Domoic acid-induced hippocampal CA1 hyperexcitability independent of region CA3 activity

Development of an LC-MS/MS method for simultaneous quantitative analysis of macromolecular pharmaceutical adjuvant 2-hydroxypropyl-β-cyclodextrin and active pharmaceutical ingredients butylphthalide in rat plasma

Hydroxypropyl-β-cyclodextrin, which possesses a high water solubility and low hemolycity, is widely used as a solubilizer and an excipient. It had also been reported that hydroxypropyl-β-cyclodextrin has the activity of regulating lipid homeostasis. In order to further understand the metabolism, the primary focus was to establish a quantitative method for hydroxypropyl-β-cyclodextrin. The analytes were extracted from plasma by protein precipitation with methanol and then carried out on a Waters CORTECS T3 column in the gradient elution of pure water and methanol. Finally, liquid chromatography-tandem mass spectrometry was applied in multiple reaction monitoring mode to complete the quantitative analysis of hydroxypropyl-β-cyclodextrin. This validated method had been successfully applied to investigate the interaction between hydroxypropyl-β-cyclodextrin and butylphthalide in vivo by optimizing the extraction reagent, simplifying the experimental procedure, and improving the sensitivity while considering the difference of drug chemical properties. Results showed that the inclusion of hydroxypropyl-β-cyclodextrin with butylphthalide significantly improved the pharmacokinetic behavior of free body hydroxypropyl-β-cyclodextrin and 3-n-butylphthalide in vivo. It had been implied that the metabolism of hydroxypropyl-β-cyclodextrin and the drug active ingredients could impact each other. It will help better application of hydroxypropyl-β-cyclodextrin and the developed method might lay the foundation for development of hydroxypropyl-β-cyclodextrin as a treatment drug for brain diseases.

Repeated low level domoic acid exposure increases CA1 VGluT1 levels, but not bouton density, VGluT2 or VGAT levels in the hippocampus of adult mice

Domoic acid (DA) is a neurotoxin produced during harmful algal blooms that accumulates in marine organisms that serve as food resources for humans. While acute DA neurotoxicity can cause seizures and hippocampal lesions, less is known regarding how chronic, subacute DA exposure in adulthood impacts the hippocampus. With more frequent occurrences of harmful algal blooms, it is important to understand the potential impact of repeated, low-level DA exposure on human health. To model repeated, low-dose DA exposure, adult mice received a single low-dose (0.75 ± 0.05 μg/g) of DA or vehicle weekly for 22 consecutive weeks. Quantitative immunohistochemistry was performed to assess the effects of repeated, low-level DA exposure on hippocampal cells and synapses. Vesicular glutamate transporter 1 (VGluT1) immunoreactivity within excitatory boutons in CA1 of DA-exposed mice was increased. Levels of other vesicular transporter proteins (i.e., VGluT2 and the vesicular GABA transporter (VGAT)) within boutons, and corresponding bouton densities, were not significantly altered in CA1, CA3, or dentate gyrus. There were no significant changes in neuron density or glial fibrillary acidic protein (GFAP) immunoreactivity following chronic, low-dose exposure. This suggests that repeated low doses of DA, unlike high doses of DA, do not cause neuronal loss or astrocyte activation in hippocampus in adult mice. Instead, these findings demonstrate that repeated exposure to low levels of DA leads to subtle changes in VGluT1 expression within CA1 excitatory boutons, which may alter glutamatergic transmission in CA1 and disrupt behaviors dependent on spatial memory.

