Effects of ethanol on the development of genetically determined epilepsies in rats

In the present study, we provide evidences for a differential effect of perinatal alcohol exposure with a direct correlation to the genetic background on the development of seizures. Ethanol (EtOH) is a widely used psychoactive substance that exerts its action by affecting multiple targets in the central nervous system. EtOH is known to interact with almost all identified neurotransmitters although its effects on excitatory and inhibitory amino acid neurotransmissions are considered to be particularly important in the mediation of its behavioural effects. Prenatal exposure to alcohol is associated with a wide variety of offspring's abnormalities which lead to the so called foetal alcohol syndrome (FAS), which is also related to a higher susceptibility to convulsions. In our study, a rat strain of convulsive epilepsy, the GEPRs rats, displayed an increase of seizure susceptibility after foetal exposure to this teratogenic drug, while a non-convulsive rat strain of absence epilepsy, the WAG/Rij rat, did not fully develop its characteristic features. However, when all groups of rat where tested for pentyletetrazole-induced convulsion, animals perinatally treated with ethanol were less responsive in comparison to their respective controls. These results are in agreement with previous reports showing how the genetic background can directly influence the teratogenic effects of alcohol, and this can be strictly related to the variability in the observation of offspring anomalies in humans which has lead to a 5-category classification system for individuals exposed to alcohol in uterus.     

Comparison of the antiepileptogenic effects of an early long‐term treatment with ethosuximide or levetiracetam in a genetic animal model of absence epilepsy

Purpose: Epilepsy is a heterogeneous syndrome characterized by recurrent, spontaneous seizures; continuous medication is, therefore, necessary, even after the seizures have long been suppressed with antiepileptic drug (AED) treatments. The most disturbing issue is the inability of AEDs to provide a persistent cure, because these compounds generally suppress the occurrence of epileptic seizures without necessarily having antiepileptogenic properties. The aim of our experiments was to determine, in the WAG/Rij model of absence epilepsy, if early long‐term treatment with some established antiabsence drugs might prevent the development of seizures, and whether such an effect could be sustained. Methods: WAG/Rij rats were treated for ～3.5 months (starting at 1.5 months of age, before seizure onset) with either ethosuximide (ETH; drug of choice for absence epilepsy) or levetiracetam (LEV; a broad‐spectrum AED with antiabsence and antiepileptogenic properties). Results: We have demonstrated that both drugs are able to reduce the development of absence seizures, exhibiting antiepileptogenic effects in this specific animal model. Discussion: These findings suggest that absence epilepsy in this strain of rats very likely follows an epileptogenic process during life and that early therapeutic intervention is possible, thereby opening a new area of research for absence epilepsy and AED treatment strategies.     

Effects of levetiracetam on spike and wave discharges in WAG/Rij rats

Effects of the novel anti-epileptic drug levetiracetam (50 and 100 mg/kg) on spike and wave discharges (SWDs) of WAG/Rij rats were studied. Levetiracetam decreased the incidence, average duration, total duration and peak frequency of the SWDs. There was no difference between the two doses. These results agree with results obtained in Genetic Absence Epilepsy Rat from Strasbourg (GAERS). Furthermore, the decrease of the SWD peak frequency might support the suggestions that levetiracetam might have a GABAergic mechanism of action.     

The brain 5HTergic response to an acute sound stress in rats with generalized (absence and audiogenic) epilepsy

The brain serotoninergic (5HTergic) system of epileptic subjects can influence their vulnerability to stress. We studied the putative dependency of 5HT neurotransmission parameters on emotional stress, and the presence, types and severity of seizures using rats with genetic generalized (absence and/or audiogenic) epilepsy, of WAG/Rij and Wistar strains. The animals were stressed by exposure to a short aversive noise or left without sound stimulation. Tissue concentrations of 5HT, tryptophan (TRT) and 5-hydroxyindolacetic acid (5HIAA) were assessed by HPLC. The stressor activated the 5HTergic system within thalamus (5HIAA elevated), frontal cortex (5HT, TRT elevated), hypothalamus (increased TRT) in all rats. However, the normal (non-epileptic) rats displayed the highest response in the frontal cortex and the lowest one in the thalamus, as compared to the epileptic rats. Absence-epileptic rats exhibited higher thalamic 5HIAA increase than their controls. Significant correlations existed between propensity of absence epilepsy and 5HTergic parameters measured in the cortex and hypothalamus of absence-epileptic rats. No major difference was found between groups with and without audiogenic epilepsy. The results imply that the stress response depends on the presence of epileptic pathology and the seizure type and severity. The brain 5HT may be involved in the control of the paroxysms and behaviour in absence-epileptic subjects.     

