Gene - environment interactions determine the individual variability in cocaine self-administration

Research into factors that determine the propensity to self-administer cocaine has shown that stressors can determine the amount of cocaine self-administered as well as the rate of acquisition. However, the interaction between the genetic make-up of the animal and stress is unknown. This study investigated this interaction by using the genetic animal model consisting of apomorphine susceptible (APO-SUS) and unsusceptible (APO-UNSUS) rats. Animals were allowed to self-administer 0.25 mg/kg cocaine under stressful and habituated conditions. This study revealed that the amount of cocaine consumed was highly dependent on the genetic make-up of the animal as well as the amount of stress during self-administration. Under habituated circumstances the APO-UNSUS rats took far more cocaine than the APO-SUS rats. Under stressful circumstances, however, the APO-SUS rats took far more cocaine than the APO-UNSUS rats. This difference in the amount consumed by APO-SUS and APO-UNSUS rats is likely to be due to the specific neurobiological features of their dopaminergic and, possibly, noradrenergic system as well as the reactivity of their HPA-axis. It is suggested that the amount of a drug consumed and, accordingly, its addictive potential and 'drug-vulnerability' are determined by the interaction between the genetic make-up of the animals and stress, and not by either component alone.