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The psychostimulants cocaine and amphetamine increase expression of the immediate early gene (IEG) c-fos indirectly, via D1 dopamine receptor activation. To determine whether dopamine transporter substrates and inhibitors can affect c-Fos expression directly, we investigated their effects on c-Fos protein and c-fos mRNA in HEK-293 (HEK) cells transfected with the human dopamine transporter (hDAT). In untransfected HEK cells, methylphenidate and cocaine produced a small but statistically significant increase in c-Fos, whereas dopamine and amphetamine did not. ⋯ Oxidative stress may partly, but not fully, account for the DA-induced c-Fos induction as an inhibitor of oxidative stress Trolox C, attenuated DA-induced c-Fos induction. Protein kinase C (PKC) may also partially account for c-Fos induction as a specific inhibitor of PKC Bisindolylmaleimide I (BIS) attenuated DA-induced c-Fos by 50%. DAT substrate and inhibitor effects on IEGs, other fos-related antigens, and possible mechanisms that contribute to c-Fos induction warrant investigation in presynaptic neurons as a potential contribution to the long-term effects of psychostimulants.
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Animal studies indicate that mu-opioids indirectly modulate neurotransmission in the nigrostriatal dopaminergic pathway. We used positron emission tomography (PET) to study the effects of alfentanil (a mu-opioid receptor agonist) on striatal dopamine D2 receptor binding in eight healthy male volunteers. D2 receptor binding was determined by using [(11)C]raclopride as radioligand. ⋯ Pain scores were significantly smaller after alfentanil PET scan (median VAS 9 (0-42) vs. 23.5 (15-52), P = 0.008). These results indicate that pharmacologically relevant concentrations of alfentanil increase D2 dopamine receptor binding in the striatum in man. This increase is assumed to reflect reduced dopamine release.