Different locomotor sensitization responses to repeated cocaine injections are associated with differential phosphorylation of GluA1 in the dorsomedial striatum of adult rats

Behavioral sensitization to psychostimulants reflects neural adaptation, which might share a common mechanism with drug addiction. Outbred male rats show different locomotor sensitization responses to cocaine, and cocaine also produces varied addictive progress in humans. We investigated whether differences in the induction of sensitization would affect the long-term persistence of sensitized locomotor activity, and we sought to determine the molecular basis for the variability in sensitization. Male Sprague-Dawley rats that showed sensitized locomotor responses over 5 consecutive daily cocaine injections (SENS) had significantly lower initial locomotor responses to the 1st cocaine exposure than did rats that did not show locomotor sensitization (NONS). Furthermore, rats that underwent 1 month of cocaine withdrawal after 5 repeated cocaine injections also exhibited sensitized or non-sensitized locomotor responses to a challenge injection of cocaine (SENS-C or NONS-C, respectively). This variability was also related to the initial responsiveness to cocaine. We examined the level of phosphorylation of the GluA1 subunit of α-amino-3-hydroxy-5-methyl-4-isoxazolepropioniate receptor (AMPAR) in the dorsal striatum and found that there were significant differences between the sensitized rats and the non-sensitized rats. pGluA1-Ser831 was increased in the SENS rats during the induction of locomotor sensitization, and pGluA1-Ser845 was increased in the SENS-C rats during the expression of locomotor sensitization. These phosphorylation changes were observed in the dorsomedial striatum (DMS) of adult rats but not in the dorsolateral striatum (DLS) of adults. Our findings suggest that differential phosphorylation of AMPAR might be an important mechanism that contributes to the development of locomotor sensitization to cocaine in adult rats.