Date Published: June 19, 2019
Publisher: Public Library of Science
Author(s): Lindsey A. Schier, Kellie M. Hyde, Alan C. Spector, John I. Glendinning.
Rats not only avoid ingesting a substance associated with LiCl toxicosis, but they display rejection reflexes (e.g., gapes) to its taste; this latter response is thought to reflect disgust or taste aversion. Prior work has shown that rats also avoid consuming foods/fluids associated with other adverse gastrointestinal (GI) effects like lactose indigestion but without the concomitant change in oromotor responses (taste reactivity; TR) indicative of aversion. Because of interpretive limitations of the methods used in those studies, we revisited the taste aversion-avoidance distinction with a design that minimized non-treatment differences among groups. Effects on intake and preference (Experiments 1a, 1b, and 2), as well as consummatory (TR, Experiment 1a and 1b) and appetitive (Progressive Ratio, Experiment 2) behaviors to the taste stimulus were assessed after training. In both experiments, rats were trained to associate 0.2% saccharin (CS) with intraduodenal infusions of LiCl, Lactose, or NaCl control. Rats trained with 18% lactose, 0.3 and 1.5 mEq/kg dose of LiCl subsequently avoided the taste CS in post-training single-bottle intake tests and two-bottle choice tests. However, only those trained with 1.5 mEq/kg LiCl displayed post-conditioning increases in taste CS-elicited aversive TR (Experiment 1a and 1b). This dose of LiCl also led to reductions in breakpoint for saccharin. The fact that conditioned avoidance is not always accompanied by changes in other common appetitive and/or consummatory indices of ingestive motivation further supports a functional dissociation between these processes, and highlights the intricacies of visceral influences on taste-guided ingestive motivation.
The gustatory system is the ultimate sentry of the gastrointestinal (GI) tract. Stimulation of its specialized chemoreceptors in the oral cavity evokes motor outputs that promote, in the case of potentially beneficial substances (e.g., nutrients), or deter, in the case of potentially harmful substances, ingestion. Consistent with the general heuristic put forth by Craig , taste-guided behaviors can be further subdivided in to those belonging to the appetitive or consummatory phases of ingestion . Appetitive behaviors are typically considered goal-directed motor sequences that bring the animal into contact with substances that are nutritious or otherwise advantageous and limits contact with substances that are linked to unfavorable consequences. Taste-driven consummatory behaviors, on the other hand, are engaged when the taste stimulus makes physical contact with the oral receptors and elicits stereotypic oromotor reflexes that facilitate ingestion (e.g., licking, swallowing) or rejection (e.g., gaping, dispelling the substance from the mouth), commonly referred to as taste reactivity (TR) [2–4]. While both appetitive and consummatory behaviors appear to be inherently linked to specific taste sensory input—e.g., “bitter” plant alkaloids are avoided and rejected in the naïve subject—such responses can also be acquired or modified through learning about the actual postingestive visceral consequences of the food or fluid [2, 5–8]
Consistent with previous work [14, 22], we found that while High LiCl (1.5 mEq/kg), intermediate LiCl (0.3 mEq/kg) and lactose all conditioned a decrease in CS intake and preference—what some term conditioned taste avoidance—only High LiCl conditioned a concomitant increase in aversive oromotor reactivity—i.e., conditioned taste aversion. Moreover, here we show that this dissociation extends to another domain of taste function. Namely, CS-High LiCl associations reduced appetitive responding for the taste CS, while CS-ID Lactose associations had no such effect on the subsequent willingness to work for the same CS, in a PR task. Thus, together, the results of these experiments provide compelling evidence that learned taste avoidance is not necessarily accompanied by a change in oromotor consummatory reactions evoked by the associated taste solution (i.e., CS) or appetitive behaviors geared towards obtaining the CS. One powerful feature of the design used here is that the postingestive consequences of the US was either absent or minimized during all of the test sessions. This allowed us to compare responses guided by the CS solution, without influence of the postoral US.