The effect of thymectomy and IL-1 on memory: implications for the relationship between immunity and depression

Macrophages produced proinflammatory cytokines and inflammatory responses can cause many symptoms of depression, including direct stimulation of the HPA axis and secretion of cortisol. In depressed patients, hypercortisolism has been well described as one of the major symptoms and also as the cause for hippocampal atrophy and memory impairment. In this paper, the relationships between thymectomy, increased IL-1 levels, and changes in corticosterone and neurotransmitter concentrations in rats are discussed, as well as their implications for memory impairments and depression. In thymectomized rats, deficits in both spatial and fear conditioned memory have been observed. Thymectomy decreases noradrenaline and dopamine levels, and increases serotonergic neurotransmission in limbic areas, without affecting corticosterone concentrations. In a depression model, thymopeptides or IL-2 treatment significantly attenuated changes in behavior, lymphocyte proliferation and neurotransmitters caused by bulbectomy. The reduction of thymic functions may increase IL-1 synthesis. Central IL-1beta administration impairs rat's spatial memory in the Morris water maze and 8 arms radial maze, but enhances conditioned memory in the passive avoidance. These changes can be reversed by either IL-1 receptor antagonist or a glucocorticoid receptor antagonist (RU 486). Furthermore, IL-1-induced changes in some neurotransmitter systems are similar to those observed in thymectomized rats. However, both acute and sub-chronic IL-1 administration increases plasma corticosterone concentrations. Together, these findings suggest that changes in the function of the thymus gland may play an important role in the unbalance between macrophages, cytokines, and lymphocytes, which induces neurotransmitter and neuroendocrine changes, and memory disturbances in depressive illness.