This review addresses the role of tight junction proteins at the blood-brain barrier (BBB). Their expression is described, and their role in physiological and pathological processes at the BBB is discussed. Based on this, new approaches are depicted for paracellular drug delivery and diagnostics in the treatment of cerebral diseases. Recent data provide convincing evidence that, in addition to its impairment in the course of diseases, the BBB could be involved in the aetiology of CNS disorders. Further progress will be expected based on new insights in tight junction protein structure and in their involvement in signalling pathways.

Permeability of pial venular capillaries to Lucifer Yellow (P_ly) was measured using the single microvessel occlusion technique. P _ly was extremely low, when measured shortly after the removal of the meninges, consistent with an intact blood‐brain barrier, but rose spontaneously to (1.65 ± 060)× 10⁻⁶cms⁻¹ (mean ±s.d.) within 20–‐60 min. This first phase of spontaneous disruption lasted 44–164 min. A second phase started when P_ly rose sharply, and was characterized by rapid permeability fluctuations with a mean of (12.31 ± 15.14) × 1CT⁻⁶ cm s⁻¹. The first phase could be mimicked by applying the divalent cation ionophore A23187 in the presence of Ca²⁺, when P_ly rose by (1.47±0–25) × 10⁻⁶ cm s⁻¹ (mean±s.e.m.). Application of histamine (10 μm) to tight vessels increased P_ly by (2.41 ± 0.22) × 10⁻⁶ cm s⁻¹. Substances that raised intraendothelial cAMP of vessels during the first phase of disruption reduced P_ly to the initial blood‐brain barrier level. The second phase could be prevented by applying catalase. Similar high and fluctuating P_ly values could be produced reversibly by applying arachidonic acid or NH₄C1. This is the first report of two distinct types of permeability increase in the cerebral microvasculature, and reasons for this are discussed.

Key points: The redox‐sensitive transcription factor NF‐E2 related factor 2 (Nrf2) plays a key role in regulating adaptive cellular antioxidant defences, and activation of Nrf2 in stroke protects the brain against oxidative stress following ischaemia‐reperfusion injury. We report the first measurements of temporal and spatial distribution of Nrf2 in nuclear and cytoplasmic compartments in cells in the ischaemic core, peri‐infarct regions and contralateral hemisphere of rat brain following cerebral ischaemia‐reperfusion injury for 4, 24 or 72 h using a novel quantitative immunohistochemical technique, which was further validated in cultured bEnd.3 murine brain endothelial cells. Nrf2 expression in brain sections was increased in core and peri‐infarct regions after 24 h reperfusion, with levels remaining elevated only in peri‐infarct regions after 72 h. Pretreatment of rats with the Nrf2 inducer sulforaphane reduced core and peri‐infarct Nrf2 levels after 24 h reperfusion. The time course of stroke‐induced changes in nuclear to cytoplasmic Nrf2 content and its modulation by pretreatment with sulforaphane provide novel insights for targeting endogenous redox sensitive antioxidant pathways to ameliorate the damaging consequences of stroke. Abstract  Activation of the redox‐sensitive transcription factor NF‐E2 related factor 2 (Nrf2) affords protection against cerebral ischaemia‐reperfusion injury via the upregulation of antioxidant defence genes. We have quantified for the first time Nrf2 content in brains from rats subjected to stroke and from cultured bEnd.3 brain endothelial cells using a novel immunohistochemical technique. Male Sprague‐Dawley rats were subjected to middle cerebral artery occlusion for 70 min followed by reperfusion for 4, 24 or 72 h. Coronal brain sections were incubated with anti‐Nrf2 primary and biotinylated‐horseradish peroxidase‐conjugated secondary antibody, after which sections were reacted with 3,3′‐diaminobenzidine (DAB) in the presence of hydrogen peroxide. The initial rates of DAB polymer formation were directly proportional to the Nrf2 protein concentration. Image processing was used to determine the temporal and spatial distribution of Nrf2 in nuclear and cytoplasmic compartments in stroke‐affected and contralateral hemispheres. Nuclear to cytoplasmic Nrf2 ratios were increased in the stroke region after 24 h reperfusion and declined after 72 h reperfusion. Pretreatment with the Nrf2 inducer sulforaphane reduced total cellular Nrf2 levels in peri‐infarct and core regions of the stroke hemisphere after 24 h reperfusion. Treatment of cultured murine brain endothelial cells with sulforaphane (2.5 μm) increased nuclear accumulation of Nrf2 over 1–4 h. We report the first quantitative measurements of spatial and temporal nuclear Nrf2 expression in rat brains following stroke, and show that sulforaphane pretreatment affects Nrf2 distribution in the brain of naïve rats and animals subjected to cerebral ischaemia. Our findings provide novel insights for targeting endogenous redox‐sensitive antioxidant pathways to ameliorate the damaging consequences of stroke.