Ultrastructural Localization of Antho-RWamides I and II at Neuromuscular Synapses in the Gastrodermis and Oral Sphincter Muscle of the Sea Anemone Calliactis parasitica

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Abstract
Light microscopic studies have shown that the sea anemone neuropeptides Antho-RWamides I (Calliactis parasitica. In the present ultrastructural study, using the immunogold technique, we found Antho-RWamide-like material in the granular vesicles of neurons that make synaptic contacts with the myonemes of both gastrodermal and oral sphincter muscle cells of Calliactis. Gastrodermal nerve cells contained immunoreactive granular vesicles averaging 149.3 +/- 4.1 nm in diameter; smaller granular vesicles (47.5 +/- 2.5 nm) were present at a labelled synapse. Neurites associated with the sphincter muscle had immunoreactive granular vesicles averaging 78.8 +/- 3.3 nm in diameter with smaller granular vesicles (63 +/- 4.4 nm) at three labelled neuromuscular synapses. All Antho-RWamide-immunoreactive vesicles were irregularly granular, unlike the typical dense-cored vesicles observed at some other synapses in sea anemones. No evidence was found of storage or release at nonsynaptic sites (paracrine secretion). The Antho-RWamide immunoreactive neurites innervate the sphincter muscle fibers directly rather than through intermediate neuronal pathways. This is the first ultrastructural evidence of a neuropeptide at a coelenterate neuromuscular synapse.

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