Shortened-duration GABA(A) receptor-mediated synaptic potentials underlie enhanced CA1 excitability in a chronic model of temporal lobe epilepsy

Interaction of GABAA and GABAB antagonists after status epilepticus in immature rats

Among neurotransmitter systems affected by status epilepticus (SE) in adult rats are both GABAergic systems. To analyze possible changes of GABA_A and GABA_B systems in developing rats lithium-pilocarpine SE was induced at postnatal day 12 (P12). Seizures were elicited by a GABA_A antagonist pentylenetetrazol (PTZ) 3, 6, 9, and 13 days after SE (i.e., in P15, P18, P21, and P25 rats), and their possible potentiation by a GABA_B receptor antagonist CGP46381 was studied. Pilocarpine was replaced by saline in control animals (lithium-paraldehyde [LiPAR]). Pentylenetetrazol in a dose of 50 mg/kg s.c. elicited generalized seizures in nearly all 15-day-old naive rats and in 40% of 18-day-old ones but not in older animals. After SE, PTZ no longer elicited seizures in these two younger groups, i.e., sensitivity of GABA_A system was diminished. The GABA_B antagonist exhibited proconvulsant effect in P15 and P18 SE as well as LiPAR rats returning the incidence of PTZ-induced seizures to values of control animals. A decrease in the incidence of minimal clonic seizures was seen in P21 LiPAR animals; these seizures in the oldest group were not affected. Change of the effect from proconvulsant to anticonvulsant (or at least to no action) took place before postnatal day 21. Both SE and LiPAR animals exhibited similar changes but their intensity differed, effects in LiPAR controls were usually more expressed than in SE rats.

Changing effect of GABA B receptor antagonist CGP46381 after status epilepticus in immature rats

Possible proconvulsant action of GABA_B receptor antagonist CGP46381 was studied 3 and 13 days after status epilepticus elicited in 12-day-old rats. GABAA-dependent activity was tested by pentylenetetrazol administration and found different in 15-day-old rats after status epiolepticus but not in the older group. The interaction of the two GABAergic systems should be studied in detail.

Higher intrinsic network excitability in ventral compared with the dorsal hippocampus is controlled less effectively by GABAB receptors

BACKGROUND

Elucidating specializations of the intrinsic neuronal network between the dorsal and the ventral hippocampus is a recently emerging area of research that is expected to help us understand the mechanisms underlying large scale functional diversification along the hippocampus. The aim of this study was to characterize spontaneous network activity between the dorsal and the ventral hippocampus induced under conditions of partial or complete blockade of GABAergic inhibition (i.e. disinhibition).

RESULTS

Using field recordings from the CA3 and CA1 fields of hippocampal slices from adult rats I found that ventral compared with dorsal hippocampus slices displayed higher propensity for and higher frequency of occurrence of spontaneous field potentials (spfps) at every level of disinhibition. Also NMDA receptor-depended spfps (spfps(-nmda)) occurred with higher probability more frequently and were larger in the ventral compared with the dorsal hippocampus. Importantly, blockade of GABA(B) receptors produced a stronger effect in enhancing the probability of generation of spfps and spfps(-nmda) in the dorsal compared with the ventral hippocampal slices and increased spfps(-nmda) only in dorsal slices.

CONCLUSION

These results demonstrate a higher intrinsic neuronal excitability of the ventral compared with the dorsal local circuitry with the considerable contribution of NMDA receptors. Furthermore, the GABA(B) receptors control the total and the NMDA receptor-dependent excitation much less effectively in the ventral part of the hippocampus. It is proposed that NMDA and GABA(B) receptors significantly contribute to differentiate local network dynamics between the dorsal and the ventral hippocampus with important implications in the information processing performed along the long hippocampal axis.

