In Vivo Electrochemical Studies of Dopamine Overflow and Clearance in the Striatum of Normal and MPTP‐Treated Rhesus Monkeys

Abstract

Rapid chronoamperometric recordings, using Nafion‐coated carbon‐fiber electrodes (30–90 µm o.d.), were used to investigate overflow and uptake of dopamine (DA) in the striatum of normal and 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP)‐treated rhesus monkeys. The monkeys were anesthetized with isoflurane and placed in a stereotaxic apparatus. Magnetic resonance imaging‐guided sterile stereotaxic procedures were used for implantations of the electrochemical electrodes coupled with single‐barrel micropipettes that were used to apply potassium or DA locally. Potassium evoked a robust overflow of DA‐like electrochemical signals into the brain extracellular space in the unlesioned or normal putamen and caudate nucleus of the rhesus monkeys. In contrast, potassium did not produce any detectable changes (> 97% depletion) of DA in the MPTP‐lesioned striatum. In addition, the diffusion/clearance of locally applied DA was markedly altered in the lesioned caudate nucleus and putamen compared with unlesioned striatum. Cell counts of the number of residual tyrosine hydroxylase‐positive neurons in MPTP‐treated monkeys, in conjunction with whole‐tissue levels of DA and its metabolites, showed that the MPTP lesions produced extensive damage of the nigrostriatal DA system. These data indicate that residual dopaminergic fibers remaining after MPTP lesions are dysfunctional and have a greatly diminished capacity for high‐affinity DA uptake.