Active transport of glycine across the frog arachnoid membrane

Postmortem image analysis of sheep cortical leptomeningeal space and vasculature: theoretical implications on brain surface dialysis

The vascular surface distribution of the subarachnoid space has not been studied extensively. The aim of our study was to develop a method of computer-assisted estimation of the distribution of the vascular network in the cortical leptomeninges and subarachnoid space and model it to aid the study of the physiology of brain surface dialysis. Nine sheets of leptomeningeal tissue were obtained from adult sheep. Fourteen image sample areas of 4 cm² each were acquired and processed with ImageJ. The vascular and nonvascular areas of the cortical subarachnoid space were identified using a "projected surface" approach. The modeling equations were used to predict the behavior of brain surface dialysis processes. The mean surface area of identified subarachnoid vessels was 0.354 ± 0.02 cm² per 1 cm² of tissue. The mean meningeal area with unidentified vessels was 0.646 ± 0.02 cm²/1 cm², and the difference between these surfaces was significant (p < 0.0001). The modeling equations used predict that modifying the vessel diameter of the subarachnoid space could alter the efficiency of brain surface dialysis. The computer-assisted modeling of the vascular surface of the cortical subarachnoid space may be a useful tool in depicting its morphology and assessing the physiology during brain surface dialysis.

Changes of bladder activity and glycine levels in the lumbosacral cord after partial bladder outlet obstruction in rats

OBJECTIVES

We investigated the time course of changes in bladder activity as well as in spinal and serum levels of glutamate and glycine after partial bladder outlet obstruction (BOO) in rats.

METHODS

A total of 36 female rats were divided into six groups: sham operation (control); 3 days, 14 days, and 28 days after BOO; 3 days and 28 days after relief of BOO. Under urethane anesthesia, isovolumetric cystometry was carried out in each group. Then, spinal and serum levels of glutamate and glycine were measured.

RESULTS

The interval between bladder contractions was shorter in all of the groups compared with the control group. The amplitude and duration of bladder contractions was decreased at 3 days, 14 days, and 28 days after BOO, and at 3 days after relief of BOO. Spinal and serum glutamate levels showed no changes. However, the spinal glycine level was decreased at 14 days and 28 days after BOO, and at 28 days after relief of BOO. Serum glycine level was also decreased at 28 days after BOO and 28 days after relief of BOO.

CONCLUSIONS

Detrusor overactivity during the chronic phase of partial BOO is partly caused by a decrease of glycinergic neuronal activity in the lumbosacral cord. A 3-day period of BOO produces detrusor overactivity, which might be due to an irreversible decrease of spinal glycinergic neuronal activity.

Neutral amino acid uptake by the isolated perfused sheep choroid plexus

1. The uptake of neutral amino acids from the blood into the cells of the choroid plexus was studied by means of the rapid (less than 40 s) single-circulation paired-tracer dilution technique in the isolated perfused choroid plexus of the sheep. 2. The study provides the first direct evidence for the carrier-mediated entry of neutral amino acids from blood into the cells of the choroid plexus. 3. In the terms of Christensen's classification the presence of L-amino acid carrier systems for large neutral amino acids with bulky side chains has been demonstrated. 4. No measurable uptake of [14C]methyl amino isobutyric acid ([14C]MeAIB) during a single passage through the choroid plexus circulation was demonstrated which indicates the probable absence of a significant 'A' transport system. 5. The uptake of small neutral amino acids such as glycine and L-alanine was shown to be carrier-mediated. Results suggest that these amino acids are mainly transported by the glycine and ASC carrier systems, respectively. 6. The results suggest that there is a similarity between the transport systems for neutral amino acids on the blood side of both the blood-brain barrier and blood-cerebrospinal fluid barrier, the exception being for the presence of a glycine carrier on the blood side of the choroid plexus.

