Evaluation of local cerebral glucose utilization and the permeability of the blood-brain barrier in the genetically epilepsy-prone rat

Comprehensive characterization of amino acids and water-soluble vitamins in a pentylenetetrazole-induced seizures rat model

Epilepsy is a complex neurological disease characterized by spontaneous recurrent seizures that affect around 1% of the global population. Despite the significant progress in the mechanisms of epileptogenesis, there is still about 60% of cases in which the cause is unknown. Thus, revealing the molecular mechanisms of epileptogenesis will greatly improve the development of epilepsy treatment. Since the comprehensive characterization of amino acids and water-soluble vitamins is important in understanding the underlying mechanisms of epilepsy or seizures, we developed two liquid chromatography-tandem mass spectrometry methods to quantify 17 water-soluble vitamins and 46 amino acids and applied them to our pentylenetetrazole-induced kindling rat model. All water-soluble vitamins were detected with a linearity of r > 0.992 and limits of quantitation between 0.1 and 5 ng/ml except for nicotinic acid. For amino acids, the linearities obtained were good with correlation coefficients higher than 0.99, and matrix effects were between 85.3% and 110%. To handle the multidimensional data more effectively, multivariate statistical analysis approaches used in non-targeted metabolomics were creatively exploited in the visualization, interpretation, and exploration of the results.

Glucose transport to the brain: a systems model

Glucose transport to the brain involves sophisticated interactions of solutes, transporters, enzymes, and cell signaling processes, within an intricate spatial architecture. The dynamics of the transport are influenced by the adaptive nature of the blood-brain barrier (BBB), the semi-impermeable membranes of brain capillaries. As both the gate and the gatekeeper between blood-borne nutrients and brain tissue, the BBB helps govern brain homeostasis. Glucose in the blood must cross the BBB's luminal and abluminal membranes to reach neural tissue. A robust representation of the glucose transport mechanism can highlight a target for brain therapeutic intervention, help characterize mechanisms behind several disease phenotypes, or suggest a new delivery route for drugs. The challenge for researchers is understanding the relationships between influential physiological variables in vivo, and using that knowledge to predict how alterations or interventions affect glucose transport. This paper reviews factors influencing glucose transport and approaches to representing blood-to-brain glucose transport including in vitro, in vivo, and kinetic models. Applications for different models are highlighted, while their limitations in answering arising questions about the human in vivo BBB lead to a discussion of an alternate approach. A developing complex systems simulation is introduced, initiating a single platform to represent the dynamics of glucose transport across the adapting human blood-brain barrier.

Regulation of amino acid and glucose transporters in endothelial and smooth muscle cells

While transport processes for amino acids and glucose have long been known to be expressed in the luminal and abluminal membranes of the endothelium comprising the blood-brain and blood-retinal barriers, it is only within the last decades that endothelial and smooth muscle cells derived from peripheral vascular beds have been recognized to rapidly transport and metabolize these nutrients. This review focuses principally on the mechanisms regulating amino acid and glucose transporters in vascular endothelial cells, although we also summarize recent advances in the understanding of the mechanisms controlling membrane transport activity and expression in vascular smooth muscle cells. We compare the specificity, ionic dependence, and kinetic properties of amino acid and glucose transport systems identified in endothelial cells derived from cerebral, retinal, and peripheral vascular beds and review the regulation of transport by vasoactive agonists, nitric oxide (NO), substrate deprivation, hypoxia, hyperglycemia, diabetes, insulin, steroid hormones, and development. In view of the importance of NO as a modulator of vascular tone under basal conditions and in disease and chronic inflammation, we critically review the evidence that transport of L-arginine and glucose in endothelial and smooth muscle cells is modulated by bacterial endotoxin, proinflammatory cytokines, and atherogenic lipids. The recent colocalization of the cationic amino acid transporter CAT-1 (system y(+)), nitric oxide synthase (eNOS), and caveolin-1 in endothelial plasmalemmal caveolae provides a novel mechanism for the regulation of NO production by L-arginine delivery and circulating hormones such insulin and 17beta-estradiol.

