Nocturnal decreases in nitric oxide and cyclic GMP contents in the chick brain and their prevention by light

The diurnal variations in the contents of nitric oxide (NO) and cyclic GMP were studied in the chick brain. NO and cyclic GMP contents in the chick brain were lower at night than during the day and were inversely correlated with high night-time tissue melatonin levels. Furthermore, when animals were kept in light at night, tissue melatonin levels remained at low diurnal values, whereas NO and cyclic GMP contents remained high. Since we have previously shown that physiological concentrations of melatonin inhibit nitric oxide synthase (NOS) activity in different brain areas, the nocturnal decrease in brain NO and cyclic GMP contents may be, in part, a consequence of the nocturnal inhibitory effect of melatonin on NOS activity.