Early molecular and behavioral response to lipopolysaccharide in the WAG/Rij rat model of absence epilepsy and depressive-like behavior,involves interplay between AMPK,AKT/mTOR pathways and neuroinflammatory cytokine release

The mammalian target of rapamycin (mTOR) pathway has been recently indicated as a suitable drug target for the prevention of epileptogenesis. The mTOR pathway is known for its involvement in the control of the immune system. Since neuroinflammation is recognized as a major contributor to epileptogenesis, we wished to examine whether the neuroprotective effects of mTOR modulation could involve a suppression of the neuroinflammatory process in epileptic brain. We have investigated the early molecular mechanisms involved in the effects of intracerebral administration of the lipopolysaccharide (LPS) in the WAG/Rij rat model of absence epilepsy, in relation to seizure generation and depressive-like behavior; we also tested whether the effects of LPS could be modulated by treatment with rapamycin (RAP), a specific mTOR inhibitor. We determined, in specific rat brain areas, levels of p-mTOR/p-p70S6K and also p-AKT/p-AMPK as downstream or upstream indicators of mTOR activity and tested the effects of LPS and RAP co-administration. Changes in the brain levels of pro-inflammatory cytokines IL-1β and TNF-α and their relative mRNA expression levels were measured, and the involvement of nuclear factor-κB (NF-κB) was also examined in vitro. We confirmed that RAP inhibits the aggravation of absence seizures and depressive-like/sickness behavior induced by LPS in the WAG/Rij rats through the activation of mTOR and show that this effect is correlated with the ability of RAP to dampen and delay LPS increases in neuroinflammatory cytokines IL-1β and TNF-α, most likely through inhibition of the activation of NF-κB. Our results suggest that such a mechanism could contribute to the antiseizure, antiepileptogenic and behavioral effects of RAP and further highlight the potential therapeutic usefulness of mTOR inhibition in the management of human epilepsy and other neurological disorders. Furthermore, we show that LPS-dependent neuroinflammatory effects are also mediated by a complex interplay between AKT, AMPK and mTOR with specificity to selective brain areas. In conclusion, neuroinflammation appears to be a highly coordinated phenomenon, where timing of intervention may be carefully evaluated in order to identify the best suitable target.     

Upholding WAG/Rij rats as a model of absence epileptogenesis: Hidden mechanisms and a new theory on seizure development

The WAG/Rij rat model has recently gathered attention as a suitable animal model of absence epileptogenesis. This latter term has a broad definition encompassing any possible cause that determines the development of spontaneous seizures; however, most of, if not all, preclinical knowledge on epileptogenesis is confined to the study of post-brain insult models such as traumatic brain injury or post-status epilepticus models. WAG/Rij rats, but also synapsin 2 knockout, Kv7 current–deficient mice represent the first examples of genetic models where an efficacious antiepileptogenic treatment (ethosuximide) was started before seizure onset. In this review, we have critically reconsidered all articles published regarding WAG/Rij rats, from the perspective that the period before SWD onset is considered as the latent period. In our new theory on seizure development, it is proposed that genes might be considered as the initial ‘insult’ responsible for all plastic changes underpinning the development of spontaneous seizures. According to this idea, in WAG/Rij rats, genetic predisposition would lead to the development of abnormal bilateral cortical epileptic foci, which would then non-genetically stimulate the rest of the brain to rearrange networks in order to phenotypically develop seizures similarly to what happens during electrical kindling.     

Dissociation between in vivo hippocampal norepinephrine response and behavioral/neuroendocrine responses to noise stress in rats

The behavioral and extracellular hippocampal norepinephrine responses to audiogenic stress were concomitantly characterized in freely moving rats using in vivo microdialysis. Noise stimulation produced a rapid, but short-lived increase in norepinephrine release from the hippocampus during the first 20 min of noise presentation that declined to baseline levels for the duration of the noise stimulation and following noise offset. In contrast, the behavioral response persisted throughout the duration of the noise stimulation. In a separate group of similarly treated animals, neuroendocrine indices of stress were monitored during exposure to noise. Consistent with the behavioral response, corticosterone and adrenocorticotropic hormone remained elevated for the duration of noise presentation. These findings support a dissociation between the hippocampal norepinephrine response and the behavioral and neuroendocrine response patterns and suggest that other systems may be involved in the regulation of behavioral responsiveness to aversive stimuli.     

