Excitatory amino acids modulate phosphoinositide signal transduction in human epileptic neocortex

Phospholipase C-β1 Hypofunction in the Pathogenesis of Schizophrenia

Schizophrenia is a mental disorder that is characterized by various abnormal symptoms. Previous studies indicate decreased expression of phospholipase C-β1 (PLC-β1) in the brains of patients with schizophrenia. PLC-β1-null (PLC-β1(-/-)) mice exhibit multiple endophenotypes of schizophrenia. Furthermore, a study of PLC-β1 knockdown in the medial prefrontal cortex of mice has shown a specific behavioral deficit, impaired working memory. These results support the notion that disruption of PLC-β1-linked signaling in the brain is strongly involved in the pathogenesis of schizophrenia. In this review, we broadly investigate recent studies regarding schizophrenia-related behaviors as well as their various clinical and biological correlates in PLC-β1(-/-) and knockdown mouse models. This will provide a better understanding of the pathological relevance of the altered expression of PLC-β1 in the brains of patients with schizophrenia. Evidence accumulated will shed light on future in-depth studies, possibly in human subjects.

Cortical and hippocampal EEG effects of neurotransmitter agonists in spontaneously hypertensive vs. kainate-treated rats

To analyze mediatory mechanisms underlying attention-deficit hyperactivity disorder (ADHD) and their association with epilepsy, the electroencephalogram (EEG) responses to various centrally applied neurotransmitter agonists were studied in spontaneously hypertensive (SH), kainate-treated (KA), and normotensive (control) rats, with chronically implanted electrodes into the frontal cortex and hippocampus and a cannula into the lateral cerebral ventricle. In SH rats, the baseline EEG showed increased delta and beta2 activity in the hippocampus and decreased alpha/beta1 activity in both brain areas. In KA rats, these delta and alpha/beta1 effects were observed 2 weeks post-kainate, while the beta2 activity increase occurred after 5 weeks in the hippocampus and, to a greater extent, 9 weeks post-injection in both brain areas. In SH rats, NMDA increased delta and decreased alpha/beta1 activity, similar to KA rats 5 weeks post-injection. In SH rats, clonidine augmented theta/beta2 increase in the cortex and alpha suppression in both brain areas, in parallel with induction of beta2 activity in the hippocampus. These beta2 effects were observed 5 and 9 weeks post-kainate. In SH rats, baclofen produced robust delta/theta enhancement and alpha/beta1 suppression in both brain areas, with additional beta2 activity increase in the hippocampus, while muscimol was ineffective in both groups of rats. In KA rats, EEG responses to GABA agonists were similar to those in control. Our results demonstrate sensitization of NMDA receptors and α2-adrenoceptors both in SH and KA rats and that of GABAb receptors specifically in SH rats.

Neurochemistry and epileptology

This paper gives an account of the global evolution of (neuro-)chemistry in epileptology with an emphasis on the role of the International League Against Epilepsy (ILAE), which declared in its constitution a mission "to make the epilepsy-problem the object of special study and to make practical use of the results of such study." As Epilepsia is the scientific journal of the ILAE, the review emphasizes papers published in the journal.

Neurotoxicity of a novel local anesthetic agent, ropivacaine: the possible roles of bursts of potential and cytoplasmic second messenger

BACKGROUND/PURPOSE

Ropivacaine has been shown to induce convulsion following overdose or accidental intravenous injection, but the mechanisms are poorly understood. Using an identifiable central neuron from giant African snail, the authors studied the mechanism of ropivacaine-elicited bursts of potential and explored the possible mechanisms of ropivacaine-induced neurotoxicity.

METHODS

Ropivacaine action on a central neuron (RP4) of the giant African snail (Achatina fulica Ferussac) was recorded by conventional electrophysiologic technique. Interactions between ropivacaine and prazosin, propranolol, atropine, d-tubocurarine, calcium-free solution, H89, U73,122, neomycin, high-magnesium solution, and chelerythrine were also observed.

