Figure 7.

Effects of insulin receptor antibody on taste aversion learning and its CTA–LTM in snails. A, Injection of saline. The saline (0.04 ml) was injected into the abdominal cavity as the control for insulin receptor antibody. After a 60 min injection, we prepared taste aversion-trained snails (blue bars), backward conditioned snails (red bars), and naive control snails (yellow bars). The y-axis shows the numbers of bites per minute elicited by the CS (sucrose) in the pretest session and the posttest sessions. The time indicated in the x-axis shows that for the posttest session, that was performed after the training. After the CS was paired 10 times with the US in the taste aversion training session, the feeding response to the CS was significantly reduced (p < 0.01, one-factor ANOVA and post hoc Scheffé's test) at the posttest of 10 min, compared with those observed for the backward conditioned and naive control snails. This aversive behavior was consolidated to CTA–LTM that was recorded at the posttests of 1 h, 1 d, and 1 week. B, Injection of IgG. The IgG (1.92 μg) was injected into the abdominal cavity as the control for insulin receptor antibody. After a 60 min injection and the training, the feeding response to the CS was also significantly reduced (p < 0.01, one-factor ANOVA and post hoc Scheffé's test) at the posttest of 10 min, compared with those observed for the backward conditioned and naive control snails. This aversive behavior was also consolidated to CTA–LTM that was recorded at the posttests of 1 h, 1 d, and 1 week. C, Injection of insulin receptor antibody for 60 min or 3 h before taste aversion training. The insulin receptor antibody, whose final concentration was 2.56 μg/ml (17.5 nm) in Lymnaea saline, was injected for 60 min (blue bars) or 3 h (pink bars) before the pretest. The CS-elicited feeding response after the training in taste aversion-trained snails, which was injected in the insulin receptor antibody for 60 min or 3 h, was significantly less (p < 0.01, one-way ANOVA followed by the post hoc Scheffé's test) than in either snails given backward conditioning or naive control. However, there were no significant differences in the elicited feeding response among the three groups (i.e., taste aversion, backward, and naive) at the posttest sessions that were performed 1 h, 1 d, and 1 week later. The red and yellow bars indicate backward conditioning control and naive control snails, respectively. D, Injection of insulin receptor antibody for 60 min before taste aversion training. The timing of injection of insulin receptor antibody and the training paradigm were the same as those shown by blue bars in C. However, we skipped the posttest of 10 min to avoid that this CS would become an extinction stimulus. There were no significant differences in the elicited feeding response among all the data. E, Injection of insulin receptor antibody for 60 min between the posttests of 24 and 25 h after taste aversion training. Without any injection of drugs, we started taste aversion training. We observed CTA–LTM at the posttests of 10 min and 24 h (blue bars; p < 0.01, one-way ANOVA followed by the post hoc Scheffé's test). We then injected the insulin receptor antibody for 60 min (left-right arrow) to examine whether this antibody would enhance the extinction of CTA–LTM. The suppressed feeding response was still observed at the posttests of 25 h and 1 week. Each bar consists of the data obtained from 10 snails.