Striatonigral Pathway Research Articles

Differential characteristics of HIV-based versus SIV-based lentiviral vector systems: Gene delivery to neurons and axonal transport of expressed gene

The differential characteristics of lentiviral vectors based on human and simian immunodeficiency viruses (HIV and SIV) were investigated in rats and monkeys. Each vector was injected into the striatum, and the expression patterns of the marker gene green fluorescent protein (GFP) were analyzed in the basal ganglia. With respect to the capability of gene delivery to neural cells, the HIV-based vector exhibited a higher tropism to neurons than to astroglias in the striatum, and vice versa for the SIV-based vector. The preferential direction of axonal transport of striatally expressed GFP was also examined in the present study. The HIV-based vector allowed for both anterograde transport via the striatopallidal and striatonigral pathways and retrograde transport via the nigrostriatal pathway. The GFP labeling of axon terminals through anterograde transport was apparent regardless of the animal species, while that of neuronal cell bodies through retrograde transport was much more prominent in monkeys than in rats. As for the SIV-based vector, on the other hand, evidence for anterograde transport was obtained much more markedly in monkeys than in rats, and only weak or no retrograde transport occurred in either monkeys or rats. Our results indicate that HIV-based, but not SIV-based, lentiviral vectors possess the high tropism to neurons and permit retrograde transport of an expressed gene, especially in primates. The latter property might carry a potential benefit in gene therapy for Parkinson's disease, as stereotaxic injections of the vectors could be performed into the striatum, spatially larger than the substantia nigra, with greater certainty.

Receptor–receptor interactions as studied with microdialysis. Focus on NTR/D2 interactions in the basal ganglia

Using mono and dualprobe(s) microdialysis in the basal ganglia of the freely moving rat evidence has been obtained that neurotensin (NT) in threshold concentrations can counteract the D(2) agonist (intrastriatally perfused) induced inhibition of striatal dopamine (DA) release and of pallidal GABA release from the striato-pallidal GABA pathway, effects that are blocked by a NTR(1) antagonist SR48692. These results indicate the existence of antagonistic intramembrane NTR/D(2) receptor interactions in the striatal DA terminals and in the somato-dendritic regions of the striato-pallidal GABA neurons. By the NT-induced reduction of the D(2) mediated signals at the striatal pre- and postjunctional level DA transmission is switched towards a D(1) mediated transmission leading to increased activity in the striatopallidal and striatonigral GABA pathways. The former action will contribute to the motor inhibition and catalepsy found with NT treatment and underlies the use of NT receptor antagonists as a treatment strategy for Parkinson's disease. Nigral NT by an antagonistic NTR/D(2) receptor interaction in the DA cell body and dendrites may also increase nigral DA release leading to a D(2) mediated inhibition of the nigrothalamic GABA pathway. Such an effect, will instead result in antiparkinsonian actions. Thus, increases in NT transmission will have different consequences for the motor system depending upon where in the basal ganglia the increase takes place.

Prenatal cadmium and ethanol increase amphetamine-evoked dopamine release in rat striatum

To explore interactive deleterious effects of the teratogens ethanol and cadmium, pregnant rats were given cadmium (CdCl 2, 50 ppm) and/or ethanol (10%), or tap water (controls) in the drinking water for the entire 21 days of pregnancy. At 3 months after birth, in vivo microdialysis was used to determine that there was a 4000% evoked release of DA by AMPH (AMPH, 4.0 mg/kg i.p.) in the striatum of rats exposed prenatally to both ethanol and cadmium, vs. a 2000% evoked release by AMPH in rats exposed prenatally to only ethanol or cadmium or tap water. Haloperidol (HAL)-evoked DA release was suppressed in groups exposed prenatally to ethanol, while HAL-evoked DOPAC and HVA release was greatest after co-exposure to prenatal cadmium and ethanol. These in vivo microdialysis results indicate that ontogenetic co-exposure to cadmium, and ethanol produces a long-lived suppressive effect on HAL-evoked DA release and a long-lived enhancing effect on AMPH-evoked DA release in rat striatum. These findings clearly demonstrate that there is marked alteration in dopaminergic regulation after ontogenetic cadmium and ethanol co-exposure, which in this regard resembles the reaction of the striatonigral pathway on AMPH-evoked DA release in rats with behavioral sensitization.

