Protein synthesis inhibition blocks maintenance but not induction of epileptogenesis in hippocampal slice

Kainic Acid Induces mTORC1-Dependent Expression of Elmo1 in Hippocampal Neurons

Epileptogenesis is a process triggered by initial environmental or genetic factors that result in epilepsy and may continue during disease progression. Important parts of this process include changes in transcriptome and the pathological rewiring of neuronal circuits that involves changes in neuronal morphology. Mammalian/mechanistic target of rapamycin (mTOR) is upregulated by proconvulsive drugs, e.g., kainic acid, and is needed for progression of epileptogenesis, but molecular aspects of its contribution are not fully understood. Since mTOR can modulate transcription, we tested if rapamycin, an mTOR complex 1 inhibitor, affects kainic acid-evoked transcriptome changes. Using microarray technology, we showed that rapamycin inhibits the kainic acid-induced expression of multiple functionally heterogeneous genes. We further focused on engulfment and cell motility 1 (Elmo1), which is a modulator of actin dynamics and therefore could contribute to pathological rewiring of neuronal circuits during epileptogenesis. We showed that prolonged overexpression of Elmo1 in cultured hippocampal neurons increased axonal growth, decreased dendritic spine density, and affected their shape. In conclusion, data presented herein show that increased mTORC1 activity in response to kainic acid has no global effect on gene expression. Instead, our findings suggest that mTORC1 inhibition may affect development of epilepsy, by modulating expression of specific subset of genes, including Elmo1, and point to a potential role for Elmo1 in morphological changes that accompany epileptogenesis.

Persistent ictal-like activity in rat entorhinal/perirhinal cortex following washout of 4-aminopyridine

Application of 4-aminopyridine (4-AP, 100μM) in a solution containing 0.6mM Mg(2+) and 1.2mM Ca(2+) to hippocampal-entorhinal-perirhinal slices of adult rat brain induced ictal-like epileptiform activity in entorhinal and perirhinal cortices as revealed by electrophysiological field potential recordings. The ictal-like activity persisted after washing out the 4-AP. This persistence indicated that a change had occurred in the slice so that it was now "epileptic" in the absence of the convulsant 4-AP. Induction of persistent ictal-like activity was dependent upon the concentration of divalent cations during 4-AP exposure; that is, although 4-AP caused ictal-like activity in approximately half the slices in solution containing 1.6mM Mg(2+) and 2.0mM Ca(2+), this ictal-like activity did not persist upon washout of the 4-AP. Expression of the persistent ictal-like epileptiform activity required ionotropic glutamate-mediated synaptic transmission: application of the AMPA/kainate receptor antagonist NBQX after 4-AP washout reduced persistent ictal-like activity, and the combined application of NBQX and the NMDA receptor antagonist d-AP5 completely blocked it. In order to investigate the mechanism of induction of persistent ictal-like activity, several agents were applied before the introduction of 4-AP. Application of d-AP5 did not block the onset of ictal-like activity upon introduction of 4-AP but did prevent the persistence of the ictal-like activity upon washout of the 4-AP. In contrast, induction of persistent ictal-like activity was not prevented by simultaneous application of the group I metabotropic glutamate receptor (mGluR) antagonists LY 367385 and MPEP or by application of the protein synthesis inhibitor cycloheximide. In conclusion, we have characterized a new in vitro model of epileptogenesis in which induction of ictal-like activity is dependent upon NMDA receptor activation but not upon group I mGluR activation or protein synthesis.

Continuous blockade of GABA-ergic inhibition induces novel forms of long-lasting plastic changes in apical dendrites of the hippocampal cornu ammonis 1 (CA1) in vitro

