Circadian covariation of norepinephrine and serotonin in the locus coeruleus and dorsal raphe nucleus in the rat.

We report robust correlations between concentrations of the neurotransmitters norepinephrine and serotonin in the locus coeruleus and the dorsal raphe nucleus of the brainstem in rats analyzed at 7 different time points over 24 h. We found similar circadian rhythmicities for both monoamines with acrophases just before the onset of the dark period. The monoamine concentrations diminished and the significant intercorrelation between norepinephrine and serotonin in the locus coeruleus disappeared during the night suggestive of a noradrenergic stimulation of dorsal raphe serotonin during the day. Timing of experiments is crucially important in studies on brain monoaminergic indices and their interrelationships.