Differential changes in the content of amino acid neurotransmitters in discrete regions of the rat brain prior to the onset and during the course of homocysteine-induced seizures

Methyl Vitamin B12 but not methylfolate rescues a motor neuron-like cell line from homocysteine-mediated cell death

Homocysteine is an excitatory amino acid implicated in multiple diseases including amyotrophic lateral sclerosis (ALS). Information on the toxicity of homocysteine in motor neurons is limited and few studies have examined how this toxicity can be modulated. In NSC-34D cells (a hybrid cell line derived from motor neuron-neuroblastoma), homocysteine induces apoptotic cell death in the millimolar range with a TC₅₀ (toxic concentration at which 50% of maximal cell death is achieved) of 2.2 mM, confirmed by activation of caspase 3/7. Induction of apoptosis was independent of short-term reactive oxygen species (ROS) generation. Methyl Vitamin B12 (MeCbl) and methyl tetrahydrofolate (MTHF), used clinically to treat elevated homocysteine levels, were tested for their ability to reverse homocysteine-mediated motor neuron cell death. MeCbl in the micromolar range was able to provide neuroprotection (2 h pretreatment prior to homocysteine) and neurorescue (simultaneous exposure with homocysteine) against millimolar homocysteine with an IC₅₀ (concentration at which 50% of maximal cell death is inhibited) of 0.6 μM and 0.4 μM, respectively. In contrast, MTHF (up to 10 μM) had no effect on homocysteine-mediated cell death. MeCbl inhibited caspase 3/7 activation by homocysteine in a time- and dose-dependent manner, whereas MTHF had no effect. We conclude that MeCbl is effective against homocysteine-induced cell death in motor neurons in a ROS-independent manner, via a reduction in caspase activation and apoptosis. MeCbl decreases Hcy induced motor neuron death in vitro in a hybrid cell line derived from motor neuron-neuroblastoma and may play a role in the treatment of late stage ALS where HCy levels are increased in animal models of ALS.

Substantia nigra osmoregulation: taurine and ATP involvement

An extracellular nonsynaptic taurine pool of glial origin was recently reported in the substantia nigra (SN). There is previous evidence showing taurine as an inhibitory neurotransmitter in the SN, but the physiological role of this nonsynaptic pool of taurine has not been explored. By using microdialysis methods, we studied the action of local osmolarity on the nonsynaptic taurine pool in the SN of the rat. Hypoosmolar pulses (285-80 mosM) administered in the SN by the microdialysis probe increased extrasynaptic taurine in a dose-dependent way, a response that was counteracted by compensating osmolarity with choline. The opposite effect (taurine decrease) was observed when osmolarity was increased. Under basal conditions, the blockade of either the AMPA-kainate glutamate receptors with 6-cyano-7-nitroquinoxaline-2,3-dionine disodium or the purinergic receptors with pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid modified the taurine concentration, suggesting that both receptors modulate the extrasynaptic pool of taurine. In addition, these drugs decreased the taurine response to hypoosmolar pulses, suggesting roles for glutamatergic and purinergic receptors in the taurine response to osmolarity. The participation of purinergic receptors was also supported by the fact that ATP (which, under basal conditions, increased the extrasynaptic taurine in a dose-dependent way) administered in doses saturating purinergic receptors also decreased the taurine response to hypoosmolarity. Taken together, present data suggest osmoregulation as a role of the nonsynaptic taurine pool of the SN, a function that also involves glutamate and ATP and that could influence the nigral cell vulnerability in Parkinson's disease.

The effect of in vitro homocystinuria on the suckling rat hippocampal acetylcholinesterase

Homocystinuria is due to enzymatic deficiencies resulting in elevated blood levels of homocysteine (Hcy), homocystine (Hci), and/or methionine (Met) and the clinical presentation of mental retardation, seizures, and cardiovascular disease. Since these symptoms may be closely implicated with acetylcholinesterase (AChE) activity, we aimed to investigate whether this metabolic disorder affects the hippocampal AChE activity in 21 days suckling Wistar rat hippocampus. Various concentrations of Hcy, Hci (0.05-0.5 mM), or Met (0.05-2 mM) as well as Mixture A (Mix A) (0.3 mM (Hcy)+0.2 mM (Hci)+1.0 mM (Met) = in vitro cystathionine beta-synthase deficiency homocystinuria), Mix B1 (Hcy 0.3 mM + Hci 0.2 mM=in vitro severe methylenetetrahydrofolate reductase deficiency homocystinuria) or Mix B2 (Hcy 0.1 mM+Hci 0.05 mM=in vitro mild methylenetetrahydrofolate reductase deficiency homocystinuria) were preincubated with homogenized hippocampii or with eel Electrophorus electricus pure AChE. AChE was evaluated spectrophotometrically. Hcy or Met stimulated hippocampal AChE by 50% (p < 0.001) at low concentrations of the amino acids (up to 0.3-0.5 mM), whereas Hci inhibited the enzyme by 40% (p < 0.001). Mix A, Mix B1, or Mix B2 activated hippocampal AChE by 40, 30, (p < 0.001), and 12% (p < 0.01), respectively. In contrast, the S-containing amino acids, Mix A, Mix B1, Mix B2 failed to affect the pure AChE activity.

CONCLUSIONS

a) The presence of -SH group in Hcy and Met may result in hippocampal AChE stimulation and the redox isomer Hci in the inhibition of the enzyme, probably by producing free radicals, and b) The SH-amino acids seem to affect the hippocampal enzyme indirectly, possibly by lipid(s)-protein modifications(s) and Hci by inducing oxidative stress, since no effect was observed on pure AChE activity.

