Serotonergic responses of the siphons and adjacent mantle tissue of the zebra mussel, Dreissena polymorpha

Bivalve siphons have important functional roles in nutritional physiology, defense, and reproductive mechanisms, yet little is known about their neuro muscular control. In the present study, tension measurements and video obse rvations of siphons and adjacent mantle tissue were used to investigate res ponses to serotonin (5-MT). 5-HT caused relaxation at 10(-6) M and 10(-5) M , contraction or biphasic responses at 10(-4) M and 10(-3) M, and siphon op ening at 10(-4) M and 10(-3) M. Responses were slow and lasted 5-10 min aft er much shorter (20 s) applications of 5-MT. The relaxation phase was enhan ced in high potassium medium. Contractile responses could be mimicked by al pha-1-methyl-5-HT and 2-methyl-5-MT but not by 8-OH-DPAT. The responses wer e not affected by methiothepin, TFMPP, 1-(1-naphthyl)piperazine, metergolin e, NAN-190, mianserin, cyproheptadine, and ketanserin. The pharmacology of the 5-HT receptor(s) mediating these siphon/mantle responses is, therefore, different from previously described 5-HT receptors involved in spawning. T he siphon/mantle contains previously undescribed longitudinal muscle fibers in the mantle and circular muscle fibers around the siphons. Serotonergic innervation of the siphon margins and mantle tissue was observed by immunoh istology. The presence of 5-HT in the siphon/mantle tissue and the responsi veness of these preparations to 5-HT suggest that 5-HT may be a physiologic al regulator of mantle and siphon movements in the zebra mussel. (C) 1999 E lsevier Science Inc. All rights reserved.