Effect of repeated seizure experiences on tyrosine hydroxylase immunoreactivities in the brain of genetically epilepsy-prone rats

Chemo-kindling in adult zebrafish alters spatial cognition but not social novelty recognition

In the past decades, zebrafish have gathered immense attention and importance in the field of neurological sciences. In the case of epilepsy, zebrafish have appeared as a promising acute animal model for the screening and identification of potential antiepileptic molecules. However, the necessity for establishing competent chronic models of epilepsy in zebrafish is apparent. In this regard, recently we developed a chemo-kindling zebrafish model with a better clinical resemblance. In the present study, an attempt to examine the effect of pentylenetetrazole (PTZ)-induced kindling on the cognitive functions of zebrafish was made. In brief, adult zebrafish were repetitively given a sub-effective concentration of PTZ, till the onset of clonic-tonic seizures, entitled as kindled. Thereafter, T-maze test and social recognition memory test were conducted to evaluate spatial memory and social novelty recognition memory of the fish. At the end, both the groups were sacrificed and the brains were isolated to estimate neurotransmitter and gene expression levels. It was observed that PTZ kindling induced spatial cognition deficits and lower social exploration in zebrafish. However, it didn't change the novelty recognition memory of kindled zebrafish. The results of genes and neurotransmitters estimations in the brain also supported the behavioural findings. The results concluded that PTZ kindling alters spatial cognitive functions in adult zebrafish without affecting the social novelty recognition memory.

Diacylglycerol Kinase-ε: Properties and Biological Roles

In mammals there are at least 10 isoforms of diacylglycerol kinases (DGK). All catalyze the phosphorylation of diacylglycerol (DAG) to phosphatidic acid (PA). Among DGK isoforms, DGKε has several unique features. It is the only DGK isoform with specificity for a particular species of DAG, i.e., 1-stearoyl-2-arachidonoyl glycerol. The smallest of all known DGK isoforms, DGKε, is also the only DGK devoid of a regulatory domain. DGKε is the only DGK isoform that has a hydrophobic segment that is predicted to form a transmembrane helix. As the only membrane-bound, constitutively active DGK isoform with exquisite specificity for particular molecular species of DAG, the functional overlap between DGKε and other DGKs is predicted to be minimal. DGKε exhibits specificity for DAG containing the same acyl chains as those found in the lipid intermediates of the phosphatidylinositol-cycle. It has also been shown that DGKε affects the acyl chain composition of phosphatidylinositol in whole cells. It is thus likely that DGKε is responsible for catalyzing one step in the phosphatidylinositol-cycle. Steps of this cycle take place in both the plasma membrane and the endoplasmic reticulum membrane. DGKε is likely present in both of these membranes. DGKε is the only DGK isoform that is associated with a human disease. Indeed, recessive loss-of-function mutations in DGKε cause atypical hemolytic-uremic syndrome (aHUS). This condition is characterized by thrombosis in the small vessels of the kidney. It causes acute renal insufficiency in infancy and most patients develop end-stage renal failure before adulthood. Disease pathophysiology is poorly understood and there is no therapy. There are also data suggesting that DGKε may play a role in epilepsy and Huntington disease. Thus, DGKε has many unique molecular and biochemical properties when compared to all other DGK isoforms. DGKε homologs also contain a number of conserved sequence features that are distinctive characteristics of either the rodents or specific groups of primate homologs. How cells, tissues and organisms harness DGKε's catalytic prowess remains unclear. The discovery of DGKε's role in causing aHUS will hopefully boost efforts to unravel the mechanisms by which DGKε dysfunction causes disease.

