Scavenging of highly reactive gamma-ketoaldehydes attenuates cognitive dysfunction associated with epileptogenesis

Cognitive dysfunction is a major comorbidity of the epilepsies; however, treatments targeting seizure-associated cognitive dysfunction, particularly deficits in learning and memory are not available. Isoketals and neuroketals, collectively known as gamma-ketoaldehydes are formed via the non-enzymatic, free radical catalyzed oxidation of arachidonic acid and docosahexaenoic acid, respectively. They are attractive candidates for oxidative protein damage and resultant cognitive dysfunction due to their formation within the plasma membrane and their high proclivity to form cytotoxic adducts on protein lysine residues. We tested the hypothesis that gamma-ketoaldehydes mechanistically contribute to seizure-associated memory impairment using a specific gamma-ketoaldehyde scavenger, salicylamine in the kainic acid and pilocarpine rat models of temporal lobe epilepsy. We show that gamma-ketoaldehydes are increased following epileptogenic injury in hippocampus and perirhinal cortex, two brain regions imperative for learning and memory. Treatment with an orally bioavailable, brain permeable scavenger, salicylamine attenuated 1) spatial memory deficits 2) reference memory deficits and 3) neuronal loss and astrogliosis in two mechanistically distinct models of epilepsy without affecting the epileptogenic injury or the development of chronic epilepsy. We have previously demonstrated that reactive oxygen species and the lipid peroxidation biomarkers, F₂-isoprostanes are produced following status epilepticus. However, which reactive species specifically mediate oxidative damage to cellular macromolecules remains at large. We provide novel data suggesting that memory impairment occurs via gamma-ketoaldehyde production in two models of epilepsy and that treatment with a gamma-ketoaldehyde scavenger can protect vulnerable neurons. This work suggests a novel target and therapy to treat seizure-induced memory deficits in epilepsy.

The role of oxidative stress in organophosphate and nerve agent toxicity

Organophosphate (OP) nerve agents exert their toxicity through inhibition of acetylcholinesterase. The excessive stimulation of cholinergic receptors rapidly causes neuronal damage, seizures, death, and long-term neurological impairment in those that survive. Owing to the lethality of organophosphorus agents and the growing risk they pose, medical interventions that prevent OP toxicity and the delayed injury response are much needed. Studies have shown that oxidative stress occurs in models of subacute, acute, and chronic exposure to OP agents. Key findings of these studies include alterations in mitochondrial function and increased free radical-mediated injury, such as lipid peroxidation. This review focuses on the role of reactive oxygen species in OP neurotoxicity and its dependence on seizure activity. Understanding the sources, mechanisms, and pathological consequences of OP-induced oxidative stress can lead to the development of rational therapies for treating toxic exposures.

Reactive oxygen species mediate cognitive deficits in experimental temporal lobe epilepsy

Cognitive dysfunction is an important comorbidity of temporal lobe epilepsy (TLE). However, no targeted therapies are available and the mechanisms underlying cognitive impairment, specifically deficits in learning and memory associated with TLE remain unknown. Oxidative stress is known to occur in the pathogenesis of TLE but its functional role remains to be determined. Here, we demonstrate that oxidative stress and resultant processes contribute to cognitive decline associated with epileptogenesis. Using a synthetic catalytic antioxidant, we show that pharmacological removal of reactive oxygen species (ROS) prevents 1) oxidative stress, 2) deficits in mitochondrial oxygen consumption rates, 3) hippocampal neuronal loss and 4) cognitive dysfunction without altering the intensity of the initial status epilepticus (SE) or epilepsy development in a rat model of SE-induced TLE. Moreover, the effects of the catalytic antioxidant on cognition persisted beyond the treatment period suggestive of disease-modification. The data implicate oxidative stress as a novel mechanism by which cognitive dysfunction can arise during epileptogenesis and suggest a potential disease-modifying therapeutic approach to target it.

Specific alterations in the performance of learning and memory tasks in models of chemoconvulsant-induced status epilepticus

Cognitive impairment is a common comorbidity in patients with Temporal Lobe Epilepsy (TLE). These impairments, particularly deficits in learning and memory, can be recapitulated in chemoconvulsant models of TLE. Here, we used two relatively low-stress behavioral paradigms, the novel object recognition task (NOR) and a spatial variation, the novel placement recognition task (NPR) to reveal deficits in short and long term memory, in both kainic acid (KA) and pilocarpine (Pilo) treated animals. We found that both KA- and Pilo-induced significant deficits in long term recognition memory but not short term recognition memory. Additionally, KA impaired spatial memory as detected by both NPR and Morris water maze. These deficits were present 1 week after SE. The characterization of memory performance of two chemoconvulsant-models, one of which is considered a surrogate organophosphate, provides an avenue for which targeted cognitive therapeutics can be tested.

