title: Modulation of 2[125I]iodomelatonin binding sites in the guinea pig spleen by melatonin injection is dependent on the dose and period but not the time.
abstract: Effects of dose, time and period of melatonin injection on 2[125I]iodomelatonin binding sites in the guinea pig spleen were studied. Guinea pigs (Dunkin-hartley), kept under 12h light/12 h darkness, were given daily intraperitoneal injections of either vehicle or 0.01, 0.1 or 1 mg melatonin/kg body weight in either early (1 hour after onset of light period) or late light period (1 hour before offset of light period) for 2 or 7 days. To study the effect of opioid antagonist on the binding, intraperitoneal injections of 2 or 20 mg naltrexone/kg body weight alone or together with 0.1 mg melatonin/kg body weight was given daily in late light period for 2 days. 2[125I]Iodomelatonin binding assays were performed on spleen membrane preparations and radioimmunoassays of melatonin levels were carried out in serum and pineal glands collected during mid-light. High dose (1 mg/kg body weight) of melatonin injection elevated the mid-light serum melatonin levels without affecting pineal melatonin levels. Early light injection group had a higher mid-light serum melatonin level. Melatonin injection for 2 days at either time points caused a dose-dependent decrease in Bmax and increase in Kd of 2[125I]iodomelatonin binding sites in the spleen. The response was independent of the time of injection. A greater suppression of binding was achieved by injecting melatonin for 7 days. Naltrexone did not affect the binding by itself and was not able to reverse the melatonin-induced suppression of binding in the spleen. The modulation of the splenic 2[125I]iodomelatonin binding sites by exogenous melatonin suggests that melatonin may act directly on the immune system to affect its function.
authors.affiliation: Department of Physiology, University of Hong Kong.
authors.forename: A M
authors.initials: AM
authors.lastname: Poon
authors.forename: S F
authors.initials: SF
authors.lastname: Pang
mesh_terms: D000818:Animals; D004305:Dose-Response Relationship, Drug; D004334:Drug Administration Schedule; D005260:Female; D006168:Guinea Pigs; D007274:Injections, Intraperitoneal; D007457:Iodine Radioisotopes; D007700:Kinetics; D008297:Male; D008550:Melatonin / Q000008:administration & dosage / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology*; D010870:Pineal Gland / Q000378:metabolism; D011956:Receptors, Cell Surface / Q000187:drug effects / Q000378:metabolism*; D044094:Receptors, Melatonin; D013154:Spleen / Q000187:drug effects / Q000378:metabolism*; D013997:Time Factors
chemical_list: D007457:Iodine Radioisotopes; D011956:Receptors, Cell Surface; D044094:Receptors, Melatonin; C044556:2-iodomelatonin; D008550:Melatonin