Failure of MK-801 to suppress D1 receptor-mediated induction of locomotor activity and striatal preprotachykinin mRNA expression in the dopamine-depleted rat

N-methyl- d-aspartate receptor antagonism exerts suppressive influences over dopamine D1 receptor-mediated striatal gene expression and locomotor behavior in the intact rat. The present study examined the effects of the N-methyl- d-aspartate receptor antagonist MK-801 on locomotor activity and striatal preprotachykinin mRNA expression stimulated by the D1 agonist (±)6-chloro-7, 8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1 H-3-benzazepine hydrobromide in rats with bilateral dopamine lesions. Two months after neonatal dopamine lesions with 6-hydroxydopamine, rats were challenged with (±)6-chloro-7, 8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1 H-3-benzazepine hydrobromide (1.0mg/kg) 15 min after administration of the N-methyl- d-aspartate receptor antagonist MK-801 (0.1mg/kg). In the intact rat, MK-801 prevented the induction of striatal preprotachykinin mRNA by D1 agonism. Similarly, direct infusion of (±)6-chloro-7, 8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1 H-3-benzazepine hydrobromide (3.0μg) into the intact striatum produced an increase in locomotor activity that was suppressed by MK-801 (1.0μg) co-infusion. In the dopamine-depleted rat, MK-801 (0.1mg/kg) administered prior to (±)6-chloro-7, 8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1 H-3-benzazepine hydrobromide (1.0mg/kg) increased, rather than suppressed, striatal preprotachykinin mRNA levels. Intrastriatal infusion of MK-801 (1.0μg) failed to inhibit D1-mediated induction of motor activity in dopamine-depleted animals. Together, these data provide further support that N-methyl- d-aspartate receptor antagonists lose their ability to block D1-mediated behavioral activation following dopamine depletion. The activation, rather than suppression, of tachykinin neurons of the direct striatonigral pathway may play a facilitatory role in this mechanism.

Subcellular localization of type 1 cannabinoid receptors in the rat basal ganglia

Endocannabinoids, acting via type 1 cannabinoid receptors (CB1), are known to be involved in short-term synaptic plasticity via retrograde signaling. Strong depolarization of the postsynaptic neurons is followed by the endocannabinoid-mediated activation of presynaptic CB1 receptors, which suppresses GABA and/or glutamate release. This phenomenon is termed depolarization-induced suppression of inhibition (DSI) or excitation (DSE), respectively.Although both phenomena have been reported to be present in the basal ganglia, the anatomical substrate for these actions has not been clearly identified. Here we investigate the high-resolution subcellular localization of CB1 receptors in the nucleus accumbens, striatum, globus pallidus and substantia nigra, as well as in the internal capsule, where the striato-nigral and pallido-nigral pathways are located.In all examined nuclei of the basal ganglia, we found that CB1 receptors were located on the membrane of axon terminals and preterminal axons. Electron microscopic examination revealed that the majority of these axon terminals were GABAergic, giving rise to mostly symmetrical synapses. Interestingly, preterminal axons showed far more intense staining for CB1, especially in the globus pallidus and substantia nigra, whereas their terminals were only faintly stained. Non-varicose, thin unmyelinated fibers in the internal capsule also showed strong CB1-labeling, and were embedded in bundles of myelinated CB1-negative axons. The majority of CB1 receptors labeled by immunogold particles were located in the axonal plasma membrane (92.3%), apparently capable of signaling cannabinoid actions. CB1 receptors in this location cannot directly modulate transmitter release, because the release sites are several hundred micrometers away. Interestingly, both the CB1 agonist, WIN55,212-2, as well as its antagonist, AM251, were able to block action potential generation, but via a CB1 independent mechanism, since the effects remained intact in CB1 knockout animals. Thus, our electrophysiological data suggest that these receptors are unable to influence action potential propagation, thus they may not be functional at these sites, but are likely being transported to the terminal fields.The present data are consistent with a role of endocannabinoids in the control of GABA, but not glutamate, release in the basal ganglia via presynaptic CB1 receptors, but also call the attention to possible non-CB1-mediated effects of widely used cannabinoid ligands on action potential generation.

