GABA and amino acid concentrations in lumbar CSF in patients with treated and untreated epilepsy

Dogs as a Natural Animal Model of Epilepsy

Epilepsy is a common neurological disease in both humans and domestic dogs, making dogs an ideal translational model of epilepsy. In both species, epilepsy is a complex brain disease characterized by an enduring predisposition to generate spontaneous recurrent epileptic seizures. Furthermore, as in humans, status epilepticus is one of the more common neurological emergencies in dogs with epilepsy. In both species, epilepsy is not a single disease but a group of disorders characterized by a broad array of clinical signs, age of onset, and underlying causes. Brain imaging suggests that the limbic system, including the hippocampus and cingulate gyrus, is often affected in canine epilepsy, which could explain the high incidence of comorbid behavioral problems such as anxiety and cognitive alterations. Resistance to antiseizure medications is a significant problem in both canine and human epilepsy, so dogs can be used to study mechanisms of drug resistance and develop novel therapeutic strategies to benefit both species. Importantly, dogs are large enough to accommodate intracranial EEG and responsive neurostimulation devices designed for humans. Studies in epileptic dogs with such devices have reported ictal and interictal events that are remarkably similar to those occurring in human epilepsy. Continuous (24/7) EEG recordings in a select group of epileptic dogs for >1 year have provided a rich dataset of unprecedented length for studying seizure periodicities and developing new methods for seizure forecasting. The data presented in this review substantiate that canine epilepsy is an excellent translational model for several facets of epilepsy research. Furthermore, several techniques of inducing seizures in laboratory dogs are discussed as related to therapeutic advances. Importantly, the development of vagus nerve stimulation as a novel therapy for drug-resistant epilepsy in people was based on a series of studies in dogs with induced seizures. Dogs with naturally occurring or induced seizures provide excellent large-animal models to bridge the translational gap between rodents and humans in the development of novel therapies. Furthermore, because the dog is not only a preclinical species for human medicine but also a potential patient and pet, research on this species serves both veterinary and human medicine.

Free-GABA levels in the cerebrospinal fluid of patients suffering from several neurological diseases Its potential use for the diagnosis of diseases which course with inflammation and tissular necrosis

Free GABA levels were measured in the cerebrospinal fluid (CSF) of 74 neurological patients suffering from cerebral cysticercosis (n = 9), Parkinson's disease (n = 5), multiple sclerosis (n = 6), epilepsy (n = 24), meningeal tuberculosis (n = 6), viral encephalitis (n = 3), cerebrovascular disease (n = 8) and several kinds of dystonia (n = 5). A statistical significant four-fold elevation in free GABA levels was found in patients with cerebral cysticercosis. A non statistical significant two-fold increase in free GABA levels was also encountered in the CSF of patients affected by cerebrovascular disease and viral encephalitis. No changes in CSF free GABA levels were found in patients suffering from any of the other disorders. It is suggested that free GABA levels may be elevated in the CSF of patients suffering from neurological diseases which course with inflammation and tissular necrosis such as cerebral cysticercosis. Much work is needed however to establishd whether CSF free GABA levels can be used as a diagnostic tool in at least some type of these patients.

Plasma and cerebrospinal fluid amino acids in epileptic patients

Altered plasma and cerebrospinal fluid amino acid levels may be associated with human epilepsy. We studied three groups of patients, those with a generalized epileptic syndrome, juvenile myoclonic epilepsy, patients with refractory localization-related epilepsies, and patients with acute seizures (within 24 h). Plasma levels of amino acids were studied in all patient groups, as were those in the cerebrospinal fluid (CSF) of patients with acute seizures. After acute seizures, the amino acid changes in the CSF were limited to a reduction in the level of taurine, whereas the levels of most amino acids in plasma were decreased. On the other hand, levels of the excitatory amino acids glutamate and aspartate were increased. The most notable finding in the juvenile myoclonic epilepsy patients was an increase in glutamate level in the plasma. Our study supports the conception of an altered metabolism of glutamate in generalized epilepsies.

GABA and glutamate in the human brain

Cortical excitability reflects a balance between excitation and inhibition. Glutamate is the main excitatory and GABA the main inhibitory neurotransmitter in the mammalian cortex. Changes in glutamate and GABA metabolism may play important roles in the control of cortical excitability. Glutamate is the metabolic precursor of GABA, which can be recycled through the tricarboxylic acid cycle to synthesize glutamate. GABA synthesis is unique among neurotransmitters, having two separate isoforms of the rate-controlling enzyme, glutamic acid decarboxylase. The need for two separate genes on two chromosomes to control GABA synthesis is unexplained. Two metabolites of GABA are present in uniquely high concentrations in the human brain. Homocarnosine and pyrrolidinone have a major impact on GABA metabolism in the human brain. Both of these GABA metabolites have anticonvulsant properties and can have a major impact on cortical excitability.