Hippocampal long-term potentiation (LTP) is considered as a fundamental mechanism for learning and memory formation. A role for GABA was reported for the induction and early but not late maintenance of LTP. We have now investigated whether GABA-receptor function is involved in the prolonged maintenance of LTP (>4 h) at afferent synapses at apical dendrites of cornu ammonis 1 (CA1)-pyramidal neurons in hippocampal slices in vitro. Our data demonstrate that GABA-receptor mediated events are not required for conventional, tetanically-induced early- or late-LTP in the hippocampal CA1-region in vitro. Inhibition of GABA-ergic transmission did not negatively influence either early- or late-LTP. In contrast, an additional facilitation was observed at time points corresponding to the establishment of late-LTP (after 3-4 h). Investigation of a second, non-tetanized control input to the same neuronal population revealed that the elevated potentiation of late-LTP in the tetanized input was not LTP-specific. Therefore, we have examined, whether continuous application of GABA-receptor inhibitors also affected the time course of the recorded potentials when a low-frequency stimulation protocol was used. Under these conditions two distinct forms of a late-onset potentiation occurred 5-6 h after drug application. Investigation of mechanisms responsible for this prolonged enhancement of potentials revealed that the higher form of potentiation (potentiation levels above 200%) was dependent on presynaptic activity and N-methyl-d-aspartate (NMDA)-receptor activation, whereas the lower form (potentiation less than 200%) did not require these mechanisms. However, the latter potentiation was prevented by nifedipine, an L-type voltage-dependent calcium channel inhibitor.

Protein synthesis is necessary for dendritic spine proliferation in adult brain slices

Dendritic spines, small protrusions from dendritic shafts, receive most of the excitatory synapses in cortical regions. Spines are highly plastic structures that can be rapidly produced or lost in response to a wide array of internal and external stimuli, and they proliferate in acute slice preparations [J. Neurosci. 19 (1999) 2876]. The goal of the present study was to determine if protein synthesis is necessary for this spine proliferation. We found that the addition of protein synthesis inhibitors to acute slices (in which spines otherwise proliferate) blocked new spine growth. Furthermore, a population of longer spines was observed after 2 h but these did not develop during protein synthesis blockade. These data suggest that protein synthesis is necessary for new spine growth in acute brain slice preparations and support literature suggesting that newly produced spines develop from filopodia-like protrusions.

Behavioral reinforcement of long-term potentiation in rat dentate gyrus in vivo is protein synthesis-dependent

A transient, protein synthesis-independent long-term potentiation (early-LTP, <4 h) can be reinforced into a maintained protein synthesis-dependent late-LTP (>4 h) by specific electrical stimulation of limbic structures (J. Neurosci. 21 (2001) 3697). Similarly, LTP-modulation can be obtained by behavioral stimuli with strong motivational content. However, the requirement of protein synthesis during behavioral reinforcement has not been shown so far. Thus, we have studied here this specific question using a behavioral reinforcement protocol, i.e. allowing water-deprived animals to drink 15 min after induction of early-LTP. This procedure transformed early-LTP into late-LTP. Anisomycin, a reversible protein synthesis inhibitor, abolished behavioral LTP-reinforcement. These results demonstrate that behavioral reinforcement depends on protein synthesis.

Activity-dependent induction of multitransmitter signaling onto pyramidal cells and interneurons of hippocampal area CA3

The granule cells of the dentate gyrus (DG) are considered to be glutamatergic, but they contain glutamic acid decarboxylase, gamma-amino butyric acid (GABA), and the vesicular GABA transporter mRNA. Their expression is regulated in an activity-dependent manner and coincides with the appearance of GABAergic transmission from the mossy fibers (MF) to pyramidal cells in area CA3. These data support the hypothesis that MF are able to release glutamate and GABA. Following the principle that a given neuron releases the same neurotransmitter(s) onto all its targets, we here demonstrate the emergence, after a generalized convulsive seizure, of MF GABAergic signaling sensitive to activation mGluR-III onto pyramidal cells and interneurons of CA3. Despite this, excitation overrides inhibition in interneurons, preventing disinhibition. Furthermore, on blockade of GABA and glutamate ionotropic receptors, an M1-cholinergic depolarizing signal is also revealed in both targets, which postsynaptically modulates the glutamatergic and GABAergic fast neurotransmission. The emergence of these nonglutamatergic signals depends on protein synthesis. In contrast to cholinergic responses evoked by associational/commissural fibers activation, cholinergic transmission evoked by DG stimulation is only observed after seizures and is strongly depressed by the activation of mGluR-II, whereas both are depressed by M2-AChR activation. With immunohistological experiments, we show that this cholinergic pathway runs parallel to the MF. Thus seizures compromise a delicate balance of excitation and inhibition, on which a complex interaction of different neurotransmitters emerges to counteract excitation at pre- and postsynaptic sites. Particularly, MF GABAergic inhibition emerges to exert an overall inhibitory action on CA3.

