Hypersynchronous ictal onset in the perirhinal cortex results from dynamic weakening in inhibition.

We obtained field, K(+) selective and "sharp" intracellular recordings from the rat entorhinal (EC) and perirhinal (PC) cortices in an in vitro brain slice preparation to identify the events occurring at interictal-to-ictal transition during 4-aminopyridine application. Field recordings revealed interictal- (duration: 1.1 to 2.2s) and ictal-like (duration: 31 to 103s) activity occurring synchronously in EC and PC; in addition, interictal spiking in PC increased in frequency shortly before the onset of ictal oscillatory activity thus resembling the hypersynchronous seizure onset seen in epileptic patients and in in vivo animal models. Intracellular recordings with K-acetate+QX314-filled pipettes in PC principal cells showed that spikes at ictal onset had post-burst hyperpolarizations (presumably mediated by postsynaptic GABAA receptors), which gradually decreased in amplitude. This trend was associated with a progressive positive shift of the post-burst hyperpolarization reversal potential. Finally, the transient elevations in [K(+)]o (up to 4.4mM from a base line of 3.2mM) - which occurred with the interictal events in PC - progressively increased (up to 7.3mM) with the spike immediately preceding ictal onset. Our findings indicate that hypersynchronous seizure onset in rat PC is caused by dynamic weakening of GABAA receptor signaling presumably resulting from [K(+)]o accumulation.

Two different interictal spike patterns anticipate ictal activity in vitro.

4-Aminopyridine (4AP, 50 µM) induces interictal- and ictal-like discharges in brain slices including parahippocampal areas such as the entorhinal cortex (EC) but the relation between these two types of epileptiform activity remains undifined. Here, by employing field potential recordings in rat EC slices during 4AP application, we found that: (i) interictal events have a wide range of duration (0.4-3.3 s) and interval of occurrence (1.4-84 s); (ii) ictal discharges are either preceded by an isolated "slow" interictal discharge (ISID; duration=1.5 ± 0.1s, interval of occurrence=33.8 ± 1.8 s) or suddenly initiate from a pattern of frequent polispike interictal discharge (FPID; duration=0.8 ± 0.1 s; interval of occurrence=2.7 ± 0.2 s); and (iii) ISID-triggered ictal events have longer duration (116 ± 7.3s) and interval of occurrence (425.8 ± 42.3 s) than those initiating suddenly during FPID (58.3 ± 7.8 s and 202.1 ± 21.8 s, respectively). Glutamatergic receptor antagonists abolished ictal discharges in all experiments, markedly reduced FPIDs but did not influence ISIDs. We also discovered that high-frequency oscillations (HFOs, 80-500 Hz) occur more frequently during ISIDs as compared to FPIDs, and mainly coincide with the onset of ISID-triggered ictal discharges. These findings indicate that interictal events may define ictal onset features resembling those seen in vivo in low-voltage fast activity onset seizures. We propose a similar condition to occur in vivo in temporal lobe epileptic patients and animal models.

Short-duration epileptic discharges show a distinct phase preference during ongoing hippocampal slow oscillations.

Non-REM (slow-wave) sleep has been shown to facilitate temporal lobe epileptiform events, whereas REM sleep seems more restrictive. This state-dependent modulation may be the result of the enhancement of excitatory synaptic transmission and/or the degree of network synchronization expressed within the hippocampus of the temporal lobe. The slow oscillation (SO), a ∼1 Hz oscillatory pattern expressed during non-REM sleep and urethane anesthesia, has been recently shown to facilitate the generation, maintenance, and propagation of stimulus-evoked epileptiform activity in the hippocampus. To further address the state-dependent modulation of epileptic activity during the SO, we studied the properties of short-duration interictal-like activity generated by focal application of penicillin in the hippocampus of urethane-anesthetized rats. Epileptiform spikes were larger but only slightly more prevalent during the SO as opposed to the theta (REM-like) state. More notably, however, epileptic spikes had a significant tendency to occur just following the peak negativity of ongoing SO cycles. Because of the known phase-dependent changes in 1) synaptic excitability (just following the positive peak of the SO) and 2) network synchronization (during the negative peak of the SO), these results suggest that it is the synchrony and not the changes in synaptic excitability that lead to the facilitation of epileptiform activity during sleep-like slow wave states.

Selective changes in inhibition as determinants for limited hyperexcitability in the insular cortex of epileptic rats.

