Protective effect of L-carnitine in ammonia-precipitated encephalopathy in the portacaval shunted rat

L-carnitine administration prevents the neurological symptoms of acute ammonia toxicity. To further evaluate its efficacy in the prevention of hepatic encephalopathy in hyperammonemic conditions, L-carnitine (16 mmol/kg, intraperitoneally [i.p.] was administered 1 hour before ammonium acetate (NH4OAc) (8.5 mmol/kg, subcutaneously) to portacaval shunted (PCS) rats. Cerebrospinal fluid (CSF) ammonia, lactate, and amino acid levels were measured in relation to deteriorating neurological status in these animals. None of 35 L-carnitine-treated animals showed neurological deterioration after NH4OAC administration compared with saline-treated controls; the latter manifested severe encephalopathy progressing through loss of righting reflex to coma. Survival rate was 100% in the L-carnitine-treated group compared with 5% in saline-treated controls. Following NH4OAC administration to PCS rats, CSF ammonia increased to 0.93 +/- 0.15 mmol/L and 1.24 +/- 0.15 mmol/L at precoma and coma stages of encephalopathy (P < .01) respectively. Treatment with L-carnitine reduced CSF ammonia at both precoma and coma stages; the time-course of this protective effect paralleled blood and CSF L-carnitine accumulation. CSF alanine and lactate increases following NH4OAC administration to PCS rats were significantly attenuated following L-carnitine treatment. However, L-carnitine treatment did not lead to significant reductions in plasma ammonia nor CSF or brain glutamine in these animals. These findings show the therapeutic efficacy of L-carnitine in ammonia-precipitated coma in PCS rats and suggest that this protective effect is centrally mediated involving improved mitochondrial respiration. L-carnitine could be of therapeutic benefit in the prevention of hepatic encephalopathy precipitated by ammoniagenic conditions in humans with chronic liver disease.

Selective loss of binding sites for the glutamate receptor ligands [3H]kainate and (S)-[3H]5-fluorowillardiine in the brains of rats with acute liver failure

There is increasing evidence that alterations of glutamatergic function are implicated in the pathogenesis of central nervous system consequences of acute liver failure. The aim of the study was to assess the integrity of glutamate receptors in the brain in experimental ischemic liver failure using quantitative receptor autoradiography and the selective ligands [3H]MK801 (for N-methyl-D-aspartate [NMDA] sites), [3H]5-fluorowillardiine (for non-NMDA, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid [AMPA] subclass sites), and [3H]kainate (for non-NMDA, kainate subclass sites). At coma stages of encephalopathy, a selective loss of up to 60% of binding sites for the kainate- and AMPA-receptor ligands was observed in cerebral cortical and hippocampal structures as well as in the hypothalamus and cerebellum. The finding of a selective loss of AMPA sites at coma stages of encephalopathy in this model of acute liver failure is consistent with previous electrophysiological reports of inhibition of AMPA-mediated neuronal depolarization resulting from exposure of hippocampal neurons to millimolar concentrations of ammonia. On the other hand, the present study showed that binding sites for the NMDA-receptor ligand [3H]MK801 at coma stages of encephalopathy in acute liver failure were within normal limits in all brain structures examined. NMDA sites are uniquely neuronal, whereas kainate and AMPA sites are localized on both neurons and astrocytes. Therefore, the selective loss of non-NMDA sites in acute liver failure may also reflect astrocytic changes in this condition. Because astrocytic glutamate receptors are implicated in K+ and neurotransmitter reuptake, alterations in their density could result in altered neuronal excitability and thus be responsible for the neurological dysfunction characteristic of hepatic encephalopathy in acute liver failure.

