The Generation of Nitric Oxide and Carbon Monoxide Produces Opposite Effects on the Release of Immunoreactive Interleukin-1 from the Rat Hypothalamus in Vitro: Evidence for the Involvement of Different Signaling Pathways

Abstract

Both the cytokine, interleukin-1 (IL-1), and the gaseous neurotransmitters, nitric oxide (NO) and carbon monoxide (CO), have been implicated in the control of neuroendocrine functions, such as the release of CRH and luteotropic hormone-releasing hormone from the hypothalamus. Though increased levels of IL-1 in this brain region are unambiguously associated with enhanced CRH and reduced luteotropic hormone-releasing hormone release, the net effects of the two gases are still unclear, but in vivo and in vitro evidence suggests that the generation of NO and CO within the hypothalamus might counteract the stimulatory effects of IL-1 and bacterial lipopolysaccharide on the neuroendocrine stress axis. In this study, we have investigated the effects of NO and CO on the release of immunoreactive (ir)-IL-1β from the rat hypothalamus in vitro. It was observed that the NO donor, sodium nitroprusside (SNP), stimulates ir-IL-1β release under basal conditions, whereas the increase in CO levels obtained with hemin, the CO precursor through the heme oxygenase pathway, has no effect on basal ir-IL-1β release but inhibits release stimulated by high K+ concentrations. The opposite effects of the two gases on cytokine release seemed to be caused by the activation of different signaling pathways, because: 1) SNP, but not CO-saturated solutions, is able to increase cyclic GMP levels in hypothalamic tissue; 2) CO-saturated solutions increase PGE2 production and release from the hypothalamic explants, whereas SNP has no effect; 3) SNP-stimulated ir-IL-1β release is counteracted by a selective inhibitor of soluble guanylyl cyclase, LY 83583, but not by a cyclooxygenase inhibitor, indomethacin; and 4) conversely, indomethacin, but not LY 83583, reverses the inhibitory effect of hemin on K+-stimulated ir-IL-1β release. It is concluded that NO and CO signal in the rat hypothalamus via the activation of soluble guanylyl cyclase and cyclooxygenase, respectively.