Effect of Hypertonicity on Augmentation and Potentiation and on Corresponding Quantal Parameters of Transmitter Release

Abstract

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.

Reduced Expression of the Vesicular Acetylcholine Transporter and Neurotransmitter Content Affects Synaptic Vesicle Distribution and Shape in Mouse Neuromuscular Junction

“…To further investigate the recycling and distribution of SV from the readily releasable pool (RRP) in nerve terminals from diaphragm muscle of VAChT KD HOM mice we used hypertonic sucrose (500 mM) as a stimulus [22] . Hypertonic extracellular solution has been shown to increase the frequency of MEPPs at the frog and rat NMJs [23] , [24] , [25] , [26] . The mechanism behind this increase is still unknown, however, it has been described that hypertonicity does not require Ca 2+ influx or release from internal stores and may facilitate fusion of docked vesicles [22] , [27] .…”

“…Furthermore, we observed a reduction in FM1-43fx staining in these mice, compatible with the reduction in the total number of SVs revealed by ultrastructural analysis when compared to WT. Hypertonic extracellular solution increases MEPP frequency at the vertebrate NMJ [23] , [24] , [25] , [26] . Although the mechanism for such enhancement is unknown, there are evidences suggesting that this stimulus does not require Ca 2+ influx or release from internal stores and consists of a calcium-independent neurotransmitter release that mobilizes specifically the RRP [22] , [27] .…”

Distribution and Kinetic Properties of GABAergic Inputs to Layer V Pyramidal Cells in Rat Auditory Cortex

“…Recent evidence (Bekkers and Stevens 1996) indicates that this method may not be a reliable measure and that it is necessary to directly stimulate the synapses at a particular location on the cell. Focal sucrose application has been used in a number of preparations (Vyklicky et al 1991;Stevens 1995, 1996;Rosemund and Stevens 1996;Cheng and Miyamoto 1999) to increase the rate of spontaneous vesicle fusion and neurotransmitter release at the presynaptic terminals in a confined region of the postsynaptic cell membrane. This leads to a focal increase in mIPSC rate postsynaptically.…”

Hyperosmolarity reduces facilitation by a Ca2+ ‐independent mechanism at the lobster neuromuscular junction: possible depletion of the releasable pool

“…Hypertonic solution increases the rate of miniature excitatory postsynaptic responses in the presence and absence of the nerve stimulation (Dudel & Kuffler, 1961; Hubbard et al 1968; Van der Kloot & Kita, 1974; Kita & Van der Kloot, 1977; Kita et al 1982; Stevens & Tsujamoto, 1995; Cheng & Miyamoto, 1999). Less consistent effects of HS have been reported for action potential‐ evoked release.…”