Effect of ethanol on entry of some substances into the brains of rats

Nervous tissue thiamine metabolism in vivo. I. Transport of thiamine and thiamine monophosphate from plasma to different brain regions of the rat

The transport of thiamine (T) and thiamine monophosphate (TMP) across the blood-brain barrier was measured in vivo in the rat. Different doses of [14C]T (15-550 nmol) and [14C]TMP (11-110 nmol) were injected into the femoral vein. The content of T and its phosphoesters in blood and brain tissue (cerebellum, pons, medulla and cerebral cortex) 20 s after the injection was determined radiometrically after electrophoretic separation. Blood flow and blood volume in the same regions of the brain was also determined. Both T and TMP entered rapidly the cerebral tissue, where they were found chemically unmodified. The cerebral tissue extracted less than 7% of plasma T. At physiological plasma T concentrations, the rate of transport ranged from 0.43 to 0.65 nmol X g-1 X h-1 with only minor differences among the various regions. T was transported into the nervous tissue by two separate mechanisms: one saturable, that at physiological plasma T levels accounted for 95% (cerebellum) to 91% (cerebral cortex) of the total T taken up, and one non-saturable, that was most efficient in the cerebral cortex. The Km (half-saturation constant) of the former transport mechanism ranged from 1.95 to 2.75 nmol X ml-1 in the 4 areas investigated. Vmax (maximal transport rate) values ranged from 6 to 9 nmol X g-1 X h-1, the highest value being found in the cerebellum. The overall transport rate of TMP was on average 5-10 times as low as that of T and also showed a saturable and a non-saturable component. Both components were slower than those observed for T.

The effect of ethanol on GABAergic transmission

One of the many actions of ethanol involves the GABAergic system. The interaction of ethanol with GABAergic neurons is a complex one involving both presynaptic and postsynaptic sites. Through a presumed fluidization of membranes after a single dose of ethanol, the available in vitro evidence suggests that ethanol disrupts the normal functioning of the GABA-benzodiazepine-chloride ionophore complex in a complicated manner involving a sequential activation of different active sites leading to the facilitation of GABA transmission. This finding has been supported in vivo using electrophysiological techniques. Presynaptic GABAergic neurons may experience a reduced activity, especially at low doses of ethanol. After chronic ethanol treatment, GABAergic transmission may be reduced, especially during an ethanol withdrawal syndrome. Also, other changes in the GABA-benzodiazepine-chloride ionophore complex suggest GABA transmission is suppressed postsynaptically. Drugs which enhance the actions of GABA may be suitable inhibitors of the ethanol withdrawal syndrome. In particular a new class of drugs, the triazolopyridazines, may be promising compounds for treatment of withdrawal with a more specific mode of action and fewer side effects.

The interaction of alcohol and stress. A review

The literature on the interaction of ethanol and stress is reviewed. Stress has amethystic properties in both experimental animals and human subjects when analgesia or various behavioral parameters were evaluated. In addition studies have shown that ethanol counteracts some effects of stress in non-alcoholic subjects. Improvement in performance and in the effective state of humans and reversal of some behavioral and pathological concomitants of stress in experimental animals have been reported. Although there is indication that ethanol improves the affective state in humans, reduction of anxiety has not been a universal finding. Recent studies have pointed out a number of variables (the drinking environment, cognitive set, mood and personality of subjects, prior experience with ethanol, sex, dose and type of beverage) which can significantly alter the effects of alcohol in human subjects. These may account for the variability in results in the literature. Although some studies have shown that alcohol ingestion increases under stressful conditions, others have failed to do so in both experimental animals and in humans. In alcoholics ethanol ingestion, in general, does not appear to relieve anxiety. In fact anxiety usually increases with time during a drinking binge. Therefore more research needs to be done to assess the validity of the anxiety-reducing theory for alcohol abuse. Possible mechanisms for the interaction of stress and alcohol are discussed.

Permeability changes in the blood-brain barrier: causes and consequences

1. Generalized changes in blood-brain barrier (BBB) permeability are accompanied by extravasation of plasma proteins; thus, they are readily studied with protein markers or protein-dye complexes. Selective changes in permeability involve alterations in BBB transport systems; they are best studied with techniques which detect the qualitative hallmarks of carrier-mediated transport, namely saturation, competition, and stereospecificity. 2. Quantitative assessments of the selective permeability of the BBB can be made from the saturation data expressed in terms of Michaelis-Menten kinetics. The advantages of the latter are twofold: (a) alterations elicited by modified barrier affinity (Km) can be distinguished from alterations in carrier capacity (Vmax); (b) the relative rates of flux of a metabolite across the BBB can be placed in the perspective of cerebral metabolism. Kinetic data on transport processes in the BBB are obtained by either constant infusion or single injection techniques. Results obtained with both methodologies have been comparable. 3. Independent transport systems for glucose, neutral amino acids, basic amino acids, and monocarboxylic acids have been identified in the BBB. The description of these transport systems in kinetic terms provides a background of information on intact mechanisms to which altered transport can be compared. 4. Experimental evidence indicates that the availability of key metabolic substrates, such as glucose or essential amino acids, may be rate-limiting in cerebral metabolism. A working hypothesis was developed that the consequences of a selective change in barrier permeability to one or more of these essential substrates are directly related to altered rates of reaction in substrate-limited pathways, e.g., cerebral protein or neuro-transmitter biosynthesis. 5. Toxicological causes of generalized changes in BBB permeability include hypertonic solutions, organic solvents, surface-active agents, enzymes, and heavy metals. Some agents, e.g., mercury or hypertonic urea, induce selective changes in BBB transport at doses much lower than those required for nonspecific barrier break-down. Subtle changes in transport of metabolic substrates may remain unrecognized unless specifically investigated, yet may have profound consequences on brain metabolism. 6. Pathological processes can also induce selective changes in BBB permeability. Such changes often temporally precede the more generalized alterations in permeability that can occur during pathogenesis. For example, in brain edema due to an ischemic infarct, glucose transport increases during the early cytotoxic phase, whereas generalized changes are not detected until the later vasogenic phase.