Pharmacological effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on striatal dopamine receptor system

In mice, chronic administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) produces an increase in the maximum number of [³H]spiperone binding sites in the striatum. The sensitivity of striatal protein phosphorylation to calcium plus calmodulin is also potentiated in MPTP-treated mice. These observations are associated with an enhancement of apomorphine-induced climbing behavior in the drug-treated animals. The results of this study suggest that in an animal model for Parkinson's disease, MPTP interrupts the dopamine (DA) transmission by chemically denervating the nigrostriatal neurons and through a compensatory mechanism, it increases the number of DA receptors as well as the sensitivity of protein phosphorylation to calcium plus calmodulin in mouse striatum. The latter two events may contribute to the development of DA receptor supersensitivity.     

Increased vulnerability of nigrostriatal terminals in DJ-1-deficient mice is mediated by the dopamine transporter

Mutations in the gene for DJ-1 have been associated with early-onset autosomal recessive parkinsonism. Previous studies of null DJ-1 mice have shown alterations in striatal dopamine (DA) transmission with no DAergic cell loss. Here we characterize a new line of DJ-1-deficient mice. A subtle locomotor deficit was present in the absence of a change in striatal DA levels. However, increased [(3)H]-DA synaptosomal uptake and [(125)I]-RTI-121 binding were measured in null DJ-1 vs. wild-type mice. Western analyses of synaptosomes revealed significantly higher dopamine transporter (DAT) levels in pre-synaptic membrane fractions. 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) exposure exacerbated striatal DA depletion in null DJ-1 mice with no difference in DAergic nigral cell loss. Furthermore, increased 1-methyl-4-phenylpyridinium (MPP(+)) synaptosomal uptake and enhanced MPP(+) accumulation were measured in DJ-1-deficient vs. control striatum. Thus, under null DJ-1 conditions, DAT changes likely contribute to altered DA neurotransmission and enhanced sensitivity to toxins that utilize DAT for nigrostriatal entry.     

Differential regulation of dopamine release by N-methyl-d-aspartate receptors in rat striatum after partial and extreme lesions of the nigro-striatal pathway

The participation of N-methyl-d-aspartate (NMDA) receptors on dopamine (DA) efflux in the striatum of anaesthetized rats, which had their DA nigrostriatal pathway previously lesioned with different doses of 6-hydroxydopamine (6-OH-DA), was assessed by in vivo microdialysis methodology. In addition, the in vivo basal DA and dihydroxy-phenyl-acetic acid (DOPAC) effluxes and the effect of local K⁺-depolarization on DA release were also evaluated in the striatum of these 6-OH-DA treated rats. Lesioned rats were divided in three groups corresponding to animals with 25–75%, 75–95% and >95% of striatum tissue DA depletion, respectively. Striatal DA tissue depletion between 25–75% occurred in parallel with a 30% reduction in DA extracellular levels, with a moderate 10% increase in basal fractional DA efflux, and with no statistical changes in the fractional DA efflux induced by NMDA (500 μM) receptor stimulation by reverse dialysis. Rats with higher DA tissue depletion (between 75–95%) exhibited a 60% reduction in DA extracellular levels in the striatum and this reduction occurred in parallel with a modest rise in basal fractional DA efflux, but with a striking decrease in the NMDA-induced fractional DA efflux. In rats with extreme or >95% of striatal DA tissue depletion, basal fractional DA efflux in the striatum increased quite substantially along with a recovery in the ability of NMDA receptor stimulation to induce fractional DA release. The >95% striatal DA-depleted rats also exhibited a significant decrease in tissue and extracellular DOPAC/DA ratio when compared to sham and partially DA-depleted rats. In contrast to the previous results, fractional DA efflux induced by reverse dialysis with K⁺ (40 mM) remained the same in the striatum of sham and all groups of DA-tissue depleted rats. The present findings suggest the existence of at least three features associated to the regulation of basal and NMDA-induced extracellular levels of DA in the striatum of rats as a function of striatal tissue DA depletion produced by 6-OH-DA. They also support the view that a differential regulation of basal and NMDA-induced DA extracellular levels occur in partial and extreme DA-depleted striatum after 6-OH-DA treatment. Such findings may have implications as regard to the participation of the NMDA receptor in the compensatory mechanisms associated to the progress of Parkinson's disease, as well as in the therapeutic treatment of this neurological disorder.     