Serotonin 5‐HT4 receptors and their mRNAs in rat and guinea pig brain: Distribution and effects of neurotoxic lesions

Serotonin 5-HT4 receptors are widely distributed in the periphery and in brain, where they modulate the release of various neurotransmitters and have been implicated in learning and memory. Nine C-terminal splice variants of this receptor have been cloned in mammalian species. In the rat, three such variants have been described: 5-HT4(a), 5-HT4(b), and 5-HT4(e). In the present study, we have examined several aspects of the distribution of these receptors in brain. First, we provide, in rat and guinea pig, a detailed comparison of the distribution of 5-HT4 receptors labeled by the antagonist [125I]-SB 207710 with the distribution of their encoding mRNA visualized by in situ hybridization histochemistry (ISHH). The results suggest that, in several projection systems (striato-nigral and striato-pallidal pathways, projection from dentate granule cells to field CA3, habenulo-interpeduncular pathway), 5-HT4 receptors are located both somatodendritically and axonally. Second, we have analyzed the distribution of mRNA for the three known rat splice variants by reverse transcription-polymerase chain reaction (RT-PCR) and by ISHH. RT-PCR indicates that all three variants are widely distributed, with 5-HT4(b) mRNA being present in all regions examined (olfactory tubercle, striatum, hippocampus, inferior colliculus, substantia nigra, parietal cortex) and 5-HT4(a) and 5-HT4(e) showing a somewhat more restricted distribution. In other regions (periaqueductal gray, reticular formation, medial septum, diagonal band), faint ISHH signals are observed for 5-HT4(a)+4(e) mRNAs, whereas 5-HT4(b) mRNA signals are almost undetectable. Finally, neurotoxic lesions of basal ganglia components in guinea pig also indicate a location of these receptors on terminals of striatal projection neurons.

Different responsiveness of striatonigral and striatopallidal neurons to L‐DOPA after a subchronic intermittent L‐DOPA treatment

Early gene induction by L-DOPA in the striatum of dopamine denervated rats represents a useful way to study long-term modifications produced by this drug. The effects of acute and subchronic L-DOPA administration on zif-268 mRNA expression were compared in 6-hydroxydopamine-lesioned rats. Rats received a subchronic intermittent L-DOPA (6 mg/kg) treatment, which produces behavioural sensitization, a correlate of dyskinetic movements. Three days after interruption of subchronic treatment, zif-268 mRNA was evaluated after an L-DOPA challenge. Zif-268 mRNA levels increased in the lesioned dorsolateral striatum after either acute or subchronic L-DOPA administration. Double labelling of striatal cells with zif-268 and enkephalin or dynorphin mRNA probes was performed to assess neuronal activation in the indirect and direct output pathway. Single acute L-DOPA significantly increased zif-268 in all striatal neurons reflecting a hyperresponsiveness of dopamine-depleted striatum. After subchronic L-DOPA, zif-268 mRNA labelling was still increased in the striatonigral pathway, limited to dynorphin(+) neurons, whereas in all other neurons it was similar to the control value. Results suggest that striatal neurons responding to acute L-DOPA differ from those responding to subchronic L-DOPA. L-DOPA-induced behavioural sensitization was associated to a down-regulation in the responsiveness of striatopallidal and striatonigral dynorphin(-) neurons, whereas in striatonigral neurons containing dynorphin a hyperresponsiveness to L-DOPA was observed. High levels of zif-268, together with a persistent hyperresponsiveness of striatonigral dymorphinergic neurons and hyporesponsiveness of striatopallidal neurons, by creating an unbalanced state of striatal efferent neurons, may be implicated in dyskinetic movements observed in Parkinson's disease (PD).

High-Dose Methamphetamine Acutely Activates the Striatonigral Pathway to Increase Striatal Glutamate and Mediate Long-Term Dopamine Toxicity

