The possible roles of the monoaminergic system in the feeding of the snailHelix pomatia

The possible role of serotonin and dopamine in the feeding of Helix pomatia was studied applying immunocytochemical, biochemical, and behavioral techn iques as well as bioassay experiments. Immunocytochemistry showed that dopa mine-containing (thyrosin-hydroxylase-immunoreactive) neuronal elements of the crop and the gizzard belong to the intrinsic part, whereas serotonin-co ntaining (serotonin-immunoreactive) neuronal elements belong to the extrins ic parr of the gastrointestinal nervous system. Bioassay studies on the spo ntaneous contractions of the crop and the gizzard showed that dopamine affe cted only the longitudinal muscle contractions by increasing both the tonus and contractility, whereas serotonin was effective on both the longitudina l and circular muscle contractions. Serotonin increased the tonus and contr actility of longitudinal muscles in the crop but decreased them in the gizz ard. Serotonin decreased the tonus and contractility of the circular muscle s in the crop but increased them in the gizzard. Serotonin effects on the c ircular 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 sa tiated animals showed that the concentration of both dopamine and serotonin significantly decreased in both the CNS and different parts of the gastroi ntestinal tract of satiated animals, suggesting a significant monoamine lib eration during feeding. The injection of monoamines (10(-3) and 10(-2) M) i nto the body cavity of starved animals showed that only dopamine was able t o induce feeding whereas serotonin increased the general activity of the an imals suggesting that the initiation of feeding is rather dopamine than ser otonin dependent.