Marin Bulat

Recent insights into a new hydrodynamics of the cerebrospinal fluid.

According to the traditional hypothesis, the cerebrospinal fluid (CSF) is secreted inside the brain ventricles and flows unidirectionally along subarachnoid spaces to be absorbed into venous sinuses across arachnoid villi and/or via paraneural sheaths of nerves into lymphatics. However, according to recent investigations, it appears that interstitial fluid (ISF) and CSF are formed by water filtration across the walls of arterial capillaries in the central nervous system (CNS), while plasma osmolytes are sieved (retained) so that capillary osmotic counterpressure is generated, which is instrumental in ISF/CSF water absorption into venous capillaries and postcapillary venules. This hypothesis is supported by experiments showing that water, which constitutes 99% of CSF and ISF bulk, does not flow along CSF spaces since it is rapidly absorbed into adjacent CNS microvessels, while distribution of other substances along CSF spaces depends on the rate of their removal into microvessels: faster removal means more limited distribution. Furthermore, the acute occlusion of aqueduct of Sylvius does not change CSF pressure in isolated ventricles, suggesting that the formation and the absorption of CSF are in balance. Multidirectional distribution of substances inside CSF, as well as between CSF and ISF, is caused by to-and-fro pulsations of these fluids and their mixing. Absorption of CSF into venous sinuses and/or lymphatics under the physiological pressure should be of minor importance due to their minute surface area in comparison to the huge absorptive surface area of microvessels.

NEW EXPERIMENTAL MODEL OF ACUTE AQUEDUCTAL BLOCKAGE IN CATS : EFFECTS ON CEREBROSPINAL FLUID PRESSURE AND THE SIZE OF BRAIN VENTRICLES

It is generally assumed that cerebrospinal fluid (CSF) is secreted in the brain ventricles, and so after an acute blockage of the aqueduct of Sylvius an increase in the ventricular CSF pressure and dilation of isolated ventricles may be expected. We have tested this hypothesis in cats. After blocking the aqueduct, we measured the CSF pressure in both isolated ventricles and the cisterna magna, and performed radiographic monitoring of the cross-sectional area of the lateral ventricle. The complete aqueductal blockage was achieved by implanting a plastic cannula into the aqueduct of Sylvius through a small tunnel in the vermis of the cerebellum in the chloralose-anesthetized cats. After the reconstitution of the occipital bone, the CSF pressure was measured in the isolated ventricles via a plastic cannula implanted in the aqueduct of Sylvius and in the cisterna magna via a stainless steel cannula. During the following 2 h, the CSF pressures in the isolated ventricles and cisterna magna were identical to those in control conditions. We also monitored the ventricular cross-sectional area by means of radiography for 2 h after the aqueductal blockage and failed to observe any significant changes. When mock CSF was infused into isolated ventricles to imitate the CSF secretion, the gradient of pressure between the ventricle and cisterna magna developed, and disappeared as soon as the infusion was terminated. However, when mock CSF was infused into the cisterna magna at various rates, the resulting increased subarachnoid CSF pressure was accurately transmitted across the brain parenchyma into the CSF of isolated ventricles. The lack of the increase in the CSF pressure and ventricular dilation during 2 h of aqueductal blockage suggests that aqueductal obstruction by itself does not lead to development of hypertensive acute hydrocephalus in cats.

Elimination of phenolsulfonphthalein from the cerebrospinal fluid via capillaries in central nervous system in cats by active transport

It was recently proposed that organic anions, such as cerebral acidic metabolites and phenolsulfonphthalein (PSP), are eliminated from cerebrospinal fluid (CSF) by diffusion into the central nervous system (CNS) and further by active transport into capillaries. To test this hypothesis, PSP was injected into cisternal CSF and its distribution into various parts of the CNS was measured 1 and 3 h later in control cats and those pretreated with probenecid, which blocks active transport of organic anions into capillaries. PSP in tissue shows an intensive pink color when exposed to 1 N NaOH. Planimetric analysis of color pictures of coronal CNS slices showed that at the first hour, diffusion and distribution of PSP into the CNS in both groups of animals was similar, while at the third hour, a great reduction of PSP distribution in the CNS in control and only a slight reduction in probenecid pretreated cats was observed. The results support the hypothesis that active transport across the capillary wall in the CNS is the main avenue for elimination of cerebral acidic metabolites from both CSF and CNS and in such a way that central homeostasis is maintained.

