Thomas L. Pazdernik

A possible role for taurine in osmoregulation within the brain

Abstract: Intracranial microdialysis was used to measure changes in extracellular amino acids within the rat brain during local osmotic alteration of the extracellular micro‐environment or during systemic water intoxication. Increased cellular hydration produced by either of these methods was accompanied by a marked increase in extracellular taurine levels without affecting the other amino acids measured. With local osmotic alteration, this increase was osmolarity dependent and reversible. The specificity, sensitivity, and reversibility of the increase in extracellular taurine strongly suggest a functional role in osmoregulation in the brain under normal as well as pathological conditions.

Changes in extracellular amino acids during soman- and kainic acid-induced seizures

Extracellular amino acid levels in the rat piriform cortex, an area highly susceptible to seizure‐induced neuropathology, were determined by means of intracranial microdialysis. Seizures were induced by systemic administration of either soman (O‐1,2,2‐trimethylpropyl methylphosphonofluoridate), a potent inhibitor of acetylcholinesterase, or the excitotoxin kainic acid. Extracellular glutamate levels increased in animals with seizures shortly after administration of either convulsant, but this change was statistically significant only in the case of soman‐treated animals. Extracellular taurine levels increased markedly, reaching two‐and fourfold baseline levels during the second hour of soman‐and kainic acid‐induced seizures, respectively. Taurine levels did not increase in the subpopulation of soman‐treated animals without seizures, a finding indicating that elevation of extracellular taurine level is seizure related. Thus, we propose that taurine efflux may be a physiological cellular response to neuronal changes produced by excito‐toxic chemicals, either directly or as a consequence of seizures.

Changes in local cerebral glucose utilization induced by convulsants

With the six convulsants studied (Soman, intrahippocampal penicillin, bicuculline, pentylenetetrazol, picrotoxin and strychnine), the anatomical distribution of changes in local cerebral glucose utilization was related to the type of seizure observed. Strychnine induced a few very intense motor convulsions during the 2-deoxyglucose experimental period without having a major effect on brain local cerebral glucose utilization, in support of the view that its actions are predominantly in the spinal cord. Pentylenetetrazol and picrotoxin induced intermittent intense seizures and marked increases in local cerebral glucose utilization in the globus pallidus and substantia nigra. Soman, intrahippocampal penicillin and bicuculline all induced persistent status epilepticus associated with increases in local cerebral glucose utilization in many brain areas; those with striking increases in glucose use include: cortical areas, the limbic system, basal ganglia and substantia nigra. The glucose use changes produced by Soman, penicillin and bicuculline greatly exceeded those induced by pentylenetetrazol and picrotoxin. Activation of the substantia nigra and basal ganglia occurred with all centrally mediated convulsions and with status epilepticus there was also marked activation of cortical and limbic structures.

Hypoxia preconditioning attenuates brain edema associated with kainic acid-induced status epilepticus in rats.

Kainic acid (KA)-induced seizures elicit edema associated with necrosis in susceptible brain regions (e.g., piriform cortex and hippocampal CA1 and CA3 regions). To test the hypothesis that hypoxia preconditioning protects against KA-induced edema formation, adult male rats were exposed to a 9% O2, 91% N2 atmosphere for 8 h. KA (14 mg/kg, i.p.) was administered 1, 3, 7, or 14 days later. Regional analysis of edema indicated that hypoxia exposure attenuated edema formation in piriform and frontal cortices and hippocampus when KA was given 1, 3, or 7 days later but not 14 days after hypoxia. Cycloheximide (2 mg/kg s.c.) given 1 h prior to hypoxia prevented the protective effect of hypoxia on KA-induced edema attenuation in the piriform cortex and hippocampus. Thus, hypoxic challenge induces a general adaptive response that protects against the seizure-associated pathophysiology, with no direct relationship to seizure intensity. This response may involve stress-related transcription factors and effector proteins.

Soman induced changes in brain regional glucose use

Soman, a potent central acetylcholine esterase inhibitor, has a greater impact on brain regional glucose use than other organophosphates, such as diisopropylfluorophosphate (DFP) or phospholinium iodide. At near-lethal doses soman induced explosive persistent seizures that were associated with a greater than fourfold increase of glucose use in many brain structures. Single near-lethal doses of soman lead to conspicuous neuronal damage and a marked reduction in brain activity, 1 to 3 days after exposure. When soman (2 X LD50) was given to TAB (an antidotal mixture of trimedoxime, atropine, and benactyzine ) pretreated rats, there was a greater than twofold reduction of glucose use in almost every brain region. We suggest that soman seizures are mediated via activation of muscarinic receptors; also, the substantia nigra has a key role in the initiation/propagation of seizures. Soman has in addition, a depressive effect on some brain components which appears not to involve muscarinic receptors. We suggest that the conspicuous pathology that follows a single, near-lethal dose of soman results from a depletion of energy flow along with an influx of Ca2+ which sets into motion a cascade of destructive reactions, such as activation of proteases.