Increase in vulnerability of middle-aged rat brain to lead by cerebral energy depletion

The neurotoxic effects of low-level lead (Pb) during senescence are increasing interests of importance. We investigated the effects of low-level Pb on the brain in a normal condition and a pathophysiological condition of energy shortage that is commonly found in age-related neurological diseases. Middle-aged rats (15 months old) were exposed to 200 mg/l Pb acetate in drinking water for 2 months and thereafter received bilateral intracerebroventricular injections of streptozotocin (STZ). After 1 month's additional exposure to the same level of Pb solution as before the rats were sacrificed. Blood and brain Pb levels were measured by graphite furnace atomic absorption spectrophotometry. Energy-rich phosphate levels in the brain were determined by high-performance liquid chromatography equipped with a UV detector. Astroglial activation and glucose-regulated protein (GRP)94 expression were examined immunohistochemically. Exposure to Pb increased the blood Pb level to 10.8 microg/dl and the brain Pb level to 0.052 microg/g. But a significant additional increase in the brain Pb level, to 0.101 microg/g, became obvious in rats treated with Pb + STZ. Both Pb and STZ induced perturbation in brain energy metabolism, but no further alteration in energy metabolite levels was found in rats treated with Pb + STZ. Astroglial activation and GRP94-positive astrocytes and neurons were found only in the brains of Pb + STZ-treated rats. These results suggest that exposure to low-level Pb can perturb brain energy metabolism and the brain becomes more vulnerable to Pb when it is under energy stress.

MR measurement of regional relative cerebral blood volume in epilepsy

The purpose of this study was to evaluate the utility of magnetic resonance (MR) relative cerebral blood volume (rCBV) maps for studying regional hemodynamic changes in interictal and ictal epilepsy patients. Ten epilepsy patients were examined on a 1.5 T MR system. Nine patients were investigated interictally and one patient ictally. In the nine interictal patients, the dynamic plane was defined coronally through the hippocampus symmetrically. For the ictal patient, an axial dynamic plane was defined and the patient was scanned during seizure. Positron emission tomography (PET) studies were performed in 8 of the 10 patients. Lower rCBV of the left hippocampus was predicted by rCBV maps in seven of the nine interictal patients. The mean ratios of rCBV were 1.96 for left hippocampus/white matter and 2.49 for right hippocampus/white matter. The difference between these two ratios is statistically significant (P = 0.01, t-test). In two of the nine interictal temporal lobe epilepsy patients, lower rCBV areas were observed in the right hippocampus. In the ictal patient, the regional rCBV map demonstrated increased blood volume in the lesions. In eight of eight patients who underwent PET studies, MR rCBV findings were consistent with PET findings. The results show that regional hemodynamic changes in epilepsy can be evaluated with dynamic contrast-enhanced MR imaging. MR rCBV maps are sensitive to characterize seizure foci both ictally and interictally.

Glucose utilization during interictal intervals in an epilepsy model induced by pilocarpine: a qualitative study

PURPOSE

Interictal intervals in pilocarpine-induced chronic epilepsy are characterized by apparent normal electrographic activity and longer sleep periods or drowsiness or both. Sparse information exists concerning the neural network activity during these seizure-free intervals. In our research, a [14C]2-deoxy-D-glucose (2DG) autoradiographic technique was used to investigate interictal changes in the metabolism of the epileptic rat brain.

METHODS

Epileptic rats were monitored by video-EEG for approximately 120 days, with [14C]2DG injected after a seizure-free interval of > or = 24 h.

RESULTS

Autoradiographic analysis revealed an increase in glucose utilization by several brain regions; the most consistent increase was found in the lateral posterior thalamic nucleus and pretectal region.

CONCLUSIONS

These findings suggest that the lateral posterior thalamic nucleus and the pretectal region may be involved in cerebral circuits inhibiting epileptic activity during interictal intervals.