Altered pharmacology and GABA-A receptor subunit expression in dorsal midline thalamic neurons in limbic epilepsy

The mediodorsal (MD) and paraventricular (PV) thalamic nuclei play a significant role in limbic epilepsy, and previous reports have shown changes in GABA-A receptor (GABAAR) mediated synaptic function. In this study, we examined changes in the pharmacology of GABAergic drugs and the expression of the GABAAR subunits in the MD and PV neurons in epilepsy. We observed nucleus specific changes in the sensitivity of sIPSCs to zolpidem and phenobarbital in MD and PV neurons from epileptic animals. In contrast, the magnitude of change in electrically evoked response (eIPSC) to zolpidem and phenobarbital were uniformly diminished in both MD and PV neurons in epilepsy. Immunohistochemical studies revealed that in epilepsy, there was a reduction in GAD65 expression and NeuN positive neurons in the MD neurons. Also, there was a decrease in immunoreactivity of the alpha1 and beta2/3 subunit of GABAARs, but not the gamma2 of the GABAAR in both MD and PV in epilepsy. These findings demonstrate significant alterations in the pharmacology of GABA and GABAARs in a key region for seizure generation, which may have implications for the physiology and pharmacology of limbic epilepsy.

Enhanced macroscopic desensitization shapes the response of alpha4 subtype-containing GABAA receptors to synaptic and extrasynaptic GABA

Up-regulation of the GABAA receptor alpha4 subunit subtype has been consistently shown in multiple animal models of chronic epilepsy. This isoform is expressed in both thalamus and hippocampus and is likely to play a significant role in regulating corticothalamic and hippocampal rhythms. However, little is known about its physiological properties, thus limiting understanding of the role of alpha4 subtype-containing GABAA receptors in normal and abnormal physiology. We used rapid GABA application to recombinant GABAA receptors expressed in HEK293T cells to compare the macroscopic kinetic properties of alpha4beta3gamma2L receptors to those of the more widely distributed alpha1beta3gamma2L receptors. These receptor currents had similar peak current amplitudes and GABA EC50 values. However, alpha4beta3gamma2L currents activated more slowly when exposed to submaximal GABA concentrations, had more fast desensitization (tau = 15-100 ms), and had less residual current during long GABA applications. In addition, alpha4beta3gamma2L currents deactivated more slowly than alpha1beta3gamma2L currents. Peak currents evoked by repetitive, brief GABA applications were more strongly attenuated for alpha4beta3gamma2L currents than alpha1beta3gamma2L currents. Moreover, the time required to recover from desensitization was prolonged in alpha4beta3gamma2L currents compared to alpha1beta3gamma2L currents. We also found that exposure to prolonged low levels of GABA, similar to those that might be present in the extrasynaptic space, greatly suppressed the response of alpha4beta3gamma2L currents to higher concentrations of GABA, while alpha1beta3gamma2L currents were less affected by exposure to low levels of GABA. Taken together, these data suggest that alpha4beta3gamma2L receptors have unique kinetic properties that limit the range of GABA applications to which they can respond maximally. While similar to alpha1beta3gamma2L receptors in their ability to respond to brief and low frequency synaptic inputs, alpha4beta3gamma2L receptors are less efficacious when exposed to prolonged tonic GABA or during repetitive stimulation, as may occur during learning and seizures.

Effects of Potassium Concentration on Firing Patterns of Low-Calcium Epileptiform Activity in Anesthetized Rat Hippocampus: Inducing of Persistent Spike Activity

PURPOSE

It has been shown that a low-calcium high-potassium solution can generate ictal-like epileptiform activity in vitro and in vivo. Moreover, during status epileptiform activity, the concentration of [K+]o increases, and the concentration of [Ca2+]o decreases in brain tissue. Therefore we tested the hypothesis that long-lasting persistent spike activity, similar to one of the patterns of status epilepticus, could be generated by a high-potassium, low-calcium solution in the hippocampus in vivo.

METHODS

Artificial cerebrospinal fluid was perfused over the surface of the exposed left dorsal hippocampus of anesthetized rats. A stimulating electrode and a recording probe were placed in the CA1 region.

