2660 INTRODUCTION The olfactory system functions similarly for most vertebrates and mediates several important life history behaviors, such as feeding, reproduction and predator avoidance (Ache and Young, 2005). Chemicals, either volatized in air for tetrapods or dissolved in water for fishes and aquatic amphibians, enter the nose and bind to molecular G-protein coupled olfactory receptors (ORs) on the cilia or microvilli of olfactory receptor neurons (ORNs) in the olfactory epithelium (Eisthen, 2002). Vertebrate groups, including lampreys, teleost fishes, lungfishes, frogs and mammals, possess two morphologically different types of ORNs coupled to specific G- protein -subunits: ciliated ORNs utilizing the G olf transduction cascade, and microvillous ORNs coupled to G o , G i or G q (Eisthen, 2004). Lampreys and actinopterygian (but not sarcopterygian) fishes also possess a third type of ORN, the crypt cell, which use the G o or G q transduction cascades (Ferrando et al., 2006; Hansen et al., 2004; Hansen and Finger, 2000; Laframboise et al., 2007; Zeiske et al., 2003). The morphologically different types of ORNs are thought to mediate responses to specific odorant classes. The olfactory system of fishes is sensitive to several types of odorants, including amino acids, polyamines, bile salts, prostaglandins, steroids and nucleotides (Hara, 1994; Rolen et al., 2003; Zielinski and Hara, 2006). Amino acids, a feeding stimulant in fishes (Zielinski and Hara, 2006), are detected primarily by microvillous ORNs (Lipschitz and Michel, 2002; Sato and Suzuki, 2001), but also possibly by ciliated and crypt ORNs (Hansen et al., 2004; Vielma et al., 2008). In contrast, several studies demonstrated that for teleosts, bile salts are detected only by ciliated ORNs (Døving et al., 2011; Hansen et al., 2003; Sato and Suzuki, 2001). Additionally, cross-adaptation and mixture experiments confirm that teleosts possess ORs for bile salts that are independent from those for amino acids, prostaglandins, gonadal steroids and polyamines (Laberge and Hara, 2004; Michel and Derbidge, 1997; Zhang and Hara, 2009). Bile salts, which are produced throughout the vertebrate clade, are biliary steroids created in the liver by the oxidation of cholesterol to facilitate intestinal absorption of lipids and fat- soluble vitamins (Hagey et al., 2010; Haslewood, 1967; Hofmann et al., 2010). Bile salts are typically reabsorbed and reused by the enterohepatic system, but teleost fishes excrete a portion of their bile salts in their urine or feces (Velez et al., 2009; Zhang et al., 2001). These excreted bile salts are potent olfactory stimuli, as several teleost species have demonstrated a high olfactory specificity and sensitivity to these compounds in the nanomolar range and lower (Huertas et al., 2010; Zhang and Hara, 2009). Similarly, sea lampreys, whose olfactory epithelium contains the same three ORN types as teleosts (Laframboise et al., 2007), are extremely sensitive to bile salts (threshold: 10 –13 mol l –1 ), employing them as pheromones to attract mates and guide adults to spawning streams (Li et al., 2002; Siefkes and Li, 2004; Sorensen et al., 2005). Teleosts may also use bile salts as pheromones (Hara, 1994; Huertas et al., 2007; Sorensen and Caprio, 1997; Sorensen and Stacey, 2004; Zhang et al., 2001). For example, bile salts released by salmonids were suggested to SUMMARY Odor detection in vertebrates occurs when odorants enter the nose and bind to molecular olfactory receptors on the cilia or microvilli of olfactory receptor neurons (ORNs). Several vertebrate groups possess multiple, morphologically distinct types of ORNs. In teleost fishes, these different ORN types detect specific classes of biologically relevant odorants, such as amino acids, nucleotides and bile salts. For example, bile salts are reported to be detected exclusively by ciliated ORNs. The olfactory epithelium of elasmobranch fishes (sharks, rays and skates) is comprised of microvillous and crypt ORNs, but lacks ciliated ORNs; thus, it was questioned whether the olfactory system of this group of fishes is capable of detecting bile salts. The present investigation clearly indicates that the olfactory system of representative shark and stingray species does detect and respond to bile salts. Additionally, these species detect glycine-conjugated, taurine-conjugated and non-conjugated bile salts, as do teleosts. These elasmobranchs are less sensitive to the tested bile salts than reported for both agnathans and teleosts, but this may be due to the particular bile salts selected in this study, as elasmobranch-produced bile salts are commercially unavailable. Cross- adaptation experiments indicate further that the responses to bile salts are independent of those to amino acids, a major class of odorant molecules for all tested fishes. Key words: electro-olfactogram, EOG, bile salt, olfaction, olfactory receptor neuron, ORN, olfactory epithelium. Received 29 September 2011; Accepted 15 April 2012 The Journal of Experimental Biology 215, 2660-2667 © 2012. Published by The Company of Biologists Ltd doi:10.1242/jeb.066241 RESEARCH ARTICLE Sensitivity and specificity of the olfactory epithelia of two elasmobranch species to bile salts Tricia L. Meredith 1, *, John Caprio 2 and Stephen M. Kajiura 1 1 Department of Biological Sciences, Florida Atlantic University, Boca Raton, FL 33431, USA and 2 Department of Biological Sciences, Louisiana State University, Baton Rouge, LA 70803, USA *Author for correspondence at present address: Department of Physiology and Biophysics, University of Miami, Leonard M. Miller School of Medicine, Miami, FL 33136, USA (tmeredith@med.miami.edu) THE JOURNAL OF EXPERIMENTAL BIOLOGY