Investigation of the signaling mechanisms of the pineal hormone melatonin in the in vitro rat suprachiasmatic nucleus.

摘要

Virtually all eukaryotic organisms on Earth experience daily oscillations in behavior, physiology, and metabolism. These circadian (Latin: circa = about, dies = day) rhythms are controlled in mammals by the master biological clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus in the brain. The SCN controls production of the pineal hormone, melatonin, thereby restricting its release to the nighttime. The SCN, in turn, expresses high-affinity receptors for melatonin, and melatonin can feed back to the SCN to regulate its timing. This study sought to examine the signaling pathways and mechanisms induced in SCN cells by melatonin. Using brain slice electrophysiology, it has been shown that the SCN exhibits a 24-h oscillation in ensemble firing rate that peaks at midday. Melatonin application to the brain slice during times restricted to subjective dusk and dawn advances the peak in firing rate by up to 4 h. This study showed that the shift induced by melatonin is dependent upon activation of protein kinase C (PKC). The necessity of PKC activation, however, does not rule out the involvement of, or interaction with, other significant pathways. This study also showed that elements of the NO/cGMP/PKG signal cascade play no role in the melatonin phase shift, but that melatonin has interactions with several neuromodulators, such as NPY, PACAP, and cAMP. Furthermore, there are at least two different melatonin receptors identified in the rodent SCN, the MT₁ and MT₂ receptors. Using specific antagonists against the MT₂ melatonin receptor, it was determined that the MT₂, and not the MT₁, receptor is responsible for the phase shift induced by melatonin. Inhibition of the MT₂ receptor also inhibited PKC activation by melatonin. At the molecular level, application of melatonin to the SCN at dusk was shown to have different effects on each of three clock gene rnRNAs—Per1, Per2, and Tim. Overall, melatonin was shown to modulate the SCN clock during two windows of sensitivity, by inducing a complex signal cascade involving specific receptors, second messengers, protein kinases, and gene transcription.