Reduction of butyrylcholinesterase activity in rat serum subjected to hyperhomocysteinemia

In the present study we investigate the effect of homocysteine (Hcy) administration, the main metabolite accumulating in homocystinuria, on butyrylcholinesterase (BuChE) activity in serum of rats. For the acute treatment, 29-day-old Wistar rats received one subcutaneous injection of Hcy (0.6 micromol/g) or saline (control) and were killed 1 h later. For the chronic treatment, Hcy was administered subcutaneously to rats from the 6th to the 28th day of life. Control rats received saline. The rats were killed 12 h after the last injection. In another set of experiments, rats were pretreated for one week with vitamins E and C or saline and 12 h after the last injection received one single injection of Hcy or saline, being killed 1 h later. Serum was used to determine BuChE activity. Our results showed that acute and chronic administration of Hcy significantly decreased BuChE activity. Furthermore, vitamins E and C per se did not alter BuChE activity, but prevented the reduction of this enzyme activity caused by acute administration of Hcy. The data suggest that the inhibitory effect of Hcy on BuChE activity is probably mediated by free radicals, since vitamins E and C administration prevented such effect.

Chronic hyperhomocysteinemia provokes a memory deficit in rats in the Morris water maze task

Homocystinuria is an inherited metabolic disease biochemically characterized by tissue accumulation of homocysteine. Affected patients present mental retardation and other neurological symptoms whose mechanisms are still obscure. In the present study, we investigated the effect of chronic hyperhomocysteinemia on rat performance in the Morris water maze task. Chronic treatment was administered from the 6th to the 28th day of life by s.c. injection of homocysteine, twice a day at 8-h intervals; control rats received the same volume of saline solution. Animals were left to recover until the 60th day of life. Morris water maze tasks were then performed, in order to verify any effect of early homocysteine administration on reference and working memory of rats. Results showed that chronic treatment with homocysteine impaired memory of the platform location and that homocysteine treated animals presented fewer crossings to the place where the platform was located in training trials when compared to saline-treated animals (controls). In the working memory task, homocysteine treated animals also needed more time to find the platform. Our findings suggest that chronic experimental hyperhomocysteinemia causes cognitive dysfunction and that might be related to the neurological complications characteristic of homocystinuric patients.

Inhibitory and excitatory neurotransmitters in the cerebrospinal fluid of epileptic dogs

OBJECTIVE

To determine concentrations of excitatory and inhibitory amino acids in CSF of a large number of dogs with idiopathic epilepsy or genetic epilepsy and to evaluate changes in CSF amino acid concentration with regard to drug treatment and sex.

ANIMALS

35 Labrador Retrievers with genetic epilepsy (20 male and 15 female), 94 non-Labrador Retrievers with idiopathic epilepsy (71 male and 23 female), and 20 control dogs (10 male and 10 female).

PROCEDURE

Collection of CSF was performed > 72 hours after the occurrence of seizures. Cerebrospinal fluid concentrations of gamma-aminobutyric acid (GABA), glutamate (GLU), aspartate (ASP), serine, and glycine were determined by use of high performance liquid chromatography with electrochemical detection.

RESULTS

CSF concentrations of GABA and GLU were significantly lower in Labrador Retrievers with genetic epilepsy (LR-group dogs) than in control-group dogs or in non-Labrador Retrievers with idiopathic epilepsy (non-LR-group dogs). The GLU-to-GABA ratio was significantly higher in LR-group dogs than in non-LR-group dogs. CSF concentrations of GLU and ASP were significantly lower when all dogs with epilepsy (non-LR- and LR-group dogs combined) were compared with control-group dogs.

CONCLUSIONS AND CLINICAL RELEVANCE

A decrease in CSF concentrations of GABA appears to play a role in the pathogenesis of genetically determined epilepsy in Labrador Retrievers. However, this decrease in CSF concentrations of GABA may also be a consequence of seizure activity. The GLU-to-GABA ratio may prove to be a useful indicator of genetic epilepsy in Labrador Retrievers.

Extracellular taurine in the substantia nigra: Taurine-glutamate interaction

Taurine has been proposed as an inhibitory transmitter in the substantia nigra (SN), but the mechanisms involved in its release and uptake remain practically unexplored. We studied the extracellular pool of taurine in the rat's SN by using microdialysis methods, paying particular attention to the taurine-glutamate (GLU) interaction. Extracellular taurine increased after cell depolarization with high-K(+) in a Ca(2+)-dependent manner, being modified by the local perfusion of GLU, GLU receptor agonists, and zinc. Nigral administration of taurine increased the extracellular concentration of gamma-aminobutyric acid (GABA) and GLU, the transmitters of the two main inputs of the SN. The modification of the glial metabolism with fluocitrate and L-methionine sulfoximine also changed the extracellular concentration of taurine. The complex regulation of the extracellular pool of taurine, its interaction with GABA and GLU, and the involvement of glial cells in its regulation suggest a volume transmission role for taurine in the SN.

Homocysteine inhibits butyrylcholinesterase activity in rat serum

In the present work we investigated the in vitro effects of homocysteine (Hcy) and methionine (Met), metabolites accumulated in homocystinuria, on butyrylcholinesterase (BuChE) activity in rat serum. We also studied the kinetics of the inhibition of BuChE activity caused by Hcy. For determination of BuChE we used serum of 60-day-old Wistar rats, which was incubated in the absence (control) or presence of Hcy (0.01-0.5 mM) or Met (0.2-2.0 mM). The kinetics of the interaction of Hcy and BuChE was determined using the Lineweaver-Burk double reciprocal plot. Results showed that serum BuChE activity was not altered by Met, but it was significantly inhibited (37%) by 500 microM Hcy, a concentration similar to those found in blood of homocystinuric patients. The apparent Km values, in the absence and presence of 500 microM of Hcy, were 0.034 and 0.142 mM, respectively, and V(max) of BuChE for acetylcholine (ACh) as substrate was 1.25 micromol ACSCh/h/mg of protein. The Ki value obtained was 120 microM, and the inhibition was of the competitive type, suggesting a common binding site for Hcy and ACh. It is proposed that inhibition of cholinesterase activity may be one of the mechanisms involved in the neurological dysfunction observed in homocystinuria.

