The field of psychoneuroimmunology deals with brainimmune system interactions. Studies involving various stress paradigms have revealed that the brain can influence the immune system through its various neuropeptides, neurohormones, and neurotransmitters. In the accompanying minireview (6), the consequences of stress on the immune system were reviewed; in general, immune suppression results from acute stress (6).Substantial animals responding with a large antibody immune response, as determined by a greater plaque-forming capacity of B cells, had more norepinephrine depletion in the hypothalamic and brain stem nuclei (34). These findings of stimulated norepinephrine metabolism and turnover were thought to be indicative of increased neurogenic activity of neurons containing norepinephrine (34). They were similar to those seen with stress and were likely to be due to stimulation of neurons that release corticotropin-releasing factor (CRF), with the subsequent activation by CRF of sympathetic nuclei in the hypothalamus and locus ceruleus. I now consider the likely mechanism(s) that is responsible for these changes in corticosteroid and norepinephrine metabolism.
CYTOKINES AND THE CRF RESPONSEIt is now known that cytokines such as interleukin-1 (IL-1), tumor necrosis factor alpha (TNF-a), and IL-6, which are released from immune cells, specifically, macrophages during immune system activation, affect the brain. Although a blood-brain barrier exists to exclude watersoluble substances such as circulating peptides and proteins from the brain, this barrier is lacking in the area of the preoptic nucleus of the hypothalamus (1, 57). Certain neurons in the preoptic nucleus have receptors for IL-1, TNF-a, and IL-6, to which the cytokines bind and thereby pass from the circulation to the brain (57). Preoptic neurons communicate with another hypothalamic nucleus, the paraventricular nucleus, which contains neurons that release CRF. As described in the accompanying minireview (6)