Different plasticity changes in D1 and D2 receptors in rat striatal subregions following impairment of dopaminergic transmission

The precise topographical changes in striatal D1 and D2 dopamine receptor density that occurred after chronic treatment with haloperidol or SCH 23390 or after 6-hydroxydopamine-induced lesion of the mesostriatal dopaminergic pathway have been studied autoradiographically in the rat. Repeated treatment with SCH 23390 (0.5 mg/kg i.p., 21 days) caused an almost similar increase in [3H]SCH 23390 binding sites in the different striatal subregions whereas lesion of the dopaminergic pathway was ineffective. Subacute administration of haloperidol (2 mg/kg i.p., 18 days) or lesion of dopaminergic afferents provoked an increase in [3H]spiperone binding which was restricted to the ventro- and dorsolateral striatal sectors.     

Dopaminergic activity in transgenic mice underexpressing glucocorticoid receptors: effect of antidepressants

Transgenic mice bearing a transgene coding for a glucocorticoid receptor antisense mRNA, which partially blocks glucocorticoid receptor expression, were used to investigate the long-term effect of hypothalamic-pituitary-adrenal axis dysfunction on brain dopamine transmission. Compared to control mice, the transgenic animals showed increased amphetamine-induced locomotor activity and increased concentrations of striatal dopamine and its metabolites dihydroxyphenylacetic acid and homovanillic acid. Binding of [3H]SCH 23390 and [3H]spiperone to, respectively, D1 and D2 dopamine receptors was increased in transgenic mice. In contrast, autoradiography of striatal [3H]GBR 12935 binding to the dopamine transporter was decreased and the mRNA levels of this transporter, measured by in situ hybridization, remained unchanged in the substantia nigra pars compacta. The effect of chronic treatment for two weeks with amitriptyline or fluoxetine was compared in control and transgenic mice. No significant changes were observed in control mice following antidepressant treatment, whereas in transgenic mice both antidepressants reduced striatal [3H]SCH 23390 and [3H]raclopride specific binding to D1 and D2 receptors. Amitriptyline, but not fluoxetine, increased striatal [3H]GBR 12935 binding to the dopamine transporter, whereas its mRNA level in the substantia nigra pars compacta was decreased in fluoxetine, compared to vehicle- or amitriptyline-treated transgenic mice. From these results we suggest that hyperactive dopaminergic activity of the nigrostriatal pathway controls motor activity in the transgenic mice. Furthermore, antidepressant treatment corrected the increased striatal D1 and D2 receptors and decreased dopamine transporter levels in the transgenic mice.     

Reserpine-induced up-regulation of dopamine D2 receptors in the rat striatum is enhanced by denervation but not by chronic receptor blockade

Compensatory increases in the density of dopamine (DA) D2 receptors in the rat striatum occur following chronic interruption of dopaminergic neurotransmission. Substantia nigra lesions, DA depletion with reserpine and D2 receptor blockade by neuroleptics increase the number of striatal D2 receptors as identified with the D2 ligand, [3H]spiperone [( 3H]SPIP). Chronic administration of haloperidol to substantia nigra-lesioned rats causes an additive increase in binding over levels obtained with one treatment alone. In this study we have found a similar response when lesioned animals are treated with reserpine. However, compensatory increases in the number of [3H]SPIP binding sites found after combined administration of reserpine and haloperidol to intact rats do not exceed levels obtained following administration of either drug alone. The data suggest that up-regulation of striatal D2 binding sites occurring after substantia nigra lesions is unique relative to other forms of up-regulation and may involve the loss of a presynaptic regulatory factor other than DA.     

