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

Abstract

At the crustacean neuromuscular junction, action potential‐evoked neurosecretion increases in proportion to stimulation frequency, a process termed frequency facilitation. In the present study we examined how frequency facilitation is affected by osmotic pressure. Hypertonic solution (HS) was applied by local superfusion of the synaptic 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 quantal content (m) together with the number of releasable units (n) and release probability (p) was evaluated for each frequency. Osmotic pressure enhanced quantal release at the lowest f tested (1 Hz) but suppressed neurosecretion at higher f (7‐10 Hz). Thus, hyperosmolarity enhanced action potential‐evoked release but suppressed frequency facilitation. Chelation of intracellular calcium by BAPTA showed that the effect of HS was calcium independent. Binomial analysis of quantal content revealed that HS suppressed the increase in the number of releasable units, which was very pronounced during facilitation under control conditions. Since HS also stimulated asynchronous quantal release, the observed effect of HS on facilitation can be explained by the depletion of the releasable pool of quanta caused by the asynchronous neurosecretion. To test this hypothesis we increased the available pool of vesicles using serotonin and demonstrated that the suppressing effect of HS on facilitation was reversed. The observed effects of HS on facilitated neurosecretion could be described quantitatively using our model for mobilization of vesicles into the releasable pool enhanced by action potentials.

Temperature dependent modulation of lobster neuromuscular properties by serotonin

“…Here we describe the temperaturedependent physiology of the lobster dactyl opener muscle and its modulation by the circulating neurohormone serotonin (5-HT). The lobster dactyl opener is a popular model system for studies of synaptic plasticity Bykhovskaia et al, 2001;Bykhovskaia et al, 2004;Glusman and Kravitz, 1982;Goy and Kravitz, 1989;Hamilton et al, 2006;Kravitz et al, 1980;Vorob'eva et al, 1999;Worden et al, 1997), as is the homologous crayfish dactyl opener muscle (Beaumont and Zucker, 2000;Delaney et al, 1991;Dixon and Atwood, 1989a;Dixon and Atwood, 1989b;Qian and Delaney, 1997;Vyshedskiy et al, 1998;Vyshedskiy and Lin, 1997;Wang and Zucker, 1998). We demonstrate that (1) thermal change within a biologically relevant range of temperatures profoundly affects the neurophysiological properties of the lobster dactyl opener neuromuscular system, and (2) the modulatory effects of serotonin on neuromuscular function have distinctly different temperature dependencies and extend the thermal range over which motor movements are effective.…”

Properties of Synaptic Vesicle Pools in Mature Central Nerve Terminals

“…However, in the latter work, the exact sucrose concentration around terminals was not defined and was probably Ͻ500 mosM. In any case, different synaptic preparations exhibit distinct sensitivities to hypertonicity (23)(24)(25), due in part to differences in synapse stabilization by the substrate in cell culture or by surrounding cells in a tissue (see also "Discussion").…”

Neuromuscular synapses on the dactyl opener muscle of the lobster Homarus americanus

“…Because the innervation pattern is relatively simple, and as the synaptic terminals are experimentally accessible, the dactyl opener has a long and rich history as a model system for studies of the morphological and physiological basis underlying synaptic plasticity (Dudel and Kuffler 1961;Zucker 1973;Jahromi and Atwood 1974;Kamiya and Zucker 1994;Tank et al 1995;Tang and Zucker 1997;Worden et al 1997;Beaumont and Zucker 2000;Bykhovskaia et al 2001). Several studies of these synapses have focused on the cellular processes by which the synaptic vesicles within motoneuron terminals become mobilized to release sites and undergo fusion with the presynaptic membrane to release transmitter.…”