Field Excitatory Potentials Research Articles

The Transcranial Light Therapy Improves Synaptic Plasticity in the Alzheimer's Disease Mouse Model.

Alzheimer’s disease (AD) is the main cause of dementia worldwide. Emerging non-invasive treatments such as photobiomodulation target the mitochondria to minimize brain damage, improving cognitive functions. In this work, an experimental design was carried out to evaluate the effect of transcranial light therapy (TLTC) on synaptic plasticity (SP) and cognitive functions in an AD animal model. Twenty-three mice were separated into two general groups: an APP/PS1 (ALZ) transgenic group and a wild-type (WT) group. Each group was randomly subdivided into two subgroups: mice with and without TLTC, depending on whether they would undergo treatment with TLTC. Cognitive function, measured through an object recognition task, showed non-significant improvement after TLTC. SP, on the other hand, was evaluated using four electrophysiological parameters from the Schaffer-CA1 collateral hippocampal synapses: excitatory field potentials (fEPSP), paired pulse facilitation (PPF), long-term depression (LTD), and long-term potentiation (LTP). An improvement was observed in subjects treated with TLTC, showing higher levels of LTP than those transgenic mice that were not exposed to the treatment. Therefore, the results obtained in this work showed that TLTC could be an efficient non-invasive treatment for AD-associated SP deficits.

Chronic treatment with a MEK inhibitor reverses enhanced excitatory field potentials in Syngap1+/- mice.

Synaptic Ras-GTPase-activating protein 1 (SYNGAP1) is an abundant brain-specific protein localized at the postsynaptic density of mammalian excitatory synapses. SYNGAP1 functions as a crucial regulator of downstream intracellular signaling triggered by N-methyl-d-aspartate receptor activation. One of the most important signaling pathways regulated by SYNGAP1 is the Ras-Raf-MEK-ERK pathway. SYNGAP1 deficiency is associated with hyperphosphorylation of MEK and ERK kinases and with altered synaptic function in Syngap1+/- mice. Loss-of-function mutations in the SYNGAP1 gene have been documented in many human cognitive and neurological disorders. However, there are currently no approaches that reverse the phenotypes of SYNGAP1 deficiency. Using electrophysiological recordings of field responses in hippocampal slices, we examined if disturbances of synaptic physiology in the hippocampus of 7-8-month old Syngap1+/- mice were sensitive to the effect of the MEK inhibitor PD-0325901 given orally for 6days. We found that in hippocampal slices from vehicle-treated Syngap1+/- mice, basal synaptic responses were higher and their long-term potentiation (LTP) was lower than in slices from wild-type littermates. Chronic administration of PD-0325901 normalized basal synaptic responses, but did not reverse LTP deficit. The differential sensitivity of basal synaptic transmission and LTP to MEK inhibition indicates that the effects of SYNGAP1 deficiency on these synaptic parameters are mediated by distinct pathways. Our findings also suggest that at least some physiological phenotypes of the germline Syngap1 mutation can be ameliorated by pharmacological treatment of adult animals.

Distinct effects of resveratrol on seizures and hyperexcitability induced by NMDA and 4-aminopyridine

Objectives: The antiepileptic activity of resveratrol has been revealed in various experimental models of epilepsy. The present study evaluated the effects of resveratrol on the seizures and hyperexcitable neuronal activity associated with activation of N-methyl-d-aspartic acid (NMDA) receptor and inhibition of voltage-gated potassium channels.Methods: The effects of resveratrol on seizure thresholds, excitatory field potentials (EFPs) and action potentials induced by NMDA and 4-aminopyridine (4-AP) were monitored in mice, the mouse cortical slices and rat cortical neurons, respectively.Results: Resveratrol increased the NMDA-induced seizure thresholds and suppressed the frequency of NMDA/glycine-evoked EFPs and action potentials. However, resveratrol lowered the 4-AP-induced thresholds for myoclonic twitch and face and forelimb clonus, yet enhanced the thresholds for running and bouncing clonus and tonic hindlimb extension at the higher dose (50 mg/kg). A similar biphasic response of resveratrol was observed in the frequency of EFPs and action potential firings evoked by 4-AP, with enhancement at lower concentrations, but suppression at higher concentrations.Discussion: These findings suggest that resveratrol might be capable of protecting against the seizure types related to neuronal excitability and progression mediated by NMDA receptor activation, but not suitable for the seizures caused by disturbance of the voltage-dependent potassium channels.

