Granisetron attenuates exercise-induced conditioned taste aversion in the rat

Preexposure to running attenuates rats' running-based flavour avoidance: Testing associative blocking with a cover cues or context change

Voluntary running in activity wheels by rats leads to a Pavlovian conditioned aversion to the flavour consumed immediately before the running, causing the rats to avoid that flavour. This learning process, known as running-based flavour avoidance learning (FAL), is weakened when the rats have had repeated exposure to the wheels before. According to the associative account, the association between the background context and running established during the preexposure phase blocks the conditioning of the target flavour because the running is highly predictable by the background context from the outset of the FAL phase. Experiments 1 and 2 examined this account by introducing another flavour as a cue signalling wheel access during the preexposure phase. In the framework of the associative account, the introduction of this cue should impede the formation of the context-running association during the preexposure phase, thereby hindering the contextual blocking of aversive conditioning for the target flavour in the FAL phase. This would result in unweakened FAL. Although the results of Experiment 1 align with this prediction, in Experiment 2, when highly distinct flavours were used as the target and second cues, the preexposure effect was not eliminated. This contradicts the predictions of the associative account, indicating that Experiment 1 may have been influenced by stimulus generalisation. In Experiment 3, changing background contexts between the preexposure and FAL phases had no impact on the preexposure effect, contrary to the predictions of the associative account. In general, the associative account was not supported.

A Handful of Details to Ensure the Experimental Reproducibility on the FORCED Running Wheel in Rodents: A Systematic Review

A well-documented method and experimental design are essential to ensure the reproducibility and reliability in animal research. Experimental studies using exercise programs in animal models have experienced an exponential increase in the last decades. Complete reporting of forced wheel and treadmill exercise protocols would help to ensure the reproducibility of training programs. However, forced exercise programs are characterized by a poorly detailed methodology. Also, current guidelines do not cover the minimum data that must be included in published works to reproduce training programs. For this reason, we have carried out a systematic review to determine the reproducibility of training programs and experimental designs of published research in rodents using a forced wheel system. Having determined that most of the studies were not detailed enough to be reproducible, we have suggested guidelines for animal research using FORCED exercise wheels, which could also be applicable to any form of forced exercise.

Further demonstration of running-based food avoidance learning in laboratory mice (Mus musculus)

Voluntary wheel running has hedonically bivalent properties in laboratory rats and mice. While it works as a reward for instrumental performance such as bar pressing, it also functions as an aversive stimulus to establish Pavlovian conditioned avoidance of the paired stimulus. The present study focused on the latter case. Running in closed wheels hampered habituation of a reluctance to eat a target snack in rats (Experiment 1A) and mice (Experiment 1B) trained by pairing access to a target snack with confinement to a wheel attached to the cage. Experiment 2 successfully confirmed and extended this finding with mice running in both open and closed wheels. A differential conditioning procedure employed in Experiment 3 ensured that this phenomenon is specific to the snack paired with running, implying that it reflects Pavlovian conditioned flavor avoidance (CFA). Free exploration in cages without wheels, however, did not results in a CFA.

Food avoidance learning based on voluntary wheel running in laboratory mice (Mus musculus)

Mice show a reluctance to eat unfamiliar food, when they first encounter it. This neophobic reaction is conventionally habituated by repeated trials: the mice gradually increase their consumption of the novel food. The new finding reported here is that the consumption remains low in mice that voluntarily run in activity wheels after the novel food access. This effect implies that running yields Pavlovian conditioned flavor aversion, which suppresses, otherwise increasing, consumption of the novel food. In the present research, the effect was demonstrated with a between-group design by pitting experimental mice receiving cheese-running paired treatment against cheese/running unpaired control mice (Experiment 1). The running-based food avoidance in mice was also shown in a differential conditioning paradigm, where one of two novel snacks (chocolate and marshmallow) was paired with running while the other was not, in non-deprived animals (Experiment 2 A) and food-deprived animals (Experiment 2B). These results concord with those previously reported in rats, indicating the generality of the phenomenon.

