Margherita D'Antuono

The Inhibitory Effects of Interleukin-6 on Synaptic Plasticity in the Rat Hippocampus Are Associated with an Inhibition of Mitogen-Activated Protein Kinase ERK

Several cytokines have short‐term effects on synaptic transmission and plasticity that are thought to be mediated by the activation of intracellular protein kinases. We have studied the effects of interleukin‐6 (IL‐6) on the expression of paired pulse facilitation (PPF), posttetanic potentiation (PTP), and long‐term potentiation (LTP) in the CA1 region of the hippocampus as well as on the activation of the signal transducer and activator of transcription‐3 (STAT3), the mitogen‐activated protein kinase ERK (MAPK/ERK), and the stress‐activated protein kinase/c‐Jun NH2‐terminal kinase (SAPK/JNK). IL‐6 induced a marked and dose‐dependent decrease in the expression of PTP and LTP that could be counteracted by the simultaneous treatment with the tyrosine kinase inhibitor lavendustin A (LavA) but did not significantly affect PPF. The IL‐6‐induced inhibition of PTP and LTP was accompanied by a simulation of STAT3 tyrosine phosphorylation and an inhibition of MAPK/ERK dual phosphorylation, in the absence of changes in the state of activation of SAPK/JNK. Both effects of IL‐6 on STAT3 and MAPK/ERK activation were effectively counteracted by LavA treatment. The results indicate the tyrosine kinases and MAPK/ERK are involved in hippocampal synaptic plasticity and may represent preferential intracellular targets for the actions of IL‐6 in the adult nervous system.

Interleukin‐6 inhibits neurotransmitter release and the spread of excitation in the rat cerebral cortex

Cytokines are extracellular mediators that have been reported to affect neurotransmitter release and synaptic plasticity phenomena when applied in vitro. Most of these effects occur rapidly after the application of the cytokines and are presumably mediated through the activation of protein phosphorylation processes. While many cytokines have an inflammatory action, interleukin‐6 (IL‐6) has been found to have a neuroprotective effect against ischaemia lesions and glutamate excitotoxicity, and to increase neuronal survival in a variety of experimental conditions. In this paper, the functional effects of IL‐6 on the spread of excitation visualized by dark‐field/infrared videomicroscopy in rat cortical slices and on glutamate release from cortical synaptosomes were analysed and correlated with the activation of the STAT3, mitogen‐activated protein kinase ERK (MAPK/ERK) and stress‐activated protein kinase/cJun NH2‐terminal kinase (SAPK/JNK) pathways. We have found that IL‐6 depresses the spread of excitation and evoked glutamate release in the cerebral cortex, and that these effects are accompanied by a stimulation of STAT3 tyrosine phosphorylation, an inhibition of MAPK/ERK activity, a decreased phosphorylation of the presynaptic MAPK/ERK substrate synapsin I and no detectable effects on SAPK/JNK. The effects of IL‐6 were effectively counteracted by treatment of the cortical slices with the tyrosine kinase inhibitor lavendustin A. The inhibitory effects of IL‐6 on glutamate release and on the spread of excitation in the rat cerebral cortex indicate that the protective effect of IL‐6 on neuronal survival could be mediated by a downregulation of neuronal activity, release of excitatory neurotransmitters and MAPK/ERK activity.

Neocortical potassium currents are enhanced by the antiepileptic drug lamotrigine.

Summary:  Purpose: We used field‐potential recordings in slices of rat cerebral cortex along with whole‐cell patch recordings from rat neocortical cells in culture to test the hypothesis that the antiepileptic drug (AED) lamotrigine (LTG) modulates K+‐mediated, hyperpolarizing currents.