Quercetin attenuates domoic acid-induced cognitive deficits in mice

Domoic acid (DA) is one of the best known marine toxins, causative of important neurotoxic alterations. DA effects are documented both in wildlife and experimental assays, showing that this toxin causes severe injuries principally in the hippocampal area. Accumulating evidence indicates that mitochondrial dysfunction and oxidative stress are involved in DA-induced cognitive functional impairment. Therefore, therapeutics targeted to improve mitochondrial function and increase oxidative stress defence could be beneficial. Quercetin, a bioflavanoid, has been reported to have potent neuroprotective effects and anti-oxidative ability, but its preventive effects on DA-induced mitochondrial dysfunction and cognitive impairment have not been well characterised. In this study, we evaluated the effects of quercetin on DA-induced cognitive deficits in mice and explored its potential mechanism. Our results showed that the oral administration of quercetin to DA-treated mice significantly improved their behavioural performance in a novel objective recognition task and a Morris water maze task. These improvements were mediated, at least in part, by a stimulation of PPARγ coactivator 1α-mediated mitochondrial biogenesis signalling and an amelioration of mitochondrial dysfunction. Moreover, quercetin activated nuclear factorerythroid-2-related factor-2 (Nrf2)-mediated phase II enzymes and decreased reactive oxygen species and protein carbonylation. Furthermore, the AMP-activated protein kinase (AMPK) activity significantly increased in the quercetin-treated group. Taken together, these findings suggest that a reduction in mitochondrial dysfunction through the increase of AMPK activity, coupled with an increase in Nrf2 pathway mediated oxidative defence, may be one of the mechanisms by which quercetin improves cognitive impairment induced by DA in mice.

Hippocampal neuropathology of domoic acid-induced epilepsy in California sea lions (Zalophus californianus)

California sea lions (Zalophus californianus) are abundant human-sized carnivores with large gyrencephalic brains. They develop epilepsy after experiencing status epilepticus when naturally exposed to domoic acid. We tested whether sea lions previously exposed to DA (chronic DA sea lions) display hippocampal neuropathology similar to that of human patients with temporal lobe epilepsy. Hippocampi were obtained from control and chronic DA sea lions. Stereology was used to estimate numbers of Nissl-stained neurons per hippocampus in the granule cell layer, hilus, and pyramidal cell layer of CA3, CA2, and CA1 subfields. Adjacent sections were processed for somatostatin immunoreactivity or Timm-stained, and the extent of mossy fiber sprouting was measured stereologically. Chronic DA sea lions displayed hippocampal neuron loss in patterns and extents similar but not identical to those reported previously for human patients with temporal lobe epilepsy. Similar to human patients, hippocampal sclerosis in sea lions was unilateral in 79% of cases, mossy fiber sprouting was a common neuropathological abnormality, and somatostatin-immunoreactive axons were exuberant in the dentate gyrus despite loss of immunopositive hilar neurons. Thus, hippocampal neuropathology of chronic DA sea lions is similar to that of human patients with temporal lobe epilepsy.

Effects of HMG-CoA reductase inhibitors on learning and memory in the guinea pig

Statins reduce the risk of death from cardiovascular disease in millions of people worldwide. Recent pharmacovigilance data has suggested that people taking statins have an increased risk of psychiatric adverse events such as amnesia and anxiety. This study aimed to investigate the possibility of statin-induced amnesia through animal models of memory and learning. We conducted extracellular field recordings of synaptic transmission in area CA1 of hippocampal slices to examine the effects of acute cholesterol lowering with lipid lowering drugs. We also assessed the effect of six weeks of simvastatin (2mg/kg/d) and atorvastatin (1mg/kg/d) treatment using the Morris water maze. Long Term Potentiation (LTP) was significantly diminished in the presence of 3µM atorvastatin or simvastatin and by the cholesterol sequestering agent methyl-β-cyclodextrin (MBCD). The effects were reversed in the MBCD but not the statin treated slices by the addition of cholesterol. In the water maze, statin treatment did not cause any deficits in the first five days of reference memory testing, but statin treated guinea pigs preformed significantly worse than control animals in a working memory test. The deficits observed in our experiments in water maze performance and hippocampal LTP are suggestive of statin induced changes in hippocampal plasticity. The effects on LTP are independent of cholesterol regulation, and occur at concentrations that may be relevant to clinical use. Our results may help to explain some of the behavioural changes reported in some people after beginning statin treatment.

