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

Effects of ectoine on behavioural, physiological and biochemical parameters of Daphnia magna

Ectoine (ECT) is a compatible solute produced by soil, marine and freshwater bacteria in response to stressful factors. The purpose of our study was to determine the possible toxic influence of ECT on Daphnia magna. We determined the following endpoints: survival rate during exposure and recovery, swimming performance, heart rate, thoracic limb movement determined by image analysis, haemoglobin level by ELISA assay, catalase and nitric oxide species (NOx) by spectrophotometric methods. The results showed 80% survival of daphnids exposed to 50mg/L of ECT after 24h and 10% after 90h, however lower concentrations of ECT were well tolerated. A concentration-dependent reduction of swimming velocity was noted at 24 and 48h of the exposure. ECT (at 2.5 and 4mg/L) induced an increase of heart rate and thoracic limb movement (at 2.5, 4 and 20mg/L) after 24h. After 10h of the exposure to ECT daphnids showed a concentration-dependent increase of haemoglobin level synthesized and accumulated in the epipodite epithelia. After 24h we noted a concentration-dependent decrease of haemoglobin level and its lowest value was found after 48h of the exposure. ECT at a concentration of 20 and 25mg/L slightly stimulated catalase activity after 24h. NOx level was also increased after 10h of the exposure to 20 and 25mg/L of ECT reaching maximal activity after 24h. Our results suggest that ECT possesses some modulatory potential on the behaviour, physiology and biochemical parameters in daphnids.

Quantal stores of excitatory transmitter in nerve-muscle synapses of crayfish evaluated from high-frequency asynchronous quantal release induced by veratridine or high concentrations of potassium

At single voltage-clamped opener muscle fibres of crayfish claw, 10-100 mumol/l veratridine increased within a few seconds the rate of asynchronous quantal release, ñ, of excitatory transmitter from ñ less than 1 quantum/s to ñ congruent to 10,000 quanta/s. Thereafter ñ declined exponentially either with a single, tau(2) congruent to 50 s, or with two time constants tau(1) congruent to 19 s, tau(2) congruent to 50 s. In total (t----infinity), about 0.3 million quanta were released by veratridine in a single short fibre of about 1 mm length. These values were estimated by means of the noise analysis technique and they agreed with equivalent parameters of release when 100 mmol/l K+ were used as release stimulus. Strong quantal release could be elicited only once in a single muscle by veratridine. Furthermore, the effect of veratridine on quantal release could be completely prevented by pretreatment with tetrodotoxin. In another nerve-muscle preparation of crayfish, the abdominal superficial extensor muscle, up to 3 million excitatory quanta could be released by veratridine in a single fibre. In the latter muscle veratridine-induced asynchronous quantal release was strongly dependent on the extracellular concentration of Ca2+ whereas in the claw opener dependence of quantal release on extracellular Ca2+ was negligible.

Effect of lithium on veratridine-induced quantal and non-quantal release from inhibitory nerve terminals in crayfish muscle

Muscle fibres of small crayfish were voltage clamped and superfused for about 10 min with Li+ saline (Na+ replaced by Li+) which contained 5 mmol/l glutamate to desensitize excitatory postsynaptic receptors. Then 100 mumol/l veratridine were added to the superfusate which caused strong asynchronous quantal release of inhibitory transmitter. However, in the presence of Li+ strong inhibitory quantal release was only transient. It could be activated a second time by removal of Li+ and readministration of Na+. From the total of 0.7 to 1.1 million quanta released by veratridine only about 30-35% could be released in Li+ saline. The voltage clamp DC-currents recorded during veratridine-induced quantal release suggested that a non-quantal release component is additionally involved. This non-quantal release component was most prominent during the period of quantal release in Li+ superfusate while it was less obvious during the second enhancement of quantal release in normal saline. Together with previous results (Martin and Finger 1988) it may be concluded that quantal release, but not non-quantal release, is decreased by Li+ in the nerve terminals.

Veratridine-induced high-frequency asynchronous release of inhibitory transmitter quanta in crayfish nerve-muscle synapses superfused with normal and low-calcium saline

Crayfish fibres of opener muscles were voltage clamped to E = -80 mV membrane potential (T = 19-22 degrees C), and veratridine (10-100 mumol/l) was added to the superfusate. Within 30-60 s this caused large fluctuations of the clamp current due to vigorous asynchronous quantal release from the inhibitory nerve terminals along the muscle fibre. Excitatory postsynaptic receptors were previously desensitized by application of 5 mmol/l glutamate. Current fluctuations were evaluated by means of the noise analysis technique. Typically, 100 mumol/l veratridine increased instantaneously the quantal release rate n from n less than 1 quantum/s to n congruent to 10,000 quanta/s. Thereafter, n declined exponentially with a time constant of congruent to 70 s. On average, about 500,000 inhibitory quanta could be liberated in this way from the terminals on a single muscle fibre of congruent to 1 mm length. Serotonin (1 mumol/l) facilitated the effect of lower veratridine concentrations (1-10 mumol/l). In opener muscles veratridine-induced asynchronous quantal release showed little dependence on the bath concentration of Ca2+. The opposite was found for fibres of the superficial abdominal extensor muscle. Beside postsynaptic current fluctuations, veratridine elicited slowly changing average postsynaptic DC-currents which could be explained partly by superposition of individual inhibitory quantal currents. These DC-currents suggest that beside inhibitory quantal release another factor activates inhibitory postsynaptic receptors after application of veratridine.

