Depletion of serotonin selectively impairs short-term memory without affecting long-term memory in odor learning in the terrestrial slug Limax valentianus

Abstract

The terrestrial slug Limax is able to acquire short-term and long-term memories during aversive odor-taste associative learning. We investigated the effect of the selective serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) on memory. Behavioral studies indicated that 5,7-DHT impaired short-term memory but not long-term memory. HPLC (high-performance liquid chromatography) analysis revealed that 5,7-DHT significantly reduced serotonin content in the central nervous system. The present study suggests that acquisition, retention, and/or retrieval of short-term memory involves serotonin, and neither acquisition nor retrieval of long-term memory requires serotonin at a level as high as that required for short-term memory.

Figure 1.

Effect of 5,7-DHT on short-term memory. (A) Time course of the experiments. Conditioning was carried out five days after injection of vehicle solution (control group) or 5,7-DHT solution (5,7-DHT group). One day after conditioning, CR, mobility, and odor sensitivity tests were performed. We assumed that acquisition of short-term memory occurs during conditioning (circle), the retrieval occurs at the CR test (star), and retention occurs between the acquisition and the retrieval (arrow). (B) Percentage of aversive response in carrot-conditioned slugs. The 5,7-DHT group showed a significantly lower percentage than the control group. P = 0.0009 by Fisher’s exact probability test. (C) Percentages of aversive response in cucumber-conditioned slugs. The 5,7-DHT group showed a significantly lower percentage than the control group. P = 0.0003 by Fisher’s exact probability test.

Figure 2.

Effect of 5,7-DHT on long-term memory. Vehicle solution was injected into the slugs in the control group, while 5,7-DHT solution was injected into the 5,7-DHT animals. We assumed that acquisition of long-term memory occurs during a period from the onset of conditioning to 2 h after the end of conditioning (circle), according to Yasui et al. (2004), that retrieval occurs at the CR test (star), and that retention occurs between the acquisition and retrieval (arrow). (A,B) Effects of 5,7-DHT injected 5 d before the conditioning. Five days after injection, conditioning was performed, and six days after conditioning, CR, mobility, and odor sensitivity tests were performed. The time course of the experiments (A) and the results (B) are shown. The percentage of slugs that showed an aversive response was not significantly different between the drug- and vehicle-injected groups. P > 0.05 by Fisher’s exact probability test. (C,D) Effects of 5,7-DHT injected one day after conditioning. One day after conditioning, drug injection was performed. Five days after drug injection, CR, mobility, and odor sensitivity tests were conducted. The time course of the experiments (C) and the results (D) are shown. The percentage of slugs that showed an aversive response was not significantly different between the drug- and vehicle-injected groups. P > 0.05 by Fisher’s exact probability test.

Figure 3.

HPLC analysis of the serotonin content of the central ganglia from control and 5,7-DHT-treated slugs. The graph shows the mean value of the serotonin content for the two groups; error bars represent the SEM. The difference in the serotonin content between the ganglia from the controls and the 5,7-DHT-treated slugs was statistically significant (P = 0.025, one-tailed unpaired Student t-test).