Hypothalamic cyclic-AMP throughout the 4-day estrous cycle of the female rat

The concentration of adenosine 3',5'-cyclic monophosphate (cAMP) in the hypothalamus was measured in the morning (10.00 h) and in the afternoon (16.00 h) during the 4-day estrous cycle of the female rat. The values were found to rise from the lowest levels on the afternoon of diestrus 1 to the highest levels on the morning of proestrus to decline thereafter. A second less marked peak was seen on the morning of diestrus 1. In rats treated with diethyldithiocarbamate to block norepinephrine (NE) synthesis, the concentration of the nucleotide decreased and remained at steady levels indicating that cyclic variations of cAMP in the hypothalamus are related to the endogenous release of NE. Blocking alpha-adrenergic receptors with phenoxybenzamine or beta-adrenergic receptors with propranolol also resulted in a drop of the concentration of cAMP in the hypothalamus. The effect of phenoxybenzamine was almost constant during the different days of the cycle whereas that of propranolol varied showing maximal effects on diestrus 2 and proestrus and minimal on estrus and diestrus 1. These results suggests that the response of endogenously released NE in the hypothalamus mediated through beta-adrenergic receptors undergo variations during the estrous cycle, whereas those mediated through alpha-adrenergic receptors remain at constant levels.