Portacaval shunting and hyperammonemia stimulate the uptake of L-[3H] arginine but not of L-[3H]nitroarginine into rat brain synaptosomes

Elevated activities of nitric oxide synthase (NOS) have been reported previously in the brains of portacaval-shunted (PCS) rats, a model of chronic hepatic encephalopathy (HE). As L-arginine availability for nitric oxide synthesis depends on a specific uptake mechanism in neurons, we studied the kinetics of L-[3H]-arginine uptake into synaptosomes prepared from the brains of PCS rats. Results demonstrate that L-arginine uptake is significantly increased in cerebellum (60%; p < 0.01), cerebral cortex (42%; p < 0.01), hippocampus (56%; p < 0.01), and striatum (51%; p < 0.01) of PCS rats compared with sham-operated controls. Hyperammonemia in the absence of portacaval shunting also stimulated the transport of L-[3H]arginine; kinetic analysis revealed that the elevated uptake was due to increased uptake capacity (Vmax) without any change in affinity (Km). Incubation of cerebellar synaptosomes with ammonium acetate for 10 min caused a dose-dependent stimulation of L-[3H]arginine uptake. Neither portacaval shunting nor hyperammonemia had any significant effect on the synaptosomal uptake of NG-nitro-L-[3H]arginine. These studies demonstrate that increased NOS activity observed in experimental HE may result from increased availability of L-arginine resulting from a direct stimulatory effect of ammonia on L-arginine transport.

Vesicular dysfunction during experimental thiamine deficiency is indicated by alterations in dopamine metabolism

Experimental and clinical studies indicate that catecholamines play an important role in the neurobehavioural symptomatology of thiamine deficiency. Given the cerebral region-selective vulnerability and the behavioural impairment commonly encountered in thiamine deficiency, we undertook to investigate regional catecholamine metabolism in the brains of pyrithiamine-induced thiamine-deficient rats. Dopamine metabolism was unaffected in the striatum. In contrast, other regions also known to be involved in sensory processing and intellectual function (e.g., frontal cortex, hypothalamus, thalamus), but having a greater noradrenergic input, had increased levels of 3,4-dihydroxyphenylacetic acid (DOPAC) and decreased levels of other dopaminergic metabolites including noradrenaline. In these regions levels of the vesicular amine transporter, defined by tetrabenazine-sensitive [3H]ketanserin binding, were also decreased. Our data suggest a region-selective vesicular dysfunction resulting in intraneuronal release, and subsequent degradation, of dopamine. These disruptions of dopamine and consequently noradrenaline metabolism may account for certain neurobehavioural deficits commonly encountered in thiamine deficiency.

Ammonium acetate challenge in experimental chronic hepatic encephalopathy induces a transient increase of brain 5-HT release in vivo

Ammonia has been shown to cause release of neurotransmitters such as serotonin (5-hydroxytryptamine; 5-HT) from synaptosomal preparations in vitro. In the present study, frontal neocortical extracellular levels of 5-HT and its major metabolite, 5-hydroxyindole-3-acetic acid (5-HIAA), were determined in vivo by the use of microdialysis in portacaval shunted (PCS) rats, an experimental model of chronic hepatic encephalopathy (HE), prior to and after an acute coma-inducing administration of ammonium acetate (NH4Ac; 5.2 mmol/kg, i.p.). PCS rats displayed elevated (P < 0.01) 5-HIAA but unaltered 5-HT extracellular levels compared with controls, supporting the contention of an increased neocortical 5-HT metabolism but unaltered neuronal 5-HT output in chronic HE. However, a transient elevation of extracellular 5-HT levels was observed when PCS-NH4Ac rats were in coma. Increased brain ammonia may thus augment neuronal 5-HT release in chronic HE, which in turn could be a causative for precipitation of more severe stages of HE.

