Properties of miniature excitatory junctional currents at the locust nerve‐muscle junction

Abstract

SUMMARY1. Miniature excitatory junctional currents (m.e.j.c.s) were examined in conditions where inward current was carried mainly by Na+ (i.e. in normal medium, Ca2+-free medium and Cl--free medium). M.e.j.c.s were also examined in isotonic Ca2+ where the inward post-synaptic current was carried mainly by Ca2+.2. In normal medium, mean m.e.j.c. amplitude = 2-34+0-05 nA. The decay time constant of m.e.j.c.s (excluding a small percentage with abnormal shapes) wasTm.e.j.c. = 2-62+0-11 msec (Vm = -80 mV, T = 22 0(). Decay-time was not markedly changed in Ca2+-free or Cl--free medium. Tm e jc approaches the life-time of glutamate activated junctional channels.3. Excitatory junctional currents, evoked by nerve impulses, decayed slightly faster than m.e.j.c.s obtained in the same fibres. Extracellularly recorded m.e.j.c.s and voltage-clamped m.e.j.c.s were similar in time course.4. Tm.ej.c decreased exponentially with membrane hyperpolarization. An e-fold change was produced by 182X1 + 24X8 mV change in Vm.5. The dependence of mean m.e.j.c. amplitude on clamp potential showed a slight non-linearity at hyperpolarized levels. The equilibrium potential for transmitter action was close to 0 mV in normal solution as well as in Ca2+-free and Cl--free solutions.6. The kinetics of junctional channels are altered in isotonic Ca2+. M.e.j.c. amplitude was reduced to about one-third normal size; mean m.e.j .c. = 0 74 + 0 03 nA.The decay time becomes markedly briefer, Tm e jic = 101 +0-08 msec, indicating a reduction in mean channel life-time (Vm = -80 mV, T = 22 C).7. A population of slow time course and composite m.e.j.c.s appear when muscle fibres are hyperpolarized in isotonic Ca2+, thus producing a prolongation in mean Tm ej c. This results from an influence of post-synaptic membrane potential on presynaptic transmitter release. If such m.e.j.c.s are ignored the voltage dependence of Tm e. c. of the remaining events is abolished or even reversed indicating that voltage sensitivity of channel life-time is altered in isotonic Ca2+. The equilibrium potential for transmitter action may be slightly more positive than normal.8. We estimate that a single packet of neurally released transmitter normally opens, on average, 250 ion channels at these junctions.

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

“…The spectra could be fitted with a single Lorentzian component. However, as previously described for miniature e.j.c.s (m.e.j.c.s) in these fibres (Cull-Candy & Miledi, 1982) the roll off of the spectra is sometimes faster than predicted by the /If 2 curve. The spectrum gives an estimate of Ti.jic (according to Ti j c = I/(27Tfc), wherefc is the half-power frequency).…”

“…E.j.c. amplitude was linearly dependent on Vm, although in some cells a slight non-linearity occurred at hyperpolarized potentials (Cull-Candy & Miledi, 1982). Outward i.j.c.s and GABA currents (Cl-influx) depended linearly on Vm, whereas inward currents consistently rectified.…”

Anomalous voltage dependence of channel blockade at a crustacean glutamate‐mediated synapse.

“…The time constant of current decay of control synaptic currents was obtained from a fit in which the amplitude, time constant and decay baseline were simultaneously adjusted until the fit was optimized. As pointed out in the Results and typical of other arthropod glutamate-activated synaptic currents (Crawford & McBurney, 1976;Onodera & Takeuchi, 1978;Cull-Candy & Miledi, 1982), the synaptic current decays typically reach exponentiality rather slowly, in the present case not until almost 50 % of the decay phase (e.g. Fig.…”

“…These results are at variance with a simple model of channel block and, as such, a more detailed analysis of this blocking phenomenon seemed warranted. (Cull-Candy & Miledi, 1982).…”

The cell polarity scaffold lethal giant larvae regulates synapse morphology and function

“…Classical interpretations of miniature event records suggest frequency reflects presynaptic fusion probability, amplitude reflects the number/function of postsynaptic receptors, and that these properties additively predict evoked neurotransmission strength (Fatt and Katz, 1952;Cull-Candy and Miledi, 1982). Miniature event recordings of lgl-null synapses indicate increased presynaptic vesicle fusion probability and postsynaptic glutamate receptor number but, conversely, evoked records show a strong reduction in neurotransmission.…”