Real-time detection of mitochondrial inhibition at frog motor nerve terminals using increases in the spatial variance in probability of transmitter release

Calmodulin increases transmitter release by mobilizing quanta at the frog motor nerve terminal

The role of calmodulin (CaM) in transmitter release was investigated using liposomes to deliver CaM and monoclonal antibodies against CaM (antiCaM) directly into the frog motor nerve terminal. Miniature endplate potentials (MEPPs) were recorded in a high K+ solution, and effects on transmitter release were monitored using estimates of the quantal release parameters m (number of quanta released), n (number of functional transmitter release sites), p (mean probability of release), and var(s) p (spatial variance in p). Administration of CaM, but not heat-inactivated CaM, encapsulated in liposomes (1000 units ml(-1)) produced an increase in m (25%) that was due to an increase in n. MEPP amplitude was not altered by CaM. Administration of antiCaM, but not heat-inactivated antiCaM, in liposomes (50 microl ml(-1)) produced a progressive decrease in m (40%) that was associated with decreases in n and p. MEPP amplitude was decreased (15%) after a 25 min lag time, suggesting a separation in time between the decreases in quantal release and quantal size. Bath application of the membrane-permeable CaM antagonist W7 (28 microM) produced a gradual decrease in m (25%) that was associated with a decrease in n. W7 also produced a decrease in MEPP amplitude that paralleled the decrease in m. The decreases in MEPP size and m produced by W7 were both reversed by addition of CaM. Our results suggest that CaM increases transmitter release by mobilizing synaptic vesicles at the frog motor nerve terminal.

Inositol trisphosphate and cyclic adenosine diphosphate-ribose increase quantal transmitter release at frog motor nerve terminals: possible involvement of smooth endoplasmic reticulum

The release of chemical transmitter from nerve terminals is critically dependent on a transient increase in intracellular Ca2+. The increase in Ca2+ may be due to influx of Ca2+ from the extracellular fluid or release of Ca2+ from intracellular stores such as mitochondria. Whether Ca2+ utilized in transmitter release is liberated from organelles other than mitochondria is uncertain. Smooth endoplasmic reticulum is known to release Ca2+, e.g., on activation by inositol trisphosphate or cyclic adenosine diphosphate-ribose, so the possibility exists that Ca2+ from this source may be involved in the events leading to exocytosis. We examined this hypothesis by testing whether inositol trisphosphate and cyclic adenosine diphosphate-ribose modified transmitter release. We used liposomes to deliver these agents into the cytoplasmic compartment and binomial analysis to determine their effects on the quantal components of transmitter release. Administration of inositol trisphosphate (10(-4)M) caused a rapid, 25% increase in the number of quanta released. This was due to an increase in the number of functional release sites, as the other quantal parameters were unaffected. The effect was reversed with 40 min of wash. Virtually identical results were obtained with cyclic adenosine diphosphate-ribose (10(-4)M). Inositol trisphosphate caused a 10% increase in quantal size, whereas cyclic adenosine diphosphate-ribose had no effect. The results suggest that quantal transmitter release can be increased by Ca2+ released from smooth endoplasmic reticulum upon stimulation by inositol trisphosphate or cyclic adenosine diphosphate-ribose. This may involve priming of synaptic vesicles at the release sites or mobilization of vesicles to the active zone. Inositol trisphosphate may have an additional action to increase the content of transmitter within the vesicles. These findings raise the possibility of a role of endogenous inositol phosphate and smooth endoplasmic reticulum in the regulation of cytoplasmic Ca2+ and transmitter release.

Effect of hypertonicity on augmentation and potentiation and on corresponding quantal parameters of transmitter release

Augmentation and (posttetanic) potentiation are two of the four components comprising the enhanced release of transmitter following repetitive nerve stimulation. To examine the quantal basis of these components under isotonic and hypertonic conditions, we recorded miniature endplate potentials (MEPPs) from isolated frog (Rana pipiens) cutaneous pectoris muscles, before and after repetitive nerve stimulation (40 s at 80 Hz). Continuous recordings were made in low Ca2+ high Mg2+ isotonic Ringer solution, in Ringer that was made hypertonic with 100 mM sucrose, and in wash solution. Estimates were obtained of m (no. of quanta released), n (no. of functional release sites), p (mean probability of release), and vars p (spatial variance in p), using a method that employed MEPP counts. Hypertonicity abolished augmentation without affecting potentiation. There were prolonged poststimulation increases in m, n, and p and a marked but transient increase in vars p in the hypertonic solution. All effects were completely reversed with wash. The time constants of decay for potentiation and for vars p were virtually identical. The results are consistent with the notion that augmentation is caused by Ca2+ influx through voltage-gated calcium channels and that potentiation is due to Na+-induced Ca2+ release from mitochondria. The results also demonstrate the utility of this approach for analyzing the dynamics of quantal transmitter release.

Niflumic acid-induced increase in potassium currents in frog motor nerve terminals: effects on transmitter release

The actions of the nonsteroidal antiinflammatory drug niflumic acid were studied on frog neuromuscular preparations by conventional electrophysiological techniques. Niflumic acid reduced the amplitude and increased the latency of endplate potentials in a concentration-dependent manner. Neuromuscular junctions pretreated with niflumic acid (0.05-0.5 mM) showed much less depression than control when they were stimulated with trains of impulses. Inhibition of acetylcholine release was reverted by raising the extracellular Ca(2+) concentration but not by simply washing out the preparations with niflumic acid-free solutions. Pretreatment with indomethacin (0.1 mM), another nonsteroidal antiinflammatory drug, did not affect the niflumic acid-induced inhibition of evoked responses. Niflumic acid (0.1 mM) did not change the amplitude of miniature endplate potentials and had a dual action on the frequency of miniatures: it decreased their frequency at 0.1 mM whereas it produced an enormous increase in the rate of spontaneous discharge at 0.5 mM. Niflumic acid (0.1 - 1 mM) reversibly increased the amplitude and affected the kinetics of presynaptic voltage-activated K+ current and Ca(2+)-activated K(+) current in a concentration-dependent manner. Niflumic acid (0.1 - 1 mM) irreversibly decreased the amplitude and reversibly affected the kinetics of the nodal Na(+) current. Indomethacin (0.1 mM) had no effect on presynaptic currents. In conclusion, niflumic acid reduces acetylcholine release by increasing presynaptic K+ currents. This may shorten the depolarizing phase of the presynaptic action potential and may reduce the entry of Ca(2+) with each impulse.