Effects of sarcosine and N, N-dimethylglycine on NMDA receptor-mediated excitatory field potentials

BackgroundSarcosine, a glycine transporter type 1 inhibitor and an N-methyl-D-aspartate (NMDA) receptor co-agonist at the glycine binding site, potentiates NMDA receptor function. Structurally similar to sarcosine, N,N-dimethylglycine (DMG) is also N-methyl glycine-derivative amino acid and commonly used as a dietary supplement. The present study compared the effects of sarcosine and DMG on NMDA receptor-mediated excitatory field potentials (EFPs) in mouse medial prefrontal cortex brain slices using a multi-electrode array system.ResultsGlycine, sarcosine and DMG alone did not alter the NMDA receptor-mediated EFPs, but in combination with glutamate, glycine and its N-methyl derivatives significantly increased the frequency and amplitude of EFPs. The enhancing effects of glycine analogs in combination with glutamate on EFPs were remarkably reduced by the glycine binding site antagonist 7-chlorokynurenate (7-CK). However, DMG, but not sarcosine, reduced the frequency and amplitude of EFPs elicited by co-application of glutamate plus glycine. D-cycloserine, a partial agonist at the glycine binding site on NMDA receptors, affected EFPs in a similar manner to DMG. Furthermore, DMG, but not sarcosine, reduced the frequencies and amplitudes of EFPs elicited by glutamate plus D-serine, another endogenous ligand for glycine binding site.ConclusionsThese findings suggest that sarcosine acts as a full agonist, yet DMG is a partial agonist at glycine binding site of NMDA receptors. The molecular docking analysis indicated that the interactions of glycine, sarcosine, and DMG to NMDA receptors are highly similar, supporting that the glycine binding site of NMDA receptors is a critical target site for sarcosine and DMG.

Microglia-dependent alteration of glutamatergic synaptic transmission and plasticity in the hippocampus during peripheral inflammation.

Peripheral inflammatory diseases are often associated with behavioral comorbidities including anxiety, depression, and cognitive dysfunction, but the mechanism for these is not well understood. Changes in the neuronal and synaptic functions associated with neuroinflammation may underlie these behavioral abnormalities. We have used a model of colonic inflammation induced by 2,4,6-trinitrobenzenesulfonic acid in Sprague Dawley rats to identify inflammation-induced changes in hippocampal synaptic transmission. Hippocampal slices obtained 4 d after the induction of inflammation revealed enhanced Schaffer collateral-induced excitatory field potentials in CA1 stratum radiatum. This was associated with larger-amplitude mEPSCs, but unchanged mEPSC frequencies and paired-pulse ratios, suggesting altered postsynaptic effects. Both AMPA- and NMDA-mediated synaptic currents were enhanced, and analysis of AMPA-mediated currents revealed increased contributions of GluR2-lacking receptors. In keeping with this, both transcripts and protein levels of the GluR2 subunit were reduced in hippocampus. Both long-term potentiation (LTP) and depression (LTD) were significantly reduced in hippocampal slices taken from inflamed animals. Chronic administration of the microglial/macrophage activation inhibitor minocycline to the inflamed animals both lowered the level of the cytokine tumor necrosis factor α in the hippocampus and completely abolished the effect of peripheral inflammation on the field potentials and synaptic plasticity (LTP and LTD). Our results reveal profound synaptic changes caused by a mirror microglia-mediated inflammatory response in hippocampus during peripheral organ inflammation. These synaptic changes may underlie the behavioral comorbidities seen in patients.