Interictal cerebral metabolic levels in Wistar rats sensitive to audiogenic seizures

In the present study, we compared interictal local cerebral metabolic rates for glucose (LCMRglcs) in a strain of audiogenic rats (Wistar AS) selected in our laboratory to interictal LCMRglcs in a strain of control non-epileptic (NE) rats. Two groups of Wistar AS were studied, one group exposed to a single audiogenic seizure and one group of kindled rats exposed to 40 daily repetitive seizures. Control NE animals were exposed to a single sound exposure which did not induce any behavioral disturbance. Interictal LCMRglcs were measured by the quantitative autoradiographic [14C]2-deoxyglucose technique 5 days after the last sound exposure. LCMRglcs were similar in the three groups of rats in 80% of the structures. Compared to the control NE strain, interictal metabolic levels were mainly decreased in auditory structures of Wistar AS, either naive or kindled, thus confirming auditory impairment in audiogenic animals. LCMRglcs were increased over control levels in both groups of Wistar AS in cerebellar regions. This increase of cerebellar functional activity in Wistar AS compared to control NE rats might reflect an increased cerebellar input which, together with auditory impairment, may facilitate the induction of seizure activity in Wistar AS. Finally, there was no difference between the interictal cerebral metabolic level of naive and kindled Wistar AS, except in the cerebellar dentate nucleus where LCMRglc was significantly higher in kindled than in naive animals.

Impairment of immunological functions in genetically epilepsy-prone rats

1. In genetically epilepsy-prone rats (GEPR-9s), which represent a natural genetic model of epilepsy, we observed that the number of peritoneal macrophages was significantly lower with respect to normal rats, and that some functional parameters (i.e. phagocytosis and intracellular killing) of these macrophages were impaired. 2. The count of lymphocyte populations showed a predominance of T-helper over T-cytotoxic/suppressor both in the spleen and lymph nodes. Moreover, an increased T-cell/B-cell ratio was observed in GEPR-9s. Flow cytometry revealed that GEPR-9s spleens possessed a large percentage of T-helper cells in comparison to normal rats. 3. By using concanavalin A-induced proliferation of GEPR-9s cultured lymphocytes, we have shown increased functional activation. 4. We suggest that the alterations in T-cell functions in GEPR-9s could be due to the involvement of the neuroendocrine system in the modulation of immunity, in the shift between Th1 and Th2, and in the activation of stress response.

Systemic cytokine administration can affect blood-brain barrier permeability in the rat

The aim of the present study was to clarify the effect of intracarotid injection of interleukin-1 beta (IL-1 beta), interleukin-2 (IL-2), interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-alpha) on the permeability of the blood-brain barrier (BBB) in the rat. A regional blood-to-brain transfer constant (Ki) for [14C] alpha-aminoisobutyric acid ([14C]AIB) and the cerebral residual blood volume were calculated 10 min following administration of cytokines (CKs; 1000 U/rat). The injection of IL-2 and IL-6 (but not of IL-1 beta) induced a significant enhancement of Ki values for [14C]AIB within several brain areas; conversely, when the rats were given TNF-alpha, a striking decrease in BBB permeability was observed. The cerebral regional blood volumes appeared significantly lower in the rats injected with IL-6 than in the control animals, but markedly increased following TNF-alpha administration. Our findings confirm the ability of some CKs to affect the permeability of the BBB and/or to act, probably indirectly, as vasomodulator agents of the cerebral microvessel endothelium.

Increased cardiovascular responsiveness to central cholinergic stimulation in the genetically epilepsy-prone rat

We sought to determine whether differences in cardiovascular responsiveness to central stimulation of the cholinergic system existed between the genetically epilepsy-prone and outbred Sprague-Dawley rats. We treated the unanaesthetized, restrained rats with the indirect cholinergic agonist physostigmine (25, 50, 100 and 200 micrograms kg-1, i.v.) and the direct muscarine agonist arecoline (50, 100 and 200 micrograms kg-1, i.v.). Blood pressure and heart rate were evaluated. Genetically epilepsy-prone rats demonstrated to be more susceptible to the action of physostigmine than the outbred Sprague-Dawley rats. Conversely, we did not note any difference between the two strains in the extent of the pressor response induced by arecoline. Moreover, we treated both strains with hemicholinium-3 (34.8 nmol, i.c.v.) to deplete endogenous stores of acetylcholine. This treatment did not affect the pressor response to arecoline, whereas it greatly reduced the response to physostigmine. The present results support an increased cardiovascular responsiveness to central cholinergic stimulation in the genetically epilepsy-prone rat which appears to be related to a pre-synaptic rather than a post-synaptic component.

