SUMMARY1. The inhibitory neuromuscular junction of the abductor muscle of the large claw of the hermit crab (Eupagurus bernhardus) was investigated using electrophysiological intracellular techniques in order to elucidate further the relative contributions ofthe pre-and post-synaptic mechanisms of action of GABA and of neural inhibition.2. The electrical constants of the post-synaptic membrane, calculated using the equations for a 'short cable' model, were characteristic of a poorly developed electrical excitability; the specific membrane resistance was usually < 1000 Q cm2 and the specific membrane capacitance was > 40 #F/cm2.3. Stimulation of the excitatory axon to the abductor muscle of the large claw at a frequency of 20 Hz evoked highly facilitating excitatory junction potentials (e.j.p.s); stimulation of the inhibitory axon (60-220 Hz) during the excitatory train elicited inhibition which was manifest as an attenuation of the e.j.p.s.4. The addition of y-aminobutyric acid (GABA) to the bathing solution produced a dose-dependent reduction of e.j.p. amplitude and membrane resistance. The inhibitory effect of concentrations (5 x 10-5 and 1 x 104 M) which caused a 40-75 % e.j.p. attenuation could largely be accounted for by a post-synaptic action on membrane conductance.5. Experiments with picrotoxin suggest that presynaptic inhibitory mechanisms have an important role in neurally evoked inhibition.6. Picrotoxin (1-5 x 10-5 M) effectively blocked neural inhibition and the actions of GABA in this preparation, whereas bicuculline proved to be considerably less potent and therefore less useful as a physiological tool for studying GABA-mediated inhibition in crustacea.
1The effects of y-aminobutyric acid (GABA), 3-guanidinopropionic acid (,BGP) and picrotoxin on the pre-and post-synaptic receptors of the hermit crab neuromuscular junction were studied quantitatively using electrophysiological techniques. Reductions in excitatory junction potential (e.j.p.) amplitude and membrane resistance were measured simultaneously from the same cells. 2 The pre-and post-synaptic receptors were activated by the same order of concentration of GABA, whereas tGP stimulated the pre-synaptic receptors at a concentration ten times lower than was required to affect the post-synaptic membrane. 3 Picrotoxin appeared to antagonize the pre-synaptic action of j3GP in a competitive manner. The affinity constants (± s.e. mean) for picrotoxin 5 x 1O-6M and 2 x 1O-4M were 6.80 (± 0.46) x 1OM 1 and 6.42 (± 1.8) x 105M-1 respectively. 4 The effect of GABA on e.j.p. amplitude also appeared to be antagonized competitively by picrotoxin whereas the post-synaptic effect was antagonized in a non-competitive manner. 5 Possible differences in the nature of the pre-and post-synaptic receptors are discussed.
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