Unifying the mathematical modeling of in vivo and in vitro microdialysis.

A unifying approach is presented for developing mathematical models of microdialysis that are applicable to both in vitro and in vivo situations. Previous models for cylindrical probes have been limited by accommodating analyte diffusion through the surrounding medium in the radial direction only, i.e., perpendicular to the probe axis, or by incomplete incorporation of diffusion in the axial direction. Both radial and axial diffusion are included in the present work by employing two-dimensional finite element analysis. As in previous models, the nondimensional clearance modulus (Θ) represents the degree to which analyte clearance from the external medium influences diffusion through the medium for systems exhibiting analyte concentration linearity. Incorporating axial diffusion introduces a second dimensionless group, which is the length-to-radius aspect ratio of the membrane. These two parameter groups uniquely determine the external medium permeability, which is time dependent under transient conditions. At steady-state, the dependence of this permeability on the two groups can be approximated by an algebraic formula for much of the parameter ranges. Explicit steady-state expressions derived for the membrane and fluid permeabilities provide good approximations under transient conditions (quasi-steady-state assumption). The predictive ability of the unifying approach is illustrated for microdialysis of sucrose in vivo (brain) and inert media in vitro, under both well-stirred and quiescent conditions.

Effect of minocycline on inflammation-induced damage to the blood-brain barrier and white matter during development.

Damage to white matter in some premature infants exposed to intrauterine infections is thought to involve disruption of the blood-brain barrier. We have examined the effect of minocycline, an agent reported to reduce brain damage resulting from inflammation, on inflammation-induced disruption of the blood-brain barrier and damage to white matter. Post-natal marsupial opossums (Monodelphis domestica) were studied as most brain development in this species occurs after birth. Single intraperitoneal lipopolysaccharide (LPS) injection (0.2 mg/kg) with or without minocycline (45 mg/kg) at post-natal day (P)35 caused short-lasting barrier breakdown to plasma proteins but not to (14)C-sucrose. By P44, blood-brain barrier integrity was intact but a reduced volume of white matter was present. At P44 after prolonged inflammation (5 x 0.2 mg/kg LPS at 48 h intervals), proteins from blood were observed within brain white matter and permeability to (14)C-sucrose in the hindbrain increased by 31%. The volume of the external capsule and the proportion of myelin were 70 and 57%, respectively, of those in control animals. Minocycline administered during prolonged inflammation restored blood-brain barrier integrity but not LPS-induced damage to white matter. These data suggest that long-term changes in blood-brain barrier permeability occur only after a prolonged period of inflammation during development; however, damage to white matter can result from even a short-lasting breakdown of the barrier. Manipulation of the inflammatory response may have implications for prevention of some developmentally induced neurological conditions.

A novel method to calculate the extent and amount of drug transported into CSF after intranasal administration.

The aim of this paper is to establish a novel method to calculate the extent and amount of drug transported to brain after administration. The cerebrospinal fluid (CSF) was chosen as the target region. The intranasal administration of meptazinol hydrochloride (MEP) was chosen as the model administration and intravenous administration was selected as reference. According to formula transform, the extent was measured by the equation of X(A)CSF, infinity/X0 = Cl(CSF) AUC(0-->infinity)CSF/X0 and the drug amount was calculated by multiplying the dose with the extent. The drug clearance in CSF (Cl(CSF)) was calculated by a method, in which a certain volume of MEP solution was injected directly into rat cistern magna and then clearance was assessed as the reciprocal of the zeroth moment of a CSF level-time curve normalized for dose. In order to testify the accurateness of the method, 14C-sucrose was chosen as reference because of its impermeable characteristic across blood-brain barrier (BBB). It was found out that the MEP concentrations in plasma and CSF after intranasal administration did not show significant difference with those after intravenous administration. However, the extent and amount of MEP transported to CSF was significantly lower compared with those to plasma after these two administrations. In conclusion, the method can be applied to measure the extent and amount of drug transported to CSF, which would be useful to evaluate brain-targeting drug delivery.

Permeability and route of entry for lipid-insoluble molecules across brain barriers in developing Monodelphis domestica.

