CGRP in relation to neurogenic inflammation and cAMP in the rabbit eye

The effects of topical application of neutral formaldehyde (1%) and intracameral administration of calcitonin gene-related peptide (CGRP, 0.5- or 2.0 micrograms) on the intraocular pressure (IOP), blood-aqueous barrier, pupil size, blood pressure and cyclic AMP (cAMP) content in the aqueous humour of a rabbit were studied. Topical chemical irritation with 1% formaldehyde caused a typical irritative response in the eye with a rise in the IOP, breakdown of the blood-aqueous barrier and miosis. The cAMP content in the aqueous humour was also increased (88.5 +/- 35.0 pmol ml-1, P less than 0.05) when compared with the control group (16.3 +/- 3.6 pmol ml-1). Intracameral administration of CGRP caused a rise in the IOP, breakdown of the blood-aqueous barrier and also systemic hypotension. Miosis was not observed after intracameral CGRP but an increase in the cAMP content in the aqueous humour was seen (130.5 +/- 30.3- and 158.7 +/- 48.1 pmol ml-1, both P less than 0.01, after 0.5 or 2.0 micrograms, respectively). The cAMP concentration in the aqueous humour after topical chemical irritation and intracameral CGRP correlated with the intensity of the breakdown of the blood-aqueous barrier. CGRP seems to cause most, but not all, of the ocular changes after sensory nerve stimulation elicited by topical neutral formaldehyde. Of these CGRP-induced changes, only the breakdown of the blood-aqueous barrier is related to an increase in the cAMP content in the aqueous humour. Contralateral responses after sensory nerve stimulation were similar to contralateral responses to intracameral CGRP.