Cerebrospinal fluid markers of neuronal and glial cell damage to monitor disease activity and predict long-term outcome in patients with autoimmune encephalitis.

BACKGROUND AND PURPOSE: Clinical symptoms and long-term outcome of autoimmune encephalitis are variable. Diagnosis requires multiple investigations, and treatment strategies must be individually tailored. Better biomarkers are needed for diagnosis, to monitor disease activity and to predict long-term outcome. The value of cerebrospinal fluid (CSF) markers of neuronal [neurofilament light chain protein (NFL), and total tau protein (T-tau)] and glial cell [glial fibrillary acidic protein (GFAP)] damage in patients with autoimmune encephalitis was investigated. METHODS: Demographic, clinical, magnetic resonance imaging, CSF and antibody-related data of 25 patients hospitalized for autoimmune encephalitis and followed for 1 year were retrospectively collected. Correlations between these data and consecutive CSF levels of NFL, T-tau and GFAP were investigated. Disability, assessed by the modified Rankin scale, was used for evaluation of disease activity and long-term outcome. RESULTS: The acute stage of autoimmune encephalitis was accompanied by high CSF levels of NFL and T-tau, whereas normal or significantly lower levels were observed after clinical improvement 1 year later. NFL and T-tau reacted in a similar way but at different speeds, with T-tau reacting faster. CSF levels of GFAP were initially moderately increased but did not change significantly later on. Final outcome (disability at 1 year) directly correlated with CSF-NFL and CSF-GFAP levels at all time-points and with CSF-T-tau at 3 ± 1 months. This correlation remained significant after age adjustment for CSF-NFL and T-tau but not for GFAP. CONCLUSION: In autoimmune encephalitis, CSF levels of neuronal and glial cell damage markers appear to reflect disease activity and long-term disability.

The diagnosis and treatment of limbic encephalitis.

The term limbic encephalitis (LE) was first introduced in 1968. While this disease was initially considered rare and is often fatal with very few treatment options, several reports published in the last decade provide a better description of this condition as well as possible causes and some cases of successful treatment. The clinical manifestation of LE is primarily defined by the subacute onset of short-term memory loss, seizures, confusion and psychiatric symptoms suggesting the involvement of the limbic system. In addition, EEG often shows focal or generalized slow wave or epileptiform activity, and MRI findings reveal hyperintense signals of the medial temporal lobes in T2-weighted or FLAIR images. The current literature suggests that LE is not a single disorder but is comprised of a group of autoimmune disorders predominantly affecting the limbic system. Before the diagnosis of LE can be determined, other causes of subacute encephalopathy must be excluded, especially those resulting from infectious aetiologies. LE has previously been regarded as a paraneoplastic phenomenon associated with the classical onconeuronal antibodies that are primarily directed against intracellular antigens. However, recent literature suggests that LE is also associated with antibodies that are directed against cell surface antigens, and these cases of LE display a much weaker association to the neoplasm. The treatment options for LE largely depend on the aetiology of the disease and involve the removal of the primary neoplasm. Therefore, a search for the underlying tumour is mandatory. In addition, immunotherapy has been successful in a significant number of patients where LE is not associated with cancer.

Don't be afraid to treat depression in patients with epilepsy!

Major depression and related depressive disorders are highly prevalent in the general population and even more so in patients with epilepsy. Yet depression in these patients remains underdiagnosed and undertreated. This is particularly worrisome as depression has greater negative impact on quality of life than seizure frequency. Additionally, depression is associated with poorer seizure control, and the risk of suicide in patients with epilepsy is greatly increased. Reluctance to treat depression results from the traditional belief that antidepressants should be restricted in epilepsy because of a supposed decrease in seizure threshold. However, there is growing evidence that many antidepressants rather have anticonvulsant effects. Experimental studies show that in critical brain regions such as the frontal lobes and the limbic system enforced serotonergic circuits increase seizure threshold. Clinical data suggest that modern antidepressants may reduce seizure frequency in patients with pharmacoresistant epilepsy. Here we review the concept that selective reuptake inhibitors of serotonin (SSRIs) have a positive effect on the mood disorder as well as on epilepsy. When adhering to the usual precautions, treatment with SSRIs in patients with epilepsy and depression is safe and should not be withheld.