Regulating hippocampal hyperexcitability through GABAB Receptors.

Abstract Disturbances of GABAergic inhibition are a major cause of epileptic seizures. GABA exerts its actions via ionotropic GABAA receptors and metabotropic G protein-coupled GABAB receptors. Malfunction of GABAA inhibition has long been recognized in seizure genesis but the role of GABAB receptors in controlling seizure activity is still not well understood. Here, we examined the anticonvulsive, or inhibitory effects, of GABAB receptors in a mouse model of hippocampal kindling as well as mouse hippocampal slices through the use of GS 39783, a positive allosteric GABAB receptor modulator, and CGP 55845, a selective GABAB receptor antagonist. When administered via intraperitoneal injections in kindled mice, GS 39783 (5 mg/kg) did not attenuate hippocampal EEG discharges, but did reduce aberrant hippocampal spikes, whereas CGP 55845 (10 mg/kg) prolonged hippocampal discharges and increased spike incidences. When examined in hippocampal slices, neither GS 39783 at 5 μmol/L nor the GABAB receptor agonist baclofen at 0.1 μmol/L alone significantly altered repetitive excitatory field potentials, but GS 39783 and baclofen together reversibly abolished these field potentials. In contrast, CGP 55845 at 1 μmol/L facilitated induction and incidence of these field potentials. In addition, CGP 55845 attenuated the paired pulse depression of CA3 population spikes and increased the frequency of EPSCs in individual CA3 pyramidal neurons. Collectively, these data suggest that GABABB receptors regulate hippocampal hyperexcitability by inhibiting CA3 glutamatergic synapses. We postulate that positive allosteric modulation of GABAB receptors may be effective in reducing seizure-related hyperexcitability.

Regulating hippocampal hyperexcitability through GABAB Receptors

Disturbances of GABAergic inhibition are a major cause of epileptic seizures. GABA exerts its actions via ionotropic GABAA receptors and metabotropic G protein‐coupled GABAB receptors. Malfunction of GABAA inhibition has long been recognized in seizure genesis but the role of GABAB receptors in controlling seizure activity is still not well understood. Here, we examined the anticonvulsive, or inhibitory effects, of GABAB receptors in a mouse model of hippocampal kindling as well as mouse hippocampal slices through the use of GS 39783, a positive allosteric GABAB receptor modulator, and CGP 55845, a selective GABAB receptor antagonist. When administered via intraperitoneal injections in kindled mice, GS 39783 (5 mg/kg) did not attenuate hippocampal EEG discharges, but did reduce aberrant hippocampal spikes, whereas CGP 55845 (10 mg/kg) prolonged hippocampal discharges and increased spike incidences. When examined in hippocampal slices, neither GS 39783 at 5 μmol/L nor the GABAB receptor agonist baclofen at 0.1 μmol/L alone significantly altered repetitive excitatory field potentials, but GS 39783 and baclofen together reversibly abolished these field potentials. In contrast, CGP 55845 at 1 μmol/L facilitated induction and incidence of these field potentials. In addition, CGP 55845 attenuated the paired pulse depression of CA3 population spikes and increased the frequency of EPSCs in individual CA3 pyramidal neurons. Collectively, these data suggest that GABABB receptors regulate hippocampal hyperexcitability by inhibiting CA3 glutamatergic synapses. We postulate that positive allosteric modulation of GABAB receptors may be effective in reducing seizure‐related hyperexcitability.

