Recent studies have revealed that the brain produces interferon-α (IFN-α) in response to noninflammatory as well as inflammatory stress and that it might have a role in normal physiology. When administered intracerebrally, IFN-α causes diverse effects including fever, anorexia, analgesia and changes in the central neuronal activities. These responses are inhibited by the opioid receptor antagonist naloxone. This is consistent with the reports suggesting that recombinant human (rh) IFN-α binds to opioid receptors in rodent brain membrane. We revealed that rhIFN-α altered the activity of thermosensitive neurons in the medial preoptic area (MPO) and glucose-responsive neurons in the ventromedial hypothalamus in an opioid-receptor-dependent way. As a stress which produces opioid-dependent analgesia is known to suppress the cytotoxicity of splenic natural killer cells, we investigated whether the administration of β-endorphin and rhIFN-α may induce a similar immunosuppression. We found that central, but not peripheral, injection of both compounds inhibited natural killer (NK) cytotoxicity. Further studies revealed that rhIFN-α decreased the activity of MPO neurons via opioid receptors and the altered activity of MPO neurons in turn resulted in the activation of corticotropin-releasing factor neurons, thereby suppressing NK cytotoxicity predominantly through activation of the splenic sympathetic nerve and β-receptor mechanisms in splenocytes. Thus, IFN-α may alter the brain activity to exert a feedback effect on the immune system. Further detailed whole-cell clamping analyses on neuronal mechanisms in rat brain tissue slices showed that the inhibitory effect of rhIFN-α on N-methyl- D-aspartate-induced membrane current responses of MPO neurons was mediated not only by opioid receptors but also by the local production of reactive oxygen intermediates, nitric oxide and prostanoids, possibly due to neuron-glial cell interaction.