Clearance of ¹²⁵I-labeled interleukin-6 from brain into blood following intracerebroventricular injection in rats

To test the hypothesis that interleukin-6 (IL-6) induced within the brain can be released into peripheral blood, ¹²⁵I-labeled IL-6 was injected into the lateral cerebral ventricle of rats, and its concentration in peripheral blood followed serially. Acid-precipitable tracer appeared within 5 min of injection and entered the blood following first-order kinetics (fractional rate, 0.0116 ± 0.0022/min). Comparison of areas under the curve of intracerebroventricular (icv) vs. iv injection showed that 37.1- 46.5% of tracer injected into the lateral cerebral ventricle appeared in the blood over a 4-h period. icv IL-6 exits at least in part via venous drainage (superior sagittal sinus/aortic concentration gradient was 1.47 ± 0.23 and 3.05 ± 0.87 in two separate groups). Prier icy injection of human IL-1β (100 ng) did not alter rate of degradation or of exit of radioiodine-labeled IL-6 from the brain. These studies indicate that a relatively high proportion of IL-6 that arises in the brain enters the peripheral circulation. Direct secretion of IL-6 from brain to blood may be a mechanism by which the brain modifies peripheral metabolic, endocrine, and immune activity.