tyrosine, in the superfusate medium. We conclude that, in brain slices, the binding of D₁ and D₂ receptor ligands but not that of DA uptake transporter ligands is readily inhibited by DA released into the synaptic cleft. Brain slices may prove to be a useful model system for the investigation of factors affecting competition between radioligand binding and endogenous neurotransmitters. © 1996 Wiley-Liss, Inc.     

The influence of endogenous dopamine levels on the density of [H]SCH23390-binding sites in the brain of the honey bee, Apis mellifera L

This paper examines the relationship between endogenous dopamine (DA) levels and the density of [³H]SCH23390-binding sites in the brain of the adult worker honey bee. DA levels were reduced pharmacologically using a single 10 μl injection of either α-methyl- -p-tyrosine (AMT; 250 μg or 500 μg) or α-methyl- -tryptophan (AMTP; 250 or 500 μg) into the haemolymph of the bee. In all cases, maximum depletion of DA was observed 3 h after treatment, but in bees treated with AMTP (250 or 500 μg) or with 250 μg AMT, DA levels returned to normal within 24 h of treatment. Neither AMT nor AMTP was selective for DA: both drugs also reduced serotonin (5-hydroxytryptamine, 5HT) levels in the brain. However, AMTP was more effective than AMT at depleting 5HT, whereas for DA, the reverse was true. Depletion of DA levels, using 250 μg AMT, led to a dramatic decline in the levels of specific binding of [³H]SCH23390, defined in this study as binding in the presence of 5×10⁻⁶ M cis-(Z)-flupentixol (see Ref. [28]). In contrast, naturally occurring diel fluctuations in DA levels, identified in the optic lobes of the brain, and changes in brain DA levels resulting from queenlessness, had no significant effect on the density of [³H]SCH23390-binding sites in the brain of the bee. Overall, these results indicate that under normal physiological conditions, there is no direct link in honey bees between changes in endogenous brain DA levels and the density of D₁-like receptors labelled by [³H]SCH23390.     

Regional adaptation of muscarinic receptors and choline uptake in brain following repeated administration of diisopropylfluorophosphate and atropine

The specific [³H]QNB binding and high-affinity uptake of [¹⁴C]choline in 8 brain regions (cerebral cortex, hippocampus, hypothalamus, thalamus, striatum, midbrain, cerebellum and brainstem) after repeated administration of DFP and atropine to guinea-pigs were simultaneously measured. Following repeated DFP administration, AChE was markedly depressed in each brain region. In these animals, there was a significant decrease in specific [³H]QNB binding in the cerebral cortex, hippocampus and striatum, whereas the [³H]QNB binding in the rest of brain regions was unchanged. Scatchard analysis revealed a 36% decrease in the B_max value for the striatal [³H]QNB binding without a change in theK_d value, suggesting a change in the receptor density. In contrast, repeated atropine administration produced a significant enhancement in the [³H]QNB binding only in the hippocampus and striatum. The B_max value in the striatum increased by 21% without a change in theK_d value.In addition to the receptor alteration, high affinity uptake of [¹⁴C]choline in the hippocampus and striatum was significantly decreased by DFP treatment, while that in the striatum increased by atropine treatment. Thus the present study has demonstrated that a prolonged activation and blockade of central muscarinic receptors resulted in specific adaptation in both the receptor density and ACh availability at the synapses in the cerebral cortex, hippocampus and striatum.     

Effects of 6R-L-erythro-5,6,7,8-tetrahydrobiopterin on the dopaminergic and cholinergic receptors as evaluated by positron emission tomography in the Rhesus monkey

Summary The effects of 6R-L-erythro-5,6,7,8-tetrahydrobiopterin (R-THBP) on the central cholinergic and dopaminergic systems in the Rhesus monkey brain were investigated by positron emission tomography (PET) with the muscarinic cholinergic receptor ligands (N-[¹¹C]methyl-benztropine) and dopaminergic receptor ligands selective for D₁ D₂, and D₃ subtypes ([¹¹C]SCH23390, N-[¹¹C]methyl-spiperone, and (+)[¹¹C]UH232, respectively). None of the doses (3, 10, and 30 mg/kg i.v.) of R-THBP used significantly affected the regional cerebral blood flow (rCBF as determined by Raichle's H₂ ¹⁵O method), and 10 mg/kg of R-THBP had little effect on the regional cerebral metabolic rate of glucose (rCMRglc) in the Rhesus monkey brain, as assessed by the graphical [¹⁸F]fluoro-deoxyglucose method. The effect of R-THBP on the muscarinic cholinergic system was dose dependent; while 3 mg/kg of R-THBP did not significantly alter the uptake ratio of N-[¹¹C]methyl-benztropine in several brain regions to that in the cerebellum, 10 and 30 mg/kg of R-THBP significantly reduced the uptake ratio in the thalamus, as well as in the frontal and temporal cortices. None of the doses (3, 10, and 30 mg/kg i.v.) of R-THBP tested affected [¹¹C]SCH23390 (dopamine D₁ receptor) binding. However, the k3 value for N-[¹¹C]methyl-spiperone (dopamine D₂ receptor) binding, which represents the association rate × B_max value, was significantly decreased in the striatum. The uptake ratio of (+)[¹¹C]UH232 (dopamine D₃ receptor) in the striatum to that in the cerebellum was also decreased by administration of R-THBP (3 and 30 mg/kg i.v.). These findings suggest that R-THBP acts on dopamine D₂ and D₃ receptors selectively without markedly affecting dopamine D₁ receptor binding. Furthermore, the changes in cholinergic and dopamine D₂ and D₃ receptors in vivo can not be attributed to a change in rCBF but may depend on the action of R-THBP.Abbreviations R-THBP 6R-L-erythro-5,6,7,8-tetrahydrobiopterin - PET positron emission tomography - rCBF regional cerebral blood flow - rCMRglc regional cerebral metabolic rate of glucose     

