CARDIAC INHIBITION IN DFCAPOD CRUSTACEA 



173 



INHIBITORY TRANSMITTERS 



Although inhibitor action is probably mediated through some chemical 

 transmitter, little is known of such a substance. Inhibitor agents have been 

 reported in the blood of darkened shrimp, Paiatya (Hara, 1952), and 

 Alexandrowicz and Carlisle (1953) find extracts of Maia pericardial organs 

 which may depress heart rate. Florey (personal communication) reports 

 that extracts of the second superior nerve (the inhibitor) of crayfish stop 

 cardiac ganglion activity. Unfortunately, none of these materials has been 

 apphed to the isolated cardiac ganghon. 



A-Aminobutyric acid (GABA) closely mimics the action of inhibitor fibers 

 in the isolated Homanis ganglion (Maynard, 1958). It not only depresses 

 burst frequency, but also reduces the number of impulses per burst by 

 cutting off the terminal discharge without drastically altering the initial 

 spike frequency or pattern (Fig. 26). When applied separately to followers 

 or pacemakers, GABA behaves as would be predicted were it the true 

 transmitter (Figs. 27. 28), and permits some direct check on earlier inferences 



GABA 



^ V^l^— 



Fig. 26. Eflfect of GABA on burst pattern, applied to entire isolated ganglion 

 {Homanis). A. Normal burst, b. Burst after 10^^ m GABA, burst frequency 

 halved, c. Burst in potassium-deficient perfusion fluid to show dilTerences 

 between low K+ and GABA effects. Upper trace of each record from posterior 

 electrode pair (/?) emphasizing pacemaker activity; lower trace from anterior 

 electrode pair (a) emphasizing follower activity. Time line, 0-1 sec, spikes 



retouched. 



