Characterization of serotonin receptors in the regulation of spawning in the zebra mussel Dreissena polymorpha (Pallas)

The serotonin (5‐HT) receptor subtype(s) mediating spawning in the zebra mussel, Dreissena polymorpha (Pallas), was investigated by examining the efficacy of specific serotonergic ligands at stimulating or blocking spawning. The receptor profile for spawning stimulation resembled that of a vertebrate 5‐HT1 type. The rank order of potency for agonists was 5‐HT (10−3 M) = 8‐hydroxydipropylaminotetral in hydrobromide (8‐OH‐DPAT) (10−4 and 10−3 M,5‐HT1A)>5‐HT (10−4 M) > 1‐(1‐naphthyl)piperazine (1‐NP) (10−4M,5‐HT1)> m‐trifluoromethylphenylpiperazine (TFMPP) (10−4M, 5‐HT1)> 2‐methylserotonin (10−4M, 5‐HT3)> alpha‐methylserotonin (10−4M, 5‐HT2) = dopamine (10−3M) = norepinephrine (10−3M). In contrast, 5‐HT2 receptor antagonists were the most effective at blocking both 5‐HT‐and 8‐OH‐DPAT‐induced spawning. Cyproheptadine (10−4M; a 5‐HT2 antagonist) reduced spawning in 10−3M 5‐HT and completely blocked spawning in 10−4M 5‐HT and 8‐OH‐DPAT (10−4 and 10−3M). Mianserin (10−4M; a 5‐HT2 antagonist) reduced the intensity of 5‐HT (10−4and 10−3M)‐induced male spawning and completely blocked 8‐OH‐DPAT (10−4 and 10−3M)‐induced spawning in both sexes. NAN‐190 (10−4 M;a 5‐HT1A antagonist) reduced spawning in 10−3M 8‐OH‐DPAT but was ineffective at blocking spawning in 5‐HT (10−4 and 10−3M). Propranolol (10−4M, 5‐HT1), ketanserin (10−4M, 5‐HT2), and 1‐NP (10−4M) did not significantly block 10−3M 5‐HT‐induced spawning. The rank order of antagonists was, for 5‐HT (both 10−4 and 10−3 M), cyproheptadine> mianserin > NAN‐190> propranolol> ketanserin = 1‐NP;for 8‐OH‐DPAT (10−4 and 10−3M) it was mianserin = cyproheptadine > NAN‐190. To our knowledge, this is the first 5‐HT receptor characterization for a freshwater bivalve. Our results suggest that spawning in zebra mussels is modulated by a serotonin receptor(s)whose profile is unlike any described for vertebrates.