Influence of estrogen in the acquisition of intravenously self-administered heroin in female rats

Previous research indicates that female rats acquire cocaine and heroin self-administration at a faster rate than male rats, and female rats with endogenous estrogen, or ovariectomized (OVX) rats with estrogen replacement acquire cocaine self-administration more rapidly than female rats with estrogen either surgically or chemically blocked. The purpose of this investigation was to extend the above findings to the acquisition of heroin (0.0075 mg/kg) self-administration in female rats. An automated autoshaping procedure was used to train rats to self-administer heroin. Three groups of female rats were compared: (1) OVX+estradiol benzoate (OVX+EB), (2) OVX+vehicle (OVX+VEH), and (3) sham-operated+vehicle (SH+VEH). Results revealed that OVX+EB rats acquired heroin self-administration in significantly fewer days compared to OVX+VEH rats. Additionally, OVX+EB rats that met the acquisition criteria self-administered a significantly greater number of heroin infusions during the last 5 days of the acquisition period compared to OVX+VEH rats. These results indicate that OVX+EB rats initiate heroin use sooner than OVX+VEH rats and consume greater amounts of heroin during the last 5 days of acquisition compared to OVX+VEH female rats.