362 H. MCLENNAN 



Fig. 6. Effect of y-aminobutyric acid on the inhibitory potential evoked in a 

 crayfish stretch receptor neurone — a before, b after 5 x 10"^ mole/1 GABA, 

 c after recovery. Calibration, 10 msec and 100 //V. Note that the amplitude and 

 half-time of decay of the potential are reduced in the presence of GABA. (Kuffler 

 and Edwards, 1958.) (Courtesy Dr. S. W. Kuffler and the Journal of Neuro- 

 physiology.) 



that such is not in fact the case. It is worth noting that other related com- 

 pounds have effects similar to those which have been described for GABA. 

 These include guanidinoacetic acid, /8-guanidinopropionic acid, /S-alanine, 

 S-aminovaleric acid, jS-guanidinobutyric acid and agmatine (y-guanidino- 

 butylamine), although all but the first two are very much less potent than is 

 GABA (Edwards and Kuffler, 1959). As in the case of the crayfish heart, 

 picrotoxin prevents the actions of Factor I, GABA and inhibitory nerve 

 stimulation. 



CRUSTACEAN NEUROMUSCULAR TRANSMISSION 



The electrical events associated with stimulation of a peripheral axon whose 

 action is to inhibit the contraction of a crustacean muscle have been studied 

 by a number of workers. Some of the inhibitory processes show similarities 

 to those discussed above for other structures, while others apparently differ. 



Marmont and Wiersma (1938) and Kuffler and Katz (1946) demonstrated 

 that inhibition of contraction could be accompanied by a reduction in amph- 

 tude of the excitatory junctional potentials evoked by motor nerve stimulation 

 ("supplemented", or a-inhibition); or that there could be inhibition without 



