Seasonal Changes in Circadian Peripheral Plasma Concentrations of Melatonin, Serotonin, Dopamine and Cortisol in Aged Horses with Cushing’s Disease under Natural Photoperiod

Abstract

Equine pituitary pars intermedia dysfunction (PPID) is a common and serious condition that gives rise to Cushing’s disease. In the older horse, it results in hyperadrenocorticism and disrupted energy metabolism, the severity of which varies with the time of year. To gain insight into the mechanism of its pathogenesis, 24-h profiles for peripheral plasma melatonin, serotonin, dopamine and cortisol concentrations were determined at the winter and summer solstices, and the autumn and spring equinoxes in six horses diagnosed with Cushing’s disease and six matched controls. The nocturnal rises in plasma melatonin concentrations, although different across seasons, were broadly of the same duration and similar amplitude in both groups of animals (P > 0.05). The plasma concentrations of cortisol did not show seasonal variation and were different in diseased horses only in the summer when they were higher across the entire 24-h period (P < 0.05). Serotonin concentrations were not significantly affected by time of year but tended to be lower in Cushingoid horses (P = 0.07). By contrast, dopamine output showed seasonal variation and was significantly lower in the Cushing’s group in the summer and autumn (P < 0.05). The finding that the profiles of circulating melatonin are similar in Cushingoid and control horses reveals that the inability to read time of year by animals suffering from Cushing’s syndrome is an unlikely reason for the disease. In addition, the results provide evidence that alterations in the dopaminergic and serotoninergic systems may participate in the pathogenesis of PPID.

Medición de hormonas tiroideas y cortisol en caballos mediante un analizador de inmunoensayo automatizado

“…In addition, the selected reference interval for FT4 was 3.60 -16.09 pmol/L (Breuhaus et al 2006;Graves et al 2006;Mendoza et al 2013). Besides, the selected reference range for cortisol, 23.45 -166.64 nmol/L, coincides with previous reports using radioimmunoassay (Haritou et al 2008;Mair & Sherlock 2009;Schenk et al 2012;Liburt et al 2013). The analysis of environmental parameters on the hormone values revealed a significant effect (p>0.05) on the following correlations: i) animals at higher altitudes ranging from 113 to 1200 masl have higher levels of cortisol; ii) animals under warmer conditions (ranging 17.7-27.9°C) have lower cortisol values; iii) under higher precipitation conditions (ranging from 0.1 to 469.8 L/m 2 ), animals show increased levels of FT4.…”

“…Basal cortisol concentration in horses is susceptible to increases induced by manipulation stress, circadian rhythm, exercise, transportation, sexual excitement, restraint via twitch, hypoglycemia and isolation stress (Haritou et al 2008;Peeters et al 2011;Ayala et al 2012). For these reasons, establishing a reference interval for basal cortisol is difficult.…”

Seasonal Variation in Serum Concentrations of Selected Metabolic Hormones in Horses

“…However, our samples were drawn at one time of day, and the effects of season and group may have been detected if multiple samples were collected at different times of day. 10 Overall serum glucose was generally higher in horses with EMS that were hyperinsulinemic than in normal horses, despite being within the normal reference range. This might reflect reduced insulin sensitivity in EMS horses in combination with a failure of hyperinsulinemia to adequately compensate.…”

Effect of Age, Season, Body Condition, and Endocrine Status on Serum Free Cortisol Fraction and Insulin Concentration in Horses

“…An effect of season on hypothalamic–pituitary–adrenal (HPA) axis activity is well documented in horses and ponies, resulting in autumn increases in circulating ACTH and α‐melanocyte‐stimulating hormone (α‐MSH) concentrations and failure to suppress endogenous cortisol in response to dexamethasone in both healthy and PPID animals 15, 23, 24, 25, 26, 27, 28. Previous studies have yielded conflicting results on the impact of season on cortisol concentrations in healthy animals, with some studies showing no effect of season on basal total cortisol concentrations15, 27, 64, 65 or 24‐hour cortisol secretion,66 and others demonstrating increased 24‐hour cortisol secretion in the spring 26. Our results differ from these previous reports because higher total cortisol concentrations were detected in this group of 57 healthy horses in the winter compared to the fall.…”