Alteration of neurotensin receptors in MPTP-treated mice

Striatal NTS1 , dopamine D2 and NMDA receptor regulation of pallidal GABA and glutamate release--a dual-probe microdialysis study in the intranigral 6-hydroxydopamine unilaterally lesioned rat

The current microdialysis study elucidates a functional interaction between the striatal neurotensin NTS(1) receptor and the striatal dopamine D(2) and N-methyl-d-aspartic acid (NMDA) receptors in the regulation of striatopallidal gamma-aminobutyric acid (GABA) and glutamate levels after an ipsilateral intranigral 6-hydroxydopamine-induced lesion of the ascending dopamine pathways to the striatum. Lateral globus pallidus GABA levels were higher in the lesioned group while no change was observed in striatal GABA and glutamate levels. The 6-hydroxydopamine-induced lesion did not alter the ability of intrastriatal NT (10 nm) to counteract the decrease in pallidal GABA and glutamate levels induced by the dopamine D(2) -like receptor agonist quinpirole (10 μm). A more pronounced increase in the intrastriatal NMDA- (10 μm) induced increase in pallidal GABA levels was observed in the lesioned group while it attenuated the increase in striatal glutamate levels and amplified the increase in pallidal glutamate levels compared with that observed in the controls. NT enhanced the NMDA-induced increase in pallidal GABA and glutamate and striatal glutamate levels; these effects were counteracted by the NTS(1) antagonist SR48692 (100 nm) in both groups. These findings demonstrate an inhibitory striatal dopamine D(2) and an excitatory striatal NMDA receptor regulation of striatopallidal GABA transmission in both groups. These actions are modulated by NT via antagonistic NTS(1) /D(2) and facilitatory NTS(1) /NMDA receptor-receptor interactions, leading to enhanced glutamate drive of the striatopallidal GABA neurons associated with motor inhibition, effects which all are counteracted by SR48692. Thus, NTS(1) antagonists in combination with conventional treatments may provide a novel therapeutic strategy in Parkinson's disease.

Effects of pallidal neurotensin on haloperidol-induced parkinsonian catalepsy: behavioral and electrophysiological studies

OBJECTIVE

The globus pallidus plays a critical role in movement regulation. Previous studies have indicated that the globus pallidus receives neurotensinergic innervation from the striatum, and systemic administration of a neurotensin analog could produce antiparkinsonian effects. The present study aimed to investigate the effects of pallidal neurotensin on haloperidol-induced parkinsonian symptoms.

METHODS

Behavioral experiments and electrophysiological recordings were performed in the present study.

RESULTS

Bilateral infusions of neurotensin into the globus pallidus reversed haloperidol-induced parkinsonian catalepsy in rats. Electrophysiological recordings showed that microinjection of neurotensin induced excitation of pallidal neurons in the presence of systemic haloperidol administration. The neurotensin type-1 receptor antagonist SR48692 blocked both the behavioral and the electrophysiological effects induced by neurotensin.

CONCLUSION

Activation of pallidal neurotensin receptors may be involved in neurotensin-induced antiparkinsonian effects.

Neurotensin receptor mechanisms and its modulation of glutamate transmission in the brain: relevance for neurodegenerative diseases and their treatment

The extracellular accumulation of glutamate and the excessive activation of glutamate receptors, in particular N-methyl-D-aspartate (NMDA) receptors, have been postulated to contribute to the neuronal cell death associated with chronic neurodegenerative disorders such as Parkinson's disease. Findings are reviewed indicating that the tridecaptide neurotensin (NT) via activation of NT receptor subtype 1 (NTS1) promotes and reinforces endogenous glutamate signalling in discrete brain regions. The increase of striatal, nigral and cortical glutamate outflow by NT and the enhancement of NMDA receptor function by a NTS1/NMDA interaction that involves the activation of protein kinase C may favour the depolarization of NTS1 containing neurons and the entry of calcium. These results strengthen the hypothesis that NT may be involved in the amplification of glutamate-induced neurotoxicity in mesencephalic dopamine and cortical neurons. The mechanisms involved may include also antagonistic NTS1/D2 interactions in the cortico-striatal glutamate terminals and in the nigral DA cell bodies and dendrites as well as in the nigro-striatal DA terminals. The possible increase in NT levels in the basal ganglia under pathological conditions leading to the NTS1 enhancement of glutamate signalling may contribute to the neurodegeneration of the nigro-striatal dopaminergic neurons found in Parkinson's disease, especially in view of the high density of NTS1 receptors in these neurons. The use of selective NTS1 antagonists together with conventional drug treatments could provide a novel therapeutic approach for treatment of Parkinson's disease.

