Assessment of Effects on Visual Function in Larval Zebrafish - Zebrafish: Methods for Assessing Drug Safety and Toxicity - page 193

requires each fish to be mounted individually, dorsal side up, in methylcellulose
and is therefore lower throughput than the OMR assay. The OKR assay has been
used as a secondary assay to analyze specific effects on visual function as a
follow-up to primary screening in the OMR assay, which may be indicative
of effects on either visual function or locomotor activity. Richards et al. (2008)
employed the OKR assay in such a manner to test two compounds, bisoprolol and
spironolactone, which had been identified as false positives in the OMR assay. The
data obtained for bisoprolol are shown in Fig. 15.5. No significant effect of the
compound on OKR was observed (b) but significant inhibition of locomotor
activity at concentrations at which inhibition of OMR was observed (c). Therefore,
in this case it was demonstrated that bisoprolol was a false positive in the OMR
assay due to detrimental effects on locomotor activity in the OMR assay rather
than specific effects on visual function.
In a further study on the effects of compounds on larval zebrafish visual function,
marketed sedatives and muscle relaxants were assessed by OKR (Liu, unpublished
data, 2007). The assay correctly predicted the effects of 8 out of the 10 drugs. The
sedatives trazodone, diphenhydramine, and haloperidol all cause blurred vision as a
known, clinical adverse effect and caused inhibition of OKR at 3-10 M. The muscle

relaxants tizanidine andmethocarbamol, both of which cause adverse visual effects in
the clinic, showed inhibition of OKR in zebrafish. Chlorzoxazone and dantrolene have
no reported adverse visual effects in the clinic and did not inhibit OKR. Diazepamwas
found to be a false negative in this study as it has been reported to cause blurred vision
in the clinic, although it did exhibit the expected sedative effect on larval zebrafish
locomotor function.

m

15.3.3 Electroretinography

Measurement of the electroretinograms of larval zebrafish retinas has been used to
localize defects in the visual pathways of mutant larval zebrafish (Brockerhoff
et al., 1995; Li and Dowling, 1997; Neuhauss et al., 1999). The ERG measures
light-evoked sum potentials at the corneal surface of the retina. The most prominent
features of the ERG are the short, negative a-wave, which originates from the
photoreceptor, followed by the extended, positive b-wave, which reflects the activity
of bipolar neurons (Neuhauss et al., 1999). Adaptation of the larval ERG apparatus has
also been made for the measurement of ERG in adult zebrafish (Makhankov
et al., 2004). The effects of a few compounds on retinal processing or development
have been assessed. The compound APB ( DL -2-amino-4-phosphonobutyric acid) has
been found to affect retinal processing of many vertebrate species by suppressing the
b-wave of the ERG, as is also the case for zebrafish (Saszik et al., 2002; Ren and
Li, 2004). Also, zebrafish exposed to ethanol from 2 to 5dpf showed a reverse
reduction in a- and b-waves and OKR response at concentrations that did not
cause obvious morphological changes to the retina (Matsui et al., 2006). However,
due to the low-throughput nature of the assay, ERG has not been used extensively to
study the effects of compounds on retinal processing.