Genetic variation, chronic stress, and the central and peripheral noradrenergic systems.

Male rats from two inbred strains genetically selected for variation in response to stress were exposed for 9 consecutive days to 50 min of intermittent foot shock and killed 24 h after the last stress session. Maudsley Nonreactive (MNRA/Har) rats, genetically selected for decreased reactivity to stress, exhibited a significantly greater elevation (114% above control levels) in tyrosine hydroxylase activity in the locus ceruleus after chronic stress than Maudsley Reactive (MR/Har) animals (63% above control levels), genetically selected for heightened reactivity to stress. In the peripheral noradrenergic system, chronic stress produced an increase in norepinephrine (NE) content of small intestine and submaxillary gland of MNRA/Har rats, whereas chronic stress had no effect on NE content of these same tissues in MR/Har animals. These results are consistent with the proposition that genetic selection for variation in susceptibility to stress has altered the capacity of the central and peripheral noradrenergic system to adapt to chronic stress.