Synaptic background noise controls the input/output characteristics of single cells in an in vitro model of in vivo activity

In vivo, in vitro and computational studies were used to investigate the impact of the synaptic background activity observed in neocortical neurons in vivo. We simulated background activity in vitro using two stochastic Ornstein-Uhlenbeck processes describing glutamatergic and GABAergic synaptic conductances, which were injected into a cell in real time using the dynamic clamp technique. With parameters chosen to mimic in vivo conditions, layer 5 rat prefrontal cortex cells recorded in vitro were depolarized by about 15 mV, their membrane fluctuated with a S.D. of about 4 mV, their input resistances decreased five-fold, their spontaneous firing had a high coefficient of variation and an average firing rate of about 5-10 Hz. Brief changes in the variance of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) synaptic conductance fluctuations induced time-locked spiking without significantly changing the average membrane potential of the cell. These transients mimicked increases in the correlation of excitatory inputs. Background activity was highly effective in modulating the firing-rate/current curve of the cell: the variance of the simulated gamma-aminobutyric acid (GABA) and AMPA conductances individually set the input/output gain, the mean excitatory and inhibitory conductances set the working point, and the mean inhibitory conductance controlled the input resistance. An average ratio of inhibitory to excitatory mean conductances close to 4 was optimal in generating membrane potential fluctuations with high coefficients of variation. We conclude that background synaptic activity can dynamically modulate the input/output properties of individual neocortical neurons in vivo.     

Alterations in hippocampal extracellular amino acids and purine catabolites during limbic seizures induced by folate injections into the rabbit amygdala

The effects on hippocampal extra- and intracellular amino acids of focal injection of folic acid into the amygdala in the rabbit were studied with brain dialysis. Folate seizures were accompanied by pronounced elevations of extracellular alanine and phosphoethanolamine. The increase of extracellular alanine was related to an enhanced level of this amino acid in total hippocampal tissue, whereas phosphoethanolamine was unaltered in tissue biopsies. Folate seizures did not significantly affect extracellular aspartate and extracellular glutamate was only slightly elevated (50-75% over baseline values). The tissue concentration of glutamate remained at control levels during the seizures and tissue aspartate was decreased by 28%. Extracellular glutamine decreased rapidly after folate injection with a concomitant increase of total hippocampal glutamine. Neither extra- nor intracellular taurine was affected by folate epilepsy. The experiments also encompassed measurements of the extracellular purine catabolites inosine, hypoxanthine and xanthine. Folic acid-induced epilepsy produced changes in these compounds indicative of moderately accelerated degradation of adenosine 5'-triphosphate. The findings support the view of glutamate as a mediator of epilepsy-related brain damage. It is, however, questionable if the small enhancement of extracellular glutamate is enough to provoke neuronal necrosis associated with folate epilepsy.     

Induction of sharp wave-ripple complexes in vitro and reorganization of hippocampal networks

Hippocampal sharp wave-ripple complexes (SPW-Rs) occur during slow-wave sleep and behavioral immobility and are thought to represent stored information that is transferred to the neocortex during memory consolidation. Here we show that stimuli that induce long-term potentiation (LTP), a neurophysiological correlate of learning and memory, can lead to the generation of SPW-Rs in rat hippocampal slices. The induced SPW-Rs have properties that are identical to spontaneously generated SPW-Rs: they originate in CA3, propagate to CA1 and subiculum and require AMPA/kainate receptors. Their induction is dependent on NMDA receptors and involves changes in interactions between clusters of neurons in the CA3 network. Their expression is blocked by low-frequency stimulation but not by NMDA receptor antagonists. These data indicate that induction of LTP in the recurrent CA3 network may facilitate the generation of SPW-Rs.     

