256 CHEMICAL SENSES 



stimuli are highly effective in stimulating olfactory receptors of sharks— the 

 most studied examples. Consequently, at least several different types of 

 transduction mechanisms might be involved. These might involve specialized 

 receptor sites for transduction of the following chemical stimuli : 



1. Electrolytes 



2. Tertiary amines and /or amino acids 



3. Certain blood factors or their breakdown products: 



(a) hemoglobin 



(b) albumen fractions. 



There may be overlaps in this list (e.g., breakdown products of blood frac- 

 tions and amino acids), and the list is almost certainly incomplete. 



Now that the effectiveness of at least these types of compounds have been 

 shown through both EEG and behavioral analyses, it should be possible to 

 conduct more extensive comparisons of stimulus effectiveness, using more 

 examples from each category of stimuli and standardized electrophysio- 

 logical recording methods. For example, Hara (1976) has shown, by com- 

 paring the effects of many amino acids and their isomers on electrical re- 

 sponses of olfactory bulbs in rainbow trout, that it is possible to draw more 

 precise conclusions about the structure activity relationships of amino acids 

 in fish olfaction. Present results with sharks are compatible with Hara's con- 

 clusion that simple, short, straight-chained amino acids are effective chemical 

 stimulants, but nothing can yet be concluded about the effects of side chains 

 attached to these amino acids, the influence of various isomers, etc. Such 

 studies will be required to characterize the hypothetical receptor sites of the 

 olfactory chemoreceptors of elasmobranchs. 



Another basic question is whether differently specialized receptor mem- 

 branes may be present on different types of chemoreceptor cells within the 

 olfactory epithelium. Pyatkina (1974) has postulated that this may be so 

 for the three forms of receptor cells found in the olfactory epithelium of 

 the sturgeon, and Kleerekoper (1978) has discussed other similar examples. 



EXPERIMENTS WITH RADIOACTIVE STIMULI 



The exact sites of effects by various chemical stimuli and the fates of the 

 stimulating ions or molecules, once they arrive at the chemoreceptor mem- 

 branes, and matters of concern in all studies on chemoreceptors. How strong 

 is the link between stimulus and receptor site? How long is the link main- 

 tained? Is the stimulus metabolized by the receptor cell or dispersed by 

 some mechanism external to the receptor? These and similar questions are 

 beginning to be answered for some chemosensory systems by the use of 

 radioactively labeled chemical stimuli known to be highly effective in the 

 particular systems studied. 



Binding of tracer -labeled pheromones onto chemoreceptor sites of insects 

 has been recorded. A natural "rinsing" mechanism of some insect chemo- 

 receptors has also been noted following the movement of tritiated water 



