PHYSIOLOGICAL REGULATION OF MELATONIN RECEPTORS IN RAT SUPRACHIASMATIC NUCLEI - DIURNAL RHYTHMICITY AND EFFECTS OF STRESS

A marked diurnal variation in high-affinity binding of 2-[I-125]iodome latonin ([I-125]MEL) in rat brain sections containing the suprachiasma tic nuclei (SCN) was observed. Binding was highest late in the light p hase and lowest during darkness, in inverse correlation to the serum m elatonin rhythm. Interestingly, only high-affinity sites were present during most of the light phase while both high- and low-affinity sites were detected just before and during darkness. Guanosine triphosphate (GTP) in combination with sodium converted all high affinity sites to a low affinity state suggesting that the two sites observed during da rkness represent the two states of the melatonin receptor. Acute swim- stress caused a significant elevation of serum melatonin, together wit h a decrease in the density of [I-125]MEL binding in the SCN. The inve rse relationship between circulating melatonin levels and binding, und er two different physiological conditions, indicates that this hormone is involved in regulating its own receptors in the SCN.