Dopamine and serotonin receptors mediating contractions of the snail, helix pomatia, salivary duct

Abstract

The combination of high performance liquid chromatography, bioassay and immunocytochemistry was applied to study the regulation of the salivary duct muscle of the snail, Helix pomatia. The major function of the duct appears to be to propel the saliva toward the buccal cavity during feeding. It has been established that serotonin and dopamine applied exogenously mimic the effect on the duct exerted by the stimulation of the salivary nerve. Immunohistochemistry revealed the presence of serotonin, but not dopaminergic nerve elements in the nerve and along the duct surface. However, both serotonin (14.9–15.5 pmol/mg) and dopamine (0.38–0.58 pmol/mg), as well as the synthesizing enzymes (tyrozine hydroxylase 0.28 pmol/mg tissue/h and DOPA 0.32 nmol synthesized DA/mg tissue/h) could regularly be assayed in the salivary duct by high performance liquid chromatography. When released following the stimulation of the salivary nerve, both monoamines were shown to interact with distinct membrane receptors. Dopamine elicited a sustained increase of the muscle tone in concentration-dependent manner ( K d=1.5 μM). Mammalian D 1 receptor antagonist flupenthixol and fluphenazine attenuated, whereas the D 1 receptor agonist SKF-38393 mimicked the effect elicited by exogenous dopamine. Serotonin had a double effect on the salivary duct: a relaxing and a contracting one with different K d values 76 nM and 2.4 μM, respectively. 5-HT 2 receptor antagonist ritanserin and ketanserin attenuated the serotonin-induced relaxation. In contrast 5-HT 3 antagonist metoclopramide and MDL2222 decreased and 5-HT 3 receptor agonist 1-(m-chlorophenyl)-biguanide mimicked the serotonin-induced contraction, suggesting that serotonin exerted its action on two different receptor subtypes. The release of radiolabeled serotonin and dopamine upon nerve stimulation was found to be Ca-dependent. Furthermore, the increase in serotonin concentration induced a decrease of the potency of dopamine to elicit sustained contraction. These results provide evidence for the transmitter role of serotonin and dopamine in salivary duct. It is concluded that receptors reveal a pharmacological profile related to vertebrate D 1, 5-HT 2 and 5-HT 3 receptor subtypes. Moreover, it was found that the process of conveying the saliva is modulated by an interaction of dopamine and serotonin.