Differential localization of alkaline phosphatase in barrier tissues of the frog and rat nervous systems: a cytochemical and biochemical study

We investigated the localization of alkaline phosphatase (AP) in the peripheral and central nervous systems of the frog (Rana pipiens) and rat. In the frog sciatic nerve, AP reaction product was seen as a precipitate within caveolae and vesicular profiles of perineurial cells, and frequently filled the extracellular space. In the rat peripheral nerve, AP reaction product appeared as small tufts on the cell surfaces and within vesicular profiles of endoneurial blood vessels. AP reaction product was not detected in the rat perineurium or in endoneurial blood vessels of the frog. In the frog central nervous system, AP reaction product was detected in the arachnoid membrane adjacent to the subarachnoid space, but not in the cerebral or pial vessels, whereas in the rat it was detected in the outer arachnoid membrane and in the cerebral and pial blood vessels. Biochemical analysis indicated a sevenfold higher AP activity in the frog perineurium over the endoneurium, whereas in the rat, threefold more activity was measured in the endoneurium over the perineurium. Levamisole, an AP inhibitor, decreased the enzyme activity by 95% in rat tissues, and by 70% in frog tissues and in plasma from both animals. Similar decrements were observed cytochemically. This study suggests that: (1) the distribution of AP varies between species, but that it is always present in at least one component of the blood-brain and blood-nerve barriers, (2) because barrier tissues of the nervous system have enzymatic activity, they may biochemically modify the adjacent environment, (3) vesicular profiles and caveolae in the blood vessels and perineurium may function as microenvironments for enzymatic activity, and (4) in the rat and frog, different isozymes of AP may be present.

Steady-state distribution of cycloleucine and alpha-aminoisobutyric acid between plasma and cerebrospinal fluid

Estimates of the steady-state distribution ratios of two nonmetabolizable amino acids, alpha-aminoisobutyric acid and aminocyclopentane carboxylic acid (cycloleucine), between plasma and cerebrospinal fluid were made with a view to establishing whether or not the low values found with metabolizable amino acids, such as glycine or leucine, could be accounted for by uptake and metabolism by the brain. The estimates, based on the ratios found after i.p. injections either in bolus form or by implantation of "osmotic pumps" containing the labeled amino acids, were comparable with those found for metabolizable amino acids.

Transport of alpha-aminoisobutyric acid across brain capillary and cellular membranes

The transport of alpha-aminoisobutyric acid (AIB), N-methyl-AIB (MeAIB), and diethylenetriaminepentaacetic acid (DTPA) from blood to brain was measured over different experimental periods in eight regions of the rat brain. Unidirectional transfer rate constants were determined from multiple-time/graphical and single-time analysis of the experimental data; values of 0.0018, 0.00057, and 0.000021 ml g-1 min-1, respectively, were obtained for the thalamus by graphical analysis. The initial distribution volume of AIB and MeAIB in brain tissue was several-fold greater than that of DTPA and the tissue plasma volume, and this difference was not accounted for by red blood cell uptake. This discrepancy could be due to rapid transport of AIB and MeAIB into brain endothelial cells in addition to the relatively rapid uptake by choroidal, meningeal, and ependymal associated tissues that was demonstrated by autoradiography. Thus, it may be misleading and erroneous to consider the blood-brain barrier (BBB) to be a simple, single-membrane structure when analyzing the blood-brain transfer data of solutes such as amino acids. The data from the ventriculocisternal perfusion experiments and previously published AIB uptake data in mouse brain slices were used to estimate the transfer rate constants across brain cell membranes. These studies indicated that the transport of AIB into brain cells was approximately 110 to 265 times greater than that across normal brain capillaries per unit mass of brain tissue, and that the BBB limits blood-to-brain cell transport of this amino acid. These observations (low rate of transport across normal brain capillaries and rapid concentrative uptake by brain cells) indicate that AIB is a good marker for measuring moderate to large increases in BBB permeability by experiments that require unidirectional flux of the tracer.

