The role of dose and restriction state on morphine-, cocaine-, and LiCl-induced suppression of saccharin intake: A comprehensive analysis

Abstract Rats avoid intake of a taste cue when paired with a drug of abuse or with the illness-inducing agent, lithium chloride (LiCl). Although progress has been made, it is difficult to compare the suppressive effects of abused agents and LiCl on intake of a gustatory conditioned stimulus (CS) because of the cross-laboratory use of different CSs, different unconditioned stimuli (USs), and different doses of the drugs, different conditioning regimens, and different restriction states. Here we have attempted to unify these variables by comparing the suppressive effects of a range of doses of morphine, cocaine, and LiCl on intake of a saccharin CS using a common regimen in non-restricted, food restricted, or water restricted male Sprague-Dawley rats. The results showed that, while the putatively aversive agent, LiCl, was effective in suppressing intake of the taste cue across nearly all doses, regardless of restriction state, the suppressive effects of both morphine and cocaine were greatly reduced when evaluated in either food or water restricted rats. Greater sensitivity to drug was revealed, at very low doses, when testing occurred in the absence of need (i.e., when the rats were non-restricted). Together, these results provide the first uniform and comprehensive analysis of the suppressive effects of morphine, cocaine, and LiCl as a function of dose and restriction state. In the present case, the suppressive effects of morphine and cocaine are found to differ from those of LiCl and, in some respects, from one another as well.