New research has found that methamphetamine improves snails’ long-term memories and also prevents them from forgetting already learned ones. The results could help explain how amphetamines improve human memory and why it’s so hard to break addiction to them (Journal of Experimental Biology, Vol. 213, No. 12).
Ken Lukowiak, PhD, at the University of Calgary in Canada, and Barbara Sorg, at Washington State University in Pullman, Wash., chose the lowly pond snail, Lymnaea stagnalis for their study. The snail uses just three neurons to decide whether, in a particular environment, it can breathe through its skin or if it needs to extend breathing tubes called pneumostomes to reach more oxygen. To see whether meth would improve the snails’ ability to remember conditioned behavior, they soaked snails in meth-laced de-oxygenated water, then 24 hours later conditioned the snails not to use their pneumostomes by poking them anytime they began extending the tubes. The next day, snails that had been trained the same way in non-meth-laced water had already forgotten the conditioning, but the meth-trained snails still remembered.
“[Meth] really does seem to enhance the snails’ ability to form memories,” Lukowiak says.
In another experiment, they again conditioned both meth-exposed and non-meth-exposed snails not to extend their pneumostomes in de-oxygenated water, then measured how long it took them to return to extending their tubes once the scientists stopped poking them. The non-meth-exposed snails reestablished their normal pneumostome-extending behavior several hours earlier than the meth-exposed snails, indicating that for the meth-exposed snail, it was harder to override their conditioning.
Lukowiak and Sorg speculate that meth could interfere with a dopamine-sensitive neuron, RPeD1, which is integral to snails’ breathing behavior memory, either by making it easier for that neuron to form long-term memories or impairing its ability to forget, or both.
Interestingly, these effects only showed up when the snails were tested in the same meth-laced water in which they were trained. If they were tested in clean water, their conditioning went out the window. That appears to mirrors humans’ tendency to relapse if they’re exposed to the same situations they lived in while addicts, Lukowiak says.