HOWARD M. LENHOFF 205 



the discovery of this phenomenon to two independent studies : one, 

 by Helen Park, who, while studying the effects of radiation on 

 Hydra, observed that the anti-radiation compound reduced gluta- 

 thione caused the Hydras mouth to open (20) ; the other by Loomis, 

 who, in a systematic search, identified reduced glutathione as the 

 substance present in crustacean extracts that activated the feeding 

 reflex in Hydra ( 17 ) . 



The significant aspects of this discovery are many. From an 

 evolutionary viewpoint, data on the distribution of the glutathione- 

 activated response has been used to deduce the sequence in geo- 

 logical time that the feeding mechanisms of some coelenterates 

 evolved (6, 15, 17). On the whole animal level, the feeding re- 

 sponse is an example of an elaborate behavioral pattern controlled 

 by a single environmental compound. At the cellular level, the glu- 

 tathione-activated feeding reflex is a clear example of chemorecep- 

 tion specific for only one molecule. 



This morning I would like to dwell on a fundamental subcellular 

 aspect: the mechanism by which glutathione combines with and 

 activates the glutathione-receptor. 



DESCRIPTION OF THE NORMAL FEEDING REFLEX 



All measurements are based on Hydra's characteristic feed- 

 ing movements, described earlier by Ewer ( 4 ) and Loomis ( 17 ) . 

 The drawings in Figure 1 illustrate each of these steps. A Hydra 

 in the absence of the glutathione has its mouth closed, and its tenta- 

 cles outstretched and relatively motionless. After the addition of 

 glutathione, the tentacles begin to writhe and sweep inwards to- 

 ward the longitudinal axis of the animal ( Fig. 1 A ) . Next, the tenta- 

 cles bend toward the mouth, and the mouth opens (Fig. IB). Shown 

 in this composite drawing (Fig. IB) are the various positions that a 

 tentacle takes before contracting. These movements, culminating in 

 mouth opening, usually all take place within half a minute. Figure 

 IC shows how a Hydra looks during the greater portion of the feed- 

 ing reflex, its mouth open wide and the tentacles in various phases 

 of contraction. Frequently, the tips of the tentacles are observed 

 within the Hydra's mouth, as shown in Figures IB and IC. 