Increased blood manganese in cirrhotic patients: relationship to pallidal magnetic resonance signal hyperintensity and neurological symptoms

Increasing evidence suggests that manganese deposition is responsible for the T1-weighted magnetic resonance imaging (MRI) signal hyperintensity consistently observed in pallidum of cirrhotic patients. However, the relationship between blood manganese and the etiology or severity of liver disease, as well as the neurological symptomatology in these patients, has not been well established. In the present study, blood manganese concentrations were measured by atomic absorption spectrometry together with MRI and neurological evaluation in 57 cirrhotic patients with various etiologies and severity of liver disease. Blood manganese concentrations were elevated in 67% of cirrhotic patients and were significantly higher in patients with previous portacaval anastomoses or transjugular intrahepatic portosystemic shunt (TIPS). Pallidal signal hyperintensity was observed in 88% of patients, and significant correlations were demonstrated between blood manganese and pallidal index (PI) (a measure of pallidal signal hyperintensity), as well as Child-Pugh score. Assessment of extrapyramidal symptoms using the Columbia rating scale revealed a significant incidence of tremor, rigidity, or akinesia in up to 89% of cirrhotic patients. However, there was no significant correlation between blood manganese and extrapyramidal symptoms, although severity of akinesia was significantly greater in Child-Pugh C patients. Extrapyramidal symptoms could result from a toxic effect of manganese on basal ganglia dopaminergic function. These findings further support a role for manganese in the etiology of pallidal MRI signal hyperintensity in patients with chronic liver disease.

Portacaval anastomosis induces region-selective alterations of the endogenous opioid system in the rat brain

Portacaval anastomosis (PCA) in the rat results in a broad spectrum of neurological and neurobehavioral changes, including alterations of circadian rhythms, impaired locomotor activity, and reflexes, as well as decreased threshold to noxious stimuli. In addition, following portacaval shunting, rats drink significantly more ethanol in a free-choice drinking paradigm. Available evidence suggests that many of these behavioral changes may be modulated by the endogenous opioid system of the brain. To evaluate this possibility, the effects of PCA on circulating beta-endorphin (beta-EP), as well as beta-EP content in the pituitary and specific brain nuclei, was evaluated using a sensitive radioimmunoassay. Furthermore, the characteristics and regional densities of mu and delta opioid receptors in the brains of PCA and sham-operated control rats were studied using an in vitro technique, as well as quantitative receptor autoradiography and the specific ligands 125I [D-Ala2, MePhe4, Met(o)ol5]enkephalin (FK 33-824) and 125I [2-D-penicillamine, 5-D-penicillamine]-enkephalin (DPDPE) for micro and delta sites, respectively. PCA resulted in region-selective modifications of beta-EP in brain, but not in pituitary or blood. Autoradiographic studies revealed a generalized decrease in mu binding sites (up to 70% decreases compared with sham-operated controls) and region-selective alterations of delta receptor densities following PCA. Portacaval-shunted rats drank significantly more ethanol in a free-choice drinking paradigm, an effect that was significantly attenuated by the administration of the opiate antagonist naloxone. Increased ethanol preference thus appeared to result from modifications of the endogenous opioid system in nucleus accumbens of rats following PCA.

Portacaval anastomosis results in altered neuron--astrocytic metabolic trafficking of amino acids: evidence from 13C-NMR studies

13C-NMR spectroscopy was used to evaluate the dynamic consequences of portacaval anastomosis on neuronal and astrocytic metabolism and metabolic trafficking between neurons and astrocytes. Glutamate is predominantly labeled from [1-13C]glucose, whereas [2-13C]acetate is more efficient in labeling glutamine, in accordance with its primary metabolism in astrocytes. Alanine and succinate labeling was only observed with [1-13C]glucose as precursor. Brain [1-13C]glucose metabolism in portacaval-shunted rats was similar to that in sham-operated controls with the exception of labeled glutamine and succinate formation, which was increased in shunted rats. The 13C enrichment was, however, decreased owing to an increase in total glutamine and succinate. Using [2-13C]acetate, on the other hand, flux of astrocytic label to neurons was severely decreased because label incorporation into glutamate, aspartate, and GABA was decreased following portacaval shunting. The latter amino acids are predominantly localized in neurons. These findings demonstrate that metabolic trafficking of amino acids from astrocytes to neurons is impaired in portacaval-shunted rats.

