Journal of Neuroscience, Vol 11, 1713-1726, Copyright © 1991 by Society for Neuroscience
Evidence for voltage-activated outward currents in the neuropilar membrane of locust nonspiking local interneurons
G Laurent
Department of Zoology, University of Cambridge, England.
Outward currents activated by depolarization were studied in the neuropilar
membrane of locust nonspiking local interneurons, using the
single-electrode voltage-clamp technique in situ. Preliminary observation
of these currents in 272 neurons revealed two families. The first and most
commonly observed (85% of recordings) showed a large transient current
followed by a slowly decaying/late current. The second (15% of recordings)
showed an additional outward current with a slow rate of activation, a peak
within 100-150 msec, and a slow rate of inactivation. Only neurons of the
first type were studied further. The transient current was activated by
depolarization around -60 mV, with a time to peak of approximately 11 msec
at -50 mV and less than 3 msec at -20 mV. This current decayed
exponentially, with a time constant of 8.1 +/- 1.6 msec (n = 8
interneurons) at -30 mV. This time constant of inactivation did not appear
to depend strongly on membrane voltage, in the range in which it was
studied. A second and longer time constant of inactivation of 50-400 msec
could not be assigned to either of the transient and late components of the
outward current. The ratio of transient-to-late current varied between 1.6
and 5.4, with a mean of about 2.5. The reversal potential for the transient
current could, on average, be shifted by 14 mV by a threefold increase in
the bath K+ concentration, indicating that K+ is a charge carrier for the
current. The transient current became inactivated with maintained
depolarization and appeared half-inactivated at about -60 mV (slope factor
k1/2 = 8 mV). This current was thus not fully inactivated at "resting"
potential (average, -58 mV). Recovery from inactivation followed a single
exponential time course, with a time constant of approximately 100 msec at
-80 mV. The time course of recovery from inactivation of the transient
current was well correlated with that of the recovery of transient outward
rectification, as measured in current-clamp recording. Tetraethylammonium,
at a bath concentration of 10 mM reduced the transient current by 70% and
the delayed current by 60%. 4- Aminopyridine, at a bath concentration of 5
mM, had a significant effect in only two of five interneurons, reducing the
transient current by approximately 85% and the late current by
approximately 15%. Quinidine at a bath concentration of 100 microM was
ineffective. Although these blockers did not allow a clear pharmacological
separation of the currents, they were effective in reducing the outward
rectification observed in current clamp during step
depolarization.(ABSTRACT TRUNCATED AT 400 WORDS)
Previous Article | Next Article
