Journal of Neuroscience, Vol 3, 2230-2239, Copyright © 1983 by Society for Neuroscience
Calcium entry causes a prolonged refractory period in peptidergic neurons of Aplysia
LK Kaczmarek and JA Kauer
A brief train of electrical stimuli to the pleuroabdominal connective of
Aplysia produces a cumulative depolarization in the peptidergic bag cell
neurons within the abdominal ganglion. This response is followed by an
afterdischarge which lasts for about 30 min, and then by a prolonged
refractory period lasting for several hours. During the refractory period
the cumulative depolarization in response to stimulation is attenuated, and
stimulation either fails to initiate afterdischarges or produces discharges
of much shorter duration. We have used the cationophore X537A to test the
hypothesis that the prolonged refractory period is caused by calcium entry
into the bag cell neurons during the afterdischarge. Exposure of intact bag
cell clusters to X537A at concentrations from 1 to 10 microM for a period
of 20 min in calcium-containing media produced no change in their resting
potentials or in their ability to generate action potentials, but induced a
state resembling natural refractoriness in response to subsequent
stimulation. Both natural refractoriness and that induced by X537A could be
overcome by extracellular tetraethylammonium ions (90 mM). Dose response
data showed that concentrations of ionophore of 2.5 to 5.0 microM produce
an attenuation of afterdischarge that is similar to that following a
stimulated afterdischarge. These concentrations of X537A also produced an
enhancement of 3H-labeled peptide release from these cells that is
comparable to that observed on stimulation of an afterdischarge. Moreover,
the time course of recovery from exposure to 5.0 microM X537A parallels
that of natural refractoriness, recovery being essentially complete about
20 hr after X537A exposure or stimulation. The ionophore did not affect the
mean duration of afterdischarge when applied in calcium-deficient media.
The electrical effects of X537A were investigated using isolated bag cell
neurons in cell culture. After treatment with the adenylate cyclase
activator, forskolin, and theophylline, such isolated cells show many of
the electrical changes occurring in intact bag cell clusters at the onset
of afterdischarge, including the enhancement of action potentials, as well
as increased input resistance and the emergence of oscillations in membrane
potential. None of these parameters was significantly affected by
concentrations of ionophore that induce refractoriness. In response to
repetitive intracellular stimulation, however, some forskolin- treated
cells undergo a cumulative depolarization which is similar to that seen at
the onset of afterdischarge in intact clusters. This cumulative
depolarization was found to be attenuated or abolished by 5.0 microM
X537A.(ABSTRACT TRUNCATED AT 400 WORDS)
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