Seizure-related activity of intralaminar thalamic neurons in a genetic model of absence epilepsy

Absence seizures are characterized by bilateral spike-and-wave discharges (SWDs) in thalamo-cortical circuits. In view of clinical studies indicating a critical involvement of intralaminar thalamic nuclei, we thought it timely to characterize the specific role and activity patterns of the respective neurons. Electrocorticographic (ECoG), intracellular, and unit activity recordings were performed in vivo from intralaminar thalamic neurons of the centrolateral (CL) and the paracentral (PC) thalamic nucleus in an established genetic rat model of absence epilepsy (WAG/Rij). Neurons in PC are depolarized to produce tonic series of action potentials at seizure-free episodes, and are rhythmically silenced concomitant with SWDs in a spike-locked manner. Rebound from spike-locked inhibition is associated with a transient increase in action potential activity. Neurons in CL possess a relatively negative membrane potential with overall low electrogenic activity at seizure-free episodes and generate burst-like discharges during SWDs that are locked to the decaying phase of the spike component on the ECoG. The SWD-locked membrane responses reverse close to the presumed chloride equilibrium potential, indicating GABA(A) receptor-mediated inhibitory postsynaptic potentials (IPSPs), with cell-type specific differences in polarity. In PC neurons, hyperpolarizing IPSPs result in spike-locked silencing of tonic firing and rebound burst discharges, while in CL neurons, IPSPs are depolarizing and trigger low-threshold burst firing likely mediated by a t-type Ca(2+) conductance. These data show a unique pattern of rhythmic SWD-locked IPSPs in PC and CL associated with paroxysms apt to impose a transient dysfunctional state to thalamo-striato-prefrontocortical networks during absence seizures.     

Epilepsy: a review of selected clinical syndromes and advances in basic science.

Epilepsy is a common neurologic disorder that manifests in diverse ways. There are numerous seizure types and numerous mechanisms by which the brain generates seizures. The two hallmarks of seizure generation are hyperexcitability of neurons and hypersynchrony of neural circuits. A large variety of mechanisms alters the balance between excitation and inhibition to predispose a local or widespread region of the brain to hyperexcitability and hypersynchrony. This review discusses five clinical syndromes that have seizures as a prominent manifestation. These five syndromes differ markedly in their etiologies and clinical features, and were selected for discussion because the seizures are generated at a different 'level' of neural dysfunction in each case: (1) mutation of a specific family of ion (potassium) channels in benign familial neonatal convulsions; (2) deficiency of the protein that transports glucose into the CNS in Glut-1 deficiency; (3) aberrantly formed local neural circuits in focal cortical dysplasia; (4) synaptic reorganization of limbic circuitry in temporal lobe epilepsy; and (5) abnormal thalamocortical circuit function in childhood absence epilepsy. Despite this diversity of clinical phenotype and mechanism, these syndromes are informative as to how pathophysiological processes converge to produce brain hyperexcitability and seizures.     

GABAergic synaptic transmission triggers action potentials in thalamic reticular nucleus neurons

GABAergic neurons in the thalamic reticular nucleus (TRN) form powerful inhibitory connections with several dorsal thalamic nuclei, thereby controlling attention, sensory processing, and synchronous oscillations in the thalamocortical system. TRN neurons are interconnected by a network of GABAergic synapses, but their properties and their role in shaping TRN neuronal activity are not well understood. Using recording techniques aimed to minimize changes in the intracellular milieu, we show that synaptic GABA(A) receptor activation triggers postsynaptic depolarizations in mouse TRN neurons. Immunohistochemical data indicate that TRN neurons express very low levels of the Cl(-) transporter KCC2. In agreement, perforated-patch recordings show that intracellular Cl(-) levels are high in TRN neurons, resulting in a Cl(-) reversal potential (E(Cl)) significantly depolarized from rest. Additionally, we find that GABA(A) receptor-evoked depolarizations are amplified by the activation of postsynaptic T-type Ca(2+) channels, leading to dendritic Ca(2+) increases and the generation of burst firing in TRN neurons. In turn, GABA-evoked burst firing results in delayed and long-lasting feedforward inhibition in thalamic relay cells. Our results show that GABA-evoked depolarizations can interact with T-type Ca(2+) channels to powerfully control spike generation in TRN neurons.     

Can GABAA conductances explain the fast oscillation frequency of absence seizures in rodents?

