Multiple synaptic connections of a single neuron change differentially with age

The efficacy of chemical synaptic connections of a single identified interneuron with different types of follower neurons was studied throughout the adult life of the pond snail Lymnaea stagnalis. Simultaneous intracellular recordings were made from the interneuron RPeD1 and its follower neurons in isolated CNS preparations from animals of different age groups (3–18 months of age). The presence of postsynaptic responses to RPeD1 action potentials was tested. With increasing age, the number of A-group neurons that was found with a response to evoked RPeD1 action potentials decreased, yet the number of HIJK-group neurons responding to RPeD1 input increased. The number of G-group neurons and the number of individual neurons VD2/3 and VD4 with RPeD1 input did not differ significantly between age groups. However, there was variability in the presence of responses in these individual neurons. Thus, synaptic connections of the single interneuron RPeD1 change differentially throughout the adult life of L. stagnalis. Within the A-group we found indications that changes in RPeD1 input apply to the entire A-group. In the A-group neurons changes in several electrical properties could not account for the observed age-related changes in the number of neurons responding to RPeD1 action potentials.