LTP of AMPA and NMDA receptor-mediated signals: evidence for presynaptic expression and extrasynaptic glutamate spill-over.

We have addressed the expression of long-term potentiation (LTP) in hippocampal CA1 by comparing AMPA and NMDA receptor-(AMPAR- and NMDAR-) mediated postsynaptic signals. We find that potentiation of NMDAR-mediated signals accompanies LTP of AMPAR-mediated signals, and is associated with a change in variability implying an increase in quantal content. Further, tetanic LTP of NMDAR-mediated signals can be elicited when LTP of AMPAR-mediated signals is prevented. We propose that LTP is mainly expressed presynaptically, and that, while AMPARs respond only to glutamate from immediately apposed terminals, NMDARs also sense glutamate released from terminals presynaptic to neighboring cells. We also find that tetanic LTP increases the rate of depression of NMDAR-mediated signals by the use-dependent blocker MK-801, implying an increase in the glutamate release probability. These findings argue for a presynaptic contribution to LTP and for extrasynaptic spill-over of glutamate onto NMDARs.

Extrasynaptic glutamate spillover in the hippocampus: dependence on temperature and the role of active glutamate uptake.

At excitatory synapses on CA1 pyramidal cells of the hippocampus, a larger quantal content is sensed by N-methyl-D-aspartic acid receptors (NMDARs) than by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs). A novel explanation for this discrepancy is that glutamate released from terminals presynaptic to one cell can diffuse to and activate NMDARs, but not AMPARs, on a neighboring cell. If this occurs in the living brain, it could invalidate the view that glutamatergic synapses function as private communication channels between neurons. Here, we show that the discrepancy in quantal content mediated by the two receptors is greatly decreased at physiological temperature, compared with conventional recording conditions. This effect of temperature is not due to changes in release probability or uncovering of latent AMPARs. It is, however, partially reversed by the glutamate uptake inhibitor dihydrokainate. The results suggest that glutamate transporters play a critical role in limiting the extrasynaptic diffusion of glutamate, thereby minimizing cross-talk between neighboring excitatory synapses.

Extrasynaptic glutamate spillover in the hippocampus: evidence and implications.

In the mammalian brain most excitatory transmission is mediated by glutamate binding to AMPA and NMDA receptors. These receptors have markedly different biophysical properties, and at synapses in the CAI region of the hippocampus they play complementary roles in long-term potentiation (LTP): while postsynaptic NMDA receptor activation is necessary for the induction of this form of plasticity, AMPA receptors play a larger role in its expression. Recent studies in hippocampal slices have revealed a further striking difference in the behaviour of the two receptor types: NMDA receptors consistently sense a larger number of quanta of glutamate released from presynaptic terminals than do AMPA receptors. Two alternative explanations for this are either that AMPA receptors are functionally silent at a proportion of synapses (although they can be uncovered by LTP), or that glutamate can spill over from neighbouring synapses and selectively activate NMDA (but not AMPA) receptors. Both of these competing hypotheses have extensive implications for the mechanisms of expression of LTP. Extrasynaptic glutamate diffusion appears to depend critically on the recording temperature, but if excitatory synapses are sufficiently close for cross-talk to occur under physiological conditions, it could have profound implications for the specificity of synaptic communication in the brain.

Rituximab treatment did not aggravate ongoing progressive multifocal leukoencephalopathy in a patient with multiple sclerosis.

