A study of the mechanism of quantal transmitter release at a chemical synapse

Abstract

SUMMARY1. The nerve-muscle preparation of the cutaneous pectoris of the frog has been used to study quantal transmitter release.2. When the osmotic pressure of the external solution is raised 15-2 fold, the frequency of miniature end-plate potentials (m.e.p.p.s) rises by 1-5-2 orders of magnitude. This effect is independent of the presence of Ca2+ ions and of the nature of the substances by which the osmotic pressure has been increased.3. In Ca2+ free hypertonic solution the nerve impulse still invades the nerve terminals but does not alter the frequency of the m.e.p.p.s.4. The arrival of the impulse in the terminals causes an immediate increase in the rate of quantal release, provided divalent cations are present whose passage through the axon membrane is facilitated by excitation (Ca2+, Sr2+, Ba2+).5. Divalent cations which penetrate only slightly (Mg2+, Be2+) lower the frequency of m.e.p.p.s and suppress the end-plate potential (e.p.p.) evoked by an impulse, in the presence of Ca2+ ions. Be2+ is a more effective inhibitor than Mg2+.6. In Ca2+ free solutions, adding Mg2+ causes an increase in the frequency of m.e.p.p.s evoked by depolarization of the nerve endings or by treatment with ethanol.7. The trivalent cation La3+ is more effective than divalent cations are in increasing the frequency of m.e.p.p.s. The tetravalent cation T-4+ also raises the m.e.p.p. frequency.8. The observations summarized in paragraphs 2-7 indicate that the frequency of m.e.p.p.s at a constant temperature depends only on the concentration of uni-, di-and trivalent cations inside the nerve ending. It ZHANNA L. BLIOCH AND OTHERS is suggested that the internal cation concentration influences the adhesion between synaptic vesicles and the membrane of the nerve ending.9. For a model experiment, artificial phospholipid membranes have been used to study the effect of uni-, di-, tri-and tetravalent cations on the adhesion process. At pH 7-7*4, the time required for adhesion to take place decreases with increasing cation concentration in the bath. Ca2+ ions are 100-1000 times more effective than K+ ions; La3+ and Th4+ ions are still more effective. The 'adhesion time' decreases when the pH is lowered; it increases greatly with lowering of temperature.10. The hypothesis is put forward that the mutual adhesion of artificial vesicles made of phospholipid membranes, and the adhesion between synaptic vesicles and the membrane of the nerve ending arise by a common mechanism. In both cases, the important factor is the influence of cations on the electric double layer at the membrane surface.

A further study of the neuromuscular effects of vesamicol (AH5183) and of its enantiomer specificity

“…Whilst our preliminary electrophysiological experiments cannot be regarded as conclusive, we believe that the observation that lanthanum treatment, which greatly increases m.e.p.p. frequency (Blioch et al, 1968;De Bassio et al, 1971;Heuser & Miledi, 1971), led to the immediate appearance of apparently normal-sized m.e.p.ps in a partially vesamicol blocked preparation may lend support to the above idea. It would appear that, despite the rundown of quantal size of m.e.p.ps emanating from the readily-releasable store in the presence of vesamicol, it is possible to induce the release of apparently normal quantal size m.e.p.ps from the depot store.…”

The effect of adenosine on the release of the transmitter from the phrenic nerve of the rat

“…An increase in frequency which outlasts the period of stimulation has already been demonstrated by Miledi & Thies (1967 and by Blioch et al (1968) in frog muscle. In the rat diaphragm (Fig.…”

“…It is well established that miniature endplate potentials continue to occur in solutions containing EGTA or EDTA without added Ca, and hence about 10-5 or 10-6 times the normal concentration of Ca2+ (Miledi & Thies, 1967Blioch, Glagoleva, Liberman & Nenashev, 1968;Hubbard et al 1968c) and it seems improbable that these miniature end-plate potentials are directly due to Ca2+ entry into the nerve terminal from the external solution. The results are shown in Table 2 (fibres 1-7).…”

“…The action of adenosine could then be explained if it reduced the intracellular Ca2+ concentration. Since a role for the intracellular Ca2+ concentration, as such, is not established (but see Blioch et al 1968;Glagoleva, Liberman & Khashaev, 1970;Crawford & Fettiplace, 1971) this possibility is highly speculative. Yet another kind of explanation has been suggested, by Christ & Nishi (1971), for the similar effect of adrenaline on synaptic transmission in sympathetic ganglia.…”

The inhibitory effect of manganese on transmitter release at the neuromuscular junction of the toad

“…Lanthanum has similar paradoxical actions (Blioch, Glagoleva, Liberman & Nenashev, 1968;Heuser & Miledi, 1971;De Bassio, Schnitzler & Parsons, 1971;Kajimoto & Kirpekar, 1972) though its effects on m.e.p.p. frequency are much more dramatic.…”