Methamphetamine (METH) has been shown to increase the extracellular concentrations of both dopamine (DA) and glutamate (GLU) in the striatum. Dopamine, glutamate, or their combined effects have been hypothesized to mediate striatal DA nerve terminal damage. Although it is known that METH releases DA via reverse transport, it is not known how METH increases the release of GLU. We hypothesized that METH increases GLU indirectly via activation of the basal ganglia output pathways. METH increased striatonigral GABAergic transmission, as evidenced by increased striatal GAD65 mRNA expression and extracellular GABA concentrations in substantia nigra pars reticulata (SNr). The METH-induced increase in nigral extracellular GABA concentrations was D1 receptor-dependent because intranigral perfusion of the D1 DA antagonist SCH23390 (10 microm) attenuated the METH-induced increase in GABA release in the SNr. Additionally, METH decreased extracellular GABA concentrations in the ventromedial thalamus (VM). Intranigral perfusion of the GABA-A receptor antagonist, bicuculline (10 microm), blocked the METH-induced decrease in extracellular GABA in the VM and the METH-induced increase in striatal GLU. Intranigral perfusion of either a DA D1 or GABA-A receptor antagonist during the systemic administrations of METH attenuated the striatal DA depletions when measured 1 week later. These results show that METH enhances D1-mediated striatonigral GABAergic transmission (1), which in turn activates GABA-A receptors in the SNr (2), leading to a decrease in GABAergic nigrothalamic activity (3), an increase in corticostriatal GLU release (4), and a consequent long-term depletion of striatal DA content (5).

Evidence for differential cortical input to direct pathway versus indirect pathway striatal projection neurons in rats.

The two main types of corticostriatal neurons are those that project only intratelencephalically (IT-type), the intrastriatal terminals of which are 0.41 microm in mean diameter, and those that send their main axon into pyramidal tract and have a collateral projection to striatum (PT-type), the intrastriatal terminals of which are 0.82 microm in mean diameter. We used three approaches to examine whether the two striatal projection neuron types (striatonigral direct pathway vs striatopallidal indirect pathway) differ in their input from IT-type and PT-type neurons. First, we retrogradely labeled one striatal projection neuron type or the other with biotinylated dextran amine (BDA)-3000 molecular weight. We found that terminals making asymmetric axospinous contact with striatonigral neurons were 0.43 microm in mean diameter, whereas those making asymmetric axospinous contact with striatopallidal neurons were 0.69 microm. Second, we preferentially immunolabeled striatonigral neurons for D1 dopamine receptors or striatopallidal neurons for D2 dopamine receptors and found that axospinous terminals had a smaller mean size (0.45 microm) on D1+ spines than on D2+ spines (0.61 microm). Finally, we combined selective BDA labeling of IT-type or PT-type terminals with immunolabeling for D1 or D2, and found that IT-type terminals were twice as common as PT-type on D1+ spines, whereas PT-type terminals were four times as common as IT-type on D2+ spines. These various results suggest that striatonigral neurons preferentially receive input from IT-type cortical neurons, whereas striatopallidal neurons receive greater input from PT-type cortical neurons. This differential cortical connectivity may further the roles of the direct and indirect pathways in promoting desired movements and suppressing unwanted movements, respectively.

Intranigral antagonism of neurokinin 1 and 3 receptors reduces intrastriatal dopamine D1 receptor-stimulated locomotion in the rat

Stimulation of striatal dopamine (DA) D1 receptors increases the activity of the direct striatonigral pathway resulting in movement. While GABA has long been considered the primary effector of this pathway, co-released tachykinin peptides and their respective nigral tachykinin receptors are also in position to influence movement. Therefore, the present studies determined to what extent nigral tachykinin receptor subtypes contribute to striatal D1-mediated locomotion. Adult male Sprague–Dawley rats bearing chronic cannulae in the dorsal striatum and/or substantia nigra (SN) were tested for locomotor responses to various drug infusions. Unilateral intranigral infusions of the neurokinin-1 (NK1) antagonist LY306740 (0 and 50 nmol) but not the neurokinin-3 (NK3) antagonist SR142801 (0 and 50 nmol) led to ipsilateral rotations. Bilateral intrastriatal infusions of the full D1 agonist SKF 82958 (0, 1.2 and 12.0 nmol) dose-dependently increased locomotion. Prior bilateral intranigral infusions of LY306740 or SR142801 (0, 5.0 and 50 nmol) dose-dependently attenuated locomotor activity induced by intrastriatal SKF 82958 (12.0 nmol). These findings indicate that NK1, but not NK3, receptors within the SN may be tonically stimulated. However, activation of both nigral NK1 and NK3 receptors appears to be required for increased locomotion in response to striatal D1 receptor stimulation.

