Unexpressed morphine conditioned salt aversion: procedural variants and hypertonicity of salt

Previous work has demonstrated that rats that received a sodium chloride (salt) solution paired with a high dose of morphine do not directly express a conditioned salt aversion (decreased intake) in a one-bottle test. Importantly, lithium conditioned salt aversions and morphine conditioned saccharin aversions are readily expressed. The present study examined two possible accounts of the failure for morphine conditioned salt aversion to be directly expressed. Experiment 1 eliminated an account based on procedural variations that were uncontrolled in the original salt and saccharin conditioning protocols (Bevins et al., 1996). Experiment 2 eliminated a hypertonic account that argues that the high concentrations of salt in the previous work (1% and greater) interfered with direct expression of salt aversion by inducing continual drinking. Rats given a non-hypertonic salt solution (0.3%) paired with morphine still did not express a salt aversion in a one-bottle test. A two-bottle testing procedure, however, revealed a robust salt aversion with the non-hypertonic solution.