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 i 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.