Isolation reduces midbrain tryptophan hydroxylase activity in mice.

Male C57 mice were divided into two experimental groups: "Isolated" and "Grouped". Isolated mice were housed individually for 30 or 45 days; Grouped mice were housed five per cage. Midbrain tryptophan hydroxylase activity was determined at the end of the isolation period. Isolated mice showed 35% less tryptophan hydroxylase activity than grouped mice (P less than 0.001). Prolongation of the isolation (45 vs 30 days) did not further reduce the enzyme activity. The changes in tryptophan hydroxylase activity may be related to behavioral changes induced by isolation.