Immunohistochemical localization and Ca2+-dependent release of Mytilus inhibitory peptides in the ABRM of Mytilus edulis

Immunohistochemical localization of Mytilus inhibitory peptides (MIPs) in the anterior byssus retractor muscle (ABRM) of Mytilus edulis was investigated by using the anti-MIP polyclonal antibody. The antibody was shown to recognize the seven members of the MIP family that had been previously isolated from the ABRM extract. The MIP-like immunoreactivity was found abundantly in neuronal fiber-like structures in the ABRM and in the connective tissue sheath covering it. The immunoreactive fibers in both areas were rich in varicosities. In addition, it was demonstrated that the MIP-like immunoreactivity was released from the neuromuscular preparation to the bathing solution in response to the repetitive electrical pulses applied to the pedal ganglion. The release was Ca2+ dependent. These findings suggest that the seven MIP-family peptides, originally isolated from the muscle extract, are the inhibitory neuropeptides involved in physiological regulation of the ABRM contraction.