Histochemical and biochemical data support the presence in hydra of classical neurotransmitters, such as dopamine, norepinephrine and serotonin, in addition to several neuropeptides. Pharmacological and physiological evidences obtained in hydra and in other coelenterates suggest for these substances a function in neurotransmission or modulation. For dopamine a role is suggested in the modulation of hydra glutathione-induced feeding response. The reduced glutathione (GSH) induces in hydra coordinated tentacles movements and mouth opening, presumably by interacting with a specific receptor, as demonstrated by the specific binding of labelled ligands to hydra membranes. Dopaminergic agonists inhibit the GSH action, as well as phosphodiesterase inhibitors. On the contrary dopaminergic blocking agents enhance and protract the feeding response. Both drugs specific for the D1 or D2 subclasses of dopaminergic receptors are active on feeding response and compete each other. cAMP levels are enhanced in hydra specimens treated with dopaminergic agents and a dopamine sensitive, G protein-coupled adenylate cyclase has been demonstrated in hydra. 6-hydroxydopamine treatment strongly enhances and protract GSH action and induces a significant reduction of dopamine levels. cAMP levels are also reduced. Tentacles` morphology is altered in 6-hydroxydopamine treated animals. These data are considered in favour of the presence of a dopaminergic receptor, incapable of discriminating between Dl selective and D2 selective agents, coupled, through a G protein, to adenylate cyclase. This dopaminergic mechanism could be responsible for a negative modulation of GSH-induced feeding reaction.
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