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This study deals with acetylcholinesterase activity in mesencephalic trigeminal nucleus neurons of the newt. These cells appear clearly positive to histochemical reaction. Electron microscope investigations revealed the fine localization of the enzyme in cell bodies. On the basis of the ultrastructural pattern of AChE distribution some considerations on the possible sites of utilization are discussed.

A histochemical light and electron microscopy study of acetylcholinesterase (AChE) was carried out on the regenerating opic tectum of adult newt. A plug of optic tectum was removed and 15 days later [6-H3] thymidine was injected. Ninety days after the lesion the brain was removed, treated for histochemical AChE-detection and autoradiographic analysis. This double treatment showed the capacity of these adult amphibians to regenerate the nervous tissue through the proliferation of undifferentiated elements and their subsequent differentiation into neurons as is shown by the presence of cells both labelled by [6-H3] thymidine and by the AChE-reaction product.

The permeability of cerebral vessels to exogenous peroxidase was studied in the newt. The reaction product was found only inside the cerebral or pial blood vessels. Electron microscope investigations revealed the presence of reaction product along the luminal area of vessels and in some parts of the intercellular spaces at the level of tight junctions joining endothelial cells. On the basis of the ultrastructural peroxidase localization, the presence of a brain-blood barrier in Triturus is discussed.

AChE distribution during the early phases of the repairing process following the removal of a plug from the optic tectum of newt has been investigated. The first AChE-positive neurons appear in the regenerating area on the 60th day after the operation. At this stage acetylcholinesterase activity is clearly seen also between the nervous fibers and in some large dendrites, present in the superficial fibrous area. The enzyme distribution and the submicroscopic organization show that in this area cholinergic connections between regenerated axons and newly-formed nervous elements are being established.

The ultrastructural localization of alkaline phosphatase and K+-NPPase was investigated in brain capillaries of newt by a cytochemical study using whole brain perfusion. The alkaline phosphatase activity was present in both luminal and antiluminal membranes of the endothelial cells. By contrast, the K+-NPPase was located only in antiluminal membranes of the brain capillaries. This distinct enzymatic distribution suggested that the luminal and antiluminal membranes are functionally different. The role of alkaline phosphatase and K+-NPPase in the blood brain barrier is discussed.

The permeability of the newt cerebral capillaries to lanthanum ion has been studied after perfusion with mannitol solutions of increasing molarity. In the control specimens lanthanum deposits were limited to the luminal side of the capillaries and tracer did not spread to the pericapillary spaces due to the tight junctions. Treatment with hypertonic solutions of mannitol (0.25M, 0.5M, 1M) caused opening of the blood brain barrier with a progressive increase in lanthanum between the endothelial cell edges, in the basal lamina and in the extracellular spaces of the nervous parenchyma in relation to the molarity of the mannitol solution. The spread of lanthanum is probably due to opening of the tight junctions between the endothelial cells, since pinocytotic vesicles labelled with tracer were not evident.

The ultrastructural distribution of alkaline phosphatase and Na+, K+-ATPase on the brain capillaries in Rana esculenta was investigated. Alkaline phosphatase activity appears both on the luminal and abluminal walls of the endothelial capillary cells; Na+, K+-ATPase is, instead, only present on the abluminal side. This different enzymatic distribution indicates that endothelial cells of the brain capillaries are polarized and the luminal and abluminal endothelial membranes are functionally different. The role of these two enzymatic activities is discussed in relation to the blood-brain barrier.