Response properties of lobster chemoreceptors : tuning of primary taste neurons in walking legs

1. We determined the tuning properties of 66 single chemoreceptor cells from the legs of the lobsterHomarus americanus. The specificity of these (taste) cells was tested electrophysiologically in the upper range of biologically relevant stimulus concentrations (3×10–4 mol/l) with 15 compounds listed in Table 1. 2. Most of the chemoreceptors surveyed were narrowly tuned to specific test chemicals (Fig. 3). The best stimuli for these chemoreceptors were glutamate (Glu), ammonium chloride (NH4), betaine (Bet) and hydroxyproline (OH-Pro) (Fig. 4). Cell populations were defined by their best stimulus. Populations of cells responding best to Glu and NH4 were extremely narrowly tuned; populations responding best to Bet and OH-Pro were less narrowly tuned (Fig. 5, Table 2). 3. Each cell population showed a range of response specificities, from cells responding to only one compound to cells responding more broadly to other compounds. Within each population cells were arranged by their tuning breadth (H). Cells that shared a best stimulus but responded also to other compounds showed no consistency for the next best stimulus (Fig. 6). This suggests that even cells responding to the same stimulus do not share the same receptor complement. 4. The response patterns to Glu, NH4 and Bet elicited across all the chemoreceptor cells surveyed can be distinguished statistically as different from each other and from all others tested. Similarly, OH-Pro can be distinguished from all other stimuli except glycine and taurine (Figs. 8 and 9). 5. Systematic prior exposure to all 15 test compounds (corresponding to 21 stimulus exposures) at our stimulus concentrations reduced the absolute sensitivity of Glu best cells but did not change the slope of their dose-response functions (Fig. 10) with respect to earlier results (Derby and Atema 1982a). 6. We suggest that extremely narrowly tuned cells in lobster legs may function to detect key compounds as feeding stimuli over a large dynamic range of mixture concentrations such as found in nature. These cells may not be affected as much as broadly tuned cells by ambient backgrounds of cross-adapting compounds. In addition, both narrowly tuned and broadly tuned cells may participate in across fiber patterns to encode the quality of naturally occurring stimulus mixtures.