Altered expression of preproenkephalin and prodynorphin mRNA in a genetic model of paroxysmal dystonia

The dt sz mutant hamster represents a model of primary paroxysmal dystonia, in which dystonic episodes occur in response to stress. Previous examinations demonstrated striatal dysfunctions in dt sz hamsters. In the present study, in situ hybridization was used to examine preproenkephalin and prodynorphin expression as potential indices of imbalances between the striatopallidal and striatonigral pathways. Brain analyses were performed in dt sz hamsters under basal conditions, i.e., in the absence of dystonia, as well as mutant hamsters that exhibited severe stress-induced dystonic attacks immediately prior to sacrifice. In the striatum the basal expression of prodynorphin tended to be higher, while that of preproenkephalin tended to be lower in mutant hamsters in comparison to non-dystonic control hamsters. Significant basal changes were restricted to higher levels of prodynorphin in the ventrolateral striatum and lower prodynorphin and preproenkephalin mRNA expression in the hippocampus and/or in subregions of the hypothalamus. After stressful stimulation, the neuropeptides increased in several regions in both animals groups. In comparison to stimulated control hamsters, a significantly lower prodynorphin expression was found in several limbic areas of stimulated mutant hamsters during the manifestation of dystonia, while preproenkephalin mRNA was significantly lower in the anterior and dorsal striatal subregions and in nucleus accumbens. Since changes in the expression of these opioid peptides have been suggested to be related to abnormal dopaminergic activity, the present findings may reflect disturbances in striatal dopaminergic systems, and also in limbic structures in the dt sz mutant, particularly during the expression of dystonia.

Selective modifications in GAD67 mRNA levels in striatonigral and striatopallidal pathways correlate to dopamine agonist priming in 6‐hydroxydopamine‐lesioned rats

The present study investigated long-term alterations in striatal gene expression after single exposure of unilaterally 6-hydroxydopamine-lesioned rats to different dopamine agonists (priming). Rats were primed with the D1 agonist SKF38393 (10 mg/kg), the D2/D3 agonist quinpirole (0.2 mg/kg), the dopamine precursor L-DOPA (50 mg/kg) or with vehicle (drug-naive), and GAD67, dynorphin and enkephalin mRNAs were evaluated in the striatum by in situ hybridization, 3 days after priming. To evaluate GAD67 mRNA in striatonigral and striatopallidal neurons, identified as enkephalin (-) and (+) neurons, double-labelling in situ hybridization was used. Drug-naive lesioned rats showed an increase in GAD67 mRNA in enkephalin (-) and (+) neurons, an increase in enkephalin and a decrease in dynorphin mRNAs. Priming with either SKF38393 or quinpirole further increased GAD67 mRNA in enkephalin (-) and (+) neurons, however, while SKF38393 produced a high and unbalanced activation toward enkephalin (-) neurons, after quinpirole the increase was of low intensity and similar in the two pathways. Dynorphin mRNA was increased by SKF38393 but not by quinpirole, whereas enkephalin mRNA was not changed by either priming. L-DOPA produced a high and similar increase in GAD67 mRNA in enkephalin (-) and (+) neurons. Priming differentially affected peptides and GAD67 mRNA in striatopallidal and striatonigral neurons depending on the dopamine agonist used. The degree of enduring overactivity of the striatopallidal and striatonigral pathways may be related to the ability of L-DOPA and D1 or D2/D3 receptor agonists to prime motor behavioural responses and to produce dyskinetic side-effects.

Immunohistochemical localization of PDE10A in the rat brain

PDE10A is a newly identified cAMP/cGMP phosphodiesterase for which mRNA is highly expressed in the mammalian striatum. In the present study, PDE10A protein and mRNA expression throughout the rat brain were determined, using a monoclonal antibody (24F3.F11) for Western blot and immunohistochemical analyses and an antisense riboprobe for in situ hybridization. High levels of mRNA are observed in most of the neuronal cell bodies of striatal complex (caudate n, n. accumbens and olfactory tubercle), indicating that PDE10A is expressed by the striatal medium spiny neurons. PDE10A-like immunoreactivity is dense throughout the striatal neuropil, as well as in the internal capsule, globus pallidus, and substantia nigra. These latter regions lack significant expression of PDE10A mRNA. Thus, PDE10A is transported throughout the dendritic tree and down the axons to the terminals of the medium spiny neurons. These data suggest a role for PDE10A in regulating activity within both the striatonigral and striatopallidal pathways. In addition, PDE10A immunoreactivity and mRNA are found at lower levels in the hippocampal pyramidal cell layer, dentate granule cell layer and throughout the cortex and cerebellar granule cell layer. Immunoreactivity is detected only in cell bodies in these latter regions. This more restricted subcellular localization of PDE10A outside the striatum suggests a second, distinct function for the enzyme in these regions.

