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The Journal of Neuroscience, December 1, 2002, 22(23):10277-10290
Roles of Tetrodotoxin (TTX)-Sensitive Na+ Current,
TTX-Resistant Na+ Current, and Ca2+ Current in
the Action Potentials of Nociceptive Sensory Neurons
Nathaniel T.
Blair and
Bruce P.
Bean
Department of Neurobiology, Harvard Medical School, Boston,
Massachusetts 20114
Nociceptive sensory neurons are unusual in expressing voltage-gated
inward currents carried by sodium channels resistant to block by
tetrodotoxin (TTX) as well as currents carried by conventional TTX-sensitive sodium channels and voltage-dependent calcium channels. To examine how currents carried by each of these helps to shape the
action potential in small-diameter dorsal root ganglion cell bodies, we
voltage clamped cells by using the action potential recorded from each
cell as the command voltage. Using intracellular solutions of
physiological ionic composition, we isolated individual components of current flowing during the action potential with the use
of channel blockers (TTX for TTX-sensitive sodium currents and a
mixture of calcium channel blockers for calcium currents) and ionic
substitution (TTX-resistant current measured by the replacement of
extracellular sodium by N-methyl-D-glucamine
in the presence of TTX, with correction for altered driving force). TTX-resistant sodium channels activated quickly enough to carry the
largest inward charge during the upstroke of the nociceptor action
potential (~58%), with TTX-sensitive sodium channels also contributing significantly (~40%), especially near threshold, and
high voltage-activated calcium currents much less (~2%). Action potentials had a prominent shoulder during the falling phase, characteristic of nociceptive neurons. TTX-resistant sodium channels did not inactivate completely during the action potential and carried the majority (58%) of inward current flowing during the shoulder, with high voltage-activated calcium current also contributing significantly (39%). Unlike calcium current, TTX-resistant sodium current is not accompanied by opposing calcium-activated potassium current and may provide an effective mechanism by which the duration of
action potentials (and consequently calcium entry) can be regulated.
Key words:
action potential; excitability; dorsal root ganglion; nociceptor; sodium channel; tetrodotoxin
Copyright © 2002 Society for Neuroscience 0270-6474/02/222310277-14$05.00/0
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