Timecourse of striatal re-innervation following lesions of dopaminergic SNpc neurons of the rat

Previously we described the extent of sprouting that axons of the rat substantia nigra pars compacta (SNpc) undergo to grow new synapses and re-innervate the dorsal striatum 16 weeks after partial lesions. Here we provide insights into the timing of events related to the re-innervation of the dorsal striatum by regenerating dopaminergic nigrostriatal axons over a 104-week period after partial SNpc lesioning. Density of dopamine transporter and tyrosine hydroxylase immunoreactive axonal varicosities (terminals) decreased up to 80% 4 weeks after lesioning but returned to normal by 16 weeks, unless SNpc lesions were greater than 75%. Neuronal tracer injections into the SNpc revealed a 119% increase in axon fibres (4 mm rostral to the SNpc) along the medial forebrain bundle 4 weeks after lesioning. SNpc cells underwent phenotypic changes. Four weeks after lesioning the proportion of SNpc neurons that expressed tyrosine hydroxylase fell from 90% to 38% but returned to 78% by 32 weeks. We discuss these phenotype changes in the context of neurogenesis. Significant reductions in dopamine levels in rats with medium (30-75%) lesions returned to normal by 16 weeks whereas recovery was not observed if lesions were larger than 75%. Finally, rotational behaviour of animals in response to amphetamine was examined. The clear rightward turning bias observed after 2 weeks recovered by 16 weeks in animals with medium (30-75%) lesions but was still present when lesions were larger. These studies provide insights into the processes that regulate sprouting responses in the central nervous system following injury.     

D2 Dopamine receptor blockade results in sprouting of DA axons in the intact animal but prevents sprouting following nigral lesions

Recently it was demonstrated that sprouting of dopaminergic neurons and a microglial and astrocyte response follows both partial lesions of the substantia nigra pars compacta and blockade of the D2 dopamine receptor. We therefore studied the effects of the combination of these two treatments (lesioning and D2 dopamine receptor blockade). Haloperidol administration caused a 57% increase in dopaminergic terminal tree size (measured as terminal density per substantia nigra pars compacta neuron) and an increase of glia in the striatum. Following small to medium nigral lesions (less than 60%), terminal tree size increased by 51% on average and returned density of dopaminergic terminals to normal. In contrast, administration of haloperidol for 16 weeks following lesioning resulted in reduced dopaminergic terminal density and terminal tree size (13%), consistent with absent or impaired sprouting. Glial cell numbers increased but were less than with lesions alone. When haloperidol was administered after the striatum had been reinnervated through sprouting (16-32 weeks after lesioning), terminal tree size increased up to 150%, similar to the effect of haloperidol in normal animals. By examining the effect of administering haloperidol at varying times following a lesion, we concluded that a switch in the effect of D2 dopamine receptor blockade occurred after dopaminergic synapses began to form in the striatum. We postulate that when synapses are present, D2 dopamine receptor blockade results in increased terminal density, whereas prior to synapse formation D2 dopamine receptor blockade causes attenuation of a sprouting response. We speculate that D2 dopamine receptors located on growth cones 'push' neurites toward their targets, and blockade of these receptors could lead to attenuation of sprouting.     

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.     

Alteration of calmodulin distribution does not accompany dopaminergic supersensitization of the mouse striatum

Membrane-bound calmodulin increases following dopaminergic supersensitization of the rat striatum. To assess the generality of this relationship, mice were treated with two different supersensitization paradigms. Calmodulin levels and subcellular distribution were determined by radioimmunoassay. Chronic haloperidol treatment increased striatal D2 dopamine receptor density by 25% but had no effect on membrane-bound calmodulin levels. Similarly, 6-hydroxy-dopamine (6-OHDA) lesions depleted striatal dopamine content greater than 95% without affecting membrane-bound calmodulin. In contrast, soluble calmodulin levels decreased by 15% in the 6-OHDA-lesioned striatum, suggesting that soluble calmodulin is enriched in presynaptic dopaminergic terminals. We conclude that dopaminergic supersensitization can occur in the mouse striatum in the absence of any change in calmodulin distribution.     

