Association of plasma interleukin-18 levels with emotion regulation and μ-opioid neurotransmitter function in major depression and healthy volunteers.

BACKGROUND: Alterations in central neurotransmission and immune function have been documented in major depression (MDD). Central and peripheral endogenous opioids are linked to immune functioning in animal models, stress-activated, and dysregulated in MDD. We examined the relationship between μ-opioid receptor (OR)-mediated neurotransmission and a proinflammatory cytokine (interleukin [IL]-18).
METHODS: We studied 28 female subjects (14 MDDs, 14 control subjects) with positron emission tomography and [(11)C] carfentanil (μ-OR selective) during neutral and sadness states. With a simple regression model in SPM2 (Wellcome Trust, London, England) we identified brain regions where baseline μ-OR availability (nondisplaceable binding potential [BP(ND)]) and sadness-induced changes in μ-OR BP(ND) were associated with baseline IL-18.
RESULTS: Baseline IL-18 was greater in MDDs than control subjects [t(25) = 2.13, p = .04]. In control subjects IL-18 was correlated with negative emotional ratings at baseline and during sadness induction. In MDDs, IL-18 was positively correlated with baseline regional μ-OR BP(ND) and with sadness-induced μ-opioid system activation in the subgenual anterior cingulate, ventral basal ganglia, and amygdala.
CONCLUSIONS: This study links plasma IL-18 with sadness-induced emotional responses in healthy subjects, the diagnosis of MDD, and μ-opioid functioning, itself involved in stress adaptation, emotion regulation, and reward. This suggests that IL-18 represents a marker associated with emotion regulation/dysregulation at least in part through central opioid mechanisms.