Brain extracellular quinolinic acid in chronic experimental hepatic encephalopathy as assessed by in vivo microdialysis: acute effects of L-tryptophan

Increased brain quinolinic acid (QUIN) levels have been suggested to play a role in hepatic encephalopathy (HE). Previous brain tissue studies have been unable to confirm this hypothesis. Because QUIn is a potent NMDA-receptor agonist, it also is relevant to determine brain extracellular QUIN levels in HE. For this purpose, we assessed frontal neocortical extracellular QUIN levels by in vivo microdialysis in rats subjected to a portacaval shunt (PCS). We also evaluated the acute effects of altered L-tryptophan (L-TRP) availability on brain extracellular QUIN levels. The basal extracellular L-TRP levels were significantly (p < .001) higher in the PCS rats than in the sham-operated controls. However, the QUIN level (p < .05) and the QUIN to L-TRP ratio (p < .01) were significantly lower in the PCS rats. Elevated L-TRP availability increased the QUIN levels to a similar degree in both sham and PCS rats. This study, in conjunction with our previous results, does thereby not support a major involvement of QUIN in the pathogenesis of HE.

Neuroactive amino acids and glutamate (NMDA) receptors in frontal cortex of rats with experimental acute liver failure

It has been proposed that alterations of excitatory and inhibitory amino acids play a role in the pathogenesis of hepatic encephalopathy in acute liver failure. To evaluate this possibility, in vivo cerebral microdialysis was used to sample extracellular concentrations of amino acids in the frontal cortex of unanesthetized rats at various times during the progression of encephalopathy resulting from acute liver failure. Liver failure was induced by portacaval anastomosis followed 24 hours later by hepatic artery ligation. Dialysate concentrations of amino acids were measured by high-performance liquid chromatography (HPLC) with fluorescence detection. Deterioration of neurological status was accompanied by two- to four-fold increases in extracellular glutamate, glutamine, and glycine; concentrations of gamma-aminobutyric acid (GABA) and taurine were unchanged. Densities of binding sites for the glutamate (N-methyl-D-aspartate [NMDA]) receptor ligand 3H-MK801, assessed using quantitative receptor autoradiography, however, were unchanged in the frontal cortex of rats at coma stages of ischemic liver failure. Increased extracellular glutamate concentrations were positively correlated with the severity of encephalopathy and with arterial ammonia concentrations. Such changes may result from an ammonia-induced reduction in the capacity for astrocytes to uptake glutamate. Increased extracellular glutamate in brain, together with increases in concentrations of glycine, a positive allosteric modulator of glutamate (NMDA) receptors, are consistent with increased NMDA-related glutamatergic neurotransmission in this model of acute liver failure. Increased extracellular glutamate, therefore, could contribute to the pathogenesis of hepatic encephalopathy and brain edema in acute liver failure.

Arginine-related guanidino compounds and nitric oxide synthase in the brain of ornithine transcarbamylase deficient spf mutant mouse: effect of metabolic arginine deficiency

The sparse-fur (spf) mouse, with an X-linked hepatic ornithine transcarbamylase (OTC, E.C.2.1.3.3) deficiency, exhibits significantly lower levels of arginine in the brain as compared to normal controls. In the present study, the effect of a sustained lower metabolic arginine was studied by measuring the levels of several arginine-related guanidino compounds in brain. The concentrations of gamma-guanidinobutyric acid (gamma-GBA), N-alpha-acetylarginine (N-alpha-AA), argininic acid (Arg-A), guanidinoacetic acid (GAA), and creatine were significantly lower in spf mice as compared to controls. Since arginine is the precursor for nitric oxide, we also measured the activity of nitric oxide synthase which was significantly reduced in cerebellum, striatum, hippocampus and cerebral cortex of spf mice. The changes seen in cerebral guanidino compound and nitric oxide metabolism of spf mice could be due to a sustained deficiency of arginine, caused by a metabolic block in the area cycle.

