Lipopolysaccharide Increases Plasma Levels of Corticotropin-Releasing Hormone in Rats

Background: Corticotropin-releasing hormone (CRH) is expressed in the brain, immune cells and the gut, where gene expression is upregulated by lipopolysaccharide (LPS) 6 h after injection. Whether these changes are reflected by increased circulating levels of CRH and adrenocorticotropic hormone (ACTH) is unknown. Methods: LPS (100 µg/kg) was injected intraperitoneally in conscious rats, and blood processed for CRH using the new RAPID (reduced temperatures, acidification, protease inhibition, isotopic exogenous controls and dilution) method compared with EDTA blood with or without plasma methanol extraction. Hormone levels were measured by commercial radioimmunoassay. Results: The RAPID method improved blood recovery of 125I-CRH in vitro compared to EDTA only added to the blood without or with methanol extraction (90.8 ± 2.0 vs. 66.9 ± 2.6 and 47.5 ± 2.0%, respectively; p < 0.001 vs. RAPID). Basal CRH levels from blood processed by the RAPID method were 28.9 ± 2.8 pg/ml, and by other methods below the radioimmunoassay detection limit (<10 pg/ml). At 6 h after LPS, CRH plasma levels increased significantly by 2.9 times, and in the proximal colon tended to decrease (–27.6 ± 5.7%; p > 0.05), while circulating levels were unchanged at 3 or 4 h. ACTH levels rose compared to control rats (135.3 ± 13.8 vs. 101.4 ± 6.0 pg/ml; p < 0.05) 30 min after the increase in CRH, while at 3 or 6 h after LPS, the levels were not changed. Conclusion: Intraperitoneal LPS induces a delayed rise in plasma CRH levels associated with an elevation in ACTH plasma levels 30 min later, suggesting that under conditions of immune challenge, CRH of peripheral origin may also contribute to pituitary activation, as detected using the RAPID method of blood processing, which improves CRH recovery.