Selective regulation of the blood-brain barrier by sensory input

Nociceptive inhibition prevents inflammatory pain induced changes in the blood-brain barrier

Previous studies by our group have shown that peripheral inflammatory insult, using the lambda-carrageenan inflammatory pain (CIP) model, induced alterations in the molecular and functional properties of the blood-brain barrier (BBB). The question remained whether these changes were mediated via an inflammatory and/or neuronal mechanism. In this study, we investigated the involvement of neuronal input from pain activity on alterations in BBB integrity by peripheral inhibition of nociceptive input. A perineural injection of 0.75% bupivacaine into the right hind leg prior to CIP was used for peripheral nerve block. Upon nerve block, there was a significant decrease in thermal allodynia induced by CIP, but no effect on edema formation 1 h post-CIP. BBB permeability was increased 1 h post-CIP treatment as determined by in situ brain perfusion of [(14)C] sucrose; bupivacaine nerve block of CIP caused an attenuation of [(14)C] sucrose permeability, back to saline control levels. Paralleling the changes in [(14)C] sucrose permeability, we also report increased expression of three tight junction (TJ) proteins, zonula occluden-1 (ZO-1), occludin and claudin-5 with CIP. Upon bupivacaine nerve block, changes in expression were prevented. These data show that the lambda-carrageenan-induced changes in [(14)C] sucrose permeability and protein expression of ZO-1, occludin and claudin-5 are prevented with inhibition of nociceptive input. Therefore, we suggest that nociceptive signaling is in part responsible for the alteration in BBB integrity under CIP.

Increased blood-brain barrier permeability following acute short-term swimming exercise in conscious normotensive young rats

The status of the blood-brain barrier (BBB) was examined following short-term forced swimming (FS) exercise in younger rats (age 8-9 wks, 80-90 g). Subjection of animals to continuous FS for 30 min duration increased the permeability of the BBB to Evans blue albumin (EBA) and 131I-sodium in 5 and 8 brain regions, respectively. Extravasation of the tracers was markedly pronounced in the cerebellum followed by the cerebral cortex. EBA staining was confined mainly to the posterior cingulate cortex, parietal and occipital cortices, whole cerebellar vermis and the mediolateral cerebellar cortices as well as the dorsal surface of the hippocampus. In addition to the above brain regions. BBB permeability to 131I-sodium extended to the caudate nucleus, thalamus and hypothalamus. At this time period, the serotonin (5-hydroxytryptamine, 5-HT) content showed a profound increase in plasma and brain of about 150% and 250% respectively from the control value. Pretreatment with p-CPA (p-chlorophenylalanine, a serotonin synthesis inhibitor) prevented both the increased permeability of the BBB and the rise in plasma and brain 5-HT level. However, prior treatment with cyproheptadine (a 5-HT2 receptor antagonist) prevented the increased permeability alone. The 5-HT level continued to remain high. These results suggest that short-term FS increases BBB permeability in specific brain regions. This increased permeability appears to be mediated through serotonin via 5-HT2 receptors.

Especial considerations for neurotoxicological research

In recent years, there has been much impetus toward a definition of behavior in terms of underlying biological events. Such correlations have been attempted in several areas ranging from learning and memory to neurological disease. Increased information concerning the relation between behavior and neurobiological mechanisms is especially important in the area of neurotoxicology. It is often abnormal behavior that is a first sign of exposure to a neurotoxic agent and such changes may give clues as to the anatomical or chemical sites of attack on the nervous system. These clues might also lead to the development of a therapeutic treatment as to the development of tests designed to reveal exposure to a toxic agent at levels below those causing gross behavioral change. Unfortunately, there is a relatively small amount of literature reporting on both behavioral and biological disturbances caused by a toxic agent in the same experimental animal. However, a variety of methodological advances combined with a growing interest in neurotoxicology is gradually changing this. Increased information concerning the role of defined nerve pathways and the means of action of their chemical constituents offers an opportunity to bring about a deepening understanding of neurotoxic events. This review will suggest how new pharmacological findings can be applied to neurotoxicology. Examples of human and animal exposure to toxic materials will be used and current problems will be shown to be major determinants of future research directions.

[3H]lysine incorporation into protein of hemisected, cycloheximide-treated spinal cord

A single application of a protein synthesis inhibitor, puromycin, in the lesion site of a spinal hemisection transiently decreases protein incorporation for less than 24 h and results in axonal sprouting and dendritic and synaptic stability for at least 60 days. In this study we characterized the protein-altering properties of single applications of a different protein synthesis inhibitor, cycloheximide, for comparison. The protein incorporation of [3H] lysine into hemisected rat spinal cord (left side, T2) treated with cycloheximide (100 micrograms/ml in Gelfoam sponge placed in wound at time of lesion) was tested 3 h, 6 h, 12 h, 1, 3, 7, and 14 days later and compared with that in 0.9% saline in Gelfoam implants as normal controls (N = 54). One hour prior to utilization, the animals were injected subcutaneously with 200 mu Ci L-[4,5(n)-3H]lysine monohydrochloride. No treatment-related differences in brain radioactivity were noted at any time postoperation. In spinal cord, inhibition of amino acid uptake by cycloheximide was not demonstrable at 3 h after hemisection, but could be detected from 6 h to 1 day after implantation. During that time, the TCA-soluble radioactivity at the lesion site was greater than in control animals (P less than 0.05). These results suggest that as with puromycin, morphologic effects produced by cycloheximide on spinal cord regeneration may correspond to chemical events which occurred within the first 24 h after cord hemisection and cycloheximide treatment.