Unilateral infusion of a dopamine transporter antisense into the substantia nigra protects against MDMA-induced serotonergic deficits in the ipsilateral striatum

The present study was designed to elucidate the consequences of antisense oligonucleotide-mediated knockdown of striatal dopamine reuptake transporters on 3,4-methylenedioxymethamphetamine (MDMA)-induced neurotoxicity. Antisense oligonucleotide complementary to the mRNA translational start site of the rat dopamine transporter was delivered by constant (7 days) intranigral infusion with an osmotic minipump. Delivery of the antisense oligonucleotide by this method resulted in a 70% reduction in the density of the dopamine transporter in the ipsilateral striatum, as measured by [(3)H]mazindol binding. The effect of this transporter knockdown on MDMA-induced serotonergic neurotoxicity was then examined. MDMA (2x20 mg/kg, s.c., given 12 h apart) administered to control rats produced hyperthermia following the first dose and led to a 45-50% reduction in striatal serotonin, 5-hydroxyindoleacetic acid, and serotonin reuptake transporter density 1 week after the second dose. Conversely, in antisense-, but not missense-treated rats, a significant attenuation of MDMA-induced neurotoxicity was observed only in the ipsilateral striatum. The hyperthermic response elicited by MDMA was not altered by prior administration of antisense. In vivo microdialysis revealed that the antisense treatment attenuated MDMA-induced dopamine release in the ipsilateral striatum.These results suggest that the dopamine transporter plays an essential role in the neurodegeneration induced by MDMA, and provides additional support for the hypothesis that extracellular dopamine is involved in the neurotoxic process, at least in the striatum.     

Pharmacological characterization of dopaminergic influence on expression of neuropeptide Y immunoreactivity by rat striatal neurons

Selective unilateral lesion of the nigrostriatal dopamine pathway by the cytotoxin 6-hydroxydopamine was previously shown to enhance the number and staining intensity of neurons expressing neuropeptide Y immunoreactivity in the ipsilateral striatum. This effect was completely reversed by treatment of the 6-hydroxydopamine-injected animals with the directly acting dopamine agonist apomorphine. This finding reinforces our previous hypothesis that changes in striatal neuropeptide Y staining subsequent to 6-hydroxydopamine lesions of this kind reflect changes in intraneuronal neuropeptide Y levels which are directly attributable to the suppression of a tonic dopaminergic control. In contrast to the effect of 6-hydroxydopamine lesion, non-destructive impairment of striatal dopamine transmission by treatments with either the dual dopamine D1/D2 receptor antagonist haloperidol or the dopamine synthesis inhibitor alpha-methylparatyrosine induced a decrease in both the number of neuropeptide Y striatal cells (-29.8% and -34.8%, respectively) and in their labeling intensity. The selective D2-antagonist sulpiride also showed a tendency to reduce the number of neuropeptide Y immunoreactive cells, whereas the selective D1 antagonist SCH 23390 induced a small but constant increase in this number. Taken as a whole, these results suggest that the dopaminergic D1 and D2 receptor subtypes play opposite roles in the dopaminergic control of the striatal neuropeptide Y neuronal system, which may account for the different changes in striatal neuropeptide Y immunostaining observed after 6-hydroxydopamine injury and after non-destructive impairment of nigrostriatal dopaminergic transmission.     

Experimental hemiparkinsonism in the rat following chronic unilateral infusion of MPP+ into the nigrostriatal dopamine pathway--II. Differential localization of dopamine and cholecystokinin receptors

