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

1 At the crustacean neuromuscular junction, action potential-evoked neurose cretion increases in proportion to stimulation frequency, a process termed frequency facilitation. In the present study we examined how frequency faci litation is affected by osmotic pressure. 2. Hypertonic solution (FIS) was applied by local superfusion of the synapt ic area. Quantal release was monitored by focal extracellular recordings of postsynaptic potentials, Several stimulation frequencies (f) in the range from 1 to 10 Hz were employed, and quant,al content (m) together with the n umber of releasable units (n) and release probability (p) was evaluated for each frequency. 3. Osmotic pressure enhanced quantal release at the lowest f tested (I Hz) but suppressed neurosecretion at higher f (7-10 Hz). Thus, hyperosmolarity enhanced action potential-evoked release but suppressed frequency facilitat ion. 4. Chelation of intracellular calcium by BAPTA showed that the effect of HS was calcium independent. 5. Binomial analysis of quantal content revealed that HS suppressed the inc rease in the number of releasable units, which was very pronounced during f acilitation under control conditions. Since HS also stimulated asynchronous quantal release, the observed effect of HS on facilitation can be explaine d by the depletion of the releasable pool of quanta caused by the asynchron ous neurosecretion, 6. To test this hypothesis we increased the available pool of vesicles usin g serotonin and demonstrated that the suppressing effect of HS on facilitat ion was reversed. 7. The observed effects of HS on facilitated neurosecretion could be descri bed quantitatively using our model for mobilization of vesicles into the re leasable pool enhanced by action potentials.