Thalamic lesions in a genetic rat model of absence epilepsy: Dissociation between spike-wave discharges and sleep spindles

Recent findings have challenged the traditional view that the thalamus is the primary driving source of generalized spike-wave discharges (SWDs) characteristic for absence seizures, and indicate a leading role for the cortex instead. In light of this we investigated the effects of thalamic lesions on SWDs and sleep spindles in the WAG/Rij rat, a genetic model of absence epilepsy. EEG was recorded from neocortex and thalamus in freely moving rats, both before and after unilateral thalamic ibotenic acid lesions. Complete unilateral destruction of the reticular thalamic nucleus (RTN) combined with extensive destruction of the thalamocortical relay (TCR) nuclei, resulted in the bilateral abolishment of SWDs and ipsilateral abolishment of sleep spindles. A suppression of both types of thalamocortical oscillations was found when complete or extensive damage to the RTN was combined with minor to moderate damage to the TCR nuclei. Lesions that left the rostral pole of the RTN and part of the TCR nuclei intact, resulted in an ipsilateral suppression of sleep spindles, but a large increase of bilateral SWDs. These findings demonstrate that the thalamus in general and the RTN in particular are a prerequisite for both the typical bilateral 7-11 Hz SWDs and natural occurring sleep spindles in the WAG/Rij rat, but suggest that different intrathalamic subcircuits are involved in the two types of thalamocortical oscillations. Whereas the whole RTN appears to be critical for the generation of sleep spindles, the rostral pole of the RTN seems to be the most likely part that generates SWDs.     

Chronic cold stress sensitizes brain noradrenergic reactivity and noradrenergic facilitation of the HPA stress response in Wistar Kyoto rats

Many psychiatric disorders, including depression, post-traumatic stress disorder and other anxiety disorders, result from an interaction between genetic factors and exposure to a sufficiently sensitizing environmental stressor. The inbred Wistar Kyoto (WKY) rat strain has been proposed as a model of stress vulnerability, exhibiting an exaggerated hypothalamic-pituitary-adrenal (HPA) response to stress and susceptibility to gastric ulceration. Previously, we showed that stress-activation of the brain noradrenergic system was deficient in WKY rats, and they lacked noradrenergic facilitation of the HPA response in the lateral bed nucleus of the stria terminalis (BSTL), compared to outbred Sprague-Dawley (SD) controls. Deficient modulatory function of the noradrenergic system may contribute to the stress susceptibility of WKY rats. Thus, we investigated the influence of a sensitizing stimulus, chronic intermittent cold exposure, on neuroendocrine and noradrenergic stress reactivity, and on noradrenergic facilitation of the HPA response in these two strains. Chronic cold exposure (7 days, 4 h/day, 4 degrees C) potentiated activation of the HPA axis by acute immobilization stress, assessed by measuring plasma adrenocorticotropic hormone (ACTH), in both strains, although to a greater extent in WKY rats, and enhanced stress-induced norepinephrine (NE) release in BSTL of WKY but not SD rats. We then compared the influence of chronic cold exposure on noradrenergic modulation of the HPA stress response in BSTL, by measuring changes in acute stress-induced elevation of plasma ACTH after microinjecting the alpha(1)-adrenoreceptor antagonist benoxathian into the BSTL. As shown previously, benoxathian attenuated stress-induced ACTH secretion in control SD but not control WKY rats. After chronic cold, the ACTH response to acute stress was attenuated by benoxathian administration into BSTL of both strains, such that the WKY response was not different from that of SD rats. Thus, chronic cold not only sensitized the release of NE in BSTL of WKY rats, but also restored noradrenergic facilitation of their already-elevated HPA response. Such functional sensitization of a previously-deficient facilitatory system may be one mechanism whereby exposure to repeated or severe stress may induce pathologic dysregulation of the stress response in susceptible individuals, resulting in psychiatric illness.     