Receptor-receptor link in membranes revealed by ligand competition: Example for dopamine D1 and D2 receptors

An interaction or link between dopamine D1 receptors and dopamine D2 receptors was found by a ligand competition method, using [³H]raclopride to label dopamine D2 receptors and SCH 23390 to block dopamine D1 receptors. In the presence of endogenous or exogenous dopamine, SCH 23390 increased the binding of [³H]raclopride in post-mortem human striata homogenates or in tissue culture cells containing human dopamine D1 and D2 receptors. In order to reveal such intramembrane receptor-receptor interactions in general, therefore, it appears essential to add two agonists, one for each receptor, and then to block one of the receptors when measuring the binding of a ligand to the second receptor. © 1994 Wiley-Liss, Inc.     

Autoradiographic analysis of D1 and D2 dopaminergic receptors in rat brain after paradoxical sleep deprivation

Previous work had shown that paradoxical sleep deprivation (PSD) results in potentiation of several apomorphine-induced behaviors, leading to the suggestion that PSD induces an upregulation of brain dopamine receptors. In this study, quantitative receptor autoradiography was used to verify whether PSD does, in fact, induce alterations in D₁ or D₂ receptor binding, and to investigate the regional brain specificity of such effects. After 96 h of PSD, [³H]SCH-23390 binding to D₁ receptors was examined in 30 different brain areas of 10 experimental and 10 cage control rats. [³H]Spiperone was used to label D₂ sites in adjacent tissue sections. Results revealed a 39% increase in [³H]SCH 23390 binding in the entorhinal cortex of PSD rats (p < 0.05), but no other changes in any of the remaining 29 brain areas examined. In contrast, [³H]spiperone binding was significantly elevated in the n. accumbens (+45%) and in all subrogions of the caudate-putamen (range: +13% to +23%). These results, thus, provide evidence that PSD increases D₂ but not D₁ receptor binding in brain. The present results also suggest that upregulated D₂ receptors can account for the previously reported changes in apomorphine-induced behaviors after PSD.     

An increase in [3H]SCH23390 binding in the cerebral cortex of postmortem brains of chronic schizophrenics

Summary. Abnormalities in neural transmission of dopamine play an important role in development of schizophrenia. Dopamine 1 (D1)-like receptors in 16 areas of the cerebral cortex were measured in the postmortem brains of 13 schizophrenics and 10 controls, using [³H]SCH23390 as a ligand for receptor binding. The specific [³H]SCH23390 bindings were statistically significantly increased in the medial and inferior cortex (Brodmann Area (BA) 20 & 21) and superior parietal cortex (BA 7) of schizophrenic patients, compared to those of the controls. The increases were also marked in the cerebral cortices of off-drug cases of patients with schizophrenia, who had not received antipsychotic drugs for more than 40 days before death. These results suggest that there are changes in dopaminergic transmission through the D1 receptors in the parieto-temporal cortex of schizophrenia, and that the altered functions are involved in the pathophysiology of the disease. Received May 14, 2001; accepted July 26, 2001     

Quantitative autoradiographic distribution of [H]mazindol-labeled dopamine uptake sites in the brains of superoxide dismutase transgenic mice

Superoxide dismutase (SOD) is an important enzyme which is involved in the dismutation of the toxic radical, Superoxide anion. The activity of CuZnSOD is increased in patients who suffer from Down's Syndrome, Alzheimer's disease, and in Parkinson's disease. In order to evaluate the contribution of this enzyme to the neuropathology of these neurodegenerative diseases, transgenic mice have been constructed which express the human CuZnSOD gene. As a first step towards exploring these issues, we have carried out an autoradiographic binding study of the distribution of the catecholaminergic uptake blocker mazindol in the brain of these transgenic mice and of their littermates. Desmethylimipramine (DMI)-insensitive [³H]mazindol binding sites which correspond to dopamine uptake sites were located in the striatum, the nucleus accumbens, the olfactory tubercle and in the substantia nigra. Within the striatum, there was a lateromedial gradient, with higher concentration of dopamine uptake sites being found laterally. These findings suggest that subregions of the basal ganglia may be more susceptible to the deleterious effects of dopaminotoxic drugs which are taken up into the dopaminergic neurons via these uptake sites. Saturation experiments revealed no differences in the characteristics of [³H]mazindol binding sites between the two groups of mice. Thus, increased activity of SOD is not associated with diffuse changes in the molecular structures of receptors in mice brain.