Distribution and origin of calcitonin gene‐related peptide (CGRP) immunoreactivity in the sensory innervation of the mammalian eye

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Abstract
The occurrence, distribution, and origin of immunoreactive calcitonin gene‐related peptide (CGRP) in nerves of rat, guinea pig, cat, and monkey eyes were investigated by immunocytochemistry, radioimmunoassay, and chromatography. A rich network of CGRP‐immunoreactive nerve fibres was noted in the anterior uvea, which was widely distributed in both dilator and constrictor papillae muscles and extended to the ciliary body and uveal blood vessels. Numerous CGRP‐immunoreactive neuronal cells were present in the trigeminal ganglion. The extractable CGRP was 8.6 ± 1.8 pmoles/gm of tissue in the iris and 44.0 ± 8.1 pmoles/gm in the trigeminal ganglion. Following damage to the Gasserian ganglion a marked decrease of CGRP immunoreactivity was observed in the anterior uvea (control 11.3± 1.6 pmoles/gm; operated 1.4 ± 0.1 pmoles/gm) confirming the origin of the immunoreactive fibres from trigeminal primary sensory neurons. The sensory nature of the CGRP‐immunoreactive fibres was substantiated by the depletion of CGRP immunoreactivity observed after treatment with capsaicin, which is known to cause selective degeneration of sensory neurons. Comparative studies on the distribution and colocalisation of CGRP and the putative sensory neurotransmitter substance P revealed a closely parallel distribution of the two peptides in certain regions of the uvea and their coexistence in a subpopulation of trigeminal primary sensory neurons. This study suggests that the sensory nervous system in the eye is more heterogeneous in terms of its putative neurotransmitters than previously indicated.