Enhanced expression of WD repeat-containing protein 35 (WDR35) stimulated by domoic acid in rat hippocampus: involvement of reactive oxygen species generation and p38 mitogen-activated protein kinase activation

Background

Domoic acid (DA) is an excitatory amino acid analogue of kainic acid (KA) that acts via activation of glutamate receptors to elicit a rapid and potent excitotoxic response, resulting in neuronal cell death. Recently, DA was shown to elicit reactive oxygen species (ROS) production and induce apoptosis accompanied by activation of p38 mitogen-activated protein kinase (MAPK) in vitro. We have reported that WDR35, a WD-repeat protein, may mediate apoptosis in several animal models. In the present study, we administered DA to rats intraperitoneally, then used liquid chromatography/ion trap tandem mass spectrometry (LC-MS/MS) to identify and quantify DA in the brains of the rats and performed histological examinations of the hippocampus. We further investigated the potential involvement of glutamate receptors, ROS, p38 MAPK, and WDR35 in DA-induced toxicity in vivo.

Results

Our results showed that intraperitoneally administered DA was present in the brain and induced neurodegenerative changes including apoptosis in the CA1 region of the hippocampus. DA also increased the expression of WDR35 mRNA and protein in a dose- and time-dependent manner in the hippocampus. In experiments using glutamate receptor antagonists, the AMPA/KA receptor antagonist NBQX significantly attenuated the DA-induced increase in WDR35 protein expression, but the NMDA receptor antagonist MK-801 did not. In addition, the radical scavenger edaravone significantly attenuated the DA-induced increase in WDR35 protein expression. Furthermore, NBQX and edaravone significantly attenuated the DA-induced increase in p38 MAPK phosphorylation.

Conclusion

In summary, our results indicated that DA activated AMPA/KA receptors and induced ROS production and p38 MAPK phosphorylation, resulting in an increase in the expression of WDR35 in vivo.

Ischemic cardiomyopathy following seizure induction by domoic Acid

Exposure to the excitotoxin domoic acid (DOM) has been shown to produce cardiac lesions in both clinical and animal studies. We have previously shown that DOM failed to directly affect cardiomyocyte viability and energetics, but the development of this cardiomyopathy has remained unexplained. The present study compared effects of high-level seizure induction obtained by intraperitoneal (2 mg/kg) or intrahippocampal (100 pmol) bolus administration of DOM on development of cardiac pathologies in a rat model. Assessment of cardiac pressure derivatives and coronary flow rates revealed a significant time-dependent decrease in combined left ventricular (LV) systolic and diastolic function at 1, 3, 7, and 14 days after intraperitoneal administration and at 7 and 14 days after intrahippocampal DOM administration. LV dysfunction was matched by a similar time-dependent decrease in mitochondrial respiratory control, associated with increased proton leakage, and in mitochondrial enzyme activities. Microscopic examination of the LV midplane revealed evidence of progressive multifocal ischemic damage within the subendocardial, septal, and papillary regions. Lesions ranged from reversible early damage (vacuolization) to hypercontracture and inflammatory necrosis progressing to fibrotic scarring. Plasma proinflammatory IL-1α, IL-1β, and TNF-α cytokine levels were also increased from 3 days after seizure induction. The observed cardiomyopathies did not differ between intraperitoneal and intrahippocampal groups, providing strong evidence that cardiac damage after DOM exposure is a consequence of a seizure-evoked autonomic response.

Domoic acid induces a long-lasting enhancement of CA1 field responses and impairs tetanus-induced long-term potentiation in rat hippocampal slices

Domoic acid (DOM) is known to cause hippocampal neuronal damage and produces amnesic effects. We examined synaptic plasticity changes induced by DOM exposure in rat hippocampal CA1 region. Brief bath application of DOM to hippocampal slices produces a chemical form of long-term potentiation (LTP) of CA1 field synaptic potentials. The potentiation cannot be blocked by NMDA receptor antagonist MK-801 but can be blocked by the calcium-calmodulin-dependent protein kinase II (CaMKII) inhibitor KN-62 or cAMP-dependent protein kinase (PKA) inhibitor H-89. DOM-potentiated slices show decreased autophosphorylated CaMKII (p-Thr286), an effect that is also dependent on the activity of CaMKII and PKA. Increased phosphorylation of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor subunit GluR1 (p-Ser831) was seen in DOM-potentiated slices. Therefore, aberrant regulation of CaMKII and GluR1 phosphorylation occurs after DOM application. In addition, tetanus-induced LTP as well as the increase of phosphorylation of CaMKII (p-Thr286) were reduced in DOM-potentiated slices. Compared with brief exposure, slices recovering from prolonged exposure did not show potentiation or altered levels of CaMKII (p-Thr286) or GluR (p-Ser831). However, decreased phosphorylation of GluR1 at Ser845 was seen. These results describe a new chemical form of LTP and uncover novel molecular changes induced by DOM. The observed impairment of tetanus LTP and misregulation of CaMKII and GluR1 phosphorylation may partially account for DOM neurotoxicity and underlie the molecular basis for DOM-induced memory deficit.

