Systemic serotonin sulfate in opisthobranch mollusks

Serotonin (5-hydroxytryptamine, 5-HT) is a ubiquitous modulatory neurotransmitter with roles as a neurohormone and neurotransmitter. However, few studies have been performed characterizing this molecule and its related metabolites in circulating fluids. Here, we demonstrate native 5-HT sulfate, but much lower levels of 5-HT, in hemolymph of the marine mollusk Pleurobranchaea californica. The metabolite 5-HT sulfate forms from 5-HT uptake and metabolism in central ganglia of Aplysia californica and in the visceral nerve and eye of Pleurobranchaea, but not in hemolymph itself. In addition, 5-hydroxyindole acetic acid (5-HIAA), while not detected in hemolymph, forms in higher quantities than does 5-HT sulfate in the eye and visceral nerve, and γ-glu-5-HT is also observed in this area but never in hemolymph. As systemic 5-HT sulfate appears not to originate from the optic region or from systemic 5-HT, 5-HT sulfate likely derives from the nervous system. Circulating 5-HT sulfate is at least 10-fold higher during the light portion of a 12: 12-h light/dark cycle than during the dark portion (p < 0.0007), but there is no obvious trend for free systemic tryptophan (Trp) (p > 0.3) in Pleurobranchaea. 5-HT in mollusks is associated with general arousal state; thus, diurnal systemic changes in a 5-HT catabolite may reflect a regulatory role for indole catabolism in behavioral rhythms.