Day- and night-time contents of monoamines and their metabolites in the medial preoptic area of the rat hypothalamus.

The present study was conducted to investigate whether monoamines and their metabolites in the medial preoptic area (mPOA) of the rat hypothalamus exhibit differences in their contents between day and night. We therefore sampled the mPOA from adult animals of either sex at the middle of the light or dark period, respectively, and analyzed the tissue by means of high performance liquid chromatography with electrochemical detection. We found that, in female animals at mid-night, dopamine and 3,4-dihydroxyphenyl acetic acid (DOPAC) was reduced to 43 and 30%, respectively, of daytime levels, while the norepinephrine content was doubled. No significant differences were observed in male animals. We also conducted immunohistochemistry of the catecholamine synthesizing enzymes, tyrosine hydroxylase (TH) and dopamine-beta-hydroxylase (DBH), in sections from perfusion-fixed male rats and showed that TH is present in neuronal perikarya and processes in the anteroventral periventricular region of the mPOA, while DBH was only seen in fibers and terminals. Our results of sex-specific and day time-dependent variations in dopamine and norepinephrine concentrations indicate that the two monoamines are candidate neurotransmitters that may transmit diurnal information to the mPOA.