Age-dependence of the anticonvulsant effects of the GABA uptake inhibitor tiagabine in vitro

Molecular Mechanism of Selective Binding of NMS-P118 to PARP-1 and PARP-2: A Computational Perspective

The nuclear protein poly (ADP-ribose) polymerase-1 (PARP-1) inhibitors have been proven effective to potentiate both chemotherapeutic agents and radiotherapy. However, a major problem of most current PARP inhibitors is their lack of selectivity for PARP-1 and its closest isoform PARP-2. NMS-P118 is a highly selective PARP inhibitor that binds PARP-1 stronger than PARP-2 and has many advantages such as excellent pharmacokinetic profiles. In this study, molecular dynamics (MD) simulations of NMS-P118 in complex with PARP-1 and PARP-2 were performed to understand the molecular mechanism of its selectivity. Alanine scanning together with free energy calculation using MM/GBSA and interaction entropy reveal key residues that are responsible for the selectivity. Although the conformation of the binding pockets and NMS-P118 are very similar in PARP-1 and PARP-2, most of the hot-spot residues in PARP-1 have stronger binding free energy than the corresponding residues in PARP-2. Detailed analysis of the binding energy shows that the 4′4-difluorocyclohexyl ring on NMS-P118 form favorable hydrophobic interaction with Y889 in PARP-1. In addition, the H862 residue in PARP-1 has stronger binding free energy than H428 in PARP-2, which is due to shorter distance and stronger hydrogen bonds. Moreover, the negatively charged E763 residue in PARP-1 forms stronger electrostatic interaction energy with the positively charged NMS-P118 than the Q332 residue in PARP-2. These results rationalize the selectivity of NMS-P118 and may be useful for designing novel selective PARP inhibitors.

Inhibition of different GABA transporter systems is required to attenuate epileptiform activity in the CA3 region of the immature rat hippocampus

GABA transporters (GATs) are an essential element of the GABAergic system, which regulate excitability in the central nervous system and are thus used as targets for anticonvulsive therapy. However, in the immature nervous system the functions of the GABAergic system and the expression profile of GATs are distinct from the adult situation, obscuring to predict how different GAT isoforms influence epileptiform activity. Therefore we analyzed the effects of subtype specific GAT inhibitors on repetitive epileptiform discharges using field potential and whole-cell patch-clamp recordings in the CA3 region of hippocampal slices of immature (postnatal days 4-7) rats. These experiments revealed that inhibition of GAT-1 with either tiagabine (30 μM) or NO-711 (10 μM) exhibited only a minor anticonvulsive effect on repetitive epileptiform discharges. Blockade of GAT-2/3 with SNAP-5114 (40 μM) had no anticonvulsive effect, but significantly prolonged the decay of spontaneous GABAergic postsynaptic currents. In contrast, the combined application of 10 μM NO-711 and 40 μM SNAP-5114 blocked epileptiform activity in 33% of all slices and reduced the occurrence of epileptiform discharges by 54% in the remaining slices. In addition, the input resistance decreased by 10.5 ± 1.0% under this condition. These results indicate that both GAT-1 and GAT-2/3 are functional in the immature hippocampus and that only the combined inhibition of GAT 1-3 is sufficient to promote a considerable anticonvulsive effect. We conclude from these results that both GAT-1 and GAT-2/3 act synergistically to regulate the excitability in the immature hippocampus.

Age-dependent changes in the susceptibility to thiopental anesthesia in mice: analysis of the relationship to the functional expression of GABA transporter

The potency of anesthetics changes during development, probably due not only to pharmacokinetic factors such as differential distribution and/or metabolism, but also to pharmacodynamic factors such as changes to the GABAergic system in the brain. To explore the latter mechanism, we focused on the GABA transporter (GAT), the uptake system for GABA, which participates in the synaptic clearance of GABA. Thiopental-induced anesthesia, as assessed by the onset and duration of loss of the righting reflex, was more pronounced in 3-week-old mice than in 7-week-old mice. Both NO-711 and SKF89976A, selective GAT-1 inhibitors, significantly enhanced the anesthesia in the 7-week-old but not in the 3-week-old mice. In synaptosomes prepared from the cerebral cortex, the kinetics of GABA transport was similar between the two age groups, as assessed by [(3)H]GABA uptake assay. In addition, expression of GAT mRNA was similar between the two age groups, as assessed by quantitative RT-PCR. Thiopental reduced [(3)H]GABA uptake only at high concentrations in a similar manner at both ages. Conversely, the ability of SKF89976A to inhibit [(3)H]GABA uptake was greater in the 7-week-old mice than in the 3-week-old mice. Based on these results, GAT seems unlikely to contribute to the greater susceptibility to thiopental anesthesia in 3-week-old mice, while the increased ability of GABA uptake inhibitors to enhance thiopental-induced anesthesia in 7-week-old mice is at least partly due to higher sensitivity of GAT to the inhibitors.

