Diurnal changes in cyclic nucleotide levels in the hypothalamus of the rat

The concentrations of cAMP and cGMP in the rat hypothalamus varied considerably over a 24-h period. The cAMP levels were found to be relatively higher in the morning and lower during the afternoon and night. Cyclic GMP levels reached highest values during the period of darkness and were lowest during the day. Deprivation of food did not have any appreciable effect on the pattern of these changes.