Ultrasonic vocalizations during male-female interaction in the mouse model of Down syndrome Ts65Dn

Down syndrome (DS) is the leading cause of genetically defined intellectual disability. Although speech and language impairments are salient features of this disorder, the nature of these phenotypes and the degree to which they are exacerbated by concomitant oromotor dysfunction and/or hearing deficit are poorly understood. Mouse models like Ts65Dn, the most extensively used DS animal model, have been critical to understanding the genetic and developmental mechanisms that contribute to intellectual disability. In the present study, we characterized the properties of the ultrasonic vocalizations (USVs) emitted by Ts65Dn males during courtship episodes with female partners. USVs emitted by mice in this setting have been proposed to have some basic correlation to human speech. Data were collected and analyzed from 22 Ts65Dn mice and 22 of their euploid littermates. We found that both the minimum and maximum peak frequencies of Ts65Dn calls were lower than those produced by euploid mice, whereas the mean individual duration of "down" and "complex" syllable types was significantly longer. Peak, minimal and maximal, and the fundamental frequencies of short syllables generated by Ts65Dn mice were lower compared to those by euploid mice. Finally, Ts65Dn males made fewer multiple jumps calls during courtship and the mean total duration of their "arc", "u", and "complex" syllables was longer. We discuss the human correlates to these findings, their translational potential, and the limitations of this approach. To our knowledge, this is the first characterization of differences between adult Ts65Dn and euploid control mice with respect to USVs.

The effects of prenatal stress on alpha4 beta2 and alpha7 hippocampal nicotinic acetylcholine receptor levels in adult offspring

Prenatal stress in humans is associated with psychiatric problems in offspring such as anxiety, depression, and schizophrenia. These same illnesses are also associated with neuronal nicotinic acetylcholine receptor (nAChR) dysfunction. Despite the known associations between prenatal stress exposure and offspring mental illness, and between mental illness and nAChR dysfunction, it is not known whether prenatal stress exposure impacts neuronal nAChRs. Thus, we tested the hypothesis that maternal stress alters the development of hippocampal alpha4 beta2 (α4β2∗) and alpha7 (α7∗) nicotinic receptor levels in adult offspring. Female Sprague-Dawley rats experienced unpredictable variable stressors two to three times daily during the last week of gestation. At weaning (21 days) the offspring of prenatally stressed (PS) and nonstressed (NS) dams were assigned to same-sex PS or NS groups. In young adulthood (56 days), the brains of offspring were collected and adjacent sections processed for quantitative autoradiography using [125I]-epibatidine (α4β2* nicotinic receptor-selective) and [125I]-α-bungarotoxin (α-BTX; α7* nicotinic receptor-selective) ligands. We found that PS significantly increased hippocampal α4β2* nAChRs of males and females in all subfields analyzed. In contrast, only females showed a trend toward PS-induced increases in α7* nAChRs in the dentate gyrus. Interestingly, NS females displayed a significant left-biased lateralization of α7* nAChRs in the laconosum moleculare of area CA1, whereas PS females did not, suggesting that PS interfered with normal lateralization patterns of α7* nAChRs during development. Taken together, our results suggest that PS impacts the development of hippocampal nAChRs, which may be an important link between PS exposure and risk for neuropsychiatric illness.

Maternal stress during pregnancy causes sex-specific alterations in offspring memory performance, social interactions, indices of anxiety, and body mass

Prenatal stress (PS) impairs memory function; however, it is not clear whether PS-induced memory deficits are specific to spatial memory, or whether memory is more generally compromised by PS. Here we sought to distinguish between these possibilities by assessing spatial, recognition and contextual memory functions in PS and nonstressed (NS) rodents. We also measured anxiety-related and social behaviors to determine whether our unpredictable PS paradigm generates a behavioral phenotype comparable to previous studies. Female Sprague-Dawley rats were exposed to daily random stress during the last gestational week and behavior tested in adulthood. In males but not females, PS decreased memory for novel objects and novel spatial locations, and facilitated memory for novel object/context pairings. In the elevated zero maze, PS increased anxiety-related behavior only in females. Social behaviors also varied with sex and PS condition. Females showed more anogenital sniffing regardless of stress condition. In contrast, prenatal stress eliminated a male-biased sex difference in nonspecific bodily sniffing by decreasing sniffing in males, and increasing sniffing in females. Finally, PS males but not females gained significantly more weight across adulthood than did NS controls. In summary, these data indicate that PS differentially impacts males and females resulting in sex-specific adult behavioral and bodily phenotypes.

