High-Voltage Activated Calcium Currents in Basal Forebrain Neurons during
Murchison, David and William H. Griffith.
Department of Medical Pharmacology and Toxicology, College of Medicine,
Texas A&M University Health Science Center, College Station, TX 77843.
APStracts 3:0023N, 1996.
SUMMARY AND CONCLUSIONS
1. Both conventional whole-cell and perforated-patch voltage-clamp recordings
were made of high-voltage activated (HVA) calcium (Ca2+) channel currents in
acutely dissociated medial septum (MS) and nucleus of the diagonal band (nDB)
neurons from young (1-3.5 mo) and aged (19-26.5 mo) Fischer 344 rats. Barium
(Ba2+) was used as the charge carrier to minimize secondary Ca2+-induced
conductances and Ca2+-induced inactivation. 2. When HVA currents generated by
voltage ramps from a holding potential (Vh) of -60 mV were recorded within
minutes following whole-cell formation, no change in peak current density was
observed between young (-44.7 + 2.5 pA/pF, n=93, mean + SE) and aged (-44.2 +
2.1 pA/pF, n=86) cells. However, currents recorded later with voltage step
protocols, revealed a reduction in peak current amplitudes and a trend towards
larger peak current densities in aged cells. From a Vh = -60 mV and with steps
to -10 mV, current densities were -21.5 + 1.9 pA/pF in young (n=55) and -25.0
+ 2.0 pA/pF in aged cells (n=44). The differences in current densities
recorded by the two protocols were explained by non-specific current rundown
and the development of a slow (minutes) inactivation process. Slow-
inactivation was different from conventional rundown of HVA currents because
it was reversible using perforated-patch recordings. 3. Perforated-patch
recordings were used to characterize slow-inactivation. There was
significantly less slow-inactivation in aged cells. When voltage steps (200 ms
duration, from -80 to -10 mV) were delivered at 12 s intervals, slow-
inactivation reduced the current after 15 minutes to 63 + 7% of control in
young and 86 + 4% in aged cells (p = 0.028). When voltage steps were delivered
at 20 s intervals, the current at the 15th step decreased to 93.4 + 1.5% of
control in aged cells compared to 86.6 + 1.6 % in young (p = 0.007). There was
less slow-inactivation with increased intervals between voltage steps and with
shorter step durations. There was also less inactivation with reduced
concentration of charge carrier, indicating a current dependent component to
slow-inactivation. Additionally, a voltage dependent component was evident, as
slow-inactivation was increased at depolarized holding potentials. 4.
Perforated-patch recordings were used to study at least four pharmacologically
distinct fractions of HVA currents in both young and aged cells. Nifedipine
(10 [mu]M) blocked 16.9 + 2.8% and 23.6 + 2.5% of the HVA in young and aged
cells, respectively. -Conotoxin GVIA (500 nM) blocked 53.2 + 5.8% in young and
53.6 + 2.9% in aged cells. In young cells, -Agatoxin IVA (200-400 nM) blocked
28.4 + 2.2% of the HVA current and 29.9 + 2.8% in aged cells. A fraction of
the current (young: 13.8 + 2.2% and aged: 11.4 + 1.6%) was resistant to a
combination of all three antagonists. Cadmium (100 [mu]M) completely blocked
the remaining HVA current. No significant age-related differences in the HVA
current fractions were observed. 5. The HVA current density, current voltage
relationship and voltage dependent activation were unchanged with age.
However, slow-inactivation of HVA currents was reduced in aged cells. The age-
related difference in HVA Ca2+ currents reported here suggests a possible
mechanism by which Ca2+ homeostasis may be altered in aged neurons.
Received 6 September 1995; accepted in final form 17 January 1996.
APS Manuscript Number J590-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 29 January 96