Journal of Neuroscience, Vol 13, 5139-5152, Copyright © 1993 by Society for Neuroscience
FMRFamide produces biphasic modulation of the LFS motor neurons in the neural circuit of the siphon withdrawal reflex of Aplysia by activating Na+ and K+ currents
KJ Belkin and TW Abrams
Department of Biology, University of Pennsylvania, Philadelphia 19104- 6018.
The molluscan neuropeptide FMRFamide has an inhibitory effect on
transmitter release from the presynaptic sensory neurons in the neural
circuit for the siphon withdrawal reflex. We have explored whether
FMRFamide also acts postsynaptically in motor neurons in this circuit,
focusing on the LFS motor neurons. FMRFamide typically produces a biphasic
response in LFS neurons: a fast excitatory response followed by a prolonged
inhibitory response. We have analyzed these postsynaptic actions and
compared them with the mechanism of FMRFamide's inhibition of the
presynaptic sensory neurons. The transient excitatory effect of FMRFamide,
which desensitizes rapidly, is due to activation of a TTX- insensitive,
Na(+)-dependent inward current. The late hyperpolarizing phase of the
FMRFamide response results from activation of at least two K+ currents. One
component of the hyperpolarizing response is active at rest and at more
hyperpolarized membrane potentials, and is blocked by 5 mM 4-aminopyridine,
suggesting that it differs from the previously described
FMRFamide-modulated K+ currents in the presynaptic sensory neurons. In
addition, FMRFamide increases a 4-aminopyridine-insensitive K+ current.
Presynaptically, FMRFamide increases K+ conductance, acting via release of
arachidonic acid. In the LFS motor neurons, application of arachidonic acid
mimicked the prolonged, hyperpolarizing phase of the FMRFamide response;
4-bromophenacyl bromide, an inhibitor of phospholipase A2, selectively
blocked this component of the FMRFamide response. Thus, FMRFamide may act
in parallel pre- and post- synaptically to inhibit the output of the siphon
withdrawal reflex circuit, producing this inhibitory effect via the same
second messenger in the sensory neurons and motor neurons, though a number
of the K+ currents modulated in these two types of neurons are different.
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