A multiple sclerosis (MS) patient developed progressive multifocal leukoencephalopathy (PML) after 43 months of natalizumab treatment. New clinical and magnetic resonance imaging (MRI) findings were initially misinterpreted as breakthrough MS disease activity and natalizumab treatment was replaced by rituximab treatment. The patient had a single infusion of rituximab 1000 mg before a definite PML diagnosis was confirmed. Despite undetectable levels of B-cells, JC virus DNA became undetectable in the cerebrospinal fluid by quantitative polymerase chain reaction. The patient partially recovered without any clinical or MRI signs of new MS activity. These findings suggest that B-cell depletion in a non-immune compromised individual did not prevent the patient from clearing the JC virus infection.

Ionotropic glutamate receptors. Their possible role in the expression of hippocampal synaptic plasticity.

In the brain, most fast excitatory synaptic transmission is mediated through L-glutamate acting on postsynaptic ionotropic glutamate receptors. These receptors are of two kinds--the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate (non-NMDA) and the N-methyl-D-aspartate (NMDA) receptors, which are thought to be colocalized onto the same postsynaptic elements. This excitatory transmission can be modulated both upward and downward, long-term potentiation (LTP) and long-term depression (LTD), respectively. Whether the expression of LTP/LTD is pre-or postsynaptically located (or both) remains an enigma. This article will focus on what postsynaptic modifications of the ionotropic glutamate receptors may possibly underly long-term potentiation/depression. It will discuss the character of LTP/ LTD with respect to the temporal characteristics and to the type of changes that appears in the non-NMDA and NMDA receptor-mediated synaptic currents, and what constraints these findings put on the possible expression mechanism(s) for LTP/LTD. It will be submitted that if a modification of the glutamate receptors does underly LTP/LTD, an increase/ decrease in the number of functional receptors is the most plausible alternative. This change in receptor number will have to include a coordinated change of both the non-NMDA and the NMDA receptors.

Onset Characteristics of Long-Term Potentiation in the Guinea-Pig Hippocampal CA1 Region in Vitro.

The temporal development of long-term potentiation (LTP) was examined in the CA1 region of the hippocampal slice preparation (bath temperature 30 degrees C). LTP was evoked by a single brief afferent tetanus (3 - 40 impulses at 50 Hz) given in the presence of picrotoxin (to facilitate LTP induction). Short-lasting potentiation processes unrelated to LTP were excluded by comparing the potentiation obtained in picrotoxin solution with that obtained in normal solution or in the presence of the N-methyl-D-aspartate receptor antagonist 2-amino-5-phosphonovalerate. LTP was also evoked by pairing single test volleys with brief (2 - 3 impulses) heterosynaptic tetani in picrotoxin solution. Both methods showed no significant rise of LTP until about 3 s after the induction event. LTP thereafter developed almost linearly towards a peak within 20 - 25 s after the tetanus, the time course being practially independent of the induction method and of the relative amount of LTP evoked. The latency and rise time of LTP depended on bath temperature, being about twice as long at 25 degrees C as at 30 degrees C. Following the peak, LTP rapidly decayed to less than half its peak value in 8 min, the decay tending to be less with longer trains. The LTP component reaching its peak 20 - 25 s after a tetanus was practically occluded after a saturating homosynaptic tetanization, and was only partially recovered 1 h afterwards. The latency to the onset of LTP suggests an indirect coupling between the calcium influx, presumed to trigger the potentiation, and the expression of LTP. The independence of the early time course with respect to the induction strength indicates that the intervening system(s) operates in a linear manner.

Increased synaptic inhibition in dentate gyrus of mice with reduced levels of endogenous brain-derived neurotrophic factor.