Topographical distribution of down-regulated muscarinic receptors in rat brains after repeated exposure to diisopropyl phosphorofluoridate

Quantitative receptor autoradiography demonstrated that muscarinic receptors were down-regulated in Wistar rats after repeated exposure to diisopropyl phosphorofluoridate. The density of receptors was decreased to 60-85% of the controls. Reductions in muscarinic receptor binding were observed in cortex, caudate-putamen, lateral septum, hippocampal formation, superior colliculus, and pons. The density of muscarinic receptors was unchanged in thalamic and hypothalamic nuclei, periaqueductal grey, cerebellum, inferior colliculus and reticular formation of the brain stem. The down-regulation of muscarinic receptors in forebrain structures, such as cortex, caudate-putamen and hippocampus, may be important in the adaptation to the behavioral effects of organophosphate poisons.

The osmotic/calcium stress theory of brain damage: Are free radicals involved?

This overview presents data showing that glucose use increases and that excitatory amino acids (i.e., glutamate, aspartate), taurine and ascorbate increase in the extracellular fluid during seizures. During the cellular hyperactive state taurine appears to serve as an osmoregulator and ascorbate may serve as either an antioxidant or as a pro-oxidant. Finally, a unifying hypothesis is given for seizure-induced brain damage. This unifying hypothesis states that during seizures there is a release of excitatory amino acids which act on glutamatergic receptors, increasing neuronal activity and thereby increasing glucose use. This hyperactivity of cells causes an influx, of calcium (i.e. calcium stress) and water movements (i.e., osmotic stress) into the cells that culminate in brain damage mediated by reactive oxygen species.

Effect of thyroidectomy and thyroxine on 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin-induced immunotoxicity

Radiothyroidectomy protected against 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced immunotoxicity in rats as assessed by the spleen anti-SRBC plaque-forming cell assay. Thyroxine (T4) replacement therapy partially reversed the effects of thyroidectomy on T4 and triiodothyronine (T3) serum levels, body weight and immune function as well as restored TCDD-induced immunotoxicity. Thus, hypothyroidism induced by TCDD exposure can be viewed as a protective response of the organism to reduce the insult caused by TCDD.

Soman-induced convulsions affect the inositol lipid signaling system: potentiation by lithium; attenuation by atropine and diazepam.

Effects of atropine or diazepam pretreatment on soman-induced convulsions and brain phosphoinositide (PI) metabolism, as assessed by brain regional inositol-1-phosphate (IP1) levels, were studied in saline and LiCl-pretreated rats. IP1, an intermediate in PI turnover, was measured in cortex, caudate, thalamus, hippocampus, and cerebellum. Soman (100 micrograms/kg; sc) produced convulsions in 63% of the saline-pretreated rats, whereas with LiCl pretreatment all rats exposed to 100 micrograms/kg of soman had tonic-clonic convulsions. Thus, LiCl pretreatment potentiated soman-induced convulsions. Tissue IP1 increased severalfold in soman-exposed convulsing rats with the highest increases being in frontal cortex and caudate. In contrast, no marked increases of IP1 occurred in similarly treated nonconvulsing rats. LiCl treatment itself increased IP1 levels without causing convulsions. In LiCl-pretreated rats, soman again markedly elevated IP1 levels above LiCl alone in convulsing rats, whereas no such effect occurred in nonconvulsing rats. In LiCl-pretreated rats, the increased IP1 levels associated with soman-induced convulsions were greatest in hippocampus and piriform cortex. Thus, LiCl appears to lower the threshold for the spread of seizure activity through limbic structures, thereby potentiating cholinergic-induced convulsions. Diazepam and atropine both blocked soman-induced convulsions, and brain regional IP1 elevations were concomitantly abolished as well. These results indicate that soman-induced convulsions involve the inositol lipid signaling system. This involvement is potentiated by lithium but attenuated by atropine and diazepam.

A global hypoxia preconditioning model: neuroprotection against seizure-induced specific gravity changes (edema) and brain damage in rats

Hypoxia preconditioning states that a sublethal hypoxia episode will afford neuroprotection against a second challenge in the near future. We describe and discuss a procedure for the development of global hypoxia preconditioning in adult male Wistar rats, using a mildly hypoxic (9% O(2), 91% N(2)) atmospheric exposure of 8 h. The persistence of neuroprotection was analyzed using a kainic acid (KA) model of brain injury. Rats were challenged with KA (14 mg/kg, i.p.) on 1-14 days post-hypoxia. The effects of hypoxia preconditioning on seizure score, weight loss, brain edema and histopathology were assessed. Brain edema, predominantly of vasogenic origin, was measured 24 h after KA administration using a reproducible and quantitative method based on the specific gravities of tissue samples. A density gradient column (1.0250-1.0650 g/cm(3)) comprised of kerosene and bromobenzene was used to assess the presence of edema in regions involved in seizure initiation and propagation that are normally extensively damaged (i.e., piriform cortex and hippocampus). Specific gravities of tissues were calculated through extrapolation with known NaCl standards. We found that hypoxia preconditioning prevented the formation of edema in these brain regions when KA challenge was given 1, 3, and 7, but not 14 days post-hypoxia exposure. Furthermore, neuroprotection was observed in animals that had robust seizures. The described procedure may be used to examine the neuroprotective mechanisms induced by global hypoxia preconditioning against many subsequent challenges reflecting a variety of experimental models of brain injury, and will provide a better understanding of the brain response to hypoxia and stress.