Serotonergic Sensory-Motor Neurons Mediate a Behavioral Response to Hypoxia in Pond Snail Embryos Shihuan Kuang, 1 Shandra A. Doran, 1 Richard J. A. Wilson, 2 Greg G. Goss, 1 Jeffrey I. Goldberg 1 1 Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E9 2 Department of Medical Physiology and Biophysics, University of Calgary, Alberta, Canada T2N 4N1 Received 7 December 2001; accepted 26 February 2002 ABSTRACT: Oxygen (O 2 ) is one of the most im- portant environmental factors that affects both physio- logical processes and development of aerobic animals, yet little is known about the neural mechanism of O 2 sensing and adaptive responses to low O 2 (hypoxia) during development. In the pond snail, Helisoma trivol- vis, the first embryonic neurons (ENC1s) to develop are a pair of serotonergic sensory-motor cells that regulate a cilia-driven rotational behavior. Here, we report that the ENC1-ciliary cell circuit mediates an adaptive behav- ioral response to hypoxia. Exposure of egg masses to hypoxia elicited a dose-dependent and reversible accel- eration of embryonic rotation that mixed capsular fluid, thereby facilitating O 2 diffusion to the embryo. The O 2 partial pressures (Po 2 ) for threshold, half-maximal, and maximal rotational response were 60, 28, and 13 mm Hg, respectively. During hypoxia, embryos relocated to the periphery of the egg masses where higher Po 2 levels occurred. Furthermore, intermittent hypoxia treatments induced a sensitization of the rotational response. In isolated ciliary cells, ciliary beating was unaffected by hypoxia, suggesting that in the embryo, O 2 sensing oc- curs upstream of the motile cilia. The rotational re- sponse of embryos to hypoxia was attenuated by appli- cation of the serotonin receptor antagonist, mianserin, correlated to the development of ENC1-ciliary cell cir- cuit, and abolished by laser-ablation of ENC1s. To- gether, these data suggest that ENC1s are unique oxygen sensors that may provide a good single cell model for the examination of mechanistic, developmental, and evolu- tionary aspects of O 2 sensing. © 2002 Wiley Periodicals, Inc. J Neurobiol 52: 73– 83, 2002 Keywords: hypoxia; O 2 sensing; serotonin (5-HT); sen- sory; motor; cilia; gastropod; Helisoma trivolvis INTRODUCTION Aquatic animals experience large fluctuations in en- vironmental oxygen (O 2 ) due to water eutrophication, changes in temperature and metabolism (Derry, 1993). While the use of external O 2 chemoreceptors to sense changes in O 2 and produce adaptive physiolog- ical responses has been well established in adult fish and other aquatic animals (Janse et al., 1985; Milsom and Brill, 1986; Burleson and Milsom, 1993; Kinkead and Milsom, 1994), little is known about how animals detect and respond to environmental O 2 during em- bryonic and larval stages. We explored this issue in embryos of the pond snail, Helisoma trivolvis. Pond snail embryos undergo direct development inside egg capsules that are embedded in gelatinous egg masses [Fig. 1(A)]. Immediately after gastrula- tion, the embryos initiate cilia-driven rotational move- ments characterized by constitutive slow rotation and periodic bursts of fast rotation (Diefenbach et al., 1991). One direct effect of the rotation behavior is to mix the surrounding capsular fluid, thus reducing the Correspondence to: J. I. Goldberg (Jeff.Goldberg@ ualberta.ca). Contract grant sponsor: Natural Science and Engineering Re- search Council of Canada (J. I. G. and G. G. G.). Contract grant sponsor: The Francis Families Foundation (R. J. A. W.). This article includes Supplementary Material available via the Internet at http://journals.wiley.com/neu © 2002 Wiley Periodicals, Inc. Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/neu.10071 73