Modulation of a rhythmic activity by serotonin via cyclic AMP in the coelenterateRenilla k�llikeri

Abstract

1. Factors influencing peristalsis-like rhythmic contractions were investigated in the rachidial tissue of a colonial anthozoan, the sea pansyRenilla kollikeri. 2. Allowing water to flow slowly over turgescent preparations resulted in transient increases in the amplitude of rhythmic contractions. Suprathreshold electrical stimulation of the rachidial tissue during relaxed periods of the cycle had a similar effect in addition to causing a rise in tonus. 3. Serotonin (5-HT) and tryptamine induced increases in the amplitude of rhythmic contractions that were similar to those obtained by mechanical (water flow) or electrical stimulation. The threshold concentration for this indolaminergic response was 1 μmol·l−1. Washing off the drugs usually further enhanced the rise in amplitude of the rhythmic contractions. The 5-HT-induced response was reduced or abolished by the 5-HT blockers naphthyl piperazine and methysergide, but not mianserin. 4. Dibutyryl or 8-bromo cyclic AMP mimicked the 5-HT-induced response. In addition, the levels of cyclic AMP, measured in rachidial tissue by radioimmunoassay, increased significantly in tissues exposed to 5-HT. 5. Incubation of rachidial tissues in the serotonergic neurotoxin 5,7-dihydroxytryptamine and the serotonin synthesis inhibitor p-chlorophenylalanine resulted in a sharp reduction of the amplitude of rhythmic and electrically stimulated contractions concomitent with potentiation of the 5-HT-induced response. The monoamine storage depletor reserpine had similar effects except for the 5-HT response. 6. This and other evidences suggest that a specific cyclic AMP-mediated serotonergic mechanism is involved in the modulation of rhythmic contractions and peristalsis in the sea pansy.