Activity-dependent expression of simultaneous glutamatergic and GABAergic neurotransmission from the mossy fibers in vitro

GABAergic transmission in the mossy fiber (MF) projection of the hippocampus is not normally detected in the rat. However, seizures induce simultaneous glutamatergic and GABAergic transmission in this projection, which coincides with an overexpression of GAD(67) and vesicular GABA transporter (VGAT) mRNA in the dentate gyrus (DG) and MF. To test whether this plastic change could be induced in an activity-dependent fashion in the absence of seizures, I recorded intracellularly from slices/cells that served as their own control, before and after direct or synaptic kindling of the DG in vitro. As expected, synaptic responses of CA3 pyramidal cells to test pulse DG stimulation were blocked by perfusion of N-methyl-D-aspartate (NMDA) and non-NMDA receptors' antagonists. However, after kindling the perforant path (3 1-s trains of 0.1-ms pulses at 100 Hz, 1 min apart from each other every 15 min for 3 h), which potentiated synaptic responses without inducing epileptiform activity, the perfusion of glutamatergic antagonists blocked the excitatory synaptic potential and isolated a fast bicuculline-sensitive inhibitory synaptic potential. Immunohistochemical experiments confirmed the overexpression of GAD(67) in the kindled slices. If kindling stimulation was provided just for 1 h or if it was completed in the presence of the protein synthesis inhibitor, cycloheximide, the expression of the GABAergic potential was prevented. Alternatively, when control synaptic responses of a given cell were first blocked, the direct kindling stimulation over the same site during perfusion of glutamatergic antagonists resulted in the induction of fast GABAergic potentials after 16.6 +/- 0.9 kindling trials. Furthermore, a high spacial specificity of this phenomenon was evidenced by recording synaptic responses of a given pyramidal cell to two different MF inputs. After blockade of all synaptic responses with the perfusion of glutamatergic antagonists, one of the inputs was kindled, while synaptic responses between the kindling trials were monitored by applying test pulse stimulation to both inputs. After 17 +/- 1 trials, test pulse stimulation provided over the kindled site evoked GABAergic potentials, whereas test pulse stimulation delivered to the alternative nonkindled parallel MF input remained ineffective. The DG-evoked GABAergic responses were inhibited by the activation of GABA(B)R and mGluR, whereby activation of group III mGluR with L-2-amino-4-phosphonobutyric acid (L-AP4) was significantly more effective than the activation of group II mGluR with DCG-IV. These data demonstrate that GABAergic transmission from the MF projection has distinctive features in the adult rat, and that its induction is dependent on protein synthesis responding in an activity-dependent fashion.

Sex differences in hippocampal slice excitability: role of testosterone

In vivo fluctuations in gonadal hormones alter hippocampal excitability and modulate both physiological and pathological hippocampal processes. To assess hormonal effects on excitability within a functional hippocampal circuit, extracellular CA1 field responses were compared in slices from intact male, intact female, orchidectomized male, and ovariectomized female rats. Secondly, the effects of in vitro applications of 17-beta estradiol, progesterone, or testosterone on baseline excitability of slices from gonadectomized rats were assessed versus pre-hormone baseline measures. Finally, using the in vitro kindling model of slice epileptogenesis, steroid hormone effects on interictal-like activity were also examined. Significant sex differences in excitatory postsynaptic potential amplitude were observed, with slices from males having larger excitatory postsynaptic potential amplitudes than those from females. Gonadectomy significantly decreased excitatory postsynaptic potential amplitude in slices from male rats. Slices from gonadectomized male and female rats also showed a decreased dendritic excitatory postsynaptic potential slope relative to slices from intact male and females rats. In vitro application of testosterone significantly increased excitatory postsynaptic potential amplitudes in slices from both orchidectomized males and ovariectomized females and the population spike amplitude of slices from ovariectomized females. Following in vitro kindling, slices from intact males showed greater spontaneous burst rates than slices from intact females, further suggesting an excitatory effect of testosterone. These results suggest: (1) a sex difference in the level of baseline excitability between slices from intact males and females as measured by excitatory postsynaptic potential amplitudes, (2) testosterone has excitatory effects on baseline physiology and kindled hippocampal responses, and (3) slices from males show a greater level of excitability than those from females in the in vitro kindling model.

