Effects of glycine on the crayfish neuromuscular junction. I. Glycine-operated inhibitory postsynaptic channels and a glycine-effected decrease in membrane conductance

Modification of chloride flux across brain membranes by inhibitory amino acids in developing and adult mice

The influx of 36Cl- was studied in membrane vesicles prepared from different brain regions from 3-day-old and adult mice. In both age groups the influx was enhanced about threefold by gamma-aminobutyric acid (GABA), which effect was blocked by bicuculline and picrotoxin but not by baclofen, characteristic of a GABAA receptor-mediated event. In samples from the adult brain stem the GABA stimulation was smaller than in samples from the other brain regions. Most of the compounds studied apparently act at the same receptor site with the following order of efficacy: muscimol greater than GABA greater than beta-alanine greater than hypotaurine greater than taurine. A number of anticonvulsant taurine derivatives were not effective and glycine only in the brain stem. The weak modulatory effects of taurine could be of significance in vivo since depolarizing stimuli release massive amounts of taurine in developing brain tissue.

Acetylcholine and GABA mediate opposing actions on neuronal chloride channels in crayfish

A central principle of neural integration is that excitatory and inhibitory neurotransmitters effect the opening of distinct classes of membrane ionic channels and that integration consists of the summation of the opposing ionic currents on the postsynaptic membrane. In tangential cells of crayfish optic lobes, a hyperpolarizing, biphasic synaptic potential is produced by the concurrent action of acetylcholine and gamma aminobutyric acid (GABA). Acetylcholine hyperpolarizes the cell and increases chlorine conductance. GABA depolarizes the cell by closing some of the same chloride channels. Therefore, in this case integration is achieved by the antagonistic actions of two transmitters on the same ionic channel.

Excitatory transmitter release induced by high concentrations of gamma-aminobutyric acid (GABA) in crayfish neuromuscular junctions

At the neuromuscular junction of very small crayfish (0.4-2 g) addition of gamma-aminobutyric acid (GABA) to the superfusing solution at concentrations exceeding 100 mmol/l elicited high frequency release of excitatory transmitter quanta. In seven experiments single application of 500 mmol/l GABA gave rise to instantaneous release of 70,000 to 130,000 quanta. These stores of transmitter were released by GABA in a first order process with time constants, tau q, of between 9 s and 20 s, the maximum rate of release, ñ0, reaching 10,000 quanta/s in some cases. After release had ceased in the presence of GABA, the preparation was allowed to recover for five minutes in normal solution. Subsequently, a second trial evoked about 50% of the release induced during the first application of GABA. Pretreatment of the preparation with 2 mumol/l serotonin (5-HT) facilitated GABA-induced transmitter release resulting in larger rates of release and consequently in a larger output of transmitter by a factor of about 3. The largest amount of transmitter released on a single application of GABA in the presence of serotonin comprised about 220,000 quanta with a maximum rate of release ñ0 approximately equal to 25,000 quanta/s. The release evoked by high GABA-concentrations did not depend markedly on extracellular Ca2+ or Mg2+, but required extracellular Na+. The effects induced by high concentrations of GABA on release of excitatory transmitter quanta were quantitatively similar to the effects of high glycine-concentrations on release of quanta from the inhibitory terminals (Finger 1983a, b).

Giant inhibitory miniature currents in crayfish muscle in the presence and absence of extracellular sodium and serotonin

At opener muscles of the claw, or first walking leg of small crayfish, giant spontaneous inhibitory postsynaptic currents (gsIPSCs) were recorded. In some experiments, the rate by which they occurred could be enhanced by application of 1 mumol/l serotonin (5-HT). The largest gsIPSCs seen were at least up to ten-fold larger than normal inhibitory miniature currents. Picrotoxin (10 mumol/l) reversibly abolished the gsIPSCs. Tetrodotoxin (0.1 mumol/l) or withdrawal of Na+ from the superfusion did not abolish gsIPSCs. Decay time constants of gsIPSCs, tau(gsIPSCs), were about two-fold larger than those of nerve evoked IPSCs and of normal inhibitory miniature currents. In Na+-free superfusion tau(gsIPSCs) was larger by a factor of about 1.8 than in normal superfusion, which might be a result of inhibition of a Na+-dependent transport process for inhibitory transmitter by removal of Na+ from the incubation medium.