Phasic GABAA -receptor activation is required to suppress epileptiform activity in the CA3 region of the immature rat hippocampus

PURPOSE

  Despite the consistent observation that γ-aminobutyric acid A (GABA(A) ) receptors mediate excitatory responses at perinatal stages, the role of the GABAergic system in the generation of neonatal epileptiform activity remains controversial. Therefore, we analyzed whether tonic and phasic GABAergic transmission had differential effects on neuronal excitability during early development.

METHODS

  We performed whole cell patch-clamp and field potential recordings in the CA3 region of hippocampal slices from immature (postnatal day 4-7) rats to analyze the effect of specific antagonists and modulators of tonic and phasic GABAergic components on neuronal excitability.

KEY FINDINGS

  The GABAergic antagonists gabazine (3 μm) and picrotoxin (100 μm) induced epileptiform discharges, whereas activation of GABA(A) receptors attenuated epileptiform discharges. Under low-Mg(2+) conditions, 100 nm gabazine and 1 μm picrotoxin were sufficient to provoke epileptiform activity in 63.2% (n = 19) and 53.8% (n = 26) of the slices, respectively. Whole-cell patch-clamp experiments revealed that these concentrations significantly reduced the amplitude of phasic GABAergic postsynaptic currents but had no effect on tonic currents. In contrast, 1-μm 4,5,6,7-tetrahydroisoxaz-olo[5,4-c]-pyridin-3-ol (THIP) induced a tonic current of -12 ± 2.5 pA (n = 6) and provoked epileptiform discharges in 57% (n = 21) of the slices.

SIGNIFICANCE

  We conclude from these results that in the early postnatal rat hippocampus a constant phasic synaptic activity is required to control excitability and prevent epileptiform activity, whereas tonic GABAergic currents can mediate excitatory responses. Pharmacologic intervention at comparable human developmental stages should consider these ambivalent GABAergic actions.

Tonic GABAergic control of mouse dentate granule cells during postnatal development

The dentate gyrus is the main hippocampal input structure receiving strong excitatory cortical afferents via the perforant path. Therefore, inhibition at this 'hippocampal gate' is important, particularly during postnatal development, when the hippocampal network is prone to seizures. The present study describes the development of tonic GABAergic inhibition in mouse dentate gyrus. A prominent tonic GABAergic component was already present at early postnatal stages (postnatal day 3), in contrast to the slowly developing phasic postsynaptic GABAergic currents. Tonic currents were mediated by GABA(A) receptors containing α(5)- and δ-subunits, which are sensitive to low ambient GABA concentrations. The extracellular GABA level was determined by synaptic GABA release and GABA uptake via the GABA transporter 1. The contribution of these main regulatory components was surprisingly stable during postnatal granule cell maturation. Throughout postnatal development, tonic GABAergic signals were inhibitory. They increased the action potential threshold of granule cells and reduced network excitability, starting as early as postnatal day 3. Thus, tonic inhibition is already functional at early developmental stages and plays a key role in regulating the excitation/inhibition balance of both the adult and the maturing dentate gyrus.

Intrinsic activation of GABA(A) receptors suppresses epileptiform activity in the cerebral cortex of immature mice

PURPOSE

Activation of ionotropic γ-aminobutyric acid type A (GABA(A) ) receptors induces in immature neocortical neurons a membrane depolarization that may contribute to the higher epilepsy susceptibility in newborns. To elucidate whether depolarizing GABAergic responses enhance or attenuate epileptiform activity in the immature neocortex, we investigated the effect of agonists, antagonists, and positive modulators of GABA(A) receptors on epileptiform activity.

METHODS

We performed in vitro field potential recordings on isolated whole neocortex preparations and whole cell recordings of identified pyramidal neurons in 400-μm slices of immature (postnatal day 1-7) mice. Epileptiform activity was induced by low Mg²(+) solutions with or without 50-100 μm 4-aminopyridine.

