Several circumventricular organs (CVO) have 'leaky' vessels, without blood-brain barrier. It has raised the question whether blood-borne substances have a free access to the surrounding –or distant – brain substance. Data published are divergent. In some opinions a glial barrier substitutes the endothelial one (see e.g. Abbott,'05). Others suppose outer barriers around the whole organs, e.g. Réthelyi ('84) suggested a barrier to the arcuate nucleus. There are data, however (Broadwell and Sofroniew, '93) on spreading into the surrounding brain tissue. The present study:

a) reports preliminary observations which suggest that spreading of blood-borne substances may alter with functional states;

b) displays local differences in the glial architecture and the gliovascular connection (the dystroglycan complex, DGC) which may cast some light on the problem.

The glial architecture was revealed by immunohistochemical reactions to GFAP, vimentin (Vim), glutamine synthetase (GS) and S-100 protein. Markers of extracellular matrix (ECM), Wisteria lectin (WFA) and chondroitine sulfate (ChS) were also investigated, as well as the DGC components b-dystroglycan and utrophin. The vascular 'leakage' was followed by intrinsic markers (immunoglobulin-Ig, and fibronectin-Fn). The investigations were performed on adult rats (xylazine-ketamine anaesthesia). Fixation occurred by immersion (4% paraformaldehyde in PB).

a) Whereas Ig immunoreactivity appeared rather intensely and widely around the median eminence (ME) and seemed to penetrate the arcuate nucleus in the intact animals, minimal spreading was observed in animals received a trauma (laparotomy) previous day. Blood-borne Fn showed no similar phenomenon, it remained within the territory of the ME.

b) Distribution of WFA (but not that of ChS) and alteration of the architecture of the Vim⁺ tanycyte system seem to correlate with the territory of Ig spreading. 'Leaky' vessels of CVOs are positive to utrophin, in contrast to other brain vessels. In the ME independent systems of GFAP⁺ and Vim⁺ glia occur, and only the latter one forms contacts with vessels. In the area postrema (AP) Vim⁺ glia was confined to the border in contrast to the even distribution of GFAP. S100 and GS were missing from the subfornical organ and AP, and GS had an uneven, layered distribution in the ME.

Further studies have to support the spreading differences by measurements, to reveal if and how functional alterations affect the spreading of blood-borne substances, and to clear the functional importance of the local differences in the glia, ECM and DGC.