1. We have studied the permeability of blood-brain barriers to small molecules such as [(14)C]sucrose, [(3)H]inulin, [(14)C]L-glucose and [(3)H]glycerol from early stages of development (postnatal day 6, P6) in South American opossums (Monodelphis domestica), using a litter-based method for estimating steady-state cerebrospinal fluid (CSF)/plasma and brain/plasma ratios of markers that were injected I.P. 2. Steady-state ratios for L-glucose, sucrose and inulin all showed progressive decreases during development. The rate of uptake of L-glucose into the brain and CSF, in short time course experiments (7-24 min) when age-related differences in CSF production can be considered negligible also decreased during development. These results indicate that there is a significant decrease in the permeability of brain barriers to small lipid-insoluble molecules during brain development. 3. The steady-state blood/CSF ratio for 3000 Da lysine-fixable biotin-dextran following I.P. injection was shown to be consistent with diffusion from blood to CSF. It was therefore used to visualise the route of penetration for small lipid-insoluble molecules across brain barriers at P0-30. The proportion of biotin-dextran-positive cells in the choroid plexuses declined in parallel with the age-related decline in permeability to the small-molecular-weight markers; the paracellular (tight junction) pathway for biotin-dextran appeared to be blocked, but biotin-dextran was easily detectable in the CSF. A transcellular route from blood to CSF was suggested by the finding that some choroid plexus epithelial cells contained biotin-dextran. 4. Biotin-dextran was also taken up by cerebral endothelial cells in the youngest brains studied (P0), but in contrast to the CSF, could not be detected in the brain extracellular space (i.e. a significant blood-brain barrier to small-sized lipid-insoluble compounds was already present). However, in immature brains (P0-13) biotin-dextran was taken up by some cells in the brain. These cells generally had contact with the CSF, suggesting that it is likely to have been the source of their biotin-dextran. Since the quantitative permeability data suggest that biotin-dextran behaves similarly to the radiolabelled markers used in this study, it is suggested that these markers in the more immature brains were also present intracellularly. Thus, brain/plasma ratios may be a misleading indicator of blood-brain barrier permeability in very immature animals. 5. The immunocytochemical staining for biotin-dextran in the CSF, in contrast to the lack of staining in the brain extracellular space, together with the quantitative permeability data showing that the radiolabelled markers penetrated more rapidly and to a much higher steady-state level in CSF than in the brain, suggests that lipid-insoluble molecules such as sucrose and inulin reach the immature brain predominantly via the CSF rather than directly across the very few blood vessels that are present at that time.

Rapid distribution of intraventricularly administered sucrose into cerebrospinal fluid cisterns via subarachnoid velae in rat.

The intracranial distribution of [14C]sucrose, an extracellular marker infused for 30 s into one lateral ventricle, was determined by autoradiography of frozen-dried brain sections. Within 3.5 min [14C]sucrose appeared in: (i) the third ventricle, including optic, infundibular and mammillary recesses; (ii) the aqueduct of Sylvius; (iii) the velum interpositum, a part of the subarachnoid space that runs along the roof of the third ventricle and contains many blood vessels; (iv) the mesencephalic and fourth ventricles; and (v) the superior medullary velum, a highly vascular extension of the subarachnoid space that terminates at the walls of the mesencephalic and fourth ventricles. Within 5 min, radioactivity was present in the interpeduncular, ambient and quadrigeminal cisterns, which encircle the midbrain. By 10 min, approximately 11% of the radioactivity had passed into the subarachnoid space via a previously undescribed flow pathway that included the velum interpositum and superior medullary velum. At many places along the ventricular system, [14C]sucrose appeared to move from cerebrospinal fluid into the adjacent tissue by simple diffusion, as reported previously (Blasberg R. G. et al. (1974) J. Pharmac. exp. Ther. 195, 73-83; Levin V. A. et al. (1970) Am. J. Physiol. 219, 1528-1533; Patlak C. and Fenstermacher J.D. (1975) Am. J. Physiol. 229, 877-884; Rosenberg G. A. and Kyner W.T. (1980) Brain Res. 193, 56-66; Rosenberg G. A. et al. (1986) Am. J. Physiol 251, F485-F489). Little sucrose was, however, taken up by: (i) circumventricular organs such as the subfornical organ; (ii) medullary and cerebellar tissue next to the lateral recesses; and (iii) the superior and inferior colliculi and cerebral peduncles. For the latter two groups of structures, entry from cerebrospinal fluid was apparently blocked by a thick, multilayered glia limitans. Although [14C]sucrose was virtually absent from the rest of the subarachnoid system after 1 h, it remained in the perivascular spaces and/or walls of pial arteries and arterioles for more than 3 h. Certain transport proteins, protease inhibitors, growth factors and other neurobiologically active materials are present in cerebrospinal fluid, and their distribution to the brain and its blood vessels may be important. The present work shows, in the rat, that the flow of cerebrospinal fluid and the disposition of its constituents is fairly complex and differs among regions. Flow was rapid throughout the ventricular system and into various subarachnoid velae and cisterns, but was surprisingly slow and slight over the cerebral and cerebellar cortices. The cerebrospinal fluid-to-tissue flux of material was relatively free at many interfaces, but was greatly restricted at others, the latter indicating that the old concept of a "cerebrospinal fluid-brain barrier" may hold at such places. Finally, radiolabeled sucrose was retained longer within the walls and perivascular spaces of pial arteries and arterioles than in other subarachnoid tissues; one function of the cerebrospinal fluid system or "third circulation" may thus be delivering factors and agents to these pial blood vessels.

