ACTIVATION AND REGULATION OF ION CHANNELS IN NEURAL CELLS 
David R Corey, Ph.D., Associate Investigator 
Dr. Corey's laboratory is interested in the regula- 
tion of membrane permeability in neural cells, 
which underlies such processes as sensory trans- 
duction, membrane excitability, and synaptic trans- 
mission. The goal is to understand the ion channel 
proteins that mediate permeability, the mechanism 
of their activation, and the processes of their ex- 
pression and regulation. Ion channels are being 
studied in two different systems: in hair cells (the 
mechanically sensitive receptor cells of the inner 
ear) and in glial cells of the optic nerve. 
I. Mechanism of Sensory Transduction in Hair Cells. 
The ion channels that mediate auditory and ves- 
tibular sensory reception are thought to be acti- 
vated directly by mechanical stress applied to them 
via fine filaments. Increased tension in the fila- 
ments, which stretch between adjacent cilia, corre- 
sponds to increased probability of opening. Prog- 
ress has been made toward understanding this 
mechanism and its regulation by an adaptation 
process. 
A. Rate and calcium control of the adaptation 
mechanism. During a maintained positive displace- 
ment there is a decline or adaptation in transduc- 
tion current; this seems to result from a relaxation 
of the gating spring. Similarly, a displacement that 
allows channels to close activates a retensioning 
mechanism that reopens channels; both are such as 
to keep constant the steady-state, channel-open 
probability. Dr. Corey and his colleagues had pre- 
viously found that the relaxation process is much 
faster than tensioning and that the tensioning rate 
and static tension are similar to those of myosin 
moving on actin in other systems. These and other 
data suggested a calcium-regulated, myosin-like 
motile system that regulates string tension. 
With a new preparation of dissociated, patch- 
clamped hair cells. Dr. Corey and his colleagues 
have found that the rate of tensioning becomes in- 
dependent of the position of the bundle, if the 
bundle is moved far enough that the filaments are 
expected to be slack. This suggests that the motile 
element underlying adaptation is attached to the 
filaments. 
The relation between displacement and channel 
opening [the P(X) curve] was also found to be volt- 
age-dependent; this was believed to follow indi- 
rectly from the voltage dependence of calcium 
entry and the calcium dependence of the adapta- 
tion rates. If the filament hypothesis of transduc- 
tion is correct, then an unrestrained bundle should 
move when the membrane potential is changed, 
since potential apparently modulates the tension in 
the filaments. This was observed: average move- 
ments were —40 nm, very close to the value pre- 
dicted from the P(X) shift and the bundle stiffness. 
In addition, the time course of the motion has been 
studied, and its asymmetry and time constants 
agree with the idea that the motion is produced by 
the same motile element. 
B. Protein constituents of purified stereocilia. To 
understand transduction or adaptation at a molecu- 
lar level, it is necessary to know what proteins are in- 
volved in these mechanisms. Dr. Corey's laboratory 
developed a method, using nitrocellulose adhesion, 
for the rapid and efficient isolation of hair cell 
stereocilia from bullfrog sacculus. Polyacrylamide 
gels revealed about a dozen protein bands; actin, 
fimbrin, calmodulin, and calbindin are among them. 
The further localization and identification of these 
proteins has been studied by producing detergent- 
insoluble cores of stereocilia; these contain actin, 
fimbrin, and three unidentified proteins, with molec- 
ular weights between —45 and 60 kDa. 
II. Function of White Matter Glia. 
The function of optic nerve glial cells and their 
interactions with retinal ganglion cells are being in- 
vestigated by Dr. Barbara A. Barres in Dr. Corey's 
laboratory. The optic nerve preparation is used be- 
cause it has a comparatively simple structure. In ad- 
dition to ganglion cell axons, three glial cell types 
are present: type 1 astrocytes, type 2 astrocytes, 
and oligodendrocytes. 
A. Ion channel phenotype of glial cells in vivo. Using 
whole-cell and single-channel patch recording, Dr. 
Barres has found that each optic nerve glial cell type 
expresses a unique set of ion channel types in cul- 
ture. These phenotypes have begun to suggest cer- 
tain hypotheses for glial function. A general concern 
in these studies is that the phenotype of cells in cul- 
ture may not accurately represent that in vivo. For 
instance, type 2 astrocytes cultured in serum-con- 
taining medium express a charybdotoxin-sensitive 
potassium channel that is not expressed by type 2 as- 
trocytes in serum-free medium. Which is the in vivo 
phenotype? 
Continued 
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