Regulation of dopamine receptor and neuropeptide expression in the basal ganglia of monkeys treated with MPTP

In Parkinson's disease (PD), striatal dopamine deficiency has been associated with complex changes in the functional and neurochemical anatomy of the basal ganglia. In this study, we simultaneously analyzed the regulation of D1 and D2 dopamine receptors and levels of the neuropeptides, substance P, and enkephalin (ENK) in various basal ganglia nuclei following 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced dopaminergic denervation of striatum in nonhuman primates. Both unilateral and bilateral lesioned animals were used for this study. Striatal dopamine deficiency resulted in distinct alterations in D1, D2, substance P, and enkephalin levels and distribution: (1) Both D1 and D2 protein levels were significantly up-regulated in striatum. (2) There was an overall up-regulation of striatal substance P expression following dopamine denervation. (3) Substance P distribution was 'reversed' in dopamine depleted striatum: striosomes, which normally express higher levels of substance P, showed decreased expression, whereas substance P expression was up-regulated in the matrix. (4) Substance P expression was up-regulated in the internal segment of the globus pallidus (GPi), but remained unchanged in substantia nigra (SN). (5) Enkephalin levels were increased in striatum and the external segment of the globus pallidus (GPe), but not in substantia nigra. All the changes were more pronounced in the bilateral lesioned monkeys, though the data represent a pooled statistical evaluation of unilateral and bilateral lesioned monkeys. Our studies indicate that D1 and D2 dopamine receptors and substance P and enkephalin undergo regulatory changes in response to nigrostriatal dopamine denervation. Simultaneous study of the alterations in these various components of the 'direct' and 'indirect' pathways in the same animals will enable better understanding of the pathophysiology of PD and its therapeutic targets.     

Estrogen down-regulates glial activation in male mice following 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine intoxication

Emerging evidence suggests beneficial effect of estrogen for Parkinson's disease (PD), yet the exact mechanisms implicated remain obscured. Activated glia observed in MPTP mouse model and in PD may participate in the cascade of deleterious events that ultimately leads to dopaminergic nigral neuronal death. In vitro studies demonstrate that estrogen can modify the microglial and astroglial expression of inflammatory mediator, such as cytokines and chemokines implicated in neuroinflammation and neurodegeneration. To determine whether estrogen-elicited neuroprotection in PD is mediated through glia, adult male C57Bl/6 mice were treated with 17beta-estradiol (E2) for a total of 11 days. Following 5 days of pretreatment with E2, they were injected with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on the sixth day. The brains were collected on day 11. Immunohistochemistry and quantitative study were used to assess the number of tyrosine hydroxylase-immunoreactive (TH-IR) neurons in the substantia nigra pars compacta (SNpc) and that of activated astrocytes and activated microglia in the SNpc and the striatum. Pretreatment with E2 decreased the loss of TH-IR nigral neurons and diminished the deficit of TH-IR striatal fibers triggered by MPTP. The neuroprotective effect of E2 was coincident with an attenuation of a glial response within the nigra and the striatum. These findings suggest that the neuroprotective effects of E2 evidenced in MPTP mouse model might mediate through an inhibition of reactive glia. However, direct neuroprotective effects of E2 upon TH-IR neurons cannot be excluded.     

The problem of antipsychotic treatment for functional imaging in Huntington's disease: receptor binding, gene expression and locomotor activity after sub-chronic administration and wash-out of haloperidol in the rat

It has been demonstrated that withdrawal from chronic treatment with haloperidol is associated with a long-lasting increase in the number of striatal dopamine D(2) receptors and variable changes in D(1) receptors. We have investigated the effect of withdrawal from sub-chronic administration of haloperidol on the density of dopamine receptors, dopamine receptor gene expression, and spontaneous locomotor activity. Following a 3-week treatment period with haloperidol (1.5 mg/kg, i.p.), spontaneous locomotor activity measurements, autoradiography of D(1) and D(2) receptors and in situ hybridisation histochemistry of D(1) and D(2) mRNA were performed. Using [3H]raclopride as the ligand, sub-chronic haloperidol administration produced a robust upregulation in D(2) binding in the striatum of rats which correlated with parallel increases in spontaneous locomotor activity from 24 h to 4 weeks. Using, [3H]SCH23390 as the ligand, D(1) binding was largely unaffected by the drug treatment. Non-significant changes were measured in the striatal expression of D(1) receptor mRNA or the nigral or striatal expression of D(2) receptor mRNA. Our findings have implications for the use of dopaminergic ligands in positron emission tomography (PET) imaging of patients under regimens of chronic neuroleptics in particular in the context of forthcoming trials of neural grafts in Huntington's disease (HD) striatum.     