The insular cortex (IC) is involved in the generalization of epileptic discharges in temporal lobe epilepsy (TLE), whereas seizures originating in the IC can mimic the epileptic phenotype seen in some patients with TLE. However, few studies have addressed the changes occurring in the IC in TLE animal models. Here, we analyzed the immunohistochemical and electrophysiological properties of IC networks in non-epileptic control and pilocarpine-treated epileptic rats. Neurons identified with a neuron-specific nuclear protein antibody showed similar counts in the two types of tissue but parvalbumin- and neuropeptide Y-positive interneurons were significantly decreased (parvalbumin, approximately -35%; neuropeptide Y, approximately -38%; P < 0.01) in the epileptic IC. Non-adapting neurons were seen more frequently in the epileptic IC during intracellular injection of depolarizing current pulses. In addition, single-shock electrical stimuli elicited network-driven epileptiform responses in 87% of epileptic and 22% of non-epileptic control neurons (P < 0.01) but spontaneous postsynaptic potentials had similar amplitude, duration and intervals of occurrence in the two groups. Finally, pharmacologically isolated, GABA(A) receptor-mediated inhibitory postsynaptic potentials had more negative reversal potential (P < 0.01) and higher peak conductance (P < 0.05) in epileptic tissue. These data reveal moderate increased network excitability in the IC of pilocarpine-treated epileptic rats. We propose that this limited degree of hyperexcitability originates from the loss of parvalbumin- and neuropeptide Y-positive interneurons that is compensated by an increased drive for GABA(A) receptor-mediated inhibition.

Network hyperexcitability within the deep layers of the pilocarpine-treated rat entorhinal cortex.

In this study we report that in the presence of normal buffer, epileptiform discharges occur spontaneously (duration = 2.60 +/- 0.49 s) or can be induced by electrical stimuli (duration = 2.50 +/- 0.62 s) in the entorhinal cortex (EC) of brain slices obtained from pilocarpine-treated rats but not in those from age-matched, nonepileptic control (NEC) animals. These network-driven epileptiform events consist of field oscillatory sequences at frequencies greater than 200 Hz that most often initiate in the lateral EC and propagate to the medial EC with 4-63 ms delays. The NMDA receptor antagonist CPP depresses the rate of occurrence (P < 0.01) of these spontaneous epileptiform discharges but fails in blocking them. Paradoxically, stimulus-induced epileptiform responses are enhanced in duration during CPP application. However, concomitant application of NMDA and non-NMDA glutamatergic antagonists abolishes spontaneous and stimulus-induced epileptiform events. Intracellular recordings from lateral EC layer V cells indicate a lower frequency of spontaneous hyperpolarizing postsynaptic potentials in pilocarpine-treated tissue than in NEC (P < 0.002) both under control conditions and with glutamatergic receptor blockade; the reversal potential of pharmacologically isolated GABA(A) receptor-mediated inhibitory postsynaptic potentials has similar values in the two types of tissue. Finally, immunohistochemical analysis shows that parvalbumin-positive interneurons are selectively reduced in number in EC deep layers. Collectively, these results indicate that reduced inhibition within the pilocarpine-treated EC layer V may promote network epileptic hyperexcitability.

GABA(B) receptor activation and limbic network ictogenesis.

Rat brain slices containing interconnected hippocampus and entorhinal cortex (EC) responded to 4-aminopyridine (50 microM) application by generating: (i) CA3-driven interictal discharges that propagated to the EC; and (ii) N-methyl-D-aspartic (NMDA) acid receptor-dependent ictal events originating in EC (cf. J. Neurosci. 17 (1997) 9308 for experiments made in brain slices). Ictal discharges disappeared within 1-2 h, but were re-established by cutting the Schaffer collaterals, which abolished CA3-driven interictal discharge propagation to EC. In intact slices, GABA(B) receptor activation by baclofen (5-40 microM): (i) depressed CA3-driven interictal activity; and (ii) disclosed non-NMDA glutamatergic receptor-dependent ictal discharges originating in CA3 and propagating to EC. These effects were reversed by the GABA(B) receptor antagonist CGP 35348 (0.5 mM). Application of increasing baclofen doses to slices in which hippocampus and EC networks were surgically isolated decreased epileptiform events with an IC50 that was lower in EC (0.6 microM; n = 12) than in CA3 (2.5 microM; n = 12). Hence, under control conditions, EC ictogenesis depends on NMDA receptor function and is controlled by CA3-driven output activity; in contrast, following GABA(B) receptor activation EC excitability is depressed to a greater extent than CA3, which leads to non-NMDA glutamatergic receptor-mediated ictogenesis in CA3. We propose that GABA(B) receptor modulation may represent an important mechanism for setting the site of initiation, the modalities of propagation and the glutamatergic receptor properties of ictogenesis in the limbic system and, perhaps, in mesial temporal lobe epilepsy patients.

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.

NMDA receptor-mediated transmission contributes to network 'hyperexcitability' in the rat insular cortex.