Inhibitory synaptic currents in voltage-clamped locust muscle fibres desensitized to their excitatory transmitter

Inhibitory junctional currents (i.j.c.s) have been examined in locust muscle fibres to give properties of GABA-channels activated by the neurally released transmitter. A nerve-muscle preparation is described which has proved suitable for voltage-clamp analysis of inhibitory transmission. I.j.c.s were recorded from fibres in which excitatory synapses had been desensitized with glutamate, to abolish excitatory junctional currents. This procedure had no apparent effect on inhibitory channel properties. The time constant of decay of the i.j.c. was 7.7 +/- 0.3 ms, slightly exceeding the time constant of the membrane noise induced by externally applied GABA. Peak i.j.c. conductance decreased with hyperpolarization. I.j.c.s showed measurable fluctuations permitting an estimate of the mean size of the quantal events composing the i.j.c. Their mean size coincided with the spontaneously occurring miniature inhibitory junctional currents that could be directly recorded in some fibres. The inhibitory nerve-impulse released an average of 35 transmitter packets at sites distributed along the muscle fibre length. Since each m.i.j.c. produced a current of about 0.6 nA (at Vm = -80 mV, ECl = -40 mV) the single quantum of inhibitory transmitter opens 600-1000 postsynaptic chloride channels. This is roughly three to four times the number of channels opened by the excitatory transmitter packet at glutamate synapses in the same fibres.

Effects of glycine on the crayfish neuromuscular junction. II. Release of inhibitory transmitter activated by glycine

Glycine applied in the bathing medium at concentrations exceeding 0.1 mol/l elicited high rates of spontaneous inhibitory postsynaptic currents (sIPSCs) in crayfish neuromuscular junctions. This effect of glycine was reversible within seconds. In several experiments on application of 0.5 mol/l glycine the rate of sIPSCs immediately increased to about 10 kHz and thereafter declined exponentially with time constants of between 10 and 20 s. This resulted in a release of about 140,000-200,000 inhibitory quanta per trial. When the readily releasable pool of transmitter had been so depleted by glycine, it was necessary to superfuse the preparation with normal solution for 5-10 min in order to be able to again evoke a high rate of sIPSCs. A similar effect of glycine on spontaneous release was also observed in some preparations which had been previously bathed in zero Ca2+ solution for up to 45 min. Addition of 25 mmol/l Mg2+ to the bathing fluid did not block the glycine evoked release of transmitter. However, in sodium-free superfusions the increase in the rate of sIPSCs induced by glycine was reduced. In the presence of 0.5 mol/l glycine no excitatory miniature currents (sEPSCs) were observed, in fact, glycine depressed excitatory synaptic transmission. In addition to the increasing the rate of sIPSCs, high concentrations of glycine evoked 'giant' sIPSCs (gsIPSCs). They were about 10-15 times larger than the normal sIPSCs and occurred at rates lower than 3 Hz, irrespective of whether the bathing medium contained sodium or not. However, in sodium-free superfusions the time constants of the decay of gsIPSCs were prolonged by a factor 2-3. These results suggest that glycine elicited sIPSCs and gsIPSCs by different mechanisms. Possible mechanisms which might explain the effects of glycine on release of inhibitory transmitter are discussed.

Glutamate activated postsynaptic channels in crayfish muscle investigated by noise analysis

Excitatory synaptic channels in crayfish muscle were investigated under various experimental conditions. Small muscle fibres of length l less than or equal to 0.6 mm were voltage clamped, spatial control of the voltage being sufficient up to at least 500 Hz. Excitatory synaptic current was induced by superfusion of glutamate. The power density spectra of this current could be fitted by single component Lorentz curves. The analysis revealed a mean open time tau noise = 0.93 ms and a conductance gamma = 32.3 pS of the glutamate operated ion channels (membrane potential E = -60 mV, temperature T = 8 degrees C). Both the conductance gamma and the channel closing rate alpha = tau -1 noise increased significantly with temperature (Q10 approximately 2). The temperature dependence of gamma and alpha could be described by Arrhenius equations with the temperature independent activation energies E gamma = 42.3 kJ/mol and E alpha = 50.2 kJ/mol. alpha also dependent on the membrane potential, increasing about e-fold when the membrane was hyperpolarized by 120 mV. The potential dependence varied considerably from fibre to fibre. The mean channel open time tau noise agreed with the time constant of decay tau (sEPSC) of spontaneous excitatory postsynaptic currents (sEPSCs).