RESULTS

  Bath application of GABA (3-100 μm, in the presence of tiagabine) attenuated epileptiform activity. The GABA transporter isoform 1 (GAT-1) inhibitor tiagabine (30 μm) and the GAT-2/3 specific inhibitor SNAP 5114 (40 μm) reduced the frequency of epileptiform activity. The benzodiazepines midazolam (0.2 μm) and zolpidem (0.5 μm) as well as the barbiturate phenobarbital (30 μm) slightly attenuated epileptiform activity. Continuous bath application of the GABAergic antagonist gabazine (SR-95531, 2-3 μm) or picrotoxin (15 μm) induced epileptiform discharges.

DISCUSSION

These results demonstrate, that (1) the activation or positive modulation of GABA(A) receptors attenuates epileptiform activity, (2) GABA(A) antagonists mediate a disinhibition, and (3) GABA uptake contributes to the regulation of extracellular GABA in immature neocortex. We conclude from these findings that a constant inhibition via GABA(A) receptors is required to suppress epileptiform activity already in the immature neocortex.

Diphenyl diselenide-induced seizures in rat pups: possible interaction with GABAergic system

OBJECTIVES

The involvement of the GABAergic system in seizures induced by diphenyl diselenide (PhSe)₂ in rat pups was investigated.

METHODS

To this end, the effect of aminooxyacetic acid hemihydrochloride (AOAA, 20 mg/kg; by intraperitoneal route, i.p.), a GABA-T inhibitor; DL-2,4-diamino-n-butyric acid hydrochloride (DABA, 16 mg/kg; i.p.), an inhibitor of GABA uptake; and γ-aminobutyric acid (GABA, 10 and 40 mg/kg; i.p.), diazepam (3 mg/kg; i.p.) and phenobarbital (40 mg/kg; i.p.), GABAergic agonists as well as picrotoxin (1 mg/kg; i.p.), a GABAA receptor antagonist on (PhSe)₂ (50 and 500 mg/kg, by oral route, p.o.)-induced seizures, were studied. The [(3)H]GABA uptake levels by cortical and hippocampal slices in rat pups exposed to (PhSe)₂ were also carried out.

RESULTS

Pre-treatment with GABA (40 mg/kg), diazepam, phenobarbital, AOAA and DABA abolished the appearance of seizures induced by 50 mg/kg (PhSe)₂ in rat pups. Picrotoxin increased the percentage of convulsing rat pups from 42 to 100% and reduced significantly the onset for the first convulsive episode induced by (PhSe)₂ at the dose of 50 mg/kg. Diazepam and phenobarbital prolonged significantly the latency for the onset of the first convulsive episode caused by 500 mg/kg (PhSe)₂ in rat pups. [(3)H]GABA uptake levels were stimulated in cerebral cortical and hippocampal slices of convulsing rat pups administered with both doses of (PhSe)₂.

DISCUSSION

Our findings demonstrated that seizures induced by (PhSe)₂ are mediated, at least in part, by an interaction with GABAergic system.

Differential regulation of psychostimulant-induced gene expression of brain derived neurotrophic factor and the immediate-early gene Arc in the juvenile and adult brain

Psychostimulant drugs are widely used in children for the treatment of attention-deficit/hyperactivity disorder. Recent animal studies have suggested that exposure to these agents in early life could be detrimental to brain development. Here, for the first time, the effect of methylphenidate (MPH) and D-amphetamine (AMPH) on the expression of two key genes for neuronal development and plasticity, brain-derived neurotrophic factor (bdnf) and the effector immediate early gene activity-regulated, cytoskeletal-associated protein (Arc), was examined in both juvenile and adult rats. Both MPH [2 mg/kg, intraperitoneal (i.p.)] and AMPH (0.5 mg/kg, i.p.) induced marked decreases of bdnf mRNA in hippocampal and cortical brain regions of juveniles, whereas effects in adults were significantly less (hippocampus) or opposite (frontal cortex). In comparison, Arc mRNA was decreased (hippocampus and parietal cortex), largely unaffected (frontal cortex) or increased (striatum) in juveniles, whereas in adults, Arc mRNA increased in most brain regions. MPH-induced locomotion was also measured, and showed a much smaller increase in juveniles than in adults. In summary, our data show that the effects of MPH and AMPH on expression of the neurodevelopmentally important genes, bdnf and Arc, differ markedly in juvenile and adult rats, with juveniles showing evidence of brain region-specific decreases in both genes. These age-dependent effects on gene expression may be linked with the reported long-term harmful effects of psychostimulants in animal models.