Thalamocortical oscillations in a genetic model of absence seizures

We used field potential recordings in an in vitro thalamocortical slice preparation to compare the rhythmic oscillations generated by reciprocally connected networks of the thalamus and cerebral cortex obtained from epileptic (> 160 days old) WAG/Rij and age‐matched, nonepileptic control (NEC) rats. To increase neuronal excitability, and thus to elicit spontaneous field potential activity in vitro, we applied medium containing: (i) zero [Mg2+]; (ii) high [K+] (8.25 mm); or (iii) low concentrations of the K+ channel blocker 4‐aminopyridine (4AP, 0.5–1 µm). Of these procedures, only the last was effective in triggering oscillatory activity that depended on the type of tissue. Thus, during 4AP application: (i) sequences of fast (intraburst frequency 9.5–16.1 Hz) and slower (5–8.9 Hz) field potential oscillations (FPOs) were recorded in WAG/Rij slices (n = 23), but (ii) only fast FPOs were seen in NEC slices (n = 7). Slower FPOs in WAG/Rij slices reflected a larger degree of thalamocortical synchronization than fast FPOs, and disappeared after surgical separation of cortex and thalamus (n = 5); under these conditions fast FPOs continued to occur in thalamus only. In addition, fast and slower FPOs disappeared in all areas of the WAG/Rij slice during thalamic application of the excitatory amino acid receptor antagonist kynurenic acid (n = 3), while fast FPOs continued to occur in thalamus when kynurenic acid was applied to the cortex (n = 4). Bath application of the N‐methyl‐d‐aspartic acid (NMDA) receptor antagonist 3,3‐(2‐carboxypiperazine‐4‐yl)‐propyl‐1‐phosphonate (CPP) abolished slower FPOs in WAG/Rij cortex and thalamus (n = 6) without infuencing fast FPOs recorded in WAG/Rij (n = 6) or NEC slices (n = 4). Moreover, cortical application of CPP (n = 6) abated slower FPOs although they persisted following CPP application to the thalamus (n = 7). Our data demonstrate that highly synchronized, slower FPOs can occur during 4AP application in WAG/Rij but not in NEC slices. This activity, which may represent an in vitro hallmark of thalamocortical epileptogenicity, requires the function of reciprocally connected thalamic and cortical networks and depends on cortical NMDA receptor‐mediated mechanisms.

Impaired activation of CA3 pyramidal neurons in the epileptic hippocampus.

We employed in vitro and ex vivo imaging tools to characterize the function of limbic neuron networks in pilocarpine-treated and age-matched, nonepileptic control (NEC) rats. Pilocarpinetreated animals represent an established model of mesial temporal lobe epilepsy. Intrinsic optical signal (IOS) analysis of hippocampal-entorhinal cortex (EC) slices obtained from epileptic rats 3 wk after pilocarpine-induced status epilepticus (SE) revealed hyperexcitability in many limbic areas, but not in CA3 and medial EC layer III. By visualizing immunopositivity for FosB/ΔFosBrelated proteins—which accumulate in the nuclei of neurons activated by seizures—we found that: (1) 24 h after SE, FosB/ΔFosB immunoreactivity was absent in medial EC layer III, but abundant in dentate gyrus, hippocampus proper (including CA3) and subiculum; (2) FosB/ΔFosB levels progressively diminished 3 and 7 d after SE, whereas remaining elevated (p<0.01) in subiculum; (3) FosB/ΔFosB levels sharply increased 2 wk after SE (and remained elevated up to 3 wk) in dentate gyrus and in most of the other areas but not in CA3. A conspicuous neuronal damage was noticed in medial EC layer III, whereas hippocampus was more preserved. IOS analysis of the stimulus-induced responses in slices 3 wk after SE demonstrated that IOSs in CA3 were lower (p<0.05) than in NEC slices following dentate gyrus stimulation, but not when stimuli were delivered in CA3. These findings indicate that CA3 networks are hypoactive in comparision with other epileptic limbic areas. We propose that this feature may affect the ability of hippocampal outputs to control epileptiform synchronization in EC.

Masking synchronous GABA-mediated potentials controls limbic seizures.

Summary:  Purpose: We determined how CA3‐driven interictal discharges block ictal activity generated in the entorhinal cortex during bath application of 4‐aminopyridine (4AP, 50 μM).

In vitro ictogenesis and parahippocampal networks in a rodent model of temporal lobe epilepsy

Temporal lobe epilepsy (TLE) is a chronic epileptic disorder involving the hippocampal formation. Details on the interactions between the hippocampus proper and parahippocampal networks during ictogenesis remain, however, unclear. In addition, recent findings have shown that epileptic limbic networks maintained in vitro are paradoxically less responsive than non-epileptic control (NEC) tissue to application of the convulsant drug 4-aminopyridine (4AP). Field potential recordings allowed us to establish here the effects of 4AP in brain slices obtained from NEC and pilocarpine-treated epileptic rats; these slices included the hippocampus and parahippocampal areas such as entorhinal and perirhinal cortices and the amygdala. First, we found that both types of tissue generate epileptiform discharges with similar electrographic characteristics. Further investigation showed that generation of robust ictal-like discharges in the epileptic rat tissue is (i) favored by decreased hippocampal output (ii) reinforced by EC-subiculum interactions and (iii) predominantly driven by amygdala networks. We propose that a functional switch to alternative synaptic routes may promote network hyperexcitability in the epileptic limbic system.