Topographical distributions of 32K and 48K cAMP-regulated phosphoproteins: Relationships to dopamine and serotonin innervations in striatum, substantia nigra and cerebral cortex

The topographical distribution of a cAMP-regulated phosphoprotein of an apparent molecular weight (M_r) of 32,000 (32K) was examined in homogenates prepared from microdiscs punched out from serial frozen slices of the striatum. The amount of this phosphoprotein progressively diminished from the rostral to the caudal part of the striatum as did both the dopamine-innervation and the dopamine (D₁)-sensitive adenylate cyclase. After kainic acid lesion of the rostral part of the striatum, the 32K phosphoprotein disappeared in this area and we observed a 48% decrease in the amount of 32K phosphoprotein found in the substantia nigra. 6-Hydroxydopamine lesions of the nigro-striatal dopaminergic pathway did not affect the 32K phosphoprotein either in striatum or substantia nigra. These results suggest that in the nigro-striatal pathway, the 32K phosphoprotein is closely associated with dopaminoceptive neurons containing D₁ receptors. In the cerebral cortex the association of 32K phosphoprotein with dopaminoceptive neurons is more questionable since we did not find a higher density of this phosphoprotein in areas containing a high amount of D₁ receptor (frontal cerebral cortex) than in areas containing a low amount of D₁ receptor (parietal cerebral cortex). In the course of this study we found another cAMP-regulated phosphoprotein of an M_r of 48,000 (48K). The amount of this phosphoprotein increased progressively from the rostral to the caudal part of the striatum, a pattern of distribution close to that of serotonin terminals. This protein was also present in the substantia nigra. Kainic acid lesioning of the rostral part of the striatum did not affect the amount of the 48K phosphoprotein within the substantia nigra. 6-Hydroxydopamine lesions of the substantia nigra also did not alter the amount of 48K phosphoprotein in this brain region.     

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.     

D1 and D2 receptor modulation in rat striatum and nucleus accumbens after subchronic and chronic haloperidol treatment

The antipsychotic effects of neuroleptic drugs are believed to be achieved by chronic blockade of dopaminergic transmission in the limbic system. Nevertheless, the effects of chronic (3-12 months) haloperidol administration on the dopaminergic transmission in the nucleus accumbens of rodents remains poorly understood. Studies of spontaneous locomotor activity (SLA), a behavioral measure related to limbic dopamine transmission, and of dopamine D2 receptor density in the nucleus accumbens after chronic oral haloperidol treatment have yielded conflicting results. We evaluated these indices after 8 months of parenteral administration of haloperidol decanoate. We report here that, after 8 months of parenteral treatment, SLA stays significantly decreased and D2 receptors in the nucleus accumbens exhibit the same up-regulation as in the striatum (about 50%). These results fail to support the notion of a different pattern of D2 receptor adaptation to neuroleptic treatment between the nucleus accumbens and the striatum. In contrast, dopamine D1 receptors were found to be unaffected in the nucleus accumbens but decreased in the striatum by 22% after 8 months of treatment. This observation could be relevant to the pathogenesis of tardive dyskinesia.     

Synergistic interaction between an adenosine antagonist and a D1 dopamine agonist on rotational behavior and striatal c-Fos induction in 6-hydroxydopamine-lesioned rats

The interaction between adenosine and D₁ dopamine systems in regulating motor behavior and striatal c-Fos expression was examined in rats with unilateral 6-hydroxydopamine (6-OHDA) lesions. These results were compared to the synergistic interaction between D₁ and D₂ dopamine systems in 6-OHDA rats. Coadministration of the adenosine antagonist 3,7-dimethyl-1-propargylxanthine (DMPX: 10 mg/kg) and the D₁ dopamine agonist SKF38393 (0.5 mg/kg) to 6-OHDA-lesioned rats produced significant contralateral rotation and c-Fos expression in the ipsilateral striatum compared to 6-OHDA rats treated with either drug alone. However, the regional pattern of striatal c-Fos activation following treatment of 6-OHDA rats with SKF38393 and DMPX was different from the dorsolateral pattern of striatal c-Fos induction observed after coadministration of D₁ and D₂ dopamine agonists (SKF38393: 0.5 mg/kg+quinpirole: 0.05 mg/kg). These data are consistent with a functional interaction between D₁ dopamine and adenosine systems in the striatum, but suggest that activation of different subsets of striatal neurons underlie the behavioral synergy observed following combined adenosine antagonist-D₁ dopamine agonist and combined D₁ dopamine agonist–D₂ dopamine agonist treatment.     

