Effects Of Melatonin On The Spontaneous Firing Of Rat Hypothalamic Suprachiasmatic Nucleus Neurons

The spontaneous firing of hypothalamic suprachiasmatic nucleus (SCN) neurons was recorded in vivo extracellularly during the subjective day (CT1O-14) and the subjective night (CT22-02) after pinealectomy (PINX) and after inhibition of endogenous melatonin (MEL) synthesis by methotrexate (MTX), and the effect of exogenous MEL on the neuronal firing of SCN was examined. The results were as follows: (1) In the spontaneous firing of SCN neurons there exist three types of discharge (regular firing, irregular firing and burst firing), which persist during both the subjective day and the subjective night, as well as after PINX and after inhibition of endogenous MEL synthesis. (2) The firing rate of SCN neurons was high during the subjective day and low during the subjective night, this circadian rhythm disappeared and the firing rate was increased after PINX and after inhibition of endogenous MEL synthesis. (3) The effect of exogenous MEL on SCN neurons was inhibitory in nature, which was blocked by ML-l receptor antagonist luzindole, but not by ML-2 receptor antagonist prazosln. The results suggested that the pineal gland ,as a regulator in circadian rhythm organization, secretes MEL, which, in turn,elicits an inhibitory regulatory effect on SCN neurons through ML-l receptor.