CHEMORECEPTOR MECHANISMS 19 



might, by blocking emptying of the sites, inhibit repeated stimulation. Further- 

 more, the reaction might reasonably be expected to be temperature dependent, 

 and subjection of the receptor to low temperature would block continued stim- 

 ulation. 



It is known, however, that the pentose fucose, which has been shown by 

 Evans in this laboratory to be non-metabolizable by the fly, stimulates just as 

 efficiently as glucose, and, upon removal of the applied solution, stimulation 

 apparently ceases just as abruptly as in the case of glucose. Thus, a sugar like 

 fucose apparently can be removed passively, and there is no need to postulate 

 a separate mechanism for the removal of glucose. This conclusion would favor 

 equations (1) and/or (2) as correct representations of the state of affairs. Al- 

 though we are not at this time able to choose between equations (1) and (2), 

 the evidence presently at hand certainly can be satisfactorily explained by the 

 simpler hypothesis (1). Tentatively, then, it is proposed that stimulation by 

 sugars, at least in insects, involves combination of the sugar molecule with a 

 specific receptor substance or site by weak forces, such as van der Waals', to 

 form a complex which depolarizes the membrane, after which (or simul- 

 taneously) sugar is removed passively by a shift in concentration gradient. 



Mechanism of Action by Salts 



The case for the salt receptors of vertebrates is different in several respects. 

 Beidler (1954) has pointed out: (1) the reactions involved are in a time-inde- 

 pendent state and stimulation is probably in thermodynamic equilibrium, (2) 

 it is very rapid (approximately 50 msec), (3) it is completely reversible, (4) 

 cations and anions both enter the reaction although the magnitude of the re- 

 sponse is determined chiefly by the cations, (5) a saturation level is reached, 

 (6) receptors respond to many diverse substances. The first statement is based 

 upon the observations that the response to 0.1 M NaCl remained constant 

 during 10 minutes of continued stimulation and the magnitude of the response 

 remained the same regardless of preceding stimulation by either higher or lower 

 concentrations of salt. The second statement reveals a latent period shorter 

 than that measured in behavioral studies, but this is to be expected. In the fly, 

 for example, the time for a complete behavioral response is approximately 0.1 

 sec; this value is not out of line. New data reported by Hodgson and Roeder 

 (in press) show values of 5-10 msec. Evidence for action by both cations and 

 anions is seen in practically all studies made regardless of the species of animal. 

 There is indirect evidence that attainment of a saturation level as described for 

 the rat receptor occurs with the insect receptor as well (Dethier, 1955). Again, 

 new data of Hodgson and Roeder show a stimulus concentration at saturation 

 to be of the same order of magnitude (e.g. ca. 0.5 M NaCl) for both insects and 

 mammals. Attainment of saturation is well illustrated by the observation that 

 in balancing salt stimulation against sugar stimulation there is a concentration 



