CHARACTERIZATION OF SEROTONIN RECEPTORS IN THE REGULATION OF SPAWNINGIN THE ZEBRA MUSSEL DREISSENA-POLYMORPHA (PALLAS)

The serotonin (5-HT) receptor subtype(s) mediating spawning in the zeb ra mussel, Dreissena polymorpha (Pallas), was investigated by examinin g the efficacy of specific serotonergic ligands at stimulating or bloc king spawning. The receptor profile for spawning stimulation resembled that of a vertebrate 5-HT, type. The rank order of potency for agonis ts was 5-HT (10(-3) M) = 8-hydroxydipropylaminotetralin hydrobromide ( 8-OH-DPAT) (10(-4) and 10(-3) M, 5-HT1A) > 5-HT (10(-4) M) > 1-(1-naph thyl)piperazine (1-NP) (10(-4) M, 5-HT1) > m-trifluoromethylphenylpipe razine (TFMPP) (10(-4) M, 5-HT1) > 2-methylserotonin (10(-4) M, 5-HT3) > alpha-methylserotonin (10(-4) M, 5-HT2) = dopamine (10(-3) M) = nor epinephrine (10(-3) M). In contrast, 5-HT2 receptor antagonists were t he most effective at blocking both 5-HT- and 8-OH-DPAT-induced spawnin g. Cyproheptadine (10(-4) M; a 5-HT2 antagonist) reduced spawning in 1 0(-3) M 5-HT and completely blocked spawning in 10(-4) M 5-HT and 8-OH -DPAT (10(-4) and 10(-3) M). Mianserin (10(-4) M; a 5-HT2 antagonist) reduced the intensity of 5-HT (10(-4) and 10(-3) M)-induced male spawn ing and completely blocked 8-OH-DPAT (10(-4) and 10(-3) M)-induced spa wning in both sexes. NAN-190(10(-4) M; a 5-HT1A antagonist) reduced sp awning in 10(-3) M 8-OH-DPAT and delayed spawning in 10(-4) M 8-OH-DPA T, but was ineffective at blocking spawning in 5-HT (10(-4) and 10(-3) M). Propranolol (10(-4) M, 5-HT,), ketanserin (10(-4) M, 5-HT2), and 1-NP (10(-4) M) did not significantly block 10(-3) M 5-HT-induced spaw ning. The rank order of antagonists was, for 5-HT (both 10(-4) and 10( -3) M), cyproheptadine > mianserin > NAN-190 > propranolol > ketanseri n = 1-NP; for 8-OH-DPAT (10(-4) and 10(-3) M) it was mianserin = cypro heptadine > NAN-190. To our knowledge, this is the first 5-HT receptor characterization for a freshwater bivalve. Our results suggest that s pawning in zebra mussels is modulated by a serotonin receptor(s) whose profile is unlike any described for vertebrates. (C) 1993 Wiley-Liss, Inc.