MEASUREMENTS OF OREXIN LEVELS, OREXINS AND COGNATE RECEPTORS IN THE EQUINE CENTRAL NERVOUS SYSTEM

Orexins, also known as hypocretins, are neurotransmitters which are spread throughout the whole body regulating important metabolic processes in mammals. Their primary role lies in the regulation of sleep rhythm, digestion and other endocrinological processes. Orexin-A and -B are known to be the main focus of research into both human and animal sleeping disorders. A lack of these neurotransmitters or their receptors is believed to be responsible for developing narcolepsy in humans, and also causes signs of narcolepsy in mice, rats and dogs, animal species in which orexins have previously been measured [1- knowledge, no studies have yet been performed on orexins presence and distribution in the nervous system of the horse. The goal of this study was to obtain an overview of physiological concentrations of orexin in the equine central nervous system. Determining Orexin-A concentrations in the cerebrospinal fluid (CSF), as well as the measurement of Orexin-A and -B neurons and receptors in the central nervous system (brain and spinal cord), of horses could be a first step towards understanding equine narcolepsy. Eleven horses, which died at the hospital of the University of Veterinary Medicine in Vienna, for reasons other than a neurological disorder, were included in this study. The CSF was collected immediately postmortem from the atlanto-occipital-approach and the brains as well as part of the spinal cord were removed and fixed in 4% formalin. In total, ten different regions of the brain (frontal, parietal, occipital, temporal, basal ganglion, thalamus and hypothalamus, hippocampus, brain stem, cerebellum, spinal cord) were prepared for histopathological examination. The brain samples were analyzed by immunohistochemistry, utilizing antibodies for Orexin-A, Orexin-B, Orexin-A-receptor and Orexin-B-receptor. Each sample was observed at light microscope and the amount of neurons positive for orexins and receptors evaluated. CSF was frozen directly after collection at - -A concentration with a specific ELISA test in the laboratory. In immunohistochemical semiquantitative assessment, positive neuron responses for orexin and receptors were determined, not only in the hypothalamus, but in all 10 regions of the central nervous system. Measured Orexin-A levels in the CSF appeared to be significantly higher in this study than previously reported in horses. However, these results differ to studies of orexin concentrations in other mammals. Based on these clinical findings, the horse might be unusual in its sleeping physiology and its orexin system.