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Journal of Neuroscience, Vol 13, 1244-1257, Copyright © 1993 by Society for Neuroscience
Multiple types of calcium channels in acutely isolated rat neostriatal neurons
K Hoehn, TW Watson and BA MacVicar
Neuroscience Research Group, University of Calgary, Alberta, Canada.
Voltage-activated high- and low-threshold Ca2+ currents were studied using
whole-cell voltage-clamp techniques and fura-2 fluorescence measurements of
intracellular Ca2+ in neurons acutely isolated from rat neostriatum.
High-threshold Ca2+ currents activated around -40 mV and were present in at
least 95% of neostriatal neurons. The maximum current, 736 +/- 44 pA (mean
+/- SEM, n = 141), was observed around 0 mV. In 70% of neurons,
high-threshold Ca2+ currents exhibited both inactivating and
noninactivating components. The majority of the high- threshold Ca2+
currents appeared to belong neither to the "L-type" nor the "N-type"
classification, since omega-conotoxin (5 microM) decreased this current by
only 29% and nimodipine (10 microM) decreased the noninactivating component
of this current by only 17%. A low-threshold transient (T-type) Ca2+
current was observed in 40% of neurons. When both T-type and high-threshold
Ca2+ currents were present, their maximum amplitudes were 78 +/- 7 pA and
800 +/- 57 pA, respectively (mean +/- SEM, n = 58). At a holding potential
of -100 mV, the T-type Ca2+ current activated around -60 mV, with maximum
current near -40 mV. Steady-state inactivation of the T-type Ca2+ current
was observed at holding potentials positive to -125 mV, and the current was
half- inactivated at -88 mV. Recovery from inactivation to 90% of maximum
occurred within 800 msec. Mn2+ or Co2+ (3 mM) blocked both high- threshold
and T-type Ca2+ currents, whereas Cd2+ (25 microM) or verapamil (50 microM
and 150 microM) preferentially blocked high- threshold over T-type Ca2+
currents. In response to depolarization by 50 mM K+, fura-2 fluorescence
measurements showed increased intracellular Ca2+ in both the soma and the
proximal dendrites of neostriatal neurons that was markedly reduced by 25
microM Cd2+. These findings suggest that high-threshold Ca channels are
present in both the soma and proximal dendrites of neostriatal neurons.
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