Abstract
1. Although monoamines are well-known to play important roles in molluscan physiology, we are far from fully understanding the synthetic and degradative pathways of these substances, particularly in commercially important bivalve species. In the present study endogenous catecholamines, indoleamines, and their possible precursors and metabolites were detected in the scallop, Placopecten magellanicus, by high-performance liquid chromatography coupled to electro-chemical detection. 2. Chromatographic analysis of CNS (cerebral, pedal, and parietovisceral combined), gill, gonad, kidney, mantle, liver, heart, fast adductor muscle, and foot disclosed the presence of the catecholamines 3,4-dihydroxyphenylalanine, dopamine, norepinephrine, and epinephrine and their metabolites normetanephrine, metanephrine, 3,4-dihydroxyphenylacetic acid, and homovanillic acid. 3. Dopamine was detected most frequently and most consistently among all catecholamines. The concentrations of dopamine (1400 pg/mg wet weight) and its major metabolite 3,4-dihydroxyphenylacetic acid (300 pg/mg wet weight) were highest in the CNS. Following the CNS, dopamine was also abundant in other tissues such as heart, foot, and gill. The concentration of norepinephrine (1000 pg/mg wet weight) was highest in the CNS followed by the heart (700 pg/mg wet weight) and gill (600 pg/mg wet weight). 4. The indoleamine, 5-hydroxytryptamine, was present in considerable amounts in all tissues, but its content was highest in the foot (2700 pg/mg wet weight) followed by the CNS (1150 pg/mg wet weight) and gonad (1000 pg/mg wet weight). The precursor 5-hydroxytryptophan was also abundant in the foot followed by the gonad, CNS, and heart. 5. The oxidative metabolite 5-hydroxy-3-indole acetic acid was detected in the largest amount in CNS (200 pg/mg wet weight), whereas N-acetyl-5-hydroxytryptamine was detected in trace amounts in CNS, gonad and foot. This study also presents evidence for gamma-glutamyl dopamine and gamma-glutamyl 5-hydroxytryptamine as the possible alternate catabolic products of dopamine and 5-hydroxytryptamine, respectively, as previously described in gastropods. 6. Thus, the detection of monoamines and their precursors and metabolites in scallop strongly suggests the presence of mammalian-type enzymic action of hydroxylation, oxidation, and methylation pathways leading to synthesis and degradation of detected compounds. Furthermore, this is the first study to disclose the evidence of nonconventional metabolic pathways for dopamine (gamma-glutamyl<--dopamine-->dihydroxyphenylacetic acid-->homovanillic acid) and 5-hydroxytryptamine (gamma-glutamyl 5-hydroxytryptamine<--5- hydroxytryptamine-->5-hydroxy-3-indoleacetic acid) inactivation in a bivalve species.