Synaptic innervation of the giant cerebral neuron in sated and hungry snails.

The giant cerebral neuron (GCN) is a serotoninergic cell that facilitates feeding behaviour in gastropod molluscs. We have examined the morphology of its cerebral arborizations after labelling them by intracellular injection of hexamminecobalt. Observations in the light microscope reveal extensive arborizations, with similar overall distributions, in the terrestrial snails Achatina fulica and Rumina decollata. All the major peripheral nerves terminate within the zone covered by the GCN arbor. In a sample of 370 synapses in which the GCN participated, the GCN was identified as the postsynaptic element in every case, thereby establishing the dendritic nature of the cerebral arborizations. A total of approximately 1,000 synapses contacts the GCN, with no evident regional differences in innervation density. The synaptic membrane of the presynaptic profile is characterized by an agglomeration of small clear vesicles and small dense vesicles. At nonsynaptic membranes there are agglomerations of larger dense and dense core vesicles, suggestive of nonsynaptic release. The dendrites of the GCN also contain vesicles. Starvation for five days (Rumina decollata) caused a significant increase in the proportion of curved synapses relative to flat synapses. This might be a plastic change allowing for a greater efficiency of transmission between sensory afferents and the GCN.