The capillary of the organum vasculosum laminae terminalis (OVLT) in rabbits is more permeable to horseradish peroxidase (HRP) than that in rats.

The distribution of the tracer, horseradish peroxidase (HRP), in the organum vasculosum laminae terminalis (OVLT) of rabbits and rats was examined by electron microscopy. In rabbits, the HRP-diaminobenzidine reaction products were heavily distributed in the OVLT and surrounding brain tissues 10 and 60 min after the injection of HRP (50 mg kg(-1), i.v.), and were retained in the parenchymal tissues at 24 h post-injection. The majority was found in numerous large phagosomes of macrophages located in the perivascular spaces of the vascular beds and in ependymal cells (tanycytes) in the parenchyma. A large amount of reaction product was also localized in the intercellular clefts of the parenchyma. HRP-incorporation was seen in both nerve cells and ependymal cells in the OVLT at 10 min post-injection, but only in nerve cells in the preoptic area at 60 min post-injection. In rats, however, a small amount of the reaction products was observed in the OVLT 10 min after the injection of HRP (50 and 70 mg kg(-1), i.v.), and the levels were markedly reduced at 60 min post-injection. No HRP-incorporation by nerve cells was seen. From these findings, we concluded that the capillary of the OVLT of the rabbit is more permeable to HRP than that of the rat.