Different cholinergic synapses converging onto neurons in Aplysia produce the same synaptic action

We have investigated neurons that receive inputs from more than one cholinergic interneuron, to see whether one cholinergic input can depolarize and the other hyperpolarize by virtue of activating different ACh receptors. We have examined synapses made in the abdominal ganglion of Aplysia californica by 3 cholinergic interneurons. Two of those interneurons have previously been shown to be cholinergic. Using both biochemical and pharmacological tests we have shown a third interneuron to be cholinergic. For 7 postsynaptic cells, we compared 6 new connections that we have identified from cholinergic interneurons, and 12 previously known connections. In each case we found that different cholinergic inputs onto any given cell produced the same synaptic action. This finding held even for L7, a cell known to have two types of ACh receptors. These data are consistent with the hypothesis that a postsynaptic cell does not segregate different types of ACh receptors. If generally true, this lack of segregation may help explain why a nervous system uses more than one neurotransmitter as well as why certain interneurons are needed in neural networks.