Alterations in serotonin parameters in brain of thiamine-deficient rats are evident prior to the appearance of neurological symptoms

Biochemical alterations of serotoninergic parameters have been demonstrated in experimental thiamine deficiency. In addition, hypophagia and hypothermia, two physiological processes associated with changes in the serotonin [5-hydroxytryptamine (5-HT)] system, are manifest early during the progression of thiamine deficiency. The binding of selected 5-HT radioligands was therefore investigated in discrete brain regions of pyrithiamine-induced thiamine-deficient rats. Using quantitative receptor autoradiography, the binding of 8-hydroxy-2-(di-n-[3H]propylamino) tetralin, a ligand used to label the somatodendritic 5-HT1A autoreceptor of the dorsal raphe nucleus, was found to be unaffected in this region, suggesting that the structural integrity of the 5-HT cell bodies is maintained throughout the course of pyrithiamine treatment. Increased binding of [3H]-ketanserin was observed in regions considered vulnerable as well as in some considered to be nonvulnerable during the course of thiamine deficiency. These binding changes, which appear to represent changes in the density of the postsynaptic 5-HT2A receptor population rather than the "tetrabenazine-sensitive" vesicular monoamine transporter, are evident before the appearance of histopathologic lesions and coincide with altered tissue concentrations of 5-HT. These data suggest that 5-HT neurons, although structurally intact, are functionally affected early during the progression of thiamine deficiency. These alterations, which are likely a part of adaptive neuronal change consequent to thiamine dysfunction, may be important in the physiological manifestations and the learning deficits commonly encountered in experimental thiamine deficiency.

L-[3H]nitroarginine and L-[3H]arginine uptake into rat cerebellar synaptosomes: kinetics and pharmacology

Characteristics of the transport of the nitric oxide synthase substrate L-arginine and its inhibitor, NG-nitro-L-arginine (L-NOARG), into rat cerebellar synaptosomes were studied. Uptake of both L-arginine and L-NOARG was linear with increasing amount of protein (up to 40 micrograms) and time of incubation (up to 5 min) at 37 degrees C. Uptake of both compounds reached a steady state by 20 min. Maximal uptake of L-NOARG (650 pmol/mg of protein) was three to four times higher than that of L-arginine (170 pmol/mg of protein). L-NOARG uptake showed biphasic kinetics (Km1 = 0.72 mM, Vmax 1 = 0.98 nmol/min/mg of protein; Km2 = 2.57 mM, Vmax2 = 16.25 nmol/min/mg of protein). L-Arginine uptake was monophasic with a Km of 106 microM and a Vmax of 0.33 nmol/min/mg of protein. L-NOARG uptake was selectively inhibited by L-NOARG, NG-nitro-L-arginine methyl ester, and branched-chain and aromatic amino acids. L-Alanine and L-serine also inhibited L-NOARG uptake but with less potency. Uptake of L-arginine was selectively inhibited by NG-monomethyl-L-arginine acetate and basic amino acids. These studies suggest that in rat cerebellar synaptosomes, L-NOARG is transported by the neutral amino acid carrier systems T and L with high affinity, whereas L-arginine is transported by the basic amino acid carrier system y+ with high affinity. These data indicate that the concentration of competing amino acids is an important factor in determining the rates of uptake of L-NOARG and L-arginine into synaptosomes and, in this way, may control the activity of nitric oxide synthase.

Central muscarinic cholinergic M1 and M2 receptor changes in congenital ornithine transcarbamylase deficiency