Desmethylimipramine, a potent inhibitor of synaptosomal norepinephrine uptake, has diverse effects on thyroid hormone processing in rat brain. II. Effect on in vivo 5'-deiodination of [125I]thyroxine

We have studied the effects of desmethylimipramine (DMI), a tricyclic antidepressant, on thyroid hormone (TH) handling in rat brain in an effort to discover a pharmacological basis for reported interactions between TH, affective disorders and psychotropic drugs. An acute dose of DMI has been used in order to determine the primary effects of the drug in brain without perturbations from secondary effects. Recently we have reported that a single dose of DMI significantly decreases brain uptake of both [125I]thyroxine (T4) and [125I]3,3',5-triiodothyronine (T3) across the spectrum of thyroid states from hypothyroid (HYPO) to euthyroid (EU) to T4-induced hyperthyroid (HYPER). To investigate further the effects of DMI on brain processing of TH, we have measured effects of the drug on in vivo rates of T4 to T3 conversion in a series of experiments in which DMI (25 mg/kg) was given to HYPO, EU and HYPER male rats in conjunction with i.v. [125I]T4. Decreased in vivo conversion ratios (T3/T4 ratios) suggest that acute DMI treatment causes a significant decrease in 5'-deiodinase activity in balance of brain (but not cerebellum) in all DMI treated rats as compared to their saline treated controls (ANOVA, P < 0.0001). For assurance that reduced T3/T4 in DMI treated rat brain is not the result of DMI enhancement of 5-deiodination of T3 or T4, the effect of DMI on concentrations of labeled I-, rT3, and T2 (3,3'- and 3',5'-) was also observed. In no case was there a significant increase in any metabolite in DMI treated rats for any tissue studied.(ABSTRACT TRUNCATED AT 250 WORDS)

Genetically epilepsy-prone rodents show some changes of ion levels in the brain

In the present study the water and ion (Na+, K+, Ca2+, Fe3+, Se4+, Mg2+, Mn2+, Mn2, Se4+, Cu2+) content in the brain of genetically epilepsy-prone rats (GEPRs) and of 21-, 45-, and 60-day-old DBA/2 mice were determined, and compared with those measured in normal controls (Sprague-Dawley rats and Swiss mice), to verify whether the predisposition to audiogenic seizures (AGS) may be partially related to changes in the cerebral osmotic and ionic state. Our findings clearly evidenziate two points: a) a more complex shift in brain ionic balance (rather than a peculiar modification in the concentration of a single ion) seems very likely involved in AGS susceptibility; (b) brain Ca2+ and Se4+ amounts, together with the water content, appear to be really important factors to which a role in abnormal seizure predisposition may be attributed.

Desmethylimipramine, a potent inhibitor of synaptosomal norepinephrine uptake, has diverse effects on thyroid hormone processing in rat brain. I. Effects on in vivo uptake of 125I-labeled thyroid hormones in rat brain

Several lines of evidence point to an interaction between amine uptake inhibitors (tricyclic antidepressants) and thyroid hormones. To examine this issue under conditions which would minimize secondary effects of drug treatment, desmethylimipramine (DMI), a highly specific norepinephrine uptake inhibitor, was given acutely as a single i.p. dose one hour before i.v. [125I]triiodothyronine (T3*) or [125I]thyroxine (T4*). Tissues were analysed after rat decapitation at 3, 5, 10, and 20 min intervals thereafter. DMI had a small but significant inhibitory effect on the brain uptake of both T3* (7.4%) and T4* (19%) over their respective 20-min time courses as indicated by two-way ANOVA. To examine the drug response further and to determine the effect of thyroid status on the response, hypothyroid (HYPO) and T4-induced hyperthyroid (HYPER) rats, were given i.v. T3* and, 5 min later, i.p. DMI or saline. They were killed 3 h later and tissue analysed. Because DMI effects on T4* uptake could not be evaluated over a 3 h period without blocking T4* to T3* conversion, sodium ipodate (60 mg/kg) was given in 2 doses before i.v. T4*. Under these conditions, DMI significantly reduced brain concentrations of the administered T3* and T4* in HYPO (15% and 19%) and in HYPER rats (13% and 25%). These results suggest that, as it does in the case of norepinephrine, DMI blocks the uptake site for T3 and T4 in rat brain. No information is available regarding the relationship, if any, between the thyroid hormone and norepinephrine uptake sites.