Inhibition of Na(+),K(+)-ATPase activity in hippocampus of rats subjected to acute administration of homocysteine is prevented by vitamins E and C treatment

In the present study we evaluated the effect of acute homocysteine (Hcy) administration on Na(+),K(+)-ATPase activity, as well as on some parameters of oxidative stress such as total radical-trapping antioxidant potential (TRAP) and on activities of antioxidant enzymes catalase (CAT), superoxide dismutase and glutathione peroxidase in rat hippocampus. Results showed that Hcy significantly decreased TRAP, Na(+),K(+)-ATPase and CAT activities, without affecting the activities of superoxide dismutase and glutathione peroxidase. We also verified the effect of chronic pretreatment with vitamins E and C on the reduction of TRAP, Na(+),K(+)-ATPase and CAT activities caused by Hcy. Vitamins E and C per se did not alter these parameters, but prevented the reduction of TRAP, Na(+),K(+)-ATPase and CAT activities caused by Hcy. Our results indicate that oxidative stress is probably involved in the pathogenesis of homocystinuria and that reduction of Na(+),K(+)-ATPase activity may be related to the neuronal dysfunction found in homocystinuric patients.

Pretreatment with vitamins E and C prevent the impairment of memory caused by homocysteine administration in rats

Homocystinuria is a metabolic disorder caused by deficiency of cystathione beta-synthase activity leading to tissue accumulation of homocysteine (Hcy); affected patients present neurological dysfunction. Considering that Hcy induces free radical formation and that memory is impaired by oxidative stress, in the present study we investigated the effect of an acute administration of Hcy on retrieval of step-down inhibitory avoidance in adult rats. The action of vitamins E and C on the effects produced by Hcy was also tested. Adult Wistar rats were pretreated for 1 week with daily i.p. administration of saline (control group) and vitamins E and C (vitamin E 40 mg/kg and vitamin C 100 mg/kg). Hcy (11 mmol/kg) or an equivalent volume of 0.9% saline were administered 1 h before training, 1 h before testing, or immediately after training sessions. Memory was significantly impaired in Hcy-treated group, whereas the rats chronically treated with vitamins E and C had this effect prevented. Present data strongly indicate that Hcy administration impairs memory, an effect probably mediated by oxidative stress since treatment with vitamins E and C prevented amnesia. Assuming the possibility that this might occur in the human condition, reported results may be relevant to explain, at least in part, neurologic dysfunction associated with homocystinuria.

Functional glycine receptor maturation in the absence of glycinergic input in dopaminergic neurones of the rat substantia nigra

The postnatal maturation pattern of glycine receptor channels (GlyRs) expressed by dopaminergic (DA) neurones of the rat substantia nigra pars compacta (SNc) was investigated using single-channel and whole-cell patch-clamp recordings in brain slices from rats aged 7-21 postnatal days (P). In neonatal rats (P7-P10), GlyRs exhibited a main conductance state of 100-110 pS with a mean open time of 16 ms. In juvenile rats (P19-P22), both the GlyR main conductance state (46-55 pS) and the mean open time (6.8 ms) were decreased. In neonatal rats, application of 30 microM picrotoxin, which is known to block homomeric GlyRs, strongly reduced glycine-evoked responses, while it was much less effective in juvenile rats. These results suggest that these GlyRs correspond functionally to alpha(2) homomeric GlyRs in neonatal rats and alpha(1)/beta heteromeric GlyRs in juvenile rats. A drastic but transient decrease in the glycine responsiveness of DA neurones occurred around P17 concomitant to the functional switch from the homomeric state to the heteromeric state. This age corresponds to a maturation phase for DA neurones. The application of 1 microM gabazine blocked spontaneous or evoked inhibitory synaptic current, while the addition of 1 microM strychnine had no effect, suggesting a lack of functional glycinergic synapses on DA neurones. Although it has been proposed that taurine is co-released with GABA at GABAergic synapses on DA neurones, in the present study the stimulation of GABAergic fibres failed to activate GlyRs. Blockade of taurine transporters and applications of high K(+) and hyposmotic solutions were also unable to induce any strychnine-sensitive current. We conclude that functional maturation of GlyRs can occur in the absence of any detectable GlyR activation in DA neurones of the SNc.

Inhibition of Na+, K+-ATPase activity by the metabolites accumulating in homocystinuria

Homocystinuria is an inborn error of sulfur amino acid metabolism characterized predominantly by vascular and nervous system dysfunction. In this study we determined the in vitro effects of homocysteine and methionine, metabolites which accumulate in homocystinuria, on Na+, K+-ATPase, and Mg2+-ATPase activities in synaptic membranes from the hippocampus of rats. The results showed that both metabolites significantly inhibit Na+, K+-ATPase but not Mg2+-ATPase activity at concentrations usually observed in plasma of homocystinuric patients. Furthermore, incubation of hippocampal homogenates with homocysteine also elicited an inhibition of the enzyme activity which was however prevented by the simultaneous addition of cysteine to the medium. In addition, cysteine or methionine per se did not modify the two enzymatic activities. These findings indicate that oxidation of critical groups in the enzyme may possibly be involved in homocysteine inhibitory effect. Moreover, kinetic studies performed to investigate the interaction between homocysteine and methionine on Na+, K+-ATPase inhibition suggested a common site for the two amino acids in the enzyme. Considering the critical role exerted by Na+, K+-ATPase in brain, it is proposed that the inhibition provoked by homocysteine and methionine on the enzyme activity may be possibly related to the brain dysfunction characteristic of homocystinuria.

