Negative regulation of melatonin secretion by melatonin receptors in ovine pinealocytes.

Julie Lépinay,Philippe Delagrange,Catherine Taragnat,Ralf Jockers,Didier Chesneau,Jean-Philippe Dubois,Véronique Bozon,Salvatore V Pizzo

doi:10.1371/journal.pone.0255249

Abstract

Melatonin (MLT) is a biological modulator of circadian and seasonal rhythms and reproduction. The photoperiodic information is detected by retinal photoreceptors and transmitted through nerve transmissions to the pineal gland, where MLT is synthesized and secreted at night into the blood. MLT interacts with two G protein-coupled receptors, MT1 and MT2. The aim of our work was to provide evidence for the presence of MLT receptors in the ovine pineal gland and define their involvement on melatonin secretion. For the first time, we identified the expression of MLT receptors with the specific 2-[125I]-MLT agonistic radioligand in ovin pinealocytes. The values of Kd and Bmax are 2.24 ± 1.1 nM and 20 ± 6.8 fmol/mg. MLT receptors are functional and inhibit cAMP production and activate ERK1/2 through pertussis toxin-sensitive Gi/o proteins. The MLT receptor antagonist/ inverse agonist luzindole increased cAMP production (189 ± 30%) and MLT secretion (866 ± 13%). The effect of luzindole on MLT secretion was additive with the effect of well-described activators of this pathway such as the β-adrenergic agonist isoproterenol and the α-adrenergic agonist phenylephrine. Co-incubation of all three compounds increased MLT secretion by 1236 ± 199%. These results suggest that MLT receptors are involved in the negative regulation of the synthesis of its own ligand in pinealocytes. While adrenergic receptors promote MLT secretion, MLT receptors mitigate this effect to limit the quantity of MLT secreted by the pineal gland.

Highlights

Melatonin (MLT) is a biological modulator of circadian and seasonal rhythms, sleep, reproduction, mood and has antioxidant activity [1,2,3,4,5,6,7,8]
Cells were characterized by immunocytochemistry using specific markers of pinealocytes (Fig 1C) and of astrocytes (Fig 1D)
Our study identifies for the first time active MT receptors in ovine pinealocytes

Summary

Introduction

Melatonin (MLT) is a biological modulator of circadian and seasonal rhythms, sleep, reproduction, mood and has antioxidant activity [1,2,3,4,5,6,7,8]. The photoperiodic information is detected by retinal photoreceptors and transmitted through nerve transmissions to the pineal gland, where MLT is synthesized and secreted at night into the blood. The synthesis of melatonin is regulated by α1- and β1-adrenoreceptors which induce the phosphorylation of Aralkylamine N-Acetyltransferase (AANAT) through cyclic AMP-dependent protein kinase (PKA), a key enzyme in the synthesis of MLT. Melatonin receptor in ovine pinealocytes body, but other tissues, such as the retina, salivary glands, platelets, lymphocytes and gastrointestinal tract, synthesize this hormone locally [9,10,11]. A cytoplasmic enzyme named MT3 or QR2, involved in the detoxification of quinones by reduction, binds MLT, albeit with a lower affinity than MT1 and MT2 receptors [17, 18]

Objectives

Methods

Results

Conclusion