Dynamics of distribution of 3H-inulin between the cerebrospinal fluid compartments

Since the distribution of substances between various cerebrospinal fluid (CSF) compartments is poorly understood, we studied (3)H-inulin distribution, over time, after its injection into cisterna magna (CM) or lateral ventricle (LV) or cisterna corporis callosi (CCC) in dogs. After the injection into CM (3)H-inulin was well distributed to cisterna basalis (CB), lumbar (LSS) and cortical (CSS) subarachnoid spaces and less distributed to LV. When injected in LV (3)H-inulin was well distributed to all CSF compartments. However, after injection into CCC (3)H-inulin was mostly localized in CCC and adjacent CSS, while its concentrations were much lower in CM and CB and very low in LSS and LV. Concentrations of (3)H-inulin in venous plasma of superior sagittal sinus and arterial plasma were very low and did not differ significantly, while its concentration in urine was very high. In (3)H-inulin distribution it seems that two simultaneous processes are relevant: a) the pulsation of CSF with to-and-fro displacement of CSF and its mixing, carrying (3)H-inulin in all directions, and b) the passage of (3)H-inulin from CSF into nervous parenchyma and its rapid distribution to a huge surface area of capillaries by vessels pulsations. (3)H-inulin then slowly diffuses across capillary walls into the bloodstream to be eliminated in the urine.

Homeostatic role of the active transport in elimination of [3H]benzylpenicillin out of the cerebrospinal fluid system.

Cerebral acidic metabolites and penicillin are organic anions which can be carried by active transport into capillaries of the central nervous system (CNS). However, it is generally believed that these metabolites are mainly delivered from CNS to cerebrospinal fluid (CSF) and eliminated by CSF circulation over cortex and its absorption into dural venous sinuses. To test this hypothesis we studied fate of penicillin ([3H]benzylpenicillin) in the CSF under control conditions and when its active transport was blocked by probenecid. After application of penicillin into cisterna magna of control dogs, it is distributed only in traces to lumbar, ventricular and cortical CSF. However, when active transport of penicillin across capillary wall is blocked by probenecid, its disappearance from cisterna is slowed down and its distribution is greatly enhanced so that at 300 min penicillin concentrations in cisternal, lumbar and cortical CSF approach or equal each other. Disappearance of penicillin from cisternal CSF shows a single exponential course (half-time 30 min) in control, while in probenecid pretreated dogs this is a slow multiexponential process. The results indicate that the active transport across capillary wall in CNS, but not generally postulated unidirectional CSF circulation over cortex and its absorption into dural venous sinuses, is instrumental in elimination of cerebral acidic metabolites and in such a way homeostasis in brain and cerebrospinal fluid is maintained.

Effect of active transport on distribution and concentration gradients of [3H]benzylpenicillin in the cerebrospinal fluid

After application of [3H]benzylpenicillin ([3H]BP) in lateral brain ventricle in dogs, the distribution of [3H]BP to contralateral ventricle and cisterna magna was much higher when its active transport from cerebrospinal fluid (CSF) was blocked by probenecid than under control conditions. Analysis of [3H]BP concentrations in both lateral ventricles and cisterna magna over time indicates that active transport restricts distribution of substances along CSF spaces and contributes to the maintenance of their concentration gradients between CSF compartments. This suggests that biochemical changes in a part of the brain and the adjacent CSF compartment may not be reflected into remote compartments of CSF such as lumbar CSF if substances in question are removed from CSF by active transport.

