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 exh ibit differences in their contents between day and night. We therefore samp led 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 perfor mance liquid chromatography with electrochemical detection. We found that, in female animals at mid-night, dopamine and 3,4-dihydroxyphenyl acetic aci d (DOPAC) was reduced to 43 and 30%, respectively, of daytime levels, while the norepinephrine content was doubled. No significant differences were ob served in male animals. We also conducted immunohistochemistry of the catec holamine synthesizing enzymes, tyrosine hydroxylase (TH) and dopamine-beta- hydroxylase (DBH), in sections from perfusion-fixed male rats and showed th at TH is present in neuronal perikarya and processes in the anteroventral p eriventricular region of the mPOA, while DBH was only seen in fibers and te rminals. Our results of sex-specific and day time-dependent variations in d opamine and norepinephrine concentrations indicate that the two monoamines are candidate neurotransmitters that may transmit diurnal information to th e mPOA, (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.