Figure 3. 10day old chicks reared in CL n17 had significantly lower IOP average of both eyes than th

1
Abstract 1990
Acetazolamide Prevents Choroidal Thickening in
Response to Plus Lens-induced Defocus in

Chicks Reared in
Constant Light.

P.Mathur, S.R.Bhat C.F.Wildsoet
School of Optometry, University of
California Berkeley, Berkeley, CA, USA
Purpose
One-day old chicks were assigned to either a 12hr
light/12 hr dark cycle (DN) or CL. After 10 days,
they were fitted with a 15D lens over one eye
the contralateral eye served as a control. Chicks
received either 70 or 100 mg/day ACZ or vehicle
(dimethyl sulfoxide) (DMSO) in 2 divided oral
doses daily for 5 days. Treatment effects were
measured at baseline and after 5 days of
treatment using high frequency A-scan
ultrasonography. IOP was measured using a Tonopen
at baseline and on day 5, 3 hrs after the last
ACZ/DMSO dose. Data were analyzed by ANOVA
followed by Tukey-Kramer post-hoc test and are
shown graphically as Mean SEM.
To investigate the effect of acetazolamide on
choroidal thickening in response to plus lenses
in chicks reared under constant light conditions.
We have shown that ACZ interferes with the
compensatory responses to imposed defocus in
chicks raised under diurnal light/dark conditions
(Wildsoet, Mathur Wong 2004). To further
investigate the mechanism of action of ACZs
effect on eye growth we employed a constant light
(CL) model of eye growth.
Emmetropization, the process by which young eyes
adjust their growth to eliminate neonatal
refractive errors, can be demonstrated
experimentally using defocusing lenses. In
response to defocus, the chick eye shows
compensatory changes in both choroidal thickness
scleral growth (axial length). (Fig.1)

Under DN, ACZ lowers IOP and decreases choroidal
thickening in response to plus lens wear. In
contrast under CL, ACZ does not lower IOP but
still inhibits the choroidal thickening response
to plus lens wear. The latter result implies that
the effect of AZM on the choroidal thickening
response is not mediated by a lowering of IOP
(CA-II). While CA-IV in the RPE represents an
alternative site of site of action that could
underlie this effect of AZM, our previous study
with benzolamide, a selective CA-IV inhibitor,
showed that inhibition of CA-IV alone has no
effect on the choroidal response to plus lenses
or on IOP (Wildsoet et al. 2004). The finding in
present study that at 10 days of age, chicks
reared in CL had significantly lower IOP than
chicks reared under DN cycle, confirmed the
findings of Lauber et al (1987). These workers
found that at 8 to 10 weeks of age, IOP was
slightly lower in CL chicks compared to DN chicks
even though aqueous outflow was markedly reduced.
This inhibitory effect of CL on aqueous outflow
might explain why AZM did not lower IOP under the
CL conditions. Because AZM inhibits the
choroidal thickening response to plus lenses
under both DN and CL conditions and it also can
be expected to decrease aqueous production under
both conditions, we speculate that these effects
of AZM are causally linked. Our model has aqueous
humor flowing into the choroid in significant
quantity via the uveoscleral outflow pathway, so
contributing to the thickening response of the
choroid.
Figure 3. 10-day old chicks reared in CL (n17)
had significantly lower IOP (average of both
eyes) than those reared in DN (n13) before
commencing drug or lens treatments (p0.0092).
Figure 4. Fall in IOP in response to AZM (70 or
100 mg) seen in DN birds but not in CL birds.
Differences between CL and DN groups
statistically significant (p0.0079). CL (DMSO,
n6), (AZM70, n5), (AZM100, n5), DN (DMSO,
n4) ,(AZM70, n3), (AZM100, n3)
Figure 1. Schematic diagram summarizing the
likely signaling pathway linking the retina with
choroid and sclera, and the effects of plus
lenses.
Acetazolamide (ACZ), a nonselective inhibitor of
carbonic anhydrase (CA), which is found in many
ocular tissues, is known to reduce intraocular
pressure (IOP), via CA II on the ciliary
epithelium it also decreases subretinal pH and
volume via inhibition of CA IV on the basolateral
membrane of the retinal pigment epithelium (RPE)
(Fig.2).
Figure 5. Both CL and DN chicks receiving DMSO
(,) show significant choroid thickening
(psignificantly (pchoroid thickening under both CL and DN
conditions (,). CL (DMSO, n6 , (AZM70, n6),
(AZM100, n6) DN (DMSO, n5), (AZM70, n3),
(AZM100, n4).
Figure 2. Schematic cell showing the location of
different isoenzymes of carbonic anhydrase (CA),
which catalyzes the formation of HCO 3 - from CO2
H2O. CAII is located intracellularly while CAIV
is membrane-bound.  
Wolfensberger TJ (1999). The role of carbonic
anhydrase inhibitors in the management of macular
edema. Doc Ophthalmol. 199997(3-4)387-97. CF
Wildsoet, P Mathur, GK Wong (2004) Acetazolamide,
a carbonic anhydrase inhibitor, interferes with
compensation to lens-induced defocus in chicks.
ARVO abstracts 1241 Lauber JK (1987)
Light-induced avian glaucoma as an animal model
for human primary glaucoma. J Ocul Pharmacol.
1987 Spring3(1)77-100..
Grant Support NEI RO1 EY12932