The central nervous system (CNS) of Aplysia is a fascinating source to identify and characterize
neuropeptides and neurotransmitters because of offering many useful divergent and convergent
neuronal aggregates. Here, two neuropeptides were isolated from the extract of CNS of the northwest
pacific sea hare, Aplysia kurodai, using HPLC system for fractionation and the anterior byssus retractor
muscle (ABRM) of the Mytilis edulis as the bioassay system. Purified peptides, myomodulin A
(MMA) and E (MME), were determined by amino acid sequencing and molecular mass analysis.
MMA showed a potentiating effect at 100 nM or lower, on the contrary, an inhibitory effect at higher
doses from 100 nM on phasic contraction elicited by repetitive electrical stimulation on the ABRM of Mytilus. However,
MME only inhibited phasic contraction with all examined concentrations. MME revealed 100 times more potent activity
than that of MMA on the relaxing catch-tension of ABRM stimulated by acetylcholine. Both MMA and MME potently
stimulated a response on the crop and penial retractor muscle of the African giant snail, Achatina fulica, compared with
other known mollusks neuropeptides. These results suggest that MMA and MME may be broadly distributed in CNS of
Aplysia to function a neuromodulatory role controlled via excitatory and inhibitory neurons, and may be involved in the
digestive and reproductive activity in other mollusk.