Distribution of serotonin uptake and binding sites in the cnidarian Renilla koellikeri: an autoradiographic study

Using [(3)H]-serotonin ([(3)H]-5-HT) as radiolabel and autoradiography, we have mapped the distribution of 5-HT uptake and binding sites in the sea pansy Renilla koellikeri in order to identify potential cellular pathways of 5-HT inactivation and to identify cellular substrates for the previously characterized 5-HT receptor involved in the modulation of peristaltic behavior. Uptake measured in fresh polyp tissues occurred via two processes: a high affinity (uptake(1)), clomipramine-sensitive process with a K(m) of 0.45 microM, and another of lower affinity (uptake(2)) with a Km of 11.6 microM. Autoradiograms of high affinity uptake sites revealed a diffuse distribution of label with higher density in the ectoderm and endoderm, and lower density in the mesoglea. No subsets of cells, including serotonergic neurons, appeared to retain label preferentially, thus suggesting that removal of 5-HT and its chemical conversion is a general property of sea pansy tissues. Under incubation conditions identical to those used in a previous radiobinding analysis, autoradiograms of binding sites were generated on sections from lightly fixed and cryosectioned polyps. In contrast to uptake sites, binding sites appeared as aggregations of label around neurons of the subectodermal, mesogleal and endodermal nerve nets. In the endoderm where the myoepithelia subtend peristalsis, myoepithelial cells appeared unlabeled, suggesting that 5-HT exerts its modulatory effect on peristalsis principally via neurons. Taken together, these results indicate that 5-HT is released as a neurohormone in the sea pansy and that it may act as a broad-range neuromodulator.