RESULTS

By elevating K+ concentration from 6 to 12 mM in the perfusate solution, the typical firing pattern of low-calcium ictal bursts was transformed into persistent spike activity in the CA1 region with synaptic transmission being suppressed by calcium chelator EGTA. The activity was characterized by double spikes repeated at a frequency approximately 4 Hz that could last for >1 h. The analysis of multiple unit activity showed that both elevating [K+]o and lowering [Ca2+]o decreased the inhibition period after the response of paired-pulse stimulation, indicating a suppression of the after-hyperpolarization (AHP) activity.

CONCLUSIONS

These results suggest that persistent status epilepticus-like spike activity can be induced by nonsynaptic mechanisms when synaptic transmission is blocked. The unique double-spike pattern of this activity is presumably caused by higher K+ concentration augmenting the frequency of typical low-calcium nonsynaptic burst activity.

Altered expression of GABA(A) and GABA(B) receptor subunit mRNAs in the hippocampus after kindling and electrically induced status epilepticus

Epilepsy may result from altered transmission of the principal inhibitory transmitter GABA in the brain. Using in situ hybridization in two animal models of epileptogenesis, we investigated changes in the expression of nine major GABA(A) receptor subunits (alpha1, alpha2, alpha4, alpha5, beta1-beta3, gamma2 and delta) and of the GABA(B) receptor species GABA(B)R1a, GABA(B)R1b and GABA(B)R2 in 1) hippocampal kindling and 2) epilepsy following electrically-induced status epilepticus (SE). Hippocampal kindling triggers a decrease in seizure threshold without producing spontaneous seizures and hippocampal damage, whereas the SE model is characterized by spontaneous seizures and hippocampal damage. Changes in the expression of GABA(A) and GABA(B) receptor mRNAs were observed in both models, and compared with those seen in other models and in human temporal lobe epilepsy. The most prominent changes were a relatively fast (24 h after kindling and electrically-induced SE) and lasting (7 and 30 days after termination of kindling and SE, respectively) reduction of GABA(A) receptor subunit delta mRNA levels (by 43-78%) in dentate granule cells, accompanied by increases in mRNA levels of all three beta-subunits (by 8-79%) and subunit gamma2 (by 11-43%). Levels of the minor subunit alpha4 were increased by up to 60% in dentate granule cells in both animal models, whereas those of subunit alpha5 were decreased 24 h and 30 days after SE, but not after kindling. In cornu ammonis 3 pyramidal cells, downregulation of subunits alpha2, alpha4, alpha5, and beta1-3 was observed in the ventral hippocampus and of alpha2, alpha5, beta3 and gamma2 in its dorsal extension 24 h after SE. Similar but less pronounced changes were seen in sector cornu ammonis 1. Persistent decreases in subunit alpha2, alpha4 and beta2 transcript levels were presumably related to SE-induced cell loss. GABA(B) receptor expression was characterized by increases in GABA(B)R2 mRNA levels at all intervals after kindling and SE. The observed changes suggest substantial and cell specific rearrangement of GABA receptors. Lasting downregulation of subunits delta and alpha5 in granule cells and transient decreases in subunit alpha2 and beta1-3 mRNA levels in cornu ammonis 3 pyramidal cells are suggestive of impaired GABA(A) receptor-mediated inhibition. Persistent upregulation of subunits beta1-3 and gamma2 of the GABA(A) receptor and of GABA(B)R2 mRNA in granule cells, however, may result in activation of compensatory anticonvulsant mechanisms.

In Situ-Produced 7-Chlorokynurenate Has Different Effects on Evoked Responses in Rats with Limbic Epilepsy in Comparison to Naive Controls

PURPOSE

Uncontrolled epilepsy remains a significant health concern and requires new approaches to therapy. N-methyl-d-aspartate (NMDA) receptor blockade has been considered, but the adverse cognitive and behavioral effects of conventional NMDA-receptor antagonists have prevented the development of clinically useful compounds. An alternative approach may be the blockade of the glycine coagonist ("glycine(B)") site of the NMDA receptor.