Chlorine conductance of the GABAA receptor of synaptoneurosomes from the brain cortex of WAG/Rij rats with absence epilepsy and wistar rats at an early period in the development of nonconvulsive or tonic-clonic kindling

We have studied muscimol-induced ³⁶Cl^? conductivity in synaptoneurosomes prepared from the frontal and somatosensory cortex of rats with three types of epileptic activity: tonic-clonic pentylenetetrazole kindling in Wistar rats, nonconvulsive absence pentylenetetrazole kindling in Wistar rats, and a genetic model of epilepsy in WAG/Rij rats. We used two concentrations of muscimol: 30 and 100 μM. The occurrence of kindling prior to tonic-clonic seizures in the Wistar rats was considerably decreased in the muscimol-induced ³⁶Cl^? conductivity as compared to the control. Development of nonconvulsive kindling considerably increased the ³⁶Cl^? conductance into the neocortical synaptoneurosomes. The control WAG/Rij rats demonstrated a significant increase in the ³⁶Cl^? conductance into neocortical synaptoneurosomes as compared to the control Wistar rats. The decrease in muscimol-induced ³⁶Cl^? conductivity after development of tonic-clonic kindling was in agreement with a large volume of literature data regarding the decrease in the activity of GABA_A receptor during tonic-clonic kindling. The high level of muscimol-induced ³⁶Cl^? conductivity in the neocortical synaptoneurosomes of the WAG/Rij rats supported the concept that absence epilepsy was induced by hyperpolarization. The high level of ³⁶Cl^? conductivity during nonconvulsive pentylenetetrazole-induced kindling suggested that the activity of the GABA_A receptor was similar in the genetic and drug-induced models of the absence epilepsy.     

Consequences of inhibition of bumetanide metabolism in rodents on brain penetration and effects of bumetanide in chronic models of epilepsy

The diuretic bumetanide, which acts by blocking the Na–K–Cl cotransporter (NKCC), is widely used to inhibit neuronal NKCC1, particularly when NKCC1 expression is abnormally increased in brain diseases such as epilepsy. However, bumetanide poorly penetrates into the brain and, in rodents, is rapidly eliminated because of extensive oxidation of its N‐butyl sidechain, reducing the translational value of rodent experiments. Inhibition of oxidation by piperonyl butoxide (PBO) has previously been reported to increase the half‐life and diuretic activity of bumetanide in rats. Here we studied whether inhibition of bumetanide metabolism by PBO also increases brain levels of bumetanide in rats, and whether this alters pharmacodynamic effects in the kindling model of epilepsy. Furthermore, we studied the effects of PBO in mice. Mice eliminated bumetanide less rapidly than rats (elimination half‐life 47 min vs. 13 min). Pretreatment with PBO increased the half‐life in mice to average values (70 min) previously determined in humans, and markedly elevated brain levels of bumetanide. In rats, the increase in plasma and brain levels of bumetanide by PBO was less marked than in mice. PBO significantly increased the diuretic activity of bumetanide in rats and, less effectively, in mice. In epileptic mice, bumetanide (with PBO) did not suppress spontaneous seizures. In the rat kindling model, bumetanide (with or without PBO) did not exert anticonvulsant effects on fully kindled seizures, but dose‐dependently altered kindling development. These data indicate that PBO offers a simple means to enhance the translational properties of rodent experiments with bumetanide, particularly when using mice.     

Enhancement of anti-absence effects of ethosuximide by low doses of a noncompetitive alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist in a genetic animal model of absence epilepsy

N-Acetyl-1-(4-chlorophenyl)-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline (THIQ-10c) is a noncompetitive α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist that has been demonstrated to antagonize generalized tonic–clonic seizures in different animal models of epilepsy. In the study described here, we tested the potential effect of such a compound alone or co-administered with ethosuximide in a genetic animal model of absence epilepsy, the WAG/Rij rat. The intraperitoneal or intracerebroventricular microinjection of THIQ-10c alone was unable to significantly modify the number and duration of spike-and-wave discharges (SWDs). In contrast, intracerebroventricular administration of AMPA induced a dose-dependent increase in the number of SWDs. THIQ-10c dose-dependently antagonized this effect. Furthermore, co-administration of THIQ-1c with ethosuximide (50 mg/kg, intraperitoneally) was able to significantly increase the efficacy of the anti-absence drug. In conclusion, although noncompetitive AMPA receptor antagonists alone might not be useful in the treatment of absence epilepsy because of their low therapeutic index, combining them with ethosuximide might be helpful in controlling absence seizures in patients not tolerating this drug or in refractory patients.     

