A long term study on the impact of sociopsychic stress in tree-shrews (Tupaia belangeri) on central and peripheral tyrosine hydroxylase activity

Summary

Experiments were carried out with male and female wild-born tree-shrews from Thailand. Both sexes arrived in a very bad shape. The males are meager, their adrenals enlarged and their testicles withdrawn into the abdomen. Tyrosine hydroxylase activity (THY) in the adrenal is high. These changes were induced by a longer period of stress caused by the housing conditions at the trader's. Animals housed singly after arrival recover. They reach normal body weights after 28 days (35% weight gain compared with newcomers). The THY activity in the adrenal decreases and reaches normal levels within 28 days. Adrenal weights reach control values after 70 days of adaptation (Fig. 1).

The THY activities in many parts of the brain are reduced in newcomers, compared with adapted controls. These changes are most prominent in the hypothalamus, in the basal ganglia and in the septum (Figs. 2, 3). These values remain unchanged until 70 days of recovery; after 119 days of recovery they are increased.

During recovery, the stress-induced THY activity changes in the brain fairly outlast the stress-induced variations of the peripheral parameters (body weight, adrenal weight, adrenal THY activity).

After more than 5 months of recovery and adaptation, the stress experiments started. To separate the more physical from mere psychic stress, which derive from certain social interactions, the male tree-shrews were exposed once a day to the attack of an experienced fighter for 2–5 min. The animals were defeated during these attacks. One group of the defeated animals rested in optical contact with the winner throughout the day (sociopsychic stress). The other group of defeated animals was optically separated from the victor after the combat (fighting-stress). The animals were killed after 3 days. Unfought animals served as controls.

During sociopsychic stress, the body weight decreases and the adrenal weight and the adrenal THY activity increase. Fighting-stress induces similar changes but to a smaller extent (Fig. 1).

The THY activity in the hypothalamus, in the basal ganglia and in the septum decrease during sociopsychic stress. Fighting-stress reduces the THY activity in the hypothalamus and — statistically not significant — in the basal ganglia. In the septum fighting-stress is not sufficient to decrease THY activity (Figs. 2, 3).

There is a correlation between the percentage of daily weight-loss of the individual and its THY activity in the basal ganglia in adapted animals serving as controls or stressed. A further correlation exists between adrenal and basal ganglia THY activities (Figs. 5, 6).

In females, group-housed for 10 days, the THY activity is reduced in the hypothalamus and in the septum, compared with singly housed controls. The peripheral parameters of females show no differences between both groups.