Withdrawal stress is a common occurrence in opioid users, yet very few studies have examined the effects of morphine withdrawal (MW) on immune functioning or the role of glucocorticoids in MW-induced immunomodulation. This study investigated for the first time the role of glucocorticoids in MW modulation of LPS-induced IL-12p40, a key cytokine playing a pivotal role in immunoprotection. Using WT and -opioid receptor knock-out mice, we show that MW in vivo significantly attenuated LPS-induced IL-12p40 mRNA and protein expression. The role of glucocorticoids in MW modulation of IL-12p40 was investigated using a murine macrophage cell line, CRL2019, in an in vitro MW model. Interestingly, MW alone in the absence of glucocorticoids resulted in a significant reduction in IL-12p40 promoter activity and mRNA and protein expression. EMSA revealed a concurrent decrease in consensus binding to transcription factors NFB, Activator Protein-1, and CCAAT/enhancer-binding protein and Western blot analysis demonstrated a significant activation of LPSinduced ERK1/2 phosphorylation. Interestingly, although glucocorticoid treatment alone also modulated these transcription factors and ERK1/2 activation, the addition of glucocorticoids to MW samples resulted in a greater than additive reduction in the transcription factors and significant hyperactivation of LPS-induced ERK1/2 phosphorylation. ERK inhibitors reversed MW and MW plus corticosterone inhibition of LPS-induced IL-12p40. The potentiating effects of glucocorticoids were non-genomic because nuclear translocation of glucocorticoid receptor was not significantly different between MW and corticosterone treatment. This study demonstrates for the first time that MW and glucocorticoids independently modulate IL-12p40 production through a mechanism involving ERK1/2 hyperactivation and that glucocorticoids can significantly augment MW-induced inhibition of IL-12p40.Drugs of abuse have long been known to contribute to immunosuppression (1) affecting both innate and adaptive immunity in experimental animals as well as in humans (2). Although a wealth of data exists on the effects of morphine treatment on immune function, very few studies have investigated the effects of morphine withdrawal on immune parameters, and of these even fewer describe the underlying mechanisms that modulate morphine withdrawal-induced immunosuppression. Morphine withdrawal can be induced either by simple cessation of morphine (abrupt withdrawal) (3) or by cessation of morphine with administration of morphine antagonists (precipitated withdrawal) (4). Elegant studies from the Eisenstein laboratory (3,(5)(6)(7)(8)(9)(10) were the first to demonstrate that both abrupt and precipitated morphine withdrawal result in significant immunosuppression with significant changes in several key cytokines, including IL-12p40.Stress has been shown to play a major role in several aspects of morphine use and abuse. These roles include precipitating abuse, enhancing drug seeking (11), potentiating responses to other stressors, a...