The effect of 6-hydroxydopamine lesions on the release of amino acids in the direct and indirect pathways of the basal ganglia: a dual microdialysis probe analysis.

The loss of dopaminergic neurons of the substantia nigra in Parkinson's disease and in animal models of Parkinson's disease is associated with an imbalance in the activity of the so-called 'direct' and 'indirect' pathways of information flow through the basal ganglia. The aim of the present study was to determine whether the imbalance is reflected in changes in the release of GABA, aspartate and glutamate in the pathways using dual probe microdialysis in freely moving rats. Control and 6-hydroxydopamine-(6-OHDA)-lesioned rats were implanted with microdialysis probes in the neostriatum and substantia nigra or globus pallidus and the release of amino acids was analysed in the dialysates. Basal levels of amino acids were largely unaltered by the 6-OHDA lesion; however, the levels of GABA in the globus pallidus dialysates were significantly elevated in the lesioned rats, indicating an imbalance in favour of the indirect pathway. Administration of kainic acid to the neostriatum enhanced the release of GABA locally and in the distal probes in the substantia nigra and globus pallidus. In 6-OHDA-lesioned rats, stimulated release of GABA in the substantia nigra was abolished, indicating a reduction in transmission along the direct pathway. Thus, consistent with the direct-indirect pathway model of the basal ganglia, the 6-OHDA lesion results in an elevation of the basal release of GABA in the striatopallidal (indirect) pathway and a reduction in the evoked release of GABA in the striatonigral (direct) pathway. These imbalances may underlie, at least in part, the motor abnormalities of Parkinson's disease and in animal models of Parkinson's disease.

Central administration of the neurotensin receptor antagonist sr48692 attenuates vacuous chewing movements in a rodent model of tardive dyskinesia

Tardive dyskinesia is a movement disorder that develops in 20–30% of patients treated with chronic neuroleptics. Whilst the pathogenesis of tardive dyskinesia remains unclear, altered expression of neuropeptides in the basal ganglia has been implicated in its emergence. The peptide neurotensin is expressed in both dopamine D1 receptor-bearing neurons of the direct striatonigral pathway and dopamine D2 receptor-bearing neurons of the indirect striatopallidal pathway. Increased levels of striatal neurotensin messenger RNA (mRNA) are reported following chronic neuroleptic therapy. Chronic treatment with the typical antipsychotic haloperidol elicits neurotensin immunoreactivity in a large number of striatopallidal and a modest number of striatonigral projection neurons, whilst treatment with the potent dopamine releaser, methamphetamine, induces intense neurotensin immunoreactivity in striatonigral projection neurons. In order to determine whether increased levels of striatal neurotensin mRNA in the direct striatonigral or the indirect striatopallidal pathway play a more influential role in the development of tardive dyskinesia, we explored the effects of a specific neurotensin antagonist in a rodent model (vacuous chewing movements [VCMs] induced by chronic neuroleptics). Three groups of animals received injections of fluphenazine decanoate (25 mg/kg) or its vehicle sesame oil every 3 weeks for at least 18 weeks. They were then surgically implanted with bilateral guide cannulae aimed at the striatum, the substantia nigra pars reticulata, or the globus pallidus respectively. After recovery, animals were infused with 2-[(1-(7-chloro-4-quinolinyl)-5-(2,6-imethoxyphenyl)pyrazol-3-yl)carbonylamino]tricyclo(3.3.1.1. 3.7)decan-2-carboxylic acid (SR48692; 0.25, 0.50, and 1.0 nmol/μl), or its vehicle (10% dimethyl sulfoxide [DMSO] in saline) and observed for 60 min. Intra-striatal, intra-nigral or intra-pallidal infusion of SR48692 attenuated neuroleptic-induced VCMs. These findings lend further support to a role for neurotensin in the development of VCMs but do not clarify which pathway plays a more important role. Thus, treatments that reduce or prevent the effects of increased neurotensin expression and release may be useful in the management of tardive dyskinesia.