Kv4.2 knockout mice demonstrate increased susceptibility to convulsant stimulation

PURPOSE

Kv4.2 subunits contribute to the pore-forming region of channels that express a transient, A-type K(+) current (A-current) in hippocampal CA1 pyramidal cell dendrites. Here, the A-current plays an important role in signal processing and synaptic integration. Kv4.2 knockout mice show a near elimination of the A-current in area CA1 dendrites, producing increased excitability in this region. In these studies, we evaluated young adult Kv4.2 knockout mice for spontaneous seizures and the response to convulsant stimulation in the whole animal in vivo and in hippocampal slices in vitro.

METHODS

Electroencephalogram electrode-implanted Kv4.2 knockout and wild-type mice were observed for spontaneous behavioral and electrographic seizures. The latency to seizure and status epilepticus onset in Kv4.2 knockout and wild-type mice was assessed following intraperitoneal injection of kainate. Extracellular field potential recordings were performed in hippocampal slices from Kv4.2 knockout and wild-type mice following the bath application of bicuculline.

RESULTS

No spontaneous behavioral or electrographic seizures were observed in Kv4.2 knockout mice. Following kainate, Kv4.2 knockout mice demonstrated a decreased seizure and status epilepticus latency as well as increased mortality compared to wild-type littermates. The background strain modified the seizure susceptibility phenotype in Kv4.2 knockout mice. In response to bicuculline, slices from Kv4.2 knockout mice exhibited an increase in epileptiform bursting in area CA1 as compared to wild-type littermates.

DISCUSSION

These studies show that loss of Kv4.2 channels is associated with enhanced susceptibility to convulsant stimulation, supporting the concept that Kv4.2 deficiency may contribute to aberrant network excitability and regulate seizure threshold.

Isodomoic acids A and C exhibit low KA receptor affinity and reduced in vitro potency relative to domoic acid in region CA1 of rat hippocampus

Several natural isomers of the seizurogenic neurotoxin domoic acid (DA) have been found to occur at up to mg/kg levels in shellfish. The aim of the current study was to assess the neurotoxic potency of isodomoic acids A and C (Iso-A and Iso-C), recently isolated from commercial shellfish. Hippocampal slices were obtained from young adult rats and maintained in a tissue recording chamber. Synaptically evoked population spikes were recorded in region CA1 before and after exposure to DA or its isomers. Both Iso-A and Iso-C produced transient neuronal hyperexcitability followed by a dose-dependent suppression of population spikes, but were, respectively, 4- and 20-fold less potent than DA (spike area: EC50 DA=237 nM; Iso-A=939 nM; Iso-C=4.6 microM). In the hippocampus, DA preconditioning induces tolerance to subsequent DA toxicity. However, in the present study neither Iso-A nor Iso-C were effective as preconditioning agents. Competitive binding studies using homomeric GluR6 kainate (kainic acid, KA) receptors showed the affinity of Iso-A to be 40-fold lower than DA (Ki DA=3.35 nM; Iso-A=130 nM). Together with earlier work showing Iso-C affinity at GluR6 receptors to be 240-fold lower than DA, our results suggest that neuroexcitatory effects of Iso-A in CA1 may involve both alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and KA receptors, while Iso-C likely involves the activation of AMPA receptors alone.