Initiation of electrographic seizures by neuronal networks in entorhinal and perirhinal cortices in vitro

The hippocampus is often considered to play a major role in the pathophysiology of mesial temporal lobe epilepsy. However, emerging clinical and experimental evidence suggests that parahippocampal areas may contribute to a greater extent to limbic seizure initiation, and perhaps epileptogenesis. To date, little is known about the participation of entorhinal and perirhinal networks to epileptiform synchronization. Here, we addressed this issue by using simultaneous field potential recordings in horizontal rat brain slices containing interconnected limbic structures that included the hippocampus proper. Epileptiform discharges were disclosed by bath applying the convulsant drug 4-aminopyridine (50 microM) or by superfusing Mg(2+)-free medium. In the presence of 4-aminopyridine, slow interictal- (duration=2.34+/-0.29 s; interval of occurrence=25.75+/-2.11 s, n=16) and ictal-like (duration=31.25+/-3.34 s; interval of occurrence=196.96+/-21.56 s, n=17) discharges were recorded in entorhinal and perirhinal cortices after abating the propagation of CA3-driven interictal activity to these areas following extended hippocampal knife cuts. Simultaneous recordings obtained from the medial and lateral entorhinal cortex, and from the perirhinal cortex revealed that interictal and ictal discharges could initiate from any of these areas and propagate to the neighboring structure with delays of 8-66 ms. However, slow interictal- and ictal-like events more often originated in the medial entorhinal cortex and perirhinal cortex, respectively. Cutting the connections between entorhinal and perirhinal cortices (n=10), or functional inactivation of cortical areas by local application of a glutamatergic receptor antagonist (n=11) made independent epileptiform activity occur in all areas. These procedures also shortened ictal discharge duration in the entorhinal cortices, but not in the perirhinal area. Similar results could be obtained by applying Mg(2+)-free medium (n=7). These findings indicate that parahippocampal networks provide independent epileptiform synchronization sufficient to sustain limbic seizures as well as that the perirhinal cortex plays a preferential role in in vitro ictogenesis.     

Senile plaques are concentrated in the subicular region of the hippocampal formation in Alzheimer's disease

The distribution of senile plaques within the hippocampal formation was examined at autopsy in the brains of 18 patients with Alzheimer's disease, ranging in age from 62 to 89 years. Wax sections were cut at the level of the central part of the cornu ammonis and stained by a silver impregnation method. Plaque surface density was determined in the stratum pyramidale of the presubiculum, subiculum, prosubiculum and CA1-4. Senile plaques were present in all 7 regions of the hippocampal formation. However, the number of senile plaques/mm2 was significantly greater in the subicular complex than in the cornu ammonis. There was no difference in plaque density between the individual regions of the subicular complex or between the 4 CA fields.     

Bactericidal activity of daptomycin against vancomycin-resistantEnterococcus faecium in an in vitro pharmacokinetic model

A dynamic in vitro model was used to determine the killing kinetics of daptomycin against 15 vancomycin-resistant clinical isolates ofEnterococcus faecium. Concentration profiles simulating those observed in serum following administration of both low-dose (2 mg/kg) and high-dose (6 mg/kg) daptomycin were bactericidal within 5.5 and 2.8 h, respectively. In contrast, when albumin was added to the growth medium, the corresponding bacterial killing times were slowed to>24 h and 7 h; these results suggest that in the clinical setting, daptomycin dosages of approximately 6 mg/kg are required to achieve bactericidal activity against vancomycin-resistantEnterococcus faecium.     

A physiologically realistic in vitro model of microvascular networks

Existing microfluidic devices, e.g. parallel plate flow chambers, do not accurately depict the geometry of microvascular networks in vivo. We have developed a synthetic microvascular network (SMN) on a polydimethalsiloxane (PDMS) chip that can serve as an in vitro model of the bifurcations, tortuosities, and cross-sectional changes found in microvascular networks in vivo. Microvascular networks from a cremaster muscle were mapped using a modified Geographical Information System, and then used to manufacture the SMNs on a PDMS chip. The networks were cultured with bovine aortic endothelial cells (BAEC), which reached confluency 3–4 days after seeding. Propidium iodide staining indicated viable and healthy cells showing normal behavior in these networks. Anti-ICAM-1 conjugated 2-μm microspheres adhered to BAEC cells activated with TNF-α in significantly larger numbers compared to control IgG conjugated microspheres. This preferential adhesion suggests that cultured cells retain an intact cytokine response in the SMN. This microfluidic system can provide novel insight into characterization of drug delivery particles and dynamic flow conditions in microvascular networks.     