Basal ganglia and processing of cortical information: functional interactions between trans-striatal and trans-subthalamic circuits in the substantia nigra pars reticulata

The substantia nigra pars reticulata (SNR), a major output station of basal ganglia, receives information from the cerebral cortex through three main pathways, i.e. a direct inhibitory trans-striatal pathway, an indirect excitatory trans-striatal pathway that involves the pallidum and the subthalamus and a direct excitatory trans-subthalamic pathway. In order to determine how cortical information flow originating from functionally distinct cortical areas and processed through the trans-striatal and trans-subthalamic pathways is integrated within the SNR, the responses induced by electrical stimulation of prefrontal, motor and auditory cortex in SNR cells were analyzed in anesthetized rats. Further confirming that direct striato-nigral pathways related to these functionally distinct cortical areas are organized in parallel channels, stimulation of the prefrontal, motor and auditory cortex induced an inhibitory response on distinct subpopulations of SNR cells. Within a given channel, the direct trans-striatal and the trans-subthalamic pathways converge on a large number of nigral cells. In addition, the present study reveals that nigral cells receiving an inhibitory input from a given cortical area through the direct trans-striatal pathway can also receive an excitatory input from a functionally distinct cortical area through the trans-subthalamic pathways. Such a convergence mainly occurred between the direct striato-nigral pathway issued from the auditory cortex and the trans-subthalamic pathways issued from the motor cortex. These data reveal the existence of a converging influence of trans-subthalamic and direct striato-nigral pathways not only within but also across channels. Within a given cortico-basal ganglia channel, the trans-subthalamic pathways likely contribute to the temporal shaping of the striato-nigral inhibition and thus of the disinhibition of the related nigral target nuclei in the thalamus and mesencephalon. Across channels, the specific interactions between trans-subthalamic and direct striato-nigral pathways could contribute to prevent inhibition of subpopulations of nigral cells implicated in competing functions.

Long-term post-synaptic consequences of methamphetamine on preprotachykinin mRNA expression.

Exposure to repeated high doses of methamphetamine produces long-term toxicity to central monoamine systems and alters striatonigral pathway function 3 weeks after exposure. To determine whether these changes in the striatonigral pathway persist for longer we examined neuropeptide mRNA expression in the striatum and cytochrome oxidase activity in the output nuclei of the basal ganglia after treatment with multiple high doses of methamphetamine. Rats exposed to multiple high doses of methamphetamine had significant depletion in dopamine and serotonin content, decreases in tyrosine hydroxylase immunoreactivity, and decreases in preprotachykinin mRNA expression, 6 and 12 weeks after methamphetamine treatment. Preprotachykinin mRNA expression was significantly reduced by approximately 20% in the middle striatum and approximately 32% in the caudal striatum, 6 weeks after treatment. Twelve weeks after treatment, preprotachykinin mRNA expression continued to be significantly reduced by approximately 20% in the middle striatum and approximately 14% in the caudal striatum. Cytochrome oxidase histochemical staining in the entopeduncular nucleus and substantia nigra pars reticulata was not significantly different from that in controls at either time point. These data suggest that neurotoxic regimens of methamphetamine induce changes in striatonigral neurons that persist for up to 3 months, although there is some recovery.

Blockade of nigral and pallidal opioid receptors suppresses vacuous chewing movements in a rodent model of tardive dyskinesia