Functional role of GABA transporters for kindling development in GLAST KO mice

Kindling-induced after discharge in electroencephalograms depends on the protein associated with glutamatergic and/or GABAergic neuronal transmission. In glutamate transporter knockout (GLAST KO) mice, the kindling phenomena in GLAST KO developed more slowly while the after discharge duration (ADD) was briefer than that of the control C57BL-6J mice. These findings indicate that either the excitatory function was suppressed or the inhibitory function was enhanced in GLAST KO kindling. To explain these phenomena, we used Western blotting to evaluate the alterations in the expression of hippocampal GABA transporter proteins, and the estimation of the effect on the process of epileptogenesis. Although no alterations were observed in the GAT-3 expression, the hippocampal GAT-1 expression was significantly suppressed in comparison to that of C57BL-6J mice. A decreased GAT-1 level in the hippocampus, which might be associated with the increased extracellular GABA level, may therefore inhibit both ADD and seizure propagation as shown by the amygdaloid kindling phenomenon observed in GLAST KO mice.

Hypoosmolar conditions reduce extracellular volume fraction and enhance epileptiform activity in the CA3 region of the immature rat hippocampus

The osmolarity of the extracellular space (ECS) compartment is an important factor determining the excitability of neuronal tissue. In the adult hippocampus an important role of osmolarity and ECS diffusion parameters on the susceptibility to epileptic events is well established, but the influence of hypo- and hyperosmolar conditions on the immature hippocampus remains elusive. To investigate the influence of osmolarity on epileptiform activity, extracellular field potentials were recorded in the CA3 region of hippocampal slices of immature (postnatal days 4-7) Wistar rats. The ECS diffusion parameters were determined by the real-time tetramethylammonium (TMA+) iontophoretic method with ion-selective microelectrodes in immature hippocampal slices and showed a lack of diffusion anisotropy; a tortuosity of about 1.39; and a volume fraction, alpha, of 0.41 +/- 0.01 (n = 10 slices). A reduction in osmolarity of -90 mOsm induced a decrease in alpha to 0.17 +/- 0.02 (n = 4 slices). The frequency of epileptiform activity elicited in 10-50 microM 4-AP-containing low-Mg2+ solution was increased under -90 mOsm and -40 mOsm hypoosmolar conditions by 39.9% +/- 8.1% (n = 16) and 24.1% +/- 4.8% (n = 10), respectively, whereas hyperosmolar solutions decreased the frequency. A -90-mOsm reduction in the osmolarity of low-Mg2+ solution induced epileptiform activity in nine of 19 slices. In summary, these results demonstrate that hypoosmolar conditions increased excitability and susceptibility to epileptiform activity in immature hippocampal slices, suggesting a functional role of the larger alpha in suppression of seizures.

Selective enhancement of tonic inhibition by increasing ambient GABA is insufficient to suppress excitotoxicity in hippocampal neurons

Gamma-aminobutyric acid (GABA) activates synaptic GABA(A) receptors to generate inhibitory postsynaptic potentials. GABA also acts on extrasynaptic GABA(A) receptors, resulting in tonic inhibition. The physiological role of tonic inhibition, however, remains elusive. We explored the neurophysiological significance of tonic inhibition by testing whether selective activation of extrasynaptic GABA(A) receptors is sufficient to curb excitotoxicity. Tonic inhibition was selectively enhanced by increasing ambient GABA. In both acute hippocampal slices and cultured hippocampal neurons, boosting tonic inhibition alone is insufficient to withstand the hyper-excitability of hippocampal neurons induced by low-magnesium (Mg2+) baths. Furthermore, selective activation of extrasynaptic GABA(A) receptors resulted in no significant neuroprotective effects against glutamate or low-Mg2+-induced neuronal cell deaths. These data imply that under physiological conditions extrasynaptic GABA(A) receptors are optimally activated by ambient GABA and that a further increase in extracellular GABA concentration will not significantly enhance the effect of tonic inhibition on neuronal excitability.

