GABAergic inhibition of crayfish deep extensor abdominal muscle exhibits a steep dose-response relationship and a high degree of cooperativity

Modulatory effects of gamma-hydroxybutyric acid on a GABA(A) receptor from crayfish muscle

The effects of gamma-hydroxybutyric acid (GHB) were evaluated with a gamma-aminobutyric acid (GABA) activated Cl- channel on crayfish deep extensor abdominal muscle. GABA and GHB were applied to outside-out patches using a fast application system. Application of GHB up to 10 mM did not result in detectable activation of the channel. After coapplication of GABA and GHB, a dose-dependent potentiation of the GABA-elicited current by GHB was observed. The maximal effect was obtained with 0.5-1 mM GHB, with which the amplitude was enhanced by about 50% with 0.4 or 1 mM GABA. Simultaneously with the potentiating effect, a decrease of the rise times was seen. Preapplication of GHB, prior to the GABA pulses, resulted in a reduction of the current amplitude elicited by GABA. This block was persistent throughout the application time of GABA. Therefore, two contrasting effects of GHB on this chloride channel, a potentiating one and a blocking one, seemed to occur simultaneously.

Block by picrotoxin of a GABAergic chloride channel expressed on crayfish muscle after axotomy

Outside-out patches containing a gamma-aminobutyric acid (GABA)-activated chloride channel expressed after axotomy on crayfish deep extensor abdominal muscle were excised. GABA and the blocker picrotoxin (PTX) were applied to the patches using a liquid filament switch to study the effects of picrotoxin on the GABA-elicited currents. Coapplication of GABA and PTX resulted in a reduction of the current amplitude compared with that elicited by the same GABA concentration alone. This reduction of the amplitude was dependent on both the GABA and PTX concentrations. The rise time of the current decreased after coapplication of GABA and PTX. Evaluation of the single channel currents and off-currents in the presence of GABA and PTX showed a dramatic shortening of the burst duration of the channel. The open time distributions were not altered, whereas in the closed time distributions a new closed time was apparent in presence of PTX. Preincubation with PTX prior to the GABA pulse resulted in an increase of the rise time. This effect was dependent on the PTX concentration only. Possible mechanisms are discussed to explain the effects of PTX and are implemented into the existing molecular reaction scheme of the channel.

Neuromuscular glutamatergic and GABAergic channels

Our laboratory has worked extensively on glutamatergic and GABA-ergic channels, predominantly in crayfish, but also in locust, Drosophila and recently Ascaris. Channel currents were recorded in the different modes of the patch-clamp technique (Hamill et al., 1981). The opening kinetics of the channels were derived from open and closed time histograms obtained from single channel recordings. From these, channel conductances could also be evaluated. The most relevant data were obtained by very rapidly rising and falling pulses (time of change about 0.1 ms) of agonists applied to outside-out patches containing the respective channels (Franke et al., 1987). From such recordings we constructed dose-response curves for peak and steady-state currents, for the rise times of the currents and for the time constants of desensitization. In double-pulse experiments we measured recovery from desensitization and predesensitization due to low agonist concentrations. For most of the channel types, we succeeded in constructing a reaction scheme which in computer simulations mimicked channel behaviour to a good approximation.

Desensitization and resensitization kinetics of glutamate receptor channels from Drosophila larval muscle

Outside-out patches from wild-type Drosophila larval muscle were exposed to L-glutamate (glu) using a piezo-driven application system. Glu receptor channels opened and desensitized in response to rapid applications of 10 mM glu. Desensitization was fitted with an exponential function with a mean time constant of desensitization (tau d) of 15 ms in response to 10 mM glu. The tau d was concentration dependent and decreased to 6 ms (on average) with 0.7 mM glu and increased again to 12 ms (on average) in response to 0.5 mM glu. Desensitization in response to longer applications of glu was almost complete, but surprisingly, even a 1-ms pulse of 3 mM glu produced about 30% desensitization. In the presence of low glu concentrations, the response to a pulse was reduced and was about halved by preequilibration with 30 microM glu. Recovery from desensitization was not concentration dependent and was fitted with an exponential function with a mean time constant of 150 ms. During recovery the channels rarely opened. Kinetic schemes were fitted to these results, and a circular reaction scheme was found to fit the data best. An important feature of the scheme is desensitization from a lower ligated closed state. This allows substantial desensitization of synaptic receptor channels in response to quantal release of transmitter, in part without opening of the channels. Desensitization reduces the probability of the channels opening in response to a subsequent release for a period of time determined by the rate of recovery from desensitization and might serve as a form of molecular short-term memory.