Newborn rabbit blood-brain barrier is selectively permeable and differs substantially from the adult

Examination of blood-brain barrier (BBB) function by the intracarotid injection technique has been utilized in studies of newborn (6-30 h) and adult rabbits. The exclusion of mannitol (mol. wt. 182), dextran (mol. wt. 60,000-90,000), and indium-bound EDTA indicate that the newborn BBB has restrictive properties similar to the adult. At birth, saturable, carrier-mediated transport mechanisms are present, regulating the entry of glucose, amino acids, organic acids, purines, nucleosides, and choline. No difference in brain uptake of glucose was observed between adult and newborn, but considerably higher uptake rates for arginine, choline, and adenine were seen in the newborn. In contrast to suggestions of an immature barrier in young animals, these studies indicate that a sophisticated, selective BBB is operative at birth. Furthermore, the specific selectivity and dramatic increases seen for certain metabolites imply a vital function in the newborn for these carrier systems.

Protein transport across cerebral vessels during metrazole-induced convulsions

The effects of pentylenetetrazole-induced convulsions on cerebrovascular permeability for horseradish peroxidase were studied in locally anesthetized, artificially respired, paralyzed cats. Horseradish peroxidase circulated for 1, 2.5, 5, 10, 30, and 60 min and accumulated in medium-size, i.e., arterial or venous, blood vessel walls. Cerebral neuropil was permeated earliest in thalamic nuclei including medial and lateral geniculate nuclei. No tight junctions containing horseradish peroxidase were found. With 1.0 and 2.5 min circulation of horseradish peroxidase, arterial adventitia and basement membranes contained more peroxidase than surrounding interstitial space in contrast to veins and capillaries. Convulsions increased the number of endothelial vesicles containing horseradish peroxidase in arteries, veins, and capillaries but total number (stained plus unstained) of endothelial vesicles was increased slightly only in capillaries. We conclude that increased permeability is initially at the arterial level, although veins may also become permeable. The role of the endothelial vesicles in transit or protein in and out of the nervous system is not clear.

Microwave alteration of the blood-brain barrier system of rats

Rats were exposed to 1.3 gHz microwave energy to assess the uptake of several neutral polar substances in certain areas of the brain. A quantitative, radioactive isotope method, which uses an internal standard, was employed to measure the loss of test substances to brain tissue. Single, 20 min exposure, to either pulsed or continuous wave (CW) microwave energy induced an increase in the uptake of D-mannitol at average power densities of less than 3.0 mW/sp. cm. The permeability change was greatest in the medulla, followed, in decreasing order, by the cerebellum and hypothalamus, with small or negligible changes in the hippocampus and cortex. Permeability increases were observed for mannitol and inulin but not for dextran. Increased permeability was observed both immediately and 4 h after exposure, but not 24 h after exposure. After an initial rise, the permeability of cerebral vessels to saccharides decreased with increasing microwave power. Differences in the level of uptake occurred between CW energy and pulsed energy of the same average power. Microwaves of the same average power but different pulse characteristics also produced different uptake levels. Our findings suggest that microwaves induce a temporary change in the permeability for small molecular weight saccharides in the blood-brain barrier system of rats.

Amino acid incorporation in medulla, pons, midbrain and cortex following spinal cord hemisection in the cebus monkey (Cebus apella)

The effects of spinal cord hemisection on protein labeling in the medulla, pons, midbrain, and cortex in the Cebus monkey were studied by measuring the uptake of [3H]lysine into the trichloroacetic acid (TCA) precipitable and non-precipitable fractions of brain. Animals were sacrificed 3, 6 or 13 days after hemisection. Two normal animals and a 3-day sham operate were utilized as controls. Differences included individual animals, operated vs. unoperated animals, brain areas, and variation of brain areas over all postoperative days. The overall trend in brain radioactivity over postoperative days indicates stress and non-stress related components of brain response to spinal injury. Left-right side differences (L greater than R) were noted in the uptake of [3H]lysine into specific areas of brain over postoperative time, including leg areas of motor cortex, occipital cortex, and superior colliculus. A biphasic action of leg sensory cortical areas over time was noted, where L greater than R for the first 6 postoperative days and R greater than L at day 13. The factors possibly mediating radiolabeled amino acid uptake into protein are discussed in relation to demonstrated anatomical and neurochemical regenerative processes.

Rapid changes in cerebral blood flow and initial visual experience in the developing chick

A single eye of young chicks was sewed shut for various times. Regions contralateral to and primarily or secondarily innervated by the sutured eye had a reduced rate of cerebral blood flow in comparison to the corresponding ipsilateral regions. However, upon reopening the sutured eye after 2 days, these contralateral regions (optic lobes and cerebral hemispheres) exhibited a rapid increase in cerebral blood flow to a level significantly above that in control ipsilateral areas. The duration of this effect was considerably prolonged in chicks that were monocularly sutured immediately after hatching without prior exposure to light. Chicks that had 1 week of normal vision before monocular suture did not show this overcompensatory effect. If the period of monocular suture was extended to 7 days before restoration of vision to the occluded eye, the overcompensatory vascular effect was delayed. The maximal effect was apparent only upon the 1st exposure of an eye to major visual input. A possible relation exists between this rapid increase in regional blood flow and the period when visual imprinting could normally be expected to be maximal.

Selective changes in the permeability of the blood-brain barrier after pharmacological or surgical deafferentation

Unilateral eye removal results in an increased delivery of blood-borne glucose, norepinephrine, choline, and tyrosine, but not of proline, to the denervated optic lobes. Administration of reserpine to chicks increases penetrance of norepinephrine into the brain but not of tyrosine, choline, or proline. Selective alterations in the permeability of the blood-brain barrier may represent a mechanism for the modulation of the delivery of critical nutrients to the brain.