Alterations in dopamine uptake sites and D1 and D2 receptors in cats symptomatic for and recovered from experimental parkinsonism

The administration of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to adult cats severely disrupts the dopaminergic innervation of the striatum. Animals display a parkinson-like syndrome, consisting of akinesia, bradykinesia, postural instability, and rigidity, which spontaneously recovers by 4–6 weeks after the last administration of MPTP. In this study we used quantitative receptor autoradiography to examine changes in DA uptake sites and DA receptors in the basal ganglia of normal, and symptomatic and recovered MPTP-treated cats. Consistent with the destruction of the nigrostriatal DA pathway, there was a severe loss of DA uptake sites, labeled with [³H]-mazindol, in the caudate nucleus (64–82%), nucleus accumbens (44%), putamen (63%), and substantia nigra pars compacta (SNc, 53%) of symptomatic cats. Following behavioral recovery, there were no significant changes in DA uptake site density. Significant increases of [³H]-SCH 23390 binding to D1 DA receptors were observed in the dorsal caudate (>24%; P < 0.05) of symptomatic cats and in all regions of the caudate-putamen (>30%; P < 0.05) of recovered animals. [³H]-SCH 23390 binding in tree substantia nigra pars reticulata was half of that in the striatum and showed no changes in symptomatic or recovered animals. No alterations in the binding of [¹²⁵1]-epidepride to D2 receptors was observed in any region of the striatum in either, symptomatic or recovered animals. [¹²⁵1]-Epidepride binding in the SNc was decreased by >36% (P < 0.05) following MPTP treatment. These data show that cats made parkinsonian by MPTP exposure have a significant decrease in the number of DA reuptake sites throughout the striatum and that recovery of sensorimotor function in these animals is not correlated with an increase in the number of striatal reuptake sites. Behavioral recovery, however, does seem to be correlated with a general elevation of Dl receptors throughout the striatal complex. The present data also show that direct correlations between changes in DA receptor regulation after a large DA depleting lesion and behavioral deficits or recovery from those deficits are difficult and that the relationships between DA receptors/transporters and behavior require further study. © 1995 Wiley-Liss, Inc.     

Striatal dopamine denervation decreases the GDP binding affinity in rat striatal membranes

The role of G-proteins in D2 receptor supersensitivity was studied in striatal membranes from rats with unilateral 6-hydroxydopamine (6-OHDA) induced lesions of the nigral dopamine (DA) system. Thirteen months after the lesion the number of [3H]raclopride binding sites was increased in the DA denervated striatum, but no changes in ligand binding affinities and in proportion of high-affinity agonist binding sites could be detected. The affinity of [35S]GTPgammaS binding was unaltered after the striatal DA denervation, whereas the binding affinity of GDP was decreased in the DA denervated as compared to the intact striatum. It is proposed that the decrease in GDP binding affinity to D2 DA receptor-coupled G proteins is an important factor in the D2 receptor supersensitivity following degeneration of the striatal DA terminals.     

Increased proenkephalin mRNA levels in the rat neostriatum following lesion of the ipsilateral nigrostriatal dopamine pathway with 1-methyl-4-phenylpyridinium ion (MPP+): reversal by embryonic nigral dopamine grafts.

In situ hybridization studies were performed to study the changes in proenkephalin mRNA levels in the neostriatum of rats with long-term (18 months) unilateral lesions of the nigrostriatal dopamine (DA) pathway induced by 1-methyl-4-phenylpyridinium ion (MPP+) and in animals bearing embryonic DA grafts implanted into the DA depleted striatum. In the ipsilateral striatum of MPP(+)-lesioned animals, there was a 2-fold increase in the levels of proenkephalin mRNA compared with those in the contralateral striatum of the same animals or the ipsilateral striatum of control animals. High resolution analysis using emulsion autoradiography showed that increase in proenkephalin gene expression in response to DA-denervation by MPP+ was due to an increase in the hybridization signal over individual expressing cells as well as to an increase in the number of labelled cells. In the DA-grafted striatum the levels of proenkephalin mRNA were significantly (P less than 0.01) reduced when compared with those in the MPP(+)-lesioned striatum due to both a decrease in the number of labelled cells as well as the hybridization density per individual cell. Moreover, when compared with the ipsilateral striatum of control animals, the levels of proenkephalin mRNA in the DA-grafted striatum was slightly lower due to a 20% decrease in the number of labelled cells rather than a decrease in the hybridization signal per individual cell. The results of this study are important in two respects. Firstly, they clearly show that the increase in proenkephalin gene expression in the striatum of rats with complete nigrostriatal DA lesions, can be maintained for many months after the lesion.(ABSTRACT TRUNCATED AT 250 WORDS)     

