Growth hormone administration to aged animals reduces disulfide glutathione levels in hippocampus

Abstract

Systemic growth hormone (GH) and insulin-like growth factor-1 (IGF-1), potent anabolic hormones, decrease with age. In humans and animal models, administration of growth hormone or IGF-1 to aged subjects improves learning and memory, suggesting that the age-related decline in cognitive performance results, in part, from peripheral GH/IGF-1 deficiency. However, the cellular mechanisms by which GH/IGF-1 effect cognitive function are unknown. We propose that the effects of these hormones may be mediated by increasing cellular redox potential resulting in reduced oxidative stress. Because the most abundant endogenous antioxidant is glutathione (GSH), we assessed GSH and disulfide glutathione (GSSH) levels in hippocampus and frontal cortex of young (4-month-old) and aged (30-month-old) male Fisher 344 × Brown Norway rats treated with porcine growth hormone (200 μg/animal, twice/daily) or vehicle. We report that hippocampal levels of GSSG increase with age (0.54 ± 0.08 to 1.55 ± 0.24 nmol GSSG/mg protein, p < 0.05) and growth hormone treatment ameliorates both the age-related rise in GSSG (1.55 ± 0.24 to 0.87 ± 0.24 nmol GSSG/mg protein, p < 0.05) and the decline in GSH/GSSG ratios. Analysis of GSSG reductase activity in aged animals indicated no effect of either age or growth hormone treatment ( p = 0.81). Although similar age-related increases in GSSG and decreases in GSH/GSSG ratios were evident in frontal cortex, growth hormone had no effect. Subsequently, we assessed whether the effects of age and growth hormone treatment result from modulating trace metal accumulation. Thirteen metals were analyzed in hippocampus and frontal cortex by inductive coupled plasma mass spectrometry. Aluminum, copper, iron, manganese and zinc levels increased with age ( p < 0.05 each) but growth hormone replacement had no effect on metal accumulation. Our results indicate that growth hormone replacement attenuates the age-related increase in oxidative stress in hippocampus without effects on glutathione reductase or trace metal accumulation. We conclude that the age-related decline in circulating growth hormone and IGF-1 contribute to increased oxidative stress in hippocampus with age.