Purpose Electroretinography (ERG) is a powerful, noninvasive tool to study the electrophysiology of synaptic activity of the outer plexiform layer of the retina. Aberrant electroretinograms (ERGs) in humans led to the discovery of Dp260, a retinal isoform of the dystrophin protein. Further investigation of Duchenne muscular dystrophy prompted the use of ERG in mouse models. Subcutaneous injection of ketamine is generally used for mouse sedation during ERG studies. With ketamine, however, it is difficult to maintain a constant depth of anesthesia, and certain mutant mouse strains are especially sensitive, resulting in intolerable mortality rates. An alternative inhalation anesthetic, isoflurane, obviates these problems. In order to validate the use of isoflurane as an alternative, the effects of anesthetic, gender, and age on mouse ERGs will be compared.
Methods Photopic and scotopic ERGs were conducted every 3-5 weeks in two cohorts consisting of four male and four female C57Bl/6J mice, randomly selected to receive either ketamine or isoflurane anesthesia. For photopic ERGs mice were light adapted for at least 30 minutes and tested with a background, white light of 30 cd/m2. Dark adaptation began the night prior to scotopic tests. Ketamine cohorts were anesthetized with 0.07-0.1 mL SC injections of a cocktail of ketamine (27.8 mg/mL), xylazine (2.8 mg/mL), and acepromazine (0.56 mg/mL). Since mice are obligate nose-breathers, the isoflurane cohorts were anesthetized with 2% isoflurane in oxygen (0.9 LPM) delivered via a nose cone. Ophthalmic solutions, 1% tropicamide (dilation), 0.5% Proparacaine (analgesic), and methylcellulose (demulcent), were applied to the eye. A platinum wire (200 μm) electrode embedded in a cellulose acetate contact lens was fixed to the cornea and the ERG signal was recorded differentially between corneal and scalp electrodes. A hind-limb electrode served as ground. An incremental series of light flashes (10 μs), from lowest (-3.04 cd-s/m2) to highest (+0.80 cd-s/m2) intensity, were delivered in a custom Ganzfeld stimulator, and the ERG signals recorded.
Results While no significant differences in ERGs were found between anesthetic types in male mice at any age, isoflurane appears to result in larger b-wave amplitudes in young, female mice than ketamine. However, there was no difference in b-wave amplitudes after 90 days of age.
Conclusions With the exception of the b-wave amplitude discrepancy in young female mice under isoflurane anesthesia, the results confirm that inhaled isoflurane is an acceptable alternative to ketamine and, in fact, a preferable alternative in mutant strains susceptible to sudden death.
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