Congenital ornithine transcarbamylase (OTC) deficiency results in neuropathologic damage to the cerebral cortex, basal ganglia, and thalamus. However, the precise nature of the cell loss, as well as the pathophysiologic mechanisms responsible for it, have not been fully elucidated. In the present study, densities of the M1 and M2 subclasses of muscarinic cholinergic binding sites were assessed using quantitative receptor autoradiography in the brains of sparse-fur (spf) mice with congenital OTC deficiency and in age-matched CD-1 controls. Densities of binding sites for the muscarinic M1 subtype ligand [3H]pirenzepine were reduced by 24-54% (p < 0.01) in frontal cortex, caudate/ putamen, and hippocampal CA1 and CA2 areas. Since muscarinic M1 sites are localized presynaptically, their selective loss, together with a previous report of reduced activities of the presynaptic cholinergic enzyme choline acetyltransferase, confirms that loss of cholinergic neurons is an important feature of congenital OTC deficiency. Densities of binding sites for the predominantly postsynaptic muscarinic M2 subtype ligand 3H-AFDX 384 were increased by up to 60% (p < 0.01) in cerebral cortex, hippocampus, globus pallidus, as well as thalamic and hypothalamic structures of OTC-deficient mice. Increased M2 sites in the cerebral cortex, hippocampus, and thalamus are most likely the result of up-regulation of these sites after the loss of the presynaptic neuron. These findings support the presence of a central muscarinic cholinergic lesion in congenital OTC deficiency.

Neuroactive amino acids in hepatic encephalopathy

There is abundant evidence to suggest that alterations of excitatory and inhibitory amino acids play a significant role in the pathogenesis of hepatic encephalopathy (HE) in both acute and chronic liver diseases. Brain glutamate concentrations are reduced in patients who died in hepatic coma as well as in experimental HE, astrocytic reuptake of glutamate is compromised in liver failure and postsynaptic glutamate receptors (both NMDA and non-NMDA subclasses) are concomitantly reduced in density. Recent studies in experimental acute liver failure suggest reduced capacity of the astrocytic glutamate transporter in this condition. Together, this data suggests that neuron-astrocytic trafficking of glutamate is impared in HE. Other significant alterations of neuroactive amino acids in HE include a loss of taurine from brain cells to extracellular space, a phenomenon which could relate both to HE and to brain edema in acute liver failure. Increased concentrations of benzodiazepine-like compounds have been reported in human and experimental HE. Clinical trials with the benzodiazepine antagonist flumazenil reveal a beneficial effect in some patients with HE; the mechanism responsible for this effect, however, remains to be determined.

Quantitative autoradiography using selective radioligands for central and peripheral-type benzodiazepine receptors in experimental Wernicke's encephalopathy: implications for positron emission tomography imaging

Wernicke's encephalopathy (WE) is difficult to diagnose during life, with up to 80% of cases being missed by routine neurological evaluation in both alcoholics and AIDS patients. Therefore, there is a need for noninvasive diagnostic procedures. Using the pyrithiamine-treated rat, an animal model of WE, we have studied, using radioligands for central (neuronal) and "peripheral-type" (glial) benzodiazepine receptors, the regional distribution of changes in the densities of these receptors in relation to the degree of reactive gliosis accompanying neuronal loss. Histological studies revealed neuronal loss in selective regions, including the thalamus, inferior colliculus, inferior olivary nucleus, and mammillary body. Autoradiographic studies demonstrated increases in densities of [3H]PK11195 binding sites that closely paralleled the topographic distribution of neuronal cell loss and reactive gliosis. In contrast, [3H]Ro15-178 showed poor spatial correlation, with the neuronal loss seen in pyrithiamine-induced thiamine deficiency. The positron emission tomography ligand [11C]PK11195 may be useful for the assessment of thiamine deficiency-induced brain damage in human alcoholics.

Nitric oxide synthase activities are selectively decreased in vulnerable brain regions in thiamine deficiency

Pyrithiamine-induced thiamine deficiency in the rat exhibits many neuropathological and biochemical similarities to Wernicke's Encephalopathy in human. Activities of constitutive nitric oxide synthase (NOS) were measured in vulnerable (thalamus and cerebellum) and non-vulnerable (hippocampus and striatum) brain regions of pyrithiamine-induced thiamine-deficient rats. NOS activities were significantly decreased in the thalamus (by 26%, P < 0.05) of presymptomatic thiamine-deficient rats compared to pair-fed controls. Following onset of symptoms, in addition to thalamus (-38%, P < 0.01), cerebellum (-50%, P < 0.01) also manifested significantly decreased activities of NOS. Hippocampal and striatal activities of NOS were unchanged at both presymptomatic and symptomatic stages of thiamine deficiency. Selectively decreased activities of neuronal NOS in the thalamus and the cerebellum extends the previous observations of region-selective metabolic changes and, ultimately, neuronal cell loss observed in thiamine deficiency.

