title: Sensitization of midbrain dopamine neuron reactivity promotes the pursuit of amphetamine.
abstract: Stimulant drugs such as amphetamine are readily self-administered by humans and laboratory animals by virtue of their actions on dopamine (DA) neurons of the midbrain. Repeated exposure to this drug systemically or exclusively in the cell body region of these neurons in the ventral tegmental area (VTA) leads to long-lasting changes in dopaminergic function that can be assessed by increased locomotor activity and enhanced DA overflow in the nucleus accumbens (NAcc) after re-exposure to the drug. Three experiments were conducted to evaluate the possibility that this enduring sensitized reactivity underlies compulsive drug self-administration. In all experiments, rats were pre-exposed to amphetamine and, starting 10 d later, their intravenous self-administration of the drug was assessed. In the first experiment, rats previously exposed to amphetamine systemically or exclusively in the VTA subsequently worked harder than untreated animals to obtain the drug when the work required to obtain successive infusions was increased progressively. In the second experiment, this progressively increasing workload was found to decrease the magnitude of amphetamine-induced DA overflow observed with successive infusions until responding ceased. Rats previously exposed to amphetamine were more resistant to this decline and more apt to maintain responding. Finally, in experiment three, a noncontingent priming injection of the drug produced a greater NAcc DA response and a greater parallel increase in lever pressing in drug compared with saline pre-exposed rats. Together, these results demonstrate a direct relation between drug-induced sensitization of midbrain dopamine neuron reactivity and the excessive pursuit and self-administration of an abused substance.
authors.affiliation: Department of Psychiatry and Committee on Neurobiology, The University of Chicago, Chicago, Illinois 60637, USA. ezina@yoda.bsd.uchicago.edu.
authors.forename: Paul
authors.initials: P
authors.lastname: Vezina
authors.forename: Daniel S
authors.initials: DS
authors.lastname: Lorrain
authors.forename: Gretchen M
authors.initials: GM
authors.lastname: Arnold
authors.forename: Jennifer D
authors.initials: JD
authors.lastname: Austin
authors.forename: Nobuyoshi
authors.initials: N
authors.lastname: Suto
mesh_terms: D000661:Amphetamine / Q000008:administration & dosage*; D019969:Amphetamine-Related Disorders / Q000503:physiopathology*; D000818:Animals; D001522:Behavior, Animal / Q000187:drug effects; D004195:Disease Models, Animal; D004298:Dopamine / Q000032:analysis / Q000378:metabolism*; D004333:Drug Administration Routes; D004334:Drug Administration Schedule; D005110:Extracellular Space / Q000737:chemistry; D007274:Injections, Intraperitoneal; D008297:Male; D008636:Mesencephalon / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D017551:Microdialysis; D009043:Motor Activity / Q000187:drug effects; D009474:Neurons / Q000187:drug effects / Q000378:metabolism*; D009714:Nucleus Accumbens / Q000187:drug effects / Q000378:metabolism; D051381:Rats; D020318:Rats, Long-Evans; D012646:Self Administration; D017557:Ventral Tegmental Area / Q000187:drug effects
chemical_list: D000661:Amphetamine; D004298:Dopamine