RESULTS

The RP4 neuron showed spontaneous firing of action potentials. Extracellular application of ropivacaine (900 microM) reversibly elicited bursts of potential in the RP4 neuron. The bursts of potential elicited by ropivacaine were not blocked after administration of: (1) prazosin, propranolol, atropine, d-tubocurarine; (2) calcium-free solution; and (3) pretreatment with H89 or chelerythrine. The bursts of potential elicited by ropivacaine were blocked by pretreatment with U73122 (30 microM) or by adding neomycin (3.5 mM) or high-magnesium solution (30 mM).

CONCLUSION

Ropivacaine reversibly elicited bursts of potential in the central snail neuron. The ropivacaine-elicited bursts of potential were associated with phospholipase C activity in the RP4 snail neuron. Our results suggest that ropivacaine-induced neurotoxicity is highly associated with phospholipase C activity and phospholipase C inhibitor may offer a novel therapeutic approach for managing local anesthetic-induced convulsion or other transient neurologic toxicity.

Metabotropic glutamate receptors and epilepsy

Metabotropic glutamate receptors (mGluRs) play an important role in the initiation of ictal discharges by participating in the interictal-ictal transition, and may play a crucial role in recruiting normal brain tissue into synchronized discharges, thereby facilitating propagation of seizure activity. In this article we present a review of mGluRs and epilepsy studies. Structural features of mGluRs offer multiple possibilities for synthetic compounds to modulate their activity, and for many reasons these compounds are good candidates for therapeutic applications. Group I mGluRs enhance excitatory transmission as much as groups II and III mGluRs can modulate those effects. Finally, main avenues to induce epileptogenesis are considered: activation of Ca2+ channels and Ca2+/CaMKII cascade, overexpression of AMPA and/or KA receptors, enhanced NMDARs function, activation of protooncogenes leading to a steady epileptogenic state, enhancement of INaP currents, blockade of A and/or M K(+) currents, calcium channelopathies, diminished number of GABARs or functions, and down-regulation of glutamate transporters. Deregulation of mGluR signaling functions including deficits in groups II and III mGluRs or hyperactivation of group I mGluRs may occur in some forms of epilepsy, therefore targeting these mechanisms with specific pharmacological tools could provide new developments for original therapeutic approaches.

Functional Pharmacology in Human Brain

Most neurological and psychiatric disorders involve selective or preferential impairments of neurotransmitter systems. Therefore, studies of functional transmitter pathophysiology in human brain are of unique importance in view of the development of effective, mechanism-based, therapeutic modalities. It is well known that central nervous system functional proteins, including receptors, transporters, ion channels, and enzymes, can exhibit high heterogeneity in terms of structure, function, and pharmacological profile. If the existence of types and subtypes of functional proteins amplifies the possibility of developing selective drugs, such heterogeneity certainly increases the likelihood of interspecies differences. It is therefore essential, before choosing animal models to be used in preclinical pharmacology experimentation, to establish whether functionally corresponding proteins in men and animals also display identical pharmacological profiles. Because of evidence that scaffolding proteins, trafficking between plasma membrane and intracellular pools, phosphorylation and allosteric modulators can affect the function of receptors and transporters, experiments with human clones expressed in host cells where the environment of native receptors is rarely reproduced should be interpreted with caution. Thus, the use of neurosurgically removed fresh human brain tissue samples in which receptors, transporters, ion channels, and enzymes essentially retain their natural environment represents a unique experimental approach to enlarge our understanding of human brain processes and to help in the choice of appropriate animal models. Using this experimental approach, many human brain functional proteins, in particular transmitter receptors, have been characterized in terms of localization, function, and pharmacological properties.