Inhibitory effect of intraterminal lithium on asynchronous release of excitatory quanta induced by veratridine in nerve-muscle synapses of crayfish

Crayfish muscle fibres were voltage-clamped at E = -80 mV membrane potential and superfused for about 10 min with Li+ saline (Na+ replaced by Li+) which contained picrotoxin to block inhibitory post-synaptic currents. Addition of veratridine (100 mumol/l) caused intense fluctuations in the voltage clamp current within 20-60 s due to vigorous asynchronous quantal release of excitatory transmitter from the nerve terminals distributed over the muscle fibre surface. Most likely, this quantal release resulted from loading the nerve terminals with Li+ via voltage-gated Na+ channels activated by veratridine. However, in the presence of Li+ quantal release was only transient; the quantal release rate, ñ, attained a maximum of congruent to 10,000 quanta/s and then declined exponentially with tau congruent to 10 to 20 s. Removal of Li+ and reapplication of normal Na+ increased ñ a second time. The amount of quanta released in the presence of Na+ was about an order of magnitude larger than that released previously in the presence of Li+. In preparations pretreated with Li+ superfusate for t greater than 45 min no marked quantal release could be elicited by veratridine. The experiments suggest an inhibitory effect of intraterminal Li+ on the quantal release process.

Differential effect of intraterminal sodium on spontaneous quantal release of transmitter in two neuromuscular junctions of crayfish

Nerve terminals on the superficial abdominal extensor muscle and the claw opener muscle of small crayfish were loaded with sodium by bath application of 100 mumol/l veratridine in superfusates where normal Ca2+ was removed (low-Ca2+ superfusate). In both preparations this caused an increase in spontaneous quantal release of excitatory and inhibitory transmitter which was evaluated by means of the noise analysis technique. About 2.5 min after application of veratridine, when spontaneous quantal release had largely ceased, the normal Ca2+ concentration was reestablished. This increased transiently the quantal release rate a second time. However, release activated by Ca2+ application was much more vigorous at the superficial abdominal extensor muscle than at the claw opener. At the superficial abdominal extensor muscle on average about 8% of the total number of quanta could be released in low Ca2+ and 92% in normal Ca2+ superfusate, while at the claw opener 75% of the quanta were released in low Ca2+ and 25% in normal Ca2+ superfusate. The experiments suggest that intraterminal sodium has a differential effect in the terminals of the two preparations. Possibly, the intraterminal source from which Na+ may liberate Ca2+ is more restricted in the superficial abdominal extensor muscle than in the opener muscle of the claw.

Miniature and evoked inhibitory junctional currents and gamma-aminobutyric acid-activated current noise in locust muscle fibres

gamma-Aminobutyric acid (GABA) current noise and inhibitory junctional currents (i.j.c.s) have been examined to give properties of the GABA receptor and its associated synaptic channel. Various procedures were used to identify muscle bundles receiving inhibitory innervation. In normal bathing medium the decay time constant of the i.j.c. was tau i.j.c. = 7.6 +/- 0.7 ms (clamp potential, Vm = -80 mV; temperature, T = 21 degrees C). Most muscle fibres were sensitive to ionophoretically applied GABA, irrespective of the presence of inhibitory innervation. GABA current noise obtained at junctional sites gave spectra which were fitted usually with a single Lorentzian component, or occasionally with the sum of two Lorentzians. The conductance of the single inhibitory channel was, gamma (GABA) = 21.6 +/- 0.9 pS (Vm = -80 mV; T = 21 degrees C). The mean 'burst length' of the openings produced by a single receptor activation was tau noise = 4.0 +/- 0.8 ms, at Vm = -80 mV. This decreased exponentially with hyperpolarization. On average tau i.j.c. exceeded tau noise although good agreement was found in some fibres. I.j.c.s were examined in greater detail after excitatory synaptic receptors had been desensitized with 10(-3) M-L-glutamate to abolish all excitatory synaptic activity. Their decay time constant was tau i.j.c. = 7.2 +/- 0.4 ms, and their rise time was 3.3 +/- 0.12 ms, at Vm = -80 mV. An e-fold decrease in tau i.j.c. resulted from a 103 +/- 7.9 mV hyperpolarization; time to peak showed a smaller dependence on Vm. The mean size of the inhibitory quantal event (i.e. response to a single transmitter packet) was estimated from fluctuations in i.j.c. amplitude. Mean quantal content of the i.j.c. was about 30 at normal levels of release. Mean amplitude of the directly measured miniature i.j.c. = 0.65 +/- 0.08 nA at Vm = -80 mV (V eq approximately equal to -40 mV). The amplitude of the quantal event showed a non-linear dependence on Vm. The burst length of the inhibitory channel, produced by a single receptor activation, is longer in duration (at -80 mV) and exhibits greater voltage dependence than the burst length of the excitatory glutamate-activated channel in these fibres. It is estimated that a single quantum of GABA opens about 600-1000 post-synaptic chloride channels.