Rodent models of absence epilepsy generate spike-and-wave oscillations at relatively fast frequency (5-10 Hz) compared with humans ( approximately 3 Hz). Possible mechanisms for these oscillations were investigated by computational models that included the complex intrinsic firing properties of thalamic and cortical neurons, as well as the multiple types of synaptic receptors mediating their interactions. The model indicates that oscillations with spike-and-wave field potentials can be generated by thalamocortical circuits. The frequency of these oscillations critically depended on GABAergic conductances in thalamic relay cells, ranging from 2-4 Hz for strong GABAB conductances to 5-10 Hz when GABAA conductances were dominant. This model therefore suggests that thalamocortical circuits can generate two types of spike-and-wave oscillations, whose frequency is determined by the receptor type mediating inhibition in thalamic relay cells. Experiments are proposed to test this mechanism.     

Transient microcircuits formed by subplate neurons and their role in functional development of thalamocortical connections

Subplate neurons are a transient population of neurons in the brain forming one of the first functional cortical circuits. Past experiments have demonstrated their importance in growth of thalamocortical afferents into the cortical plate and later segregation of thalamocortical afferents. Recently, subplate neurons have been shown to be required for the functional maturation of both thalamocortical connections and mature visual responses in visual cortex. These findings suggest that thalamocortical afferents might not segregate properly in the absence of subplate neurons because the thalamocortical synapse does not mature. Subplate neurons are unique in that they form a circuit that appears to promote synaptic scaling and maturation. Although the precise contribution of subplate neurons within the context of cortical development is unknown, they might play an early role in providing thalamic input to cortex that then interacts with learning rules governing synaptic strengthening at the thalamocortical synapse. Because they appear to play multiple key roles at different stages of development, subplate neurons might also play a role in the pathology of developmental disorders, such as epilepsy and schizophrenia.     

Attenuation of inhibitory synaptic transmission by glial dysfunction in rat thalamus

The thalamus serves as the obligatory gateway to the neocortex for sensory processing, and also serves as a pathway for corticocortical communication. In addition, the reciprocal synaptic connectivity between the thalamic reticular nucleus (TRN) and adjacent thalamic relay nuclei generates rhythmic activities similar to that observed during different arousal states and certain neurological conditions such as absence epilepsy. Epileptiform activity can arise from a variety of neural mechanisms, but in addition glia are thought to have an important role in such activities as well. Glia serve a central role in glutamine synthesis, a precursor for glutamate or GABA in nerve terminals. While alterations in glutamine shuttling from glia to neurons can influence GABA and glutamate neurotransmission; the consequences of such action on synaptic transmission and subsequent network activities within thalamic circuits is less understood. We investigated the consequences of altering glutamine transport on inhibitory transmission and intrathalamic activities using the in vitro thalamic slice preparation. Disruption of the glutamine shuttling by the neuronal glutamine transporter (system A transporter) antagonist, α-(methylamino)isobutyric acid (MeAIB), or the selective gliotoxic drug, fluorocitric acid (Fc) dramatically decreased intrathalamic rhythmic activities. At the single cell level, MeAIB and Fc significantly attenuated electrically evoked inhibitory postsynaptic currents (eIPSCs) in thalamic relay neurons; however, miniature IPSCs were unaffected. These data indicate that glutamate-glutamine shuttle is critical for sustaining thalamic synaptic transmission, and thereby alterations in this shuttle can influence intrathalamic rhythmic activities associated with absence epilepsy.     

Sleep, epilepsy and thalamic reticular inhibitory neurons

Thalamic reticular neurons release the potent inhibitory neurotransmitter GABA and their main targets are thalamocortical neurons in the dorsal thalamus. This article focuses on two topics: (i) the role of thalamic reticular neurons in the initiation of spindles, a hallmark oscillation during early sleep stages; and (ii) the reticular-induced inhibition of thalamocortical neurons during cortically generated spike-wave seizures. Although hotly debated during the past decade, the idea of spindle generation by a network of GABAergic reticular neurons was recently supported by in vivo and in computo studies demonstrating interactions between inhibitory reticular neurons that lead to spindle sequences. During spike-wave seizures and electrical paroxysms of the Lennox-Gastaut type, which arise in the neocortex, reticular neurons are powerfully excited through corticofugal projections and they produce prolonged inhibitory postsynaptic potentials in thalamocortical neurons. Thus, GABAergic reticular neurons are crucial in the generation of some sleep rhythms, which produce synaptic plasticity, and in inhibiting external signals through thalamocortical neurons, which leads to unconsciousness during absence epilepsy.     