Inhibition of rat brain Na+, K+-ATPase activity induced by homocysteine is probably mediated by oxidative stress

The objective of the present study was to investigate the effects of preincubation of hippocampus homogenates in the presence of homocysteine or methionine on Na+, K+-ATPase and Mg2+-ATPase activities in synaptic membranes of rats. Homocysteine significantly inhibited Na+, K+-ATPase activity, whereas methionine had no effect. Mg2+-ATPase activity was not altered by the metabolites. We also evaluated the effect of incubating glutathione, cysteine, dithiothreitol, trolox, superoxide dismutase and GM1 ganglioside alone or incubation with homocysteine on Na+, K+-ATPase activity. Tested compounds did not alter Na+, K+-ATPase and Mg2+-ATPase activities, but except for trolox, prevented the inhibitory effect of homocysteine on Na+, K+-ATPase activity. These results suggest that inhibition of this enzyme activity by homocysteine is possibly mediated by free radicals and may contribute to the neurological dysfunction found in homocystinuric patients.

Apoptotic cell death in the mouse retinal ganglion cell layer is induced in vivo by the excitatory amino acid homocysteine

Homocysteine, an excitatory amino acid and a homolog of cysteine, induces neuronal cell death in brain via stimulation of N-methyl-D-aspartate (NMDA) receptors. It also selectively activates NMDA receptors of retinal ganglion cells, but it is not known if high levels of homocysteine are toxic to these cells. The purpose of this study was to determine whether increased levels of homocysteine caused death of neurons in the ganglion cell layer; if so whether this death occurred via an apoptotic mechanism and to determine the consequences of simultaneous elevation of homocysteine and glutamate, a known retinal excitotoxin, on the viability of neurons of the ganglion cell layer. C57BL/6 mice were injected intravitreally with either homocysteine or glutamate/homocysteine combined (final concentrations: 25, 75, and 200 microM); injection of glutamate (25 and 200 microM) served as a positive control. Eyes were harvested and cryosections prepared 5-6 days post-injection. Systematic morphometric analysis of retinas of mice injected with homocysteine indicated that the total number of cells in the ganglion cell layer decreased by about 23% following exposure to 200 microM homocysteine. To determine whether the neurons of the ganglion cell layer were dying by apoptosis, the TUNEL method was used and was confirmed by immunohistochemical studies of caspase-3, known to be expressed at high levels during retinal ganglion cell apoptosis. Microscopic analysis revealed significantly more TUNEL-positive cells in the ganglion cell layer in homocysteine-injected eyes than in contralateral PBS-injected eyes. Retinas injected with 75 and 200 microM homocysteine displayed significantly more TUNEL-positive neurons in the ganglion cell layer (2 and 2.9, respectively) than PBS-injected retinas (0.25). In eyes injected simultaneously with homocysteine/glutamate, the number of apoptotic cells in the ganglion cell layer almost doubled that for homocysteine or glutamate injections alone. Immunohistochemical analysis of activated caspase-3 revealed numerous positively labelled neurons in the ganglion cell layer in homocysteine and homocysteine/glutamate-injected eyes, but not in PBS-injected eyes. Quantification of this data revealed a significantly greater number of caspase-3-positive neurons in the ganglion cell layer of retinas injected with 75 and 200 microM homocysteine (2.9 and 4.4, respectively) than for PBS-injected retinas (0.5). This confirms that death of neurons in the ganglion cell layer is occurring by apoptosis. The present study provides the first evidence that homocysteine is toxic to neurons of the ganglion cell layer. In addition, it provides evidence that these retinal neurons are dying by apoptosis and it demonstrates for the first time that excitotoxic damage to neurons of the ganglion cell layer is potentiated by simultaneous elevation of homocysteine and glutamate. These findings are relevant to retinal ganglion cell death characteristic of diabetic retinopathy, which is thought to be mediated by overstimulation of the NMDA receptor.

Effect of chronic hyperprolactinemia on daily changes of glutamate and aspartate concentrations in the median eminence and different hypothalamic areas of male rats

The 24h changes of glutamate (GLU) and aspartate (ASP) were studied in the median eminence (ME) and hypothalamic areas. It was analyzed whether prolactin may change their daily patterns. The hypothalamic concentration of these amino acids was measured by high-performance liquid chromatography (HPLC) with fluorometric detection. Plasma prolactin levels increased over the 24h light-dark cycle after pituitary grafting compared to controls, and its circadian rhythm was disrupted. In controls, aspartate and glutamate in the hypothalamic areas studied followed a specific daily variation or showed no rhythmicity. In the median eminence, hyperprolactinemia seem to phase advance the aspartate or glutamate peaks from 16:00 to 12:00. In the mediobasal hypothalamus, hyperprolactinemia altered daily changes of aspartate and significantly decreased its concentration. Also, it seems to delay the nocturnal glutamate peak compared to controls. In the posterior hypothalamus, hyperprolactinemia did not change aspartate and glutamate concentrations and their daily changes, although it increased the glutamine concentration. These data show the existence of 24h changes of amino acid concentration in three of the hypothalamic regions studied. Increased plasma prolactin levels differentially affected these patterns depending on the hypothalamic area analyzed.

Involvement of N-methyl-d-aspartate receptor and free radical in homocysteine-mediated toxicity on rat cerebellar granule cells in culture

The present study investigates the possible mechanism responsible for the neurotoxicity of D,L-homocysteine in primary culture of rat cerebellar granule cells. Neurotoxicity was assessed by measuring the amount of lactate dehydrogenase released from the cells following homocysteine treatment. D,L-Homocysteine (> 300 microM; 16-22 h) induced the release of lactate dehydrogenase from the cells in a concentration-dependent manner. The N-methyl-D-aspartate (NMDA) antagonist (+/-)-2-amino-5-phosphonopentanoic acid (APV) partially blocked the homocysteine-mediated neurotoxicity. However, the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) did not block the homocysteine-mediated toxicity. The homocysteine-mediated neurotoxicity was mostly prevented by the co-administration of superoxide dismutase and catalase or catalase alone. The results suggest that homocysteine induces neuronal cell death by stimulating NMDA receptor as well as by producing free radicals.

