Growth hormone and insulin-like growth factor-1 (IGF-1) and their influence on cognitive aging.

The concept that growth hormone and IGF-1 are required for normal development of the mammalian body and, more recently the brain, is supported by a vast experimental literature. IGF-1 crosses the blood-brain barrier and in recent years, much attention has focused on age-related decreases in serum growth hormone and IGF-1 as potential mechanisms that may influence cognitive function in the elderly. However, interventional studies are needed to establish a definite link between these hormones and function of the aging brain. In rodents, long-term growth hormone/IGF-1 replacement improves learning and memory in aged rats. While the exact mechanism underlying these cognitive improvements is unknown, growth hormone and IGF-1 replacement to aged animals increases neurogenesis, vascular density, and glucose utilization, and alters NMDA receptor subunit composition in brain areas that are implicated in learning and memory. While these observations offer valuable insight into the influence of growth hormone and IGF-1 on neuronal events in the aged mammal, additional functional studies are required to link these changes to cognitive improvements.