Anti-epileptiform effects of audiogenic seizure priming on in vitro kindling in rat hippocampus

The effect of priming for audiogenic seizures (AGS) on the development of epileptiform activity in the hippocampus was studied using in vitro kindling (IVK) in Long-Evans rats. AGS priming consists of intense auditory stimulation during a critical period of auditory development, resulting in sound-induced clonic convulsions upon subsequent testing. Between postnatal day (PND) 28 and 50, slices from subjects primed and sham-primed for AGS on PND 18 were used for recording responses in area CA1 of hippocampus following Schaffer collateral stimulation from stratum radiatum of area CA2/CA3. The developmental priming procedure, which enhances auditory brainstem excitability, resulted in fewer afterdischarges in slices from primed subjects across initial IVK stimulation sequences. These results suggest that changes in excitability that occur with acoustic priming can initially diminish selective epileptiform response characteristics in forebrain areas such as the hippocampus.

Cytokine involvement in dynorphin-induced allodynia

Dynorphin A is an endogenous opioid peptide, which has previously been shown to produce a long-lasting allodynia and hyperalgesia in mice, behavioral states consistent with signs of clinically observed neuropathic pain. This dynorphin-induced allodynia was used as a pharmacological, central model of neuropathic pain. In this study, we examined the involvement of the cytokine IL-1beta, the transcription factor nuclear factor kappa B (NF-kappaB), and de novo protein synthesis in the development of allodynia induced by intrathecal (i.t.) administration of dynorphin in male ICR mice. Pretreatment with the protein synthesis inhibitor cycloheximide (0. 3-85nmol), the NF-kappaB inhibitor pyrrolidinedithiocarbamate (PDTC) (0.001-1000pmol), the IL-1 receptor antagonist (IL-1ra) protein (0. 01-100ng), the caspase-1 inhibitor (YVAD) (0.1-300pmol), and the anti-inflammatory cytokine IL-10 (0.1-300ng) all dose-dependently reduced the induction of dynorphin-induced allodynia. Finally, IL-10 administered within the first 24h after the dynorphin insult prevented the development of chronic allodynia. These results demonstrate that the anti-inflammatory cytokines IL-10 and IL-1ra impede the development of dynorphin-induced allodynia. These results also suggest that production of new proteins through NF-kappaB activation is required for the induction of allodynia. We speculate that IL-1ra, IL-10, PDTC and cycloheximide interfere with the central pro-inflammatory cascade. Modulation of cytokine activity in the spinal cord may therefore prove to be an effective therapeutic strategy for the treatment of chronic pain.

Requirement of protein synthesis for group I mGluR-mediated induction of epileptiform discharges

Picrotoxin (50 microM) elicited rhythmic synchronized bursting in CA3 pyramidal cells in guinea pig hippocampal slices. Addition of the selective group I metabotropic glutamate receptor (mGluR) agonist (S)-3,5-dihydroxyphenylglycine (25 microM) elicited an increase in burst frequency. This was soon followed by a slowly progressive increase in burst duration (BD), converting the brief 250-520 ms picrotoxin-induced synchronized bursts into prolonged discharges of 1-5 s in duration. BD was significantly increased within 60 min and reached a maximum after 2-2.5 h of agonist exposure. The protein synthesis inhibitors anisomycin (15 microM) or cycloheximide (25 microM) significantly impeded the mGluR-mediated development of the prolonged bursts; 90-120 min of agonist application failed to elicit the expected burst prolongation. By contrast, the mGluR-mediated enhancement of burst frequency progressed unimpeded. Furthermore, protein synthesis inhibitors had no significant effect on the frequency or duration of fully developed mGluR-induced prolonged discharges. These results suggest that the group I mGluR-mediated prolongation of synchronized bursts has a protein synthesis-dependent mechanism.

RGDS tetrapeptide and hippocampal in vitro kindling in rats: evidence for integrin-mediated physiological stability

We have examined a potential role for integrins in an animal model of epileptogenesis termed in vitro kindling. Integrins mediate cell-cell and cell-matrix interactions, and also participate in the transduction of information from the extracellular environment to the intracellular milieu. As many extracellular matrix (ECM) molecules contain the conserved amino acid sequence arg-gly-asp-ser (RGDS) at the integrin recognition site, integrin-ECM binding can be disrupted using RGDS peptides. Hippocampal slices were washed in either RGDS, gly-gly-gly-gly (GGGG), vehicle or artificial cerebral spinal fluid (ACSF) for 1 h prior to in vitro kindling. Baseline electrophysiological responses were unaltered by RGDS peptide. The RGDS-treated slices displayed a significant decrease in the rate of spontaneous bursts, whereas the period of spontaneous bursting increased dramatically. Our results indicate that the competitive peptide, RGDS, changed hippocampal slice excitability over time, indicating that interference with ECM-integrin binding may alter neuronal signaling through an RGDS binding site.