Seizures induced by homocysteine in rats during ontogenesis

We studied the convulsant action of homocysteine in 211 immature and adult Wistar albino rats. Homocysteine elicited minimal, predominantly clonic, and major generalized tonic-clonic seizures at six different developmental stages, from 7 days to adulthood. Nevertheless, some age-dependent differences in the seizure pattern were apparent. Minimal seizures in immature rats lasted < or = 20 min, thus representing an epileptic status, whereas in adult animals these seizures were much shorter, lasting only < or = 40 s. In addition, flexion seizures were observed in 7- and 12-day-old rats, only rarely in 15- and 18-day-old animals, and never in the 25-day-old and adult rats. ECoG recordings demonstrated a nearly isoelectric pattern during homocysteine-induced seizures in 7- and 12-day-old rat pups. In older rats, spikes or sharp waves were recorded, but precise electroclinical correlations were poor. The greater sensitivity of younger animals to kainic acid (KA) and N-methyl-D-aspartate (NMDA), as reported previously, was not evident in the case of homocysteine-induced seizures. This observation, together with a different behavioral pattern, suggests that homocysteine cannot be considered a simple agonist of the kainate or NMDA type of excitatory amino acid receptors. The exact mechanism of the convulsant action of homocysteine, both during development and in adulthood, remains to be clarified.

Toxicity, biodistribution and radioprotective capacity of L-homocysteine thiolactone in CNS tissues and tumors in rodents: comparison with prior results with phosphorothioates

L-Homocysteine thiolactone (L-HCTL) was evaluated for its potential as an intravenously-administered central nervous system (CNS) radioprotector in C3H mice and F344 rats. Toxicity assessments in the mouse yielded a LD50 of 297 mg/kg and in the rat 389 mg/kg. Biodistribution studies in tumor-bearing mice showed that brain specimens contained more label at 10 min than the tumors but less at 30 or 60 min. Brain uptake relative to the tumors, the brain/tumor ratio, ranged between 0.5 and 3.3. The cervical spinal cord of non-tumor-bearing rats was irradiated with 32 Gy 137Cs with or without prior treatment with L-HCTL following which the time to forelimb or hindlimb paralysis was measured to determine the relative protective factors (RPFs) for this radiation dose. For forelimb paralysis the RPF was 1.9 (+/- 1.0, SD) and for hindlimb it was 2.0 (+/- 1.1, SD). 36B-10 glioma cells irradiated in vitro with or without L-HCTL and assayed for colony forming capacity demonstrated a dose modifying factor (DMF) of only 1.15 (+/- 0.16, SE). Rats bearing intracerebral 36B-10 glioma received 137Cs irradiation with or without L-HCTL after which the tumors were similarly assayed in vitro. From this the glioma DMF was 1.2 (+/- 0.30, SE). Compared to prior results with phosphorothioates our data show that the toxicity of L-HCTL is roughly the same as WR2721, WR77913 and WR3689 and that it distributes at higher levels in the CNS after systemic administration. L-HCTL may well equal these phosphorothioates at protecting normal CNS tissue without requiring administration directly into the cerebrospinal fluid-containing spaces and it does not protect the 36B-10 glioma.

Chemical models of epilepsy with some reference to their applicability in the development of anticonvulsants

This paper reviews chemical models of epilepsy and their relevance in the identification and characterization of anticonvulsants. For each convulsant we discuss possible modes of administration, clinical type(s) of seizures induced, proposed mechanism(s) of epileptogenesis and, where available, responsiveness of the induced seizures to anticonvulsants. The following compounds are reviewed: pentylenetetrazol, bicuculline, penicillin, picrotoxin, beta-carbolines, 3-mercaptopropionic acid, hydrazides, allylglycine; the glycine antagonist strychnine; gamma-hydroxybutyrate; excitatory amino acids (glutamate, aspartate, N-methyl-D-aspartate, quisqualate, kainate, quinolinic acid); monosubstituted guanidino compounds, metals (alumina, cobalt, zinc, iron); neuropeptides (opioid peptides, corticotropin releasing factor, somatostatin, vasopressin); cholinergic agents (acetylcholine, acetylcholinesterase inhibitors, pilocarpine); tetanus toxin; flurothyl; folates; homocysteine and colchicine. Although there are a multitude of chemical models of epilepsy, only a limited number are applied in the routine screening of potential anticonvulsants. Some chemical models have a predictive value with regard to the clinical profile of efficacy of the tested anticonvulsants. Some chemical models may contribute to a better understanding of possible mechanisms of epileptogenesis.

Selective changes in GABAergic transmission in substantia nigra and superior colliculus caused by ethanol and ethanol withdrawal

One of ethanol's actions after acute exposure is anticonvulsant activity whereas withdrawal from chronic ethanol exposure increases convulsant activity. An increase in neuronal transmission in the GABAergic pathways from striatum to the substantia nigra (SN) and a decrease in GABAergic transmission from SN to superior colliculus (SC) both appear to play a major role in inhibiting seizure propagation. If this is the case, then the changes in seizure sensitivity caused by ethanol may be expected to affect GABAergic transmission in opposite ways in SN and SC. We measured the effects of in vitro ethanol on pre- and postsynaptic indices of GABA transmission using SN and SC tissue from both ethanol-naive rats and rats given ethanol in their drinking water for 24 days and then withdrawn for 24 hr, a treatment that decreases seizure latency. While ethanol inhibited 3H-GABA release from slices of SC at low concentrations (20-100 nM), much higher concentrations were required to inhibit release from SN (100-500 mM). In fact, release from SN was increased by low concentrations of ethanol. Ethanol in vitro (20-1000 mM) also inhibited specific binding of 35S-TBPS to the GABAA receptor but this effect was similar in both potency and efficacy in SC and SN. Next, the in vitro effects of ethanol were measured in rats that had consumed an average of 9.8 g ethanol/kg body weight/day and were then withdrawn for 24 hr. Ethanol inhibition of 3H-GABA release from SC was significantly less in ethanol-treated rats compared to controls whereas the inhibitory effect of ethanol was increased in SN from ethanol-treated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of soman-induced seizures on different extracellular amino acid levels and on glutamate uptake in rat hippocampus