Regulation of the high-affinity ammonium transporter (BnAMT1;2) in the leaves of Brassica napus by nitrogen status

Substantial concentrations of NH4+ are found in the apoplast of the leaves of Brassica napus. Physiological studies on isolated mesophyll protoplasts with 15NH4+ revealed the presence of a high-affinity ammonium transporter that shared physiological similarity to the high-affinity NH4+ transporters in Arabidopsis thaliana (AtAMT1;3). PCR techniques were used to isolate a full-length clone of a B. napus homologue of AMT1 from shoot mRNA which showed 97% similarity to AtAMT1;3. The full-length cDNA when cloned into the yeast expression vector pFL61 was able to complement a yeast mutant unable to grow on media with NH4+ as the sole nitrogen source. Regulatory studies with detached leaves revealed a stimulation of both NH4+ uptake and expression of mRNA when the leaves were supplied with increasing concentrations of NH4+. Withdrawal of NH4+ supply for up to 96 h had little effect on mRNA expression or NH4+ uptake; however, plants grown continuously at high NH4+ levels exhibited decreased mRNA expression. BnAMT1;2 mRNA expression was highest when NH4+ was supplied directly to the leaf and lowest when either glutamine or glutamate was supplied to the leaves, which directly paralleled chloroplastic glutamine synthetase (GS2) activity in the same leaves. These results provide tentative evidence that BnAMT1;2 may be regulated by similar mechanisms to GS2 in leaves.

Scutellospora calospora (Nicol. & Gerd.) Walker & Sanders associated with subterranean clover produces non-infective hyphae during sporulation

Subterranean clover (Trifolium subterraneum L.) inoculated with either Scutellospora calospora (Nicol. & Gerd.) Walker & Sanders or Glomus sp. (WUM 10(1)) was grown in 3 kg of Lancelin sand surrounding a fine mesh bag (< 38 μm) filled with 400 g of steamed sand. The mesh size was chosen to allow hyphae to penetrate but to exclude plant roots. Every week, from 3 wk after planting the original plants, subterranean clover seeds were sown into the sand inside the mesh bags. The last sowing date was 8 wk from sowing of the original plants. The seedlings, with their roots growing inside the mesh bags, were allowed to grow for 3 wk. At that stage the seedlings and the original plants were harvested and their roots assessed for mycorrhizal colonization. The percentage of root length of the original plants colonized by S. calospora decreased from the first harvest onwards, while colonization by Glomus sp. maintained a similar level at all harvests. Seedlings growing in pots whose original plants were colonized by S. calospora had a her percentage of root length colonized at the first two harvests but levels of Colonization decreased substantially thereafter. Seedlings in the Glomus sp. treatment on the other hand, had similar levels of colonization at all but the last harvest. The length of hyphae g^-1 soil in the mesh hues increased throughout the experiment for both fungi. However, the length of hyphae in the S. calospora treatment increased at a faster rate. Spore production by S. calospora commenced 7 wk after sowing the original plants. These results indicate that the hyphae produced by S. colospora after internal colonization had stopped increasing were largely non-infective, and that these hyphae are primarily involved in the production of spores.

Phosphorus, soluble carbohydrates and the competition between two arbuscular mycorrhizal fungi colonizing subterranean clover

We investigated the competition between two arbuscular mycorrhizal fungi, Glomus sp. [isolate WUM 10(1)] and Scutellospora calospora [isolate WUM 12(3)] during the colonization of subterranean clover at four concentrations of soil phosphorus (P). When the clover seedlings were 4 wks old we collected data for plant growth, P uptake, mycorrhizal colonization and soluble carbohydrate status of the roots. The percent and total colonization by Glomus sp. was reduced to a much greater extent than S. calospora when the soil P status was increased. When both fungi were inoculated together, S. calospora was unaffected by the presence of Glomus sp. or the P status of the soil; Glomus sp., however, was reduced to a far greater extent when in the presence of S. calospora than compared to when it was inoculated alone, Associated with the decreases in colonization by Glomus sp., when inoculated alone, with S. calospara and with increases in the level of P applied to the soil, were decreases in the concentration of soluble carbohydrates within the root. We propose that root carbohydrates directly influence the outcome of competition between these two isolates during the colonization of subterranean clover.

Scutellospora calospora (Nicol. & Gerd.) Walker & Sanders associated with subterranean clover: dynamics of colonization, sporulation and soluble carbohydrates

Two time-course experiments were conducted to investigate the relationship between colonization and sporulation by Scutellospora calospora (Nicol. & Gerd.) Walker & Sanders colonizing roots of subterranean clover (Trifolium subterraneum L.). The percentage of root length colonized by S. calospora increased up to 49 d in expt 1 and 32 d in expt 2 after which it declined. At this stage, there was no further increase in mycorrhizal root length but a sharp increase in non-mycorrhizal root length. In expt 1, the soluble carbohydrate concentration of the roots decreased during the first 49 d, followed by a sudden increase, coinciding with the decline in the percentage of root length colonized. One week before this the shoot P inflow also decreased. In both experiments, sporulation by S. calospora occurred after the decline in colonization. During sporulation there is no further spread in colonization within the root, which may result in increases in soluble carbohydrate concentration within the root. Spore formation may be aided by carbon stores located in the external auxiliary bodies formed by species of the genus Scutellospora.