Mediation of the ocular response to injury and irritation: peptides versus prostaglandins.
The immediate response of the rabbit eye to noxious stimulation is mediated neurogenically through antidromic sensory activation and subsequent action of neuropeptides, CGRP, and substance P or a related tachykinin. The rise in ocular tension and breakdown in the BAB in response to injury are initiated by dilation and increased permeability of the ciliary vessels, which bring about disruptive alterations in the squamous epithelium covering the anterior part of the iridial processes and ciliary ridge. PGs formed during the response to certain types of irritation can enhance the ocular changes to trauma by facilitating the neurogenic pathway. In the rabbit eye substance P acts as a strong miotic in response to injury but has no effect on the IOP rise or disruption of the BAB. Conversely, CGRP is a potent vasodilator that displays no spasmogenic action. Species differences in the ocular response to injury have been observed, with lower mammals such as rabbit and rat generally being more responsive than higher mammals. For the most part, these differences are related to the organization of the iris-ciliary complex and to the sensitivity of the effector elements to the neuromediators. Reciprocal modulation exists between the autonomic and sensory nervous elements and the tissue they innervate, so that altered sensitivity of the effector cells develops in selectively denervated eyes. Chronic sympathetic denervation causes hypersensitivity of the eye to irritation. This increase is owing largely to hypertrophied sensory elements with increased sensory neuropeptide levels and consequently augmented sensory activity. Conversely, chronic sensory denervation is accompanied by an increase in catecholamine-forming enzymes and in the neuropeptide NPY. Future research will no doubt reveal neuromodulation of homeostatic, inflammatory, and vegetative processes and the ability of ocular tissues to recover from wounding. Both the discovery of biogenic neuropeptides in peripheral nervous elements of the eye alongside the classical transmitters and the elucidation of their profound effects on the eye afford scope for the development of potential new therapeutic agents.
Unger
W G
WG
Institute of Ophthalmology, London, United Kingdom.
D000818:Animals; D001082:Aqueous Humor / Q000032:analysis; D002415:Cats; D002417:Cattle; D002924:Ciliary Body / Q000378:metabolism; D006224:Cricetinae; D005123:Eye / Q000503:physiopathology*; D005131:Eye Injuries / Q000209:etiology*; D006168:Guinea Pigs; D006801:Humans; D007498:Iris / Q000378:metabolism; D008251:Macaca; D009479:Neuropeptides / Q000494:pharmacology*; D011453:Prostaglandins / Q000494:pharmacology*; D011680:Pupil / Q000187:drug effects; D011817:Rabbits; D051381:Rats; D013045:Species Specificity
D009479:Neuropeptides; D011453:Prostaglandins