Active versus passive cocaine administration: differences in the neuroadaptive changes in the brain dopaminergic...

Area of Science:

• Neuroscience
• Pharmacology
• Addiction Research

Background:

• Chronic cocaine use is linked to reduced dopamine D2 receptor availability in the striatum.
• The precise mechanisms driving these neuroadaptive changes remain unclear, with debate on whether they stem from direct drug effects or motivational states associated with drug-seeking.

Purpose of the Study:

• To differentiate between direct pharmacological effects of cocaine and motivational states in causing neuroadaptive changes in dopamine D2 receptor densities.
• To investigate the relationship between these receptor changes and the expression of dopamine D2 receptor mRNA.

Main Methods:

• Utilized a "yoked" procedure in rats, comparing active cocaine self-administration with passively yoked cocaine or saline injections.
• Employed in vitro quantitative autoradiography to measure dopamine D2 receptor levels.
• Applied in situ hybridization analysis to assess dopamine D2 receptor mRNA expression.

Main Results:

• Passively administered cocaine, but not actively self-administered cocaine, led to decreased dopamine D2 receptor levels in the caudate/putamen and nucleus accumbens.
• Active cocaine self-administration was associated with increased dopamine D2 receptor mRNA levels in the ventral tegmental area.
• These findings indicate a dissociation between receptor density changes and gene expression.

Conclusions:

• Reductions in striatal dopamine D2 receptor density are likely attributable to the chronic administration of cocaine itself, independent of motivated drug-seeking behavior.
• Increased dopamine D2 receptor mRNA levels in limbic regions do not invariably translate to higher receptor densities and may reflect underlying motivational states during active drug self-administration.