Convergent cholinergic neurons produce similar postsynaptic actions in Aplysia: implications for neural organization.

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Abstract
Neurons that receive convergent inputs from 2 or more cholinergic interneurons were investigated to see whether different cholinergic inputs to the same cell can activate selectively different types of acetylcholine (ACh) receptors. Synapses made in the abdominal ganglion of A. californica were examined by 5 cholinergic interneurons and 3 noncholinergic neurons. Three of those neurons have previously been shown to be cholinergic. Using pharmacological and/or biochemical tests it was shown that neuron L16 and interneuron XX are also cholinergic, and that neurons L28, L32 and LE are noncholinergic. Synapses made by these neurons in the abdominal ganglion were investigated and compared to previously studied synapses. These 8 neurons make a total of 44 synaptic connections onto a population of 14 different identified postsynaptic neurons in the abdominal ganglion. For each postsynaptic cell, it was found that the cholinergic inputs to a particular cell produce the same synaptic action, and synaptic actions that differ from those of cholinergic inputs are produced by noncholinergic inputs. These data are consistent with the hypothesis that a postsynaptic cell in Aplysia provides the same type or types of ACh receptors to all synapses made onto it by cholinergic interneurons. If generally true, this hypothesis would have important implications for the organization of nervous systems. It may help explain why a nervous system uses many neurotransmitters and why certain interneurons are needed in neural networks.