P.1.017 Isoflurane anesthesia rapidly activates TrkB receptor signaling and produces antidepressant-like behavioral effects

Purpose of the study: Emerging evidence suggests that induction of TrkB signaling and the subsequent changes in synaptic plasticity are involved in the therapeutic effects of antidepressant drugs, including rapid-acting antidepressant ketamine [1]. Intriguingly, isoflurane, a commonly used volatile anesthetic, has been shown to produce rapid antidepressant effects in treatment resistant depressive patients [2]. The mechanism of isoflurane’s antidepressant action, however, is completely unknown. We have here investigated the effects of isoflurane anesthesia on TrkB activation and on its downstream signaling pathways related to synaptogenesis and neuronal plasticity. Furthermore, we have studied the effects of single isoflurane treatment on cortical spine formation and morphology, hippocampal long-term potentiation and antidepressant-like behavior in mice. Methods: Adult mice were anesthetized with isoflurane (maximum 30min). For biochemical experiments the animals were killed during the anesthesia or after described recovery period. Hippocampus and prefrontal cortex were dissected out and protein and mRNA levels were analysed using western blotting/ELISA and qPCR, respectively. For electrophysiological recordings mice were decapitated by guillotine 24 h after isoflurane treatment. Parasagittal 400mm hippocampal slices were cut using vibroslicer. Field excitatory potentials (fEPSP) were evoked in Schaffer collateral pathway of the hippocampal slices and LTP was induced by 100 Hz tetanic stimulation. For spine density analysis animals expressing YFP in a subset of cortical layer V pyramidal neurons were perfused with PFA 24 hours after isoflurane treatment. Brains were extracted and 70mm sections were cut using a vibratome. Immunohistochemistry was performed to enhance the fluorescence and sections were analyzed using confocal microscope. Isoflurane-induced antidepressantlike behavior in the forced swim test was monitored 15 minutes and 2 weeks after anesthesia. Results: Isoflurane anesthesia (30min) increased TrkB phosphorylation and its downstream signaling related to neuronal plasticity (pCREB, pAkt) and to synaptogenesis (pmTOR, pP70S6k) in the mouse hippocampus and prefrontal cortex but BDNF protein and mRNA levels remained unaltered. Increased pTrkB, pCREB and pmTOR levels were detected already within 2 minutes from isoflurane treatment onset. Isoflurane’s effects on TrkB signaling could not be blocked with pre-treatment of AMPA receptor blocker NBQX (10mg/kg, ip). LTP was facilitated in hippocampal slices obtained from mice treated with isoflurane 24 h before but any change in cortical spine density and morphological types could not be observed. A single isoflurane anesthesia led to rapid antidepressant-like behavioral effect in the forced swim test that persisted for at least 2 weeks. Conclusions: We show that isoflurane anesthesia rapidly activates TrkB neurotrophin signaling in the mouse brain. This phenomenon is followed by antidepressantlike behavioral responses and facilitation of LTP but not by changes in spine formation or morphology. The biochemical, functional and behavioral effects of isoflurane partially resemble those of rapid-acting antidepressant ketamine. Our findings may provide the neurobiological basis for the previously observed rapid antidepressant effect of isoflurane narcotherapy and encourage investigating further the therapeutic potential of isoflurane.

N-acetylcysteine modulates hallucinogenic 5-HT2A receptor agonist-mediated responses: Behavioral, molecular, and electrophysiological studies

N-acetylcysteine (NAC) has been reported to reverse the psychotomimetic effects in the rodent phencyclidine model of psychosis and shown beneficial effects in treating patients with schizophrenia. The effect of NAC has been associated with facilitating the activity of cystine-glutamate antiporters on glial cells concomitant with the release of non-vesicular glutamate, which mainly stimulates the presynaptic metabotropic glutamate receptor subtype 2 receptors (mGluR2). Recent evidence demonstrated that functional interactions between serotonin 5-HT2A receptor (5-HT2AR) and mGluR2 are responsible to unique cellular responses when targeted by hallucinogenic drugs. The present study determined the effects of NAC on hallucinogenic 5-HT2AR agonist (±)1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI)-elicited behavioral and molecular responses in mice and DOI-evoked field potentials in the mouse cortical slices. NAC significantly attenuated DOI-induced head twitch response and expression of c-Fos and Egr-2 in the infralimbic and motor cortex and suppressed the increase in the frequency of excitatory field potentials elicited by DOI in the medial prefrontal cortex. In addition, the cystine-glutamate antiporter inhibitor (S)-4-carboxyphenylglycine (CPG) and the mGluR2 antagonist LY341495 reversed the suppressing effects of NAC on DOI-induced head twitch and molecular responses and increased frequency of excitatory field potentials, supporting that NAC attenuates the 5-HT2AR-mediated hallucinogenic effects via increased activity of cystine-glutamate antiporter followed by activation of mGluR2 receptors. These findings implicate NAC as a potential therapeutic agent for hallucinations and psychosis associated with hallucinogen use and schizophrenia.

