Projections from the hypothalamus to the posterior lobe in rats during ontogenesis: 1,1 '-dioctadecyl-3,3,3 ',3 '-tetramethylindocarbocyanine perchlorate tracing study

The objective of this study was to determine the schedule of the arrival of the axons from the hypothalamus to the posterior lobe of the pituitary (PL ) in rats during ontogenesis by using the fluorescent Lipophilic carbocyani ne dye 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarboxyanine perchlorate ( DiI) as a retrograde tracer. After preliminary fixation of the brain, DiI c rystals were implanted in the PL on embryonic day 15 (E15), E16, E17, and E 19 as well as on postnatal day 2 (P2) and P9. This was followed by a DiI re trograde diffusion along the plasma membrane and subsequent staining of hyp othalamic neuronal cell bodies. The supraoptic nucleus (SO) contained an ac cumulation of fluorescent cells that extended toward the diamond-like swell ing of the third ventricle as early as E15. These data suggest that the mag nocellular neurons of the SO send their axons to the PL at the very beginni ng of differentiation, perhaps even before reaching their final position. T he initial axons of the neurons of the paraventricular nucleus proper (PV) appeared to reach the PL significantly later, at E17. In addition to the SO and the PV, accessory magnocellular nuclei contributed to the innervation of the PL in perinatal rats. The neurons of the retrochiasmatic accessory n ucleus first sent their axons to the PL on E16-E17. Axons that originated f rom other accessory hypothalamic nuclei reached the PL after birth, suggest ing a delay in their involvement in the regulation of visceral functions co mpared with other magnocellular nuclei. Thus, the axons of magnocellular ne urons reach the PL unexpectedly early in embryogenesis, raising the possibi lity of the functional significance of vasopressin and oxytocin as fetal ne urohormones. J. Comp. Neurol. 422:327-337, 2000. (C) 2000 Wiley-Liss, Inc.