Neurochemical and morphological changes associated with human epilepsy

To date a multitude of studies into the morphology and neurochemistry of human epilepsy have been undertaken with variable, and often inconsistent, results. This review summarises these studies on a range of neurotransmitters, neuromodulators, neuropeptides and their receptors. In addition to this, novel changes in cell viability and sprouting have been identified and are discussed. Whether the alterations observed are a result of the seizures or are a contributory factor is unclear. However, it may be that following an initial insult (such as febrile convulsions, status epilepticus or head injury) secondary processes occur both of an anticonvulsant nature in an attempt to compensate for seizure activity, and in a kindling type of fashion, resulting in an increased susceptibility to seizures, leading to future seizures. Many of the alterations documented in this study probably represent one or both of these processes. Clearly no single chemical abnormality or morphological alteration is going to explain the clinically diverse disorder of epilepsy. However, by drawing together the neurochemistry and morphology of epilepsy, we may begin to understand the mechanisms involved in seizure disorders.

Determination of in vivo amino acid neurotransmitters by high-performance liquid chromatography with o-phthalaldehyde-sulphite derivatisation

The measurement of amino acid neurotransmitters by high-performance liquid chromatography (HPLC) has emerged as a reliable and sensitive method. This paper describes a method which employs electrochemical (EC) detection of amino acid derivatives formed by a reaction with o-phthalaldehyde (OPA) in the presence of sulphite ions. This is discussed in relation to the problems of previously reported methods based on OPA derivatisation. Precise separation of the following 7 amino acid standards is achieved using isocratic elution: serine, glycine, taurine, glutamate, arginine, alanine and GABA, in order of increasing retention time. Total elution time is 25 min. Derivatisation proceeds at room temperature and the derivatives are stable for up to 5 h. This technique has the sensitivity to determine the concentrations of amino acid neurotransmitters in cerebrospinal fluid (CSF) and an in vivo microdialysis method is discussed for the detection of basal and potassium-stimulated levels of gamma-aminobutyric acid and glutamate from rat hippocampus.

Platelet and brain GABA-transaminase and monoamine oxidase activities in patients with complex partial seizures

The activities of gamma-aminobutyrate aminotransferase (GABA-T) and monoamine oxidase (MAO-A and -B) were measured in blood platelets from 27 patients and hippocampal tissues from eight (GABA-T) and ten (MAO) patients with complex partial seizures. The activity of platelet GABA-T was found to be higher in the epileptic patients (43.37 +/- 13.53 pmol/min/mg protein, P < 0.005) in comparison with that found in 14 healthy volunteer subjects (29.59 +/- 13.14 pmol/min/mg protein). This difference was most pronounced in patients treated with carbamazepine (CBZ) (P < 0.01) and phenytoin (PHT) (P < 0.01). Contrary to the platelets, the activity of GABA-T in the hippocampi from the epileptic patients (6.937 +/- 2.204 nmol/min/mg protein) did not differ significantly from that found in seven non-epileptic control cases (7.158 +/- 0.951 nmol/min/mg protein). The increase in GABA-T activity in the blood platelets from the epileptic patients could not be explained by a direct effect of the antiepileptic compounds, since there were no changes in the activities on exposure to PHT, CBZ or VPA in vitro, either of blood platelets or of brain tissue. With regard to platelet MAO, no difference in the activity was found between the two groups, whereas the MAO-B activity in the hippocampi was significantly higher in the epileptic patients (3.51 +/- 1.32 nmol/mg/mg protein) than in the control cases (1.21 +/- 0.73 nmol/mg/mg protein) (P < 0.0004). There was no difference in MAO-A activity in the hippocampi between epileptic patients and controls.

CONCLUSION

In the hippocampi from patients with complex partial seizures the activities of the mitochondrial enzymes GABA-T and MAO-A were similar to those found in control subjects. The activity of MAO-B, however, was significantly higher indicating that there is an increased proportion of reactive astrocytes in epileptic hippocampus.

Amino acid levels in the cerebrospinal fluid of newly diagnosed epileptic patients: effect of vigabatrin and carbamazepine monotherapies

We studied the CSF amino acid levels of 42 patients with newly diagnosed epilepsy before treatment with antiepileptic medication and during monotherapy with either vigabatrin or carbamazepine. The present study shows that patients with newly diagnosed epilepsy have elevated levels of the excitatory amino acid glutamate in CSF. Vigabatrin monotherapy effectively prevents the appearance of seizures in patients with high baseline CSF glutamate levels. In these patients, vigabatrin not only elevates the levels of gamma-aminobutyric acid, but also decreases the elevated levels of glutamate in CSF, which may also be important to the antiepileptic efficacy of vigabatrin. Patients with low CSF glutamate levels did not benefit from vigabatrin-induced changes in amino acid levels and successful monotherapy with carbamazepine did not affect CSF amino acid levels. The elevation of gamma-aminobutyric acid is thus not the only way to achieve seizure control and there are several factors underlying the generation and control of seizures. Follow-up of the patients with high baseline glutamate CSF levels will show if the observed abnormalities are related to the severity of epilepsy in individual patients and if early treatment with vigabatrin of these patients could prevent the development of intractable epilepsy.