Bereitschaftspotential augmentation by neuro-feedback training in Parkinson’s disease

ObjectiveDecreased early Bereitschaftspotential (BP) is one of the electrophysiological characteristics in patients with Parkinson’s disease (PD). We examined whether PD patients could increase BP amplitude by means of neuro-feedback (NFB) training for their slow cortical potentials (SCPs). MethodsWe worked with 10 PD patients and 11 age-matched controls. BP was measured for self-paced button pressing by their right thumb. The subjects were instructed to make the introspective efforts to produce negative SCPs (negativation). The one-day session consisted of three trials, that is, the first BP, NFB training and the second BP, and each patient performed this routine for 2–4days. Amplitudes of the first and second BPs were compared between the two groups that were divided depending on NFB performance. ResultsGood NFB performance had the tendency of larger early BP in the second BP recording than in the first one, whereas in the poor NFB performance the early BP was smaller in the second BP recording than in the first one in both patient and normal groups (p<0.001). ConclusionsGood NFB performance of negativation could increase excitatory field potentials of pyramidal cells for the generation of early BP. SignificanceVoluntary regulation of SCPs could enhance BP in PD patients and in aged controls.

N‐ethyl lidocaine (QX‐314) protects striatal neurons against ischemia: An in vitro electrophysiological study

In this study, we have investigated the neuroprotective actions of the membrane impermeable, lidocaine analog, N-ethyl lidocaine (QX-314) in the striatum. The effects of this drug were compared with those caused by the strictly-related-compound and sodium channel blocker lidocaine. To address this issue, electrophysiological recordings were performed in striatal slices, in control condition (normoxia) and during combined oxygen and glucose deprivation (in vitro ischemia). Either QX-314 or lidocaine induced, to some extent, a protection of the permanent electrophysiological alteration (field potential loss) caused by a period (12 min) of ischemia. Thus, both compounds permitted a partial recovery of the ischemic depression of the corticostriatal transmission and reduced the amplitude of the ischemic depolarization in medium spiny neurons. However, while QX-314, at the effective concentration of 100 microM, slightly reduced the amplitude of the excitatory field potential and did not affect the current-evoked spikes discharge of medium spiny striatal neurons, equimolar lidocaine depressed the field potential and eliminated repetitive spikes on a depolarizing step. On the basis of these observations, our results suggest the use of QX-314 as a neuroprotective agent in ischemic brain disorders.

Repeated hypoxic episodes induce seizures and alter hippocampal network activities in mice

We examined intra-cranial electroencephalographic activity in a mouse model of global hypoxia in an attempt to explore the pathophysiology underlying hypoxic seizures. Mice between the ages of 22 and 41 days underwent repeated hypoxic challenges, and their behavioral and electroencephalographic activities were monitored continuously before, during and after the challenge. We found that the animals exhibited convulsive behaviors during the hypoxic challenge, but hippocampal/cortical electrographic ictal discharges were recognizable only after the repeated hypoxic challenge. When examined in hippocampal slices in vitro, the post-hypoxic hippocampal circuit was hyper-excitable and had a higher propensity than the control to generate repetitive excitatory field potentials. We postulate that an ictogenic process develops after an initial hypoxic challenge rendering an increase of seizure susceptibility to recurrent hypoxic insults.