Autoradiography of dopamine receptors and dopamine uptake sites in the spontaneously hypertensive rat

We examined the status of dopamine (DA) D1 and D2 receptors by using [3H]SCH 23390 and [3H]spiperone binding, respectively, and DA uptake sites by using [3H]mazindol binding in spontaneously hypertensive rats (SHR) and Sprague-Dawley (SD) rats. SHR showed significantly higher [3H]SCH 23390 and [3H]spiperone binding in the caudate-putamen (CPu), the nucleus accumbens (NAc) and the olfactory tubercle (OT) in comparison to the SD rats. There were no significant differences in [3H]mazindol-labeled DA uptake sites between the two strains. Unilateral 6-hydroxydopamine (6-OHDA) injection into the striatum resulted in more than 90% depletion of DA uptake sites in the CPu in both strains. 6-OHDA-induced DA depletion was associated with significant increases in striatal [3H]spiperone binding which were of similar magnitude in the SD rats (+64.1%) and SHR (+51.3%). There were only small decreases (-5.4%) in D1 receptor binding in the dorsolateral aspect of the CPu in the SHR, whereas there were no changes in striatal D1 receptors in the SD rats. These results indicate that, although the SHR have higher concentrations of both D1 and D2 receptors in the basal ganglia, these receptors are regulated in a fashion similar to DA receptors in SD rats after 6-OHDA-induced striatal DA depletion.     

In vivo PET studies of the dopamine D2 receptors in rhesus monkeys with long-term MPTP-induced parkinsonism

Studies of dopamine (DA) receptor binding in early parkinsonian patients, or in models of Parkinson's disease, have revealed a supersensitivity of the D2-like receptor subtype as compared to age-matched controls. The lack of upregulation in advanced patients is often attributed to the effects of prolonged antiparkinsonian therapy, but the impact of therapy vs. intrinsic mechanisms in untreated patients or animals with long-term lesions of the DA nigrostriatal pathway has been difficult to address. We studied, in vivo, by PET using the DA D2 receptor ligand raclopride, the status of the DA receptors in normal rhesus monkeys and those with acute (3 months) or long-term (10 years) MPTP-induced nigrostriatal lesions. Compared to age-matched controls, there was no change in raclopride binding in MPTP-treated animals without parkinsonian symptoms. There was a significant increase in raclopride binding in the putamen (but not caudate nucleus) of all the animals displaying rigidity, hypo- and bradykinesia. This increase was greater in the animals with acute lesions (32%) than with established, long-term lesions (18%). There was no correlation between the postmortem striatal DA concentrations and in vivo raclopride binding but there was a correlation between PET raclopride binding and [(3)H]raclopride binding in vitro. Complex changes in D2 receptor binding occur in various stages of parkinsonism. Antiparkinsonian therapy is unlikely to be solely responsible for the lack of upregulation found in advanced parkinsonian patients but may be a contributing factor.     

Asymmetrical changes of dopamine receptors in the striatum after unilateral dopamine depletion

