Objective To test the hypothesis that the distribution, density, and subtype of opioid and alpha (α)-2 adrenergic receptors within the central nervous system (CNS) are significantly different between horse and dog.
Study design Prospective experimental study.
Animals Three dogs (3 years of age) and three horses (2–5 years of age). Animals were opioid- and α-2 agonist-free at the time of euthanasia.
Methods 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 °C. Receptor autoradiography was performed using established techniques. [3H]DAMGO, [3H]U-69593, and [3H]RX821002 were used for mu (µ)-opioid, kappa (κ)-opioid, and α-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.
Results There was higher binding of µ-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 κ-opioid receptors in the frontal cortex of dogs compared to horses, whereas binding to κ-opioid receptors in the cerebellum was higher in horses. Binding to α-2 adrenergic receptors in the mid-brain was significantly higher in dogs than in horses. There was higher binding of α-2 adrenergic receptors in the dorsomedial and dorsolateral periaqueductal grey of dogs as compared to that of horses.
Conclusion 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 α-2 adrenergic agonists in these species.