Rat brain opioid peptides-circadian rhythm is under control of melatonin

• Published in: Neuropeptides (Edinburgh) Vol. 41; no. 6; pp. 389 - 397
• Main Authors: Miguel Asai, M.A, Lilian Mayagoitia, L.M, David García, D.G, Gilberto Matamoros-Trejo, G.M.-T, Marcela Valdés-Tovar, M.V.-T, Phillipe Leff, P.L
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01.12.2007; Elsevier
• Subjects: Advanced Basic Science; Animals; Biological and medical sciences; Brain Chemistry; Chronobiology; Circadian Rhythm; Dose-Response Relationship, Drug; Endocrinology & Metabolism; Enkephalins; Functional pinealectomy; Fundamental and applied biological sciences. Psychology; Hippocampus - chemistry; Hypothalamus - chemistry; Light; Melatonin; Melatonin - administration & dosage; Melatonin - physiology; Opioid Peptides - metabolism; Rats; Synthesis; Vertebrates: anatomy and physiology, studies on body, several organs or systems; Vertebrates: endocrinology; Melatonin; Synthesis; Circadian rhythm; Functional pinealectomy; Enkephalins; Rat; Rodentia; Central nervous system; Enkephalin; Biological rhythm; Biosynthesis; Neuropeptide; Opioid peptide; Encephalon; Vertebrata; Mammalia; Pinealectomy; Animal; Pineal hormone
• ISSN: 0143-4179; 1532-2785
• DOI: 10.1016/j.npep.2007.09.001

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 15 days (<50 lux), displayed a disruption of the EOS-circadian rhythm, showing that absence of MEL induced a significant decrease of tissue content of enkephalin peptides at 01:00 h during the dark-phase of the 24-h circadian rhythm, when compared to control rats. Functional pinealectomized rats exposed to 4 or 6 h 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, 600 μg/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 (150 μg/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.