Cortisol (CORT), also known as stress hormone, plays a vital role in physiological processes such as electrolyte and fluid balance, cardiovascular homeostasis, carbohydrate, protein and lipid metabolism, immune and inflammatory responses, and sexual development and reproduction. Cortisol levels are influenced by various physiological factors such as race, age, circadian rhythm, seasonality, exercise and pregnancy. Also, some stressful conditions including isolation or transport, among others, modify levels of this hormone in the body. Excesses or deficiencies of cortisol cause important clinical problems such as Cushing's and Addison's syndromes, which contribute substantially to morbidity in equine medicine. Thus, in this review, we will develop the mechanisms of synthesis and regulation, as well as the physiological factors involved and the most important diseases related to the alteration of cortisol secretion in horses and foals.

Serotonin (5-HT) is a neurotransmitter that has important functions such as the physiological regulation of hemostasis, blood clotting, bone metabolism, cardiovascular growth, contractile activity and gastrointestinal motility, renal function, and stress and sexual behavior, among others. In this review, we consider the potential of 5-HT to contribute to the development of various pathological conditions, including metabolic, vascular, and nervous disorders in horses. The values of 5-HT in circulation are modified under common pathological conditions. Thus, laminitis, endotoxemia, surgical cramps, recurrent airway obstruction, Cushing's syndrome, central fatigue, and certain behavioral alterations such as stereotypes and other acute or chronic conditions can cause increased levels of 5-HT.

Dopamine (DA) is an important endogenous catecholamine that exerts generalized effects on both neuronal (as a neurotransmitter) and non-neuronal tissues (as an autocrine or paracrine agent). In the central nervous system (CNS), DA binds to specific membrane receptors present in neurons and plays a key role in the control of motor activity, learning, cognition, affectivity and attention. Horses can also present with hyper-and hypodopaminergic conditions, including stereotypic behaviors and pituitary pars intermedia dysfunction and Parkinsonian's syndrome, respectively. DA biosynthesis also occurs in peripheral tissues, and receptors in various organs such as the kidney, pancreas, lungs and blood vessels outside the CNS have been detected. DA emulates the actions related to the sympathetic nervous system (SNS), promoting the increase in heart rate, blood pressure, electrolyte balance and gastrointestinal (GI) motility. In fact, GI alterations in dopaminergic transmission have been directly or indirectly related to hypomotility and/ or postoperative ileus (POI). On the other hand, there are physiological factors, such as breed, age, exercise and reproductive status that modify DA concentrations. In reproduction, the administration of DA antagonists in the middle/end of the spring and anestrus transition period advances the first ovulation of the year in mares. This chapter offers a brief description of the importance of DA as a neurotransmitter and peripheral hormone. Special attention is paid to: (1) functional alterations that occur in the brain and GI tract in various diseases and (2) current therapy to correct alterations in DA systems.