Changes in responsiveness of the beta-adrenergic and serotonergic pathways of the rabbit corneal epithelium

Adrenergic agonists stimulate the synthesis of cyclic AMP by incubated rabbit corneas with the following order of potency: isoproterenol greater than epinephrine greater than norepinephrine. These agonists have the same order of potency when displacing the specific, beta-adrenergic radioligand, 3H-dihydroalprenolol, from beta-adrenergic receptors on membranes prepared from corneal epithelium. At another locus, serotonin stimulates cyclic AMP synthesis. Inhibition of stimulation in vitro by lysergic acid diethylamide, methysergide, cyproheptadine, and spiroperidol demonstrates the specificity of this pathway for serotonin. Topical epinephrine causes subsensitivity or decreased responsiveness of the beta-adrenergic pathway. There is loss of approximately half the beta-adrenergic receptors from the cornea and a similar loss of epinephrine-stimulated cyclic AMP synthesis, both of which return to control levels in 96 hrs. There is no change in affinity for catecholamines and no loss of responsiveness to prostaglandin E2 or serotonin. Pretreatment with nialamide or subsequent treatment with additional epinephrine does not cause further loss of responsiveness. Supersensitivity or increased responsiveness of this pathway occurs following superior cervical ganglionectomy. Topical serotonin causes decreased responsiveness of the serotonergic pathway. When potentiated by nialamide, serotonin causes almost complete loss of serotonin-stimulated cyclic AMP synthesis for 24-48 hrs. There is no loss of responsiveness to epinephrine. Increased responsiveness of this pathway does not occur following superior cervical ganglionectomy. The authors conclude that the corneal epithelium has both beta 2-adrenergic and serotonin-2 pathways, and each pathway exhibits altered responsiveness by similar mechanisms. In response to exogenous or endogenous stimulation, the beta-adrenergic responsive cells and the serotonergic responsive cells apparently regulate the total number of pathway-specific receptors on their surfaces. Furthermore, the authors postulate that two populations of beta-adrenergic responsive cells exist; those on the apical surface of the epithelium that respond to catecholamine in the tears and those near the basal surface that respond to neuronal catecholamine.