Metabolic parameters of epilepsy: adjuncts to established antiepileptic drug therapy

Hughlings Jackson at the turn of the century defined epilepsy as a disorder originating in a "morbid nutrition" of the neuron. With the advances in modern neurochemistry, it is becoming increasingly clear that a chronic seizure predisposition or a lowering of the brain's discharge threshold can be demarcated by a number of biochemical markers. They include a tendency for an increased release of glutamate with or without GABAergic impairment, (intra)neural tissue alterations in water redistribution/osmolarity or other distortions of the cytoarchitecture, and an elevation of ionic calcium inside the cell. These changes are dominantly shared parameters of the seizure prone brain. Magnetic resonance spectroscopy (MRS) shows that cerebral levels of glutamate + glutamine (Glx) are increased interictally in epileptogenic regions in human partial epilepsy; other findings using this technique suggest damage to (cellular/mitochondrial) membranes, denoted by N-acetyl-aspartic acid (NAA) changes and a decreased energy capability. The merging of previous in vitro and ex vivo findings in neurophysiology and neurochemistry with magnetic resonance spectroscopy technology provides a powerful new methodology to interpret and to obtain clinical insight into the metabolic alterations that underlie an epileptogenic process. In this review some of these basic neurochemical and electrophysiological mechanisms are discussed. In addition, certain adjuncts to established antiepileptic drug therapy are suggested in the hope that over the long term they may help in correcting the primary metabolic deficits.

Update on the pathophysiology of the epilepsies

The pathophysiology of convulsive and non-convulsive epilepsies is discussed in its primary generalised forms. Focal, clinical and experimental epilepsies, with emphasis placed on the temporal lobe epilepsies (TLE) and their pathophysiologies are also reviewed. Neurotransmitters and neuromodulators and between them, the second messenger systems are considered in the generation, maintenance or inhibition of the epileptic discharge. Action mechanisms of the more classic antiepileptic drugs are briefly summarized along with the therapeutic strategies that might achieve the final control of abnormal discharges, including genetic control as a promising alternative in the current state of research. We emphasized the study of all type of glutamate and GABA receptors and their relation with mRNA editing in the brain. Some of the genetic studies which have been so fruitful during the last ten years and which have brought new insights regarding the understanding of epileptic syndromes are summarized in this article.

Neuroactive amino acids in focally epileptic human brain: a review

Studies of neuroactive amino acids and their regulatory enzymes in surgically excised focally epileptic human brain are reviewed. Concentrations of glutamate, aspartate and glycine are significantly increased in epileptogenic cerebral cortex. The activities of the enzymes, glutamate dehydrogenase and aspartate aminotransferase, involved in glutamate and aspartate metabolism are also increased. Polyamine synthesis is enhanced in epileptogenic cortex and may contribute to the activation of N-methyl-D-aspartate (NMDA) receptors. Nuclear magnetic resonance spectroscopy (NMRS) reveals that patients with poorly controlled complex partial seizures have a significant diminution in occipital lobe gamma aminobutyric acid (GABA) concentration. The activity of the enzyme GABA-aminotransaminase (GABA-T) which catalyzes GABA degradation is not altered in epileptogenic cortex. NMRS studies show that vigabatrin, a GABA-T inhibitor and effective antiepileptic, significantly increases brain GABA. Glutamate decarboxylase (GAD), responsible for GABA synthesis, is diminished in interneurons in discrete regions of epileptogenic cortex and hippocampus. In vivo microdialysis performed in epilepsy surgery patients provides measurements of extracellular amino acid levels during spontaneous seizures. Glutamate concentrations are higher in epileptic hippocampi and increase before seizure onset reaching potentially excitotoxic levels. Frontal or temporal cortical epileptogenic foci also release aspartate, glutamate and serine particularly during intense seizures or status epilepticus. GABA in contrast, exhibits a delayed and feeble rise in the epileptic hippocampus possibly due to a reduction in the number and/or efficiency of GABA transporters.

AMPA receptor activation regulates the glutamate metabotropic receptor stimulated phosphatidylinositol turnover in human cerebral cortex slices