Extracellular amino acid levels in CA3 and CA1 fields of rat hippocampus, an area highly sensitive to seizures, were determined by intracranial microdialysis during seizures induced by systemic administration of soman (o-1,2,2-trimethylpropyl methylphosphonofluoridate), a potent inhibitor of acetylcholinesterase. The glutamate uptake level was determined on another series of animals in hippocampus homogenates. An early and transient increase in the extracellular glutamate level occurred in CA3 within 30 min of seizures, with correlated brief elevations of taurine, glycine and glutamine levels. The glutamate level increased early in CA1, declined and then became more sustained (after 50 min of seizures). Apparent elevations of taurine, glycine and glutamine levels in CA1 accompanied changes in glutamate concentrations. Changes of glutamate level correlated with an increase in the glutamate uptake which rapidly declined after 40 min of seizures. The role of the transient release of glutamate in CA3 and of the sustained release in CA1 in prolonged soman-induced seizures is considered. The correlation between glutamate and other amino acid release is studied.

Long-lasting effects of audiogenic seizures on neurotransmitter amino acids in Rb mice

The existence of long-lasting (15-18 h) alterations of neurotransmitter amino acid levels following a single or repeated acoustic stimulations in audiogenic seizure-prone Rb1 and Rb2 mice and seizure-resistant Rb3 mice were investigated. The levels of glutamate, aspartate, glycine, taurine, and of some of their precursors: glutamine and serine were determined. Fourteen brain areas were examined. Alterations were found only in 6 brain areas (pons, olfactory bulbs, superior colliculus, inferior colliculus, olfactory tubercles and raphe). Most frequent occurring changes were observed in pons and olfactory tubercles. These changes concerned mainly the excitatory amino acids, glutamate, and aspartate. Alterations of taurine, glycine and serine were also recorded.

Changes with aging in the levels of amino acids in rat CNS structural elements: IV. Methionine and basic amino acids

This paper reports the distribution of methionine, histidine, lysine, arginine, and ornithine in 53 discrete brain areas of 3- and 29-month-old male Fischer 344 rats microdissected by the punch technique. Like that of the other amino acids were reported in previous papers of this series, the distribution of methionine and the basic amino acids was regionally highly heterogeneous. The ratios of levels in the areas of highest concentrations to levels in the areas of lowest concentration varied from 10 to 15 for these amino acids, except that it was 23 for arginine. This heterogeneity is also illustrated by the finding that in some areas arginine was more than 5% of the total amino acid content and in others was less than 0.5%. The distribution of methionine differed from that of the basic amino acids, which were high in hypothalamic areas and low in the limbic system. With aging, methionine and basic amino acids, like the other amino acids studied in this series, mainly decreased in level, although in a few cases increases could be seen. Significant decreases were noted more than five times as often as increases.

Brain amino acid concentration changes during status epilepticus induced by lithium and pilocarpine

Amino acid concentrations were measured in specific structures from the brains of rats decapitated before and during the course of status epilepticus induced by lithium and pilocarpine, with the stages of status defined by the electroencephalographic (EEG) pattern displayed. Early status was marked by discrete seizures on EEG, mid status by continuous spiking, and late status by periodic epileptiform discharges. Aspartate levels were lower than control levels in most regions prior to the onset of status. The decline continued and reached statistical significance in different regions at times from early to late status. Glutamate concentrations were typically higher than control just prior to status onset and then decreased in a manner similar to aspartate, but with less percentage change. gamma-Aminobutyric acid increased during status, with the earliest statistically significant differences observed in mid status. These changes were observed in most forebrain structures studied, but the largest percentage changes in excitatory amino acid concentration were found in substantia nigra, where they fell to less than half of control.

Neurotransmitter changes in guinea-pig brain regions following soman intoxication

The effects of the organophosphate acetylcholinesterase (AChE) inhibitor soman (31.2 micrograms/kg s.c.) on guinea-pig brain AChE, transmitter, and metabolite levels were investigated. Concentrations of acetylcholine (ACh) and choline (Ch), noradrenaline (NA), dopamine (DA), 5-hydroxytryptamine (5-HT), and their metabolites, and six putative amino acid transmitters were determined concurrently in six brain regions. The brain AChE activity was maximally inhibited by 90%. The ACh content was elevated in most brain areas by 15 min, remaining at this level throughout the study. This increase reached statistical significance in the cortex, hippocampus, and striatum. The Ch level was significantly elevated in most areas by 60-120 min. In all regions, levels of NA were reduced, and levels of DA were maintained, but those of its metabolites increased. 5-HT levels were unchanged, but those of its metabolites showed a small increase. Changes in levels of amino acids were restricted to those areas where ACh levels were significantly raised: Aspartate levels fell, whereas gamma-aminobutyric acid levels rose. These findings are consistent with an initial increase in ACh content, resulting in secondary changes in DA and 5-HT turnover and release of NA and excitatory and inhibitory amino acid transmitters. This study can be used as a basis to investigate the effect of toxic agents and their treatments on the different transmitter systems.