GABA uptake via GABA transporter-1 modulates GABAergic transmission in the immature hippocampus

GABA uptake limits GABA actions during synaptic responses when the density of active release sites is high or multiple axons are synchronously activated. GABA transporter-1 (GAT-1) is the main neuronal GABA transporter subtype and is already expressed in the early postnatal rat hippocampus. However, previous studies have demonstrated little functional role for the transporter during this developmental period. We used whole-cell voltage-clamp and field-potential recordings in hippocampal slices of neonatal rats (postnatal day 4-5) to study whether GAT-1 plays a role in GABAergic transmission during spontaneous population oscillations, which are seen as "giant depolarizing potentials" (GDPs) in intracellular recordings. We show that the GDP-associated GABAergic current observed in CA3 pyramidal neurons is strongly enhanced by the GAT-1-specific blocker NO-711 (1-[2-[[(diphenylmethylene)imino]oxy]ethyl]-1,2,5,6-tetrahydro-3-pyridinecarboxylic acid hydrochloride). Our results indicate a novel role for GAT-1 in the control of endogenous activity of the immature hippocampus.

Effects of antiepileptic drugs on refractory seizures in the intact immature corticohippocampal formation in vitro

PURPOSE

We developed a new in vitro preparation of immature rats, in which intact corticohippocampal formations (CHFs) depleted in magnesium ions become progressively epileptic. The better to characterize this model, we examined the effects of 14 antiepileptic drugs (AEDs) currently used in clinical practice.

METHODS

Recurrent ictal-like seizures (ILEs, four per hour) were generated in intact CHFs of P7-8 rats, and extracellular recordings were performed in the hippocampus and neocortex. AEDs were applied at clinically relevant concentrations (at least two), during 30 min after the third ILE. Their ability to prevent or to delay the next ILE was examined.

RESULTS

Valproic acid and benzodiazepines (clobazam and midazolam) but also phenobarbital and levetiracetam prevent the occurrence of seizures. In contrast, usual concentrations of carbamazepine (CBZ), phenytoin, vigabatrin, tiagabine, gabapentin, lamotrigine (LTG), topiramate, felbamate, and ethosuximide did not suppress ILEs. In addition, LTG and CBZ aggravate seizures in one third of the cases.

CONCLUSIONS

This intact in vitro preparation in immature animals appears to be quite resistant to most AEDs. Blockade of seizures was achieved with drugs acting mainly at the gamma-aminobutyric acid (GABA)A-receptor site but not with those that increase the amount of GABA. Drugs with a broad spectrum of activity are efficient but not those preferentially used in partial seizures or absences. We suggest that this preparation may correspond to a model of epilepsy with generalized convulsive seizures and could be helpful to develop new AEDs for refractory infantile epilepsies.

Unaltered control of extracellular GABA-concentration through GAT-1 in the hippocampus of rats after pilocarpine-induced status epilepticus

The uptake of the inhibitory transmitter GABA (gamma-aminobutyric acid) limits the efficacy of synaptic and tonic inhibition in brain tissue. It has been reported that GABA-uptake is down-regulated in temporal lobe epilepsy. This down-regulation may increase the inhibitory action of GABA but may also limit the anticonvulsant activity of GABA-uptake blockers. We have directly compared the function of GABA-uptake in hippocampal slices from normal and chronically epileptic rats. We raised the global extracellular concentration of GABA by bath-application of the agonist in the absence and presence of the GABA-uptake blocker tiagabine. GABA-induced currents were measured in dentate granule cells and CA1 pyramidal neurons in hippocampal slices. The potentiation of currents by tiagabine was taken as a measure for the efficacy of GABA-uptake in the hippocampal tissue. There was no difference between cells from control- or pilocarpine-treated animals in the response to GABA or in the conductance increase following application of tiagabine. Our data show that in the chronic phase of the pilocarpine-model GABA-uptake maintains its ability to control the extracellular background concentration of GABA.

Pre- rather than co-application of vigabatrin increases the efficacy of tiagabine in hippocampal slices

PURPOSE

The antiepileptic drug vigabatrin (VGB) increases intracellular availability of the inhibitory transmitter gamma-aminobutyric acid (GABA) by inhibition of GABA-transaminase. A blockade of the GABA uptake is the main mechanism of action of tiagabine (TGB). Based on this, the two antiepileptic drugs (AEDs) can be speculated to act synergistically so that their combined antiepileptic efficacy is supraadditive.