Mammalian Taste Bud Cells Utilize Extragemmal 5-Hydroxy-L-Tryptophan to Biosynthesize the Neurotransmitter Serotonin

Serotonin or 5-hydroxytryptamine (5-HT) is an important neurotransmitter that is found in mammalian taste buds and can regulate the output of intragemmal signaling networks onto afferent nerve fibers. However, it is unclear how 5-HT is produced, synthesized locally inside taste buds or absorbed from outside sources. In this study, we attempt to address this question by delineating the process of possible 5-HT biosynthesis within taste buds. First, we verified that the rate-limiting enzyme tryptophan hydroxylase (TPH2) responsible for converting L-tryptophan into the intermediate 5-hydroxy-L-tryptophan (5-HTP) is expressed in a subset of type II taste bud cells (TBCs) whereas the enzyme aromatic L-aromatic amino acid decarboxylase (AADC) capable of converting 5-HTP into 5-HT is found in type III TBCs. And abolishment of TPH2 did not affect the production of intragemmal 5-HT or alter TBCs; the mutant mice did not show any changes in behavioral responses to all five primary taste qualities: sweet, umami, bitter, salty, and sour. Then we identified that 5-HTP as well as AADC are abundant in type III TBCs; and application of an AADC inhibitor significantly blocked the production of 5-HT in taste buds. In contrast, administration of an inhibitor on serotonin-reuptake transporters had minimal impact on the 5-HT amount in taste buds, indicating that exogenous 5-HT is not a major source for the intragemmal transmitter. Taken together, our data indicate that intragemmal serotonin is not biosynthesized de novo from tryptophan; instead, it is produced by AADC-mediated conversion of 5-HTP absorbed from the plasma and/or nerve fibers into 5-HT. Thus, our results suggest that the overall bodily 5-HTP level in the plasma and nervous system can regulate taste buds' physiological function, and provide an important molecular mechanism connecting these peripheral taste organs with the circulatory and nervous systems.

Running-based pica and taste avoidance in rats

Running in an activity wheel generates pica behavior (kaolin clay intake) in rats. Wheel running also results in Pavlovian conditioned avoidance of the taste solution consumed immediately before the running. Since pica has been considered a behavioral marker of nausea in rats, these findings suggest that wheel running induces nausea, which is the underlying physiological state for establishing taste avoidance. This article reports a replication of running-based pica in rats (Experiment 1) and concurrent demonstrations of running-based pica and taste avoidance in the same animals (Experiments 2 and 3). Also shown is that pica does not alleviate running-based taste avoidance (Experiment 3). Another finding is that pica is generated by a nausea-inducing lithium chloride injection but not by a pain-inducing hypertonic saline injection (Experiment 4). These results, when taken together, support the hypothesis that pica behavior generated by wheel running reflects nausea in rats.

Taste aversion in rats induced by forced swimming, voluntary running, forced running, and lithium chloride injection treatments

The present experiment compared the strengths of taste aversion learning in rats induced by forced swimming in a water pool (5, 15, 30, or 60 min), voluntary running in an activity wheel (15, 30, 60, or 120 min), forced running in a motorized wheel (60 min at the speed of 8 m/min), optional running in the apparatus consisting of an activity wheel and a side room (120 min), and a lithium chloride (LiCl, 0.15 M LiCl at 2% of body weight) injection. The rats were given an access to saccharin solution immediately followed by one of the above treatments or simply returned back to the home cages for the control group. On the next 2 days, aversion to the saccharin solution was assessed by two-bottle choice testing between it and tap water. The following results were obtained. (1) The saccharin aversion was a positive function of exercise durations in the forced swimming and voluntary running rats, and the exercise of more than 30 min induced statistically significant saccharin aversion, compared with the control rats. (2) The forced running caused relatively strong saccharin aversion. The group of forced running rats acquired the numerically strongest saccharin aversion on average among all exercised rats. (3) The optional running treatment had little effect. (4) The LiCl injection resulted in the strongest aversion among the all treatments explored here.