Facilitation of membrane electrical excitability in Drosophila.

Abstract

Prior electrical activity in the indirect flight muscles of Drosophila facilitates membrane excitability. The mechanism of facilitation involves the inactivation of an early, fast, transient outward current by prior membrane depolarization. In the facilitated state the ca cium-dependent spikelike response has a decreased current and voltage threshold. The facilitated state persists for 1.5 sec after a membrane active response. A single nerve-driven spike is sufficient to facilitate membrane excitability.Most known cases of changes in neural function with use depend on synaptic mechanisms. There are some examples, however, showing that the excitable membrane itself can be plastic in its response to excitation (1-5). As a prelude to genetic analysis of excitable membranes in Drosophila mlanogaster, physiological analysis of the dorsal longitudinal flight muscle (DLM) was undertaken. In the course of these studies we found striking changes in the excitability characteristics of the membrane, depending on prior use. For example, it was found that a single prior spike may evoke a large active membrane response to a stimulus that previously elicited no active response at all. The mechanism that is apparently responsible for these phenomena involves the inactivation of a transient inhibitory outward current similar to the "A" current first analyzed by Conner and Stevens (6) in molluscan neurons. METHODS AND MATERIALSAdult female D. melanogaster of wild-type strain Canton-S or the temperature-sensitive paralytic mutant stock shi(ts-1) were used in these experiments. Muscle membrane properties were found to be the same for these strains. In experiments requiring decreased transmitter release, the shi(ts-1) strain was used. The shi(ts-1) mutation confers a presynaptic temperature-sensitive defect that suppresses neurotransmitter release at temperatures higher than 290C (7,8). Thus, the use of this mutant together with appropriate temperature controls permits the experimental regulation of the size of the excitatory junction potentials (ejps). The techniques for evoking muscle responses via nerve stimulation are published (8).For recording and current injection, a single muscle fiber was impaled with two 3 M KCl-filled glass microelectrodes. Current was measured with a Ag-AgCl ground electrode by monitoring the voltage across a 10 kW resistor to ground. Flies were mounted so that the spiracles had access to air, and the dorsal thorax was opened and exposed to saline (128 mM NaCI/4.7 mM KCI/1.8 mM CaCI2/1.0 mM phosphate, pH 6.9). For experiments requiring low sodium and the complete removal of calcium, 1.4 mM [ethylenebis(oxyethylenenitrilo)]tetraacetic acid was added and NaCl was partially replaced with choline chloride (to 114 mM). Magnesium chloride (14 mM) was added for membrane stability. Muscle fibers had resting membrane potentials ranging from 60 to 90 mV. All fibers with resting potentials within this range showed the excitability changes described here. Within this range those with higher resting potentia...

Ion currents in Drosophila flight muscles

“…depends on many diverse factors such as the resting potential of the cell, the age of the fly, and whether the excitatory input is from a motorneurone or from micro-electrode current injection. Some ofthe first observations of graded responsiveness in electrically active cells were made in insect muscle cells (Cerf, Grundfest, Hoyle & McCann, 1959;Piek & Njio, 1979 (Salkoff & Wyman, 1980). As shown, this current activates rapidly and opposes the depolarizing force of the inward calcium current.…”

“…Just before the last nerve stimulus a conditioning current pulse was delivered intracellularly to the muscle (not shown in Figure). The size of the pulse was large enough to inactivate a sufficient amount of the A-current to lower the threshold of the spike-like response effectively (Salkoff & Wyman, 1980). It can now be seen that the final e.p.s.p.…”

“…A prior publication dealt with the effect of A-current inactivation during repetitive current injection into the muscle (Salkoff & Wyman, 1980). During such stimulation an initial depolarizing stimulus is largely shunted by the rapid turn-on of IA, thus aborting a large active response.…”

“…A recent study showed that the dorsal longitudinal flight muscles (d.l.m.) ofDrosophila were suitable for voltage-clamping (Salkoff & Wyman, 1980). Use of this system for studying the development of membrane ion currents revealed that a single ion channel type developed before all other types (Salkoff & Wyman, 1981 a).…”

Characterization of three types of potassium current in cultured neurones of rat supraoptic nucleus area.

“…The role of IA may be complex but it has been suggested that this current controls the rate of firing by modulating the rate of depolarization of the membrane between the after-hyperpolarization of one action potential and the threshold for the next (Connor, 1978;Salkoff & Wyman, 1980;Gustafsson et al 1982;Hille, 1984). A depolarizing stimulus will be reduced in effectiveness when IA is completely noninactivated but the same stimulus will depolarize the neurone to a greater extent if IA is inactivated or blocked (Gustafsson et al 1982) and thus may induce firing.…”