Effect of the antiepileptic di-n-propylacetamide on 5-hydroxytryptamine turnover in the brain and 5-hydroxyindoleacetic acid level in the cerebrospinal fluid

The effect of antiepileptic drug di-n-propylacetamide (DPM) on 5-hydroxytryptamine (5-HT) turnover in rat brain and 5-hydroxyindoleacetic acid (5-HIAA) in cat cerebrospinal fluid (CSF) was investigated. DPM (200 mg/kg) increased brain 5-HIAA without altering the 5-HT level. DPM augmented the accumulation of 5-HT induced by monoamine oxidase inhibition with pargyline (80 mg/kg) and enhanced the accumulation of 5-HIAA in the brain following blockade of transport of this metabolite by probenecid (200 mg/kg). Prior inhibition of 5-HT synthesis by p-chlorophenylalanine (300 mg/kg) abolished the DPM-induced increase in cerebral 5-HIAA. DPM (100 mg/kg) given daily for 5 days considerably elevated 5-HIAA in the CSF of cat during the treatment period. We conclude that DPM increases the turnover of 5-HT in brain and that this can be observed by monitoring the 5-HIAA content of CSF.

Spinal contribution to CSF pressure lowering effect of mannitol in cats.

OBJECTIVES After application of hyperosmolar mannitol the cerebrospinal (CSF) pressure is usually lowered within 30 min but this effect cannot be explained either by changes in intracranial blood volume and flow or by changes in brain volume. We assume that this effect of mannitol my be consequence of CSF volume decrease primarily in the spinal CSF due to high compliance of the spinal dura. METHODS To explore such a possibility we planned to separate spinal and cerebral CSF. In chloralose anaesthetized cats dorsal laminectomy of C2 vertebrae was performed and a plastic semi ring was positioned extradurally separating cranial and spinal CSF. CSF pressures were recorded via cannulas positioned in lateral ventricle and lumbar subarachnoid space at L3 vertebrae, respectively. RESULTS After intravenous bolus of 20% mannitol (0.5 or 1.0 g/kg/ 3 min) in control animals without cervical stenosis, the fall of both ventricular and lumbar CSF pressures was equal over time. At 15 min after mannitol application in cats with cervical stenosis an slight increase of ventricular and a fall of lumbar CSF pressures were observed, while at 30 min a gradient of these pressures of 5.5 and 7 cm H2O at lower and higher dose of mannitol, respectively, were registered. However, after removal of cervical stenosis these gradients disappeared. CONCLUSION The observed changes of CSF pressures in spinal and intracranial space indicate that spinal subarachnoid space contributes a great deal to overall fall of CSF pressure and volume in the early period after mannitol application probably due to high compliance of the spinal dura.

The investigation of cerebrospinal fluid formation by ventriculo-aqueductal perfusion method in cats

OBJECTIVES The perfusion of cerebrospinal fluid (CSF) spaces by artificial CSF (aCSF) containing an indicator, is an indirect method used to calculate CSF formation. To evaluate this method, we have developed a ventriculo-aqueductal perfusion method, which enables a direct measurement of CSF formation in the ventricles. METHODS In chloralose anaesthetized cats, the aqueduct of Sylvius was cannulated so that the outflow end of the plastic cannula was positioned extracranially. Both lateral ventricles were also cannulated, with one cannula for infusion of aCSF containing blue dextrane and the other for measurement of CSF pressure. RESULTS During ventriculo-aqueductal perfusion (direct method) under physiological CSF pressure, the outflow rate from aqueductal cannula did not differ significantly from the inflow rate, i.e. no CSF formation was observed. When the indirect method based on dilution of blue dextran in the outflowing perfusate was used, the formation of approximately 5 microl/min of CSF was obtained. CONCLUSION Results of the direct method indicate that net CSF formation inside brain ventricles does not exist. The opposite results obtained by the indirect method questions this method as a reliable study of CSF formation.

Osmotic Force of the CSF and Intracranial Pressure in Health and Disease

The role of the osmotic pressure of CSF in physiological and pathological conditions is unknown. In our preliminary experiments in cats we have observed that slow infusion of different hyperosmolal solutions in the lateral ventricle increases the ICP [1], In the present experiments in dogs we have studied the effect of hyperosmolal sucrose solution and innoculum of Streptoccocus pneumoniae applied into the lateral ventricle on ICP and osmolality of CSF.