CHEMORECEPTION 151 



reaction being involved. It was proposed tentatively (Dethier, 1956) 

 that stimulation by sugars involves combination of the sugar mole- 

 cules with a specific receptor site or substance by weak forces, such as 

 van der Waal, to form a complex which depolarizes the membrane, 

 after which (or simultaneously) sugar is removed passively by a shift 

 in concentration gradient. 



At this point current knowledge of the sugar receptor ends. Few 

 electrophysiological investigations of stimulation by carbohydrates 

 have been reported thus far. Hodgson (1957) examined the effects of 

 twenty-three carbohydrates. At that time the existence of a water 

 fibre was not suspected, and the lack of this information complicated 

 the interpretation of records. It appears, nevertheless, that all accept- 

 able sugars stimulate one neuron, while unacceptable sugars either 

 fail to stimulate or actually inhibit. 



In Vanessa indica the sugar receptor responds to i)-glucose, D- 

 fructose, and sucrose in increasing order of effectiveness. Lactose, 

 Z)-galactose, inositol, trehalose, and L-sorbitol are non-stimulating 

 (Takeda, 1961). Maltose and turanose elicit small deformed spikes 

 which have been interpreted by Takeda as indicative of a combined 

 stimulatory and inhibitory action. The sugar receptor is sensitive to 

 concentrations of sucrose as low as 2-'^ M. In addition to this receptor 

 there is one that responds in identical fashion to sucrose and to 

 NaCl. 



The Salt Receptor 



It was originally believed that rejection of a solution was triggered 

 solely by activity in one neuron, and since so many compounds elicited 

 rejection, it was assumed that the single neuron was grossly non- 

 specific. Studies on competitive inhibition and on the action of heavy 

 metal salts, however, hinted that rejection was not a single modality. 

 As a consequence, it was no longer possible to assume that every 

 compound that was rejected acted on the same receptor cell. 



Electrophysiological studies, while only just beginning, have par- 

 tially answered the question of the adequacy of stimuH for rejection. 

 They indicate that many rejected compounds block activity in all fibres 

 of the chemoreceptive hairs, but that monovalent salts stimulate one 

 cell actively (Hodgson, 1956 a; Morita, 1959). It has been possible to 

 come to some understanding of the mechanism of action of this salt 

 fibre by acquiring enough data to make the same kind of theoretical 

 analysis which Beidler (1954) made of mammalian salt receptors 

 (Evans and Mellon, 1962 b). 



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