The insular cortex (IC) plays distinct roles under physiological and pathological conditions. However, the mechanisms regulating excitability in this area remain unknown. By employing field potential and sharp-electrode intracellular recordings in horizontal rat brain slices comprising the IC and the perirhinal cortex, we studied here the intrinsic and network characteristics of neurons in the agranular IC. These cells generated regular action potential firing with weak adaptation during intracellular injection of depolarizing current pulses, and were pyramidal in shape when neurobiotin filled. Spontaneous, field events (duration = 2.3 +/- 0.25 s; intervals of occurrence = 44.9 +/- 6.3 s) were identified in 22/52 slices and corresponded in IC neurons to intracellular depolarizations with action potential firing. Similar field and intracellular discharges were elicited in all slices by electrical stimuli. Antagonizing N-methyl-d-aspartate (NMDA) receptors blocked the spontaneous activity and reduced or abolished the stimulus-induced discharges. In the latter cases, stimuli elicited depolarizing events that became hyperpolarizing at about -64 mV, suggesting the contribution of gamma-aminobutyric acid (GABA)(A) receptor-mediated conductances. Our findings identify for the first time some functional properties of agranular IC neurons and point at a powerful NMDA receptor-mediated mechanism implementing network hyperexcitability. This feature may contribute to the role of IC in neurological disorders.

Subiculum network excitability is increased in a rodent model of temporal lobe epilepsy.

In this study, we used in vitro electrophysiology along with immunohistochemistry and molecular techniques to study the subiculum--a limbic structure that gates the information flow from and to the hippocampus--in pilocarpine-treated epileptic rats. Comparative data were obtained from age-matched nonepileptic controls (NEC). Subicular neurons in hippocampal-entorhinal cortex (EC) slices of epileptic rats were: (i) hyperexcitable when activated by CA1 or EC inputs; and (ii) generated spontaneous postsynaptic potentials at higher frequencies than NEC cells. Analysis of pharmacologically isolated, GABA(A) receptor-mediated inhibitory postsynaptic potentials revealed more positive reversal potentials in epileptic tissue (-67.8 +/- 6.3 mV, n = 16 vs. -74.8 +/- 3.6 mV in NEC, n = 13; P < 0.001) combined with a reduction in peak conductance (17.6 +/- 11.3 nS vs. 41.1 +/- 26.7 nS in NEC; P < 0.003). These electrophysiological data correlated in the epileptic subiculum with (i) reduced levels of mRNA expression and immunoreactivity of the neuron-specific potassium-chloride cotransporter 2; (ii) decreased number of parvalbumin-positive cells; and (iii) increased synaptophysin (a putative marker of sprouting) immunoreactivity. These findings identify an increase in network excitability within the subiculum of pilocarpine-treated, epileptic rats and point at a reduction in inhibition as an underlying mechanism.

Ripple activity in the dentate gyrus of dishinibited hippocampus-entorhinal cortex slices.

Fast oscillations at approximately 200 Hz, termed ripples, occur in the hippocampus and cortex of several species, including humans, and are thought to play a role in physiological (e.g., sensory information processing or memory consolidation) and pathological (e.g., seizures) processes. Blocking gamma-aminobutyric acid type A (GABA(A)) receptor-mediated inhibition represents one of the most often used models of epileptiform discharge. Here we found that bath application of the GABA(A) receptor antagonist picrotoxin (50 microM) to mouse hippocampus-entorhinal cortex slices induced spontaneous epileptiform activity (duration 536.6 +/- 146.1 msec, mean +/- SD; interval of occurrence 14.8 +/- 3.3 sec, n = 12) with two distinct phases of discharge; the first was characterized, in the dentate gyrus only, by high-frequency, field oscillations (ripples) at 206.3 +/- 23.4 Hz (n = 12), whereas the second component corresponded to afterdischarges in the theta range frequency. Ripples, which were also recorded in "minislices" only of the dentate gyrus, were unaffected by application of the mu-opioid receptor agonist (D-Ala2-N-Me-Phe,Gly-ol)enkephalin (10 microM; n = 6) or the N-methyl-D-aspartate (NMDA) receptor antagonist 3-(2-carboxy-piperazine-4-yl)-propyl-l-phosphonate (10 microM; n = 5). In contrast, the non-NMDA glutamatergic receptor antagonist 6-cyano-7-nitro-quinoxaline-2,3-dione (10 microM; n = 5) completely blocked all picrotoxin-induced activities. In addition, application of the GABA(B) receptor agonist baclofen (0.01-0.5 microM; n = 6) dose dependently and reversibly abolished all picrotoxin-induced activities. We also found that application of the gap-junction decouplers carbenoxolone (0.2-0.5 mM; n = 6) or octanol (0.2-0.5 mM; n = 3) blocked the second phase while leaving ripples unchanged. These findings demonstrate that the disinhibited dentate gyrus can generate ripple activity at approximately 200 Hz that is contributed by ionotropic glutamatergic mechanisms and is not dependent on either GABA(A) receptor-mediated or gap-junction mechanisms.