Effects of different types of physical exercise on the staining of parvalbumin-positive neurons in the hippocampal formation of rats with epilepsy

Effects of exercise in animals with epilepsy have been demonstrated. To investigate whether the type of physical activity, voluntary or forced, would promote different morphological changes in hippocampal formation we performed an immunocytochemical study using the parvalbumin (PV) distribution as a marker. Control rats and rats with epilepsy were submitted to a voluntary (wheel running) and forced (treadmill) exercise for 10 days (acute physical exercise) or 45 days (chronic physical exercise). It was observed in normal rats a higher number of PV-positive cells in the hilus of dentate gyrus (DG) in the voluntary and forced exercise groups (acute and chronic physical exercise), when compared to the control group. In animals with epilepsy the number of PV-positive cells and staining intensity of PV-fibers in the hilus was significantly higher only in the acute physical exercise (voluntary and forced). These findings demonstrate that acute physical exercise, both voluntary and forced results in increased number of PV-positive cells and staining intensity of PV-fibers in the hilus of rats with epilepsy and the occurrence of these changes takes place only in the early phase of epilepsy.     

Voltage-gated calcium channels in the etiopathogenesis and treatment of absence epilepsy

Voltage-gated calcium channels are key elements in regulating neuronal excitability and are thus of central importance in the pathogenesis of various forms of epilepsies. Among these, absence epilepsies represent about 10% of epileptic seizures in humans. They are electroencephalographically characterized by bilateral synchronous spike-wave discharge activity associated with loss or severe impairment of consciousness. Extensive studies during the last decades revealed that pathophysiologically increased oscillatory activity, i.e., hyperoscillation within the reticulothalamocortical circuitry, is the electrophysiological correlate of absence epilepsy, with extrathalamocortical structures, e.g., brainstem and cerebellum, projecting to the thalamocortical circuitry, thereby modulating its activity. Voltage-gated calcium channels are one of the central players regulating the transition from tonic to rebound burst-firing modes in both thalamic relay and reticular thalamic nucleus neurons, the burst-firing mode being the substrate of the thalamocortical oscillation. Thus, pharmacological interference with these channels enables effective control of spike-wave discharge activity in patients suffering from absence seizures. In this review, we summarize the medical history of absence epilepsies, their classification and terminology, the diagnostic armamentarium available today and the etiopathogenesis of absences. Finally, various antiepileptic drugs that have been proven to or are supposed to exert anti-absence effects are discussed with respect to their pharmacodynamics and pharmacokinetics.     

Neonatal 5HT activity antagonizes the masculinizing effect of testosterone on the luteinizing hormone release response to gonadal steroids and on brain structures in rats

Hypothalamic 5HT concentrations are transiently lower in male compared to female Wistar rats in the second week post partum (pp) and our previous findings have shown that pharmacologically potentiating 5HT activity over this period feminizes certain aspects of sexually differentiated behaviours in adult males and androgenized females. In order to investigate whether neonatal testosterone and 5HT interact to influence physiological and morphological brain sexual differences, females, androgenized females and males were treated with the 5HT2 agonist (-) [2,5 dimethoxy-4-iodophenyl]-2-amino propane HCl [(-) DOI], over days 8-16 pp. In androgenized females (250 microg testosterone proprionate, day 2 pp) (-) DOI prevented the delay in vaginal opening, but did not prevent the androgen-induced constant oestrus in females treated with 100 microg TP, day 2 pp. (-) DOI overcame the neonatal androgen effect in suppressing the positive feedback of ovarian steroids in a few males and androgenized females. (-) DOI had a feminizing effect on the volume of the anteroventral periventricular nucleus (normally smaller in males), by significantly increasing its volume in male and androgenized females. It also had a significant antagonistic effect on the testosterone-induced increase in the volume of the sexually dimorphic nucleus of the preoptic area in males and androgenized females. These findings support the view that raised 5HT activity in the second week of life antagonizes the masculinizing effect of neonatal testosterone.     

