Discriminated conditioned taste aversion for studying multi-element stimulus control

The effects of morphine pre-treatment interval on the stimulus control exerted by a multi-element stimulus consisting of morphine (5.6 mg/kg), saccharin (0.2%, w/v), and a ball-bearing drinking nozzle in a discriminated taste aversion procedure were examined. In this discriminated aversion procedure, rats received infections of LiCl following presentation of this multi-element stimulus, and injections of saline following the saline, water, and non-ball-bearing nozzle composite stimulus. These paired rats were compared to unpaired rats that received saline injections rather than LiCl injections following presentation of the multi-element stimulus. Morphine pre-treatment times of 5, 10, and 20 min were examined in groups of 12 paired and 6 unpaired rats. The discrimination was rapidly learned under all three pre-treatment intervals. In subsequent testing with each individual stimulus element and combinations of two stimulus elements, stimulus control was clearly exerted by both morphine and saccharin. Paired rats drank less saccharin than unpaired rats, and less saccharin than water. Similarly, paired rats drank less fluid following morphine administration than following saline administration, and less fluid than unpaired rats following morphine administration. Control by the nozzle type was also apparent in significant interactions between the nozzle and morphine or saccharin and pairing with LiCl. In general, pre-treatment time did not influence the stimulus control that developed. However, at the two shorter pre-treatment times there was some indication that a conditioned taste aversion to morphine was developing in the unpaired rats. These experiments indicate that such discriminated taste aversion procedures may be viable methods for studying the contextual control of how drugs function as discriminative stimuli, and that longer drug pre-treatment times may be desirable in such procedures.