The Journal of Neuroscience, December 1, 2002, 22(23):10134-10141
Prolonged Activation of Ca2+-Activated K+
Current Contributes to the Long-Lasting Refractory Period of
Aplysia Bag Cell Neurons
Yalan
Zhang1,
Neil S.
Magoski2, and
Leonard K.
Kaczmarek
1 Department of Pharmacology, Yale University School of
Medicine, New Haven, Connecticut 06520, and 2 Department of
Physiology, Queen's University, Kingston, Ontario, Canada K7L 3N6
Stimulation of the bag cell neurons of Aplysia
activates several biochemical pathways, including protein kinase C
(PKC), and alters their excitability for many hours. After an ~30 min
afterdischarge, these neurons enter an ~18 hr inhibited state during
which additional stimulation fails to evoke discharges. In
vivo, this refractory period limits the frequency of
reproductive behaviors associated with egg laying. We have now examined
the role of Ca2+-activated K+
(BK) currents in the refractory period. Outward currents gated by both
intracellular Ca2+ and depolarization, with
pharmacological characteristics of BK currents, were recorded in
isolated bag cell neurons. These currents were enhanced by the BK
channel activators phloretin and
1,3-dihydro-1-[2-hydroxy-5-(trifluoro-methyl)phenyl]-5-trifluoromethyl-2H-benzimidazol-2-one and inhibited by the BK blocker paxilline. The BK component of K+ current was enhanced by
12-O-tetradecanoyl-phorbol-13-acetate, an activator of
PKC, and this effect was blocked by sphinganine and
PKC19-36, inhibitors of PKC in bag cell neurons. To test
whether the BK current is altered during the refractory period, intact
clusters were stimulated to afterdischarge, and neurons were isolated
after the clusters had entered the refractory period. Compared with
unstimulated cells, current density was almost doubled in refractory
neurons. This increase in current was inhibited by preincubating
clusters in sphinganine. Treatment of refractory clusters with
paxilline significantly restored the ability of stimulation to evoke
afterdischarges. Conversely, application of phloretin to previously
unstimulated clusters inhibited the onset of afterdischarges. These
results indicate that a prolonged increase in BK channel activity
contributes to the prolonged refractory period of the bag cell neurons.
Key words:
mollusk; BK channels; refractory state; excitability; afterdischarge; TPA
Copyright © 2002 Society for Neuroscience 0270-6474/02/222310134-08$05.00/0
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