Adverse Effects of Methylene Blue on the Central Nervous System

An increasing number of clinical observations suggest adverse neurologic outcome after methylene blue (MB) infusion in the setting of parathyroid surgery. Hence, the aim of the current study was to investigate the potentially neurotoxic effects of MB using a combination of in vivo and in vitro experimental approaches. Isoflurane-anesthetized adult rats were used to evaluate the impact of a single bolus intravascular administration of MB on systemic hemodynamic responses and on the minimum alveolar concentration (MAC) of isoflurane using the tail clamp test. In vivo, MB-induced cell death was evaluated 24 h after MB administration using Fluoro-Jade B staining and activated caspase-3 immunohistochemistry. In vitro, neurotoxic effects of MB were examined in hippocampal slice cultures by measuring excitatory field potentials as well as propidium iodide incorporation after MB exposure. The impact of MB on dendritic arbor was evaluated in differentiated single cell neuronal cultures. Bolus injections of MB significantly reduced isoflurane MAC and initiated widespread neuronal apoptosis. Electrophysiologic recordings in hippocampal slices revealed a rapid suppression of evoked excitatory field potentials by MB, and this was associated with a dose-dependent effect of this drug on cell death. Dose-response experiments in single cell neuronal cultures revealed that a 2-h-long exposure to MB at non-cell-death-inducing concentrations could still induce significant retraction of dendritic arbor. These results suggest that MB exerts neurotoxic effects on the central nervous system and raise questions regarding the safety of using this drug at high doses during parathyroid gland surgery.

High-Frequency Afferent Stimulation Induces Long-Term Potentiation of Field Potentials in the Ventral Tegmental Area

Excitatory synapses on dopamine neurons in the VTA can undergo both long-term potentiation and depression. Additionally, drug-induced plasticity has been found at VTA synapses, and is proposed to play a role in reward-related learning and addiction by modifying dopamine cell firing. LTP at these synapses is difficult to generate experimentally in that it requires an undisturbed intracellular milieu and is often small in magnitude. Here, we demonstrate the induction of LTP as a property of evoked field potentials within the VTA. Excitatory field potentials were recorded extracellularly from VTA neurons in acute horizontal midbrain slices. Using extracellular and intracellular recording techniques, we found that evoked field potentials originate within the VTA itself and are largely composed of AMPA receptor-mediated EPSPs and action potentials triggered by activation of glutamatergic synapses on both dopamine and GABA neurons. High-frequency afferent stimulation (HFS) induced LTP of the field potential. The induction of this LTP was blocked by application of the NMDAR antagonist, d-APV, prior to HFS. As reported previously, glutamatergic synapses on GABA neurons did not express LTP while those on dopamine neurons did. We conclude that the potentiation of glutamatergic synapses on dopamine neurons is a major contributor to NMDA receptor-dependent LTP of the field potential. Field potential recordings may provide a convenient approach to explore the basic electrophysiological properties of VTA neurons and the development of addiction-related processes in this brain region.

Amphetamine depresses excitatory synaptic transmission at prefrontal cortical layer V synapses

Dopamine modulates the function of glutamatergic synapses in prefrontal cortex, modifying synaptic strength and influencing synaptic plasticity. Here we have explored the ability of endogenous dopamine, present in slices containing the prefrontal cortex, to influence excitatory synaptic transmission. We found that 10 μM amphetamine, which releases and blocks the reuptake of dopamine from dopaminergic nerve terminals, significantly depressed excitatory field potentials recorded in layer V during stimulation of layer II/III. The depression was reversible, dose dependent and correlated with increased paired pulse facilitation, suggesting that amphetamine inhibits the presynaptic release of glutamate. Pharmacological dissection of this response showed that dopamine D1 receptors are likely to mediate the effects of endogenous dopamine on excitatory synaptic transmission, with little effect of alpha2 adrenergic receptors, serotonin receptors, or D2 dopamine receptors. The time to peak amphetamine effect was longer than expected based on diffusion, suggesting that to raise dopamine levels in brain slices amphetamine may need to be transported into the presynaptic terminals. These results provide evidence that D1/D5 receptors depress glutamate release at this cortical synapse, and suggest that amphetamine will have profound and persistent effects on PFC functioning in vivo. Dysregulation of this mechanism could contribute to the impairment in cognitive performance associated with abnormal PFC dopamine receptor activation.