Diacylglycerol Kinase-ε: Properties and Biological Roles

In mammals there are at least 10 isoforms of diacylglycerol kinases (DGK). All catalyze the phosphorylation of diacylglycerol (DAG) to phosphatidic acid (PA). Among DGK isoforms, DGKε has several unique features. It is the only DGK isoform with specificity for a particular species of DAG, i.e., 1-stearoyl-2-arachidonoyl glycerol. The smallest of all known DGK isoforms, DGKε, is also the only DGK devoid of a regulatory domain. DGKε is the only DGK isoform that has a hydrophobic segment that is predicted to form a transmembrane helix. As the only membrane-bound, constitutively active DGK isoform with exquisite specificity for particular molecular species of DAG, the functional overlap between DGKε and other DGKs is predicted to be minimal. DGKε exhibits specificity for DAG containing the same acyl chains as those found in the lipid intermediates of the phosphatidylinositol-cycle. It has also been shown that DGKε affects the acyl chain composition of phosphatidylinositol in whole cells. It is thus likely that DGKε is responsible for catalyzing one step in the phosphatidylinositol-cycle. Steps of this cycle take place in both the plasma membrane and the endoplasmic reticulum membrane. DGKε is likely present in both of these membranes. DGKε is the only DGK isoform that is associated with a human disease. Indeed, recessive loss-of-function mutations in DGKε cause atypical hemolytic-uremic syndrome (aHUS). This condition is characterized by thrombosis in the small vessels of the kidney. It causes acute renal insufficiency in infancy and most patients develop end-stage renal failure before adulthood. Disease pathophysiology is poorly understood and there is no therapy. There are also data suggesting that DGKε may play a role in epilepsy and Huntington disease. Thus, DGKε has many unique molecular and biochemical properties when compared to all other DGK isoforms. DGKε homologs also contain a number of conserved sequence features that are distinctive characteristics of either the rodents or specific groups of primate homologs. How cells, tissues and organisms harness DGKε's catalytic prowess remains unclear. The discovery of DGKε's role in causing aHUS will hopefully boost efforts to unravel the mechanisms by which DGKε dysfunction causes disease.

Role of oxidative stress in epileptic seizures

Oxidative stress resulting from excessive free-radical release is likely implicated in the initiation and progression of epilepsy. Therefore, antioxidant therapies aimed at reducing oxidative stress have received considerable attention in epilepsy treatment. However, much evidence suggests that oxidative stress does not always have the same pattern in all seizures models. Thus, this review provides an overview aimed at achieving a better understanding of this issue. We summarize work regarding seizure models (i.e., genetic rat models, kainic acid, pilocarpine, pentylenetetrazol, and trimethyltin), oxidative stress as an etiologic factor in epileptic seizures (i.e., impairment of antioxidant systems, mitochondrial dysfunction, involvement of redox-active metals, arachidonic acid pathway activation, and aging), and antioxidant strategies for seizure treatment. Combined, this review highlights pharmacological mechanisms associated with oxidative stress in epileptic seizures and the potential for neuroprotection in epilepsy that targets oxidative stress and is supported by effective antioxidant treatment.

The WAG/Rij strain: a genetic animal model of absence epilepsy with comorbidity of depression [corrected]