Neurotensin: role in psychiatric and neurological diseases

Neurotensin (NT), an endogenous brain-gut peptide, has a close anatomical and functional relationship with the mesocorticolimbic and neostriatal dopamine system. Dysregulation of NT neurotransmission in this system has been hypothesized to be involved in the pathogenesis of schizophrenia. Additionally, NT containing circuits have been demonstrated to mediate some of the mechanisms of action of antipsychotic drugs, as well as the rewarding and/or sensitizing properties of drugs of abuse. NT receptors have been suggested to be novel targets for the treatment of psychoses or drug addiction.

Neurotensin selectively facilitates glutamatergic transmission in globus pallidus

The tridecapeptide neurotensin has been demonstrated to modulate neurotransmission in a number of brain regions. There is evidence that neurotensin receptors exist in globus pallidus presynaptically and postsynaptically. Whole-cell patch-clamp recordings were used to investigate the modulatory effects of neurotensin on glutamate and GABA transmission in this basal ganglia nucleus in rats. Neurotensin at 1 microM significantly increased the frequency of glutamate receptor-mediated miniature excitatory postsynaptic currents. In contrast, neurotensin had no effect on GABA(A) receptor-mediated miniature inhibitory postsynaptic currents. The presynaptic facilitation of neurotensin on glutamatergic transmission could be mimicked by the C-terminal fragment, neurotensin (8-13), but not by the N-terminal fragment, neurotensin (1-8). The selective neurotensin type-1 receptor antagonist, SR48692 {2-[(1-(7-chloro-4-quinolinyl)-5-2(2,6-dimethoxyphenyl)pyrazol-3-yl)carbonylamino]-tricyclo(3.3.1.1.(3.7))-decan-2-carboxylic acid}, blocked this facilitatory effect of neurotensin, and which itself had no effect on miniature excitatory postsynaptic currents. The specific phospholipase C inhibitor, U73122 {1-[6-[[17beta-3-methoyyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione}, significantly inhibit neurotensin-induced facilitation on glutamate release. Taken together with the reported postsynaptic depolarization of neurotensin in globus pallidus, it is suggested that neurotensin excites the globus pallidus neurons by multiple mechanisms which may provide a rationale for further investigations into its involvement in motor disorders originating from the basal ganglia.

Species differences in brain pre-pro-neurotensin/neuromedin N mRNA distribution: the expression pattern in mice resembles more closely that of primates than rats

We have used in situ hybridization to determine the distribution of pre-pro-neurotensin/neuromedin N (NT/N) mRNA in the brain of mice and rats. In rats and mice, expression was observed in the lateral septal nucleus, nucleus accumbens, medial preoptic area, bed nucleus of the stria terminalis, lateral hypothalamus, central amygdaloid nucleus and the subicilum. However, several differences in the NT/N mRNA distribution were observed between rats and mice in other brain areas. In mice, NT/N expression was detected in the subthalamic nucleus and geniculate nucleus, whereas expression was not observed in these brain areas in rats. Surprisingly, expression was not observed in mouse mesencephalic dopaminergic (mesDA) neurons and the CA1 area of the hippocampus, areas known to contain NT/N mRNA in the rat brain. Taken together, these results show that although the brain NT/N mRNA distribution largely overlaps in mice and rats, species differences exist in specific brain areas in rodents. Moreover, these data indicate that the distribution in mice resembles most that of primates than rats.

