Serotonergic projections to the suprachiasmatic nucleus and the median eminence of the rat: identification by fluorescence and electron microscope.

The identification of serotonergic input to the rat median eminence and nucleus suprachiasmaticus was carried out by fluorescence and electron microscopy. The clear differentiation of serotonin (5-HT) from catecholamine (CA) neurons was possible with an improved new filter system. 6-hydroxytryptamine (6-HT) was selectively taken up by 5-HT terminals and produced an intense yellow fluorescence. The yellow fluorescence of 5-HT disappeared 2 weeks after treatment with 5,6-dihydroxytryptamine (5,6-DHT), whereas the blue-green fluorescence of CA persisted. Combined treatment with either 6-HT and reserpine with addition to alpha-methyl-p-tyrosine, or with 6-HT and 6-hydroxydopamine, caused a marked reduction of CA and yellow fluorescence prominent only in 5-HT terminals. Under the electron microscope, 6-HT and 5,6-DHT were selectively taken up by the 5-HT terminals and produced electron-dense cores in the small vesicles with a diameter of 50 nm. The quantitative study revealed that the density of serotonin boutons marked with such dense-cored vesicles was 4.3 +/- 0.5% (mean +/- S.E.M.) in the suprachiasmatic nucleus and 4.8 +/- 0.8% in the perivascular region of the median eminence. Several studies have indicated an important functional role of 5-HT in the control of gonadotrophin secretion. The present study suggests that the serotonergic terminals exert their influence on the anterior pituitary function, not only at the level of the suprachiasmatic nucleus, but also at the level of the perivascular region of the median eminence, either through the axo-axonic synapses with the axons containing the releasing hormone or by the direct release of 5-HT into the portal vessels.