Striatal glutamatergic function: Modifications following specific lesions

The effects of specific lesions of the striatum: (a) hemidecortication; (b) striatal injection of(±) ibotenate; and (c) 6-hydroxydopamine injections into the substantia nigra, were investigated on specific [³H]glutamate binding to striatal membranes. One month after decortication, there was a substantial reduction of calcium-dependent, stimulated glutamate release from striatal slices, indicating effective loss of glutamatergic fibres. Striatal glutamate binding increased by approximately 30% and this supersensitivity could be attributed solely to an increased receptor density. Ibotenate lesions which destroy target neurones for the glutamatergic fibres (sparing terminals), reduced glutamate binding in the striatum, as did nigral 6-OHDA lesions which delete striatal dopaminergic terminals. This finding supports the concept of there being glutamate receptors on pre-synaptic dopamine terminals in the striatum, involved in regulation of dopamine release. 6-OHDA lesions also result in a supersensitivity of the dopamine receptors localized on the cortico-striatal afferent terminals, as evidenced by the enhanced ability of dopamine to inhibit the K⁺-evoked, calcium-dependent release of endogenous striatal glutamate.     

Dopaminergic inhibition of striatal GABA release after 6-hydroxydopamine

We have examined the regulation of striatal GABA release by endogenous dopamine in rats with partial degeneration of dopamine-containing neurons. 6-Hydroxydopamine was administered into the lateral ventricles or medial forebrain bundle. Either 3 days or 3 weeks later, slices of neostriatum were prepared, preloaded with [³H]GABA, and superfused in order to measure [³H]GABA overflow in response to electrical stimulation (8 Hz). The loss of dopaminergic terminals was estimated by measuring tissue levels of dopamine. The impact of endogenous dopamine on [³H]GABA was evaluated by measuring the ability of sulpiride, a D₂ dopamine receptor antagonist, to increase the depolarization-induced [³H]GABA overflow. In non-treated or vehicle-pretreated rat neostriatum, sulpiride (10 μM) increased the depolarization-induced [³H]GABA overflow to 193% of control. Three days after lesioning, the stimulatory effect of sulpiride on [³H]GABA overflow was identical to that seen in control rats so long as the loss of tissue dopamine did not exceed 60%, although with larger lesions the sulpiride-induced response was reduced. Three weeks after lesioning, however, the stimulatory effect of sulpiride on electrically evoked [³H]GABA overflow remained at the level seen in control tissue even in cases where tissue dopamine was reduced to 13% of normal. In contrast, no sulpiride-induced increase in [³H]GABA overflow was detected 3 weeks after nearly complete lesions which reduced tissue dopamine to 2% of normal. These data suggest that short- and long-term compensatory changes maintain dopaminergic control over GABAergic projection neurons and interneurons until the loss of dopamine innervation is almost complete.     

Manganese neurotoxicity: Effects ofl-DOPA and pargyline treatments

Single, monolateral injection into rat substantia nigra of manganese chloride produced within two weeks from its administration a loss of dopamine in the striatum ipsilateral to the injected side. The effect was dose-dependent and was not extended to serotoninergic terminals present in this brain area, whose content in serotonin and 5-hydroxyindoleacetic acid was not affected. Whenl-DOPA + carbidopa or pargyline were given to these animals the decrease of striatal dopamine was more marked. Moreover, rats treated two weeks before with a dose of manganese chloride that produced a 70–80% drop in striatal dopamine concentrations, rotated ipsilaterally to the dopamine-depleted striatum when injected with apomorphine, suggesting that in these animals the stimulatory effects of apomorphine were more relevant in striatum where presynaptic dopaminergic neurons were not affected by manganese chloride. These data indicate that the alterations of dopaminergic postsynaptic receptors may be different in parkinsonian and in manganese-intoxicated patients and that current therapy used for Parkinson's disease could be a hazard in treating manganese poisoning.     

