Degeneration of possible serotonergic nerves in the myenteric plexus of Mytilus induced by 5,7-dihydroxytryptamine (5,7-DHT).

Initial observations on the effects of 5,7-DHT on the myenteric plexus of Mytilus reveal extensive lesioning of monoamine nerves containing dense-cored vesicles. At least, two types of monoaminergic nerves could be recognized among the degenerating axons. The most affected were profiles with a predominance of large granular vesicles (56-200 nm) which resemble possible indoleamine nerves in the lamprey gut. The other type had small granular vesicles (40-56 nm) resembling catecholamine nerves of vertebrates. These two types of nerves possibly represent serotonergic and dopaminergic nerves, in accordance with pharmacological, biochemical and physiological evidence available in molluscs. There were, however, nerves with a population of medium-sized granular vesicles (56-130 nm) which were also damaged. Two other types of nerve profiles were unaffected by the drug. One conforms to cholinergic nerves while the other resembles "purinergic" nerves in the vertebrate gut. These results confirm previous studies with dihydroxytryptamines on molluscan smooth muscle.