The role of inducible transcription factors in apoptotic nerve cell death

Recent studies have shown that certain types of nerve cell death in the brain occur by an apoptotic mechanism. Researchers have demonstrated that moderate hypoxic-ischemic (HI) episodes and status epilepticus (SE) can cause DNA fragmentation as well as other morphological features of apoptosis in neurons destined to die, whereas more severe HI episodes lead to neuronal necrosis and infarction. Although somewhat controversial, some studies have demonstrated that protein synthesis inhibition prevents HI-and SE-induced nerve cell death in the brain, suggesting that apoptotic nerve cell death in the adult brain is de novo protein synthesis-dependent (i.e., programmed). The identity of the proteins involved in HI-and SE-induced apoptosis in the adult brain is unclear, although based upon studies in cell culture, a number of potential cell death and anti-apoptosis genes have been identified. In addition, a number of studies have demonstrated that inducible transcription factors (ITFs) are expressed for prolonged periods in neurons undergoing apoptotic death following HI and SE. These results suggest that prolonged expression of ITFs (in particular c-jun) may form part of the biological cascade that induces apoptosis in adult neurons. These various studies are critically discussed and in particular the role of inducible transcription factors in neuronal apoptosis is evaluated.

Time-related antiepileptic effects of the synthetic glucocorticoid dexamethasone in rat hippocampal slices

The in vitro antiepileptic activity of the synthetic glucocorticoid dexamethasone (DEX) was tested in rat hippocampal slices on the CA1 epileptiform activity induced by sodium penicillin (PEN). Slice perfusion with 1 mM PEN produced within 60 min the development of a CA1 epileptiform bursting made up of an increase of the primary CA1 population spike followed by the appearance of secondary epileptiform population spikes. Slice perfusion with 100 microM DEX together with PEN (1 mM) partially prevented but did not block the expression of the CA1 epileptiform bursting as evidenced by a significant (P < 0.05) reduction of the duration of the bursting due to the epileptogenic agent. Slice perfusion with 50 microM DEX together with PEN (1 mM) failed to prevent or block the expression of the CA1 penicillin-induced epileptiform bursting. A 60 min slice pretreatment with 50-100 microM DEX followed by a slice perfusion with 50-100 microM DEX together with PEN (1 mM) prevented the expression of the CA1 epileptiform bursting. Cycloheximide (1 microM), a protein synthesis inhibitor, perfused together with DEX reverted the inhibitory effects of dexamethasone on the expression of the penicillin-induced CA1 epileptiform bursting. The results indicate that the synthetic glucocorticoid DEX presents concentration- and time-related in vitro antiepileptic effects. In addition, the data suggest that this inhibitory effect occurs via a protein synthesis-dependent mechanism.

Selective regulation of dendritic MAP2 mRNA levels in hippocampal granule cells by nitric oxide

Application of NMDA, or agents releasing nitric oxide (NO), onto the dendrites of hippocampal granule cells increased the levels of the mRNA encoding MAP2, a cytoskeletal component induced during periods of neurite outgrowth. Furthermore, local increases in the hybridisation signal in the molecular layer, representing dendritic MAP2 mRNA, occurred independently of changes in MAP2 mRNA levels in the cell body layer. The selective modulation of MAP2 mRNA in dendrites reveals a mechanism allowing a sustained stimulation of dendritic outgrowth to be confined to those regions of a neuron's dendritic arbour local to glutamate receptor stimulation.

Nitric oxide alters proenkephalin and prodynorphin gene expression in hippocampal granule cells

Application of N-methyl-D-aspartate on to the dendrites of hippocampal granule cells dramatically decreased prodynorphin messenger RNA levels in the affected cells while increasing proenkephalin messenger RNA levels. Sin-1 molsidomine (an agent which releases nitric oxide) and 8-bromo-cyclic GMP were similarly effective, and the actions of sin-1 molsidomine were blocked by inhibition of cyclic GMP-dependent protein kinase. Since, in this region, dynorphins act to inhibit potentiation of synaptic transmission, while enkephalins have excitatory effects, this switch in opioid gene expression is likely to have a prolonged effect on the efficiency of the mossy fibre synapses. In addition, the results demonstrate a powerful role for nitric oxide in the long-term regulation of hippocampal excitability.