METHODS

As a first step in the exploration of this approach, we examined the effect of 4-chloro-kynurenine (4-Cl-KYN), which is converted by astrocytes to the potent NMDA glycine-site antagonist 7-chloro-kynurenic acid (7-Cl-KYNA), on the in vivo epileptiform evoked potentials in the CA1 region of rats with chronic limbic epilepsy (CLE). 4-Cl-KYN (100 mg/kg) was administered intraperitoneally to naive and epileptic rats. Evoked potentials were induced in area CA1 of the hippocampus by electrical stimulation of the midline region of the thalamus. Simultaneous microdialysis was performed in the contralateral hippocampus to determine the extracellular levels of 7-Cl-KYNA over the course of the experiment.

RESULTS

Administration of 4-Cl-KYN caused a significant reduction in the amplitude of the population spike and in the number of population spikes in epileptic animals (p < 0.01) but had no effect on the evoked response in naive rats. In contrast, 4-Cl-KYN significantly altered the paired response in naive animals (p < 0.01), but had no significant effect on this parameter in epileptic animals. The levels of 7-Cl-KYNA measured achieved known pharmacologically effective concentrations and paralleled the observed physiological effects.

CONCLUSIONS

The use of glial cells for the neosynthesis and local delivery of neuroactive compounds may be a viable strategy for the treatment of limbic epilepsy. These results also underscore the unique pharmacology of neurons in epilepsy.

Greater contribution of N-methyl-D-aspartic acid receptors in ventral compared to dorsal hippocampal slices in the expression and long-term maintenance of epileptiform activity

Functional segregation along the dorso-ventral axis of the hippocampus is a developing concept. The higher susceptibility of the ventral hippocampus to epileptic activity compared with dorsal hippocampus is one of the main features, which still has obscure mechanisms. Using the model of magnesium-free medium and field recordings, single epileptiform discharges displayed higher incidence (77% vs 57%), rate (41.7+/-3.1 vs 13.5+/-0.7 events/min), duration (173.9+/-17.7 vs 116.8+/-13.6 ms) and intensity (coastline, 25.4+/-2.5 vs 9.5+/-1.8) in ventral compared with dorsal rat hippocampal slices. In addition, the decay phase of the evoked synaptic potentials was 110% slower in ventral slices. The N-methyl-D-aspartate (NMDA) receptor antagonist d-(-)-2-amino-5-phosphonopentanoic acid (50-100 microM) decreased the discharge rate and coastline similarly in ventral and dorsal slices, but it shortened the discharges in ventral slices (by 40%) only. The NMDA receptor antagonist 3-((R)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (10 microM) decreased the rate in both groups and additionally shortened discharges in both kinds of slices, an effect which was greater in ventral ones (31% vs 13%). Furthermore, both drugs shortened the evoked potentials more in ventral (77%) than in dorsal slices (52%). On the other hand, 1 microM of 3-((R)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid shortened the discharges and evoked synaptic potentials only in ventral slices, and slowed down the discharge rate only in dorsal slices. Addition of NMDA, in the magnesium-free medium, enhanced activity in both kinds of slices. At 5 and 10 microM of NMDA 51% of the ventral but only 9% of the dorsal slices displayed persistent epileptiform discharges, which were recorded for at least one hour after reintroduction of magnesium in the medium. At 10-20 microM the enhancement of activity was transient, followed by suppression of discharges in 40% and 76% of the ventral and dorsal slices, respectively. Most of the slices having experienced suppression did not develop persistent activity. We propose that the NMDA receptors contribute to the higher susceptibility of the ventral hippocampus to expression and long-term maintenance of epileptiform discharges. This diversification may be related to other aspects of hippocampal dorso-ventral functional segregation.