Dopamine plays an important role in modulating synaptic transmission in the striatum and has great influence on the function of the basal ganglia. Degeneration of dopamine neurons in the substantia nigra (SN) is the major cause of many neurological disorders, and the reduction of dopamine innervation results in alterations of dopamine receptors in the striatum. It has been shown that the nigrostriatal dopamine system has functional and neurochemical asymmetry. To investigate the lateralization of dopamine receptors in the striatum after dopamine denervation, the present study used quantitative autoradiography to compare the changes in dopamine receptor binding in the left and right striatum in rats after unilateral dopamine depletion. In comparison to control levels, dopamine D1)-like receptor binding, labeled with [3H]-SCH23390, in the dorsal striatum was reduced 2 weeks after unilateral lesions of the SN with 6-hydroxydopamine. D1-like receptor binding was decreased in the ipsilateral striatum following unilateral lesions of either the left or right SN. The left and right striatum responded similarly to unilateral SN lesions, as there were no significant differences in the percent decrease in D1-like binding in the two striata. In contrast, D2-like receptor binding, labeled with [3H]-spiroperidol, was significantly increased in the dorsal striatum following an ipsilateral SN lesion. Furthermore, the up-regulation of D2-like receptors in the right striatum was significantly greater than that in the left striatum after an ipsilateral lesion. The asymmetrical up-regulation of striatal D2 receptors after extensive dopamine depletion might contribute to the lateralization of the nigrostriatal system observed in some pathological conditions.     

Alterations in expression of dopamine receptors and neuropeptides in the striatum of GTP cyclohydrolase-deficient mice

The hph-1 mice have defective tetrahydrobiopterin biosynthesis and share many neurochemical similarities with l-dopa-responsive dystonia (DRD) in humans. In both, there are deficiencies in GTP cyclohydrolase I and low brain levels of dopamine (DA). Striatal tyrosine hydroxylase (TH) levels are decreased while the number of DA neurones in substantia nigra (SN) appears normal. The hph-1 mouse is therefore a useful model in which to investigate the biochemical mechanisms underlying dystonia in DRD. In the present study, the density of striatal DA terminals and DA receptors and the expression of D-1, D-2, and D-3 receptors, preproenkephalin (PPE-A), preprotachykinin (PPT), and nitric oxide synthase (NOS) mRNAs in the striatum and nucleus accumbens and nigral TH mRNA expression were examined. Striatal DA terminal density as judged by specific [3H]mazindol binding was not altered while the levels of TH mRNA were elevated in the SN of hph-1 mice compared to control (C57BL) mice. Total and subregional analysis of the striatum and nucleus accumbens showed that D-2 receptor ([3H]spiperone) binding density was increased while D-1 receptor ([3H]SCH 23390) and D-3 receptor ([3H]7-OH-DPAT) binding density was not altered. In the striatum and nucleus accumbens, expression of PPT mRNA was elevated but PPE-A mRNA, D-1, D-2 receptor, and nNOS mRNA were not changed in hph-1 mice compared to controls. These findings suggest that an imbalance between the direct strionigral and indirect striopallidal output pathways may be relevant to the genesis of DRD. However, the pattern of changes observed is not that expected as a result of striatal dopamine deficiency and suggests that other effects of GTP cyclohydrolase I deficiency may be involved.     

Striatal D1- and D2-dopamine receptor sites are separately detectable in vivo

We have characterized in particulate fractions of normal rat striatum the in vivo binding kinetics, binding affinity, and pharmacological profiles of [3H]SCH 23390, a ligand selective for the D1-subtype of dopamine (DA) receptor, and compared these to [3H]spiperone, a ligand classically associated with the D2 DA receptor subtype. The pharmacological specificity of each ligand's in vivo binding is very similar to binding to striatal homogenates in vitro. While similar maximum numbers (Bmax) of striatal binding sites exist in vivo compared to in vitro for both ligands, binding affinities in vivo for both ligands are reduced 125- to 200-fold compared to in vitro. In vivo binding of [3H]SCH 23390 to striatum is not increased by dopamine denervation produced by 6-hydroxydopamine lesions of the nigrostriatal pathway. In vivo binding of [3H]SCH 23390 and [3H]spiperone to striatum is not significantly reduced by increased synaptic concentration of dopamine following D-amphetamine administration. 125I-SCH 23982, the iodinated analogue of SCH 23390, localizes very highly to dopaminergic forebrain areas following i.v. administration. External imaging of mammalian and human brain D1-receptors is potentially feasible with this ligand.     