The autoradiographical localization of dopamine D1, D2 and cholecystokinin receptors has been investigated in rat brain 6 months following unilateral infusion of 1-methyl-4-phenyl pyridinium ion (MPP+) (10 micrograms/day for 7 days) into the nigrostriatal dopamine pathway. Treatment with 1-methyl-4-phenyl pyridinium ion produced a marked depletion of dopamine cell bodies in the substantia nigra together with greater than 95% loss of tyrosine hydroxylase immunoreactivity in the striatum. Measurement of specific [3H]spiperone binding to D2 receptors indicated a 38% increase (P less than 0.01) in the maximal binding capacity of [3H]spiperone to striatal membrane homogenates and a 13% increase (P less than 0.05) in specific [3H]spiperone binding to striatal tissue sections, verifying striatal D2 receptor denervation supersensitivity. In contrast, MPP+ lesion of the nigrostriatal tract had no effect on the autoradiographical localization of striatal D1 or cholecystokinin receptors. In addition, there was a 38% loss (P less than 0.05) of D2 receptor binding sites in the substantia nigra pars compacta, whilst D1 receptors remained unchanged. Similar changes in dopamine and cholecystokinin receptor number were found following 6-hydroxydopamine lesion of the nigrostriatal dopamine pathway. These results provide further evidence that 1-methyl-4-phenyl pyridinium ion treatment in rats produces extensive destruction of the dopaminergic nigrostriatal tract and supports the differential anatomical localization of striatal and nigral D1, D2 and cholecystokinin receptors.     

Effect of chronic calcium antagonist treatment on dopamine recognition sites in rat striatum

The effects of nimodipine and flunarizine administration (18 days 15 mg/kg/day p.o.) on striatal dopamine recognition sites in rats were investigated in vivo. In vitro flunarizine but not nimodipine displaces [3H]spiroperidol binding. After in vivo treatment both drugs induce a significant increase in the number of sulpiride displaceable spiroperidol binding sites (flunarizine, +114%, nimodipine +61%) concomitant with an increase in the dissociation constant. Binding parameters return toward control values after 1 week of suspension of the treatment. The results suggest that the repeated in vivo treatment with nimodipine and flunarizine may significantly interact with dopaminergic transmission leading to adaptive changes of the dopamine recognition sites.     

Early direct and transneuronal effects in mice with targeted expression of a toxin gene to D1 dopamine receptor neurons

The neurochemical profile was examined at postnatal day 3-4 in mutant mice generated by in vivo Cre mediated activation of an attenuated diphtheria toxin gene inserted into the D1 dopamine receptor gene locus. An earlier study of this model had shown that D1 dopamine receptor, substance P and dynorphin were not expressed in the striatum. Quantitative in situ hybridization analysis showed an increase in D2 dopamine receptor and enkephalin messenger RNA expression. The nigrostriatal pathway in the mutant pups was intact with a normal number of dopaminergic neurons in the substantia nigra and the ventral tegmental area in addition to a normal pattern of striatal dopamine transporter and tyrosine hydroxylase immunoreactivity. Quantitative analysis of striatal dopamine transporter density using [3H]mazindol showed a reduction of 26% suggesting a degree of transneuronal down-regulation. There was also a 49% reduction of striatal GABA receptor binding and a 36% reduction of striatal muscarinic receptor binding in mutant pups. The number of healthy striatal neuropeptide Y-containing interneurons was also substantially down-regulated in the mutant striatum. In contrast, there was an increase in the number of striatal cholinergic interneurons. Down-regulated cortical GABA receptor and muscarinic receptor binding was also observed in addition to subtle morphological changes in the neuropeptide Y-expressing population of cortical neurons. The changes reflect the early cascade of events which follows the ablation of D1 dopamine receptor-positive cells. Although extensive changes in a number of striatal and cortical neurons were demonstrated, only subtle transneuronal effects were seen in the nigrostriatal pathway.     

Long-term changes in striatal D1 dopamine receptor distribution after dopaminergic deafferentation

