EFFECT OF INTERLEUKIN-1-BETA, TUMOR-NECROSIS-FACTOR-ALPHA AND INTERLEUKIN-6 ON THE CONTROL OF THYROTROPIN SECRETION

Since serum concentrations of tumour necrosis factor-alpha (TNF-alpha), interleukin-1 beta (IL-1 beta), and interleukin-6 (IL-6) are elevated in infectious and inflammatory illnesses, we examined their potential role in contributing to the low TSH concentrations associated with such conditions, both at the level of the pituitary and the hypothalamus. 20 hours exposure to recombinant murine TNF-alpha (10(-11) to 10(-10)mol/l) enhanced the basal and the TRH-stimulated release of TSH by cultured rat anterior pituitary cells, but 4 hours exposure increased only basal. TSH secretion. Recombinant human (rh) IL-1 beta, at a dose of 10(-11)mol/l only, produced a very small increase in basal TSH secretion after 4h, but not 20h, exposure. TRH-stimulated TSH secretion was not affected by IL-1 beta in concentrations up to 10(-10)mol/l, at either exposure time. Rh IL-6 (10(-12) to 10(-9)mol/l), had no effect on basal or TRH-stimulated TSH secretion at either exposure time. TNF-alpha, IL-1 beta and IL-6 all failed to modify the inhibitory response to triiodothyronine (T-3) and thyroxine (T-4) on TSH secretion, under basal or TRH-stimulated conditions. Indirect effects of the cytokines on the stimulation or inhibition of TSH secretion, via TRK or SRIF respectively, were tested in isolated rat hypothalamic slices. 30 min exposure to TNF-alpha, IL-1 beta, or IL-6 had no effect on the basal release of SRIF. However, IL-1 beta, from 2.5x10(-12) to 10(-10)mol/l, produced a dose-dependent enhancement of the SRIF released by 5x10(-2)mol/l extracellular K+. The effect appeared to be mediated via IL-1 receptors, and to involve prostanoid formation, since it was inhibited by IL-1 receptor antagonist protein, 10(-7)mol/l, and indomethacin, 2.8x10(-5)mol/l, respectively. Neither basal nor K+-stimulated TRH release was influenced by TNF-alpha, IL-1 beta, or IL-6. The results indicate that direct effects of these cytokines on the pituitary do not contribute to reduced circulating TSH concentrations during inflammation and infection, but that enhanced hypothalamic release of SRIF, in response to elevated IL-1 beta, could contribute to such a decrease in TSH. None of the cytokines tested decreased hypothalamic TRH release in vitro. However, further in vivo experiments would be required to determine whether a longer exposure to these agents could reduce TRH release either directly, or indirectly via inputs from outside the hypothalamus.