Shift of adenosine kinase expression from neurons to astrocytes during postnatal development suggests dual functionality of the enzyme

Adenosine is a potent modulator of excitatory neurotransmission, especially in seizure-prone regions such as the hippocampal formation. In adult brain ambient levels of adenosine are controlled by adenosine kinase (ADK), the major adenosine-metabolizing enzyme, expressed most strongly in astrocytes. Since ontogeny of the adenosine system is largely unknown, we investigated ADK expression and cellular localization during postnatal development of the mouse brain, using immunofluorescence staining with cell-type specific markers. At early postnatal stages ADK immunoreactivity was prominent in neurons, notably in cerebral cortex and hippocampus. Thereafter, as seen best in hippocampus, ADK gradually disappeared from neurons and appeared in newly developed nestin- and glial fibrillary acidic protein (GFAP)-positive astrocytes. Furthermore, the region-specific downregulation of neuronal ADK coincided with the onset of myelination, as visualized by myelin basic protein staining. After postnatal day 14 (P14), the transition from neuronal to astrocytic ADK expression was complete, except in a subset of neurons that retained ADK until adulthood in specific regions, such as striatum. Moreover, neuronal progenitors in the adult dentate gyrus lacked ADK. Finally, recordings of excitatory field potentials in acute slice preparations revealed a reduced adenosinergic inhibition in P14 hippocampus compared with adult. These findings suggest distinct roles for adenosine in the developing and adult brain. First, ADK expression in young neurons may provide a salvage pathway to utilize adenosine in nucleic acid synthesis, thus supporting differentiation and plasticity and influencing myelination; and second, adult ADK expression in astrocytes may offer a mechanism to regulate adenosine levels as a function of metabolic needs and synaptic activity, thus contributing to the differential resistance of young and adult animals to seizures.

Topiramate alters excitatory synaptic transmission in mouse hippocampus

Antiepileptic drugs may exert neuroprotective effects by decreasing excessive membrane excitability, neurotransmitter release, or postsynaptic Ca 2+ entry. To assess these sites of action, we combined fluorescence Ca 2+ imaging with extracellular field recording to analyze axonal excitability, evoked presynaptic Ca 2+ entry through presynaptic Ca 2+ channels, postsynaptic excitatory field potentials (fEPSP), and postsynaptic Ca 2+ buildup ([Ca post]) at the mouse hippocampal CA3–CA1 synapse exposed to topiramate (TPM). Topiramate had no effect on presynaptic Ca 2+ entry, and produced only a minor inhibition of axonal excitability. Topiramate at concentrations up to 100 μM only slightly reduced the amplitude of the evoked fEPSP, but strongly inhibited the [Ca post] evoked by repetitive synaptic activation. Postsynaptically, the action of TPM on the fEPSP and [Ca post] was not mediated by an inhibition of the NMDA receptor, or by direct modulation of voltage-dependent Ca 2+ channels, but reflected reduced somatic or dendritic membrane depolarization by AMPA and kainate receptors. These results are consistent with the known anticonvulsant properties of TPM. In addition, the ability of TPM to reduce postsynaptic Ca 2+ buildup may provide a potential mechanism for neuronal protection during paroxysmal firing associated with epileptic seizures.

Nicotinic effects on excitatory field potentials recorded from the immature CA3 area of rat hippocampal slices.

