Re: Seasonal variations dopamine, cortisol and serotonin
Eleanor Kellon, VMD
One problem I have with this study is that the horses were all older (21 to 36) and with an upper normal for ACTH of 50 pg/mL there could easily have been some early PPID animals in the control group.
It has been known for a very long time for both human and equine that cortisol is not a reliable marker. There are simply too many things that can influence it, it's secretion is pulsatile and metabolism by the peripheral tissues prevents single "spot" samples from accurately predicting how much is being produced. Serotonin changes between the two groups did not reach statistical significance and with groups this small I would forget about that.
The dopamine data is the most significant. What it says to me is that the dopamine levels in PPID horses do not change significantly with the seasons, but they do in normal horses. There is less variability over the day in PPID horses and less effect of season/day length. Melatonin alone does not explain the pattern in the normal horses - ? maybe because they are not all normal and maybe because, as Jaini said, other hormones are involved. We know thyroid hormone levels change with season and that prolactin is involved in seasonal reproduction and coat changes.
Eleanor in PA