Arthur M. Butt

Effect of histamine and antagonists on electrical resistance across the blood-brain barrier in rat brain-surface microvessels

The effect of histamine on blood-brain barrier permeability was investigated using in situ measurement of transendothelial electrical resistance in brain-surface microvessels of anaesthetized rats. Mean resistance of vessels superfused with artificial cerebrospinal fluid was 1500 omega.cm2, indicating a tight barrier with low ion permeability. The addition of 10(-4) M histamine resulted in a 75% decrease in resistance, in both arterial and venous vessels, indicating a marked increase in barrier permeability. To determine the nature of the response to histamine, rats were given presurgical intraperitoneal injections of promethazine (H1 receptor antagonist), cimetidine (H2 receptor antagonist) or indomethacin (cyclo-oxygenase inhibitor), singularly and in combinations. Cimetidine completely blocked the histamine-mediated increase in barrier permeability whereas promethazine only had a small effect and indomethacin was ineffective. In addition, cimetidine treatment resulted in a 100% increase in basal resistance in both arterial and venous vessels, suggesting endogenous histamine was acting to increase blood-brain barrier permeability. It is concluded that histamine causes an increase in blood-brain barrier permeability which is mediated via endothelial H2 receptors, and that the electrical resistance in cimetidine-treated rats most closely represents the true permeability of the blood-brain barrier.

Platelet-derived growth factor delays oligodendrocyte differentiation and axonal myelination in vivo in the anterior medullary velum of the developing rat.

The AA dimeric form of platelet‐derived growth factor (PDGF‐AA) is implicated in the differentiation of cells of the oligodendrocyte lineage, which express PDGF receptors of the alpha subunit type (PDGF‐αR). In the present study, we show that a single injection of PDGF‐AA into the cerebrospinal fluid of neonatal rats delays oligodendrocyte differentiation and interrupts the progress of myelination in the anterior medullary velum (AMV), a white matter tract roofing the IVth ventricle of the brain. PDGF‐AA or saline was injected intrathecally in postnatal day (P) 7 rats, and the AMV was subsequently removed and immunolabelled with the oligodendrocyte‐specific antibody Rip, at P9, P12, and P21, corresponding to postinjection days (PID) 2, 5, and 14. At P9 (PID2), myelination was retarded in PDGF‐AA‐treated rats as opposed to saline‐treated controls but progressed rapidly after P12 (PID5). Quantification supported the qualitative observations that PDGF‐AA mediated an acute decrease in the number of Rip+ oligodendrocytes at P9–12, which largely recovered by P21, suggesting that PDGF‐AA may have delayed recruitment of myelinating oligodendrocytes. However, the definitive number of Rip+ oligodendrocytes in the AMV was not increased, suggesting that its action as a promoter of early oligodendrocyte survival may not ultimately affect the definitive number of myelinating oliogdendrocytes in vivo. We discuss the possibilities that excess PDGF‐AA may have acted on early oligodendrocytes (precursors or preoligodendrocytes) to either (1) delay their differentiation by maintaining them in the cell cycle or (2) accelerate their differentiation, which may result in premature cell death in the absence of synchronised survival signals. This study supports a role for PDGF‐AA in the timing of oligodendrocyte differentiation in vivo, as has been shown in vitro. J. Neurosci. Res. 48:588–596, 1997.

Optic neuritis in chronic relapsing experimental allergic encephalomyelitis in Biozzi ABH mice: Demyelination and fast axonal transport changes in disease

The encephalitogenicity of optic nerve tissue was demonstrated in Biozzi ABH (H-2(dq1)) mice. Acute experimental allergic encephalomyelitis (EAE) occurred in 11/14 animals and 4/5 exhibited relapse. The involvement of the optic nerve in spinal cord homogenate induced chronic relapsing EAE (CREAE) was demonstrated by mononuclear cell infiltration and myelin degradation in the optic nerve prior to and during clinical disease. During the relapse phase gross pathological assessment revealed swollen and translucent plaques on the optic nerves. Advanced lesions showed widespread demyelination, astrocytic gliosis and fibrotic changes of the blood vessels. Physiologically, the fast axonal transport of proteins from the retina to the optic nerve and superior colliculus was significantly decreased during relapse. The association of inflammation and demyelination with physiological deficit in the optic nerve highlights the usefulness of this model in the study of multiple sclerosis in which acute monosymptomatic unilateral optic neuritis is a common manifestation. Furthermore, the novel induction of CREAE with optic nerve homogenate suggests that optic neuritis is a common significant role in the pathophysiology and progression of neurological disease in CREAE which may be relevant to studies of optic neuritis in multiple sclerosis.