Cerebrovascular permeability changes during experimental meningitis in the rat.

Alterations in the blood-brain and blood-cerebrospinal fluid barriers occur during bacterial meningitis. Preventing barrier alterations is important as the increases in barrier permeability are thought to contribute to adverse neurological outcomes. The objective of this study was to characterize pharmacokinetically cerebrovascular permeability alterations during meningeal inflammation. 14C-Sucrose was used as a quantitative marker of cerebrovascular integrity 8 hr after induction of experimental meningitis by intracisternal injection of 0, 25 or 200 micrograms lipopolysaccharides. Serum and brain tissues were obtained after tracer dosing. 14C-Sucrose influx transfer coefficients (Kin(app)) and cerebrovascular volumes (Vbr) were calculated for each brain region. Regional Vbr values were unaffected by lipopolysaccharide pretreatment. However, statistically significant increases in 14C-sucrose K(in)(app) values were observed in various brain regions (1.6- to 3.3-fold from control; P < .05). These permeability alterations cannot be attributed to changes in the systemic pharmacokinetics of 14C-sucrose as total clearance and the volume of distribution were unaffected by lipopolysaccharide treatment. This approach can be used in future studies to examine the contribution of various inflammatory mediators to altered cerebrovascular permeabilities during experimental meningitis.

The nature of the decrease in blood-cerebrospinal fluid barrier exchange during postnatal brain development in the rat.

1. The blood-cerebrospinal fluid (CSF) exchange of a wide range of passively transported lipid insoluble compounds (0.43-5.4 nm molecular radius) has been investigated in rats at different stages of postnatal development (2 days old to adult). A novel 'litter-based' model for investigating blood-CSF barrier exchange in immature animals is described. 2. At each age investigated there was a clear inverse correlation between molecular radius and blood-CSF barrier exchange, in addition to an overall decrease in blood-CSF barrier exchange with increasing age. 3. The decrease in blood-CSF barrier exchange with age was not consistent with a reduction in pore diameters, nor does it appear to be due to an increase in the CSF sink effect with age. It seems likely to be due to a relative decrease in the number of a population of large diameter pores.

Species-specific transfer of plasma albumin from blood into different cerebrospinal fluid compartments in the fetal sheep.

1. The blood-cerebrospinal fluid (CSF) transfer of endogenous sheep albumin and several exogenous species of albumin has been investigated in different CSF compartments of the immature fetal sheep brain, at an early stage of development (60 days gestation, term is 150 days) when the CSF concentration of total protein is high. 2. There were marked differences in the steady-state CSF/plasma ratios for all species of albumin (including endogenous sheep albumin) between different CSF compartments. Ratios measured in the cisterna magna were significantly higher than those in the dorsal subarachnoid space, which in turn were higher than those in the lateral ventricles. The ratios for endogenous sheep albumin were (%; mean +/- S.E.M.): lateral ventricle (LV), 4.0 +/- 0.03; dorsal subarachnoid (DSA), 6.1 +/- 1.0; cisterna magna (CM), 13.7 +/- 0.8. 3. Three hours after I.V. injection, the CSF/plasma ratios for bovine albumin (LV, 2.0 +/- 0.2; DSA, 2.4 +/- 0.1; CM, 7.2 +/- 0.7%) were significantly lower than the ratio for endogenous sheep albumin in all three compartments. The ratios for human albumin (LV, 0.7 +/- 0.2; DSA, 1.0 +/- 0.2; CM, 3.9 +/- 0.4%) were significantly lower than those for bovine albumin. 4. In all three CSF compartments, the endogenous sheep albumin ratios were higher than would be expected on the basis of transfer by passive mechanisms. Conversely, steady-state CSF/plasma ratios for [3H]sucrose and [14C]inulin were consistent with passive transfer, and there were no differences between the ratios for these markers measured in each of the three CSF regions. 5. Goat albumin and [35S]sheep albumin ratios were not significantly different, 5 h after injection, from the endogenous sheep albumin levels in each of the three CSF compartments. 6. It is concluded that in the 60-day-old fetal sheep, all of the endogenous albumin in CSF is derived from the plasma by a specific transfer mechanism that can distinguish between different species of the same protein. There is also some evidence of a small passive component of blood-CSF albumin transfer. 7. Immunocytochemical evidence suggests that the route of transfer from blood to CSF is transcellular, through the choroid plexus epithelial cells. 8. Regional variations in albumin ratios are probably due to differences in specific transfer into each CSF compartment. This is reflected in a differential immunocytochemical staining for albumin in choroid plexus epithelial cells from different regions of the brain. 9. The results are discussed in terms of differences in albumin amino acid sequences, structural homologies, and transfer by a specific transcellular mechanism.