Kinetics of homomeric GluR6 glutamate receptor channels

We studied the kinetics of the unedited version of rat GluR6 glutamate (glu) receptor channels, GluR6Q, in outside-out patches using a system for submillisecond solution exchange. Half-maximum activation of the channels was reached with approximately 0.5 microM glu. The maximum slope of the double-logarithmic plot of the peak current versus glu was approximately 1.3, indicating that at least two binding steps are necessary to open the channels. Currents in response to a pulse of 10 microM glu had a short rise time (10-90% of peak current) of approximately 220 microseconds at approximately 20 degrees C. The rise time increased with falling glu concentration, reaching approximately 6.0 ms with 10 microM glu. In the continued presence of glu, the channels desensitized, and this desensitization can be described with a single time constant of approximately 7.0 ms for a pulse of 10 microM glu. The steady-state current in response to a long pulse of 10 microM glu was below 1/280th of the peak current. The time constant of desensitization was found to be independent of concentration between 30.0 and 0.3 microM glu, but to be increased for lower concentrations. After a short pulse of 1 ms duration and 10 or 0.3 microM glu, currents decayed with a time constant of approximately 2.5 ms. Recovery from desensitization after a pulse took approximately 5 s, and the half-time of recovery was approximately 2.2 s. Continuous application of low concentrations of glutamate reduced the peak currents in response to a pulse of 10 microM glu markedly. Fifty percent response reduction was observed in the continuous presence of approximately 0.3 microM glu. Our results for homomeric GluR6 agree with a cyclical reaction scheme developed for completely desensitizing, glu-activated channels on crayfish muscles.

Activation kinetics of glutamate receptor channels from wild-type Drosophila muscle

Outside-out patches from wild-type Drosophila larval muscle were exposed briefly to L-Glutamate (Glu) using a piezo-driven application system. Glu in concentrations of 0.1 to 30 mM was applied and the responses to repeated applications of a given concentration were averaged. The peak current, î, and the current rise time, tr, from 0.1 î to 0.9 î were determined from the averages. Half-maximum activation of the channels was reached with approximately 2 mM Glu. î increased proportional to the power n = 3. 5 to n = 5.8 (average of four experiments, n = 4.4) for Glu concentrations between 0.3 and 0.5 mM. tr increased from approximately 0.2 ms at 10 mM Glu to a value of approximately 3.5 ms at 0.2 mM Glu. A linear reaction scheme with five binding steps preceding the channel-opening conformational change is proposed as the kinetic mechanism of channel activation and investigated in computer simulations. A set of rate constants assuming the same affinity for each binding site is found to describe the data better than one assuming positive cooperativity. The results are very similar to those for Glu-gated channels of crayfish and locust muscle, which is evidence for a common kinetic mechanism of these channels.

Characterization and molecular reaction scheme of a chloride channel expressed after axotomy in crayfish

The nerve to the deep extensor abdominal muscle (DEAM) in crayfish species Astacus astacus, containing four excitatory and one inhibitory motor axons, was cut in the third segment on one side of the animal. The distal axon stump was not subject to phagocytosis but was present for months after the axotomy. The two lateral bundles of the DEAM were prepared 4-6 weeks after the axotomy. The gamma-aminobutyric-acid-(GABA-) activated chloride channel of these bundles was characterized by applying pulses of GABA to outside-out patches of the muscle membrane and measuring the responses. Based on the dose/response relationship of the peak current and of the rise time as well as on single-channel kinetics, a detailed molecular scheme for the reaction of the channel with GABA was derived. This scheme contains four binding steps of the agonist to the receptor and two open states. Simulations of the dose/response relationships with this model resulted in a set of rate constants which generate proper fits. In comparison to the channels present in innervated muscles, the channels of denervated muscles have a higher affinity for GABA, a lower single-channel conductance, four versus five binding steps, and non-cooperative binding. The first three of these adaptations of denervated muscles correspond to similar changes in denervated vertebrate muscles.

A molecular scheme for the reaction between gamma-aminobutyric acid and the most abundant chloride channel on crayfish deep extensor abdominal muscle

Single-channel measurements were performed with the aim of constructing a detailed molecular scheme for the reaction between gamma-aminobutyric acid (GABA) and a chloride channel of crayfish deep extensor abdominal muscle (DEAM). GABA was applied in pulses to outside-out patches of muscle membrane, and, based on the dose-response of the peak currents and of their rise times, a linear model with five binding steps has been proposed. Evaluation of the single-channel kinetics indicated at least three open states. Two of them originate most probably from the fully liganded receptor state and are grouped in mixed bursts due to their different life times. The third one appears independently, outside the bursts, and originates from a lower liganded receptor state. Simulations of the dose-responses and the open time distributions with this model led to a set of rate constants which generated relatively optimal fits.