The aim of this study was to explore the role of endogenous neurotrophins for inhibitory synaptic transmission in the dentate gyrus of adult mice. Heterozygous knockout (+/-) mice or neurotrophin scavenging proteins were used to reduce the levels of endogenous brain-derived neurotrophic factor and neurotrophin-3. Patch-clamp recordings from dentate granule cells in brain slices showed that the frequency, but not the kinetics or amplitude, of miniature inhibitory postsynaptic currents was modulated in brain-derived neurotrophic factor +/- compared to wild-type (+/+) mice. Furthermore, paired-pulse depression of evoked inhibitory synaptic responses was increased in brain-derived neurotrophic factor +/- mice. Similar results were obtained in brain slices from brain-derived neurotrophic factor +/+ mice incubated with tyrosine receptor kinase B-immunoglobulin G, which scavenges endogenous brain-derived neurotrophic factor. The increased inhibitory synaptic activity in brain-derived neurotrophic factor +/- mice was accompanied by decreased excitability of the granule cells. No differences in the frequency, amplitude or kinetics of miniature inhibitory postsynaptic currents were seen between neurotrophin-3 +/- and +/+ mice. From these results we suggest that endogenous brain-derived neurotrophic factor, but not neurotrophin-3, has acute modulatory effects on synaptic inhibition onto dentate granule cells. The site of action seems to be located presynaptically, i.e. brain-derived neurotrophic factor regulates the properties of inhibitory interneurons, leading to increased excitability of dentate granule cells. We propose that through this mechanism, brain-derived neurotrophic factor can change the gating/filtering properties of the dentate gyrus for incoming information from the entorhinal cortex to hippocampus. This will have consequences for the recruitment of hippocampal neural circuitries both under physiological and pathological conditions, such as epileptogenesis.

Long-term potentiation and paired-pulse facilitation in the hippocampal CA1 region.

We have investigated the interaction between long-term potentiation (LTP) and paired-pulse facilitation (PPF) in the hippocampal CA1 region under different conditions of initial PPF. PPF is a short lasting presynaptic alteration in synaptic efficacy, determined by the presynaptic release probability, where a high and low PPF indicate a low and high release probability, respectively. LTP was associated with no, or only little, change in PPF, independent of the initial value of PPF and of the time of measurement within the first hour of LTP. These results support the notion that the expression mechanism of LTP is not regulated by the initial release probability and that LTP is due to a single expression mechanism within its first hour.

The early decay of long-term potentiation in the hippocampal CA1 region in vitro is reduced by activators of protein kinase C.

The effect of the exogenous protein kinase C (PKC) activator phorbol-12,13-diacetate (PDAc) on the early (0-10 min) time course of long-term potentiation (LTP) has been studied in the CA1 region of the guinea pig hippocampal slice. As shown previously, following a brief tetanus LTP develops almost linearly towards a peak value within 20-25 s, and decays thereafter rapidly to about a third of the peak value within 10 min after tetanization before a more stable level is reached. In the presence of 1.0 microM PDAc the growth phase of LTP is prolonged to 40-50 s, and the subsequent early decay is reduced. This reduction of the early decay resembles that previously found when increasing the number of afferent impulses of the LTP-generating tetanus. Examination of the early time course in solutions with different calcium-magnesium concentration ratios suggests that the observed effect of PDAc is not directly mediated via a change in presynaptic release probability, another effect observed after phorbol ester application. The results show that PKC activity is involved in the early stage of LTP development and support the idea that the early phase of LTP represents the same modification process as that underlying the more sustained phase of LTP.

Long-lasting potentiations evoked by a brief heterosynaptic tetanus in the guinea pig dentate gyrus in vitro.

The heterosynaptic effects induced by a brief afferent tetanization in the molecular layer of the dentate gyrus were investigated in the guinea pig hippocampal slice preparation using extracellular recording technique. At a brief interval (5 ms) between a single stimulation of the test afferents and the tetanus evoked in the conditioning afferents, a long-lasting (greater than 1 h) potentiation of the test field excitatory postsynaptic potential (EPSP) initial slope and amplitude was observed. This potentiation was occluded by prior homosynaptic tetanization of the test afferents, suggesting that it represents long-term potentiation (LTP). Thus, in the dentate gyrus, a single activation of a single test EPSP suffices to induce LTP when coinciding in time with a brief tetanus to other afferents. When not temporally paired with the test stimulation, i.e. at longer test-conditioning intervals (greater than 50 ms), the conditioning tetanus also elicited a long-lasting potentiation of the test field EPSP. This potentiation was, however, seen as a prolongation of the rising phase with no change in the field EPSP initial slope, and may represent a potentiation distinct from LTP.