GABA and its receptors in epilepsy

Gamma-aminobutyric acid (GABA) is the principal inhibitory neurotransmitter in the mammalian brain. It acts through 2 classes of receptors, GABAA receptors that are ligand-operated ion channels and the G-protein-coupled metabotropic GABAB receptors. Impairment of GABAergic transmission by genetic mutations or application of GABA receptor antagonists induces epileptic seizures, whereas drugs augmenting GABAergic transmission are used for antiepileptic therapy. In animal epilepsy models and in tissue from patients with temporal lobe epilepsy, loss in subsets of hippocampal GABA neurons is observed. On the other hand, electrophysiological and neurochemical studies indicate a compensatory increase in GABAergic transmission at certain synapses. Also, at the level of the GABAA receptor, neurodegeneration-induced loss in receptors is accompanied by markedly altered expression of receptor subunits in the dentate gyrus and other parts of the hippocampal formation, indicating altered physiology and pharmacology of GABAA receptors. Such mechanisms may be highly relevant for seizure induction, augmentation of endogenous protective mechanisms, and resistance to antiepileptic drug therapy. Other studies suggest a role of GABAB receptors in absence seizures. Presynaptic GABAB receptors suppress neurotransmitter release. Depending on whether this action is exerted in GABAergic or glutamatergic neurons, there may be anticonvulsant or proconvulsant actions.

Different reactions of control and epileptic rats to administration of APV or muscimol on thalamic or CA3-induced CA1 responses

The physiology and pharmacology of CA1 is changed in epilepsy. There is evidence that the thalamic input to CA1 has a somewhat different physiological effect compared with the CA3 input. In this study we sought to determine whether this difference in physiology persists in epilepsy, and whether there are changes in the pharmacologic profile of these responses. Under urethane two stimulating electrodes were placed in mid to ventral CA3 and in the midline thalamus of control or epileptic rats. One glass micropipette electrode was placed into CA1 for recording. After the baseline acquisition of CA1-evoked responses to single- or paired-pulse stimulation, the stimuli were repeated with local application of either the GABAA agonist muscimol or the NMDA antagonist dl-2-amino-5-phosphonovalerate (APV). The CA1 response of epileptic rats had a smaller population postsynaptic potential (PSP) and spike amplitudes, longer PSP duration, multiple spikes, and the paired-pulse (at 20-ms intervals) facilitation in contrast to the paired-pulse depression seen in control and kindled rats. The duration of the PSP as well as the amplitude and number of spikes were reduced by administration of APV or muscimol into CA1 in both control and epileptic rats. In control rats, APV enhanced the depression induced by maximal paired thalamic or CA3 stimulation at 20-ms intervals and reduced the facilitation of threshold stimulation into no change. In contrast, muscimol in control rats reversed the depression induced by paired maximal stimulation into a mild facilitation and reduced the facilitation of threshold stimulation. In epileptic rats neither APV nor muscimol had a significant effect on the changes of the CA1 responses induced by maximal or threshold paired stimulation. This initial in vivo study demonstrated that the physiology and pharmacology of CA1 in epileptic rats are different from control rats. Although there are physiological differences in the evoked responses that are linked to the site of stimulation in the control and epileptic group, the pharmacology in each condition is independent of the site of stimulation.

Molecular biology and ontogeny of gamma-aminobutyric acid (GABA) receptors in the mammalian central nervous system

gamma-Aminobutyric acid (GABA) is the predominant inhibitory neurotransmitter in the mammalian central nervous system. After release from nerve terminals, GABA binds to at least two classes of postsynaptic receptors (ie, GABAA and GABAB), which are nearly ubiquitous in the brain. GABAA receptors are postsynaptic heteropentameric complexes that display unique physiologic and pharmacologic properties based on subunit composition. Activation of GABAA receptors in mature neurons results in membrane hyperpolarization, which is mediated principally by inward chloride flux, whereas in early stages of brain development, GABAA receptor activation causes depolarization of the postsynaptic membrane. GABA, receptors reside both presynaptically and postsynaptically, exist as heterodimers and are coupled to voltage-dependent ion channels through interactions with heterotrimeric G proteins. This review summarizes the molecular biology and ontogeny of GABAA and GABAB receptors, highlighting some of their putative roles during normal brain development as well as in disease states such as epilepsy.

