How stress alters memory in 'smart' snails

Abstract

Cognitive ability varies within species, but whether this variation alters the manner in which memory formation is affected by environmental stress is unclear. The great pond snail, Lymnaea stagnalis, is commonly used as model species in studies of learning and memory. The majority of those studies used a single laboratory strain (i.e. the Dutch strain) originating from a wild population in the Netherlands. However, our recent work has identified natural populations that demonstrate significantly enhanced long-term memory (LTM) formation relative to the Dutch strain following operant conditioning of aerial respiratory behaviour. Here we assess how two populations with enhanced memory formation (i.e. 'smart' snails), one from Canada (Trans Canada 1: TC1) and one from the U.K. (Chilton Moor: CM) respond to ecologically relevant stressors. In control conditions the Dutch strain forms memory lasting 1-3 h following a single 0.5 h training session in our standard calcium pond water (80 mg/l [Ca(2+)]), whereas the TC1 and CM populations formed LTM lasting 5+ days following this training regime. Exposure to low environmental calcium pond water (20 mg/l [Ca(2+)]), which blocks LTM in the Dutch strain, reduced LTM retention to 24 h in the TC1 and CM populations. Crowding (20 snails in 100 ml) immediately prior to training blocks LTM in the Dutch strain, and also did so in TC1 and CM populations. Therefore, snails with enhanced cognitive ability respond to these ecologically relevant stressors in a similar manner to the Dutch strain, but are more robust at forming LTM in a low calcium environment. Despite the two populations (CM and TC1) originating from different continents, LTM formation was indistinguishable in both control and stressed conditions. This indicates that the underlying mechanisms controlling cognitive differences among populations may be highly conserved in L. stagnalis.

Figure 1. Response to operant conditioning and yoked controls at 24 h.

Number of average (± SEM) pneumostome openings in 30 min during training (TR) and the test for LTM 24 h later (test @ 24 h) in the CM (Chilton Moor) and TC1 (Trans Canada 1) populations following contingent (white bars) or yoked (grey bars) training. ** = significantly different from TR (P<0.01).

Figure 2. Duration of memory retention following operant conditioning.

Number of average (± SEM) pneumostome openings in 30 min during training (TR: average response across four training groups is displayed) and the test for LTM (test @ 24 h, 3 d, 5 d or 8 d) in the CM (Chilton Moor) and TC1 (Trans Canada 1) populations following training in control conditions. ** = significantly different from TR (P<0.01).

Figure 3. Effect of low calcium on the duration of memory retention.

Number of average (± SEM) pneumostome openings in 30 min during training (TR: average response across two training groups is displayed) and the test for LTM (test @ 24 h or 3 d) in the CM (Chilton Moor) and TC1 (Trans Canada 1) populations following exposure to low environmental calcium (pale grey: 20 mg/l) for 1 week before and during training/testing. ** = significantly different from TR (P<0.01).

Figure 4. Effect of crowding on long-term memory formation.

Number of average (± SEM) pneumostome openings in 30 min during training (TR) and the test for LTM 24 h later (test @ 24 h) in the CM (Chilton Moor) and TC1 (Trans Canada 1) populations following crowding (CR: 20 snails/100 ml) for 1 h immediately prior to training. There was no significant decline in pneumostome opening attempts between TR and the test at 24 h following crowding prior to TR.