Regionally specific expression of high-voltage-activated calcium channels in thalamic nuclei of epileptic and non-epileptic rats

The polygenic origin of generalized absence epilepsy results in dysfunction of ion channels that allows the switch from physiological asynchronous to pathophysiological highly synchronous network activity. Evidence from rat and mouse models of absence epilepsy indicates that altered Ca^2 + channel activity contributes to cellular and network alterations that lead to seizure activity. Under physiological circumstances, high voltage-activated (HVA) Ca^2 + channels are important in determining the thalamic firing profile. Here, we investigated a possible contribution of HVA channels to the epileptic phenotype using a rodent genetic model of absence epilepsy. In this study, HVA Ca²⁺ currents were recorded from neurons of three different thalamic nuclei that are involved in both sensory signal transmission and rhythmic-synchronized activity during epileptic spike-and-wave discharges (SWD), namely the dorsal part of the lateral geniculate nucleus (dLGN), the ventrobasal thalamic complex (VB) and the reticular thalamic nucleus (NRT) of epileptic Wistar Albino Glaxo rats from Rijswijk (WAG/Rij) and non-epileptic August Copenhagen Irish (ACI) rats. HVA Ca^2 + current densities in dLGN neurons were significantly increased in epileptic rats compared with non-epileptic controls while other thalamic regions revealed no differences between the strains. Application of specific channel blockers revealed that the increased current was carried by L-type Ca²⁺ channels. Electrophysiological evidence of increased L-type current correlated with up-regulated mRNA and protein expression of a particular L-type channel, namely Ca_v1.3, in dLGN of epileptic rats. No significant changes were found for other HVA Ca²⁺ channels. Moreover, pharmacological inactivation of L-type Ca^2 + channels results in altered firing profiles of thalamocortical relay (TC) neurons from non-epileptic rather than from epileptic rats. While HVA Ca^2 + channels influence tonic and burst firing in ACI and WAG/Rij differently, it is discussed that increased Ca_v1.3 expression may indirectly contribute to increased robustness of burst firing and thereby the epileptic phenotype of absence epilepsy.     

The midline thalamus: alterations and a potential role in limbic epilepsy

PURPOSE: In limbic or mesial temporal lobe epilepsy, much attention has been given to specific regions or cell populations (e.g., the hippocampus or dentate granule cells). Epileptic seizures may involve broader changes in neural circuits, and evidence suggests that subcortical regions may play a role. In this study we examined the midline thalamic regions for involvement in limbic seizures, changes in anatomy and physiology, and the potential role for this region in limbic seizures and epilepsy. METHODS: Using two rat models for limbic epilepsy (hippocampal kindled and chronic spontaneous limbic epilepsy) we examined the midline thalamus for evidence of involvement in seizure activity, alterations in structure, changes in the basic in vitro physiology of the thalamic neurons. We also explored how this region may influence limbic seizures. RESULTS: The midline thalamus was consistently involved with seizure activity from the onset, and there was significant neuronal loss in the medial dorsal and reuniens/rhomboid nuclei. In addition, thalamic neurons had changes in synaptically mediated and voltage-gated responses. Infusion of lidocaine into the midline thalamus significantly shortened afterdischarge duration. CONCLUSIONS: These observations suggest that this thalamic region is part of the neural circuitry of limbic epilepsy and may play a significant role in seizure modulation. Local neuronal changes can enhance the excitability of the thalamolimbic circuits.     

Development of firing patterns and electrical properties in neurons of the rat ventrobasal thalamus

The electrophysiological properties of thalamic neurons of the rat ventrobasal complex (VB) in vitro were studied during early postnatal development. Current clamp recordings using the whole cell patch clamp method revealed that immature thalamic neurons had less negative membrane potential and higher input resistance than mature neurons. One of the most remarkable differences was the absence of spike bursts riding on the low threshold calcium spike (LTS) in VB neurons before postnatal day 12 (P12). Action potentials recorded from immature neurons had longer duration than those of mature cells and were followed by a longer afterhyperpolarization (spike-ahp). The spike-ahp became shorter as maturation progressed, reaching mature characteristics around P12, coinciding with the appearance of spike bursting on the LTS. The calcium activated potassium conductance, I_C, played a prominent role in the spike-ahp in immature neurons. In conclusion, the major differences in intrinsic membrane properties of VB neurons occur during the first 12 postnatal days. The appearance of spike bursting riding on the LTS at P12 is consistent with the emergence of synchronized thalamocortical oscillations in rats around that age.     

