When rats are allowed increased access time to cocaine for self-administration (from 1-hr/day to 6-hr/day), they exhibit an escalation in their average daily intake of the drug [64,65]. Rats maintained on 1-hr/day access do not show an escalation. We have replicated this finding in our laboratory (See Figure 7; red circles, solid line). The escalation of drug intake phenomenon is a potential animal model with strong construct validity for the behavior of some human drug users who, as addiction develops, tend to increase drug intake when cocaine is readily available over extended time periods [66–68].