Domoic acid preconditioning and seizure induction in young and aged rats

Clinical reports suggest that the elderly are hypersensitive to the neurological effects of domoic acid (DOM). In the present study we assessed DOM-induced seizures in young and aged rats, and seizure attenuation following low-dose DOM pretreatment (i.e. preconditioning). Seizure behaviours following saline or DOM administration (0.5-2mg/kg i.p.) were continuously monitored for 2.5h in naïve and DOM preconditioned rats. Competitive ELISA was used to determine serum and brain DOM concentrations. Dose- and age-dependent increases in seizure activity were evident in response to DOM. Lower doses of DOM in young and aged rats promoted low level seizure behaviours. Animals administered high doses (2mg/kg in young; 1mg/kg in aged) progressed through various stages of stereotypical behaviour (e.g., head tics, scratching, wet dog shakes) before ultimately exhibiting tonic-clonic convulsions. Serum and brain DOM analysis indicated impaired renal clearance as contributory to increased DOM sensitivity in aged animals, and this was supported by seizure analysis following direct intrahippocampal administration of DOM. Preconditioning young and aged animals with low-dose DOM 45-90 min before high-dose DOM significantly reduced seizure intensity. We conclude that age-related supersensitivity to DOM is related to reduced clearance rather than increased neuronal sensitivity, and that preconditioning mechanisms underlying an inducible tolerance to excitotoxins are robustly expressed in both young and aged CNS.

Mutations in amyloid precursor protein and presenilin-1 genes increase the basal oxidative stress in murine neuronal cells and lead to increased sensitivity to oxidative stress mediated by amyloid beta-peptide (1-42), HO and kainic acid: implications for Alzheimer's disease

Oxidative stress is observed in Alzheimer's disease (AD) brain, including protein oxidation and lipid peroxidation. One of the major pathological hallmarks of AD is the brain deposition of amyloid beta-peptide (Abeta). This 42-mer peptide is derived from the beta-amyloid precursor protein (APP) and is associated with oxidative stress in vitro and in vivo. Mutations in the PS-1 and APP genes, which increase production of the highly amyloidogenic amyloid beta-peptide (Abeta42), are the major causes of early onset familial AD. Several lines of evidence suggest that enhanced oxidative stress, inflammation, and apoptosis play important roles in the pathogenesis of AD. In the present study, primary neuronal cultures from knock-in mice expressing mutant human PS-1 and APP were compared with those from wild-type mice, in the presence or absence of various oxidizing agents, viz, Abeta(1-42), H2O2 and kainic acid (KA). APP/PS-1 double mutant neurons displayed a significant basal increase in oxidative stress as measured by protein oxidation, lipid peroxidation, and 3-nitrotyrosine when compared with the wild-type neurons (p < 0.0005). Elevated levels of human APP, PS-1 and Abeta(1-42) were found in APP/PS-1 cultures compared with wild-type neurons. APP/PS-1 double mutant neuron cultures exhibited increased vulnerability to oxidative stress, mitochondrial dysfunction and apoptosis induced by Abeta(1-42), H2O2 and KA compared with wild-type neuronal cultures. The results are consonant with the hypothesis that Abeta(1-42)-associated oxidative stress and increased vulnerability to oxidative stress may contribute significantly to neuronal apoptosis and death in familial early onset AD.

Reduction in functional potency of the neurotoxin domoic acid in the presence of cadmium and zinc ions

The tricarboxylic neurotoxin domoic acid (DA) binds trace metals such as iron and copper. In vitro brain slice recording (area CA1 of rat hippocampal slices) was used to assess changes in DA potency in the presence of cadmium and zinc. Cadmium or zinc alone had little or no effect on CA1 responses. DA alone produced hyperexcitability and, with prolonged administration, a robust suppression of CA1 responses. Coadministration of DA with either 2 or 4μM Cd(2+) produced significant reductions in the potency of DA; less striking effects were seen in the presence of 4μM Zn(2+). These findings suggest that interactions of Cd(2+) and Zn(2+) with DA result in the formation of trace metal-neurotoxin complexes which are either unavailable for binding to ionotropic glutamate receptors, or bind without producing full agonist activity.

