analysis. Anat Embryol 1995;191:101–118.
7. Cunningham CD, Schulte BA, Schmiedt BN. Immunohisto-
chemical localization of connexin 26 in the human inner
ear. Abst Assoc Res Otolaryngol 2001;24:532.
showing that heptanol, another well-known uncoupler of
gap junctions, produces effects similar to proadifen when
applied to the round window membrane.27
These data help shed light on probable mechanisms
involved in hearing loss caused by defective gap junctions
resulting from GJB2 gene mutations. The findings indi-
cate that hearing loss may ensue from the failure to main-
tain an adequate EP secondary to a diminished capacity to
recycle Kϩ. The findings also demonstrate the exquisite
sensitivity of the cochlea to metabolic changes from both
physiological and morphologic perspectives. In particular,
it appears that the type II and V fibrocytes play a signif-
icant role in cochlear Kϩ homeostasis and are reliant on
the proper functioning of gap junctions.
The chronic exposure trials suggest that the cochlea
has the ability to adapt to gap junction insults, perhaps
either by repairing or replacing damaged channels. The
alterations in DPOAE amplitudes additionally suggest
that the apical cochlea can, under some conditions, be
more sensitive to interruption of Kϩ recycling than the
base. This may be related to the possibility that apical
hair cells use more Kϩ than basal hair cells owing to their
larger size and the fact that there is more low- than
high-frequency energy in the environment.
8. Schulte BA, Steel KP. Expression of ␣ and  subunit isoforms
of Na,K-ATPase in the mouse inner ear and changes with
mutations at the Wv or Sld loci. Hear Res 1994;78:65–76.
9. Spicer SS, Schulte BA. The fine structure of spiral ligament
cells relates to ion return to the stria and varies with
place-frequency. Hear Res 1996;100:80–100.
10. Spicer SS, Schulte BA. Evidence of a medial Kϩ recycling
pathway from inner hair cells. Hear Res 1998;118:1–12.
11. Weber PC, Cunningham CD, Schulte BA. Potassium recy-
cling pathways in the human cochlea. Laryngoscope 2001;
111:1156–1165.
12. Salt AN, Thalmann R. Cochlear fluid dynamics. In: Jahn AF,
Santos-Sacchi J, eds. Physiology of the Ear. New York:
Raven Press, 1988:341–357.
13. Schmiedt RA. Spontaneous rates, thresholds and tuning of
auditory-nerve fibers in the gerbil. Hear Res 1989;42:23–36.
14. Mills JH, Schmiedt RA, Kulish LF. Age related changes in
auditory potentials of Mongolian gerbil. Hear Res 1990;46:
201–210.
15. Hellstrom LI, Schmiedt RA. Frequency selectivity in single-
fiber and whole-nerve turning curves in young and aged
gerbils. J Acoust Soc Am 1996;100:3275–3285.
16. Boettcher FA, Schmiedt RA. Distortion-product otoacoustic
emissions in Mongolian gerbils with resistance to noise-
induced hearing loss. J Acoust Soc Am 1995;98:3215–3222.
17. Chamberlain SC. Neuroanatomical aspects of the gerbil inner
ear: light microscope observations. J Comp Neurol 1977;
171:193–204.
Fibrocyte density decreases dramatically from the
base to the apex of the normal cochlea.28 Lateral wall
degeneration first appears in the apex and then the base
in the gerbil model of presbycusis,29,30 suggesting that the
fibrocyte pathway for Kϩ homeostasis may be an impor-
tant mechanism underlying age-related hearing loss.
18. Tarnowski BI, Schmiedt RA, Hellstrom LI, Lee F, Adams JC.
Age-related changes in cochleas of Mongolian gerbils. Hear
Res 1991;54:123–134.
19. Spicer SS, Thomopoulos GN, Schulte BA. Structural evidence
for ion transport and tectorial membrane maintenance in
the gerbil limbus. Hear Res 2000;143:147–161.
20. Sato Y, Santos-Sacchi J. Cell coupling in the supporting cells
CONCLUSION
of Corti’s organ: sensitivity to intracellular Hϩ and Ca2ϩ
Hear Res 1994;80:21–24.
.
Gap junctions play a significant role in maintaining
normal cochlear function. In particular, they appear to be
essential for maintaining the EP. This may be related to
their role in the Kϩ recycling pathway. Hearing losses
associated with mutations in the GJB2 gene may result
from declines in the EP consequent to a diminished capac-
ity to recycle Kϩ.
21. Salt AN, Melichar I, Thalmann R. Mechanisms of endoco-
chlear potential generation by stria vascularis. Laryngo-
scope 1987;97:984–991.
22. Rozental R, Srinivas M, Spray DC. How to close a gap junc-
tion channel. Efficacies and potencies of uncoupling
agents. Methods in Molecular Biology 2001;154:447–476.
23. Valiunas V, Bukauskas FF, Weingart R. Conductances and
selective permeability of connexin 43 gap junction chan-
nels examined in neonatal rat heart cells. Circ Res 1997;
80:708–719.
24. Schmilinsky-Fluri G, Valiunas V, Willi M, Weingart R. Mod-
ulation of cardiac gap junctions: the mode of action of
arachidonic acid. J Molec Cell Cardiol 1997;29:1703–1713.
25. Song X-Z, Pedersen SE. Electrostatic interactions regulate
desensitization of the nicotinic acetylcholine receptor. Bio-
physical Journal 2000;78:1324–1334.
Acknowledgments
The authors thank Ms. Nancy Smythe and Mrs. Mar-
tha Harvey for technical and administrative assistance.
The work was supported by Grants RO1 AG14748 from
the NIH/NIA (R.A.S.) and RO1 DC00713 (B.A.S.) from the
NIH/NIDCD.
26. Papineni R, Sanchez JU, Baksi K, Willcockson IU, Pedersen
SE. Site-specific charge interactions of ␣-cenotoxin MI with
the nicotinic acetylcholine receptor. J Biol Chem 2001;276:
23589–23598.
27. Noone MC, Lang H, Meetze KA, Schulte BA, Schmiedt RA.
The effects of heptanol, a gap-junction uncoupler, on co-
chlear function in the gerbil. Assoc for Res in Otolaryng
2002;532:139.
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