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Comparison of opioid receptor binding in horse, guinea pig, and rat
Are there differences in the opioid receptors in the cerebral cortex and cerebellum of horse, rat, and guinea pig? A very interesting question! In this study, the density and binding characteristics of opioid receptor subtypes in these species were compared. Horses show more ì-receptors in the cerebral cortex but nobody knows the significance of this finding.

Whole brains were obtained from four neurologically normal adult horses during necropsy. Rat and guinea pig brains were obtained commercially.

The cerebellum and cerebral cortex were dissected from each brain, and tissue homogenates prepared. A radioligand binding technique with the highly selective ligands [3H]-DAMGO, [3H]-U69593, and [3H]-DPDPE was used to identify the mu- (ì), kappa- (ê) and delta- (ä) opioid receptors, respectively.

Competitive binding assays were performed with these ligands and varying concentrations of one of multiple unlabeled ligands.

Results: While there were marked species differences in relative densities of opioid receptors, all radioligands interacted with their binding sites with high, nanomolar affinity in both the cerebral cortex and cerebellum.

In the horse cerebral cortex, the percentages of total opioid binding sites for the ì-, ê- and ä-receptors were 71%, 14% and 15%, respectively. In the rat and guinea pig cerebral cortex, the corresponding values were 56% ì-, 4% ê- and 40% ä-receptors, and 25% ì-, 37% ê- and 38% ä-receptors, respectively. In horse and guinea pig cerebellum, the binding was 37% ì-, 59% ê- and 4% ä-receptors, and 15% ì-, 76% ê- and 10% ä-receptors, respectively.

For competitive analysis, all competitors of the ì-, ê- and ä-receptors completely displaced [3H]-DAMGO, [3H]-U69593, and [3H]-DPDPE and had inhibitory constants in the nanomolar range.

Conclusion and clinical relevance: Horses used in this study had a greater density of ì-receptors in the cerebral cortex compared with rats and guinea pigs but without further characterization of the functional role of these receptors it is impossible to determine the clinical significance of these data.



Source: Sara M Thomasy, Benjamin C Moeller, Scott D Stanley (2007): Comparison of opioid receptor binding in horse, guinea pig, and rat cerebral cortex and cerebellum. In: Veterinary Anaesthesia and Analgesia 34 (5), 351–358.



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EQUINE

Theiler´s disease in a Trakehner caused by contaminated tetanus vaccinemembers
An 11‐year‐old Trakehner gelding was presented for evaluation of lethargy, decreased appetite, mild icterus, and elevated hepatic enzyme activities. Physical examination, serum chemistry results, and liver biopsy histopathologic findings were supportive of Theiler`s disease. Polymerase chain reaction (PCR) testing results of serum and liver tissue were positive for nonprimate (equine) hepacivirus (NPHV) and a novel equine parvovirus‐hepatitis virus (EqPV‐H). A serious and finally fatal problem, caused by contaminated vaccine.

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