Ionic mechanism of the excitatory synaptic membrane of the crayfish neuromuscular junction

An analysis of the inhibitory post-synaptic current in the voltage-clamped crayfish muscle

1. Inhibitory post-synaptic currents (i.p.c.s) were recorded from the feed-back current through a wire electrode inserted longitudinally into the opener muscle fibre of the claw in the crayfish (Cambarus clarkii). 2. I.p.s.c. rose to its peak in about 3-4 msec and decayed approximately exponentially. The decay time constant at -100 mV was 9.4 msec. 3. The decay time constant decreased as the membrane was hyperpolarized and increased during depolarization. The time constant (tau) depends on voltage (V) according to the relation tau = a exp (AV), with a = 18.6 msec and A = 0.0065 mV-1. Voltage dependence was opposite in direction to that seen at frog end-plates, but in the same direction as that of e.p.s.c. in crayfish muscle. 4. At lower temperatures, the rise and fall times of i.p.s.c.s were prolonged. Q10 for the decay time constant was 2.4 between 22.6 and 12.5 degrees C. 5. When pH was decreased from 7.2 to 5.5, the decay time constant increased by about 50%, with little change in the voltage dependence of the time course. 6. When chloride in the solution was changed to iodide, the decay time constant was increased by a factor of 3, while voltage dependence of the time course was not changed. In bromide solution the decay time constant increased by about 50%. 7. Peak amplitudes of i.p.s.c.s were approximately linear as the membrane was depolarized, but they levelled off as the membrane was hyperpolarized beyond reversal potential. The non-linear I-V relation did not result from inadequate voltage clamping, nor from a change in the inside concentration of chloride. After equilibration with iodide solution the I-V relation was approximately linear. 8. The decay time constant was increased after repetitive nerve stimulation. This prolongation became more pronounced at lower temperatures. 9. The kinetic process of the transmitter action is discussed. It is suggested that the rate limiting process for i.p.s.c. is binding and unbinding of the transmitter to the receptor.

Effects of membrane potential and temperature on the excitatory post-synaptic current in the crayfish muscle

1. Effects of membrane potential and temperature on the excitatory post-synaptic current (e.p.s.c.) were studied in the voltage-clamped crayfish muscle. E.p.c. was recorded either by measuring the feedback current through an intracellular wire electrode or by focal recording with an extracellular micro-electrode. 2. The amplitude of the e.p.s.c. obtained by the voltage clamp method varied almost linearly with membrane potential between -100 mV and +70 mV, whilst the reversal potential was +23.8 +/- 3.9 mV (S.E. of mean). 3. The declining phase of the extracellular e.p.s.c. was slightly prolonged by depolarization and shortened by hyperpolarization. Potential dependence of the decay time constant was expressed by tau = a exp (AV), with a = 2.78 msec and A = 0.0037 mV-1. 4. The decay time constant had a Q10 of 2.3 and the growth time had a Q10 of 1.5. 5. The voltage dependence of the decay phase of the e.p.s. was the reverse of that found in frog end-plate. It is concluded that the voltage dependence of the time course is not related either to the charge of ions which carry the synaptic current or to the charge of the transmitter.

Permeability of the post-synaptic membrane of an excitatory glutamate synapse to sodium and potassium

1. The changes in permeability of the post-synaptic membrane at the insect skeletal neuromuscular junction caused by the excitatory transmitter and L-glutamate have been studied using the voltage clamp technique. 2. The reversal potential (ER) of the excitatory post-synaptic current and the glutamate current was +3 and +4 mV respectively. 3. ER of the synaptic current did not change when external K was altered between 0 and 20 mM, but did show a small positive shift in 40 mM external K. Reducing external Na to 1-10 mM changes ER by 12-18 mV. Reducing external Cl to to zero caused no change in ER. 4. It is proposed that the transmitter and L-glutamate cause an increase in permeability to Na and K, but not to Cl. 5. In normal saline, the ratio of the permeability increase to Na and K (delta PNa/delta PK) is 0.9. 6. The changes in ER caused by altering external K were similar to those predicted by the Goldman-Hodgkin-Katz equation, assuming delta PNa/delta PK stays constant. 7. The changes in ER caused by alterations of external Na are much less than those predicted by the Goldman equation. 8. No glutamate current could be recorded in Na- and Ca-free saline either at the resting potential or at depolarized or hyperpolarized membrane potentials. 9. It is proposed that the outward K current is dependent upon the inward Na current, and that the increase in K permeability is abolished in zero external Na.

