The relation between transmitter release and Ca2+ entry at the mouse motor nerve terminal: role of stochastic factors causing heterogeneity.
The relation between quantal transmitter release and presynaptic Ca2+/Ba2+ entry at the mouse neuromuscular junction was studied, making use of the finding that in the presence of Ba2+ trains of nerve stimuli or brief nerve terminal depolarizations elicit "tails" of raised miniature end-plate potential frequency (fm) that reflect entry of Ba2+ per pulse, and hence effectiveness of pulses in opening Ca2+/Ba2+ channels; at the same time these pulses elicit end-plate potentials. With nerve stimulation in the presence of Ba2+ and Ca2+ and modulation of release by raised Mg2+ or bekanamycin, slopes of log quantal content (m) vs log apparent Ba2+ entry per pulse were close to 4, which is the same as the Hill coefficient for Ba2+ cooperativity derived from other data. With depolarizing pulses of varied intensity, however, similar plots gave slopes close to 2, with Ba2+ alone or in a mixture of Ca2+ and Ba2+. Thus, the relation between transmitter release and Ca2+ (or Ba2+) entry apparently depends upon how entry is varied; varying the numbers of channels opened is not the same as varying ion entry per channel. A mathematical model was developed to examine the consequences of heterogeneity of local Ca2+ (or Ba2+) between release sites, arising because of stochastic variation of number and time course of Ca2+ channels opened per site; the experimental results were consistent with this model. It was therefore concluded that release is normally governed by intracellular Ca2+ close to points of Ca2+ entry through channels; stochastic factors give rise to more release than if Ca2+ were homogeneously distributed. If Ca2+ channels are uniformly close to release sites the average number of channels opened per site per action potential may be as low as 4.
Department of Pharmacology & Therapeutics, Faculty of Medicine, University of British Columbia, Vancouver, Canada.
D M
DM
Quastel
Y Y
YY
Guan
D A
DA
Saint
D000818:Animals; D001464:Barium / Q000378:metabolism; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000502:physiology; D004558:Electric Stimulation; D005071:Evoked Potentials / Q000187:drug effects; D066298:In Vitro Techniques; D051379:Mice; D009045:Motor Endplate / Q000187:drug effects; D009046:Motor Neurons / Q000187:drug effects / Q000378:metabolism*; D009411:Nerve Endings / Q000187:drug effects / Q000378:metabolism*; D018377:Neurotransmitter Agents / Q000378:metabolism*; D011189:Potassium Chloride / Q000494:pharmacology
D015220:Calcium Channels; D018377:Neurotransmitter Agents; D001464:Barium; D011189:Potassium Chloride; D002118:Calcium