Sensory innervation of the guinea pig extraocular muscles: a 1,1'-dioctadecyl-3,3,3'3'-tetramethylindocarbocyanine perchlorate tracing and calcitonin gene-related peptide immunohistochemical study.

The sensory apparatus of the extraocular muscles attains special interest because of the great variation among different species with respect to the proprioceptors. The sensory innervation of the guinea pig extraocular muscles, lacking both muscle spindles and tendon organs, was investigated with a fluorescence double-labelling method. Primary sensory perikarya were assessed by postmortem application of 1,1'-dioctadecyl-3,3,3'3'-tetramethylindocarbocyanine perchlorate (Di-I) to the extraocular muscle nerves. Traced neurons were found in the ipsilateral ophthalmic part of the trigeminal ganglion. This is in line with findings in other species. Calcitonin gene-related peptide (CGRP) was detected immunohistochemically within the trigeminal ganglion. No somatotopic organization was observed for CGRP-like immunoreactive perikarya. Small (maximal diameter below 30 microm), medium (maximal diameter between 30 and 50 microm), and large (maximal diameter larger than 50 microm) trigeminal ganglion cells were found among the primary afferent perikarya from extraocular muscles. Among CGRP-like immunoreactive cells, only small and medium cells were observed. Double-labelling experiments indicated the CGRP content of primary afferents of the guinea pig extraocular muscles. The relationship to former morphological categories of ganglion cells is discussed. Primary afferent neurons with CGRP-like immunoreactivity might have efferent functions and might also be involved in inflammatory processes of extraocular muscles.