A central mechanism is involved in the secretion of ACTH in response to IL-6 in rats: comparison to and interaction with IL-1 beta

Abstract

The cytokines interleukin-1 beta (IL-1 beta), interleukin-6 (IL-6) and tumor necrosis factor-alpha are known to be potent effectors of ACTH secretion. Some of the peripheral effects of IL-1 beta appear to be related to the secretion of IL-6 induced by IL-1 beta. Thus, we evaluated the effect of IL-6 on ACTH secretion and its interaction with IL-1 beta. Rats received recombinant human (rhIL-6) or murine (rmIL-6) IL-6 through indwelling jugular cannulae. rhIL-6 (200 ng or 2 micrograms/rat) produced peak plasma ACTH levels which were 3- to 4-fold greater than basal levels. rmIL-6 produced similar responses. Neither species of IL-6 affected plasma prolactin levels. Comparison of rhIL-1 beta (200 ng) to rhIL-6 (200, 100 or 50 ng) showed that IL-6 elevated ACTH in a dose-dependent manner and that IL-1 beta was significantly more effective. IL-1 beta was also administered concomitantly with or 10 min after IL-6. Delivered together, IL-1 beta (100, 30 or 10 ng) and IL-6 (100 ng) produced significantly higher ACTH levels than when given alone. This additivity was also evident when IL-6 was given 10 min prior to IL-1 beta. The coadministration of IL-6 (2 micrograms) with corticotropin-releasing factor (CRF, 1 micrograms/kg, b.w.) also had an additive effect on ACTH secretion (at 20 min: 300 +/- 40 pg/ml for CRF; 320 +/- 83 pg/ml for IL-6; and 540 +/- 44 pg/ml for CRF + IL-6), whereas a higher dose of CRF (10 micrograms/kg b.w.) yielded ACTH levels of 1,000 +/- 107 pg/ml at 20 min, with no further enhancement by IL-6. Incubation of pituitary cells with IL-6 alone (0.1, 1.0 or 3.0 nM) produced a slight but significant stimulation of ACTH secretion within 2 h in response to the higher doses of IL-6 only (p < 0.05), but did not modify the effect of CRF in vitro. To determine if the action of IL-6 was at a site(s) within the brain, IL-6 (30 or 100 ng/0.5 microliters) was injected into the third cerebroventricle of alert rats. 100 ng IL-6 elicited peak plasma ACTH levels (300 +/- 65 pg/ml) within 30 min; these were significantly higher than the buffer responses (90 +/- 25 pg/ml, p < 0.01), and lower than the responses to 30 ng IL-1 beta (530 +/- 50 pg/ml, p < 0.001). 30 ng IL-6 was ineffective.(ABSTRACT TRUNCATED AT 400 WORDS)