Does noradrenaline behave as a neurotransmitter or hormone in the mammalian retina?

Topical administration of adrenergic receptor pharmaceutics and nerve growth factor

Topical application of nerve growth factor (NGF) and adrenergic receptor pharmaceutics are currently in use for corneal ulcers and glaucoma. A recent interest in the neuroprotective abilities of NGF has led to a renewed interest in NGF as a therapeutic for retinal and choroidal diseases. NGF can promote cell proliferation through actions of the TrkA receptor or promote apoptosis through receptor p75^NTR. This understanding has led to novel interest in the role of NGF for diseases of the posterior eye. The role of β-adrenergic receptor agonists and antagonists for treatments of glaucoma, diabetic retinopathy, and their potential mechanisms of action, are still under investigation. This review discusses the current knowledge and applications of topical NGF and adrenergic receptor drugs for ocular disease.

Beta-adrenergic receptor stimulation modulates iNOS protein levels through p38 and ERK1/2 signaling in human retinal endothelial cells

Diabetic retinopathy is the leading cause of blindness in working-age adults. Recently, data has suggested that some of the pathological changes that occur in the diabetic retina may be due to increased expression of inflammatory markers. We have previously reported that human retinal endothelial cells cultured in high glucose show increased protein levels of iNOS, which were reduced when cells were pre-treated with a beta-1-adrenergic receptor agonist, xamoterol. The cellular signaling involved in this response is not known. The hypothesis of this study was that modulation of PKA activity through beta-adrenergic receptor stimulation can alter members of the mitogen-activated protein kinase (MAP kinase) family to regulate iNOS protein levels. To address this hypothesis, human retinal endothelial cells were grown in high glucose medium, treated with xamoterol, and immunoblotting and ELISA analyses were done to evaluate cellular signaling of PKA, p38 MAP kinase, and p42/p44 MAP kinase. The results indicate that treatment with xamoterol significantly reduced PKA levels in the retinal endothelial cells cultured in high glucose. This reduction in PKA was followed by decreased ratios of phosphorylated p42/p44 and p38 MAP kinases. Blockade of p42/p44 or p38 signaling could restore iNOS protein levels to those prior to xamoterol stimulation, suggesting that beta-adrenergic receptor regulates iNOS protein levels through regulation of PKA and MAP kinase signaling. These results offer new avenues for therapeutic intervention for retinal diseases that involve altered inflammatory marker expression.

Alpha-1-adrenergic modulation of K and Cl transport in bovine retinal pigment epithelium

Intracellular microelectrode techniques were used to characterize the electrical responses of the bovine retinal pigment epithelium (RPE)-choroid to epinephrine (EP) and several other catecholamines that are putative paracrine signals between the neural retina and the RPE. Nanomolar amounts of EP or norepinephrine (NEP), added to the apical bath, caused a series of conductance and voltage changes, first at the basolateral or choroid-facing membrane and then at the apical or retina-facing membrane. The relative potency of several adrenergic agonists and antagonists indicates that EP modulation of RPE transport begins with the activation of apical alpha-1-adrenergic receptors. The membrane-permeable calcium (Ca2+) buffer, amyl-BAPTA (1,2-bis(o-aminophenoxy)-ethane-N,N,N',N' tetraacetic acid) inhibited the EP-induced voltage and conductance changes by approximately 50-80%, implicating [Ca2+]i as a second messenger. This conclusion is supported by experiments using the Ca2+ ionophore A23187, which mimics the effects of EP. The basolateral membrane voltage response to EP was blocked by lowering cell Cl, by the presence of DIDS (4,4'-diisothiocyanostilbene-2,2'-disulfonic acid) in the basal bath, and by current clamping VB to the Cl equilibrium potential. In the latter experiments the EP-induced conductance changes were unaltered, indicating that EP increases basolateral membrane Cl conductance independent of voltage. The EP-induced change in basolateral Cl conductance was followed by a secondary decrease in apical membrane K conductance (approximately 50%) as measured by delta [K]o-induced diffusion potentials. Decreasing apical K from 5 to 2 mM in the presence of EP mimicked the effect of light on RPE apical and basolateral membrane voltage. These results indicate that EP may be an important paracrine signal that provides exquisite control of RPE physiology.