The morphochemical disposition of the adenylate cyclase-linked dopamine receptor (D1 type) in the rat striatum has been assessed at various time points after a neurotoxic lesion of the dopaminergic afferent pathway to the caudate nucleus. D1 receptor binding sites in the caudate nucleus were determined by in vitro autoradiography of the substituted benzazepine D1 antagonists, [3H]SCH 23390 or [125I]SCH 23982, and contrasted to the pattern of striatal immunohistochemical reactivity of the second messenger compound, cyclic 3',5'-adenosine monophosphate. The results demonstrate that the specific association of this dopamine receptor type with cyclic 3',5'-adenosine monophosphate-stained neurons is abolished at 7 days following chemical interruption of the nigrostriatal pathway, and the receptor disruption is persistent for durations as long as 20 weeks. This investigation suggests that once the postsynaptic receptor pathology is produced by deafferentation, it does not recover the selective morphochemical relationship normally established with the target cell containing the second messenger. This is in contrast to modest biochemical recuperation in D1 dopamine receptor binding seen using this experimental paradigm. This change in D1 dopamine receptor morphochemistry is discussed in relation to the neurochemical deficits produced by dopaminergic denervation and in Parkinson's disease.     

Alterations of the muscarinic cholinergic (mACh) receptors in the striatum of the MPTP-induced parkinsonian model in mice: in vitro quantitative autoradiographical analysis

The alteration of muscarinic cholinergic (mACh) receptors in the striatum of mice after administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) was examined using a [3H]quinuclidinyl benzilate [( 3H]QNB) in vitro labeling macro-autoradiographic technique and mACh receptor concentration was quantitatively analyzed using a computer analysis system. Two weeks after cessation of MPTP administration, the striatal mACh receptors were significantly increased. In the subchronic phase, at 6 weeks, the striatal mACh receptors were significantly decreased but recovered to the normal level by the treatment with L-dihydroxyphenylalanine (L-DOPA) for two weeks before sacrifice. These findings indicated that the striatal mACh receptors are strongly regulated by the nigrostriatal dopaminergic function.     

Neonatal hypothyroidism induces striatal dopaminergic dysfunction

Oral administration of the antithyroid drug methimazole (50 mg/kg per day) to rats during the last six days of pregnancy, and subsequent daily s.c. injection of methimazole (20-30 mg/kg) to their pups from birth to postnatal day 30 provoked hormonal and somatic alterations resembling (with all caution to any association between rodent and human data) those of congenital hypothyroidism. The steady-state concentrations of striatal dopamine were similar in hypothyroid and euthyroid, 32-day-old rats, while the levels of the dopamine metabolites 3,4-dihydroxyphenylacetic and homovanillic acids were markedly decreased in hypothyroidism. The results of this and our earlier study [Vaccari A. and Gessa G. L. (1989) Neurochem. Res. 14, 949-955] show that the maximal synaptosomal uptake of [3H]dopamine, an index for the density of nigrostriatal dopaminergic terminals, and the maximum number of membrane [3H]tyramine binding sites, reflecting the concentration of the vesicular transporter for dopamine, were decreased in the hypothyroid striatum. There was also a loss of those D1-type dopaminergic receptors claimed to be located on neurons intrinsic to the striatum, and, consequently, dopamine-stimulated, D1-regulated adenylate cyclase activity was depressed. It is suggested that individual dopaminergic nerve endings in the neonatal hypothyroid striatum must contain more dopamine, owing to some loss of pertinent innervation and, therefore, to the presence of less vesicular transport sites for dopamine. Hypothyroidism-related decreases in the maximum number of striatal D1- and, reportedly, D2-receptors, plus the impairment of D1-coupled second messenger activity, may play a role in the derangement of those neurobehavioural patterns where a dopaminergic regulation is putatively implied.     

Striatal opiate mu-receptors are not located on dopamine nerve endings in the rat