A chronic sheep preparation for the study of drug pharmacokinetics in spinal and ventricular CSF.

We describe a sheep preparation utilizing chronic vascular and subarachnoid catheterization and ventriculocisternal perfusion. This preparation allows simultaneous, atraumatic sampling of plasma and CSF after drug administration by the intravenous, intracerebroventricular, or lumbar intrathecal (i.t.) routes in an unanesthesized animal. This sheep preparation provides a convenient means of studying the CSF distribution of exogenous and/or endogenous substances. During intravenous infusion at a rate of 2.2 micrograms/kg/min, morphine appears in cisternal CSF within 15 min. The steady-state plasma concentration and CSF flux (or appearance rate) of morphine was 0.037 and 0.009 micrograms/min, respectively. At steady state, 0.008% of the administered dose appears in CSF/min. The coadministration of morphine, methadone, and [14C]sucrose into the fifth lumbar subarachnoid space is associated with the simultaneous appearance of morphine and [14C]sucrose, but not methadone, in cisternal CSF. The ratio of [14C]sucrose to morphine increased by nearly sevenfold in cisternal CSF, indicating clearance of morphine relative to [14C]sucrose as the compounds ascend in the CSF axis. The simultaneous appearance of morphine and [14C]sucrose in cisternal CSF after lumbar subarachnoid administration indicates that morphine, like sucrose, is distributed within the CSF by bulk flow. This sheep preparation can be used to provide the quantitative data necessary for the development of pharmacokinetic-pharmacodynamic models that relate plasma and CSF concentrations of opiates to their pharmacological effects. These studies will help to provide the pharmacological rationale for the administration of opiates by novel routes for pain management in man.

Drinking in the pigeon (Columba livia) in response to water deprivation and the influence of intracerebroventricular infusions of NaCl or sucrose solutions.

Pigeons were deprived of fluid for 24 hours then allowed access to water and a 2.7% NaCl solution, the intakes of both solutions during the subsequent 60 min were observed. In a second experiment hypertonic or isotonic solutions of NaCl or sucrose were infused ICV, starting 5 min before access to the solutions and finishing 10 min after access, and the consummatory behaviour observed. Pigeons rehydrated themselves after the period of deprivation with a volume of water equal to the weight lost during the deprivation. Pigeons never drank the hypertonic NaCl solution. ICV infusions of hypertonic or isotonic sucrose attenuated drinking in response to 24 hours water deprivation. Isotonic or hypertonic NaCl infused ICV, on the other hand, had no significant effect. Thus, in the pigeon, drinking in response to 24 hours of water deprivation appears to be controlled by one mechanism, possibly osmoreceptor in nature (with the permissive control of CSF sodium concentration), since the birds drank a volume of water equal to their weight loss and changes in CSF sodium concentration influenced drinking in a manner similar to that described previously for osmotically induced drinking in pigeons.

CSF distribution of morphine, methadone and sucrose after intrathecal injection.

The lumbar to cisternal CSF distribution of morphine and methadone were compared to C-14 sucrose, a standard marker of CSF bulk flow, after lumbar subarachnoid injections in a sheep preparation. Morphine appeared and peaked simultaneously with C-14 sucrose in cisternal CSF at 90 to 190 minutes. The mean peak cisternal CSF morphine concentrations were sustained for 30-40 minutes, and averaged 148 ng/ml, representing 0.3% of the administered dose. Methadone was not detectable in cisternal CSF up to 240-300 minutes after lumbar subarachnoid administration. The C-14 sucrose/morphine ratio was increased an average of 6.7 times in cisternal CSF as compared to the ratio of the two compounds injected into the lumbar subarachnoid space. These studies demonstrate that morphine, a hydrophilic opioid, given intrathecally moves rostrally and appears in cisternal CSF by bulk flow. Furthermore the rostral redistribution of morphine is associated with the clearance of morphine from CSF. Methadone, a lipophilic opioid, appears to be completely cleared from CSF before it reaches the cisterna magna. These pharmacokinetic studies support a contribution of supraspinal sites to the analgesic and adverse effects produced by morphine given by spinal routes of administration. In contrast methadone appears to exert its effects predominantly at spinal sites.

