Evidence for peptide-mediated neurotransmission in a molluskan brain

The previous report (Snow, 1982) characterized the monosynaptic actions of an identified cerebral interneuron (C2) in the marine mollusk Tritonia. The C2 neurons produce four types of postsynaptic potentials in an identified pedal neuron (Pd5). A high-molecular-weight (approximately 1400 daltons by Sephadex G-15 gel filtration) compound, which could mimic the four postsynaptic responses in Pd5, was isolated from C2 somata. The C2 somata had the ultrastructural characteristics of peptide-secreting cells, including profuse rough endoplasmic reticulum and large (170 nm average diameter) dense secretory vesicles. These data are consistent with the hypothesis that the synaptic transmitter of C2 neurons is a peptide(s).