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ORIGINAL RESEARCH ARTICLE

Inhibition of Melatonin-Induced Ascorbic Acid and LHRH Release by a Nitric Oxide Synthase and Cyclic GMP Inhibitor

Pennington Biomedical Research Center, Louisiana State University, Baton Rouge, Louisiana 70808

To whom requests for reprints should be addressed at ¹ Pennington Biomedical Research Center, Louisiana State University, 6400 Perkins Road, Baton Rouge, LA 70808-4124. E-mail: karants{at}pbrc.edu

Melatonin (MEL), the principle secretory product of the pineal gland, has been shown to function as an antioxidant and free-radical scavenger. We previously showed that the release of ascorbic acid (AA) and luteinizing hormone releasing hormone (LHRH) from medial basal hypothalamus (MBH) was mediated by nitric oxide (NO) that released cyclic guanosine 3'5'-mono-phosphate (cGMP). Therefore, it was of interest to evaluate the effect of MEL on AA and LHRH release and study the effect of a nitric oxide synthase (NOS) inhibitor, 6-anilino-5,8-quinoline-dione (LY 83583), and a guanylyl cyclase (GC) inhibitor, 1H-[1,2,4] oxadiazolo [4,3-a] quinoxalin-1-one (O.D.Q.), on the release process. Because NO has been shown to activate soluble guanylyl cyclase that elicited an elevation of cGMP in target cells, in the current investigation LY 83583, O.D.Q., or N^G-monomethyl-L-arginine (NMMA), a competitive inhibitor of NOS, were used to evaluate their effects on MEL-induced AA and LHRH release. Medial basal hypothalami were incubated in 0.5 ml of Krebs-Ringer bicarbonate (KRB) buffer for 1 hr. Subsequently, the tissues were incubated with graded concentrations of MEL (10^–8 to 10^–4 M), MEL + NMMA (3 x 10^–4 M), MEL + LY 83583 (10^–6 M), or MEL + O.D.Q. (10^–5 M) for 1 hr. Ascorbic acid and LHRH released into the medium were measured by high-performance liquid chromatography (HPLC) and radio-immunoassay (RIA), respectively. Melatonin (10^–6 and 10^–5 M) significantly stimulated both AA and LHRH release, but the lower and the highest concentrations were ineffective. A combination of MEL + NMMA completely blocked both AA and LHRH release, supporting a role for NO in the releasing action. Both LY 83583 and O.D.Q. significantly suppressed MEL-induced AA and LHRH release, emphasizing the role of NOS, GC, and cGMP in mediating the action of MEL. The data of these in vitro experiments support a role for MEL in the hypothalamic control of AA and LHRH release.

Key Words: melatonin • N^G-monomethyl-L-arginine (NMMA) • LY 83583 (NOS inhibitor) • O.D.Q. (GC inhibitor)

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