Mechanisms of peptidergic regulation of muscle movements in molluscs.

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1. More than twenty kinds of excitatory or inhibitory neuropeptides have been isolated from the central nervous system, heart and buccal muscles of the African giant snail Achatina fulica. Among them, the function and distribution of FMRFamide, ACEP-1, achatin-I and fulicin were investigated.2. Although FMRFamide, ACEP-1 and achain-I were isolated from the atria of this animal, they did not show any significant direct effects on the atrium but did show strong excitatory effects on the ventricle.Immunohistochemical investigation has revealed that FMRFamide- and ACEP-1-like substances are richly distributed in many neurons in the cerebral and subesophageal ganglia, neuropile, intestinal nerve going to the heart, and atrium, but very scarcely in the ventricle. It is probable that these peptides synthesized in the ganglia are transported to the nerve fibers in the atrium and then released to the ventricle where they exhibit their actions.3. In relation to the buccal muscle movements, two pairs of cerebral ganglionic neurons and ten pairs of buccal ganglionic neurons have been identified and their morphology and function have been investigated. Among them two systems are characteristic. One; the contraction of the radula retractor elicited by the firing of cholinergic motoneuron B4 is modulated excitatorily by serotonergic neuron v-CDN. The other; a cholinergic motoneuron B10 contains neuropeptide ACEP-1, and the contraction of the radula protractor is modulated by ACEP-1 possibly acting presynaptically to the nerve terminals to increase the release of ACh.4. Structure-activity relationships of fulicin were studied by using the penis retractor muscle as a bioassay system, and the distribution and localization of this peptide were investigated immunohistochemically. It was concluded that fulicin is involved physiologically in the regulation of contraction of the penis retractor muscle.