Behavioral sensitization to cocaine in rats: evidence for temporal differences in dopamine D3 and D2 receptor sensitivity

Rationale

Cocaine-induced changes in D₂ receptors have been implicated in the expression of sensitized behavioral responses and addiction-like behaviors; however, the influence of D₃ receptors is less clear.

Objectives

To characterize the effects of repeated cocaine administration on the sensitivity of rats to D₂- and D₃-mediated behaviors, as well as the binding properties of ventral striatal D₂-like and D₃ receptors.

Methods

Pramipexole was used to assess the sensitivity of rats to D₃/D₂ agonist-induced yawning, hypothermia, and locomotor activity, 24 h, 72 h, 10, 21, and 42 days after repeated cocaine or saline administration. The locomotor effects of cocaine (42 day) and the binding properties of ventral striatal D₂-like and D₃ receptors (24 h and 42 days) were also evaluated.

Results

Cocaine-treated rats displayed an enhanced locomotor response to cocaine, as well as a progressive and persistent leftward/upward shift of the ascending limb (72 h–42 day) and leftward shift of the descending limb (42 days) of the pramipexole-induced yawning dose–response curve. Cocaine treatment also decreased B _max and K _d for D₂-like receptors and increased D₃ receptor binding at 42 days. Cocaine treatment did not change pramipexole-induced hypothermia or locomotor activity or yawning induced by cholinergic or serotonergic agonists.

Conclusions

These studies suggest that temporal differences exist in the development of cocaine-induced sensitization of D₃ and D₂ receptors, with enhancements of D₃-mediated behavioral effects observed within 72 h and enhancements of D₂-mediated behavioral effects apparent 42 days after cocaine. These findings highlight the need to consider changes in D₃ receptor function when thinking about the behavioral plasticity that occurs during abstinence from cocaine use.