In rat striatal slices, the autoradiographic analysis of [3H]naloxone binding allows one to define highly labelled patches corresponding to the striosomes and representing about 17% of the total striatal volume, surrounded by a poorly labelled zone, the matrix. Previous studies have shown that the density of these mu-opiate receptor binding sites is decreased by about 28% following destruction of the striatal dopamine innervation suggesting a partial localization of these receptors on dopamine presynaptic nerve endings. These results were confirmed but, in addition, we have shown that a chronic (30 days) blockade of dopamine transmission obtained by treatment of the animals with a long acting neuroleptic induces a similar decrease of mu binding sites. Further experiments made with D-Pen2,D-Pen5-[tyrosyl-3-5(n)-3H] enkephalin, a selective delta opiate receptor agonist, have revealed that the density of delta opiate binding sites is decreased (30%) in rats with striatal dopamine denervation but not in those treated with the long acting neuroleptic. These data indicate that part of these delta receptors is located on dopamine nerve terminals but are not in favour of the presence of mu receptors on these nerve terminals. The decrease in [3H]naloxone binding sites induced by prolonged interruption of dopamine transmission can be attributed to postsynaptic events.     

Autoradiographic study of striatal dopamine re-uptake sites and dopamine D1 and D2 receptors in a 6-hydroxydopamine and quinolinic acid double-lesion rat model of striatonigral degeneration (multiple system atrophy) and effects of embryonic ventral mesencephalic, striatal or co-grafts

The influence of embryonic mesencephalic, striatal and mesencephalic/striatal co-grafts on amphetamine- and apomorphine-induced rotation behaviour was assessed in a rat model of multiple system atrophy/striatonigral degeneration type using dopamine D1 ([3H]SCH23390) and D2 ([3H]spiperone) receptor and dopamine re-uptake ([3H]mazindol) autoradiography. Male Wistar rats subjected to a sequential unilateral 6-hydroxydopamine lesion of the medial forebrain bundle followed by a quinolinic acid lesion of the ipsilateral striatum were divided into four treatment groups, receiving either mesencephalic, striatal, mesencephalic/striatal co-grafts or sham grafts. Amphetamine- and apomorphine-induced rotation behaviour was recorded prior to and up to 10 weeks following transplantation. 6-Hydroxydopamine-lesioned animals showed ipsiversive amphetamine-induced and contraversive apomorphine-induced rotation behaviour. Amphetamine-induced rotation rates persisted after the subsequent quinolinic acid lesion, whereas rotation induced by apomorphine was decreased. In 11 of 14 animals receiving mesencephalic or mesencephalic/striatal co-grafts, amphetamine-induced rotation scores were decreased by >50% at the 10-week post-grafting time-point. In contrast, only one of 12 animals receiving non-mesencephalic (striatal or sham) grafts exhibited diminished rotation rates at this time-point. Apomorphine-induced rotation rates were significantly increased following transplantation of mesencephalic, striatal or sham grafts. The largest increase of apomorphine-induced rotation rates approaching post-6-hydroxydopamine levels were observed in animals with striatal grafts. In contrast, in the co-graft group, there was no significant increase of apomorphine-induced rotation compared to the post-quinolinic acid time-point. Morphometric analysis revealed a 63-74% reduction of striatal surface areas across the treatment groups. Striatal [3H]mazindol binding on the lesioned side (excluding the demarcated graft area) revealed a marked loss of dopamine re-uptake sites across all treatment groups, indicating missing graft-induced dopaminergic re-innervation of the host. In eight (73%) of the 11 animals with mesencephalic grafts and reduced amphetamine-induced circling, discrete areas of [3H]mazindol binding ("hot spots") were observed, indicating graft survival. Dopamine D1 and D2 receptor binding was preserved in the remaining lesioned striatum irrespective of treatment assignment, except for a significant reduction of D2 receptor binding in animals receiving mesencephalic grafts. "Hot spots" of dopamine D1 and D2 receptor binding were observed in 10 (83%) and nine (75%) of 12 animals receiving striatal grafts or co-grafts, consistent with survival of embryonic primordial striatum grafted into a severely denervated and lesioned striatum. Our study confirms that functional improvement may be obtained from embryonic neuronal grafts in a double-lesion rat model of multiple system atrophy/striatonigral degeneration type. Co-grafts appear to be required for reversal of both amphetamine- and apomorphine-induced rotation behaviour in this model. We propose that the partial reversal of amphetamine-induced rotation asymmetry in double-lesioned rats receiving mesencephalic or mesencephalic/striatal co-grafts reflects non-synaptic graft-derived dopamine release. The changes of apomorphine-induced rotation following transplantation are likely to reflect a complex interaction of graft- and host-derived striatal projection pathways and basal ganglia output nuclei. Further studies in a larger number of animals are required to determine whether morphological parameters and behavioural improvement in the neurotransplantation multiple system atrophy rat model correlate.     