Neurotransmitter actions in the thalamus and cerebral cortex.

The postsynaptic actions of glutamate, gamma-aminobutyric acid (GABA), acetylcholine, norepinephrine, serotonin, and histamine in the cerebral cortex and thalamus and their relevance to the control of thalamocortical activity are reviewed. Excitatory and inhibitory amino acids (such as glutamate and GABA) are proposed to form the neurotransmitters by which the executative neural networks of the neocortex and thalamus process synaptic information. In contrast, the more slowly acting neurotransmitters, acetylcholine, norepinephrine, serotonin, and histamine, are proposed to control the state of activity and excitability of thalamic and cortical neurons and thereby modulate the state of thalamocortical activity. Specific examples of the involvement of fast and slow transmitter actions in the genesis of epileptic seizures and the determination of sleep-wake cycles are given.     

Correlation of T-channel coding gene expression, IT, and the low threshold Ca2+ spike in the thalamus of a rat model of absence epilepsy

T-type Ca(2+) current-dependent burst firing of thalamic neurons is thought to be involved in the hyper-synchronous activity observed during absence seizures. Here we investigate the correlation between the expression of T-channel coding genes (alpha1G, -H, -I), T-type Ca(2+) current, and the T-current-dependent low threshold Ca(2+) spike in three functionally distinct thalamic nuclei (lateral geniculate nucleus; centrolateral nucleus; reticular nucleus) in a rat model of absence epilepsy, the WAG/Rij rats, and a non-epileptic control strain, the ACI rats. The lateral geniculate nucleus and centrolateral nucleus were found to primarily express alpha1G and alpha1I, while the reticular thalamic nucleus expressed alpha1H and alpha1I. Expression was higher in WAG/Rij when compared to ACI. The T-type Ca(2+) current properties matched the predictions derived from the expression pattern analysis. Current density was larger in all nuclei of WAG/Rij rats when compared to ACI and correlated with LTS size and the minimum LTS generating slope, while T-type Ca(2+) current voltage dependency correlated with the LTS onset potential.     

Generation of spike latency tuning by thalamocortical circuits in auditory cortex

In many sensory systems, the latency of spike responses of individual neurons is found to be tuned for stimulus features and proposed to be used as a coding strategy. Whether the spike latency tuning is simply relayed along sensory ascending pathways or generated by local circuits remains unclear. Here, in vivo whole-cell recordings from rat auditory cortical neurons in layer 4 revealed that the onset latency of their aggregate thalamic input exhibited nearly flat tuning for sound frequency, whereas their spike latency tuning was much sharper with a broadly expanded dynamic range. This suggests that the spike latency tuning is not simply inherited from the thalamus, but can be largely reconstructed by local circuits in the cortex. Dissecting of thalamocortical circuits and neural modeling further revealed that broadly tuned intracortical inhibition prolongs the integration time for spike generation preferentially at off-optimal frequencies, while sharply tuned intracortical excitation shortens it selectively at the optimal frequency. Such push and pull mechanisms mediated likely by feedforward excitatory and inhibitory inputs respectively greatly sharpen the spike latency tuning and expand its dynamic range. The modulation of integration time by thalamocortical-like circuits may represent an efficient strategy for converting information spatially coded in synaptic strength to temporal representation.     

Relations between cortical and thalamic cellular activities during absence seizures in rats

