Long-delay learning in intraoral conditioned taste aversion
The cardinal feature of conditioned taste aversion (CTA) learning is the ability of animals to associate the taste or flavor of a food (the conditioned stimulus; CS) with a subsequent toxic effect (unconditioned stimulus; US), even if the toxicity occurs hours later, i.e. after a long delay. Two experiments were conducted which took advantage of the stimulus control afforded by intraoral catheterization to establish the parameters of long-delay learning in intraoral CTA. First, to determine the range of CS-US intervals which supports intraoral conditioning, rats received infusions of 5% sucrose paired with LiCl (76 mg/kg, ip) across a range of delays (0-6 h). Second, to determine the interaction of US dose and delay, rats were conditioned with sucrose paired with different doses of LiCl (19, 38 or 76 mg/kg) at several CS-US intervals (0, 10, or 60 min). Con-ditioning, assessed during a second infusion of sucrose at 48 h post-conditioning, was optimal at 10 min (although not significantly different at intervals between 0 and 60 min). Effectiveness declined at longer delays, such that CTA was not supported at intervals of 3h or greater. The dose-interval function suggested that an increased US can compensate for a longer CS-US interval. Low doses of LiCl induced a long-term CTA at 0-min (19 and 38 mg/kg) or 10-min delays (38 mg/kg), but were not sufficient to induce CTA at longer delays, which required the highest dose (76 mg/kg).