Electrophysiological differences in the CpG aerial respiratory behavior between juvenile and adult Lymnaea

Intact, freely moving juvenile Lymnaea perform aerial respiration significantly less often than do adults. We therefore hypothesized that RPeD1, the central pattern generator (CPG) neuron that initiates rhythmogenesis, would be less active in juveniles than adults. Using both isolated and semi-intact preparations to directly test this hypothesis, we found the opposite; juvenile RPeD1s were significantly smaller and more excitable than RPeD1s from adults. Significant age-related differences were found in the membrane resistance (greater in juveniles), time constant (smaller in juveniles), and rheobase current (lower in juveniles), all of which would tend to make juvenile cells significantly more excitable. However, there were significant age-related differences in the synaptic connectivity within the CPG and in peripheral input to the CPG, all which favor more rhythmic activity in the adult CPG. As was the case for intact Lymnaea, juvenile semi-intact preparations perform aerial respiration less often than do adults. The difference in excitability between juvenile and adult RPeD1s is therefore not sufficient to cause increased rhythmogenesis. Age-related changes in synaptic connectivity within the respiratory CPG and in peripheral modulation allow respiratory rhythmogenesis to be more easily expressed in adults which may compensate for their decreased dependence on cutaneous respiration as their surface to volume ratio changes as the grow in size.