Abstract
Extrasynaptic glutamate has been shown to regulate dopamine function in the mesocorticolimbic pathway, which plays an important role in the behavioral pharmacology of psychostimulants. Basal levels of glutamate are primarily regulated by the cystine-glutamate transporter and provide glutamatergic tone on extrasynaptic glutamate receptors. The present study examined the effects of a cystine-glutamate transporter enhancer on the neurochemical and behavioral effects of cocaine and amphetamine in nonhuman primates. It was hypothesized that augmenting extrasynaptic glutamate release with N-acetyl-L-cysteine (NAC), a cystine prodrug, would attenuate cocaine- or amphetamine-induced increases in extracellular dopamine and their corresponding behavioral-stimulant and reinforcing effects. In vivo microdialysis was used to evaluate cocaine-induced changes in extracellular dopamine (DA) in the caudate nucleus (n=3). NAC significantly attenuated cocaine-induced increases in dopamine but had inconsistent effects on amphetamine-induced increases in dopamine (n=4). Separate groups of subjects were either trained on a fixed-interval schedule of stimulus termination (n=6) or on a second-order schedule of self-administration (n=5) to characterize the behavioral-stimulant and reinforcing effects of psychostimulants, respectively. Systemic administration of NAC did not alter the behavioral-stimulant effects of either cocaine or amphetamine. Furthermore, cocaine self-administration and reinstatement of previously extinguished cocaine self-administration were not altered by pretreatment with NAC. Hence, drug interactions on caudate neurochemistry in vivo were not reflected in behavioral measures in squirrel monkeys. The present results in nonhuman primates do not support the use of NAC as a pharmacotherapy for cocaine abuse, although rodent and clinical studies suggest otherwise.