Calcium-Independent Depolarization-Activated Potassium Currnets in Superior
Colliculus-Projecting Rat Visual Cortical Neurons.
Albert, Jennifer L., and Jeanne M. Nerbonne.
Department of Molecular Biology and Pharmacology, Washington University
School of Medicine, St. Louis, MO 63110, USA.
APStracts 2:0010N, 1995.
SUMMARY AND CONCLUSIONS
1. K+ conductances were characterized in isolated, identified superior
colliculus-projecting (SCP) rat visual cortical neurons. SCP neurons were
identified in vitro under epifluorescence illumination following in vivo
retrograde labelling with rhodamine-labelled microspheres or "beads". For
experiments, SCP neurons were isolated from the primary visual cortex of
postnatal day 7 to 16 (P7-P16) Long Evans rat pups following bead injections
into the ipsilateral superior colliculus at postnatal day 5. 2. Recording
conditions were optimized to allow the characterization of Ca++-independent K+
conductances. SCP cells that were largely devoid of processes were selected
for recording and experiments were completed 2 to 30 hrs after cell isolation.
Ca++-independent, depolarization-activated K+ currents were routinely recorded
during 200 ms voltage steps to potentials positive to -50 mV from a holding
potential of -70 mV. 3. Peak outward current densities and the relative
amplitudes of the peak and plateau outward currents evoked during 200 ms
voltage steps varied among SCP cells. Although cells were isolated from
animals at different ages (P7-P16) and maintained for varying times in vitro
(2-30 hrs), no correlations were found between the variations in peak current
densities or peak to plateau current ratios and the age of the animal from
which the cell was isolated or the length of time the cell was maintained in
vitro prior to recording. 4. Pharmacological experiments revealed the
coexpression of three K+ current components in SCP cells that could be
separated based on differing sensitivities to the K+ channel blockers, 4-
aminopyridine (4-AP) and tetraethylammonium (TEA). Varying the concentration
of 4-AP, for example, facilitated the separation of two rapidly-activating K+
currents similar to A (IA) and D (ID) type currents in other cells. ID in SCP
neurons is blocked by [mu]M concentrations of 4-AP, whereas mM concentrations
of 4-AP are required to effect complete block of IA in these cells. The
current component remaining in the presence of high concentrations (5-10 MM)
of 4-AP is slowly activating outward K+ current, similar to delayed rectifier
(IK) currents in other cells. IK in SCP neurons is blocked by mM
concentrations of TEA. 5. Activation of IA, ID and IK in SCP neurons is
voltage-dependent, although the three current components display distinct
time- and voltage- dependent properties. For example, although both IA and ID
begin to activate at approximately -50 mV, IA activates two to three times
faster than ID. In addition, the threshold for activation of IK (-30 mV) is
approximately 20 mV depolarized from that of IA (or ID), and the voltage
dependence of IK activation is steeper than that of IA and ID. 6. In
contrast to activation, current inactivation rates are voltage- independent.
Inactivation of IA is fast, proceeding with a time constant of approximately
20 ms at all test potentials; the rates of inactivation of ID and IK are
approximately three and 100 fold slower, respectively. The voltage-
dependences of steady-state inactivation of the three current components are
quite similar, although the voltage at which half the channels are
inactivated is slightly more depolarized for I than for IA and IK. 8. The
experiments here reveal that there is some variability in the expression of
IA, ID and IK among SCP cells. For example, although IA and IK are
coexpressed in all SCP neurons, ID was not evident in approximately 20 % of
the cells studied. In addition, the relative densities of IA, ID and IK vary
among cells although, in all cells, IK is the dominant outward K+ current
component. 9. The properties of ID, IA and IK in SCP neurons are compared to
those of similar currents characterized in other mammalian neurons, and the
physiological significance of coexpression of these Ca 2+-independent,
depolarization-activated K+ currents in SCP neurons is discussed.
Received 15 August 1994; accepted in final form 18 January 1995.
APS Manuscript Number J512-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 3 April 1995.