Learning and memory in Lymnaea are negatively altered by acute low-level concentrations of hydrogen sulphide

Hydrogen sulphide (H2S) is a common industrial pollutant as well as an endogenous neural transmitter/ neural modulator. Experiments were performed on the pond snail Lymnaea stagnalis to determine the acute effects of low-level exposure to H2S (50-100 mumol l(-1)) on aerial respiratory behaviour, associative learning, and its subsequent consolidation into long-term memory (LTM). A 3-neuron network whose sufficiency and necessity have been demonstrated drives aerial respiratory behaviour in Lymnaea. In the presence of 100 mumol l(-1) H2S the number of bouts of aerial respiration and the total breathing time were significantly increased compared to the control hypoxic situation, but were equivalent to those observed in snails that had been subjected to a 'more intense hypoxic challenge'. In addition, at a concentration of 100 mumol l(-1) H2S neither associative learning nor long-term memory (LTM) were observed. However, snails subjected to a 'more intense hypoxic challenge' still had the capacity to learn and form LTM. These snails, in fact, showed statistically the best learning and memory performance of any group. While learning and memory were observed at 50 and 75 mumol l(-1) H2S, respectively, they were statistically poorer than the learning and memory exhibited by snails in the standard hypoxia condition. Hence the ability to learn and form memory was compromised by H2S. Thus an invertebrate model system with a well-defined neural network can be used to study of the effects of H2S on the processes of learning and memory.