GABAERGIC INHIBITION OF CRAYFISH DEEP EXTENSOR ABDOMINAL MUSCLE EXHIBITS A STEEP DOSE-RESPONSE RELATIONSHIP AND A HIGH-DEGREE OF COOPERATIVITY

A patch-clamp study was done to characterize the recently found GABAergic (i.e. gamma-aminobutyric acid) inhibitory synaptic channels of crayfish deep extensor abdominal muscle. Outside-out patches were rapidly activated by GABA to measure the dose/response curves for the open probability of the channels, P-o, and the rise time, t(r), (time from P-o=0.1 to P-o=0.9). In some of the patches the GABA-activated currents decayed due to desensitization and such patches were not studied further. Rare channel openings were elicited with 0.1 mM GABA. The P-o at this low concentration of GABA was 0.0005 to 0.01. Application of 10 mM GABA was necessary to reach the maximal P-o of 0.9. The slope of the dose/response relationship in the double logarithmic plot was 5.4+/-1.1 (mean+/-SD; n=9) between 0.1 mM and 0.2 mM GABA. The plot of t(r) versus GABA concentration had a peculiar shape, recently found to be characteristic for positive cooperativity of the binding sites. t(r) increased from a minimum at 10 mM GABA with declining concentrations of GABA and reached a peak at 0.4 mM GABA. Below 0.4 mM GABA, t(r) decreased again. With 0.2 mM GABA t(r) was 0.40+/-0.1 (mean+/-SD; n=4) of the peak value measured at 0.4 mM GABA. Simulations were compared with the experimental results and a linear reaction scheme with five binding sites for GABA was established to describe the dose/response curves for P-o and t(r).