Reduced humidity experienced by mice in vivo coincides with reduced outflow facility measured ex vivo

Abstract

Mice are routinely used to study aqueous humour dynamics. However, physical factors such as temperature and hydration affect outflow facility in enucleated eyes. This retrospective study examined whether differences in temperature and relative humidity experienced by living mice within their housing environment in vivo coincide with differences in outflow facility measured ex vivo. Facility data and environmental records were collected for one enucleated eye from 116 mice (C57BL/6J males, 9-14 weeks) at two institutions. Outflow facility was reduced when relative humidity was below the lower limit of 45% recommended by the UK Code of Practice, but there was no detectable effect of temperature on outflow facility. Even when accounting for effects of humidity, there were differences in outflow facility measured between institutions and between individual researchers at the same institution. These data indicate that humidity, as well as additional environmental factors experienced by living mice within their housing environment, may significantly affect outflow facility measured ex vivo.

Consensus Recommendation for Mouse Models of Ocular Hypertension to Study Aqueous Humor Outflow and Its Mechanisms

“…To test drug (or other treatment) effects, paired experiments in which both eyes are perfused simultaneously are highly recommended, whenever appropriate, due to the large inter-animal range in normal outflow facility observed even in inbred mice. 110 , 112 When this is not possible, studies should be adequately powered to account for this variability. The pharmacokinetics and pharmacodynamics of drug within the eye must be considered carefully in such studies.…”

“…Poor maintenance of humidity can lead to significant over- or underestimates of facility. 105 , 111 , 112 Aqueous gels or repeated saline drops to the cornea are not necessarily a sufficient solution, as evaporation of the aqueous phase of gels or drops can create an osmotically driven flow across the cornea. 6 A potential alternative option would be to place the mouse eye on the surface of a pool of buffered saline in a humidified chamber or have saline continually applied to the cornea.…”

“…Environmental stresses during the housing of the living animals (e.g., low humidity, noise) affects outflow facility measured ex vivo, 112 so housing conditions should be monitored and carefully controlled.…”

“…Ex vivo outflow facility in mouse and rat eyes is lognormally distributed when measured using the iPerfusion system ( Fig. 2 ) 110 , 112 , 122 ; that is, the distribution of outflow facility values within a population will be asymmetric and skewed toward lower values when plotted on linear axes (but symmetric when plotted on logarithmic axes. This is a consequence of the small mean outflow facility measured in mice, combined with the physical constraint that the facility must remain positive and depends on several multiplicative factors.…”

Smarce1 and Tensin 4 Are Putative Modulators of Corneoscleral Stiffness

“…While QTL analysis may be carried out with even one sample per BXD strain, attempts were made to include n ≥ 8 eyes per BXD strain, as permitted by the breeding propensity of the animals. Environmental factors have been shown to significantly influence outflow facility (Reina-Torres et al, 2019 ), and may contribute significantly to variations in other ocular properties, such as ocular compliance, as well. The ability to resample members of individual BXD strains decreases the influence of environmental and experimental variance, providing for a more accurate assessment of genetic variance.…”