Differences in hippocampal mitotic activity within the dorsal and ventral hippocampus following flurothyl seizures in mice

To date, there is little information regarding the regional division (dorsal and ventral) of hippocampal mitotic activity at either baseline or following seizures. To determine the regional distribution of hippocampal mitotic activity, animals were given seizures followed by injections of 5-bromo-2'-deoxyuridine (label for mitotic cells). Our results indicate that the baseline and seizure-induced levels of mitotic activity (neurogenesis) are greater in the dorsal hippocampus as compared to the ventral hippocampus. Moreover, seizures were able to significantly increase mitotic activity in both dorsal and ventral hippocampus with a greater proportional increase from baseline in the ventral hippocampus as compared to the dorsal hippocampus. These results suggest that the amount of mitotic activity differs between the dorsal and ventral hippocampus and suggests that seizure activity is capable of differentially driving mitotic activity in the dorsal and ventral hippocampus.     

Long-term potentiation in the prefrontal cortex following stimulation of the hippocampal CA1/subicular region

We have examined single cell activity and field potentials in the prelimbic area of the prefrontal cortex of the rat to electrical stimulation of the CA1/subicular region of the temporal hippocampus. Excitatory unit responses were found in 50 out of 120 neurons recorded in the prelimbic area. Paired-pulse facilitation was found for both single cell responses and field potentials. High-frequency, tetanic stimulation of the temporal hippocampus produced a significant and persistent potentiation of prelimbic field potentials. The evidence suggests that the direct pathway from the temporal hippocampus to the prelimbic area of the prefrontal cortex in the rat is excitatory and can undergo long-term potentiation (LTP).     

Comparative activity of cefixime and cefaclor in an in vitro model simulating human pharmacokinetics

The dependence of the antibacterial activity of the two oral cephalosporins cefixime and cefaclor on pharmacokinetic properties was investigated in an in vitro model using strains of enterobacteria and a streptococcal strain. In the cultures the course of serum concentrations of the respective antibiotic was simulated. The more rapidly attained (1 h) high peak levels (17.5 µg/ml) of cefaclor (500 mg dose) in no case showed an advantage over the more slowly reached (3 h) low peak levels (2.5 µg/ml) of cefixime (200 mg dose). Cefixime was comparable to cefaclor with respect to its initial killing velocity, whereas it was generally superior with respect to maximum values for reduction of bacterial counts. Due to its long elimination half-life (2.5 h) cefixime prevented regrowth for at least twice as long as cefaclor, which has a short half-life (0.7 h). As a result of its antibacterial activity and pharmacokinetic properties cefixime can be administered less frequently than cefaclor.     

Alkylxanthine adenosine antagonists and epileptiform activity in rat hippocampal slices in vitro

Despite its potent proconvulsant effects in vitro, the adenosine A1 receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) does not induce seizures when administered in vivo. This contrasts with the effects of less selective adenosine antagonists such as theophylline or cyclopentlytheophylline, and led us to reexamine the nature of DPCPX-induced epileptiform activity. In the present study, we report that proconvulsant effects of bath-applied DPCPX in rat hippocampal slices are only observed after a preceding stimulus such as NMDA receptor activation or brief tetanic stimulation. While this may be due to the absence of a basal “purinergic tone”, the relatively high interstitial concentrations of adenosine present in the slice suggest that access of the drug to A1 receptors may instead be prevented by tightly coupled endogenous adenosine, with the ternary adenosine-A1 receptor-G protein complex stabilised in the high-affinity conformation by a coupling cofactor. This implies that a substantial percentage of adenosine A1 receptors are inactive under physiological conditions, but that access of adenosine A1 receptor antagonists may be facilitated under pathological conditions. Once induced, DPCPX-evoked spiking persists for long periods of time. A “kindling” effect of A1 receptor blockade is unlikely, since persistent spiking is not usually observed with less selective A1 antagonists even after prolonged application. Alternatively, endogenous adenosine released during increased neuronal activity may activate A2 receptors during selective A1 blockade. The most important factor determining the duration of DPCPX-induced spiking, however, may be a persistence of the drug in the tissue and subsequent access to the A1 receptor via a membrane-delineated pathway, since DPCPX-induced spiking could be shown to decrease markedly after a transient superfusion of theophylline. This hypothesis, which implies that the apparent affinity of adenosine antagonists for the A1 receptor is in part a function of their membrane partitioning coefficient, is supported by a close correlation between alkylxanthine logP values obtained from the literature and theirK _i value at A1 receptors, but not at the enzyme phosphodiesterase, whose xanthine binding site is presented to the cytosol. The implications for the therapeutic value of purinergic drugs are discussed.     