Lipid peroxidation and glutathione peroxidase, glutathione reductase, superoxide dismutase, catalase, and glucose-6-phosphate dehydrogenase activities in FeCl3-induced epileptogenic foci in the rat brain

This study investigated the relationship between lipid peroxidation, subsequent activation of antioxidative enzymes, and development of iron-induced epilepsy in the rat. Epileptic foci were produced in rat cerebral cortex by intracortical injection of ferric chloride (FeCl3). The epileptic foci were identified by electrocorticography (ECoG). Epileptiform ECoG activity was shown to occur in the contralateral homotopic cerebral cortex as well. We measured levels of lipid peroxides and changes in the activities of the enzymes: superoxide dismutase (SOD), glutathione peroxidase (GP), glutathione reductase (GR), catalase (CA), and glucose-6-phosphate dehydrogenase (G6P) in the epileptogenic focus (both ipsilateral and contralateral) at days 3, 8, 15, and 23 after FeCl3 injection. Biochemical estimations were made in subcellular fractions, and changes in the ipsilateral site were compared with those in the contralateral site. The results of this study showed that large increases in lipid peroxidation were associated with development and buildup of the ECoG epileptiform discharges. Lipid peroxides increased in the ipsilateral focus by approximately 100% as compared with control. In the contralateral site, however, the increase in lipid peroxides was marginal only. The increase in lipid peroxidation was concomitant with development of the high level of epileptiform activity. The time course of changes in lipid peroxidation paralleled the time course of development and persistence of the epileptiform activity. Regarding changes in the enzyme activities accompanying development of iron epilepsy, the data showed that although SOD and G6P increased by approximately 60% and GR increased by approximately 40%, the increases in the enzyme GP and CA were much lower, less than 20%. Thus, comparatively less increase in CA and GP activities produces a deficiency of these two enzymes in the iron (ipsilateral) focus. Among the various biochemical disturbances that have been identified as involved in epileptogenesis, peroxidative injury resulting from lipid peroxidation in neural plasma membrane may be causally related to development of paroxysmal epileptiform activity in the iron focus. Since GP is an enzyme of major importance in detoxification of lipid peroxides in the brain, based on the results presented in this article, it appears reasonable to suggest that GP deficiency causes lipid peroxidation to increase tremendously during iron epileptogenesis.

The effects of aging on MAO activity and amino acid levels in rat brain

The possibility that age-related changes in amino acid levels in various rat brain areas might correlate with modifications of monoamine oxidase (MAO) activity, already found with aging, has been examined. Taurine, aspartic acid and glutamic acid levels were found to be unchanged or decreased with age, whereas GABA and glutamine concentrations increased or remained unchanged. Serine and glycine (except in pons-medulla) levels were found to be unaffected by age. The increase in total MAO activity with aging in some brain areas might contribute to the changes in amino acid levels. Likewise, the possible influence of age-induced changes in activity of various enzymes involved in H2O2 and NH3 detoxication and in amino acid biosynthesis on rat brain amino acid levels are considered. Oral administration of clorgyline or 1-deprenyl to young rats did not significantly modify the concentrations of most brain amino acids.

Excitatory sulphur amino acid-evoked neurotransmitter release from rat brain synaptosome fractions

The neuroactive sulphur-containing amino acids L-cysteate (CA), L-cysteine sulphinate (CSA), L-homocysteine sulphinate (HSA), S-sulpho-L-cysteine (SC) and L-homocysteate (HCA) evoked the release of previously accumulated D-[3H]aspartate from rat brain cerebrocortical and cerebellar synaptosome fractions in a manner that was wholly Ca2+-independent. However, analysis of endogenous release by hplc revealed the presence of both Ca2+-dependent and -independent component of L-glutamate release but only a Ca2+-independent component of L-aspartate release. CA, CSA, HSA and SC but not HCA evoked the release of previously accumulated [3H]GABA from synaptosome fractions by a mechanism shown to comprise both a Ca2+-dependent and -independent component. The specific antagonists of the N-methyl-D-aspartate (NMDA) receptor, 3-[(+/-)-2-carboxypiperazin-4-yl]propyl-l-phosphonic acid (CPP) and the relatively selective competitive quisqualate (QUIS)/kainate (KA) receptor antagonist, 6-cyano-7-dinitroquinoxalinedione (CNQX), were ineffective in blocking the excitatory sulphur amino acid-evoked release of either D-[3H]aspartate, [3H]GABA or of endogenous established transmitter amino acids.

Regional changes in the concentrations of glutamate, glycine, taurine, and GABA in the vitamin B-6 deficient developing rat brain: association with neonatal seizures

It is well known that a dietary restriction of vitamin B-6 during gestation and lactation produces spontaneous seizures in neonatal animals. Since pyridoxal phosphate, one of the biologically active forms of vitamin B-6, is the cofactor for GAD the neonatal seizures have been attributed to low levels of brain GABA as a result of cofactor depletion. Although GABA levels are significantly lower in B-6 restricted neonatal rats with spontaneous seizures, seizure activity is not present in B-6 deficient adult rats or 28 day old rats in the present study, despite significantly low levels of brain GABA. These facts suggest that depletion of GABA is not the only biochemical alteration essential for the emergence of seizures. In the present study, the effect of vitamin B-6 undernutrition on the concentrations of the neuroactive amino acids, Glu, Gly, Tau, and GABA was determined in selected regions of the developing rat brain. The results show that the concentrations of Glu, Tau, and GABA were significantly lower and GLY significantly higher in selected brain regions of the B-6 restricted 14 day old rat compared to control tissue. Most of these changes were unique to 14 days of age, the time when spontaneous seizures are observed, and not present at 28 or 56 days of age when seizures are absent. This pattern of amino acid changes in the brain and the magnitude of the changes was consistent with those measured in a variety of chemically-induced animal models of epilepsy and in human epileptic foci.(ABSTRACT TRUNCATED AT 250 WORDS)

Animal models of the epilepsies

The study of mechanisms of the epilepsies requires employment of animal models. Choice of a model system depends upon several factors, including the question to be studied, the type of epilepsy to be modelled, familiarity and convenience. Over 50 models are reviewed. Major categories of models are those for simple partial seizures: topical convulsants, acute electrical stimulation, cortically implanted metals, cryogenic injury; for complex partial seizures: kainic acid, tetanus toxin, injections into area tempesta, kindling, rodent hippocampal slice, isolated cell preparations, human neurosurgical tissue; for generalized tonic-clonic seizures: genetically seizure-prone strains of mouse, rat, gerbil, fruitfly and baboon, maximal electroshock seizures, systemic chemical convulsants, metabolic derangements; and for generalized absence seizures: thalamic stimulation, bilateral cortical foci, systemic penicillin, gamma-hydroxy-butyrate, intraventricular opiates, genetic rat models. The lithium-pilocarpine, homocysteine and rapid repetitive stimulation models are most useful in studies of status epilepticus. Key findings learned from each of the models, the model's strengths and weaknesses are detailed. Interpretation of findings from each of these models can be difficult. Do results pertain to the epilepsies or to the particular model under study? How important are species differences? Which clinical seizure type is really being modelled? In a model are behavior or EEG findings only similar superficially to epilepsy, or are the mechanisms comparable? The wealth of preparations available to model the epilepsies underscores the need for unifying themes, and for better understanding of basic mechanisms of the epilepsies.