Genetic control of dopamine and serotonin receptors in brain regions of inbred mice

In this report the genetic determinants of dopamine and serotonin receptors are investigated. We have used two types of radioreceptor binding assays to identify and quantify these neurotransmitter receptors in various brain regions of inbred mice. In the first method dopamine and serotonin sites are quantified using [3H]spiperone in the presence of appropriate blanking agents. These results are compared with those obtained by the use of [3H]domperidone and [3H]mianserin to label D2 and S2 sites, respectively. Both methods yield nearly identical results. Strain differences in D2 sites are found in the striatum, olfactory tubercle and pituitary. The density of dopaminergic sites is uncorrelated in the 3 brain regions in all mouse strains studied, suggesting that genetic determination of receptor density is independently regulated in each region. Similar observations have been made for S2 receptors in the striatum, hypothalamus, olfactory tubercle and frontal cortex. Analysis of D3 and D2 binding sites in recombinant inbred lines suggests that each site may be determined monogenically.     

The 6‐hydroxydopamine‐induced nigrostriatal neurodegeneration produces microglia‐like NG2 glial cells in the rat substantia nigra

Neuron/glial 2 (NG2)‐expressing cells are often referred to as oligodendrocyte precursor cells. NG2‐expressing cells have also been identified as multipotent progenitor cells. However, microglia‐like NG2 glial cells have not been fully examined in neurodegenerative disorders such as Parkinson's disease (PD). In the present study, we chose two rat models of PD, i.e., intranigral or intrastriatal injection of 6‐hydroxydopamine (6‐OHDA), since the cell bodies of dopamine (DA) neurons, which form a nigrostriatal pathway, are in the substantia nigra pars compacta (SNpc) while their nerve terminals are in the striatum. In the nigral 6‐OHDA‐injected model, activated NG2‐positive cells were detected in the SNpc but not in the striatum. In contrast, in the striatal 6‐OHDA‐injected model, these cells were detected in both the SNpc and the striatum. In both models, activated NG2‐positive cells were located close to surviving tyrosine hydroxylase (TH)‐positive neurons in the SNpc. In addition, activated NG2‐positive cells in the SNpc coexpressed ionized calcium‐binding adaptor molecule 1 (Iba1), a microglia/macrophage marker. Interestingly, these double‐positive glial cells coexpressed glial cell line‐derived neurotrophic factor (GDNF). These results suggest that microglia‐like NG2 glial cells may help protect DA neurons and may lead to new therapeutic targets in PD. © 2010 Wiley‐Liss, Inc.     

Denervation accelerates the reappearance of neostriatal D-2 receptors after irreversible receptor blockade

The effect of denervation on the turnover of striatal dopaminergic D-2 receptors was examined by determining the rate of receptor reappearance in vivo after administration of the irreversible receptor antagonist, N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) to rats that received prior unilateral intracerebral injection of 6-hydroxydopamine (6-OHDA). Initial experiments confirmed that EEDQ (10 mg/kg i.p.) induces a severe, prolonged blockade of D-2 receptors. Recovery of [3H]spiroperidol binding occurred at a rate of approximately 9% of control binding per day. 6-OHDA injection into the ascending dopaminergic projection 3 or 5 days prior to EEDQ administration revealed that denervation had no effect on the rate of D-2 receptor recovery during the first post-operative week. By 4-5 weeks postoperatively, however, denervation enhanced the rate of recovery of [3H]spiroperidol binding. These results are consistent with our finding that, when homogenized tissue preparations are used, steady-state receptor density does not change within the first postoperative week but increases by 3-4 weeks after the injury. Saturation analysis determined that both EEDQ and denervation altered the density of binding sites, whereas neither treatment significantly affected the affinity of the receptor for [3H]spiroperidol. By 4-5 weeks postoperatively, the receptor degradation rate constant in the denervated striatum (0.0054/h) was equal to that in the intact striatum (0.0052/h). Thus, only the receptor reappearance rate was elevated in the denervated striatum (3.8 fmol/mg protein/h) relative to the intact striatum (2.8 fmol/mg protein/h).     