Clomethiazole: mechanisms underlying lasting neuroprotection following hypoxia-ischemia

Damage after hypoxia-ischemia (HI) is observed in both cortical and subcortical regions. In this study, we employed a "Levine" rat model of HI (left carotid ligation + 1 h global hypoxia on PND-26) and used histological and electrophysiological paradigms to assess the long-term neuroprotective properties of clomethiazole (CMZ; a GABA(A) receptor modulator). Key enzymes involved in inflammation, namely nitric oxide synthase (NOS) and arginase, were also examined to assess potential CMZ mechanisms not involving GABA-R activation. Assessments were carried out 3 and 90 days post-HI. Extensive CNS lesions were evident after HI ipsilaterally at both short- and long-term intervals. CMZ significantly decreased the lesion size at 3 and 90 days (P<0.01; P<0.05). Evoked field potential analyses were used to assess hippocampal CA1 neuronal activity ex vivo. Electrophysiological measurements contralateral to the occlusion revealed impaired neuronal function after HI relative to short- and long-term controls (P<0.001, 3 and 14 days; P<0.01, 90 days), with CMZ treatment providing near complete protection (P<0.001 at 3 and 14 days; P<0.01 at 90 days). Both NOS and arginase activities were significantly increased at 3 days (P<0.01), with arginase remaining elevated at 90 days post-HI (P<0.05) ipsilaterally. CMZ suppressed the HI-induced increase in iNOS and arginase activities (P<0.001; P<0.05). These data provide evidence of long-term functional neuroprotection by CMZ in a model of HI. We further conclude that under conditions of HI, functional deficits are not restricted to the ipsilateral hemisphere and are due, at least in part, to changes in the activity of NOS and arginase.

Differential upregulation of extracellular matrix molecules associated with the appearance of granule cell dispersion and mossy fiber sprouting during epileptogenesis in a murine model of temporal lobe epilepsy

We have investigated changes in the extracellular matrix of the hippocampus associated with the early progression of epileptogenesis in a murine model of temporal lobe epilepsy using immunohistochemistry. In the first week following intrahippocampal injection of the glutamate agonist, domoate, there is a latent period at the end of which begins a sequential upregulation of extracellular matrix (ECM) molecules in the granule cell layer of the dentate gyrus, beginning with neurocan and tenascin-C. This expression precedes the characteristic dispersion of the granule cell layer which is evident at 14 days post-injection when the first recurrent seizures can be recorded. At this stage, an upregulation of the chondroitin sulfate proteoglycan, phosphacan, the DSD-1 chondroitin sulfate motif, and the HNK-1 oligosaccharide are also observed. The expression of these molecules is localized differentially in the epileptogenic dentate gyrus, especially in the sprouting molecular layer, where a strong upregulation of phosphacan, tenascin-C, and HNK-1 is observed but there is no expression of the proteoglycan, neurocan, nor of the DSD-1 chondroitin sulfate motif. Hence, it appears that granule cell layer dispersion is accompanied by a general increase in the ECM, while mossy fiber sprouting in the molecular layer is associated with a more restricted repertoire. In contrast to these changes, the expression of the ECM glycoproteins, laminin and fibronectin, both of which are frequently implicated in tissue remodelling events, showed no changes associated with either granule cell dispersion or mossy fiber sprouting, indicating that the epileptogenic plasticity of the hippocampus is accompanied by ECM interactions that are characteristic of the CNS.

Kainate receptor agonists and antagonists mediate tolerance to kainic acid and reduce high-affinity GTPase activity in young, but not aged, rat hippocampus

Domoic acid acts at both kainic acid (KA) and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)-sensitive glutamate receptors and induces tolerance against subsequent domoic acid insult in young but not aged rat hippocampus. To determine the receptor specificity of this effect, tolerance induction was examined in hippocampal slices from young and aged rats. Slices were preconditioned by exposure to low-dose KA to activate kainate receptors, or the AMPA-receptor selective agonist (S)-5-fluorowillardiine (FW), and following washout, tolerance induction was assessed by administration of high concentrations of KA or FW (respectively). FW preconditioning failed to induce tolerance to subsequent FW challenges, while KA-preconditioned slices were significantly resistant to the effects of high-dose KA. KA preconditioning failed to induce tolerance in aged CA1. Given the lasting nature of the tolerance effect, we examined G-protein-coupled receptor function. A number of ionotropic KA receptor agonists and antagonists significantly reduced constitutive GTPase activity in hippocampal membranes from young but not aged rats. Furthermore, in young CA1, low concentrations of the AMPA/KA blocker GYKI-52466 also induced tolerance to high-dose KA. Our findings suggest that tolerance is triggered by a selective reduction in constitutive KA-sensitive G-protein activity, and that this potential neuroprotective mechanism is lost with age.