The effect of caffeine on some mouse brain free amino acid levels

Changes in free amino acids were examined in the central nervous system of mice treated with caffeine for three weeks. Caffeine was administered in the drinking water, and at the end of three weeks the level of caffeine in the cerebral cortex was 113 +/- 19 micrograms/g. When amino acid levels in cerebral hemispheres, midbrain, pons and medulla, and cerebellum were measured a significant increase in glutamine levels was found in all four regions. Glycine, alanine, serine, threonine, and GABA were significantly reduced in some regions. Caffeine appears to alter some of the metabolic or transport processes regulating amino acid pools in the brain. The decrease of GABA found in pons and medulla may contribute to the observed increase in reflex excitability after caffeine.

Convulsant effect of lindane and regional brain concentration of GABA and dopamine

Lindane (gamma-hexachlorocyclohexane) is an organochlorine insecticide with known neurotoxic effects. Its mechanism of action is not well understood although it has been proposed that lindane acts as a non-competitive antagonist at the gamma-aminobutyric acid (GABA)-A receptor. We studied the effect of lindane (150 mg/kg) on the GABAergic and dopaminergic systems by measuring the concentration of GABA, dopamine and its metabolites in 7 brain areas at the onset of seizures. All animals suffered tonic convulsions at 18.3 +/- 1.4 min after lindane administration. The concentration of GABA was only slightly but significantly decreased in the colliculi without modifications in the other areas. The concentration of dopamine was increased in the mesencephalon and that of its metabolite DOPAC was also increased in the mesencephalon and the striatum.

Aspartic acid aminotransferase activity is increased in actively spiking compared with non-spiking human epileptic cortex

Increased concentration of the excitatory neurotransmitter aspartic acid in actively spiking human epileptic cerebral cortex was recently described. In order to further characterise changes in the aspartergic system in epileptic brain, the behaviour of aspartic acid aminotransferase (AAT), a key enzyme involved in aspartic acid metabolism has now been examined. Electrocorticography performed during surgery was employed to identify cortical epileptic spike foci in 16 patients undergoing temporal lobectomy for intractable seizures. Patients with spontaneously spiking lateral temporal cortex (n = 8) were compared with a non-spiking control group (n = 8) of patients in whom the epileptic lesions were confined to the hippocampus sparing the temporal convexity. Mean activity of AAT in spiking cortex was significantly elevated by 16-18%, with aspartic acid concentration increased by 28%. Possible explanations for the enhanced AAT activity include increased proliferation of cortical AAT-containing astrocytes at the spiking focus and/or a generalised increase in neuronal or extraneuronal metabolism consequent to the ongoing epileptic discharge. It is suggested that the data provide additional support for a disturbance of central excitatory aspartic acid mechanisms in human epileptic brain.

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 excitotoxic chemicals, either directly or as a consequence of seizures.

Determination of 4-aminobutyric acid, aspartate, glutamate and glutamine and their 13C stable-isotopic enrichment in brain tissue by gas chromatography-mass spectrometry

A selected-ion monitoring method was developed for measuring 4-aminobutyric acid, aspartate, glutamate, and glutamine in brain tissue. Natural isotopes of these amino acids and their stable-isotopic enrichment following intravenous infusion of a precursor, [13C]glucose, were quantitated. Frozen mouse brain tissue was homogenized in cold 80% ethanol, and the supernatant, equivalent to 1 mg of wet weight brain tissue, was extracted using solid-phase bonded silica ion-exchange columns. Aspartate and glutamate (dicarboxylic acids) were isolated from strong anion-exchange columns, whereas 4-aminobutyric acid and glutamine (neutral amino acids) were isolated from strong-cation exchange columns. n-Butyl ester pentafluoropropionyl amide derivatives of these amino acids were analyzed by gas chromatography-mass spectrometry using a methane positive chemical ionization mode after gas chromatographic separation on a wide-bore, fused-silica capillary column. The method is applicable to determination of brain concentrations of these amino acids as well as their fluxes following administration of a stable-isotopic tracer.

Reduced uptake and release of 5-hydroxytryptamine and taurine in the cerebral cortex of epileptic El mice

Inbred mutant El mice are highly susceptible to convulsive seizures upon 'throwing' stimulation, and the inhibition of 5-hydroxytryptamine (5-HT) and taurine activities appears to be involved in the El mouse seizures. Uptake and release of [3H]5-HT and [3H]taurine into and from cerebral neurocortical slices using a superfusion system were investigated in both non-stimulated and stimulated El mice [El(-), El(+)] and in ddY mice, which do not have a convulsive disposition. Release was defined as 40 mM K+-stimulated release. 5-HT and taurine uptake in El(+) was lower than El(-) but no difference in either uptake was found between ddY and El(-). Release of 5-HT and taurine in El(-) was higher than in ddY whereas their release in El(+) was lower than in El(-). The taurine level in the cerebral neocortex of El(-) and El(+) was higher than in ddY. These results suggest that the synaptic function of the 5-HT and taurine containing neurons is suppressed and that dysfunction of these inhibitory neurons is involved in the seizure susceptibility in the El mice.