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Properties of miniature excitatory junctional currents at the locust nerve‐muscle junction
Author(s) -
CullCandy S. G.,
Miledi R.
Publication year - 1982
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.1982.sp014210
Subject(s) - excitatory postsynaptic potential , chemistry , hyperpolarization (physics) , biophysics , reversal potential , neuromuscular junction , membrane potential , anatomy , patch clamp , neuroscience , stereochemistry , biochemistry , biology , receptor , nuclear magnetic resonance spectroscopy
1. 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, Ca 2+ ‐free medium and Cl − ‐free medium). M.e.j.c.s were also examined in isotonic Ca 2+ where the inward post‐synaptic current was carried mainly by Ca 2+ . 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) was τ m.e.j.c. = 2.62±0.11 msec ( V m = ‐80 mV, T = 22 °C). Decay‐time was not markedly changed in Ca 2+ ‐free or Cl − ‐free medium. τ m.e.j.c. 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. τ m.e.j.c. decreased exponentially with membrane hyperpolarization. An e ‐fold change was produced by 182.±24.8 mV change in V m . 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 Ca 2+ ‐free and Cl − ‐free solutions. 6. The kinetics of junctional channels are altered in isotonic Ca 2+ . 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, τ m.e.j.c. = 1.01±0.08 msec, indicating a reduction in mean channel life‐time ( V m = ‐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 Ca 2+ , thus producing a prolongation in mean τ m.e.j.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 τ m.e.j.c. of the remaining events is abolished or even reversed indicating that voltage sensitivity of channel life‐time is altered in isotonic Ca 2+ . 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.