Movement initiation characteristics in young adult rats in relation to the high- and low-affinity agonist states of the striatal D2 dopamine receptor

Changes in the speed of movement initiation as a function of age, brain damage, or rat strain are associated with altered characteristics of nigrostriatal dopamine (DA) neurons and of striatal D2 DA receptors. In the present study we investigated the relationship between movement initiation (response parameters: percent of successful responses and response latency) and the agonist binding states of the D2 DA receptor in corpus striatum in 3-month-old Sprague-Dawley rats (n = 51). In contrast to the typical experimental procedure, the variances of the behavioral and receptor binding data were intentionally made as small as possible to provide the most stringent test of putative relationships among variables. Rats were trained to release a lever as rapidly as possible in response to a light/buzzer (CS) combination in order to avoid a mild footshock (UCS). Percent avoidance scores, latencies of the fastest successful trials (successful latencies) and mean latencies for all responses (mean latencies) were collected for 1000-, 500-, 300- and 200-ms CS-UCS intervals. Twenty-four hours following the last behavioral test, animals were euthanized for measurements of the high- and low-affinity binding of DA to D2 receptors in corpus striatum. The standard errors of the mean for both behavioral and receptor binding parameters were, generally, less than 10%. The tightness of the receptor binding data appeared to be related to a lack of biological variance in the animals rather than to an artifact associated with the behavioral testing procedure, since a parallel experiment indicated that different numbers of behavioral shaping sessions had no effects on striatal D2 binding characteristics.(ABSTRACT TRUNCATED AT 250 WORDS)     

Kinetic and pharmacologic characterization of dopamine binding in the mouse cerebellum and the effects of the reeler mutation

The purpose of this study was to characterize the dopaminergic system in the mouse cerebellum and to determine whether the dyskinesia of the reeler mutant is accompanied by alterations in cerebellar and/or striatal dopamine binding. From the analysis of (3H) dopamine ((3H)DA) and (3H)spiperone ((3H)Sp) binding, the study of the effects of several drugs on this binding, and the comparison of these parameters between the cerebellum and striatum, we conclude that a dopaminergic system exists in the cerebellum with properties common to the striatal system but also with some differences. That is, 1) with (3H)DA as ligand, we find two binding sites in cerebellum with similar Kd to those of striatum but of lower density, 2) with (3H)Sp as ligand we observe two binding sites in cerebellum and one in striatum, and 3) the competition of (3H)DA binding by various drugs shows that among the cerebellar sites, relative to striatum, there is a higher proportion that corresponds to high affinity D3 and D4 (D2 high) binding sites. In cerebellum and striatum of reeler mice, (3H)DA binding increases 125-174% and 14%, respectively.     

Effects of para-chlorophenylalanine on amphetamine and haloperidol-induced changes in striatal dopamine turnover

Para-chlorophenylalanine (PCPA) was administered to rats, and the animals were sacrificed 48 h later. Animals received various doses of S(+)-amphetamine (AMPH) 31 min before sacrifice or of haloperidol (HAL) 46 min before sacrifcie and an intracerebral injection of [³H]tyrosine 15 min prior to sacrifice. PCPA pretreatment potentiated the increased accumulation of striatal [³H]dopamine (DA) induced by doses of AMPH to 1.5 mg/kg, and inhibited the decrease in striatal [³H]DA accumulation observed at higher doses (> 2.5 mg/kg) of AMPH. Furtherm the AMPH-induced increase in endogeneous levels of striatal DA was inhibited by PCPA pretreatment. Similarily, PCPA pretreatment potentiated the HAL-induced increase in striatal [³H]DA accumulation. The data are discussed with respect to an inhibitory serotonergic influence on nigrostriatal DA function, and the relationship of nigrostriatal biochemical events to AMPH-induced behavioral changes.     

Development of D1 and D2 dopamine receptors and associated second messenger systems in fetal striatal transplants.