Na+,K(+)-ATPase activity is selectively increased in thalamus in thiamine deficiency prior to the appearance of neurological symptoms

The relationship between progression of neurological status and the activities of both Na+,K(+)- and Mg(2+)-dependent-ATPase (adenosine 5'-triphosphate phosphohydrolase) was investigated in brain regions of pyrithiamine-induced thiamine deficient rats. Thalamic Na+,K(+)-ATPase activity was selectively increased by 200% (P < 0.01) prior to the appearance of symptoms of thiamine deficiency and normalized in symptomatic rats. This selective transitory activation precludes a mediation by brain soluble fraction Na+,K(+)-ATPase modifiers as does the unaltered distribution in regional high-affinity [3H]ouabain binding densities observed throughout the time-course used in these experiments. Na+,K(+)-ATPase maintains cellular ionic gradients and has been implicated in neurotransmitter uptake and release mechanisms. The fact that the increased thalamic Na+,K(+)-ATPase activity coincides with the early alterations in serotonin metabolism observed in similarly treated animals and the concomitantly early increase in glucose utilization previously observed in the thalamus of thiamine-deficient rats is discussed.

Regional distribution of binding sites for the nitric oxide synthase inhibitor L-[3H]nitroarginine in rat brain

The regional distribution of NG-nitro-L-[(3)H]arginine (L-[(3)H]NOARG) binding to different regions of rat brain was studied by quantitative autoradiography. These studies revealed highest density of binding sites in cerebellum, anterior olfactory nucleus, islands of Calleja and substantia nigra with appreciable binding site densities in inferior colliculus, superior colliculus, olfactory tubercle and dorsal tegmental nucleus. The regional distribution of L-[(3)H]NOARG binding, is in good agreement with the distribution of nitric oxide synthase studied previously by NADPH-diaphorase staining and immunohistochemistry using antibodies against neuronal nitric oxide synthase. The kinetics of L-[(3)H]NOARG binding to the cytosolic preparations of cerebral cortex, cerebellum, hippocampus and striatum was studied using an in vitro binding technique. Specific L-[(3)H]NOARG binding was of nanomolar affinity, saturable, and best fit to a single-site model in all four brain regions. These studies support the potential use of L-[(3)H]NOARG binding as a tool for further elucidation of the regional distribution and functional properties of NOS in the central nervous system.

Neuronal cell death in Wernicke's encephalopathy: pathophysiologic mechanisms and implications for PET imaging

Thiamine deficiency in humans is associated with Wernicke's encephalopathy (WE) which is characterized neuropathologically by neuronal loss in selective brain regions. Pyrithiamine-induced thiamine-deficiency in the rat results in lesions which are similar in nature and distribution to those seen in human WE. Several mechanisms have been implicated in the pathogenesis of neuronal loss in thiamine deficiency including, (i) impaired cerebral energy metabolism, (ii) focal lactic acidosis, (iii) NMDA-receptor mediated excitotoxicity and (iv) blood-brain barrier breakdown. WE is difficult to diagnose during life and a large number of cases are missed by routine clinical neurological evaluation. Recently, non-invasive diagnostic procedures such as CT and MRI have been used for the evaluation of acute and chronic WE. Autoradiographic studies reveal that increased densities of binding sites for the astrocytic ligand 3H-PK11195 closely parallel the topographic distribution of reactive gliosis and neuronal loss in selective brain regions of pyrithiamine-induced thiamine-deficient rats. In contrast, binding sites for the neuronal ligand 3H-Ro15-1788 show poor regional correlation with neuronal loss in thiamine deficiency. Both of these ligands are available, and have been used in PET assessment of various disorders in humans. The results of autoradiographic studies suggest that 11C-PK11195 may offer a useful PET ligand for the assessment of brain damage in WE in humans.

