106 ERNST FLOREY AND GRAHAM HOYLE 



The action of GABA was therefore interpreted as activation of inhibitory 

 subsynaptic membrane and that of picrotoxin as inactivation of inhibitory 

 synapses. 



Lobster muscle is known to show marked a-inhibition. It appeared desirable 

 to have data comparable to those obtained by Grundfest et al., but from one 

 or more of the many crustaceans in which a-inhibition is very slight or un- 

 obtainable. This might help to resolve the question as to the importance of 

 the attenuation of excitatory junction potentials in achieving mechanical 

 inhibition. 



We have, therefore, examined the membrane changes produced in muscle 

 fibers of two species of crab which do not show much a-inhibition. Cancer 

 magister and Cancer productiis, both by natural excitatory and inhibitory 

 action and also by GABA and picrotoxin. 



METHODS 



The excitatory and inhibitory nerve fibers supplying the adductor muscle 

 of the dactylus of walking legs were isolated and stimulated through platinum 

 electrodes. The muscle was exposed by cutting away the carapace covering it. 

 The tendon of the adductor muscle was cut. Movements of the dactylus were 

 transduced by allowing it to press against a strain gauge. Routinely two 

 intracellular glass capillary microelectrodes were inserted into the same single 

 muscle fibers, one for recording potential changes and the other for passing 

 current. 



A saline medium, composed of 466 niM NaCl, 8 mM KCl. 10 mM CaCIo 

 and 12 mM MgCl2 was used throughout. The saline and also the drugs, in 

 the same medium, were introduced by perfusion through two hypodermic 

 needles inserted into the carpo-dactylopodite joint. This permitted rapid 

 flushing of all fibers of the test muscle. The preparations were maintained at 

 a temperature of 15-19°C. 



RESULTS 



As a preliminary we repeated the experiments of Grundfest et al. (1959) 

 on the lobster, Hoiuarus americanits (the animals were flown to Friday 

 Harbor from the east coast). Their results were confirmed: large decrease in 

 membrane resistance occurred following the application of GABA, together 

 with attenuation of the junctional potentials. 



With crab muscle fibers, GABA (10-* g/ml) produced a marked imme- 

 diate attenuation of the excitatory junctional potentials and eventually 

 obliterated them (Fig. 1); this action was associated with a progressive fall in 

 tension. The eff'ects were completely reversible by thorough washing with fresh 

 sahne medium and they could be repeated several times on the same prepara- 

 tion. In contrast, normal inhibitory transmitter action was not accompanied 



