Serotonergic regulation of the buccal (feeding) rhythm of the pond snail, Lymnaea stagnalis. An immunocytochemical, biochemical and pharmacological approach.

Hatching is an important phase of the development of pulmonate gastropods followed by the adult-like extracapsular foraging life. Right before hatching the juveniles start to display a rhythmic radula movement, executed by the buccal complex, consisting of the buccal musculature (mass) and a pair of the buccal ganglia. In order to have a detailed insight into this process, we investigated the serotonergic regulation of the buccal (feeding) rhythm in 100% stage embryos of the pond snail, Lymnaea stagnalis, applying quantitative immunohistochemistry combined with the pharmacological manipulation of the serotonin (5-HT) synthesis, by either stimulating (by the 5-HT precursor 5-hydroxytryptophan, 5-HTP) or inhibiting (by the 5-HT synthesis blocker para-chlorophenylalanine, pCPA) it. Corresponding to the direction of the drug effect, significant changes of the fluorescence intensity could be detected both in the cerebral ganglia and the buccal complex. HPLC-MS assay demonstrated that 5-HTP increased meanwhile pCPA decreased the 5-HT content both of the central ganglia and the buccal complex. As to the feeding activity, 5-HTP induced only a slight (20%) increase, whereas the pCPA resulted in a 20% decrease of the radula protrusion frequency. Inhibition of 5-HT re-uptake by clomipramine reduced the frequency by 75%. The results prove the role of both central and peripheral 5-HTergic processes in the regulation of feeding activity. Application of specific receptor agonists and antagonists revealed that activation of a 5-HT1-like receptor depressed the feeding activity, meanwhile activation of a 5-HT6,7-like receptor enhanced it. Saturation binding plot of [3H]-5-HT to receptor and binding experiments performed on membrane pellets prepared from the buccal mass indicated the presence of a 5-HT6-like receptor positively coupled to cAMP. The results suggest that 5-HT influences the buccal (feeding) rhythmic activity in two ways: an inhibitory action is probably exerted via 5-HT1-like receptors, while an excitatory action is realized through 5-HT_6,7-like receptors.