The possible roles of the monoaminergic system in the feeding of the snail Helix pomatia

The possible role of serotonin and dopamine in the feeding of Helix pomatia was studied applying immunocytochemical, biochemical, and behavioral techniques as well as bioassay experiments. Immunocytochemistry showed that dopamine-containing (thyrosin-hydroxylase-immunoreactive) neuronal elements of the crop and the gizzard belong to the intrinsic part, whereas serotonin-containing (serotonin-immunoreactive) neuronal elements belong to the extrinsic part of the gastrointestinal nervous system. Bioassay studies on the spontaneous contractions of the crop and the gizzard showed that dopamine affected only the longitudinal muscle contractions by increasing both the tonus and contractility, whereas serotonin was effective on both the longitudinal and circular muscle contractions. Serotonin increased the tonus and contractility of longitudinal muscles in the crop but decreased them in the gizzard. Serotonin decreased the tonus and contractility of the circular muscles in the crop but increased them in the gizzard. Serotonin effects on the circular muscle of the gizzard were concentration dependent between a range of 10(-5) M-3 x 10(-5) M. HPLC measurements of monoamines in starved and satiated animals showed that the concentration of both dopamine and serotonin significantly decreased in both the CNS and different parts of the gastrointestinal tract of satiated animals, suggesting a significant monoamine liberation during feeding. The injection of monoamines (10(-3) and 10(-2) M) into the body cavity of starved animals showed that only dopamine was able to induce feeding whereas serotonin increased the general activity of the animals suggesting that the initiation of feeding is rather dopamine than serotonin dependent.