TRANSECTED AXONS OF ADULT HYPOTHALAMONEUROHYPOPHYSEAL NEURONS REGENERATE ALONG TANYCYTIC PROCESSES

In the intact hypothalamo-neurohypophysial system, oxytocinergic or va sopressinergic neurons project their axons throughout the internal lay er of the median eminence towards the blood vessels of the hypophysial neural lobe, When transected at the level of the median eminence, the se axons undergo massive sprouting towards the external layer of the o rgan and the underlying perivascular region containing hypophysial por tal vessels, The present study was designed to explore the possible ro les of median eminence glial cells in such a reorganization of transec ted neurohypophysial axons, The relationships between regenerating axo ns and glial cells were studied by laser scanning confocal microscopy and electron microscopy on vibratome sections immunostained with speci fic antibodies against neurohypophysial peptides and/or against glial markers. All along the intact median eminence, two main types of glial cells were identified: (1) tanycytes immunoreactive to vimentin and s lightly immunoreactive to glial fibrillary acidic protein, and (2) cla ssical astrocytes immunoreactive to glial fibrillary acidic protein bu t vimentin-negative, In the intact median eminence, neurohypophysial a xons were associated with astrocytic processes located in the internal layer, After a lesion of the hypophysial stalk, peptidergic regenerat ing axonal sprouts were found to project massively towards the externa l layer and to penetrate the underlying perivascular region in close a ssociation with tanycytic-like processes immunoreactive to both viment in and to glial fibrillary acidic protein, In contrast, regenerating s prouts were absent from those regions of the lesioned median eminence containing astrocytic processes immunoreactive to glial fibrillary aci dic protein but vimentin-negative, When fixed lesioned median eminence s were treated by placing crystals of the lipophilic dye DiI on their ventricular surface, regenerating axons were found to be closely assoc iated with DiI-labelled tanycytic-like end feet terminating in the ext ernal layer, and with connected thin processes projecting through the external vascular region, These data indicate that in the median emine nce of the adult rat, lesioned neurohypophysial axons regenerate in cl ose association with tanycytic processes. (C) 1995 Wiley-Liss, Inc.