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Journal of Neuroscience, Vol 14, 4393-4411, Copyright © 1994 by Society for Neuroscience
Enhancement of Ca current in the accessory radula closer muscle of Aplysia californica by neuromodulators that potentiate its contractions
V Brezina, CG Evans and KR Weiss
Department of Physiology and Biophysics, Mount Sinai School of Medicine, New York, New York 10029.
A major goal of neuroscience is to identify the neural and cellular
mechanisms of behavior and its plasticity. Progress toward this goal has
come particularly from work with a small number of tractable model
preparations. One of these is the simple neuromuscular circuit consisting
of the accessory radula closer (ARC) muscle of the mollusk Aplysia
californica and its innervating motor and modulatory neurons. Contraction
of the ARC muscle underlies a component of Aplysia feeding behavior, and
plasticity of the behavior is in large part due to modulation of the
amplitude and duration of the contractions of the muscle by a variety of
modulatory neurotransmitters and peptide cotransmitters, among them the
small cardioactive peptides (SCPs), myomodulins (MMs), and serotonin
(5-HT). We have studied single dissociated ARC muscle fibers in order to
determine whether modulation of membrane ion currents in the muscle might
underlie these effects. First, we confirmed that the dissociated fibers
were functionally intact: just as with the whole ARC muscle, their
contractions were potentiated by 5-HT and SCPB and potentiated as well as
depressed by MMA, and their cAMP content was greatly elevated by 5-HT, SCPA
and SCPB, and to a lesser extent by MMA and MMB. Next, using voltage-clamp
techniques, we found that two ion currents present in the fibers were
indeed modulated. The fibers possess a dihydropyridine-sensitive, high-
threshold "L"-type Ca current. This current was enhanced by the modulators
that potentiate ARC-muscle contractions--5-HT, SCPA and SCPB, and MMA and
MMB--but not by buccalinA, a modulator that does not act directly on the
ARC muscle. All of the potentiating modulators, as well as elevation of
cAMP in the fibers by forskolin or a cAMP analog, maximally enhanced the
current about twofold and mutually occluded each other's effects. Since the
Ca current supplies Ca2+ necessary for contraction of the muscle, the
enhancement of the current is a good candidate to be a major mechanism of
the potentiation of the contractions. In the following article we report
that the modulators also, to different degrees, activate a distinctive K
current and thereby depress the contractions. Net potentiation or
depression then depends on the balance between the relative strengths of
the modulation of the two ion currents.
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