Researchers are now equipped with additional insight into how poisonous frogs may have evolved resistance against their own toxins, thanks to the results of a new study. Neotropical poisonous frogs produce a neurotoxin called epibatidine - which attaches to cell membrane proteins called acetylcholine receptors - that protects them from harm and predation. This defense mechanism also required the organisms to adapt a tolerance to epibatidine (to avoid the risk of poisoning themselves). As a direct consequence, frogs' sensitivity to acetylcholine (a key neurotransmitter needed for nerve cells to communicate with each another) was decreased, but at the expense of acetylcholine receptor function. In search of an explanation underlying this paradox, Rebecca Tarvin and colleagues closely analyzed the electrical properties of frog acetylcholine receptors. The scientists expressed frog acetylcholine receptors in human cells, finding that a single amino acid replacement within the acetylcholine receptor evolved three independent times, contributing to reduced epibatidine and acetylcholine sensitivity. Interestingly, the receptor's functionality was eventually rescued by additional amino acid replacements that differed among poison frog lineages, allowing them to resist their self-generated toxins, while still maintaining normal operation of their target neurotransmitters. Tarvin et al. argue that adaptation and protein evolution in general must balance many evolutionary pressures, and that some adaptations come at a cost, at least initially. The authors further note that agonists like epibatidine are particularly effective toxins because resistance to such compounds is relatively complex.