Fecal Glucocorticoid Analysis: Non-invasive Adrenal Monitoring in Equids

The overall goal of this procedure is to demonstrate how adrenal activity can be assessed non-invasively in equines through the extraction and analysis of fecal glucocortocoid metabolites. This method can help us answer key questions in the area of welfare assessment by looking at the physiological impacts of husbandry protocols on domestic horses, all the way through to wild populations of African zebras. The main advantage of this technique is it offers a non-invasive option for measuring the long-term adrenal activity in both domestic and free-ranging horses.

On the day of the experiment, after thawing at room temperature, manually homogenize the fecal samples by mixing within each individual sample bag. Then, for each sample, measure out the appropriate amount of feces on a micro balance in a clean weigh boat, and transfer the samples into individual eight milliliter glass vials. Next, mix five milliliters of 90%methanol in water with each sample, and shake the samples overnight on an orbital shaker at room temperature.

The following morning, vortex the samples and spin them down. At the end of the centrifugation, decant the methanol fractions into individual 16 by 125 millimeter glass tubes. Then briefly place the tubes in a 37-degree Celsius water bath, followed by evaporation of the methanol under air in a fume cupboard.

When all of the methanol has dispersed, resuspend the fecal extracts in one milliliter of 100%methanol. Then transfer the extracts into individual 12 by 75 millimeter polypropylene tubes, cap the tubes, and store the samples at minus 20 degrees Celsius until their analysis. Before beginning the analysis, use a repetitive pipette with a 50 microliter pipette tip to load 50 microliters of polyclonal corticosterone CJM006 antiserum, diluted in coating buffer, per well, in a 96 well microtiter plate.

Then cover the plate with a plate sealer and incubate the antiserum overnight at four degrees Celsius. The next day, immediately before beginning the analysis, use an automated microtiter plate washer to wash the plate five times with wash solution. Then load the nonspecific binding and zero wells with 50 microliters of enzyme-linked immunoassay, or EIA buffer, per well, the standard wells with the appropriate concentrations of corticosterone, and the experimental wells with the fecal extract samples diluted in EIA buffer.

After loading the last sample, immediately add 50 microliters of horseradish perodixase conjugate diluted in EIA buffer to each well, and incubate the microtiter plate in the dark for two hours at room temperature. At the end of the incubation, wash the plate with wash solution five times. Then incubate the samples with 100 microliters per well of freshly prepared, room temperature substrate in the dark at room temperature, with periodic reading of the plate at 405 nanometers on a spectrophotometer.

The incubation is considered complete when the optical density of the zero wells reaches 0.8 to 1.0. To perform a biochemical validation for parallelism, perform a serial dilution on pooled fecal extracts, ideally taken from suspected high-and low-hormone concentration samples from several individuals. Then run the samples on the EIA as just demonstrated.

A parallelism is considered achieved when the serial dilutions of the fecal extracts yield a displacement curve parallel to the standard curve, and the linear regression results demonstrate an R squared greater than 0.9, a slope greater than 0.5, and a P less than 05. To perform a biochemical validation for interference, spike 200 microliters of serial diluted standards with 200 microliters of the appropriate pooled fecal extract, diluted at the 50%binding as determined by the parallelism assay. Then run the samples on the EIA as demonstrated.

Following the EIA analysis, plot the observed concentration minus the background concentration of the zero sample versus the expected concentration. No interference is considered achieved if the addition of diluted pooled fecal extract to the standards does not significantly alter the amount expected, and linear regression results yields R squared greater than 0.9, a slope close to one, and P less than 05. To perform a biological validation, run samples extracted before, and after a challenging event on the EIA as just demonstrated.

The biological validation is a critical step for the validation of EIA, and is considered achieved when a significant rise in glucocorticoid metabolites is observed after a challenging event. The data in graph A and B demonstrate the biochemical validation of adult male and female domestic horses, by demonstrating parallelism with a corticosterone standard curve. No biochemical validation with the cortisol EIA was achieved, as there was no parallelism between the pooled fecal extract and the cortisol standard curve.

These data from challenged male and female domestic horses demonstrate biological validation, as the concentration of fecal corticosterone increased as the level of isolation increased in these animals. Indeed, the horses in the isolated housing situation exhibited significantly higher levels of fecal corticosterone, compared to all of the other housing designs. So assessing hormone concentrations in saliva or plasma usually involves us looking at the native hormone, but when we're assessing it in fecal material, we're looking at a breakdown of that product and looking at the metabolic end products of that hormone.

Indeed, fecal sampling offers a pooled glucocortocoid concentration over time, rather than the single point reflected by saliva or plasma. This makes it more appropriate for measuring long term, chronic or seasonal adrenal activity. So as fecal sample collection is non-invasive to the animal, it's been used extensively to monitor equine adrenal activity in numerous natural and experimental circumstances.

When running EIA, it is important to minimize the time of loading the microtiter plate to under 10 minutes, as longer times will result in drift. It is also important to ensure that duplicate samples have a coefficient of variation of less than 10%and controls have an intra-and inter-assay coefficient variation of 10%and 15%respectively. So after watching this video, you should have a really good understanding of how to extract fecal glucocortocoid metabolites, analyze those extracts in an enzyme immunoassay, and also perform the required validation steps.