The effect of excitatory amino acids (EAA) on phosphatidylinositol (PI) turnover in human cerebral cortical slices was investigated. Trans-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) increased inositol phosphate (IP) formation in the 1-1000 microM range. Quisqualic acid (QA) was maximally effective at 10-100 microM, showing an inverse correlation between concentration and effect in the 100-1000 microM range. The glutamate metabotropic receptor antagonist 2-amino-3-phosphonopropionic acid (AP3), the ionotropic non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and the NMDA channel blocker dizolcipine (MK-801) failed to prevent the PI response to ACPD (1000 microM). However, CNQX (100 microM) modified the concentration-response curve of QA reducing the effect of QA 10 microM by approx. 50% and enhancing that of QA 1000 microM by 2-fold. In addition, CNQX (100 microM) together with MK-801 (100 microM) unmasked the ability of L-glutamate (L-GLU) 3000 microM to stimulate PI turnover. The effect of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) on the EAA-induced PI turnover was also studied. AMPA (0.1-1 microM) potentiated the response to submaximal (30 microM) ACPD and (1 microM) QA concentrations. However, higher AMPA concentrations (10 microM) failed to synergize with ACPD 30 microM and, in addition, inhibited the PI turnover maximally stimulated by QA 10 microM. These results further support the presence of the glutamate metabotropic receptor in the human neocortex. In addition, they show the occurrence of a concentration-related dual interaction between AMPA and glutamate metabotropic receptor activation in the IP formation in this brain area.

Roles of second-messenger systems and neuronal activity in the regulation of lordosis by neurotransmitters, neuropeptides, and estrogen: a review

Many neurotransmitters and neuropeptides can affect the rodent feminine sexual behavior, lordosis, when administered in the ventromedial hypothalamus (VMH), midbrain central gray (MCG), or other brain regions. A survey of the electrophysiological and biochemical actions of these neural agents revealed that there is a very consistent association between lordosis facilitation with both the activation of the phosphoinositide (PI) pathway and the excitation of VMH and MCG neurons. In contrast, lordosis inhibition is associated, less consistently, with alterations of the adenylate cyclase (AC) system and the inhibition of neuronal activity. The findings that lordosis could be facilitated by going beyond membrane receptors and directly activating the PI pathway, suggest that this second-messenger pathway is a common mediator for the lordosis-facilitating agents. Furthermore, as in the case of stimulating membrane receptors, direct activation of this common mediator also requires estrogen priming for lordosis facilitation. Therefore, it is likely that the PI pathway is modulated by estrogen in the permissive action of estrogen priming. Indeed, a literature review shows that estrogen can affect selective isozymes of key enzyme families of the PI pathway at various levels. Such selective modulations, at several levels, could easily alter the course of a PI cascade; thence, the eventual functional outcome. These findings prompt us to propose that estrogen enables lordosis to be facilitated by a selective modulation of the PI pathway.

Functional studies of neurotransmitter receptors in human brain

Understanding synaptic transmission in the human brain is of the uppermost importance due to the involvement of neurotransmitters in several neurological and psychiatric disorders. Studies of animal pharmacology and of molecular biology are revealing that transmitter receptors are highly heterogeneous. It is therefore essential, also in view of using animal models in the development of therapeutically useful drugs, to establish if functionally corresponding receptors in men and animals also display identical pharmacological profiles. Using human brain tissue samples removed during neurosurgery and monitoring transmitter release as a functional response, a number of neurotransmitter receptors have been identified, localized and pharmacologically characterized as types and subtypes.

Excitotoxicity and neurological disorders: involvement of membrane phospholipids

Excitatory amino acids and their receptors play an important role in membrane phospholipid metabolism. Persistent stimulation of excitatory amino acid receptors by glutamate may be involved in neurodegenerative diseases and brain and spinal cord trauma. The molecular mechanism of neurodegeneration induced by excitatory amino acids is, however, not known. Excitotoxin-induced calcium entry causes the stimulation of phospholipases and lipases. These enzymes act on neural membrane phospholipids and their stimulation results in accumulation of free fatty acids, diacylglycerols, eicosanoids, and lipid peroxides in neurodegenerative diseases and brain and spinal cord trauma. Other enzymes, such as protein kinase C and calcium-dependent proteases, may also contribute to the neuronal injury. Excitotoxin-induced alterations in membrane phospholipid metabolism in neurodegenerative diseases and neural trauma can be studied in animal and cell culture models. These models can be used to study the molecular mechanisms of the neurodegenerative processes and to screen the efficacy of therapeutic drugs.