Equal sensitivity to cocaine reward in addiction-prone and addiction-resistant rat genotypes.

Rat genotypes tentatively identified as addiction-prone or addiction-resistant on the basis of alcohol preference and locomotor responsiveness to novelty - Lewis versus Fischer strains and Nijmegen high versus low responder lines - differed in time to develop intravenous cocaine self-administration habits, but did not differ in sensitivity to the ability of cocaine reward to summate with lateral hypothalamic brain stimulation reward. Moreover, rats from the Nijmegen low-responder line that initiated self-administration came to do so compulsively and to the same degree as did the Nijmegen high-responder rats. Thus the differences between both sets of genotypes appeared to reflect differences in reactions to the testing situation more than differences in reaction to the reinforcing drug per se.