The WAG/Rij strain: a genetic animal model of absence epilepsy with comorbidity of depression [corrected

A great number of clinical observations show a relationship between epilepsy and depression. Idiopathic generalized epilepsy, including absence epilepsy, has a genetic basis. The review provides evidence that WAG/Rij rats can be regarded as a valid genetic animal model of absence epilepsy with comorbidity of depression. WAG/Rij rats, originally developed as an animal model of human absence epilepsy, share many EEG and behavioral characteristics resembling absence epilepsy in humans, including the similarity of action of various antiepileptic drugs. Behavioral studies indicate that WAG/Rij rats exhibit depression-like symptoms: decreased investigative activity in the open field test, increased immobility in the forced swimming test, and decreased sucrose consumption and preference (anhedonia). In addition, WAG/Rij rats adopt passive strategies in stressful situations, express some cognitive disturbances (reduced long-term memory), helplessness, and submissiveness, inability to make choice and overcome obstacles, which are typical for depressed patients. Elevated anxiety is not a characteristic (specific) feature of WAG/Rij rats; it is a characteristic for only a sub-strain of WAG/Rij rats susceptible to audiogenic seizures. Interestingly, WAG/Rij rats display a hyper-response to amphetamine similar to anhedonic depressed patients. WAG/Rij rats are sensitive only to chronic, but not acute, antidepressant treatments, suggesting that WAG/Rij rats fulfill a criterion of predictive validity for a putative animal model of depression. However, more and different antidepressant drugs still await evaluation. Depression-like behavioral symptoms in WAG/Rij rats are evident at baseline conditions, not exclusively after stress. Experiments with foot-shock stress do not point towards higher stress sensitivity at both behavioral and hormonal levels. However, freezing behavior (coping deficits) and blunted response of 5HT in the frontal cortex to uncontrollable sound stress, increased c-fos expression in the terminal regions of the meso-cortico-limbic brain systems and greater DA response of the mesolimbic system to forced swim stress suggest that WAG/Rij rats are vulnerable to some, but not to all types of stressors. We propose that genetic absence epileptic WAG/Rij rats have behavioral depression-like symptoms, are vulnerable to stress and might represent a model of chronic low-grade depression (dysthymia). Both 5HT and DAergic abnormalities detected in the brain of WAG/Rij rats are involved in modulation of vulnerability to stress and provocation of behavioral depression-like symptoms. The same neurotransmitter systems modulate SWDs as well. Recent studies suggest that the occurrence and repetition of absence seizures are a precipitant of depression-like behavior. Whether the neurochemical changes are primary to depression-like behavioral alterations remains to be determined. In conclusion, the WAG/Rij rats can be considered as a genetic animal model for absence epilepsy with comorbidity of dysthymia. This model can be used to investigate etiology, pathogenic mechanisms and treatment of a psychiatric comorbidity, such as depression in absence epilepsy, to reveal putative genes contributing to comorbid depressive disorder, and to screen novel psychotropic drugs with a selective and/or complex (dual) action on both pathologies.     

Differential regulation of HCN channel isoform expression in thalamic neurons of epileptic and non-epileptic rat strains

Hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channels represent the molecular substrate of the hyperpolarization-activated inward current (I_h). Although these channels act as pacemakers for the generation of rhythmic activity in the thalamocortical network during sleep and epilepsy, their developmental profile in the thalamus is not yet fully understood. Here we combined electrophysiological, immunohistochemical, and mathematical modeling techniques to examine HCN gene expression and I_h properties in thalamocortical relay (TC) neurons of the dorsal part of the lateral geniculate nucleus (dLGN) in an epileptic (WAG/Rij) compared to a non-epileptic (ACI) rat strain. Recordings of TC neurons between postnatal day (P) 7 and P90 in both rat strains revealed that I_h was characterized by higher current density, more hyperpolarized voltage dependence, faster activation kinetics, and reduced cAMP-sensitivity in epileptic animals. All four HCN channel isoforms (HCN1-4) were detected in dLGN, and quantitative analyses revealed a developmental increase of protein expression of HCN1, HCN2, and HCN4 but a decrease of HCN3. HCN1 was expressed at higher levels in WAG/Rij rats, a finding that was correlated with increased expression of the interacting proteins filamin A (FilA) and tetratricopeptide repeat-containing Rab8b-interacting protein (TRIP8b). Analysis of a simplified computer model of the thalamic network revealed that the alterations of I_h found in WAG/Rij rats compensate each other in a way that leaves I_h availability constant, an effect that ensures unaltered cellular burst activity and thalamic oscillations. These data indicate that during postnatal developmental the hyperpolarizing shift in voltage dependency (resulting in less current availability) is compensated by an increase in current density in WAG/Rij thereby possibly limiting the impact of I_h on epileptogenesis. Because HCN3 is expressed higher in young versus older animals, HCN3 likely does not contribute to alterations in I_h in older animals.