Classical Proprioceptors and Palisade Endings Have Distinct Molecular Profiles in Pig Eye Muscles

Classical proprioceptors, such as muscle spindles and Golgi tendon organs, are absent from the extraocular muscles (EOMs) of most mammals, whereas palisade endings are present in most mammalian EOMs. Even-toed ungulates such as pigs are the only species with classical proprioceptors together with palisade endings in their EOMs. This study aimed to compare the molecular phenotype of classical proprioceptors and palisade endings in pig EOMs.

EOMs from both eyes of nine pigs were analyzed. EOM cross sections and EOM whole mount preparations were immunolabeled with antibodies against neurofilament, glucose transporter 1 (GLUT1), choline acetyltransferase (ChAT), synaptophysin, synaptobrevin, complexin, and vesicular glutamate transporter 1 (VGLUT1). These were used together with the toxins phalloidin and α-bungarotoxin. The analyses were conducted using confocal laser scanning microscopy.

A capsule, expressing GLUT1, was present in muscle spindles and Golgi tendon organs but not in palisade endings. Conversely, palisade endings express ChAT, whereas muscle spindles and Golgi tendon organs lack it. An exception was the paraequatorial and polar regions of muscle spindles, where ChAT immunoreactivity was present. VGLUT1 was present in Golgi tendon organs and muscle spindles but absent from palisade endings. In classical proprioceptors as well as palisade endings, nerve terminals exhibited synaptophysin, synaptobrevin, and complexin immunoreactivity. Alpha-bungarotoxin was present in the motor terminal of the muscle spindle's paraequatorial and polar regions.

Palisade endings and classical proprioceptors in pig EOMs differ regarding the capsular envelope and neurotransmitters. However, they equate each other with respect to proteins implicated in neurotransmitter release.