Relationship between sensorimotor gating deficits and dopaminergic neuroanatomy in Nurr1-deficient mice

Nurr1 (NR4A2) is an orphan nuclear receptor highly essential for the development and maintenance of dopaminergic neurons. Reduced expression of Nurr1 has been linked to the etiopathogenesis of Parkinson's disease and other dopamine-related disorders such as schizophrenia. Recent experimental work in mice with a heterozygous constitutive deletion of Nurr1 has revealed that this genetic manipulation leads to the presence of sensorimotor gating dysfunctions in the form of reduced prepulse inhibition of the acoustic startle reflex. However, the neuronal substances for this behavioral manifestation remain essentially unknown. Since converging evidence supports a key role of the central dopamine system in the regulation of prepulse inhibition, we hypothesized that the emergence of prepulse inhibition deficits in adult Nurr1-deficient mice may be linked to dopaminergic neuroanatomical changes. To test this hypothesis, we followed a within-subject approach in which sensorimotor gating performance was correlated with post-mortem expression of several dopaminergic markers in relevant striatal and midbrain regions. We found that prepulse inhibition deficits in Nurr1-deficient mice were paralleled by reduced numbers of substantia nigra dopamine cells expressing tyrosine hydroxylase, and by decreased tyrosine hydroxylase and dopamine transporter immunoreactivity in ventral parts of the striatum. Most interestingly, we also revealed a striking negative correlation between prepulse inhibition levels and tyrosine hydroxylase immunoreactivity in Nurr1-deficient mice in dorsal striatal regions (caudate putamen) and ventral striatal regions (nucleus accumbens core and shell). Our findings thus suggest that the emergence of prepulse inhibition deficits induced by heterozygous constitutive deletion of Nurr1 is, at least in part, related to alterations in presynaptic components of the striatal dopamine system. The constellation of neuroanatomical and behavioral alterations in Nurr1-deficient mice observed here confirms previous impressions that the consequences of Nurr1 down-regulation capture neuronal and behavioral pathologies relevant especially for (but not limited to) Parkinson's disease.     

Localization of striatal and nigral tachykinin receptors in the rat

The effects of lesioning mesostriatal dopamine projections or striatal neurons on tachykinin binding in the basal ganglia were assessed in the rat. 6-Hydroxydopamine lesions of the medial forebrain bundle destroyed striatal dopamine terminals as assessed by [³H]mazindol autoradiography, but did not significantly affect the binding of NK-1 ([³H][Sar⁹, Met(O₂)¹¹]substance P) or NK-3 ([³H]senktide) tachykinin ligands in the striatum. 6-Hydroxydopamine lesions significantly reduced NK-3 binding in the substantia nigra pars compacta, but not the ventral tegmental area. In contrast, striatal quinolinic acid lesions reduced both NK-1 and NK-3 binding in the striatum, but failed to affect NK-3 binding in the substantia nigra. These findings suggest that both NK-1 and NK-3 receptors within the striatum are predominantly post-synaptic with respect to dopamine neurons, whereas nigral NK-3 receptors are located on dopaminergic neurons.     

Differential vulnerability of dopamine receptors in the mouse brain treated with MPTP

We investigated the chronological changes of dopamine D1 and D2 receptors and dopamine uptake sites in the striatum and substantia nigra of mouse brain treated with 1-methyl-4-phenyl-1,2,3, 6-tetrahydropyridine (MPTP) by quantitative autoradiography using [3H]SCH23390, [3H]raclopride and [3H]mazindol, respectively. The mice received i.p. injections of MPTP (10 mg/kg) four times at intervals of 60 min, the brains were analyzed at 6 h and 1, 3, 7 and 21 days after the last the injection. Dopamine D2 receptor binding activity was significantly decreased in the substantia nigra from 7 to 21 days after MPTP administration, whereas such binding activity was significantly increased in the medial part of the striatum at 21 days. There was no alteration of dopamine D1 receptor binding activity in either the striatum or the substantia nigra for the 21 days. The number of dopamine uptake sites gradually decreased in the striatum and the substantia nigra, starting at 6 h after MPTP administration, and the lowest levels of binding activity were observed at 3 and 7 days in the striatum (18% of the control values in the medial part and 30% in the lateral part) and at 1 day in the substantia nigra (20% of the control values). These results indicate that severe functional damage to the dopamine uptake sites occurs in the striatum and the substantia nigra, starting at an early stage after MPTP treatment. Our findings also demonstrate the compensatory up-regulation in dopamine D2 receptors, but not dopamine D1 receptors, in the striatum after MPTP treatment. Furthermore, our results support the existence of dopamine D2 receptors, but not dopamine D1 receptors, on the nigral neurons. The present findings suggest that there are differential vulnerabilities to MPTP toxicity in the nigrostriatal dopaminergic receptor systems of mouse brain.