Reference: Biol. Bull. 174: 364-372. (June, 1988) Response Properties of Chemoreceptors from the Medial Antennular Filament of the Lobster Homarus americanus ANN JANE TIERNEY1, RAINER VOIGT, AND JELLE ATEMA2 Boston University Marine Program, Marine Biological Laboratory, Woods Hole, Massachusetts 02543 Abstract. We determined the spectral tuning proper ties of 53 single cells from the medial antennular filament of the lobster Homarus americanus. Test stimuli were 15 single compounds and a mixture that included all 15 test compounds. Three main cell populations were found: hydroxyproline best (14 cells), taurine best ( 13 cells), and arginine best ( 11 cells). Most hydroxyproline and taurine best cells were narrowly tuned and had no consistent next best stimulus. In contrast, arginine best cells were generally broadly tuned and had consistent second (leu- cine) and third (lysine) best stimuli. Mixture suppression occurred in most cells. Responses of hydroxyproline, taurine, and arginine best cells to the mixture were 25%, 38%, and 50%, respectively, relative to the responses of these cells to their best compound alone (100%). In a sec ond experiment, we tested 12 arginine sensitive cells with a series of arginine analogs. Most cells were broadly tuned and as a population showed a similar response ra tio to arginine, leucine, and lysine. Introduction Amino acid receptors occur in the olfactory and taste systems of numerous species. Crustaceans have proved to be particularly useful models for investigating proper ties of such external amino acid receptors. Behavioral studies have identified stimulatory amino acids and de scribed responses to these compounds in marine, fresh water, and semi-terrestrial species (Carter and Steele, Received 11 January 1988; accepted 25 March 1988. 1Present address: Section of Neurobiology and Behavior, Division of Biological Sciences, Cornell University, Ithaca, NY 14850. 2To whom reprint requests should be addressed. 1982; Zimmer-Faust et ai, 1984; Johnson and Atema, 1986; Tierney and Atema, 1988; review: Ache, 1982). These studies have generally supported the idea that amino acid chemoreception is related to the detection of food. Electrophysiological studies, particularly with lob sters and crayfish, have provided additional information on the location, sensitivity, and specificity of single amino acid receptors. Two significant properties of lob ster chemoreceptors have been identified. First, unlike most chemoreceptors that function in feeding behavior in vertebrates and insects, lobster cells are typically nar rowly tuned. Many receptors respond strongly to only one or a few compounds (Derby and Atema, 1982b; Johnson and Atema, 1983; Johnson et ai, 1984). Sec ond, most crustacean chemoreceptors display mixture interactions in which responses to a single compound are reduced or enhanced when this compound is presented in a mixture (Johnson and Atema, 1983; Johnson et ai, 1985; Gleeson and Ache, 1985). The discovery of these properties has generated new ideas about how chemore- ceptor systems may resolve stimulus quality (Atema et al, 1988; Derby and Atema, 1988). Electrophysiological studies of crustacean chemore ception have primarily focused on cells in the lateral antennular filament (Shepheard, 1974; Johnson and Ache, 1978; Johnson and Atema, 1983; Derby and Ache, 1984a; Gleeson and Ache, 1985; Johnson et ai, 1985; Spencer, 1986) and in the walking legs (Bauer et ai, 1981; Derby and Atema, 1982a, b; Johnson # ai, 1984; Hatt, 1984). However, other structures, including the medial antennular filament, the antennae, and the mouthparts are also chemosensory (Ache, 1982; Derby and Atema, 1982c). Little is known about the specificity and sensitivity of chemoreceptors in these latter append- 364