A great number of clinical observations show a relationship between epilepsy and depression. Idiopathic generalized epilepsy, including absence epilepsy, has a genetic basis. The review provides evidence that WAG/Rij rats can be regarded as a valid genetic animal model of absence epilepsy with comorbidity of depression. WAG/Rij rats, originally developed as an animal model of human absence epilepsy, share many EEG and behavioral characteristics resembling absence epilepsy in humans, including the similarity of action of various antiepileptic drugs. Behavioral studies indicate that WAG/Rij rats exhibit depression-like symptoms: decreased investigative activity in the open field test, increased immobility in the forced swimming test, and decreased sucrose consumption and preference (anhedonia). In addition, WAG/Rij rats adopt passive strategies in stressful situations, express some cognitive disturbances (reduced long-term memory), helplessness, and submissiveness, inability to make choice and overcome obstacles, which are typical for depressed patients. Elevated anxiety is not a characteristic (specific) feature of WAG/Rij rats; it is a characteristic for only a sub-strain of WAG/Rij rats susceptible to audiogenic seizures. Interestingly, WAG/Rij rats display a hyper-response to amphetamine similar to anhedonic depressed patients. WAG/Rij rats are sensitive only to chronic, but not acute, antidepressant treatments, suggesting that WAG/Rij rats fulfill a criterion of predictive validity for a putative animal model of depression. However, more and different antidepressant drugs still await evaluation. Depression-like behavioral symptoms in WAG/Rij rats are evident at baseline conditions, not exclusively after stress. Experiments with foot-shock stress do not point towards higher stress sensitivity at both behavioral and hormonal levels. However, freezing behavior (coping deficits) and blunted response of 5HT in the frontal cortex to uncontrollable sound stress, increased c-fos expression in the terminal regions of the meso-cortico-limbic brain systems and greater DA response of the mesolimbic system to forced swim stress suggest that WAG/Rij rats are vulnerable to some, but not to all types of stressors. We propose that genetic absence epileptic WAG/Rij rats have behavioral depression-like symptoms, are vulnerable to stress and might represent a model of chronic low-grade depression (dysthymia). Both 5HT and DAergic abnormalities detected in the brain of WAG/Rij rats are involved in modulation of vulnerability to stress and provocation of behavioral depression-like symptoms. The same neurotransmitter systems modulate SWDs as well. Recent studies suggest that the occurrence and repetition of absence seizures are a precipitant of depression-like behavior. Whether the neurochemical changes are primary to depression-like behavioral alterations remains to be determined. In conclusion, the WAG/Rij rats can be considered as a genetic animal model for absence epilepsy with comorbidity of dysthymia. This model can be used to investigate etiology, pathogenic mechanisms and treatment of a psychiatric comorbidity, such as depression in absence epilepsy, to reveal putative genes contributing to comorbid depressive disorder, and to screen novel psychotropic drugs with a selective and/or complex (dual) action on both pathologies.

Decrease in the kainate-induced wet dog shake behavior in genetically epilepsy-prone rats: possible involvement of an impaired synaptic transmission to the 5-HT(2A) receptor

Genetically epilepsy-prone rats (GEPR-9s) were derived from Sprague-Dawley rats (SD). The number of kainate-induced wet dog shake behavior (WDS) responses was found to decrease significantly in GEPR-9s compared to SD. WDS responses were potentiated by 5-hydroxytryptophan or 2,5-dimethoxy-4-iodoamphetamine and antagonized by ritanserin. The antagonizing effect of ritanserin on WDS latency was more evident in GEPR-9s than in SD, and hippocampal expression of activity-regulated cytoskeleton-associated protein paralleled the severity of WDS. The results suggest that downstream serotonergic synaptic activation is less pronounced in GEPR-9s than in SD and that the serotonergic agent may directly activate postsynaptic 5-HT2A receptors in both strains.

Reduced immunoreactivity of tyrosine hydroxylase in the hypothalamic paraventricular nucleus of the seizure sensitive gerbil

We compared the immunoreactivity and numbers of tyrosine hydroxylase (TH) immunoreactive neurons and neuropil in the paraventricular nucleus (PVN) of the hypothalamus between the seizure sensitive (SS) and seizure resistant (SR) gerbils. The distributional pattern of TH immunoreactivity was similar in both groups: TH immunoreactivity was seen mainly in magnocellular neurons of the PVN. However, total TH immunoreactivity in the neurons and neuropil in the SS gerbils was significantly lower than that in the SR gerbils. In addition, the number of TH immunoreactive neurons in the SS gerbils was also much lower than those in the SR gerbils. These results indicate that SS gerbils have a low TH immunoreactivity in the hypothalamic PVN compared with that in SR gerbils.

Tyrosine hydroxylase in rat auditory midbrain: distribution and changes following deafness