Conditional stimulus probability and activity of hippocampal, cingulate cortical, and limbic thalamic neurons during avoidance conditioning in rabbits

Past studies of the neuronal correlates of avoidance conditioning in rabbits have led to a model of information flow among structures of the limbic system. A hypothesis of the model is that unexpected stimuli activate certain hippocampal and cingulate cortical neurons. This activity in turn suppresses or "limits" the firing of limbic thalamic neurons. This hypothesis is tested in relation to stimuli classified as unexpected or expected on the basis of their incidence or "probability." Multi-unit and field potential responses in the anterior and posterior cingulate cortices (AC and PC), the dentate gyrus (DG), and the anterior ventral (AV) and medial dorsal (MD) thalamic nuclei were recorded during the acquisition and performance of a locomotor conditioned response (CR). The CR, stepping in an activity wheel in response to a 0.5-s tone (CS+), prevented the occurrence of a shock US scheduled 5 s after CS+ onset. The rabbits also learned to ignore a different tone (CS-), not predictive of the US. Training was given daily (120 trials, 60 with each CS in an irregular sequence) until behavioral discrimination reached criterion. After criterion, asymmetric probability (AP) sessions were given, in which the CS+/CS- proportions were .2/.8 or .8/.2. The AP sessions were the same as conditioning sessions except for the probability manipulation. A significant discriminative response, i.e., a greater neuronal discharge to the CS+ than to the CS-, developed in all regions during behavioral acquisition. The unit response in the AP sessions was enhanced in all areas by rare presentation of the CS-, compared with the equal and frequent CS- conditions. Rare presentation of the CS+ enhanced the unit response in the cortical areas (AC, PC, and DG), but it suppressed the firing of limbic thalamic (AV and MD) neurons. These results were supportive of the model. Rare CS+ presentations did not alter AV and PC neuronal activity in rabbits with subicular lesions, a result suggesting that an intact hippocampus is essential for normal neuronal responses to stimulus probability in the cingulate cortex and limbic thalamus.     

Wi-Fi decreases melatonin protective effect and increases hippocampal neuronal damage in pentylenetetrazole induced model seizures in rats

AimEpilepsy is a common brain disorder in which the seizures could cause a neuronal loss in the hippocampus. Oxidative stress has an important role in the pathology of epilepsy. Some studies indicate that Wi-Fi increases oxidative stress and suppresses antioxidant systems. The aim of this study is to investigate the effect of Wi-Fi on melatonin anticonvulsive effect and oxidative damage in pentylenetetrazole-induced epileptic seizures in rats.MethodsIn our study, we used 30 male Wistar Albino rats, 230?250 grams of the body weight. The animals were divided into five groups as control, saline (1 ml/kg/day olive oil for 30 days), Wi-Fi (12 h/day for 30 days), melatonin (10 mg/kg/day for 30 days) and melatonin + Wi-Fi (10 mg/kg/day +12 h/day for 30 days). In the thirtieth day, thirty minutes after the last drugs administration at the indicated doses, PTZ in 45 mg/kg was administered to induce epileptic seizure. The animals were observed for 30 min during the seizure stages (according to the Racine Scale) and first myoclonic jerk times (FMJ). Twenty-four hours after PTZ injection, brain tissues were removed for biochemical and histopathological evaluation. The hippocampal Cornu Ammonis (CA) 1, CA3 and DG (dentate gyrus) regions were histopathologically evaluated in terms of a neuronal damage in addition that oxidative stress markers (total antioxidant status (TAS), total oxidant status (TOS) and oxidative stress index (OSI)) were measured in brain tissues.ResultsWi-Fi was not found to affect behavioral changes associated with epilepsy (p > 0.05). However, Wi-Fi reduced anticonvulsive and antioxidant effect of melatonin (p < 0.05). Moreover, Wi-Fi increased neuronal damage in hippocampus (p < 0.05).ConclusionWi-Fi did not directly affect epileptic seizures. Nevertheless, it inhibits the positive effects of melatonin on epilepsy and it also has negative effects on hippocampal neuronal damage. These effects of Wi-Fi may occur via oxidative pathways.     

