Isolation stress increases tyrosine hydroxylase mRNA in the locus coeruleus and midbrain and decreases proenkephalin mRNA in the striatum and nucleus accumbens.

Isolation of adult animals represents a form of psychological stress from which the animals cannot escape. In order to assess the effect of this stressor on neurochemical substrates in the brain, we assessed behavior and measured tyrosine hydroxylase and proenkephalin mRNA levels in selected brain areas by in situ hybridization histochemistry. Tyrosine hydroxylase (TH) mRNA levels in the locus coeruleus (LC) were significantly and progressively increased by 18, 42 and 68% after 7, 14 or 28 days of isolation, respectively. TH mRNA in the midbrain was transiently increased by isolation. Levels were significantly elevated by 34 and 48% above group-housed controls in the ventral tegmentum and the substantia nigra, respectively, after 14 days of isolation. In the forebrain, proenkephalin (PE) mRNA levels were found to be transiently decreased by 29% in the anterior and medial aspects of the caudate-putamen and the nucleus accumbens after 7 or 14 days of isolation stress, but the levels returned toward control levels after 28 days of isolation. Behavioral tests indicate that isolated animals progressively became more aggressive with duration of stress and showed a small but significant decrease in locomotor activity. The results demonstrate that a physically noninvasive stressor such as isolation of adult male rats can produce significant alterations in brain neurochemistry. The neurochemical responses observed may represent a brain mechanism designed to help the organism adapt to or protect from the deleterious effects of chronic psychological stress.