Melatonin desensitizes endogenous MT₂ melatonin receptors in the rat suprachiasmatic nucleus: relevance for defining the periods of sensitivity of the mammalian circadian clock to melatonin

Abstract

The hormone melatonin phase shifts circadian rhythms generated by the mammalian biological clock, the suprachiasmatic nucleus (SCN) of the hypothalamus, through activation of G protein-coupled MT2 melatonin receptors. This study demonstrated that pretreatment with physiological concentrations of melatonin (30-300 pM or 7-70 pg/mL) decreased the number of hMT2 melatonin receptors heterologously expressed in mammalian cells in a time and concentration-dependent manner. Furthermore, hMT2-GFP melatonin receptors heterologously expressed in immortalized SCN2.2 cells or in non-neuronal mammalian cells were internalized upon pretreatment with both physiological (300 pM or 70 pg/mL) and supraphysiological (10 nM or 2.3 ng/mL) concentrations of melatonin. The decrease in MT2 melatonin receptor number induced by melatonin (300 pM for 1 h) was reversible and reached almost full recovery after 8 h; however, after treatment with 10 nM melatonin full recovery was not attained even after 24 h. This recovery process was partially protein synthesis dependent. Furthermore, exposure to physiological concentrations of melatonin (300 pM) for a time mimicking the nocturnal surge (8 h) desensitized functional responses mediated through melatonin activation of endogenous MT2 receptors, i.e., stimulation of protein kinase C (PKC) in immortalized SCN2.2 cells and phase shifts of circadian rhythms of neuronal firing in the rat SCN brain slice. We conclude that in vivo the nightly secretion of melatonin desensitizes endogenous MT2 melatonin receptors in the mammalian SCN thereby providing a temporally integrated profile of sensitivity of the mammalian biological clock to a melatonin signal.

International Union of Basic and Clinical Pharmacology. LXXV. Nomenclature, Classification, and Pharmacology of G Protein-Coupled Melatonin Receptors

“…Physiological concentrations of nocturnal melatonin (100 -400 pM) are already well above the potency (EC 50 ) for the melatonin receptors, which are activated by picomolar concentrations of melatonin Dubocovich et al, 1997). Daytime concentrations typically fall below 30 pM and yet they can still induce activation and desensitization of melatonin receptors upon prolonged exposure to the hormone (ϳ8 h) (Gerdin et al, 2004b). Blood melatonin levels after administration of an oral dose of 0.3 mg are similar to endogenous levels found in humans at night (Dollins et al, 1994).…”

“…However, oral doses of melatonin or other ligands at Ն1 mg may increase blood levels several times above the concentration necessary to activate melatonin receptors and therefore may alter receptor sensitivity (Dollins et al, 1994;Vachharajani et al, 2003;Mulchahey et al, 2004;Karim et al, 2006). hMT 1 melatonin receptors expressed in heterologous mammalian cells show no observable changes in melatonin-receptor density, affinity, or functional sensitivity after exposure to physiological concentrations of melatonin for a period of time that mimics normal nocturnal exposure (i.e., 8 h) (Gerdin et al, 2004b). By contrast, exposure to supraphysiological concentrations of melatonin (100 nM) increases MT 1 receptor density and decreases receptor affinity, but there is no detectable internalization or loss of MT 1 melatonin membrane receptors in CHO cells (MacKenzie et al, 2002;Gerdin et al, 2003Gerdin et al, , 2004b.…”

The role of the melatoninergic system in epilepsy and comorbid psychiatric disorders

“…However, agomelatine was ineffective in a chronic treatment protocol, a result possibly due to agomelatine-induced receptor desensitization or internalization. In line with this, the receptor-mediated melatonin effect was also found to involve a fine-tuning mechanism of desensitization or internalization, which was strictly dependent upon the treatment protocol (Gerdin et al, 2004;Delagrange and Guardiola-Lemaitre, 1997;Witt-Enderby et al, 2003). Moreover, the anticonvulsant efficacy of agomelatine against i.v.-injected PTZ in mice was reported to involve iNOS or nNOS induction, because selective iNOS and nNOS inhibitors attenuated the effect of agomelatine (Dastgheib and Moezi, 2014).…”

Low Doses of Melatonin Promote Sleep Onset and Maintenance in Older People—An Update

“…Or perhaps because the decrease in melatonin-receptor-sensitivity that very large doses produce [12][13][14] FDA regulations nor university-held patents constrained purveyors of melatonin from selling whatever dose above 1 mg that they might desire. ln any event, the patents on melatonin-for-sleep have now expired, so the hormone's patent status should no longer be a factor diminishing the availability of low-dose preparations.…”