The effect of hypercapnia on a blood-brain barrier mechanism in foetal and new-born sheep.

1. The effect of marked hypercapnia (arterial PCO2 100 mmHg), nonrespiratory acidosis (pH 6-95-7-15) or hypoxia (arterial PO2 10-15 mmHg) upon penetration of labelled sucrose from blood into brain and c.s.f. has been investigated in exteriorized foetal sheep and new-born lambs. 2. In hypercapnia there was a consistent increase in c.s.f./plasma sucrose ratio after 90 min I.V. sucrose to four to five times control. Brain/plasma sucrose ratios were more variable. Usually there was an increase (up to three-and-a-half-times control); sometimes there was no change or even a decrease. The effect of hypercapnia on sucrose penetration was reversible. 3. Hypercapnia reduced c.s.f. secretion rate to approximately half the control value. Hypercapnia also caused a decrease in brain extracellular space. 4. Non-respiratory acidosis did not affect sucrose penetration. Hypoxia caused a decrease in brain/plasma sucrose ratio. 5. It is concluded that hypercapnia can cuase an increase in cerebral vascular permeability to sucrose in foetal and new-born sheep. Some possible mechanisms are discussed.

Nervous system involvement in systemic lupus erythematosus.

In a retrospective analysis of 80 patients with systemic lupus erythematosus (SLE) seen over a 10-year period, 41 (51%) exhibited neurological manifestations. Nervous system involvement was characterized by a significantly greater involvement of black patients (P less than 0-02), a higher incidence of renal failure after the first appearance of neurological features (P less than 0-05), and a significantly worse 10-year survival rate (P less than 0-05). Specific neurological investigations were generally unhelpful in diagnosis. The finding of hypoglycorrhachia in 4 patients was striking and may have pathogenetic significance. The lack of a specific diagnostic test for central nervous system involvement may have hindered recognition of a cerebral abscess in one patient. Treatment with massive doses of corticosteroids was not obviously more effective than treatment with smaller doses. Autopsy findings showed scattered small cerebral infarcts but vasculitis was apparent in only one case.

The development of a blood-brain barrier mechanism in foetal sheep.

1. The penetration of a metabolically inert, small molecular radius lipid insoluble substance ([(13)C] and [(4)H]sucrose), from blood into brain and c.s.f., has been studied in developing sheep from 50 days gestation (term, 150 days) through to the new-born stage. Around 50 days gestation sucrose accumulated rapidly into brain and c.s.f., and reached a steady-state level in brain of about 12% of the plasma level by 3 hr. By 60 days sucrose penetrated less freely into brain and c.s.f.; the brain steady-state level was 10% by 4(1/2) hr. A large decrease in sucrose penetration occurred by 70 days gestation, and by 123 days (just before the time when a foetal lamb becomes viable) both the rate of penetration and the brain steady-state level of sucrose were similar to those of the adult of other species.2. The rate of c.s.f. secretion at different ages has been estimated by dye dilution during ventriculo-cisternal perfusion. The turnover of c.s.f. in 60 day foetuses was high (1.36%/min.g wet weight brain). From 123 days gestation to the adult stage the turnover was much lower, 0.02%/min.g at 123 and 137 days gestation and 0.01%/min.g in the adult ewe.3. A simple new method for measuring c.s.f. volume is described. The volume at 51 days was estimated to be 0.14 ml., S.E. +/- 0.03, n = 4 (brain weight = 0.87 g +/- 0.11), at 59 days it was 0.45 ml., S.E. +/- 0.04, n = 6 (brain weight = 2.0 g +/- 0.1) and near term it was 7.28 ml S.E. +/- 1.29, n = 4 (brain weight 42.0 g +/- 0.5).4. The results are discussed in relation to possible changes in permeability of the cerebral capillary endothelium, the sink effect of c.s.f., and changes in extracellular space of the brain during its development. It is concluded that the high rate of penetration and raised brain steady-state level of sucrose in immature sheep foetuses is probably due to immaturity of a permeability barrier at the level of the cerebral capillary endothelium or its associated glial processes. Some clinical implications of these findings are considered briefly.