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Journal of Neuroscience, Vol 15, 4023-4032, Copyright © 1995 by Society for Neuroscience
Individual variation in and androgen-modulation of the sodium current in electric organ
MB Ferrari, ML McAnelly and HH Zakon
Department of Zoology, University of Texas at Austin 78712, USA.
Electric fish of the genus Sternopygus produce a sinusoidal electric organ
discharge (EOD) of low frequencies in males, high frequencies in females,
and overlapping and intermediate frequencies in juveniles. Correspondingly,
the cells of the electric organ, the electrocytes, generate action
potentials which are of long duration in mature males, short duration in
females, and intermediate duration in immatures. The androgen
dihydrotestosterone (DHT) lowers EOD frequency and increases electrocyte
action potential duration. We examined the electrocytes under voltage clamp
to determine whether variations in the kinetic properties of the Na+
current might underlie these phenomena. We found that the fast inactivation
time constants of the peak Na+ current (0 mV) ranged from 0.5 to 4.7 msec
and varied systematically with EOD frequency and action potential duration.
Voltage dependence of steady- state inactivation also varied with EOD
frequency with the midpoint of inactivation being more positive in fish
with low EOD frequencies. There was no correlation between the voltage at
which the Na+ current activates, voltage at peak current, reversal
potential, rate of recovery from inactivation, or TTX sensitivity and EOD
frequency. We tested whether DHT influenced Na+ current inactivation by
recording from electrocytes before and after juvenile fish of both sexes
were implanted with a DHT-containing or empty capsule. We found that
inactivation time constants were significantly slower in DHT implanted, but
not control, fish. This is the first observation of functionally relevant
individual variation in the kinetics of a Na+ current and the first
demonstration that the kinetics of a Na+ current may be modulated by an
androgen.
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