The distribution of intravenously-injected inulin in the fluids of the nervous system of the dog and rat

Retinoic acid is detected at relatively high levels in the CNS of adult rats

Retinoic acid (RA) is essential for cellular growth and differentiation in developing and adult animals. The central nervous system (CNS) suffers developmental defects if embryonic levels of RA are too high or too low. The production and function of RA in adult brain are unclear. We report that RA is present throughout the brain and spinal cord of adult, vitamin A-deficient (VAD) rats treated with a physiological amount of all-trans-retinol. The hippocampus/cortex contained the highest proportion of RA in the brain (27.2 +/- 2.9% of the organic phase radioactivity, and 23.5 +/- 0.8% of the organic phase radioactivity extracted from spinal cord was RA). RA comprises a higher proportion of the retinoid pool in the CNS compared with amounts reported in other target tissues (E Werner and HF DeLuca. Arch Biochem Biophys 393: 262-270, 2001). However, RA is not preferentially transported from the blood to the brain. There were 2.90 +/- 0.20 fmol RA/g tissue transported to the brain of VAD rats treated with 2.00 nmol [20-(3)H]all-trans-retinoic acid, but higher amounts of RA were delivered to the liver, testis, and spleen. Because RA is not transported preferentially to brain, this tissue likely synthesizes RA more efficiently than other target tissues.

Drugs in the brain

In our studies on the entry of drugs into the central nervous system we have found the technique of autoradiography combined with radioassay to be a valuable research tool. It has disclosed such unsuspected phenomena as the dual routes of entry into the brain of acetazolamide. Although many factors controlling drug entry remain to be studied, we propose certain general conclusions. 1) The anatomical boundaries of brain are clearly reflected by the penetration and accumulation of all compounds we have studied-a finding that confirms the original proposition that whole-brain homogenates are inadequate for the study of drug and brain relationships. 2) Circulation, expressed as egional blood flow or volume of capillary blood, was seldom decisive in nfluencing entry or accumulation of exogenous substances in the brain. To date, the only compounds demonstrated to be circulation-dependent are trifluoroiodomethane and thiopental. Both are extremely fat-soluble. Tissue binding appears to be an additional factor in the case of thiopental. 3) Penetration is retarded by myelin. All substances we have studied have shown a relatively slower rate of entry into this tissue. In immature brain, before myelinization has taken lace, the primordial white matter is readily penetrated. We have suggested that entry into mature white matter is retarded by the lamellated membranes of the myelin sheath, which should be regarded, therefore, as a component of the blood-brain barrier. The small interstitial space indicated by the limited entry of sulfate ion is an additional hindrance to dispersal of exogenous substances into brain parenchyma. The blood-brain barrier is a complex anatomical, physiological, and biochemical phenomenon, and no unitary hypothesis is adequate to embrace all the observed events. 4) Accumulation of a drug in the brain implies some form of binding or interaction between drug and tissue. Findings on injection of phenobarbital, thiopental, or diphenylhydantoin illustrate such an accumulation. These binding interactions may be nonspecific, as is probable in the case of drugs bound to plasma protein. However, a more fundamental significance is suggested when a drug is found to bind, react with, or accumulate in, a specific anatomical structure of the brain. We have made reference to this possibility in connection with the localization of isonicotinic acid hydrazide or its metabolites in the hippocampus (46), and we have also reported the striking accumulation of acetazolamide in hippocampus, caudate nucleus, and hypothalamus. Although the binding process is poorly understood, further investigation of these phenomena should lead to a clearer understanding of regional variations in brain chemistry. While one should not assume that the demonstration of a focal concentration of a drug implies site of action, correlation between pharmacological action, electrophysiological events, biochemical changes, and temporal and regional drug concentrations may indeed exist (47).

Studies on the permeability of the blood-brain barrier in experimental diabetes

Whether the increased capillary permeability characteristic of diabetes extends to the blood-brain barrier is presently unclear. We have examined in streptozotocin-diabetic rats the permeability of the blood-brain barrier at the level of 12 discrete brain regions employing 3 intravenous tracers of different molecular weight: sucrose, insulin and horseradish peroxidase. In animals killed 5 min after tracer injection both the sucrose and the inulin spaces were similar to controls. In order to assess whether more prolonged circulation of tracers would uncover leakage, we studied brain spaces at longer intervals after tracer injections. When inulin was allowed to circulate for 15 min prior to killing, animals with 4 weeks of diabetes (but not 2 weeks) exhibited larger inulin spaces at the level of the medio-basal hypothalamus (p less than 0.01), medio-dorsal hypothalamus (p less than 0.05) and periaqueductal gray (p less than 0.01). Horseradish peroxidase, even after 75 min of perfusion, remained confined in both diabetic and control animals to central nervous system areas devoid of blood-brain barrier. Thus, after a relatively short duration of diabetes the blood-brain barrier manifests an increased permeability. It is subtle, limited to some brain regions and selective for low molecular weight tracers.

Factors that limit brain volume changes in response to acute and sustained hyper- and hyponatremia

Rats were made acutely hyper- or hyponatremic by infusion of hypertonic saline or water, respectively. Other rats were maintained in these states from 1 to 7 days to observe the effects of time. Brain tissue water, Na, Cl, and K were compared with serum Na and Cl concentration (Na(E) and Cl(E)). The following observations are noted: Brain Cl content varies directly with Cl(E) and brain Na content in the Cl space (Na(e)) varies directly with Na(E), indicating little or no restraint on the inward or outward movement of Na or Cl from the Cl space of brain. The intracellular volume of brain fluid (V(i)) derived as the difference between total water and Cl space, decreases with hypernatremia and increases with hyponatremia. The changes in V(i) in the acute studies are not accompanied by any change in brain K content, or calculated intracellular Na content, and are approximately 0.6 the changes predicted from osmotic behavior of cells, which apply four assumptions: (a) Na(E) is proportional to osmolality; (b) brain osmolality remains equal to plasma osmolality; (c) V(i) is osmotically active; and (d) there is no net gain or loss of solute from V(i). The validity of these assumptions is considered. When changes in osmolality are sustained, V(i) is much closer to control values than when in the acute phase. K content increases in hypernatremia and decreases in hyponatremia. The changes in K content can account for some of the adjustment in V(i) observed over the extended period of hyper- or hyponatremia. The regression of (Na + K)/v upon Na(E) describes a slope less than 1.0 and an intercept of (Na + K)/v equal to 40% of the control (Na + K)/v. These characteristics are interpreted to mean that significant quantities of Na and K in brain are osmotically inactive. The brain protects itself from acute volume changes in response to change in Na(E) by the freedom for Na and Cl to move from the Cl space, by V(i) not changing acutely to the degree predicted from osmotic properties of cells in general, and by significant quantities of Na + K in V(i) being osmotically inactive. With sustained changes in osmolality, V(i) approaches normal values and brain K changes to account for part of this later adjustment.