Abstract
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‐HT~1~ 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‐HT~1A~)>5‐HT (10^−4^ M) > 1‐(1‐naphthyl)piperazine (1‐NP) (10^−4^M,5‐HT~1~)> m‐trifluoromethylphenylpiperazine (TFMPP) (10^−4^M, 5‐HT~1~)> 2‐methylserotonin (10^−4^M, 5‐HT~3~)> alpha‐methylserotonin (10^−4^M, 5‐HT~2~) = dopamine (10^−3^M) = norepinephrine (10^−3^M). In contrast, 5‐HT~2~ receptor antagonists were the most effective at blocking both 5‐HT‐and 8‐OH‐DPAT‐induced spawning. Cyproheptadine (10^−4^M; a 5‐HT~2~ antagonist) reduced spawning in 10^−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‐HT~2~ antagonist) reduced the intensity of 5‐HT (10^−4^and 10^−3^M)‐induced male spawning and completely blocked 8‐OH‐DPAT (10^−4^ and 10^−3^M)‐induced spawning in both sexes. NAN‐190 (10^−4^ M;a 5‐HT~1A~ antagonist) reduced spawning in 10^−3^M 8‐OH‐DPAT but was ineffective at blocking spawning in 5‐HT (10^−4^ and 10^−3^M). Propranolol (10^−4^M, 5‐HT~1~), ketanserin (10^−4^M, 5‐HT~2~), and 1‐NP (10^−4^M) did not significantly block 10^−3^M 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^−3^M) 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. © 1993 wiley‐Liss, Inc.