Motor behaviour deficits and their histopathological and functional correlates in the nigrostriatal system of dopamine transporter knockout mice

Chronic dysregulation of dopamine homeostasis has been shown to induce behavioural impairment in dopamine transporter knockout mutant mice arising from the dysfunction of the mesolimbic and hypothalamo-infundibular system. Here, we assessed whether there are also any motor consequences of a chronic and constitutive hyperdopaminergia in the nigrostriatal system in dopamine transporter knockout mutant mice. For this, we analysed motor performances using tests assessing balance, coordinated motor skills (rotarod, pole test), stride lengths and locomotor activity. Dopamine transporter knockout mutant mice were markedly hyperactive in the open field with central compartment avoidance, as previously shown. However, sensorimotor integration was also found to be altered in dopamine transporter knockout mutant mice which displayed a reduced fore- and hind-limb mean stride length, impaired motor coordination on the pole test and reduced rearings in the open field. Moreover, dopamine transporter knockout mutant mice showed a slower task acquisition on the rotarod. Six-week-old dopamine transporter knockout wild type mice having the same femur size as adult dopamine transporter knockout mutant mice ruled out a possible size-effect bias. Whilst there was no significant difference in the striatal volume, we found a slight but significant reduction in neuronal density in the striatum but not in the nucleus accumbens of dopamine transporter knockout mutant mice. There was a reduced binding in the striatum and nucleus accumbens of dopamine(1) receptors ([(3)H]SCH 23390) and dopamine(2) receptors ([(3)H]YM-09151-2). There was no significant difference in the number of dopaminergic neurons in the substantia nigra between dopamine transporter knockout mutant mice and dopamine transporter knockout wild type mice. These results suggest an impaired functioning of the nigrostriatal system in dopamine transporter knockout mutant hyperdopaminergic mice, as illustrated by motor and sensorimotor integration deficits, despite their apparent hyperactivity. These dysfunctions may arise from combined striatal cell loss and/or functional changes of dopaminergic neurotransmission.     

AMPA receptor blockade potentiates the stimulatory effect of L-DOPA on dopamine release in dopamine-deficient corticostriatal slice preparation

The release of [3H]dopamine was measured in rat corticostriatal slice preparations that contained the striatum and the adjacent prefrontal cortex to maintained glutamatergic corticostriatal afferentation. These slices were prepared from either nontreated or 6-hydroxydopamine-pretreated rats. The slices were loaded with [3H]dopamine, submerged in a two-compartment bath so that the cortical region was contained in one compartment, the corpus callosum was passed through a silicone greased slot, and the striatal region was contained in the other compartment. The cortical and the striatal parts were superfused with Krebs-bicarbonate buffer independently. The release of [3H]dopamine was determined from the striatal part at rest and in response to electrical stimulation of the cortical area. Electrical stimulation of the cortical part increased the release of [3H]dopamine from the striatal part of the slices, and this release was found to be higher after lesion of the nigrostriatal dopaminergic pathway with 6-hydroxydopamine. Cortically evoked [3H]dopamine release was even higher in the presence of the dopamine precursor L-DOPA after 6-hydroxdopamine lesion. Perfusion of GYKI-53405, a noncompetitive AMPA receptor antagonist, in combination with L-DOPA further increased both basal and stimulation-evoked [3H]dopamine release, whereas GYKI-53405 by itself did not influence basal [3H]dopamine outflow from striatum. These findings indicate that, in parkinsonian striatum, the stimulatory effect of L-DOPA on dopamine release is potentiated by AMPA receptor blockade, and the antiparkinsonian effect of GYKI-53405 may be due to its L-DOPA sparing effect.     