METHODS

To test this, experiments were performed on hippocampal slices of guinea-pigs. As an epilepsy model, epileptiform field potentials (EFPs) were induced by omission of Mg2+ from the bath solution and recorded in stratum pyramidale of the CA3 region. VGB (7.5 microM) and TGB (0.75 microM) were added to the superfusate.

RESULTS

VGB, given alone, failed to decrease the repetition rate of EFPs. Similarly, TGB applied alone only transiently led to a nonsignificant reduction of the EFP frequency. Combining VGB and TGB, their suppressive efficacy increased, yielding a significant reduction of EFP frequency, which, however, again did not persist. Pretreatment of the preparations with VGB for 2 h, followed by additional application of TGB, or TGB alone, drastically and persistently potentiated the effects.

CONCLUSIONS

These results demonstrate that VGB and TGB show favorable pharmacodynamic interactions, provided VGB is allowed to block intracellular GABA degradation before GABA uptake block by TGB.

Persistent epileptiform activity induced by low Mg2+ in intact immature brain structures

We have determined the properties of seizures induced in vitro during the first postnatal days using intact rat cortico-hippocampal formations (CHFs) and extracellular recordings. Two main patterns of activity were generated by nominally Mg2+-free ACSF in hippocampal and cortical regions: ictal-like events (ILEs) and late recurrent interictal discharges (LRDs). They were elicited at distinct developmental periods and displayed different pharmacological properties. ILEs were first observed in P1 CHFs 52 +/- 7 min after application of low-Mg2+ ACSF (frequency 1.5 +/- 0.3 h-1, duration 86 +/- 3 s). There is a progressive age-dependent maturation of ILEs characterized by a decrease in their onset and an increase in their frequency and duration. ILEs were abolished by d-APV and Mg2+ ions. From P7, ILEs were followed by LRDs that appeared 89 +/- 8 min after application of low-Mg2+ ACSF (frequency approximately 1 Hz, duration 0.66 s, amplitude 0.31 +/- 0.03 mV). LRDs were no longer sensitive to d-APV or Mg2+ ions and persisted for at least 24 h in low-Mg2+ or in normal ACSF. ILEs and LRDs were synchronized in limbic and cortical regions with 10-40 ms latency between the onsets of seizures. Using a double chamber that enables independent superfusion of two interconnected CHFs, we report that ILEs and LRDs generated in one CHF propagated readily to the other one that was being kept in ACSF. Therefore, at a critical period of brain development, recurrent seizures induce a permanent form of hyperactivity in intact brain structures and this preparation provides a unique opportunity to study the consequences of seizures at early developmental stages.

A new subdivision of anterior piriform cortex and associated deep nucleus with novel features of interest for olfaction and epilepsy

The anterior part of the piriform cortex (the APC) has been the focus of cortical-level studies of olfactory coding and associative processes and has attracted considerable attention as a result of a unique capacity to initiate generalized tonic-clonic seizures. Based on analysis of cytoarchitecture, connections, and immunocytochemical markers, a new subdivision of the APC and an associated deep nucleus are distinguished in the rat. As a result of its ventrorostral location in the APC, the new subdivision is termed the APC(VR). The deep nucleus is termed the pre-endopiriform nucleus (pEn) based on location and certain parallels to the endopiriform nucleus. The APC(VR) has unique features of interest for normal function: immunostaining suggests that it receives input from tufted cells in the olfactory bulb in addition to mitral cells, and it provides a heavy, rather selective projection from the piriform cortex to the ventrolateral orbital cortex (VLO), a prefrontal area where chemosensory, visual, and spatial information converges. The APC(VR) also has di- and tri-synaptic projections to the VLO via the pEn and the submedial thalamic nucleus. The pEn is of particular interest from a pathological standpoint because it corresponds in location to the physiologically defined "deep piriform cortex" ("area tempestas") from which convulsants initiate temporal lobe seizures, and blockade reduces ischemic damage to the hippocampus. Immunostaining revealed novel features of the pEn and APC(VR) that could alter excitability, including a near-absence of gamma-aminobutyric acid (GABA)ergic "cartridge" endings on axon initial segments, few cholecystokinin (CCK)-positive basket cells, and very low gamma-aminobutyric acid transporter-1 (GAT1)-like immunoreactivity. Normal functions of the APC(VR)-pEn may require a shaping of neuronal activity by inhibitory processes in a fashion that renders this region susceptible to pathological behavior.