A CONFOCAL APPROACH TO THE MORPHOFUNCTIONAL CHARACTERIZATION OF THE TRANSIENT TYROSINE-HYDROXYLASE SYSTEM IN THE RAT SUPRACHIASMATIC NUCLEUS

The suprachiasmatic nucleus (SCN) of the neonatal rat is transiently i nnervated by tyrosine hydroxylase (TH) fibers of unknown origin and wh ose catecholaminergic nature is rather doubtful. In order to character ize this system morphofunctionally, immunocytochemical double labellin g and confocal laser scanning microscopy analysis were employed on cry ostat brain sections of 10-day-old rats. Simultaneous stainings for ne uropeptide Y (NPY) and tyrosine hydroxylase (TH) immunoreactivity show ed that they are not colocalized, neither in the SCN fibers nor in the intergeniculate leaflet (IGL) neurons, site of origin of the NPY proj ection to the SCN. Therefore, the possibility that SCN transient TH fi ber system originates from the IGL could be excluded. Double labelling for TH and aromatic L-aminoacid decarboxylase (AADC) demonstrated tha t transient SCN TH immunoreactive (IR) fibers are AADC negative, thus supporting the hypothesis of their non-catecholaminergic nature. Moreo ver two new group of cells which are TH positive and AADC negative wer e found: one in the SCN and the other in the periventricular hypothala mic nucleus (PHN). The presence of somatostatin (SRIF) and TH in PHN n eurons and SCN fibers suggested their possible colocalization, but dou ble immunolabellings gave negative results. Simultaneous immunocytoche mical staining for vasoactive intestinal polypeptide (VIP) and TH show ed that TH fibers may interact with ventrolateral SCN VIP neurons. Thi s result suggests a possible involvement of TH fibers in regulating VI P cells activity in the entrainment of circadian rhythms.