Tyrosine hydroxylase (TH), a key enzyme in the catecholaminergic pathway, allows for the differentiation of dopaminergic neurons. We previously showed decreases in TH gene expression in the rat inferior colliculus (IC) 3 and 21 days following deafness. In the present study, we characterized the normal distribution of TH as well as changes following deafness (bilateral cochlear ablation) in the IC and nuclei of the lateral lemniscus. Immunostaining was compared in three groups of rats: normal hearing (n=8), 21 day deaf (n=5) and 90 days following deafening (n=5). Many TH immunoreactive fibers and puncta were identified in the IC and nuclei of the lateral lemniscus of normal hearing animals and labeling was most dense in the external cortex of the IC. We also identified immunolabeling for fibers and puncta for another catecholaminergic enzyme, dopamine beta hydroxylase (DBH), but not phenylethanolamine-N-methyltranferase (PNMT). Neurons immunopositive for TH but not DBH or PNMT were observed in the dorsal cortex and dorsal horn of the central nucleus of the IC and ventral and intermediate lemniscus. In the central nucleus of the IC and dorsal lateral lemniscus many lightly labeled TH neurons were also DBH positive. Although the number of immunopositive cells in the IC and lemniscus declined 3 weeks and 3 months after deafening, the decline was not significant at three weeks in the VNLL nor after three months in the dorsal cortex. Immunolabeling for TH decreased significantly in IC and lemniscus 3 weeks and 3 months following deafening. These results suggest a role for dopaminergic neurons and fibers in deafness-related plasticity.

Epileptogenesis in diacylglycerol kinase epsilon deficiency up-regulates COX-2 and tyrosine hydroxylase in hippocampus

Diacylglycerol kinase (DGK) phosphorylates the second messenger diacylglycerol (DAG) to yield phosphatidic acid, two neural signaling elements that function to modulate synaptic activity. Of the nine mammalian DGK isotypes known, DGK epsilon (DGKepsilon) shows specificity for arachidonoyldiacylglycerol (20:4-DAG) and selectively contributes to modulate brain signaling pathways linked to synaptic activity and epileptic seizure activity. In this study, we examined changes in gene transcription in a mouse kindling model of epileptogenesis using control DGKepsilon (+/+) and DGKepsilon-knockout (-/-) mice. Total RNA was isolated from the hippocampus and analyzed using RNA and DNA arrays. Significantly altered gene-expression levels were confirmed independently using Western immunoblot analysis. In agreement with our previous studies, a very few number of genes reached a significance of twofold or greater (either up- or down-regulated; p<0.05). Among the most significantly up-regulated genes in DGKepsilon (+/+) mice included those encoding the inducible prostaglandin synthase cyclooxygenase-2 (COX-2) and tyrosine hydroxylase (TH), also known as tyrosine 3-monooxygenase, the rate-limiting enzyme of catecholamine biosynthesis. Kindled DGKepsilon (-/-) animals exhibited no large increases in COX-2 or TH gene expression. These data, plus our previous findings that DGKepsilon (-/-) mice show higher resistance to electroconvulsive shock, suggest an interplay between and regulatory role for DGKepsilon, COX-2, and catecholamine signaling during kindling epileptogenesis.

Analytical characterization of a sensitive radioassay for tyrosine hydroxylase activity in rodent striatum

Several buffer compositions with a wide range of pH values have been reported for radiometric assay of tyrosine hydroxylase (TH) in biological samples. Assay sensitivity becomes a prime concern while analyzing TH in minute samples like tissue biopsies or discrete regions of rodent brain wherein lower enzyme levels are anticipated due to smaller sample sizes. It was therefore rationalized to evaluate relative affinities of three commonly used assay buffers (sodium phosphate, sodium acetate, and Tris-acetate) with TH enzyme activity. The impact of buffer pH and cofactor concentration on the sensitivity of TH assay was also investigated. Striata from rats or mice were homogenized, respectively, with 1.0 or 0.5 ml of the assay buffer containing 0.5% Triton X-100. The supernatants (200 microl) were incubated (20 min, 37 degrees C) with 0.8 microCi [3H] L-tyrosine, 1.5 mM DL-6-methyl-5,6,7,8-tetrahydropterine (6-MPH4), 100 U catalase, and 1.0 microM dithiothreitol in a total volume of 300 microl. The reaction was terminated by 1-ml suspension of activated charcoal in 0.1 M HCl. After centrifugation, 200-microl aliquots were mixed with 5 ml of cocktail for quantitation of [3H] H2O in supernatant. The results showed significant impact of pH rather than the buffer composition on the sensitivity of TH assay. An optimal pH range was found to be 5.5-6.0, whereas TH activity was significantly inhibited at pH 5.0 and pH 6.8 (F = 55.09, P = 0.000). A significantly high TH activity was observed with 1.5 mM 6-MPH4, whereas higher concentrations (3.0-4.5 mM) inhibited TH activity (F = 7.47, P = 0.005). Analysis of serially diluted striatal homogenates showed a significant correlation between TH activity and sample amount. The assay reaction was linear for 20- and 30-min incubation for rat and mice striata, respectively.