Monosynaptic transmission during epileptiform activity induced by penicillin in hippocampal slices in vitro

Synaptic transmission was studied in the CA1 region of transverse hippocampal slices in vitro before and after the addition of the epileptogenic agent sodium benzyl penicillin. The presynaptic fibre volley and the field potential associated with the intracellular EPSP, 'field EPSP', were recorded from the layer of the activated synapses. Addition of penicillin did not change either response. The rising phase of the intracellularly recorded EPSP did not change. However, the peak amplitude and, particularly, the duration of the EPSP both increased. The prolongation of the EPSP may be of importance for the triggering of epileptiform bursts.     

Progestin receptors mediate progesterone suppression of epileptiform activity in tetanized hippocampal slices in vitro

Clinical and laboratory studies suggest that progesterone reduces epileptic seizure activity. The mechanisms underlying this effect are not known. The present study determined the effects of progesterone on extracellular evoked responses recorded in the CA1 field of hippocampal slices, as well as epileptiform responses recorded from tetanized slices. Slices were prepared from ovariectomized rats, with or without estrogen replacement. Hippocampal slices were superfused in vitro with one of the following treatments: progesterone with or without RU486 (a progesterone receptor antagonist); allopregnanolone (a progesterone metabolite that potentiates GABA action at GABA(A) receptors); RU5020 (a high-affinity progesterone receptor agonist); or cholesterol (control). In non-tetanized slices, a twofold increase in the excitatory postsynaptic field potential and population spike amplitude occurred during both cholesterol and progesterone superfusion. In contrast, under the same conditions, exposure to allopreganolone caused a 25% reduction in both field potential and population spike amplitude of evoked responses within 30min of treatment. In tetanized slices, progesterone and RU5020, but not allopregnanolone or cholesterol, caused significant reductions in the field potential and population spike amplitude of evoked responses. Progesterone and RU5020 also significantly reduced the duration of tetanic stimulus-induced afterdischarges and the frequency of spontaneous interictal discharges. The effects of allopregnanolone were restricted to a reduction in the primary afterdischarge duration. Estrogen replacement slightly attenuated progesterone's suppression of spontaneous discharges and depression of evoked responses. All responses to progesterone were blocked by prior or concurrent exposure to RU486.These data indicate that allopregnanolone suppresses evoked potentials in non-tetanized hippocampal slices, consistent with previous reports that this neurosteroid has marked anxiolytic and anticonvulsant effects. After tetanization, however, progesterone receptor-mediated responses become quantitatively more important as a mechanism for suppressing hippocampal electrical activity.     

Complex synaptic responses of entorhinal cortical cells in the rat to subicular stimulation in vitro: demonstration of an NMDA receptor-mediated component

Intracellular recordings from neurones in the medial entorhinal cortex of the rat in vitro showed that the lowest threshold response to stimulation in the subiculum was a long latency, prolonged depolarization, which in extracellular records corresponded to a late, negative DC shift. These responses were reduced or abolished by 2-aminophosphonovalerate suggesting mediation via activation of amino acid receptors of the 'N-methyl-D-aspartate'-type.