Effect of chain length of short-chain fatty acids on their effect on intracranial pressure in rabbits

Novel use of cranial epidural space in rabbits as an animal model to investigate bone volume augmentation potential of different bone graft substitutes

Background

The success of bone augmentation to a major degree depends on the biomechanics and biological conditions of the surrounding tissues. Therefore, an animal model is needed providing anatomical sites with similar mechanical pressures for comparing its influence on different biomaterials for bone regeneration. The present report describes the new bone formation associated to biomaterial in a bursa created in the epidural space, between dura mater and cranial calvaria, under the constant pressure of cerebrospinal fluid.

Methods

Five adult California rabbits were used for the trial. In each animal, two bursae were created in the epidural spaces, in the anterior part of the skull, below both sides of the interfrontal suture. The spaces between dura mater and cranial calvaria were filled with in-situ hardening biphasic calcium phosphate containing hydroxyapatite and beta tricalcium-phosphate (BCP), in-situ hardening phase-pure beta-tricalcium phosphate (β-TCP) or without any biomaterials (sham). After 90 days, the animals were sacrificed, and the defect sites were extracted and processed for histomorphometric analysis by optical and backscattered electron microscopy.

Results

The cranial epidural spaces created (n = 10) could be preserved by the application both BCP (n = 3) and β-TCP biomaterials (n = 3) in all experimental sites. The sites augmented with BCP showed less new bone formation but a trend to better volume preservation than the sites augmented with β-TCP. However, the bone in the BCP sites seemed to be more mature as indicated by the higher percentage of lamellar bone in the sites. In contrast, the created space could not be preserved, and new bone formation was scarce in the sham-operated sites (n = 4).

Conclusion

The experimental bursae created bilaterally in the epidural space allows comparing objectively bone formation in relation to biomaterials for bone regeneration under permanent physiological forces from cerebrospinal fluid pressure.

Astrocyte volume regulation and ATP and phosphocreatine concentrations after exposure to salicylate, ammonium, and fatty acids

Cellular volume regulation following swelling in hypo-osmotic phosphate-buffered saline (PBS) and ATP and phosphocreatine concentrations of cells incubated in iso-osmotic or hypo-osmotic PBS were measured in primary cultured rat cerebral astrocytes exposed for 30 min to NH4Cl, salicylate, hexanoate, octanoate, and/or dodecanoate. These compounds have been implicated in the pathogenesis of cerebral edema in Reye's Syndrome. NH4Cl (0.10 - 10 mM) had no effect on astrocyte volume regulation or ATP concentration. Salicylate significantly reduced ATP concentrations at 3.0 mM and 10 mM but had no effect on volume regulation. Hexanoate (10 mM and 30 mM) decreased astrocyte ATP content by over 80% while octanoate (10 mM) reduced ATP content by more than 50%. Concentrations of these fatty acids at or below 3.0 mM had no effect on ATP content. Volume regulation was inhibited by 3.0 mM hexanoate and 3.0 mM octanoate but not lower concentrations. Dodecanoate (0.1-3.0 mM) decreased cellular ATP content by 33-51% in iso-osmotic PBS solutions. Phosphocreatine content was reduced by exposure to salicylate or octanoate at concentrations which had no effect on ATP content. These results indicate that astrocyte energy metabolism and volume regulation may be compromised by agents associated with cerebral edema in Reye's Syndrome. Analysis of the dose-dependence of these effects suggests that inhibition of astrocyte energy metabolism is not sufficient to affect volume regulation.

Influence of short-chain fatty acids on vascular tone

Results from research on the influence of short-chain fatty acids (SCFA) (C3-C6) on vascular tone are reported. Isolated vascular smooth muscle strips were studied in vitro and the arterial blood pressure of guinea-pigs injected with SCFA was taken. On the basis of results from these first two methods, the level of PG F2 alpha was determined by radioimmunoassay. The experiments indicate that in vitro the SCFA have spasmogenic effects which are blocked by indomethacin and aspirin and which are reduced by the inhibitor of the activating effect of PG F2 alpha, PG E2, and PG I2-SC19220. The effects of SCFA on smooth muscle strips used as a bioassay are analogous to the effects of PG F2 alpha on the same tissue. The injection of butyric acid into guinea-pigs causes hypertension which is not manifest if indomethacin pretreatment is carried out. Radioimmunoassay results indicate that the level of PG F2 alpha in the blood of animals treated with butyric acid is significantly increased.

Brain water content, brain blood volume, blood chemistry, and pathology in a model of cerebral edema

STUDY OBJECTIVES

The objective was to correlate regional changes during brain water content with alterations in blood chemistry and cerebral pathology during hypo-osmotic edema.