Morphology of oligodendrocytes during demyelination in optic nerves of mice infected with Semliki Forest virus

Multiple sclerosis (MS) is a demyelinating disease which affects oligodendrocytes, the myelinating cells of the CNS. Demyelination is known to occur in the optic nerves of Balb/c mice infected with the avirulent A7(74) strain of Semliki Forest virus (SFV), and many of the changes are similar to those of patients with MS. The aim of the present study was to determine how demyelination proceeds in individual oligodendrocytes in SFV infection, to help in understanding the pathology of demyelination and remyelination in MS. The whole‐cell morphology of individual oligodendrocyte units (defined as the oligo‐dendrocyte, its processes and the internodal myelin segments of the axons it ensheaths) was characterized using intracellular dye injection in isolated intact optic nerves. In untreated control mice, oligodendrocytes had a relatively uniform morphology and each cell on average provided 20 or so nearby axons with single myelin sheaths with internodal lengths of ∼ 150 μm. In SFV infected mice, during the peak of demyelination at post inoculation days 14–21, 55% of oligodendrocytes displayed a range of morphological abnormalities, which most likely represented sequential changes in oligodendrocytes during demyelination. Thus, at the earliest stage of demyelination oligodendrocytes developed swellings or vacuolations along their internodal myelin sheaths, which became gradually attenuated and were completely lost in extreme cases. The results show that whole oligodendrocyte units were affected during SFV‐induced demyelination and this is the basis of the focal nature of lesions in this viral model of MS. Individual oligodendrocyte units which had lost their full complement of myelin sheaths had the appearance of immature oligodendrocytes, suggesting they had undergone de‐differentiation. We concluded that these cells may not be destroyed during demyelination and it is possible they are capable of remyelination which is a feature of SFV infection in mice and MS in humans.

Transfer of horseradish peroxidase from oligodendrocyte to axon in the myelinating neonatal rat optic nerve : Artefact or transcellular exchange ?

In this paper we make the surprising observation that intracellular injection of horseradish peroxidase (HRP) into a single myelinating oligodendrocyte also resulted in localised HRP labelling at the nodes of Ranvier of some axons of the unit. It appeared that HRP had been transferred to the nodal axoplasm from the paranodal loops of the HRP‐filled oligodendrocyte. Three HRP‐filled oligodendrocytes from isolated optic nerves of 14‐day‐old rats were analysed by serial section electron microscopy, and HRP was observed in the axonal cytoplasm at three of the nodes of Ranvier delineated by one of the cells. At labelled nodes, HRP was of a uniform intensity throughout the nodal axoplasm. Axonal labelling gradually diminished along the paranodal regions and was not evident in the contiguous internodal axoplasm beyond 20 μm from the node. The myelin sheaths, paranodal loops, and axons appeared normal at labelled nodes, and the paranodal loops and astrocyte perinodal processes adjacent to those of the HRP‐filled oligodendrocyte unit did not contain HRP. There was no evidence of extracellular HRP or tissue damage in the surrounding neuropil, and axons neighbouring those enwrapped by the HRP‐filled oligodendrocyte did not contain HRP. The possibility that axonal labelling was an artefact of either iontophoretic injection or tissue preparation is discussed. This provocative finding is not definite proof of exchange, but the balance of evidence supports the possibility that there was transcellular exchange of HRP at paranodes between the labelled oligodendrocyte and some of the axons in the unit. The rarity of HRP transfer to axons suggests that it may be a transient or labile event. It is not clear whether oligodendrocyte to axon macromolecular exchange has real physiological and/or pathological significance.

Electrical Resistance Measurements of Blood-Brain Barrier Permeability

In adult mammals, homeostasis of the brain interstitial fluid is provided by the blood-brain barrier, protecting neurones from disturbances in blood composition.1 The exceptionally low permeability of the blood-brain barrier is determined by tight intercellular junctions between the endothelial cells of the cerebral vasculature which greatly restrict the movement of substances between blood and brain.2

Blood-Brain Barrier Permeability Changes during Semliki Forest Virus-Induced Encephalomyelitis in the Balb/c Mouse

We have measured blood-brain barrier permeability changes during Semliki Forest Virus infections in the Balb/c mouse, and shown that the permeability changes have a biphasic response peaking at post inoculation days 5 and 14. The histamine H2 receptor antagonist, cimetidine reduced the permeability changes at post inoculation day 5.

Time-Course of Demyelination and Blood-Brain Barrier Disruption in the Semliki Forest Virus Model of Multiple Sclerosis in the Mouse

Increased permeability of the blood-brain barrier (BBB) is a prominent feature of multiple sclerosis (MS). However, the cause of the BBB leak and its role in the pathogenesis of MS are unknown. The present study addressed these questions in the Semliki Forest virus (SFV) model of MS in optic nerves of Balb/C mice, using the unidirectional transfer coefficient for [14C]mannitol as a measurement of BBB permeability and immunolabelling with anti-myelin basic protein (MBP) to determine the time-course and extent of demyelination. In SFV, there was a significant increase in BBB permeability in the optic nerve, prior to the onset of demyelination. The BBB leak was blocked by treatment with cimetidine, an antihistamine which acts on H2 receptors at the BBB. Preliminary results suggested that cimetidine may also have delayed the onset or reduced the level of demyelination in the optic nerve. The results support a role for histamine in BBB leak in the SFV mouse model of MS and indicate that BBB leak may be integral to the pathogenesis of demyelination.