Stereotactic implantation of fetal brain regional anlage into adult host brain ("brain transplantation") appears to be an increasingly viable strategy for therapy of neurodegenerative diseases. We have studied implantation of fetal striatum into adult striatum, previously lesioned by neurotoxic amino acid injection, as a model for transplantation therapy of Huntington's disease. The beginning of behavioral recovery to apomorphine is not apparent until 6.5 months after implantation. By 4 months after implantation cerebral blood flow through the implants appears equal to that in the intact contralateral striatum. At this time, cerebral glucose utilization is reduced in the implants but increases following apomorphine treatment. The development of D1 and D2 dopamine (DA) receptors is markedly deficient in the striatal grafts at both 4 and 6.5 months after implantation. Very little D2 radioligand binding was observed in the grafts at either time point; D1 receptors appeared in a patchy fashion by 6.5 months at densities approaching normal striatum. In situ hybridization of D2 dopamine receptor mRNA demonstrated robust hybridization signal in normal striatum and accumbens but no signal in 6.5-month-old striatal grafts. Adenylate cyclase (AC) activity, examined with high-affinity [3H]forskolin binding, also appeared in patches similar to D1 receptors at 6.5 months. In contrast, protein kinase C activity, labeled with [3H]phorbol ester, was very apparent in the grafts at both time points. Higher and generally homogenous densities of muscarinic cholinergic receptors, assessed with [3H]QNB binding, develop in the grafts, but there appear to be few functioning cholinergic terminals, as measured by [3H]hemicholinium binding.(ABSTRACT TRUNCATED AT 250 WORDS)     

Long-term striatal overexpression of GDNF selectively downregulates tyrosine hydroxylase in the intact nigrostriatal dopamine system

Sustained neurotrophic factor treatment in neurodegenerative disorders such as Parkinson's disease is likely to affect both degenerating and intact neurons. To investigate the effect of long-term glial cell line-derived neurotrophic factor (GDNF) overexpression on intact nigrostriatal dopamine neurons, we injected a recombinant lentiviral vector encoding GDNF, or green fluorescent protein, in the right striatum of young adult rats. Thirteen months after viral injection GDNF levels were 4.5 ng/mg tissue in the striatum and 0.9 ng/mg in the substantia nigra as measured by ELISA, representing a 25-100-fold increase above control vector- or nontransduced tissue. GDNF overexpression significantly reduced tyrosine hydroxylase mRNA levels (by 39-72%) in the substantia nigra and ventral tegmental area neurons, and the optical density of tyrosine hydroxylase-immunoreactive innervation in the striatum was reduced by 25-52% with the most prominent reductions appearing caudally. No significant reduction was seen in striatal vesicular monoamine transporter 2-immunoreactivity or [3H]mazindole binding autoradiography to dopamine uptake sites, two other presynaptic markers in dopamine axon terminals. The striatal D1 and D2 receptor binding as determined by [3H]SCH23390 and [3H]spiperone binding, respectively, was unaltered relative to the intact side in both treatment groups. Preproenkephalin mRNA levels in postsynaptic striatal neurons, which increase upon removal of striatal dopamine, were also unaffected by the GDNF treatment. Taken together our findings indicate that sustained GDNF administration to intact nigrostriatal dopamine neurons selectively reduces tyrosine hydroxylase expression, without altering striatal dopamine transmission to the extent that compensatory changes in several other components related to dopamine storage and signalling occur.     

Differential response of striatal dopamine and muscarinic cholinergic receptor subtypes to the loss of dopamine: I. Effects of intranigral or intracerebroventricular 6-hydroxydopamine lesions of the mesostriatal dopamine system

Quantitative autoradiography was utilized to examine the response of the dopamine (DA) and muscarinic cholinergic system within the striatum to lesions of the mesostriatal DA system following intranigral 6-hydroxydopamine (6-OHDA) injections. In addition, the response of DA system was examined in the striatum of animals treated with low, medium, or high doses of 6-OHDA made intracerebroventricularly (icv). Three weeks following removal of the mesostriatal DA fibers with intranigral 6-OHDA, there was an almost complete depletion of DA and [3H]mazindol binding throughout the striatum. The resulting increase in D2 receptors labeled with [3H]spiroperidol (27%) was most evident in the lateral striatum and topographically correlated with an increase in choline uptake sites labeled with [3H]hemicholinium-3 (20%). There was a smaller but significant decrease in D1 receptors labeled with [3H]SCH 23390 (15-18%) that was not topographically related to changes in [3H]spiroperidol or [3H]hemicholinium-3 binding. All doses of icv 6-OHDA produced a significant loss of DA and of [3H]mazindol binding as compared to vehicle injections that was more pronounced in the medial than in the lateral striatum. No increase in D1 receptors was observed with any dose of 6-OHDA and greater than 90% loss of DA and [3H]mazindol resulted in an increase in D2 receptors in the lateral striatum and a reduction in D1 receptors in the dorsal striatum. These data are consistent with the evidence that there is independent regulation of the two subtypes of the DA receptor. Moreover, the distribution and regulation of the subtypes of the muscarinic receptor were independent. Muscarinic M2 receptors ([3H]N-methylscopolamine in presence of excess pirenzepine) showed a lateral to medial gradient (highest laterally) that was related to the pattern of choline uptake sites and D2 receptors. Loss of DA resulted in a reduction in M2 receptors (24-30%) that was correlated with the increase in choline uptake sites. In contrast, M1 ([3H]pirenzepine) receptors showed a reverse gradient from the M2 receptor and a smaller reduction following loss of DA.     