The inhibitory role of methylation on the binding characteristics of dopamine receptors and transporter

Excess methylation has been suggested to play a role in the pathogenesis of Parkinson's disease (PD), since the administration of S-adenosylmethionine (SAM), a biological methyl donor, induces PD-like changes in rodents. It was proposed that SAM-induced PD-like changes might be associated with its ability to react with the dopaminergic system. In the present study the effects of SAM on dopamine receptors and transporters were investigated using rats and cloned dopamine receptor proteins. Autoradiographic examination of SAM indicated its tendency to be localized and accumulated in rat striatal region after the intracerebroventricular injection into rat brain. Moreover, results showed that SAM significantly decreased dopamine D1 and D2 receptor binding activities by decreasing the Bmax and increasing the Kd values. At concentrations of 0.1, 0.25 and 0.5 mM, SAM was able to reduce the Bmax from the control value of 848.1 for dopamine D1-specific ligand [3H] SCH 23390 to 760.1, 702.6 and 443.0 fmol/mg protein, respectively. At the same concentrations, SAM was able to increase the Kd values from 0.91 for the control to 1.06, 3.84 and 7.01 nM of [3H] SCH 23390, respectively. The effects of SAM on dopamine D2 binding were similar to those of dopamine D1 binding. SAM also decreased dopamine transporter activity. The interaction of SAM with dopamine receptor proteins produced methanol from methyl-ester formation and hydrolysis. We propose that the SAM effect might be related to its ability to react with dopamine receptor proteins through methyl-ester formation and methanol production following the hydrolysis of the carboxyl-methylated receptor proteins.     

Correlation between decreased motor activity and dopaminergic degeneration in the ventrolateral putamen in monkeys receiving repeated MPTP administrations: a positron emission tomography study

Parkinson's disease (PD) patients have remarkably reduced levels of dopaminergic biomarkers in the caudal putamen. However, the relationship between motor impairments and the localization of intrastriatal dopaminergic degeneration in monkey PD models remains unclear. To identify the striatal areas with dopaminergic dysfunction responsible for motor impairments, we measured changes in both general motor activity and in vivo dopaminergic biomarkers in three cynomolgus monkeys that repeatedly received 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), starting in the normal state and continuing until after tremor appearance. Binding of dopamine transporters (DAT) and D(2) receptors were measured by positron emission tomography (PET) using [(11)C]PE2I and [(11)C]raclopride, respectively. Region-of-interest-based regression analysis demonstrated the degree of general motor activity reduction to be explained by striatal DAT binding but not by D(2) receptor binding. Furthermore, voxel-based analysis revealed a significant correlation between reduced general motor activity and decreased DAT binding, specifically in the ventrolateral putamen, which corresponds to the area receiving upper body motor inputs from the primary motor cortex. These results suggest that specific functional deficits in PD models are closely related to the degeneration of dopaminergic terminals in the striatal subregion responsible for these functions and that the level of deficit is dependent on the degree of degeneration.     

