Systemic Propranolol Reduces b-Wave Amplitude in the ERG and Increases IGF-1 Receptor Phosphorylation in Rat Retina

As the overall age of the population increases, so does the need for a better understanding of the aging process in each tissue. We have previously reported that aging is associated with loss of dopamine β-hydroxylase levels in the retina.1 Recently, we have shown that increasing age in rats also produced increased tyrosine phosphorylation of insulin-like growth factor-1 receptor (IGF-1R), a dysfunctional electroretinogram (ERG), and increased apoptosis in retinal lysates.2 Both IGF-1R and vascular endothelial cell growth factor (VEGF) are increased in the aging rat retina, suggesting that normal aging may promote the vascular growth of normally quiescent tissue.3 Occurring concurrently with the increase in VEGF and IGF-1R is an increase in β-1-adrenergic receptor levels, likely because of denervation sensitivity in response to reduced norepinephrine.1 It is unclear whether the changes in IGF-1R and VEGF are related to this loss of norepinephrine in aging. The role of IGF-1R in the retina is still unclear. IGF-1 receptor (IGF-1R) mutations allow for increased life spans in multiple organisms,4,5 which may occur because these animals are resistant to cancer.6,7 IGF-1R may promote cancer and other age-related diseases through its ability to activate both the mitogen activated kinase pathway and the phosphatidylinositol-3-kinase cascade.8,9 In the retina, IGF-1R signaling has been shown to be involved with neovascularization in retinopathy of prematurity,10 diabetic retinopathy,11,12 normal development,13 and normal aging.2 However, the regulation of IGF-1R in the retina is unknown. Insulin-like growth factor binding proteins (IGFBPs) regulate circulating IGF-I bioavailability. All IGFBPs function at some level to regulate the delivery of IGF molecules to local tissues,14 with the predominating IGFBP species determined by cell type and local conditions. Both IGFBP-3 and IGFBP-5 bind the extracellular matrix,14 which provides a physiological “sink” in which to store these IGFBPs. The extracellular matrix is disrupted in normal aging15,16; hence, protein levels of IGFBP-3 and IGFBP-5 may be decreased because of abnormal extracellular matrix. Work in RPE samples of elderly humans has shown that aging can regulate the expression of IGFBP-2 in vitro.17 In other work, all six of the IGFBPs have been localized in the RPE in culture.18 Retinal Muller cells in culture express IGFBP-2, -3, -4, and -6.19 IGFBP-3 is expressed on the surface and in the cytoplasm of human retinal endothelial cells.20 We have previously shown that both retinal Muller cells and endothelial cells have β-adrenergic receptors and, therefore, may regulate IGFBP levels in the retina.21–23 In addition to the IGFBPs, limited work has demonstrated that β-adrenergic receptors may regulate IGF-1 levels and signaling. Evidence from β-2-adrenergic receptor knockout mice demonstrated that loss of β-2-adrenergic receptors in astrocytes increases IGF1R gene expression.24 Whether these findings can be extended to the retina is not known because most work on IGF-1R signaling has investigated the ability of IGF-1R to regulate β-adrenergic receptor signaling.25 Our goal was to determine whether β-adrenergic receptors can regulate IGF-1R levels in the rat retina. Given that aging is associated with a loss of norepinephrine, we hypothesized that the blockade of β-adrenergic receptors in young rats will induce changes in the retina similar to those noted in aging, specifically increased IGF-1R signaling. To test our hypothesis, we placed osmotic pumps containing saline, phentolamine (nonselective α-adrenergic receptor antagonist), or propranolol (nonselective β-adrenergic receptor antagonist) into 8-week-old rats for 21 days of systemic delivery of the drug. The doses and time course for treatment were based on previous work with this technique, which demonstrated that propranolol at 1 mg/d for 21 days could elicit histologic changes in the rat retina and choroid.26 We evaluated IGF-1R signaling and electroretinographic findings of rats in each treatment group to measure whether the blockade of adrenergic receptor signaling in youth could produce changes similar to those that occur during normal aging of the retina.