Progesterone withdrawal increases the alpha4 subunit of the GABA(A) receptor in male rats in association with anxiety and altered pharmacology - a comparison with female rats

Withdrawal from the neurosteroid 3alpha,5alpha-allopregnanolone after chronic administration of progesterone increases anxiety in female rats and up-regulates the alpha4 subunit of the GABA(A) receptor (GABA(A)-R) in the hippocampus. We investigated if these phenomena would also occur in male rats. Progesterone withdrawal (PWD) induced higher alpha4 subunit expression in the hippocampus of both male and female rats, in association with increased anxiety (assessed in the elevated plus maze) comparable to effects previously reported. Because alpha4-containing GABA(A)-R are insensitive to the benzodiazepine (BDZ) lorazepam (LZM), and are positively modulated by flumazenil (FLU, a BDZ antagonist), we therefore tested the effects of these compounds following PWD. Using whole-cell patch clamp techniques, LZM-potentiation of GABA ((EC20))-gated current was markedly reduced in CA1 pyramidal cells of male rats undergoing PWD compared to controls, whereas FLU had no effect on GABA-gated current in control animals but increased it in PWD animals. Behaviorally, both male and female rats were significantly less sensitive to the anxiolytic effects of LZM. In contrast, FLU demonstrated significant anxiolytic effects following PWD. These data suggest that neurosteroid regulation of the alpha4 GABA(A)-R subunit may be a relevant mechanism underlying anxiety disorders, and that this phenomenon is not sex-specific.

Epilepsy-associated plasticity in gamma-aminobutyric acid receptor expression, function, and inhibitory synaptic properties

Although epilepsy is fundamentally a circuit phenomenon, the most basic manifestation of the hyperexcitability characteristic of epilepsy must be evident at the level of a single neuron. Furthermore, in the future, manipulations of surviving neurons within the epileptic focus will constitute one of the best therapeutic targets for intervention to cure this devastating disease. Therefore, the more that can be learned about epileptogenic alterations in this population of surviving focal neurons, the more potential avenues for therapeutic intervention will emerge. This chapter has summarized one aspect of postsynaptic neuronal function that undergoes dramatic alterations in the epileptic brain: the properties of inhibitory neurotransmitter (i.e., GABA) receptors in surviving focal neurons. GABARs in these neurons undergo significant alterations in their function and pharmacology, which appear to be mediated, at least in part, by alterations in the transcriptional production of GABAR subunits. These GABAR alterations fulfill many of the requirements for an epileptogenic mechanism: they are consistent with the hyperexcitability characteristic of epilepsy; the changes develop prior to the onset of recurrent spontaneous seizures; and the elevated zinc sensitivity of epileptic GABARs combined with epilepsy-associated mossy fiber sprouting (a zinc "delivery mechanism") can account for the existence of a prolonged latent period. Although GABAR alterations in DGCs of the epileptic hippocampus may be consistent with hyperexcitability and therefore contribute to epileptogenesis, many other processes undoubtedly also contribute, including (but not limited to) neuronal loss, circuit rearrangements, alterations in other membrane proteins, and birth of new neurons. Assuming any single change is both necessary and sufficient to fully account for epilepsy is undoubtedly an oversimplification. The initial precipitating events associated with the subsequent development of epilepsy are often traumatic events and associated with changes in many processes in widespread areas of the brain. Some of these processes may contribute to excitability changes, some may resist the development of epilepsy, and some may be unrelated to epileptogenesis. Characterizing the critical processes initiated during epileptogenesis remains an important and challenging research endeavor for the foreseeable future.

