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Vet Anaesth Analg. 2003 Jul;30(3):172-82.

Comparison of opioid and alpha-2 adrenergic receptor binding in horse and dog brain using radioligand autoradiography.

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Department of Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, CO 80523, USA.



To test the hypothesis that the distribution, density, and subtype of opioid and alpha (alpha)-2 adrenergic receptors within the central nervous system (CNS) are significantly different between horse and dog.


Prospective experimental study.


Three dogs (3 years of age) and three horses (2-5 years of age). Animals were opioid- and alpha-2 agonist-free at the time of euthanasia.


Brain tissue was obtained at 126 days post-surgery from dogs and 72 days post-surgery from horses. The brains were removed, sectioned coronally into 1-cm slabs, frozen in methylbutane, which was cooled by liquid nitrogen, and stored at -70 degrees C. Receptor autoradiography was performed using established techniques. [3H]DAMGO, [3H]U-69593, and [3H]RX821002 were used for mu ( micro )-opioid, kappa (kappa)-opioid, and alpha-2 adrenergic-binding assays, respectively. Species differences were analyzed separately for each major brain region by repeated measures anova for subregions followed by Fisher's protected Latin square design (LSD). p < 0.05 was considered significant.


There was higher binding of micro -opioid receptors in the frontal cortex, left somatosensory cortex, colliculus (mid-brain), and granule cell layer of the cerebellum of horses than that of dogs. There was higher binding to kappa-opioid receptors in the frontal cortex of dogs compared to horses, whereas binding to kappa-opioid receptors in the cerebellum was higher in horses. Binding to alpha-2 adrenergic receptors in the mid-brain was significantly higher in dogs than in horses. There was higher binding of alpha-2 adrenergic receptors in the dorsomedial and dorsolateral periaqueductal grey of dogs as compared to that of horses.


The results of this study show that the distribution of these receptors is different between horses and dogs. Further work is needed to understand the relevance of these differences to clinical responses to opioids and alpha-2 adrenergic agonists in these species.

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