In a rat model of generalized absence epilepsies (Genetic Absence Epilepsy Rats from Strasbourg, GAERS), multiunit activity was recorded simultaneously at different sites of the thalamocortical system under neurolept anaesthesia (fentanyl-droperidol). Under these conditions, bilaterally synchronized spike-and-wave-discharges (SWDs) occurred spontaneously on the electroencephalogram (EEG) that were in principle identical to those reported earlier from unanaesthetized preparations. The generation of SWDs on the EEG was associated with spike-concurrent, rhythmic burst-like activity in (mono-)synaptically connected regions of specific (somatosensory) thalamic regions and layers IV/V of the somatosensory cortex, and the reticular thalamic nucleus. Precursor activity was typically recorded in cortical units, concomitant with ‘embryonic’ SW seizures on the EEG, before the paroxysm was evident on the gross EEG and in the thalamus. On average, SWD-correlated activity in layers IV/V of the somatosensory cortex started significantly earlier than correlated burst-like firing in reticular and in ventrobasal thalamic neurons. Cellular peak firing in thalamus and cortex during bilaterally synchronized SWDs was related to the spike component on the gross EEG with the temporal rank order ventroposteromedial > ventrolateral ≥ ventroposterolateral thalamic > > rostral reticular thalamic nuclei ≥ cortex (layers IV/V) = caudal reticular thalamic nucleus. A spike-related depression and wave-related increase in firing was recorded in anteroventral ventrolateral thalamic areas, presumably reflecting their peculiar anatomical arrangement within the thalamus. These results from an in vivo preparation with intact synaptic connections that spontaneously produces SWDs indicate that SWDs spread within the thalamocortical network, involving short and long delays. The order of concurrent rhythmic firing observed in thalamocortical circuits during SW seizures are supportive of the hypothesis that the processes of rhythmogenesis recruit local thalamic networks, while cortical mechanisms appear to synchronize rhythmic activities on a larger spatiotemporal scale, thereby providing an important contribution to the generalization of epileptiform activity and expression of SWDs on the EEG.     

The role of T-channels in the generation of thalamocortical rhythms

The presence of T-channels in thalamic cells allows for the generation of rhythmic bursts of spikes and the existence of two firing modes in thalamic cells: tonic and bursting. This intrinsic electrophysiological property has fundamental consequences for the functional properties of the thalamus across waking and sleep stages and is centrally implicated in a growing number of pathological states. Rhythmic bursting brings about highly synchronized activity throughout corticothalamic circuits which is incompatible with the relay of information through the thalamus. Understanding the conditions that determine the change in firing mode of thalamic cells as well as the role of bursting in the generation of synchronized oscillations is critical to understand the function of the thalamus. The functional properties of T-channels and the resulting low threshold spike are discussed here with emphasis on the differences in the bursting properties of reticular thalamic and thalamocortical neurons. The role of thalamic bursting in the generation of sleep oscillations and their specific sequence during slow wave sleep will also be discussed.     

Transgenic mice over-expressing GABA(B)R1a receptors acquire an atypical absence epilepsy-like phenotype

In this study, we tested whether over-expressing the GABA(B) receptor R1a subtype in transgenic mouse forebrain neurons would be sufficient to induce spontaneous absence seizures. As hypothesized, these transgenic mice develop spontaneous, recurrent, bilaterally synchronous, 3-6 Hz slow spike and wave discharges between 2 and 4 months of age. These discharges are blocked by ethosuximide and exacerbated by baclofen confirming their absence nature. The discharges occur coincident with absence-like behaviors such as staring, facial myoclonus, and whisker twitching. However, in contrast to typical absence epilepsy models, these mice move during the ictal event, display spike and wave discharges in both thalamocortical and limbic circuitry, exhibit impaired hippocampal synaptic plasticity, and display significantly impaired learning ability. Collectively, these features are more characteristic of the less common but more debilitating atypical form of absence epilepsy. Thus, these data support a role for the GABA(B)R1a receptor subtype in the etiology of atypical absence epilepsy.     

Neuronal sensitivity to GABA and glutamate in generalized epilepsy with spike and wave discharges

In awake but painlessly immobilized cats the extracellular activity of the same cortical neurons was recorded before and for 2 to 5 h after the injection of penicillin G (350,000 IU/kg, i.m.) during the development of generalized epilepsy with bilaterally synchronous spike and wave discharges. Possible changes in their sensitivity to microiontophoretically applied glutamate and GABA during this period were searched for using computer-generated periejection histograms at intervals of about 30 min. In contrast to reported studies in other models of epilepsy, glutamate excited and GABA depressed virtually all neurons tested during fully developed spike and wave epilepsy. Spike height was not apparently affected either by the amino acids or by the development of epilepsy. Comparison of relative thresholds for the above effects on rhythmical neuronal activity associated with spike and wave discharge versus effects on random neuronal activity during the interburst periods, supported the idea that spikes and waves result from strong excitatory and inhibitory synaptic drives of the neurons. In all neurons until the appearance of spike and wave discharges, changes in the effect of amino acids, if observed, were small and statistically nonsignificant. This suggests that the hyperexcitability of cortical neurons which reportedly leads to the appearance of spike and wave discharges depends on mechanisms other than an increase in sensitivity to glutamate or a desensitization to GABA. Sometimes the sensitivity to GABA decreased later in this experimental model when the very frequent appearance of spike and wave discharges eventually led to EEG tonic-clonic seizures.     