Aberrant neuronal physiology in the basal nucleus of the amygdala in a model of chronic limbic epilepsy

Limbic epilepsy is a chronic condition associated with a broad zone of seizure onset and pathology. Studies have focused mainly on the hippocampus, but there are indications that changes occur in other regions of the limbic system. This study used in vitro intracellular recording and histology to examine alterations to the physiology and anatomy of the basal nucleus of the amygdala in a rat model of chronic limbic epilepsy characterized by spontaneously recurring seizures. Epileptic pyramidal neuron responses evoked by stria terminalis stimulation revealed hyperexcitability characterized by multiple action potential bursts and no evident inhibitory potentials. In contrast, no hyperexcitability was observed in amygdalar neurons from kindled (included as a control for seizure activity) or control rats. Blockade of ionotropic glutamate receptors unmasked inhibitory postsynaptic potentials in epileptic pyramidal neurons. Control, kindled and epileptic inhibitory potentials were predominantly biphasic, with fast and slow components, but a few cells exhibited only the fast component (2/12 in controls, 0/3 in kindled, 3/10 in epileptic). Epileptic fast inhibitory potentials had a more rapid onset and shorter duration than control and kindled. Approximately 40% of control neurons exhibited spontaneous inhibitory potentials; no spontaneous inhibitory potentials were observed in neurons from kindled or epileptic rats. A preliminary histological examination revealed no gross alterations in the basal amygdala from epileptic animals. These results extend previous findings from this laboratory that hyperexcitability is found in multiple epileptic limbic regions and may be secondary to multiple alterations in excitatory and inhibitory efficacy. Because there were no differences between control and kindled animals, the changes observed in the epileptic animals are unlikely to be secondary to recurrent seizures.

Status epilepticus in epileptogenesis

There has been direct evidence of gamma-aminobutyric acidA receptor modification during status epilepticus. Neuropeptides galanin and neuropeptide Y were demonstrated to play a role in terminating status epilepticus. Many of the CA3 pyramidal neurons destined to die as a consequence of status epilepticus were demonstrated to diminish expression of the GluR2 subunit of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptors. It was demonstrated that the pattern of cell loss due to status epilepticus is distinct in immature pups compared with adult rats. The genetic basis for susceptibility to neuronal loss during status epilepticus was described. There was increasing evidence of unique receptors and ion channels in the epileptic brain. The molecular studies of epileptic gamma-aminobutyric acidA receptors present on dentate granule cells of rats with temporal lobe epilepsy revealed altered gene and receptor expression before onset of recurrent spontaneous seizures. They also revealed insertion of new gamma-aminobutyric acidA receptors in the inhibitory synapses present on soma and proximal dendrites of dentate granule cells.

Ontogeny of altered synaptic function in a rat model of chronic temporal lobe epilepsy

In the limbic status model of chronic temporal lobe epilepsy, hippocampal stimulation induces acute status epilepticus in rats; recurrent, spontaneous seizures develop following an asymptomatic silent period lasting several weeks. Previous work has shown increased excitability and decreased inhibition in CA1 pyramidal neurons in chronically epileptic animals. To determine the relationship of altered cellular responses to seizure onset, in vitro intracellular recording was used to follow the evolution of changes in synaptic physiology occurring during the seizure-free silent period. Pyramidal cells displayed increasing epileptiform activity throughout the period investigated, 3-14 days following status; the mean number of evoked action potentials from 1.1+/-0.05 in control cells to 2.4+/-0.4 early (3 days after status) and 4. 3+/-0.7 late (14 days) in the silent period. Monosynaptic inhibitory postsynaptic potentials mediated by gamma-aminobutyric acid-A receptors in silent period cells differed markedly from controls. Area, rise time, and duration of these potentials decreased by 40-60% within 3 days following status and to values commensurate with chronically epileptic animals in 7 to 10 days. gamma-Aminobutyric acid-B receptor-mediated IPSPs diminished more gradually in the silent period, reaching a minimum at day 14. In contrast, presynaptic gamma-aminobutyric acid-B receptor function showed maximum impairment 3 days after status. The benzodiazepine type 1 receptor agonist zolpidem reduced hyperexcitability in both silent period and chronically epileptic cells, but was more effective at unmasking the underlying IPSP in silent period neurons. The results indicate that changes in different components of pyramidal cell inhibitory synaptic physiology associated with chronic epilepsy in this model evolve individually at different rates, but are all complete before seizure onset. Although the results do not imply causality, they do suggest that the development of physiological changes in CA1 pyramidal cells may contribute to the lag period preceding the onset of chronic seizures.