Sadahiko Nakajima

Summation of Overshadowing and Latent Inhibition in Rats» Conditioned Taste Aversion: Scapegoat Technique Works for Familiar Meals

In rats taste aversion learning, presentation of another taste with a target taste alleviates aversion for the target taste (overshadowing), and exposure of a target taste prior to its conditioning alleviates aversion for that taste (latent inhibition). The present study demonstrated summation of these effects, resulting in the least aversion in the rats that had received both overshadowing and latent inhibition treatments. The finding that overshadowing and latent inhibition summate is contrary to the prediction by the comparator hypothesis that they counteract, as recently reported in conditioned suppression of licking in thirsty rats. The present result supports the employment of the so-called scapegoat technique in cancer patients receiving chemotherapy after taking familiar meals.

Running-based pica in rats. Evidence for the gastrointestinal discomfort hypothesis of running-based taste aversion ☆

Voluntary running in an activity wheel establishes aversion to paired taste in rats. A proposed mechanism underlying this taste aversion learning is gastrointestinal discomfort caused by running. We tested this hypothesis by measuring the pica behavior (kaolin clay intake) of rats, because it is known that rats engage in pica behavior after various nausea-inducing treatments including irradiation, motion sickness, and injection of emetic drugs such as lithium chloride (LiCl). Following a demonstration of the already-known phenomenon of LiCl-based pica in Experiment 1, we successfully showed running-based pica behavior in Experiment 2 where the running treatment was compared with a non-running control treatment (i.e., confinement in a locked wheel). These results suggest that not only LiCl but also running induces nausea in rats, supporting the gastrointestinal discomfort hypothesis of running-based taste aversion learning.

Running-based taste aversion learning in five strains of rats

Although it is well known that voluntary wheel running works as an effective unconditioned stimulus to cause conditioned taste aversion (CTA) in several strains of rats, there is no study that explores strain differences in running-based CTA. The present study examines this issue with regard to five frequently used rat strains. Experiment 1 compared Sprague-Dawley versus Wistar rats from two suppliers, with the target taste being salty (NaCl+MSG) and then sweet (saccharin). Experiments 2, 3, and 4 tested rats of Wistar versus Long-Evans, Lewis versus Fischer, and Sprague-Dawley versus Lewis strains, respectively, with sweet and then salty solutions. None of the experiments showed any reliable strain differences in the strength of running-based CTA, suggesting the robustness of this learning phenomenon.

Calorie supply does not alleviate running-based taste aversion learning in rats

Voluntary running establishes aversion to the paired taste in rats. A proposed mechanism underlying this taste aversion learning is energy expenditure caused by the running. The energy expenditure hypothesis predicts that running-based taste aversion should be alleviated by a calorie supply since this would compensate for the energy expended by running. Accordingly, running-based taste aversion would be less readily established to a caloric substance (20% sucrose solution) than to a noncaloric substance (0.2% sodium saccharin solution). Because the sucrose and saccharin aversions were equivalent in Experiment 1, the validity of the energy expenditure hypothesis was questioned. Experiments 2 and 3 also pose a problem for this hypothesis, as post-session calorie supply by glucose tablets failed to alleviate running-based aversion to salty water.

Running induces nausea in rats: Kaolin intake generated by voluntary and forced wheel running.

Three experiments were conducted showing rats pica behavior (kaolin clay intake) due to running in activity wheels. The amount of kaolin consumed was a positive function of the available time of voluntary running (20, 40, or 60xa0min), although this relationship was blunted by a descending (i.e., 60xa0→xa040xa0→xa020xa0min) test series of execution (Experiment 1). Pica was also generated by forced running in a motorized wheel for 60xa0min as a positive function of the speed of wheel rotations at 98, 185, or 365xa0m/h, independent of the order of execution (Experiment 2). Voluntary running generated more pica than did forced running at 80xa0m/h, although the distance travelled in the former condition was 27% lesser than that in the latter condition (Experiment 3). Because kaolin intake is regarded as a reliable measure of nausea in rats, these results show that wheel running, either voluntary or forced, induces nausea in rats.

Does Conspecific Fighting Yield Conditioned Taste Aversion in Rats

Running in an activity wheel yields conditioned aversion to a taste solution consumed before the running, but its underlying physiological mechanism is unknown. According to the claim that energy expenditure or general stress caused by physical exercise is a critical factor for this taste-aversion learning, not only running but also other stressful exercises should yield conditioned aversion to the paired taste. This prediction was disconfirmed in two experiments, because stressful conspecific fighting did not work as an effective agent to establish taste aversion in rats.

Swimming-based pica in rats.

We have recently demonstrated that voluntary or forced running in activity wheels yields pica behavior (kaolin clay intake) in rats (Nakajima, 2016; Nakajima and Katayama, 2014). The present study provides experimental evidence that a single 40-min session of swimming in water also generates pica in rats, while showering rats with water does not produce such behavior. Because kaolin intake has been regarded as a measure of nausea in rats, this finding suggests that swimming activity, as well as voluntary or forced running, induces nausea in rats.

Overshadowing of running-based taste aversion learning by another taste cue

Training rats with serial presentations of two taste solutions before confinement in an activity wheel (X-->A-->running) resulted in weak aversion to taste X, compared to a training procedure without the presentation of A. Demonstration of the overshadowing effect in the present study provides another parallel feature between running-based taste aversion learning and Pavlovian conditioning preparations including poison-based taste aversion learning. It also indirectly supports the claim that cue competition causes degraded contingency effect and cover-cue effect in rats running-based taste aversion (Nakajima, 2008).