Serotonergic activation stimulates the pituitary-adrenal axis and alters interleukin-1 mRNA expression in rat brain

Interactions between neurotransmitters and immunomodulators within the central nervous system may be functionally relevant for communication between the immune system and the brain. Previous studies indicate that cytokines such as interleukin-1 (IL-1) alter activity of the serotonergic system at multiple levels. This study tested the hypothesis that serotonergic activation modulates cytokine mRNA expression in brain. Serotonergic activation was induced by injecting rats intraperitoneally (i.p.) prior to dark onset with the serotonin precursor -5-hydroxytryptophan (5-HTP; 100 mg/kg). Cytokine mRNA expression in discrete brain regions at selected time points was determined by means of ribonuclease protection assay. Plasma corticosterone concentrations were also measured to determine if the hypothalamic-pituitary-adrenal axis is activated in response to this treatment, which potentially could exert feedback regulating cytokine message expression in brain. Plasma corticosterone was elevated for 4 h after 5-HTP administration. At this time IL-1a mRNA expression was reduced in the hippocampus, hypothalamus, and brainstem, and IL-1b mRNA was reduced in the hippocampus. Six hours after 5-HTP injection, IL-1b mRNA increased in the hypothalamus. These results show that activation of the serotonergic system affects cytokine message expression in rat brain, possibly by actions of corticosterone.