Neuroadaptations in the dopaminergic system after active self-administration but not after passive administration of...

Methamphetamine is a strong and long-lasting stimulant that can be easily synthesized and is effective when taken either orally, intravenously, or smoked as 'ice'. Due to it's escalating abuse, a clear need exists for laboratory procedures to evaluate motivational components of methamphetamine abuse and their underlying neurobiological mechanisms. In the present experiment, we utilized a 'yoked' procedure in which rats were run simultaneously in groups of three, with two rats serving as yoked controls which received an injection of either 0.1 mg/kg methamphetamine or saline which was not contingent on responding each time a response-contingent injection of 0.1 mg/kg methamphetamine was self-administered by the third paired rat. Rats that had actively self-administered methamphetamine for 5 weeks and were then withdrawn from methamphetamine for 24 h showed marked decreases in somatodendritic dopamine D2 autoreceptors levels in the ventral tegmental area (34%) and medial (31%) and dorsal (21%) part of the substantia nigra zona compacta with a corresponding down-regulation of dopamine D1 receptors in the shell of the nucleus accumbens (15%), as measured by in vitro quantitative autoradiography. Since the decreases in levels of dopamine D1 and D2 receptors which occurred in rats self-administering methamphetamine did not occur in littermates that received either yoked injections of methamphetamine or saline, these changes likely reflect motivational states that were present when methamphetamine injection depended on active drug self-administration behavior.