Synaptic potentiation in the hippocampal CA1 region induced by application of N-methyl-D-aspartate.

The effect of local pressure application of N-methyl-D-aspartate (NMDA) in the synaptic layer of CA1 pyramidal cells was investigated in the guinea pig hippocampal slice preparation using extracellular recording technique. Application of NMDA produced a transient depression and a subsequent 30-60 min potentiation of the field excitatory postsynaptic potential (EPSP) seen as an increase of the initial slope and amplitude of the EPSP. The increase in amplitude was consistently greater than that of the initial slope. Prior tetanization that caused saturation of long-term potentiation prevented the generation of an NMDA-induced potentiation of the initial slope for more than 1-2 h, but not the generation of an increase of the amplitude.

Dissociation between long-term potentiation and associated changes in field EPSP waveform in the hippocampal CA1 region: an in vitro study in guinea pig brain slices.

The present study examines changes in field excitatory postsynaptic potential (EPSP) waveform in association with long-term potentiation (LTP) in the CA1 region of the hippocampal slice preparation. Experiments were performed in the presence of the GABAA-antagonist picrotoxin. With test field EPSP about one-third the size of that evoking spike activity (measured in the cell body layer along the same somatodendritic axis as the dendritic recording) a decreased decay time constant (approximately 8%) was seen in association with LTP. This change in field EPSP waveform was not associated with any apparent spike activity in the cell body recording. Nevertheless, several findings suggest that increased spike activity explains the change in the decay time constant of the field EPSP during LTP. First, when reducing the stimulation strength after the induction of LTP to obtain a field EPSP of the same magnitude as the pretetanus one, the change of the decay time constant was reduced to only 2.8%. Second, when using small test field EPSP (about one-fourth the size of that evoking spike activity) the decay time constant was not significantly affected in association with LTP. Third, when cutting the slice in such a manner that spike activity originating from somatic regions closer to the stimulating electrode was removed, the EPSPs decay time constant was not significantly affected in association with LTP. It is concluded that LTP is not associated with a change in the shape of the field EPSP.

Effect of adenosine-induced changes in presynaptic release probability on long-term potentiation in the hippocampal CA1 region.

In the present study some characteristics of long-term potentiation (LTP) in the hippocampal CA1 region were examined under different conditions of transmitter release. Adenosine A1 agonist/antagonists, or in some instances changes in the extracellular calcium/magnesium ratio, were used to alter release probability. The overall LTP time course (onset latency, growth phase, and subsequent decay for both the non-NMDA and NMDA receptor-mediated EPSPs) following a brief tetanus was essentially the same over an almost 10-fold variation in release probability (measured as change in field EPSP magnitude). The major difference observed was a faster initial decay of LTP evoked at low levels of release probability, possibly related to impaired induction conditions. It was also observed that LTP induced at one level of release probability occluded that induced at a lower (or higher) level, and that changes in release probability induced by adenosine agonist/antagonists affected potentiated and "naive" EPSPs to an equal extent. Taken together, these data do not provide support for the notion of different locations for LTP expression at different conditions of release probability. The results are also more compatible with the notion of a single, rather than several, expression mechanism(s) within the first hour of LTP in the hippocampal CA1 region.

Hyperammonemic encephalopathy induced by a combination of valproate and pivmecillinam.