Chronic neuroleptic treatment leads to the development of tardive dyskinesia in 20–30% of patients. While the pathogenesis of tardive dyskinesia remains elusive, altered opioid peptide function in striatal projection pathways of the basal ganglia has been implicated. Using a rodent model of vacuous chewing movements induced by chronic neuroleptic administration, we investigated regional involvement of opioid transmission in tardive dyskinesia. We examined the role of dynorphin in the direct striatonigral pathway by infusing nor-binaltorphimine, a selective κ opioid receptor antagonist, into the substantia nigra pars reticulata. As well, infusions of naloxone (a non-specific opioid receptor antagonist), D-Phe-Cys-Tyr- D-Trp-Orn-Thr-Pen-Thr amide (CTOP; a μ opioid receptor antagonist) or naltrindole (a δ opioid receptor antagonist) into the globus pallidus were used to establish the contribution of the striatopallidal pathway. Chronic fluphenazine treatment (25 mg/kg i.m. every 3 weeks for 18 weeks) resulted in a robust increase in vacuous chewing movements. Infusion of nor-binaltorphimine (5.0 nmol) into the substantia nigra pars reticulata significantly attenuated vacuous chewing movements. Infusion of naloxone (0.5 and 2.0 nmol) into the globus pallidus also significantly attenuated vacuous chewing. Infusion of naltrindole into the globus pallidus blocked vacuous chewing at all doses administered (0.5, 1.0, 2.0 nmol) while CTOP was only effective at the two higher doses. From these results we suggest that increases in dynorphin in the direct striatonigral pathway and enkephalin in the indirect striatopallidal pathway following chronic neuroleptic administration are both likely to contribute to tardive dyskinesia.

Persistent Behavioral Sensitization to Chronic l-DOPA Requires A2AAdenosine Receptors

To investigate the role of A(2A) adenosine receptors in adaptive responses to chronic intermittent dopamine receptor stimulation, we compared the behavioral sensitization elicited by repeated l-DOPA treatment in hemiparkinsonian wild-type (WT) and A(2A) adenosine receptor knock-out (A(2A) KO) mice. Although the unilateral nigrostriatal lesion produced by intrastriatal injection of 6-hydroxydopamine was indistinguishable between WT and A(2A) KO mice, they developed strikingly different patterns of behavioral sensitization after daily treatment with low doses of l-DOPA for 3 weeks. WT mice initially displayed modest contralateral rotational responses and then developed progressively greater responses that reached a maximum within 1 week and persisted for the duration of the treatment. In contrast, any rotational behavioral sensitization in A(2A) KO mice was transient and completely reversed within 2 weeks. Similarly, the time to reach the peak rotation was progressively shortened in WT mice but remained unchanged in A(2A) KO mice. Furthermore, daily l-DOPA treatment produced gradually sensitized grooming in WT mice but failed to induce any sensitized grooming in A(2A) KO mice. Finally, repeated l-DOPA treatment reversed the 6-OHDA-induced reduction of striatal dynorphin mRNA in WT but not A(2A) KO mice, raising the possibility that the A(2A) receptor may contribute to l-DOPA-induced behavioral sensitization by facilitating adaptations within the dynorphin-expressing striatonigral pathway. Together these results demonstrate that the A(2A) receptor plays a critical role in the development and particularly the persistence of behavioral sensitization to repeated l-DOPA treatment. Furthermore, they raise the possibility that the maladaptive dyskinetic responses to chronic l-DOPA treatment in Parkinson's disease may be attenuated by A(2A) receptor inactivation.

Mice with very low expression of the vesicular monoamine transporter 2 gene survive into adulthood: potential mouse model for parkinsonism.

We have created a transgenic mouse with a hypomorphic allele of the vesicular monoamine transporter 2 (Vmat2) gene by gene targeting. These mice (KA1) have profound changes in monoamine metabolism and function and survive into adulthood. Specifically, these animals express very low levels of VMAT2, an endogenous protein which sequesters monoamines intracellularly into vesicles, a process that, in addition to being important in normal transmission, may also act to keep intracellular levels of the monoamine neurotransmitters below potentially toxic thresholds. Homozygous mice show large reductions in brain tissue monoamines, motor impairments, enhanced sensitivity to dopamine agonism, and changes in the chemical neuroanatomy of the striatum that are consistent with alterations in the balance of the striatonigral (direct) and striatopallidal (indirect) pathways. The VMAT2-deficient KA1 mice are also more vulnerable to the neurotoxic effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in terms of nigral dopamine cell death. We suggest that the mice may be of value in examining, long term, the insidious damaging consequences of abnormal intracellular handling of monoamines. On the basis of our current findings, the mice are likely to prove of immediate interest to aspects of the symptomatology of parkinsonism. They may also, however, be of use in probing other aspects of monoaminergic function and dysfunction in the brain, the latter making important contributions to the pathogenesis of schizophrenia and addiction.