Contribution of intralaminar thalamic nuclei to spike-and-wave-discharges during spontaneous seizures in a genetic rat model of absence epilepsy

In an epileptic rat model of generalized absence epilepsies, the genetic absence epilepsy rats from Strasbourg (GAERS), simultaneous recordings of bilateral epidural electroencephalogram (EEG) of the prefrontal cortex and unit activity of neurons in the intralaminar centrolateral (CL) and paracentral thalamic nucleus (PC) were performed under neurolept-anaesthesia (fentanyl-dehydrobenzperidol analgesia). Spike-and-wave (SW) seizures in these rats are characterized by generalized 7-10 Hz spike-and-wave discharges (SWDs) on the EEG. All neurons recorded in intralaminar thalamic nuclei during spontaneous SWDs showed high-frequency (average 368 Hz, range 200-500 Hz), burst-like activity, which occurred in a highly synchronized fashion with every SWD or with alternating SWD-complexes. Burst discharges in intralaminar neurons were delayed by 13.1 ms (CL) and 12.7 ms (PC), with respect to the spike component of a given SWD on the EEG, whereas burst discharges in the ventrobasal thalamus (VB) and in the rostral nucleus reticularis thalami (rRT) preceded the spike component by 17.8 ms and 8.3 ms, respectively. The onset of SWDs on the EEG was preceded by a tonic firing pattern (20-50 Hz) in about one third of CL and PC neurons. Microiontophoretic application of the gamma-aminobutyric acid (GABA)A receptor antagonist bicuculline aggravated, whereas, the glutamate receptor antagonists DNQX and APV dampened, SWD-related discharges in PC and CL; the GABAB receptor antagonist CGP 35347 had no measurable effect. These data indicate that intrathalamic nuclei are recruited rhythmically during SWDs, through mechanisms that seem to rely on a delayed glutamatergic excitation modulated by GABAergic influences, rather than a GABA-mediated rebound burst activity typical of relay cells. The finding of a temporal delay of SWD-related activity in intrathalamic, compared with "specific" thalamic relay nuclei, does not support the notion of a leading or pacemaker role in SWD generation. It is, however, rather suggestive of a function of intrathalamic neurons during synchronization and maintenance of neuronal oscillations, and these intrathalamic neurons may be recruited through glutamatergic corticofugal inputs.     

Two differential frequency-dependent mechanisms regulating tonic firing of thalamic reticular neurons

Transmission through the thalamus activates circuits involving the GABAergic neurons of the thalamic reticular nucleus (TRN). TRN cells receive excitatory inputs from thalamocortical and corticothalamic cells and send inhibitory projections to thalamocortical cells. The inhibitory output of TRN neurons largely depends on the level of excitatory drive to these cells but may also be partly under the control of mechanisms intrinsic to the TRN. We examined two such possible mechanisms, short-term plasticity at glutamatergic synapses in the TRN and intra-TRN inhibition. In rat brain slices, responses of TRN neurons to brief trains of stimuli applied to glutamatergic inputs were recorded in voltage- or current-clamp mode. In voltage clamp, TRN cells showed no change in α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor-mediated excitatory postsynaptic current amplitudes to stimulation at non-gamma frequencies (< 30 Hz), simulating background activity, but exhibited short-term depression in these amplitudes to stimulation at gamma frequencies (> 30 Hz), simulating sensory transmission. In current clamp, TRN cells increased their spike outputs in burst and tonic firing modes to increasing stimulus-train frequencies. These increases in spike output were most likely due to temporal summation of excitatory postsynaptic potentials. However, the frequency-dependent increase in tonic firing was attenuated at gamma stimulus frequencies, indicating that the synaptic depression selectively observed in this frequency range acts to suppress TRN cell output. In contrast, intra-TRN inhibition reduced spike output selectively at non-gamma stimulus frequencies. Thus, our data indicate that two intrinsic mechanisms play a role in controlling the tonic spike output of TRN neurons and these mechanisms are differentially related to two physiologically meaningful stimulus frequency ranges.     

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.