Nociceptive and sympathetic innervations in the abaxial part of the cranial horn of the equine medial meniscus: an immunohistochemical approach

In athletic horses, diseases leading to lameness are of major importance due to the loss of performance and the resultant economic concerns. Although, in the hindlimb, stifle lesions are frequent due to the large and complex joint, and meniscal tears have been identified as the most common soft tissue injuries in this joint, little is known about the mechanism that causes the painful sensation and thus the lameness. The aim of our study was therefore to highlight any peripheral fibres involved in meniscal nociception in five healthy anterior horns of the equine medial meniscus, which is the most common locale reported for equine meniscal injuries. Immunohistochemical stainings were performed using antibodies against Substance P in order to identify nociceptive fibres; antibodies against tyrosine hydroxylase for detecting postganglionic sympathetic fibres; and against glial fibrillary acidic proteins in order to identify Schwann cells. Fundamentally, our work highlights for the first time the presence of nociceptive and sympathetic fibres in equine menisci. They were found in the abaxial part of the anterior horn of the equine medial meniscus. This study suggests thus that when the abaxial part is injured, the meniscus itself could be the source of pain. This finding
provides a better understanding of the nociceptive and sympathetic innervation of the equine meniscus which could contribute towards improving the therapeutic strategies in order to alleviate pain in cases of equine meniscal injury.