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).

Tonic depolarization of excitatory nerve terminals in crayfish muscle by high concentrations of extracellular potassium

At voltage-clamped fibres of the claw opener muscle of small crayfish, spontaneous quantal release of excitatory transmitter elicited by raising extracellular K+ to 100 mM was investigated. On application of the high K+ concentration, the rates of quantal release increased to n = 10,000-25,000 quanta/s within 10 s, and thereafter declined exponentially, either with a single (tau congruent to 15-40 s) or with two (tau 1 congruent to 15-40 s, tau 2 greater than 70 s) time constants. The total number of quanta released per trial ranged from s = 200,000 to 800,000 quanta. The results were derived by means of the fluctuation analysis technique.

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.

High rates of excitatory miniature currents in crayfish claw opener muscle evoked by high concentrations of gamma-aminobutyric acid (GABA) in normal and Ca2+-deficient superfusions

High concentrations (0.5 mol/l) of the neutral amino acid GABA were used to evoke release of transmitter quanta from excitatory terminals at voltage clamped crayfish muscle fibres in normal and Ca2+-deficient superfusions. An experiment in which the release of transmitter quanta proceeded at high rates in both normal and Ca2+-deficient superfusion was analyzed in detail indicating a Ca2+-independent mechanism of release. In the normal superfusion, on application of GABA, the release rates ñ increased within a few seconds up to about 6000 quanta/s and thereafter declined exponentially with a time constant tau q = 18.5 s, most likely due to depletion of a readily releasable store of transmitter in the excitatory nerve terminals comprising at least 110,000 quanta per muscle fibre. Assuming that about 1900 excitatory synapses exist per muscle fibre [9], it results that about 58 quanta can be associated with each synapse in agreement with morphological data [15] which show that between 47-117 vesicles exist in a single glutamatergic synapse of crayfish.

Postsynaptic actions of ethanol and methanol in crayfish neuromuscular junctions

Actions of ethanol and methanol on excitatory postsynaptic channels activated by quisqualate were investigated in opener muscles from the first walking leg and the claw of crayfish. Both ethanol and methanol reduced the elementary currents [i] that flow through channels operated by quisqualate in a concentration-dependent manner but did not affect the apparent mean open time, tau noise, of the channels estimated from power spectra. 0.26 mol/l ethanol, or 1 mol/l methanol, respectively, reduced [i] e-fold. Ethanol also markedly decreased the size and the decay time constant tau (sEPSCs) of spontaneous excitatory postsynaptic currents (sEPSCs). At ten fibres, on the average, 0.26 mol/l ethanol decreased tau (sEPSCs) by a factor 1.56 +/- 0.24 (SD). tau (sIPSCs) and tau noise of inhibitory postsynaptic currents apparently were not affected by ethanol. Moreover the size of elementary inhibitory postsynaptic currents did not decrease in the presence of this alcohol. Thus, in crayfish opener muscles ethanol seems to selectively depress excitatory postsynaptic currents.

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

Inhibitory postsynaptic membrane channels which are activated by glycine were investigated by means of the noise analysis technique. Dose-response curves were obtained for gamma-aminobutyric acid (GABA) in the presence and in the absence of glycine, and it was concluded that GABA and glycine are likely to activate the same receptors. However, glycine proved to have a very low affinity for the inhibitory postsynaptic receptors; this affinity was smaller than that of GABA by a factor of 1 . 10(3)-2 . 10(3). The mean open time tau of the postsynaptic Cl- channels activated by glycine at E = -100 mV and E = -60 mV membrane potentials were tau = 6.1 ms +/- 1.5 ms and tau = 17.7 ms +/- 2.2 ms, respectively. These values are in agreement with the tau obtained by activation with GABA (Dudel et al. 1980); however, on activation by glycine the potential dependence of tau was larger by a factor of 1.35. At E = -100 mV the conductance gamma of glycine-operated channels was about 3 pS which is a third of the respective conductance elicited by GABA. In the presence of high concentrations of glycine (0.1-0.5 mol/l) spontaneous inhibitory postsynaptic currents (sIPSCs) and 'giant' spontaneous inhibitory postsynaptic currents (gsIPSCs) were observed. Furthermore at high concentrations of glycine an additional glycine-induced noise component was found in the power spectra of current fluctuations at higher frequencies. It was concluded that this spectral component resulted from the closing of otherwise open K+ channels, as has been observed already on application of GABA (Dudel and Finger 1980). The mean duration of the low conductance state was tau- = 2.2 ms +/- 0.9 ms and the conductance decrease gamma- coupled to this process was estimated to be about 3 pS. In Na+ free- and Ca2+-enriched bathing solutions the glycine-induced conductances gamma and gamma- were reduced by a factor of about 1.7 while tau and tau- remained unchanged. The decrease in gamma and gamma- was most likely effected by the increase in concentration of divalent cations.