Distinct Neurochemical Adaptations Within the Nucleus Accumbens Produced by a History of Self-Administered vs Non-Contingently Administered Intravenous Methamphetamine

Methamphetamine is a highly addictive psychomotor stimulant yet the neurobiological consequences of methamphetamine self-administration remain under-characterized. Thus, we employed microdialysis in rats trained to self-administer intravenous (IV) infusions of methamphetamine (METH-SA) or saline (SAL) and a group of rats receiving non-contingent IV infusions of methamphetamine (METH-NC) at 1 or 21 days withdrawal to determine the dopamine and glutamate responses in the nucleus accumbens (NAC) to a 2 mg/kg methamphetamine intraperitoneal challenge. Furthermore, basal NAC extracellular glutamate content was assessed employing no net-flux procedures in these three groups at both time points. At both 1- and 21-day withdrawal points, methamphetamine elicited a rise in extracellular dopamine in SAL animals and this effect was sensitized in METH-NC rats. However, METH-SA animals showed a much greater sensitized dopamine response to the drug challenge compared with the other groups. Additionally, acute methamphetamine decreased extracellular glutamate in both SAL and METH-NC animals at both time-points. In contrast, METH-SA rats exhibited a modest and delayed rise in glutamate at 1-day withdrawal and this rise was sensitized at 21 days withdrawal. Finally, no net-flux microdialysis revealed elevated basal glutamate and increased extraction fraction at both withdrawal time-points in METH-SA rats. Although METH-NC rats exhibited no change in the glutamate extraction fraction, they exhibited a time-dependent elevation in basal glutamate levels. These data illustrate for the first time that a history of methamphetamine self-administration produces enduring changes in NAC neurotransmission and that non-pharmacological factors have a critical role in the expression of these methamphetamine-induced neurochemical adaptations. Neuropsychopharmacology (2012) 37, 707-722; doi: 10.1038/npp.2011.248; published online 26 October 2011