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Endogenous opioids and attenuated hypothalamic-pituitary-adrenal axis responses to immune challenge in pregnant rats

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Original languageEnglish
Pages (from-to)5117-26
Number of pages10
JournalJournal of Neuroscience
Issue number21
Publication statusPublished - 2005


In late pregnant rats, the hypothalamic-pituitary-adrenal (HPA) axis is hyporesponsive to psychogenic stressors. Here, we investigated attenuated HPA responses to an immune challenge and a role for endogenous opioids. ACTH and corticosterone were assayed in blood samples from virgin and 21 d pregnant rats before and after endotoxin [lipopolysaccharide (LPS); 1 microg/kg, i.v.], interleukin-1beta (IL-1beta; 500 ng/kg, i.v.), or vehicle. In virgins, plasma ACTH concentrations increased 1 h after LPS and 15 min after IL-1beta, as did corticosterone, with no responses in pregnant rats. In situ hybridization revealed increased corticotrophin releasing hormone (CRH) mRNA expression in the dorsomedial parvocellular paraventricular nucleus (pPVN) and increased anterior pituitary pro-opiomelanocortin mRNA expression 4 h after IL-1beta in virgins; these responses were absent in pregnant rats. In contrast, immunocytochemistry showed that Fos expression was similarly increased in the nucleus tractus solitarius (NTS) A2 region in virgin and pregnant rats 90 min and 4 h after IL-1beta. Naloxone pretreatment (5 mg/kg, i.v.) restored ACTH and pPVN CRH mRNA responses after IL-1beta in pregnant rats but reduced the CRH mRNA response in virgins without affecting ACTH. Proenkephalin-A and mu-opioid receptor mRNA expression in the NTS was significantly increased in the pregnant rats, indicating upregulated brainstem opioid mechanisms. IL-1beta increased noradrenaline release in the PVN of virgin, but not pregnant, rats. However, naloxone infused directly into the PVN increased noradrenaline release after IL-1beta in pregnant rats. Thus, the HPA axis responses to immune signals are suppressed in pregnancy at the level of pPVN CRH neurons through an opioid mechanism, possibly acting by preterminal autoinhibition of NTS projections to the pPVN.

    Research areas

  • Adrenocorticotropic Hormone, Age Factors, Analysis of Variance, Animals, Cell Count, Chromatography, High Pressure Liquid, Corticotropin-Releasing Hormone, Drug Interactions, Enkephalins, Female, Gene Expression Regulation, Developmental, Hypothalamo-Hypophyseal System, Immunohistochemistry, In Situ Hybridization, Interleukin-1, Lipopolysaccharides, Microdialysis, Naloxone, Narcotic Antagonists, Narcotics, Oncogene Proteins v-fos, Pituitary-Adrenal System, Pregnancy, Pro-Opiomelanocortin, Protein Precursors, Radioimmunoassay, Rats, Rats, Sprague-Dawley, Receptors, Opioid, mu, Solitary Nucleus, Time Factors

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