Serotonin localization in the gills of the freshwater mussel, Ligumia subrostrata

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Abstract
Exogenous serotonin stimulates sodium influx in Unionid mussels and in isolated gill tissue, which suggests that sodium transport in mussels is regulated by serotonin. The present study was an attempt to localize endogenous serotonin within the gill of Ligumia subrostrata. Examination of the gills at the light and electron microscopic levels revealed extensive branchial nerve tracts lying anterior–posterior between the adjacent gill filaments of each lamellar surface. In addition, there was a smaller nerve tract lying along the base of the water channel epithelium. These nerve tracts have a distinctive organization (containing nerve fibers and peripheral glial interstitial cells with gliosomes) and are capable of incorporating the vital dye procion yellow. Serotonin fluorescence following exposure to formaldehyde vapor was limited to the area of the nerve tracts. The identification of serotonergic neurons was further confirmed by light microscopic autoradiographs displaying silver grains localized principally over nerve tracts in gills exposed to tritiated serotonin. These morphologic data are consistent with the hypothesis, generated by previous physiologic data, that Na transport in freshwater Unionids is regulated by a neural serotonergic (cAMP-mediated) mechanism.

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