The presence of miniature end plate potentials (mEPPs) can be observed in the small spontaneous depolarizations of the postsynaptic membrane (Fig. 6.5). These mEPPs have an amplitude ≤1 mV, which corresponds to the smallest evoked EPP under low [Ca++] conditions. It has been proposed that each mEPP corresponds to the release of neurotransmitter from a single vesicle, and the amplitudes of other EPPs are integral multiples of the mEPP amplitude. The suggestion that each mEPP could be caused by a single transmitter molecule binding to and opening a single postsynaptic receptor was rejected based on experimental evidence where responses smaller than mEPPs could be generated by applying dilute solutions of acetylcholine directly to the muscle. In fact, calculations indicate that mEPPs are caused by the action of approximately 10,000 molecules, which aligns with estimates of the number of neurotransmitter molecules in a single vesicle.
Numerous studies have further supported the vesicle hypothesis of neurotransmitter release. Biochemical studies have demonstrated that neurotransmitter is concentrated in vesicles, and electron microscopic techniques have revealed the fusion of vesicles to the plasma membrane and their depletion in the terminal cytoplasm following action potentials.
Fig. 6.5 ). Moreover, small spontaneous depolarizations of the postsynaptic membrane, termed miniature end plate potentials (mEPPs), are observable. The amplitude of the mEPP (≤1 mV) corresponds to that of the smallest EPP evoked under low [Ca++], and the amplitudes of other EPPs were shown to be integral multiples of the mEPP amplitude; thus, it was proposed that each mEPP corresponded
SynthesizedandpackagedinthenerveterminalSynthesizedandpackagedinthecellbody;transportedtothenerveterminalbyfastaxonaltransportSynthesizedinactiveformActivepeptideformedwhenitiscleavedfromamuchlargerpolypeptidethatcontainsseveralneuropeptidesUsuallypresentinsmallclearvesiclesUsuallypresentinlargeelectron-densevesiclesReleasedintoasynapticcleftMaybereleasedsomedistancefromthepostsynapticcellTheremaybenowell-definedsynapticstructureActionofmanyterminatedbecauseofuptakebypresynapticActionterminatedbyproteolysisorbythepeptidediffusingawayterminalsviaNa+-poweredactivetransportTypically,actionhasshortlatencyandshortduration(msec)Actionmayhavelonglatencyandmaypersistformanyseconds •Fig. 6.4 PresynapticCa++ currentanditsrelationshiptothepostsynapticresponse.A, Schematicofasquidgiantsynapsepreparation.Electrodes1and2areusedtovoltage-clampthepresynapticterminalandrecorditsvoltageandcurrent.(NotethattetrodotoxinandtetraethylammoniumwerepresenttoblockNa+ andK+ conductancetoisolatetheCa++ conductance.)Electrode3recordsthemembranepotentialofthepostsynapticaxon.Thepresynapticterminalwasvoltage-clampedtoincreasinglymoredepolarizedlevels(blue traces). Withasmalldepolarization(B), asmallCa++ currentstartsshortlyafterthevoltagestep,continuestogrowforthedurationofthestep(oncurrent),andthendecaysexponentiallyafteritstermination(offortailcurrent).Alargervoltagestep(C) increasesboththeonandtheoffcomponentsoftheCa++ current,andnowdistinctonandoffresponsesareobservedinthepostsynapticresponse.D, ThevoltagestepistotheNernstpotentialforCa++ ,sothereisnoCa++ currentduringthestep,butalargetailcurrentandoffresponseareobserved.(BasedondataofLlinasRetal.Biophys J 1981;33:323.) to the release of transmitter from a single vesicle and that EPPs represented the combined simultaneous release of transmitter from many vesicles.
This linking of mEPPs and vesicles implies that each mEPP is caused by the action of many molecules of neurotransmitter binding to postsynaptic receptors. The alternative that each mEPP could be caused by a single transmitter molecule binding to and opening a single postsynaptic receptor was rejected, in part because responses smaller in amplitude than mEPPs could be generated experimentally by directly applying dilute solutions of acetylcholine to the muscle. In fact, mEPPs were calculated to be caused by the action of approximately 10,000 molecules, which corresponds well to estimates of the number of neurotransmitter molecules contained within a single vesicle.
Many additional studies have confirmed the vesicle hypothesis of neurotransmitter release. For example, biochemical studies have shown that neurotransmitter is concentrated in vesicles, and fusion of vesicles to the plasma membrane and their depletion in the terminal cytoplasm after action potentials have been shown with electron microscopic techniques.