Granulocyte-macrophage colony stimulating factor suppresses LHRH release by inhibition of nitric oxide synthase and stimulation of gamma-aminobutyric acid release.

Several cytokines produced by immune cells act within the hypothalamus and/or on the pituitary to produce the pattern of pituitary hormone secretion that characterizes infection. Granulocyte-macrophage colony stimulating factor (GMCSF) was first described as a hematopoietic cytokine; however, its synthesis is also stimulated during infection, and it has been found in glia in the brain. Previous research indicates that interleukin-1 inhibits release of luteinizing hormone-releasing hormone (LHRH) both in vivo and in vitro. In the present study, we determined that GMCSF inhibited the release of LHRH in vitro and evaluated the mechanisms involved. After a 1-hour preincubation in Krebs-Ringer bicarbonate glucose buffer (KRB), medial basal hypothalamic explants were incubated in KRB together with recombinant murine GMCSF for 0.5 h in a Dubnoff metabolic shaker (50 cycles/min) in an atmosphere of 95% O2/5% CO2. LHRH release into the media was determined by radioimmunoassay. At concentrations of 10(-12) and 10(-11) M, GMCSF significantly inhibited LHRH release. There was a U-shaped dose-response curve and LHRH release was not inhibited at lower or higher cytokine concentrations. The inhibition was specific since it was completely blocked by GMCSF antiserum. Since sodium nitroprusside (NP; 300 microM), a releaser of nitric oxide (NO), stimulates LHRH, presumably by acting within the LHRH neurons, we examined the effect of GMCSF (10(-11) M) on NP-induced LHRH release. It completely suppressed NP-induced release of LHRH. Bicuculline (10(-5) M), a gamma-aminobutyric acid (GABA) receptor antagonist, partially reversed the inhibitory effects of GMCSF on LHRH release. This dose completely reversed the suppression of LHRH release induced by GABA. The present results indicate that the inhibitory effects of GMCSF on LHRH release are partially caused by blockade of NO-induced LHRH release by its activation of GMCSF receptors on GABAergic neurons. The stimulated release of GABA acts on the GABA-a receptors on the LHRH terminals to inhibit their response to NO. At the end of the experiment, NO synthase (NOS) activity was measured in the tissue homogenate by the citrulline method. NOS activity was highly significantly reduced by GMCSF (10(-11) M) indicating that part of its suppressive action on LHRH release is mediated by reduction in NOS activity in the medial basal hypothalamus.