title: Rat brain opioid peptides-circadian rhythm is under control of melatonin.
abstract: Several experiments have revealed an Endogenous Opioid System (EOS)-circadian rhythm. The brain-borne hormone, melatonin (MEL) has been shown to regulate the organism photoperiodic activity and may be implicated in the EOS-circadian rhythm. To explore this hypothesis, we studied the effect of functional pinealectomy on the EOS-circadian rhythm by measuring the immunoreactive content of Met-Enkephalin, Leu-Enkephalin and Synenkephalin in both hypothalamus and hippocampus of the rat brain, using standard radioimmunoassay procedures. Experimental animals exposed to white fluorescent light (WFL) for 15days (<50lux), displayed a disruption of the EOS-circadian rhythm, showing that absence of MEL induced a significant decrease of tissue content of enkephalin peptides at 01:00h during the dark-phase of the 24-h circadian rhythm, when compared to control rats. Functional pinealectomized rats exposed to 4 or 6h period of darkness (used to revert the effects induced by the absence of melatonin) significantly increased the tissue content of ME-IR and LE-IR, when compared to both controls and non-exposed WFL-treated rats. In addition, subcutaneous administration of exogenous melatonin (10, 100, 150, 300, 600microg/kg), in WFL-treated animals produced significant dose-dependent increases of ME-IR in both brain regions tested. Finally, luzindole (melatonin receptor antagonist) administration, was not able to prevent the enkephalin tissue increase, induced with the MEL administration (150microg/kg). This data suggest that MEL not only regulates the EOS-circadian rhythm, but also appears to modulate their synthesis in the rat brain from their respective neurons.
authors.affiliation: Laboratorio de Análisis Químicos, Instituto Nacional de Psiquiatría, Calzada México-Xochimilco #101, Col. San Lorenzo Huipulco, C.P. 14370, México D.F., Mexico. asaicm@imp.edu.mx
authors.forename: M A
authors.initials: MA
authors.lastname: Miguel Asai
authors.forename: L M
authors.initials: LM
authors.lastname: Lilian Mayagoitia
authors.forename: D G
authors.initials: DG
authors.lastname: David García
authors.forename: G M-T
authors.initials: GM
authors.lastname: Gilberto Matamoros-Trejo
authors.forename: M V-T
authors.initials: MV
authors.lastname: Marcela Valdés-Tovar
authors.forename: P L
authors.initials: PL
authors.lastname: Phillipe Leff
mesh_terms: D000818:Animals; D001923:Brain Chemistry; D002940:Circadian Rhythm; D004305:Dose-Response Relationship, Drug; D006624:Hippocampus / Q000737:chemistry; D007031:Hypothalamus / Q000737:chemistry; D008027:Light; D008550:Melatonin / Q000008:administration & dosage / Q000502:physiology*; D018847:Opioid Peptides / Q000378:metabolism*; D051381:Rats
chemical_list: D018847:Opioid Peptides; D008550:Melatonin