Alterations of thiamine phosphorylation and of thiamine-dependent enzymes in Alzheimer's disease

There is a growing body of evidence to suggest that thiamine neurochemistry is disrupted in Alzheimer's Disease (AD). Studies in autopsied brain tissue from neuropathologically proven AD patients reveal significantly reduced activities of the thiamine phosphate dephosphorylating enzymes thiamine diphosphatase (TDPase) and thiamine monophosphatase (TMPase) as well as the thiamine diphosphate-dependent enzymes, pyruvate dehydrogenase complex, alpha-ketoglutarate dehydrogenase (alpha KGDH) and transketolase. Reductions in enzyme activities are present both in affected areas of AD brain as well as in relatively well conserved tissue. Decreased TDP concentrations and concomitantly increased TMP in autopsied brain tissue from AD patients and in CSF from patients with Dementia of the Alzheimer Type suggests that CNS thiamine phosphorylation-dephosphorylation mechanisms are disrupted in AD. alpha KGDH is a rate-limiting enzyme for cerebral glucose utilization and decreases in its activity are associated with lactic acidosis, cerebral energy failure and neuronal cell loss. Deficiencies of TDP-related metabolic processes could therefore participate in neuronal cell death mechanisms in AD.

Kinetics, pharmacology, and autoradiographic distribution of L-[3H]nitroarginine binding sites in rat cerebellum

The kinetics and pharmacology of NG-nitro-L-[2,3,4,5-3H]arginine (L-[3H]NOARG) binding to rat cerebellum were investigated using in vitro radioligand binding. Specific L-[3H]NOARG binding in cerebellum was of nanomolar affinity, reversible, saturable, and best fit to a single-site model. Specific binding was Ca2+ dependent and sensitive to pH (with an optimum of 5.5-7.0). Added calmodulin (1.5-40 micrograms/ml) had no influence on specific L-[3H]NOARG binding. However, the calmodulin antagonists W-5, W-13, and calmidazolium inhibited L-[3H]-NOARG binding with IC50 values in the micromolar range, and calmodulin (10 micrograms/ml) competitively reversed this inhibition. Nitric oxide synthase (NOS) inhibitors (NG-nitro-L-arginine methyl ester and NG-monomethyl-L-arginine acetate) and L-arginine displaced L-[3H]NOARG binding with IC50 values in the nanomolar range, whereas D-arginine and basic amino acids (L-lysine and L-histidine) displaced L-[3H]NOARG binding with IC50 values in the millimolar range. A comparison of the NOS functional assay with L-[3H]NOARG binding in rat cerebellum showed similar profiles of Ca2+ dependency and inhibitory kinetics. Quantitative autoradiographic distribution of L-[3H]NOARG binding sites was significantly higher in the molecular layer than in the granular layer of cerebellum. These studies confirm the potential use of L-[3H]NOARG binding to study the regional distribution and functional properties of NOS.

Taurine in hepatic encephalopathy

Liver and brain are the major organs responsible for taurine synthesis. In both acute and chronic liver failure, brain taurine concentrations are decreased and, since taurine appears to be implicated in K+ and Ca2+ homeostasis in brain, such losses could contribute to the pathophysiology of hepatic encephalopathy. Furthermore, taurine concentrations in cerebrospinal fluid in experimental acute liver failure are increased early in the progression of encephalopathy and prior to the onset of cerebral edema, a potentially fatal complication of acute liver failure. These findings suggest an osmoregulatory role for taurine in brain in acute liver failure. Monitoring of cerebrospinal fluid taurine may be of prognostic value in this severe, frequently fatal disorder.