Neurotensin receptors and dopamine transporters: effects of MPTP lesioning and chronic dopaminergic treatments in monkeys.

The effect of denervation with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) of the dopamine (DA) nigrostriatal pathway on neurotensin (NT) receptor and DA transporter (DAT) in basal ganglia of monkeys (Macaca fascicularis) was investigated. The MPTP lesion induced a marked depletion of DA (90% or more vs. control) in the caudate nucleus and putamen. The densities of NT agonist binding sites labeled with [125I]NT and the NT antagonist binding sites labeled with [3H]SR142948A decreased by half in the caudate-putamen of MPTP-monkeys. In addition, the densities of [125I]NT and [3H]SR142948A binding sites markedly decreased (-77 and -63%, respectively) in the substantia nigra of MPTP-monkeys. Levocabastine did not compete with high affinity for [125I]NT binding in the monkey cingulate cortex, suggesting that only one class of NT receptors was labelled in the monkey brain. An extensive decrease of [3H]GBR12935 DAT binding sites (-92% vs. Control) was observed in the striatum of MPTP-monkeys and an important loss of DAT mRNA(-86% vs. Control) was observed in substantia nigra. Treatments for 1 month with either the D1 agonist SKF-82958 (3 mg/kg/day) or the D2 agonist cabergoline (0.25 mg/kg/day) had no effect on the lesion-induced decrease in NT and DAT binding sites or DAT mRNA levels. The decrease of striatal NT binding sites was less than expected from the decrease of DA content in this nucleus, suggesting only partial localization of NT receptors on nigrostriatal DAergic projections. These data also suggest that under severe DA denervation, treatment with D1 or D2 DA agonists does not modulate NT receptors and DAT density.     

Functional recovery of supersensitive dopamine receptors after intrastriatal grafts of fetal substantia nigra

Interruption of the ascending dopamine neurons of the nigrostriatal pathway, by 6-hydroxydopamine (6-OHDA) lesion in rats, produced a significant loss of the dopamine transport complexes labeled with the phencyclidine derivative [3H]BTCP. This loss of dopamine innervation in the striatum was present at least 12 to 14 months after lesioning and was functionally manifested by ipsilateral rotation of the animals in response to amphetamine. In these same animals, in comparison to controls, there was a significant increase in the number (Bmax) of [3H]SCH 23390-labeled D-1 receptors in the striatum (36.7%) and the substantia nigra (35.1%) and a 54.4% increase in the number (Bmax) of [3H]sulpiride-labeled striatal D-2 receptors without an apparent change in affinity (Kd). Ten to twelve months after the transplantation of homologous fetal substantia nigra into the denervated striatum, there was a significant decrease in amphetamine-induced turning behavior. In these animals, there was an ingrowth of dopamine nerve terminals in the striatum as demonstrated by a return of [3H]BTCP binding. Accompanying this reinnervation was the normalization of D-1 and D-2 receptors to control values in the striatum as well as the return of D-1 receptors to prelesion densities in the substantia nigra. In a subgroup of transplanted rats, amphetamine continued to induce ipsilateral turning. In these animals both D-1 and D-2 receptors remained supersensitive. These results support the hypothesis that the functional recovery of transplanted animals is due, in part, to reinnervation of the striatum. In addition, long-term alterations in receptor density may be related to the behavioral deficits that are associated with the 6-OHDA-lesioned rat. Furthermore, dopamine receptor plasticity may play a role in the functional recovery of substantia nigra transplanted animals and graft viability seems to be a prerequisite for behavioral recovery as well as receptor normalization.