We describe the clinical and neurophysiological findings in a case of hyperammonemic encephalopathy. A 72-year-old woman taking valproate (VPA), as monotherapy for her partial epilepsy developed urinary tract infection. She was treated with pivmecillinam 600 mg daily. The following days she deteriorated and became stuporous. At admission her serum ammonia level was increased (113 mmol/l) but the liver function appeared normal. EEG showed bilateral triphasic waves and continuous high-amplitude delta-theta wave. The patient recovered rapidly after discontinuation of VPA and i.v. treatment with cefuroxime for her urinary tract infection. VPA-induced hyperammonemic encephalopathy in adults is a rare phenomenon, especially when VPA is used as monotherapy. It has been suggested that the VPA-induced hyperammonemic encephalopathy is due to reduced serum carnitine concentration. Pivmecillinam, a widely used antibiotic for treatment of urinary tract infections, is also known to decrease the serum carnitine concentration. Our case shows that caution is required when treatment with VPA is combined with pivmecillinam due to the risk of developing hyperammonemic encephalopathy.

The role of mammalian ionotropic receptors in synaptic plasticity: LTP, LTD and epilepsy.

Synaptic plasticity is the foremost candidate mechanism to explain the rapid acquisition of memories. In the mammalian brain, the NMDA subclass of glutamate receptors plays a central role in the induction of several forms of use-dependent plasticity. The finding that modifications in synaptic strength are largely expressed by receptors of the AMPA subclass has focused attention on molecular mechanisms that affect their function and targeting. Receptor plasticity has also been reported in pathological situations, notably in animal and human forms of epilepsy. Which of these changes are causally implicated in the generation of seizures, and which may be compensatory or neuroprotective adaptations, has not been fully resolved.

Endogenous neurotrophin-3 regulates short-term plasticity at lateral perforant path-granule cell synapses.

In the adult brain, neurotrophin-3 (NT-3) is mainly localized in dentate granule cells, and its expression is decreased by various stimuli, e.g., seizure activity. We have examined the role of endogenous NT-3 for excitatory synaptic transmission at lateral perforant path-dentate granule cell synapses using hippocampal slices from NT-3 knock-out (+/-) and wild-type (+/+) mice. Paired-pulse facilitation (PPF) and also short-term synaptic plasticity induced by a brief, high-frequency train of afferent stimulation were reduced, but the expression of long-term potentiation was not affected in the NT-3+/- mice. Incubation of the slices with recombinant NT-3 reversed the deficit in PPF through a mechanism requiring de novo protein synthesis, implying that the impaired short-term plasticity does not result from a developmental alteration. No changes of overall presynaptic release probability, measured by the progressive block of NMDA receptor-mediated synaptic currents by MK-801, or desensitization of AMPA receptors were detected. Because NT-3 expression is reduced after focal seizures, impaired short-term facilitation may represent a protective response that limits the propagation of epileptiform activity from the entorhinal cortex to the hippocampus.

Extracellular glutamate diffusion determines the occupancy of glutamate receptors at CA1 synapses in the hippocampus.

Following exocytosis at excitatory synapses in the brain, glutamate binds to several subtypes of postsynaptic receptors. The degree of occupancy of AMPA and NMDA receptors at hippocampal synapses is, however, not known. One approach to estimate receptor occupancy is to examine quantal amplitude fluctuations of postsynaptic signals in hippocampal neurons studied in vitro. The results of such experiments suggest that NMDA receptors at CA1 synapses are activated not only by glutamate released from the immediately apposed presynaptic terminals, but also by glutamate spillover from neighbouring terminals. Numerical simulations point to the extracellular diffusion coefficient as a critical parameter that determines the extent of activation of receptors positioned at different distances from the release site. We have shown that raising the viscosity of the extracellular medium can modulate the diffusion coefficient, providing an experimental tool to investigate the role of diffusion in activation of synaptic and extrasynaptic receptors. Whether intersynaptic cross-talk mediated by NMDA receptors occurs in vivo remains to be determined. The theoretical and experimental approaches described here also promise to shed light on the roles of metabotropic and kainate receptors, which often occur in an extrasynaptic distribution, and are therefore positioned to sense glutamate escaping from the synaptic cleft.