Glutamate-operated postsynaptic channels and spontaneous excitatory postsynaptic currents in crayfish claw opener muscle

Prolonged time course of glutamate-operated single channel currents in neuromuscular preparations of small crayfish and a membrane current triggered by glutamate channel gating

Single channel currents activated by glutamate were recorded by means of the patch-clamp technique in the abdominal superficial extensor muscle and the claw opener muscle of small (1-3 months old) and large (greater than 16 months old) crayfish. It was found that in small crayfish the time course of glutamate-operated single channel currents was prolonged by a factor of about 4 in these two preparations. In the abdominal superficial extensor muscle, single channel currents activated by 5 mmol/l glutamate had a mean burst length of tau = 2-3 ms in large crayfish and a mean burst length of tau = 8-9 ms in small crayfish. In the claw opener, for large crayfish tau congruent to 0.5 ms and for small crayfish tau = 1.5-2.5 ms resulted (500 mumol/l glutamate). Moreover, single channel currents with long time courses often slowly increased their amplitudes during the open time of the channel and current amplitudes did not decline completely to the baseline after channel closing. In addition, single channel currents with relatively constant amplitude were often followed by a small increasing and decreasing membrane current. The latter results suggest that glutamate channel gating might trigger a membrane current.

Quisqualate-activated single channel currents in neuromuscular preparations of small and large crayfish

Single channel currents elicited by 1-5 mumol/l quisqualate in neuromuscular preparations in large (greater than 16 month old) and small (1-3 month old) crayfish were recorded by means of the patch-clamp technique. In preparations from large crayfish single channel currents of variable amplitude (-1 to -12 pA) were induced by quisqualate. The mean burst lengths of these currents were tau approximately equal to 1-2 ms. In the opener muscle of the first walking leg and the contractor epimeralis muscle of small crayfish the mean burst lengths of single channel currents evoked by quisqualate were prolonged by a factor of about 4 (tau approximately equal to 5 ms). Moreover, in the opener muscle of the first walking leg of small crayfish single channel currents of small amplitude (-0.5 to -2.5 pA) were preferentially evoked by quisqualate. By contrast, in the contractor epimeralis muscle of small crayfish mainly single channel currents of large amplitude (-10 to -12 pA) were elicited by quisqualate. The results suggest that at the stage of neuromuscular development characterizing the small crayfish, gating properties of excitatory postsynaptic channels are different from those in adult crayfish. Furthermore, the results obtained in the opener muscle of the first walking leg of small crayfish are consistent with those obtained previously by means of the noise analysis technique.

Spontaneous excitatory postsynaptic currents in crayfish neuromuscular junctions in the absence and presence of serotonin and 3,4-diaminopyridine

Spontaneous excitatory postsynaptic currents (sEPSCs) were recorded under voltage clamp in short fibres (l less than or equal to 0.6 mm) from opener muscles and the contractor epimeralis muscle of small crayfish. From the amplitude distributions of sEPSCs which could be approximated by a Gaussian function, a mean amplitude ã = -1.16 nA +/- 0.28 (SE) was found for sEPSCs in 16 fibres of the claw opener voltage clamped to E = -60 mV (19-22 degrees C). In the opener of the first walking leg and in the contractor epimeralis muscle ã = -1.1 nA +/- 0.21 (SE; n = 6, -100 mV less than or equal to E less than or equal to -60 mV, 5-10 degrees C) and ã = -2.0 nA +/- 0.2 (SE; n = 4, E = -60 mV, 19-22 degrees C) were obtained. On average about 300-500 synaptic channels were estimated to open during a sEPSC. 'Giant' sEPSCs (gsEPSCs) were also observed. The amplitudes of gsEPSCs were up to 14 times larger than the amplitude of an average normal sEPSC. Moreover, the lifetime of gsEPSCs was up to about 3 times longer than that of sEPSCs. Like sEPSCs, gsEPSCs could not be abolished by 0.1 mumol/l tetrodotoxin. The rate at which sEPSCs and gsEPSCs occurred could be markedly enhanced by serotonin (1 mumol/l) and 3,4-diaminopyridine (1 mmol/l).

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).

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.

Single synaptic channels recorded at glutamate sensitive patches on a crayfish muscle

When a patch clamp pipette filled with 50 mumol/l glutamate was placed on a muscle fiber of the deep abdominal extensor of crayfish, in some locations current pulses were recorded which were identified as synaptic, glutamate-operated ionic channel openings. At a given site all current pulses had approximately the same amplitude. At resting potential and 19 degrees C, their mean amplitudes were 7-8 pA, corresponding to channel conductances of 70-80 pS. The distribution of open times of the channels could be described by the sum of two exponentials with time constants tau 1 of 0.3-0.5 ms for the longer, and tau 2 of 0.03-0.06 ms for the shorter component. Bursts of channel openings interrupted by gaps occurred in about 10% of the events only. The longer time constant tau 1 conforms to the channel open times estimated by noise analysis [11].