title: NMDA antagonist effects on striatal dopamine release: microdialysis studies in awake monkeys.
abstract: Brain imaging studies have suggested that the NMDA antagonist ketamine is as potent a releaser of striatal dopamine as amphetamine. This conclusion contradicts microdialysis findings in the rodent that NMDA antagonists, in contrast to amphetamine, have little or no effect on striatal dopamine release. The present study addressed two mechanisms that could account for this discrepancy: 1) whether there is a species difference, i.e., rodents vs. primates, in the responsivity of striatal dopamine to NMDA antagonists, and 2) whether rapid uptake of dopamine prevents reliable measures of synaptic dopamine release by microdialysis in response to NMDA antagonists. MRI-directed in vivo microdialysis was used to compare the effects of psychotomimetic NMDA antagonists phencyclidine (PCP), ketamine, and amphetamine on extracellular striatal dopamine levels in awake rhesus monkeys. The effect of PCP was also investigated in the presence of intrastriatally applied nomifensine, a dopamine uptake blocker. Amphetamine (0.1 or 0.4 mg/kg) produced robust and dose-dependent increases in dopamine release ranging 2-10-fold above baseline. PCP at 0.1 mg/kg had no effect and at 0.3 mg/kg produced a small 50% increase over baseline. Ketamine, at the relatively high dose of 5 mg/kg, produced only a 30% increase in dopamine release. Intrastriatal application of nomifensine did not influence the effect of PCP, suggesting that rapid uptake of dopamine is not preventing the detection of a PCP-induced increase in dopamine release. These findings suggest that in the primate, ketamine and PCP are not effective dopamine releasers, as has been suggested by previous imaging studies.
authors.affiliation: Department of Psychiatry, Yale University School of Medicine, Veterans Administration Medical Center, West Haven, Connecticut 06516, USA.
authors.forename: Barbara W
authors.initials: BW
authors.lastname: Adams
authors.forename: Charles W
authors.initials: CW
authors.lastname: Bradberry
authors.forename: Bita
authors.initials: B
authors.lastname: Moghaddam
mesh_terms: D000661:Amphetamine / Q000494:pharmacology*; D000818:Animals; D004298:Dopamine / Q000378:metabolism*; D018765:Dopamine Uptake Inhibitors / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology*; D005110:Extracellular Space / Q000187:drug effects / Q000378:metabolism; D005260:Female; D007649:Ketamine / Q000494:pharmacology; D008253:Macaca mulatta / Q000033:anatomy & histology / Q000378:metabolism*; D008279:Magnetic Resonance Imaging; D008297:Male; D017551:Microdialysis; D017072:Neostriatum / Q000033:anatomy & histology / Q000187:drug effects* / Q000378:metabolism; D009474:Neurons / Q000187:drug effects* / Q000378:metabolism; D009627:Nomifensine / Q000494:pharmacology; D010622:Phencyclidine / Q000494:pharmacology; D016194:Receptors, N-Methyl-D-Aspartate / Q000037:antagonists & inhibitors* / Q000378:metabolism; D013238:Stereotaxic Techniques; D014851:Wakefulness / Q000187:drug effects / Q000502:physiology
chemical_list: D018765:Dopamine Uptake Inhibitors; D018691:Excitatory Amino Acid Antagonists; D016194:Receptors, N-Methyl-D-Aspartate; D009627:Nomifensine; D007649:Ketamine; D000661:Amphetamine; D010622:Phencyclidine; D004298:Dopamine