Improving Effects of Long-Term Growth Hormone Treatment on Monoaminergic Neurotransmission and Related Behavioral Tests in Aged Rats

An age-related decline in cognitive functions and physical performance has been associated with reductions in growth hormone (GH) secretion and brain neurotransmitter function. In vivo experiments were performed to study the long-term effects of exogenously administered GH on the central monoaminergic neurotransmitters serotonin, dopamine, and noradrenaline and behavioral tests in old Wistar rats. The accumulation of 5-hydroxytryptophan (5-HTP) and L-3,4-dihydroxyphenylalanine (DOPA) after decarboxylase inhibition was used as a measure of the rate of tryptophan and tyrosine hydroxylation in vivo. Also, the content of the neurotransmitters serotonin, dopamine, and noradrenaline and some metabolites was measured by high-pressure liquid chromatography (HPLC) in the hippocampus and striatum, brain regions involved in adult memory processing and motor coordination. The age-related decline observed in all the neurochemical parameters in control rats was significantly reversed after repeated subcutaneous administration of GH (2 mg/kg per day, 4 weeks). Thus, GH treatment exerted a long-term effect on serotonin, dopamine, and noradrenaline neurotransmission by enhancing neurotransmitter synthesis and metabolism in aged rats. The results obtained after examining working memory tasks in the eight-radial maze and motor ability in the Rotarod treadmill in aged rats were consistent with these neurochemical data; both tests were significantly improved after chronic GH treatment. Overall, these in vivo findings suggest that the positive effects induced by GH on serotonin, dopamine, and noradrenaline neurotransmitters might explain, at least in part, the effects of chronic GH treatment in improving cognitive and motor ability in aged rats, and could aid in preventing or delaying deficits in monoamines associated with learning or motor disabilities.