Ventriculo-cisternal perfusion of twelve amino acids in the rabbit

The clearances of twelve amino acids from the ventricles during ventriculo-cisternal perfusion in the rabbit have been measured; uptake by the brain was also measured and this permitted the separate computation of loss to brain and loss to blood during the perfusion. Clearance under carrier-free conditions was greater than when a concentration of 5mM unlabeled amino acid was present in the perfusion fluid. Brain uptake was also usually reduced by the presence of unlabeled amino acid due presumably to suppression of accumulation by brain cells. Reduction of transport across the blood-brain barrier would tend to increase brain uptake, and there was some evidence for a balance between the two opposing tendencies. Inhibition of clearance of a given labeled amino acid could be brought about by unlabeled amino acids of different molecular species. In general, the amino acids fell into three categories: neutral, acidic, and basic, and there was some overlap between them; of the neutral amino acids the A- and L-classification of Christensen was valid, although once again there was some overlap. If, during ventriculo-cisternal perfusion of a labeled amino acid, the activity of this labeled amino acid in the blood was raised well above that in the inflowing perfusion fluid, the labeled amino acid continued to be cleared from the perfusion fluid, suggesting uphill transport. On this basis it was suggested that the normally low concentrations of amino acids in the cerebrospinal fluid (CSF), by comparison with those in plasma, were due to an active transport from the CSF to the blood. Substrate-facilitated transport, whereby the penetration of labeled amino acid into the perfusion fluid from blood could be accelerated by adding unlabeled amino acid to the perfusion fluid, or vice versa, was demonstrated.

Role of carbon dioxide fixation, blood aspartate and glutamate in the adaptation of amphibian brain tissues to a hyperosmotic internal environment

Mechanisms have been examined by which hyperosmotic blood plasma might elevate the levels of aspartate and glutamate in the brain of the toad Bufo boreas. CO2 fixation was assessed by two in vivo methods using [2-14 C]glucose injected intracisternally. Thirty minutes after injection, the 14C labeling of glutamate and aspartate was more than 100 times greater in brain than in liver. In brain tissues, 40 + % of 14C atoms appeared to be incorporated into aspartate via the pyruvate carboxylase pathway. Brain tissues of control toads and toads adapting or adapted to hyperosmotic plasma osmolality revealed no differences in the rate of CO2 fixation as related to glucose utilization or tissue pool sizes of glutamate and aspartate. Elevated levels of these amino acids in blood plasma preceded increases in brain tissues. Carbon atoms required during hyperosmotic adaptation for expansion of amino acid pools in brain tissues may, in part, originate from amino acids in blood but apparently not from CO2 fixation in brain.

Colloid osmotic and hydrostatic pressures in chronic subdural haematomas

Intracranial pressure (ICP) has been measured in eight patients with chronic subdural haematomas (CSH) for 24 hours prior to and for 24 hours following evacuation of the haematomas. In all patients ICP was increased prior to surgery (mean: 23 mm Hg), and it rose again to the preoperative level following evacuation of the haematomas in spite of disappearance of most symptoms and signs. In two of these patients and in another eight patients with CSH, colloid osmotic pressures were measured in samples obtained simultaneously from haematoma fluid and venous blood during surgery. The colloid osmotic pressure in haematoma fluid varied considerably between patients (lowest: 11.8 mm Hg, highest: 60 mm Hg), and thus the difference between colloid osmotic pressures in haematoma fluid and plasma also varied considerably. We suggest that the increased ICP in patients with CSH is the consequence of an increased resistance to absorption of cerebrospinal fluid due to compression of the underlying subarachnoid space. The findings with regard to colloid osmotic pressures suggest that osmotic gradients may be maintained across the boundaries of a chronic subdural haematoma.

Electrical properties and structure of the frog arachnoid membrane

We have used an in vitro preparation of the frog arachnoid membrane to study the role of this membrane in the maintenance of the "blood-cerebrospinal fluid (csf) barrier". Electron microscopy showed that the membrane was made up of 10-15 layers of flat epithelial cells joined together by numerous cell junctions. The electrical resistance of the preparation was about 2000 ohms cm2. The steady-state transmural potential difference (pd) ranged up to 45 mV, csf positive, and this eliminated by either the addition of ouabain to the csf, or by replacing the NaC1 with TEA C1. The pd across the membrane increased when bicarbonate was added to the external bathing solutions. We conclude that this pd is due to the active transport of sodium from the subural fluid to the csf. In some preparations the transmural pd was reversed, i.e., csf negative, and this was also abolished by the addition of ouabain to the csf, or by replacing chloride with isethionate. We conclude that this pd is related to active chloride transport. These, and other experiments, lead us to the conclusion that the arachnois membrane is involved in the production of the cerebrospinal fluid and the maintenance of the blood-cerebrospinal fluid barrier.