Cerebrospinal fluid levels of neuropeptides, cortisol, and amino acids in patients with epilepsy

We measured lumbar cerebrospinal fluid (CSF) levels of somatostatin, cholecystokinin, neurotensin, atrial natriuretic factor, vasoactive inhibitory peptide, neuropeptide Y, adrenocorticotrophic hormone, corticotropin releasing hormone, beta-endorphin, metenkephalin, cortisol, alanine, glycine, aspartate, glutamate, taurine, and gamma-aminobutyric acid in 25 inpatients with epilepsy at known interictal and postictal times and in 11 neurologically normal volunteers. There were no significant differences between interictal or postictal complex partial seizures (CPS), postictal generalized tonic-clonic seizures (GTC), and control CSF neuropeptide, cortisol, and amino acid (AA) levels. However, there were nonsignificant trends for CSF levels of several neuropeptides to be increased after CPS and GTC as compared with interictal baseline levels. There were significant correlations between levels of certain CSF neuropeptides or (AAs) and serum antiepileptic drug (AED) levels. Several correlations were noted between CSF levels of AAs, including a correlation between the excitatory neurotransmitters aspartate and glutamate identified only after CPS.

Platelet GABA-transaminase in epileptic children: influence of epilepsy and anticonvulsants

The relationship between platelet GABA-transaminase (GABA-T) activity and either epilepsy or its treatment has been studied in 281 epileptic children: 55 were newly diagnosed untreated patients and 226 were chronically receiving anticonvulsants (154 in monotherapy and 72 in polytherapy). Results were compared with those from 48 control children. Untreated children had a GABA-T activity of 9.1 +/- 3.7 pmol/min/mg protein, lower than the control group (10.6 +/- 3.8 pmol/min/mg, P < 0.05), whereas treated epileptic children had higher values (11.9 +/- 6.3 pmol/min/mg) than those untreated (P < 0.01). In untreated children, the seven with absences and the nine with simple partial seizures had a GABA-T activity of 6.9 +/- 3.3 and 7.8 +/- 3.2 pmol/min/mg, respectively, lower than the control group (P < 0.05). In treated patients, those receiving valproate (VPA) in monotherapy had a GABA-T activity of 15.3 +/- 7.5 pmol/min/mg, higher than both the control group and the untreated children (P < 0.001). All patients receiving VPA in mono- or polytherapy had a higher activity than those receiving other anticonvulsants (16.4 +/- 8.4 vs. 9.9 +/- 3.9 pmol/min/mg, P < 0.001), the activity in Lennox syndrome and myoclonic epilepsies being significantly higher than in those with absences and partial epilepsy. GABA-T activity did not correlate with doses or trough steady-state serum levels of VPA. Platelet GABA-T could be useful as a peripheral marker of GABAergic alterations and GABAergic effects of antiepileptic drugs in epileptic patients.

Amino acids in the neuronal microenvironment of focal human epileptic lesions

Extracellular fluid was topically sampled with a dialysis probe during electrocorticography from the exposed cerebral cortex in 23 patients undergoing epilepsy surgery. Sampling was done in parallel from epileptiform regions and from non-epileptic areas. The former were classified according to the histopathology, into neoplastic, non-tumoral or 'special cases'. The epileptiform regions had significantly higher extracellular concentrations of alanine, glycine and phosphoethanolamine in the majority of the cases. The excised epileptic lesions were analyzed to provide the corresponding intracellular concentrations of amino acids. Several of the non-tumoral group showed high concentrations of GABA, ethanolamine and alanine. The intra- to extracellular concentration ratio for amino acids was low for phosphoethanolamine, glycine, serine and glutamine in most of the samples of epileptiform cortex, while the intracellular accumulative ability for ethanolamine apparently was stronger in epileptiform than in normal cortex.

Inhibitory and excitatory amino acids in cerebrospinal fluid of chronic epileptic patients

We studied the levels of excitatory and inhibitory amino acids in the cerebrospinal fluid (CSF) of 28 epileptic patients (24 with partial type seizures, 4 with primary generalized seizures) and 12 controls. The levels of aspartate were 63% (p less than 0.01), glutamine 129% (p less than 0.001), and homocarnosine 127% (p less than 0.005) that of controls. The concentrations of glutamate, asparagine, total GABA, free GABA, taurine, and glycine did not differ between epileptic patients and controls. Patients with partial epilepsy had a pattern of amino acids in CSF similar to that in patients with primary generalized seizures. In the present study we did not observe increased excitation or decreased inhibition in the seizure-active brains of epileptics, as far as the CSF levels of amino acids reflect their levels in the brain.