Inhibitory Effects of Endotoxin on LH Secretion in the Ovariectomized Monkey Are Prevented by Naloxone but Not by an Interleukin-1 Receptor Antagonist

Abstract

Endotoxin (lipopolysaccharides, LPS), the pathogenic moiety of gram-negative bacteria, is a well-known trigger for the central release of cytokines. The objective of this study is to evaluate the effects of systemic endotoxin administration on LH and cortisol secretion in a non-human primate model and to investigate whether these endocrine effects are mediated by centrally released interleukin-1 (IL-1) using the receptor antagonist to IL-1 (IL-1ra). An additional objective is to investigate whether endogenous opioid peptides mediate these endocrine effects of LPS, using the opiate antagonist naloxone. The experiments were performed in long-term-ovariectomized rhesus monkeys. Blood samples for hormone determination were obtained at 15-min intervals for a period of 8 h, which included a 3-hour baseline period. Since the effective central dose of IL-1ra in the monkey was unknown, in the first experiment we tested the potency of several doses of this antagonist in preventing the effects of centrally administered IL-1α, a cytokine which is known to inhibit LH and stimulate cortisol release. Rhesus monkeys received a 30-min intracerebroventricular infusion of IL-1α (4.2 μg/30 min) alone or together with various doses of IL-1ra (30–180 μg/h i.c.v.). IL-1ra infusion was initiated 1 h before IL-1 and extended over the experimental period. As previously reported, IL-1α induced a significant inhibition of LH, to 36.5 ± 3.3% (mean ± SE) by 5 h as a percentage from the 3-hour baseline. This inhibitory effect was reversed by cotreatment with the 180 µg/h dose of IL-1ra (to 82.5 ± 3.8% by 5 h; NS vs. saline) but not with the lower doses. IL-1 stimulated cortisol release to 165.9 ± 7.7%, but this increase was prevented by IL-1ra (66.6 ± 8.9%; p < 0.05 vs. IL-1, NS vs. saline). In the second experiment, LPS (50 μg) was administered intravenously, alone or in combination with intracerebroventricular IL-1ra infusion. LPS induced a significant decrease in LH secretion (to 57.1 ± 5.2%). These effects were not reversed by intracerebroventricular administration of IL-1ra (52.5 ± 9.6%). Cortisol secretion increased in response to LPS, but this stimulatory effect was not affected by IL-1ra (178.3 ± 13.4 vs. 166.9 ± 5.7%). There were no effects of IL-1ra alone. In experiment 3, we investigated whether the opiate antagonist naloxone reverses the endocrine effects of endotoxin. Both LPS (50 μg) and naloxone (5-mg bolus + 5 mg/h) were infused intravenously. Naloxone was effective in preventing the inhibitory effect of LPS on LH (to 124.6 ± 22.1%, NS vs. saline) but not the increase in cortisol (to 166.7 ± 16.7%; p < 0.05 vs. saline, NS vs. LPS). Naloxone alone has no significant effect on LH or cortisol secretion. These data demonstrate that, in the ovariectomized monkey, a systemic inflammatory/immune- like stress challenge acutely inhibits tonic LH secretion while concomitantly stimulating cortisol release. Although endotoxin is known to affect central cytokine release, these endocrine effects do not require a mediatory role of central IL-1 in the primate. In contrast, endogenous opioid pathways appear to be involved in this process.