Objective—To evaluate μ-opioid receptors in synovial
membranes of horses and determine whether these
receptors are up-regulated in nerve endings during
Sample Population—Synovial tissue obtained from
39 client-owned horses during arthroscopy and 14
research horses during necropsy; brain and synovial
tissues were obtained during necropsy from 1 horse,
and control tissues were obtained from a mouse.
Procedure—Horses were classified into 7 groups on
the basis of histologically determined degree of
inflammation. Binding of primary rabbit antibody
developed against μ-opioid receptors in equine synovial
tissue was studied, using western blot analysis.
Synovial membranes were tested for μ-opioid receptors
by immunohistochemical staining, using a
diaminobenzidine-cobalt chloride chromogen.
Homogenates of synovial membranes were evaluated
by use of radioligand binding.
Results—Examination of western blots of equine
thalamus revealed that rabbit antibody developed
against μ-opioid receptors yielded a band (molecular
weight, 55 kd) that corresponded with that of other
opioid receptors. Use of immunohistochemical staining
of synovial tissue revealed considerable staining
in the proliferative lining layer and in regions surrounding
vascular structures. Specific radioligand
binding of tissue homogenates was found in all
groups. We did not detect significant differences in
binding between horses with inflammation and horses
Conclusions and Clinical Relevance—Results of
immunohistochemical analysis and radioligand binding
of tissue homogenates suggest that there are opioid
receptors in synovial membranes of horses. Our
results support the practice of intra-articular administration
of opioids to relieve pain after arthroscopic
surgery in horses. (Am J Vet Res 2001;62:1408–1412).