Antagonism by some antihistamines of the amino acid-evoked responses recorded from the lobster muscle fibre and the frog spinal cord

1 The effects of some antihistamines on the lobster muscle fibre and the frog spinal cord were investigated using intracellular and extracellular recordings, respectively. 2. On lobster muscle, histamine H1-blockers reversibly antagonized responses to bath-applied glutamate, aspartate and quisqualate but not responses to gamma-aminobutyric acid (GABA). Iontophoretic glutamate potentials were also reduced. Histamine (up to 1 mM) had no effect on this preparation. 3 The H1-antagonists produced a small increase in muscle membrane conductance and a slight hyperpolarization. These effects were largely unchanged in a low C1- bathing solution. Procaine (1 mM) decreased membrane conductance and did not affect responses to GABA or glutamate. 4 The H2-antagonist burimamide blocked both glutamate and GABA-evoked responses on the lobster muscle without affecting resting potential or conductance. 5 In the frog cord, bath-applied histamine produced ventral root depolarizations and dorsal root hyperpolarizations (sometimes biphasic responses). These effects were reduced by tetrodotoxin (TTX) but not by antazoline (H1-blocker) or burimamide; the latter reversibly antagonized responses to both glutamate and GABA on TTX-treated cords while antazoline was ineffective. 6 It is suggested that antihistamines can act as non-specific amino acid antagonists by interacting at the level of the receptor-coupled ionophores.

L-glutamate as an excitatory transmitter at the Drosophila larval neuromuscular junction

The possibility that L-glutamate is the excitatory transmitter at the Drosophila larval neuromuscular junction and the ionic basis of its action on the muscle membrane are examined. 2. Iontophoretically applied L-glutamate causes muscle depolarization (L-glutamate potential) if and only if the L-glutamate pipette is within a few mum of the nerve ending. D-glutamate, substance P, ACh and GABA are ineffective. 3. Bath-applied L-glutamate produces similar changes in the time course and amplitude of miniature excitatory junctional potential (m.e.j.p.), excitatory junctional potential (e.j.p.) and the L-glutamate potential. 4. Neuromuscular transmission and excitation-contraction coupling are operative in a haemolymph-like solution containing 1 mM L-glutamate. 5. The reversal potentials of the e.j.p. and the L-glutamate potential are identical to each other, changing similarly with changes in the ionic compositions of the external medium (twelve solutions). 6. The ionic dependence of the reversal potentials is predicted from an extended constant-field equation using a ratio of sodium:potassium permeabilities of PNa/PK=1-3, and a ratio of magnesium:potassium permeabilities of PMg/PK=4-7. 7. It is concluded that L-glutamate is, or is an agonist of, the excitatory transmitter at certain Drosophila larval neuromuscular junctions.

Permeability changes produced by L-glutamate at the excitatory post-synaptic membrane of the crayfish muscle

1. Permeability changes produced by L-glutamate at the neuromuscular junction of the crayfish (Cambarus clarkii) were investigated by application of the drug iontophoretically to the voltage-clamped junction and measuring the resulting 'glutamate current'. 2. Reversal potentials were determined by measuring the glutamate current at different membrane potentials. They were +39-1 +/- 3-6 mV (mean +/- S.E. of mean) in normal solution and +16-5 +/- 2-0 mV in solutions made twice as hypertonic by the addition of sucrose. 3. Decreasing external Na+ concentration shifted the reversal potential in the negative direction; increased Na+ in the positive direction. 4. The relation between the amplitude of the glutamate current and extracellular Na+ concentration was approximately linear. 5. Alteration of the external K+ or Cl- concentration did not affect the amplitude or reversal potential of glutamate current. 6. In Na+-free solution the application of L-glutamate produced a small inward current at the resting potential and its amplitude was augmented by increasing the external Ca2+ concentration. 7. Increasing the Ca2+ concentration in the normal Na+ media produced no appreciable effect on the reversal potential but decreased the amplitude of glutamate current. 8. The results indicate that L-glutamate increases the membrane permeability mainly to Na+ and slightly to Ca2+. 9. The time course of glutamate current was shorter than that of the concentration calculated from the diffusion equation and it was simulated more closely by the square of the concentration.