Changes with age in rat central monoaminergic system responses to cold stress.

Changes with age in responses to stress of certain central monoaminergic systems were investigated. Three groups of rats, 4, 18 and 29 months old, were exposed to cold and the effect of this stress on hypothalamic tyrosine hydroxylase, and on the metabolism of DA and 5HT in different brain regions was evaluated. Senescent rats were unable for several hours to compensate the loss of body heat. Corticosterone secretion however was equally stimulated. Hypothalamic tyrosine hydroxylase activity was enhanced in the young rats but not in the old ones. However, the two groups of senescent rats did not show the increase in HVA levels noted in striata of young rats 2 hours after cold exposure. In contrast, the 18 and 29-month-old rats presented enhanced serotonergic tonus, indicated by the greater increase in 5HIAA determined by stress.