Electroencephalographic, behavioral, and c-fos responses to acute domoic acid exposure

Domoic acid, a potent excitotoxic analogue of glutamate and kainate, may cause seizures, amnesia, and sometimes death in humans consuming contaminated shellfish. Continuous behavioral observations and recordings of the electrocorticogram (ECoG, via bipolar, epidural electrodes) were obtained from nonanesthetized rats for 2 h after intraperitoneal injection with either saline, 2.2, or 4.4 mg/kg of domoic acid. Rats were then sacrificed for c-fos immunohistochemistry. Fast Fourier transformation (FFT) of the ECoG data to obtain the voltage as a function of frequency indicated that the lower frequency bands (theta, 4.75-6.75 Hz and delta, 1.25-4.50 Hz) were the first to respond, with a significant elevation by 30 min after the high dose of domoic acid. The lower dose of domoic acid also caused a significant elevation of ECoG voltage, but not until later in the session. Sixty minutes after dosing, the behavioral biomarkers of "ear scratching" and "rearing, praying" (RP) seizures became significantly elevated in the high-dose rats. The low-dose rats showed no significant alterations in behavior at any time during the session. In postmortem brains obtained immediately after the sessions, c-fos was activated in the anterior olfactory nucleus by both the low and high doses of domoic acid. However, only the high dose increased c-fos immunoreactivity in the hippocampus, affecting both the granule and pyramidal neurons. These data indicate that electroencephalographic and c-fos responses can be obtained at a dose of domoic acid that fails to activate the behavioral response most commonly used as a bioassay for this marine toxin: ear scratching with the ipsilateral foot.

Unilateral inner ear damage results in lasting changes in hippocampal CA1 field potentials in vitro

We investigated the effects of a surgical lesion of one vestibular inner ear (unilateral vestibular damage [UVD]) on the field potential responses of CA1 neurons in vitro. Hippocampal slices were removed from rats at 4-6 weeks or 5-6 months post-UVD, and the field responses of CA1 neurons to electrical stimulation of the Schaffer collateral commissural pathway were analyzed. Compared with slices from sham and naive control animals, slices from UVD animals at 5-6 months post-UVD exhibited decreases in the population spike amplitude, the somal field excitatory postsynaptic potential (sfEPSP) slope, and the field EPSP (fEPSP) slope. For the population spike amplitude and fEPSP slope, this effect was observed in both CA1 ipsilateral and contralateral to the UVD. On both the ipsilateral and contralateral sides, paired-pulse testing showed increases in paired-pulse inhibition at the shortest interstimulus intervals (ISIs), with increases in paired-pulse facilitation at longer ISIs. This study provides the first evidence that peripheral vestibular damage can produce long-term changes in hippocampal electrophysiological activity in vitro.

Age-related changes in tolerance to the marine algal excitotoxin domoic acid

During an incident of toxic mussel poisoning, the epileptogenic excitotoxin domoic acid (DOM) was associated with lasting neurological deficits mainly in older patients (), suggesting supersensitivity to excitotoxins is a feature of brain aging. Here, hippocampal slices from young (3 months) and aged (26-29 months) Sprague Dawley rats were assessed by CA1 field potential analysis before and after preconditioning with DOM. In naïve slices from young animals, DOM produced initial hyperexcitability followed by significant dose-dependent reductions in population spike amplitude during prolonged application. Following toxin washout, only small changes in neuronal activity were evident during a second application of DOM, suggesting that a resistance to the effects of DOM occurs in hippocampal slices which have undergone prior exposure to DOM. This inducible tolerance was not antagonized by the NMDA receptor blockers APV or MK-801, nor was it diminished by the group I, II or III mGluR blockers AIDA, CPPG and EGLU. Likewise, neither the AMPA/KA blocker CNQX nor the VSCC blocker nifedipine were effective in blocking tolerance induction in young slices. Field potential analysis revealed significant age-related reductions in CA1 EPSP strength, population spike amplitude and paired-pulse inhibition, but aged slices did not differ in sensitivity to DOM relative to young. However, aged CA1 failed to exhibit any tolerance to DOM following preconditioning, suggesting that a loss of inducible neuroprotective mechanisms may account for increased sensitivity to excitotoxins during aging.