Common pathogenic mechanisms between depression and epilepsy: an experimental perspective

Affective disorders and the epilepsies appear to share partially similar pathogenic mechanisms. Predisposition to both disorders is determined genetically and experientially. A component of the shared predisposition appears to arise from noradrenergic and serotonergic deficits. Shared GABAergic deficits coupled with CRHergic and glutamatergic excesses may trigger and maintain seizures as well as dysfunctional affective episodes, albeit via dissimilar neuronal interplay.

Regional expression of multidrug resistance genes in genetically epilepsy-prone rat brain after a single audiogenic seizure

PURPOSE

The multidrug resistance (mdr) gene family encodes the drug transport macromolecule P-glycoprotein (P-gp), which contributes to the functionality of the blood-brain barrier. Recent evidence suggests that P-gp-mediated drug extrusion may play a facilitatory role in refractory epilepsy. We investigated the regional expression of mdr genes in genetically epilepsy-prone rat (GEPR) brain after a single audiogenic seizure.

METHODS

Three groups of adult male GEPRs (n = 5/group) were exposed to a seizure-inducing audiogenic stimulus and killed at 4 h, 24 h, and 7 days thereafter. A further group (n = 5) served as a stimulus-naïve control. Expression of mdr1a and mdr1b in distinct anatomic brain regions (cortex, midbrain, pons/medulla, hippocampus) was determined by quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) in the presence of competitive internal standards.

RESULTS

When compared with control, mdr1a expression in cortex and midbrain was significantly (p < 0.05) increased at 24 h after a single audiogenic seizure. Cortical mdr1a expression remained elevated at 7 days after stimulus. In contrast, mdr1a expression in pons/medulla and hippocampus was unchanged. The mdr1b isoform was quantifiable in hippocampus alone and not influenced by seizure activity.

CONCLUSIONS

These findings suggest that acute seizures in the GEPR can induce the expression of mdr genes. The pattern of increased expression appears to follow the anatomic pathway of audiogenic seizures in these animals, with initiation in the midbrain and propagation to the cortex. Further studies are required to investigate the effects of recurrent seizure activity and to characterise mdr expression in other experimental seizure models.

Abnormal postnatal ontogeny of the locus coeruleus in the epileptic mutant mouse quaking

The tonic-clonic convulsions of the quaking mutant mice have been shown to be associated with the hyperplasia of the nucleus locus coeruleus, the origin of most brain noradrenergic neurons. In the present study, the postnatal ontogeny of the locus coeruleus has been studied by tyrosine hydroxylase immunolabeling in the mutant mice quaking and their controls at postnatal days 1, 30 and 90. In the control mice, the number of immunoreactive neuronal cell bodies increased significantly in the rostral half of the locus coeruleus between birth and postnatal day 30, while it decreased significantly in the caudal half between birth and adulthood. Thus, during postnatal maturation, the distribution of locus coeruleus neurons was shifted in the rostral direction. In the quaking mutant mice, while the increase of immunolabeling between birth and postnatal day 30 was observed in the rostral half of the locus coeruleus, no diminution could be found in the caudal half between birth and adulthood. As a result, the rostral shift of tyrosine hydroxylase immunoreactivity was not observed. Consequently, in adult mice, the caudal part of the mutants locus coeruleus appeared to contain significantly more neurons than the corresponding region in the controls. These results indicate that the hyperplasia of the locus coeruleus of the quaking mice that we had previously reported results from an alteration of the postnatal maturation of this nucleus. This developmental abnormality might be a primary determinant of the inherited epilepsy of the quaking mutant mice.