Neurotensin enhances glutamate excitotoxicity in mesencephalic neurons in primary culture

The tridecapeptide neurotensin has been demonstrated to increase glutamate release in discrete rat brain regions, leading to the hypothesis of a possible involvement of the peptide in neurodegenerative pathologies. The role of neurotensin in modulating glutamate excitotoxicity and the possible neuroprotective action of the neurotensin receptor antagonist SR48692 were investigated in primary cultures of mesencephalic neurons by measuring [(3)H]dopamine uptake and tyrosine hydroxylase immunocytochemistry 24 hr after glutamate treatment. The exposure to glutamate (30 and 100 microM, 10 min) decreased [(3)H]dopamine uptake into mesencephalic neurons. Neurotensin (10 and 100 nM), added before glutamate (30 microM) exposure, significantly enhanced the glutamate-induced reduction of [(3)H]dopamine uptake. In addition, the peptide (10 nM) also significantly enhanced the effect of 100 microM glutamate. The effects of neurotensin were counteracted by the neurotensin receptor antagonist SR48692 (100 nM) and by the protein kinase C inhibitor calphostin C. The exposure to 100 microM, but not 30 microM, glutamate significantly reduced the number of tyrosine hydroxylase-immunoreactive cells, and neurotensin (10 nM) significantly enhanced this effect. SR48692 (100 nM) prevented the neurotensin-induced action. These findings support the view of a possible pathophysiological role of neurotensin in mesencephalic dopamine neuronal function. Furthermore, selective neurotensin antagonists in combination with conventional drug treatments could provide a novel therapeutic approach for the treatment of neurodegenerative disorders, such as Parkinson's disease.

Neuroprotective effect of riluzole in MPTP-treated mice

The neuroprotective effects of riluzole, a Na(+) channel blocker with antiglutamatergic activity, and MK-801, a blocker of N-methyl-D-aspartate (NMDA) receptors, were compared in the model of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced depletion of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) levels in mice. The mice were injected intraperitoneally (i.p.) with four administrations of MPTP (10 mg/kg) at 1 h intervals and then the brains were analyzed 1, 3 and 7 days after the treatment. Dopamine and DOPAC levels were significantly decreased in the striatum from 1 day after MPTP treatment. A severe depletion in dopamine and DOPAC levels was found in the striatum 3 and 7 days after MPTP treatment. Riluzole antagonized the MPTP-induced decrease in dopamine, DOPAC and HVA levels in the striatum. On the other hand, MK-801 prevented the MPTP-induced decrease in DOPAC levels, but not in dopamine levels in the striatum. An immunohistochemical study indicated that riluzole can protect against MPTP-induced neuronal damage in the substantia nigra. These results suggest that riluzole is effective against MPTP-induced neurodegeneration of the nigrostriatal dopaminergic neuronal pathway.

Nigral neurotensin receptor regulation of nigral glutamate and nigroventral thalamic GABA transmission: a dual-probe microdialysis study in intact conscious rat brain

Dual-probe microdialysis in the awake rat was employed to investigate the effects of intranigral perfusion with the tridecapeptide neurotensin on local dialysate glutamate and GABA levels in the substantia nigra pars reticulata and on dialysate GABA levels in the ventral thalamus. Intranigral neurotensin (10-300nM, 60min) dose-dependently increased (+29+/-3% and +46+/-3% vs basal for the 100 and 300nM concentrations, respectively) local dialysate glutamate levels, while the highest 300nM concentration of the peptide exerted a long-lasting and prolonged reduction in both local and ventral thalamic (-20+/-4% and -22+/-2%, respectively) GABA levels. Intranigral perfusion with the inactive neurotensin fragment neurotensin(1-7) (10-300nM, 60min) was without effect. Furthermore, the non-peptide neurotensin receptor antagonist SR 48692 (0.2mg/kg) and tetrodotoxin (1microM) fully counteracted the intranigral neurotensin (300nM)-induced increase in local glutamate. SR 48692 (0.2mg/kg) also counteracted the decreases in nigral and ventral thalamic GABA release induced by the peptide. In addition, intranigral perfusion with the dopamine D(2) receptor antagonist raclopride (1microM) fully antagonized the neurotensin (300nM)-induced decreases in nigral and ventral thalamic GABA levels. The ability of nigral neurotensin receptor activation to differently influence glutamate and GABA levels, whereby it increases nigral glutamate and decreases both nigral and ventral thalamic GABA levels, suggests the involvement of neurotensin receptor in the regulation of basal ganglia output at the level of the nigra.