Vendor differences in cocaine-induced behavioral activity and hormonal interactions in ovariectomized Fischer rats

Contradictory effects of ovarian hormone on cocaine-induced behaviors have been reported in ovariectomized Fischer rats. To determine if these discrepancies are based on where the rats were purchased, Charles River Laboratories and Taconic Fischer rats were randomly assigned to either cocaine (15 mg/kg, intraperitoneal) or saline treatment; and one of four hormone-pretreatment subgroups: vehicle, estrogen, progesterone or estrogen + progesterone. Vendor differences were observed in cocaine-induced locomotor activities; overall, Taconic rats demonstrated less locomotor activity than Charles River rats. Furthermore, vendor differences in ambulatory activity were also observed after steroid replacement treatment. In Charles River rats, estrogen + progesterone co-administration suppressed cocaine-induced increases in ambulatory activity when compared to other hormone-treated groups given cocaine. In contrast, Taconic rats showed an increase in ambulations after this drug/hormone treatment. Vendor differences were also observed in steroid effects on cocaine-induced rearing activity, where estrogen + progesterone and cocaine caused an increase in rearing in Charles River rats, but not in Taconic rats. No differences between the vendors were observed in saline- or cocaine-treated animals' stereotypic activity. Vendor differences in cocaine-induced locomotor activity were not due to differences in cocaine metabolism, as no differences in plasma levels of benzoylecgonine were observed. Interestingly, Taconic animals had overall higher plasma levels of corticosterone than Charles River rats. Thus, intrinsic differences between different lines of Fischer rats may affect the outcome of ovarian hormone interactions in cocaine-induced behavioral alterations.