Alterations in dopamine D3 receptors in the circling (ci3) rat mutant

We have previously described a black-hooded mutant rat (BH.7A/Ztm-ci3/ci3) that displays abnormal lateralized circling behavior, but normal auditory and vestibular functions. Neurochemical determination of dopamine and dopamine metabolite levels in striatum, nucleus accumbens and substantia nigra showed that ci3 rats have a significant asymmetry in striatal dopamine in that dopamine levels were significantly lower in the hemisphere contralateral to the preferred direction of turning. Consistent with this finding, immunohistological examination of dopaminergic neurons in substantia nigra and ventral tegmental area yielded a significant laterality in the medial part of substantia nigra pars compacta with a lower density of tyrosine hydroxylase-positive neurons in the contralateral hemisphere of mutant circling rats, while no laterality was seen in unaffected rats of the background strain. In the present study, quantitative autoradiography was used to examine the binding of [(3)H]SCH 23390, [(3)H]raclopride and [(3)H]7-OH-DPAT (7-hydroxy-N,N-di-n-propyl-2-aminotetralin) to dopamine D1, D2, and D3 receptors, respectively, in various brain regions of ci3 rats and unaffected rats of the background strain (BH.7A(LEW)/Won). No significant differences between circling rats and controls were obtained for D1 and D2 receptor binding in any region, but mutant rats differed from controls in dopamine D3 binding in several regions. A significant decrease in D3 binding was seen in the shell of the nucleus accumbens, the islands of Calleja, and the subependymal zone of ci3 mutant rats. Furthermore, a significant laterality in D3 binding was determined in ci3 rats in that binding was lower in the contralateral hemisphere in the shell of the nucleus accumbens and the islands of Calleja. Our data indicate that alterations of dopamine D3 receptors may be involved in the behavioral phenotype of the ci3 rat, thus substantiating the findings from a recent genetic linkage analysis that indicated the D3 receptor gene as a candidate gene in this rat mutant.     

Effects of repeated treatment with haloperidol on rat striatal neutral endopeptidase, and on mu- and delta-opioid binding sites: comparison with chronic morphine and chronic kelatorphan

Neutral endopeptidase (NEP) and mu- and delta-opioid receptor densities were measured in sections of rat striatum by in vitro radioautography with the selective ligands: 3 nM [3H]N-[(2RS)-3-hydroxyaminocarbonyl-2-benzyl-1-oxopropyl]glycine ([ 3H]HACBO-Gly), 3 nM [3H]Tyr-D.Ala-Gly-(NMe)Phe-Gly-ol ([ 3H]DAGO) and 3 nM [3H]Tyr-D-Thr-Gly-Phe-Leu-Thr ([ 3H]DTLET), respectively. Haloperidol treatment (2 mg/kg/day, i.p., 3 weeks), which has been reported to increase enkephalin levels in the striatum, induced a 23% decrease in striatal (posterior level A= +8.4-8.6 mm) NEP labeling (but no change of mu- and delta-sites). In contrast, no change in NEP occurred after chronic morphine (40-160 mg/kg/day, s.c., 4 days) or kelatorphan (10 nmol/h, i.c.v., 7 days), a mixed inhibitor of enkephalin-degrading peptidases.     

The cellular localization of adenosine receptors in rat neostriatum

Using quantitative autoradiography of ligand binding sites combined with lesions of specific neuronal pathways, the cellular locations of A1 and A2 adenosine receptors, as well as a third binding site for the adenosine receptor ligand, [3H]N-ethylcarboxamidoadenosine, and a nucleoside transporter were investigated in rat neostriatum. Intrastriatal kainic acid administration resulted in the loss of 50% of A1 adenosine receptors and virtually abolished ligand binding to A2 receptors. A small reduction in [3H]cyclohexyladenosine binding to striatal A1 receptors was found after lesioning the corticostriatal input. A2 receptor sites were unaffected by this treatment. Destruction of dopaminergic neurons using 6-hydroxydopamine or the raphestriatal serotoninergic input using 5,7-dihydroxytryptamine affected neither A1 nor A2 binding sites. These results indicate the localization of both A1 and A2 adenosine receptors on neurons intrinsic to the neostriatum and probably postsynaptic to the dopaminergic input. In addition, a binding site for [3H]N-ethylcarboxamidoadenosine which is not affected by the adenosine receptor agonist, R-phenylisopropyladenosine, was also partly abolished after kainic acid injection. In contrast, no significant change in the binding of the nucleoside transporter ligand, [3H]nitrobenzylthioinosine, was observed after any lesions, indicating the widespread association of this site with various cell types.