PARTICIPANTS

Sprague-Dawley male adult rats were used in these studies.

DESIGN

Animals were block-randomized to receive either an intraperitoneal distilled water injection equivalent to 5% or 15% of their body weight or no injection (controls). Rats were sacrificed 15 or 60 minutes after water injection or at an equivalent time for controls.

INTERVENTIONS

No interventions were performed.

MEASUREMENTS AND MAIN RESULTS

Water content of cerebral cortical gray and white matter was calculated from measurements of tissue specific gravity. Blood plasma osmolality and sodium and potassium concentrations were determined at various times after water injection. An index of blood-brain barrier permeability was obtained by measuring brain red blood cell and plasma volumes. A qualitative assessment of edema was made from light and electron micrographs of the cerebral cortex. We found that water injection produced a dose-dependent decrease in plasma osmolality and sodium concentration within 15 minutes. Cortical water content was unchanged after this period. An influx of water into cerebral gray, and, less readily, into cerebral white matter occurred during the next 15 minutes. Whole blood specific gravity and brain blood content were unchanged and thus did not confound the measurement of cerebral water content. Hematocrit was increased 60 minutes after a 15% water injection. The blood-brain barrier remained intact throughout this period. Microscopy revealed astrocytic swelling with slight extracellular fluid accumulation 60 minutes after the water injection.

CONCLUSIONS

Homeostatic mechanisms in the cerebral cortex can maintain constant water content for at least 15 minutes during maintained intravascular hypo-osmolality. Fluid that subsequently moves into the tissue primarily enters an intracellular compartment. This model will be useful in investigating physiological mechanisms of brain water regulation and the pathogenesis of brain edema, a common clinical entity in emergency conditions.

[1-14C]Octanoate: a fast functional marker of brain activity

[1-14C]Octanoate (OCTO) is rapidly extracted from blood and metabolized by brain. Up to 6 min following an intravenous pulse injection the autoradiographic pattern of brain labeling reflects differences in blood flow and energy metabolism. When animals are given OCTO during visual or somatosensory stimulation the autoradiograms reveal differential labeling of activated pathways. Studies can be performed in 7-20 s allowing fast labeling of functional brain anatomy.

Repetitive measurements of intracranial pressure in awake rabbits

Previous measurements of the intracranial pressure in experimental animals suffer from acute and subacute effects of cannulation. In order to obtain reliable, repetitive or continuous values, we measured the intracranial pressures in awake rabbits with a new permanent adjustable ventricular cannula that included a separate entrance to the subarachnoid space. The mean intraventricular pressure ten days after the operation was 5.2 +/- 1.1 mmHg (SD) (70 animals). Manipulation of the cannula system and infusion of artificial cerebrospinal fluid did not damage the blood-brain barrier (indicated by extravasation of Evans Blue). The intracranial pressure was constant for as long as 6 months and as many as 22 separate measurements and infusions. The cerebrospinal fluid cells and protein content did not change in animals with permanent cannulae and in animals perfused with 2-4 ml artificial cerebrospinal fluid. In 30 animals the ventricular cannula functioned for 10-180 (median 65) days and the subarachnoid entrance for 11-23 (median 16) days.

Induction of brain edema following intracerebral injection of arachidonic acid

The effects of polyunsaturated fatty acids on brain edema formation have been studied in rats. Intracerebral injection of polyunsaturated fatty acids (PUFAs), including linolenic acid (18:3) and arachidonic acid (20:4), caused significant increases in cerebral water and sodium content concomitant with decreases in potassium content and Na+- and K+- dependent adenosine triphosphatase activity. There was gross and microscopic evidence of edema. Saturated fatty acids and monounsaturated fatty acid were not effective in inducing brain edema. The [125I]-bovine serum albumin spaces increased twofold and threefold at 24 hours with 18:3 and 20:4, respectively, indicating vasogenic edema with increased permeability of brain endothelial cells. Staining of the brain was observed five minutes after injection of Evans blue dye followed by arachidonic acid perfusion. A major decrease in brain potassium content was evidence of concurrent cellular (cytotoxic) edema as well. The induction of brain edema by arachidonic acid was dose dependent and maximal between 24 and 48 hours after perfusion. Dexamethasone (10 mg/kg) was effective in ameliorating the brain edema, whereas a cyclooxygenase inhibitor, indomethacin (10 mg/kg), was not. These data indicate that arachidonic acid and other PUFAs have the ability to induce vasogenic and cellular brain edema and further support the hypothesis that the degradation of phospholipids and accumulation of PUFAs, particularly arachidonic acid, initiate the development of brain edema in various disease states.