We investigated the nicotinic modulation of the excitatory field potentials recorded from the immature (postnatal day 10-20) hippocampal CA3 area, in the presence of the GABA(A) antagonist bicuculline methiodide (BMI, 10 microM). Nicotine (50 microM) enhanced the evoked field potentials; its effects were also observed in the presence of the GABA(B) antagonist 2-hydroxy-saclofen (250 microM; added to BMI) and were blocked by pre-perfusion with the nicotinic antagonist hexamethonium (HXM, 50 microM). The potentiating effects of nicotine in BMI persisted during prolonged perfusion (more than 20 min), while those in control perfusion medium were transient. The nicotinic antagonists HXM (50 microM), methyllycaconitine (MLA, 0.01 microM) and dihydro-beta-erythroidine (DHbetaE, 50 microM) potentiated CA3-evoked field potentials. Perfusion of HXM in the presence of the anticholinesterase eserine (1 microM) or the muscarinic antagonist atropine (1 microM) did not alter its effects. None of the nicotinic agents tested changed the frequency of spontaneous BMI-induced epileptiform discharges (nicotine, HXM, MLA, DhbetaE), suggesting that nicotinic receptors do not drive spontaneous epileptiform discharges in this in vitro model. These experiments demonstrate that nicotinic receptors are activated tonically during disinhibition and modulate the activity of excitatory synapses in the immature CA3 hippocampal area. The persistent nicotinic facilitatory effects during disinhibition versus the transient in control conditions indicate that nicotinic modulation depends on environmental conditions and also that nicotinic receptors may be a contributing factor in early-life seizures.

Field potential analysis of synaptic transmission in spiking neurons in a sparse and irregular neuronal structure in vitro

Extracellular field potential (FP) recording of dendritic depolarizations evoked by afferent stimulation is widely used as a quantitative measure of excitatory synaptic transmission in brain slices in-vitro for structures with high neuronal density and regularity such as hippocampus, neocortex and cerebellum. On the other hand, FP recordings of somatic depolarizations induced by orthodromic or antidromic stimulation of afferent or efferent nerves have been used in some in-vivo preparations for mapping the central projections of these nerves. In this study, we examined the applicability of somatic FPs as a measure of excitatory synaptic transmission in a sparse and irregular brain structure. Excitatory FPs were induced in nucleus tractus solitarius (NTS) in the dorsal medulla by electrical impulse stimulation of primary afferent fibers in the tractus solitarius (TS) in rat brainstem slices in vitro. The evoked FP was rapid and biphasic, and was stimulus-intensity dependent and saturable. The morphology of these somatic FPs resembled the dendritic FPs found in hippocampal and neocortical slices, with an excitatory postsynaptic component that exhibited similar pharmacological and stimulus frequency-dependent properties as found in NTS cells with intracellular or whole-cell recordings. Simultaneous FP and whole-cell recordings revealed that the postsynaptic component of FP was associated with neuronal firing rather than subthreshold membrane depolarizations. We conclude that somatic FP recording provides a simple and reliable measure of excitatory neurotransmission in the TS-NTS pathway and is a useful alternative or adjunct to intracellular or whole-cell recordings especially for studies of long-term synaptic plasticity in spiking neurons. This technique may also be applicable to other brain regions that lack the regular and dense organization of hippocampal and neocortical structures.

Induction of stable long-term depression in vivo in the hippocampal-prefrontal cortex pathway.

We studied excitatory field potentials in the medial prefrontal cortex (mPFC, prelimbic area) to electrostimulation of the ventral hippocampus (CA1/subicular region) in the anaesthetized rat. Nine hundred stimulus trains (5 pulses at 250 Hz) applied at 1 Hz to the ventral hippocampus significantly and persistently depressed the amplitude and maximal slope ( approximately 55% for each index) of the prelimbic field potentials, but did not change the latency of the maximal slope or peak negativity. Twelve stimulus trains (50 pulses at 250 Hz) applied subsequently at 0.1 Hz restored the depression back to control level, and this reversible depression was maintained for at least 13 h. Cumulative depressive effects on the prelimbic field potential amplitude and maximal slope were observed upon addition of stimulus trains in the hippocampus. An important implication of the results is that the direct pathway from the hippocampus to the mPFC in the rat retains long-term depression (LTD) as a neuroplastic form in vivo. This form could cooperate with long-term potentiation (LTP) and such a bi-directional synaptic plasticity in the prefrontal cortex contributes to how cortical neural networks store information.