Perirhinal cortex and temporal lobe epilepsy.

The perirhinal cortex—which is interconnected with several limbic structures and is intimately involved in learning and memory—plays major roles in pathological processes such as the kindling phenomenon of epileptogenesis and the spread of limbic seizures. Both features may be relevant to the pathophysiology of mesial temporal lobe epilepsy that represents the most refractory adult form of epilepsy with up to 30% of patients not achieving adequate seizure control. Compared to other limbic structures such as the hippocampus or the entorhinal cortex, the perirhinal area remains understudied and, in particular, detailed information on its dysfunctional characteristics remains scarce; this lack of information may be due to the fact that the perirhinal cortex is not grossly damaged in mesial temporal lobe epilepsy and in models mimicking this epileptic disorder. However, we have recently identified in pilocarpine-treated epileptic rats the presence of selective losses of interneuron subtypes along with increased synaptic excitability. In this review we: (i) highlight the fundamental electrophysiological properties of perirhinal cortex neurons; (ii) briefly stress the mechanisms underlying epileptiform synchronization in perirhinal cortex networks following epileptogenic pharmacological manipulations; and (iii) focus on the changes in neuronal excitability and cytoarchitecture of the perirhinal cortex occurring in the pilocarpine model of mesial temporal lobe epilepsy. Overall, these data indicate that perirhinal cortex networks are hyperexcitable in an animal model of temporal lobe epilepsy, and that this condition is associated with a selective cellular damage that is characterized by an age-dependent sensitivity of interneurons to precipitating injuries, such as status epilepticus.

Neocortical Hyperexcitability in a Genetic Model of Absence Seizures and Its Reduction by Levetiracetam

Summary:  Purpose: To study the effect of the antiepileptic drug levetiracetam (LEV) on the patterns of intrinsic optical signals (IOSs) generated by slices of the somatosensory cortex obtained from 3‐ and 6‐month‐old WAG/Rij and age‐matched, nonepileptic control (NEC) rats.

Epileptiform Synchronization and GABAB Receptor Antagonism in the Juvenile Rat Hippocampus

The GABAB receptor agonist baclofen enhances the epileptiform activity induced by 4-aminopyridine (4AP) in juvenile rat hippocampal slices. In this study, we used a similar experimental approach (i.e., field potential, intracellular, and [K+]o recordings in the CA3 area of slices obtained from 15–23-day-old rats) to establish whether antagonizing GABAB receptors could exert opposite (presumably anticonvulsant) effects. Bath application of 4AP (50 μM) induced spontaneous interictal and ictal discharges along with synchronous GABA receptor-mediated potentials. All types of 4AP-induced synchronous activity occurred more frequently during application of the GABAB receptor antagonistp3-amino-propyl,p-diethoxymethylphosphonic acid (CGP 35348) (0.1–1 mM; EC50 = 0.25 mM). Moreover, CGP 35348 augmented the frequency and, to a lesser extent, the duration of the asynchronous synaptic activity recorded intracellularly from CA3 pyramids (n = 19). In medium containing 4AP + ionotropic glutamatergic antagonists (which abolished interictal and ictal activity), CGP 35348 prolonged both GABA-receptor-mediated field potentials and the accompanying intracellular long-lasting depolarizations without modifying their rate (n = 12). The transient elevations in [K+]o associated with GABA receptor-mediated potentials in 4AP-containing medium (n = 7 slices) became larger during CGP 35348 application. Similar findings were obtained when CGP 35348 was applied to medium containing 4AP + ionotropic glutamatergic antagonists (n = 6). Thus, the effect of CGP 35348 on 4AP-induced epileptiform activity is not anticonvulsant and to some extent similar to what was reported in this model during GABAB receptor activation. We propose that the facilitation of ictal activity by CGP 35348 is mainly caused by the blockade of presynaptic GABAB receptor, leading to an increase in GABA release and subsequent larger [K+]o elevations.