2ND-ORDER CONDITIONING DETECTS UNEXPRESSED MORPHINE-INDUCED SALT AVERSION

Morphine failed to condition a salt taste aversion at a dose (15 mg/kg ) sufficient to produce a robust aversion to a saccharin taste. Indeed , three different concentrations of salt (1%, 1.5%, and 2%) paired wit h the same morphine dose yielded no direct evidence for conditioned av ersion. Yet, when a novel saccharin taste was paired in compound with the previously conditioned salt conditioned stimulus, we found evidenc e for a conditioning to the saccharin cue alone in three separate expe riments. Control groups eliminated alternative accounts such as neopho bia and differential exposure to morphine. Combined, these findings in dicate that morphine conditioned a salt aversion. Although this aversi on was not directly expressed, a second-order conditioning procedure w as able to provide a more sensitive index of conditioning.