Vortioxetine increases absence-like seizures in WAG/Rij rats but decreases penicillin- and pentylenetetrazole-induced seizures in Wistar rats.

AIM: Depression is the major psychiatric disorder in patients with epilepsy. Vortioxetine is a novel antidepressant drug for the treatment of major depressive disorders. In the present study, effects of vortioxetine were evaluated in different experimental epilepsy models of rats. MATERIALS AND METHODS: Fifty-six adult male Wistar rats and 28 WAG/Rij rats were divided into 12 groups of 7 rats each. Experiments were conducted with penicillin (500 IU, i.c.) and pentylenetetrazole models (50 mg/kg, intraperitoneally (i.p.)) in Wistar rats and genetic absence epileptic WAG/Rij rats. The vortioxetine (1, 5, or 10 mg/kg, i.p.) was evaluated in these three models. All groups were compared with their control groups. RESULTS: In the penicillin-induced seizure model, 1, 5, or 10 mg/kg vortioxetine administration significantly decreased mean spike frequency. In the pentylenetetrazole-induced seizure model, 1, 5, or 10 mg/kg vortioxetine demonstrated a significant dose-dependent decrease in mean spike frequency, an increase in the latency to minor and major seizures, and a decrease in total duration of major seizure and convulsion stage. In genetic absence epileptic WAG/Rij rats, 1 mg/kg vortioxetine caused no significant alteration in the number and duration of SWDs compared to the controls, while 5 and 10 mg/kg doses of vortioxetine increased the number and duration of SWDs. Amplitude of the epileptiform activity did not change in any of the experimental epilepsy models. CONCLUSION: The results of this study suggested that vortioxetine has anticonvulsant activity in penicillin- and pentylenetetrazole-induced seizure models. However, it exhibited proconvulsant activity in the absence epileptic WAG/Rij rats.

Trazodone increases seizures in a genetic WAG/Rij rat model of absence epilepsy while decreasing them in penicillin-evoked focal seizure model.

AIM: Psychiatric disorders, especially depression and anxiety, are among the most disabling comorbidities in patients with epilepsy, and they are difficult to treat because many antidepressants cause proconvulsive effects. Thus, it is important to identify the seizure risks associated with antidepressants. Trazodone is one of the most frequently prescribed selective serotonin reuptake inhibitor (SSRI) antidepressant drugs for the treatment of depression and anxiety. The aim of the present study was to evaluate the effects of trazodone on epileptiform activity in a penicillin-evoked focal seizure model in Wistar rats and in a genetic absence epilepsy model in Wistar Albino Glaxo/Rijswijk strain (WAG/Rij) rats. METHODS: Trazodone at 5-, 10-, and 30-mg/kg doses was injected intraperitoneally in Wistar rats 30 min after penicillin injection, and spike frequency and amplitude of penicillin-induced epileptiform activity were evaluated. In a separate experimental model, the same trazodone doses were injected in WAG/Rij rats to elucidate their effects on number, duration, and amplitude of spike-and-wave discharges (SWDs) and on depression-anxiety like behavior. In both experimental groups, after trazodone injections recordings were made for 3 h. Depression-anxiety like behaviors in WAG/Rij rats were examined using forced swim test and open-field test. RESULTS: Trazodone at 10- and 30-mg/kg doses significantly reduced the frequency of penicillin-induced epileptiform activity without changing the amplitude. Trazodone at a 5-mg/kg dose had no effect on either frequency or amplitude of epileptiform activity. Trazodone at all doses significantly increased number and duration of SWDs without changing the amplitude. In addition, all doses of trazodone decreased the number of squares crossed and duration of grooming in open-field test, and reduced swimming time activity and increased immobility time in forced swim test. CONCLUSION: Our results suggest that depending on the dose used, trazodone had an anticonvulsant effect or no effect on penicillin-evoked focal seizure model, but all trazodone doses resulted in proconvulsant and depression-anxiety like behavior in WAG/Rij rats, which represent a genetic absence model of epilepsy.

Hsp90ß inhibitors prevent GLT-1 degradation but have no beneficial efficacy on absence epilepsy.

The loss of glutamate transporter-1 (GLT-1) is associated with temporal lobe epilepsy (TLE). A recent study reported that Hsp90ß interacted with GLT-1 and recruited it to 20S proteasome for degradation. Therefore, inhibiting Hsp90ß may be a new strategy for treating epilepsy. So far, no studies have shown whether the inhibition of Hsp90ß had therapeutic effects on absence epilepsy. Using a model of absence epilepsy, we demonstrated that 17-allylamino-17-demethoxygeldanamycin (17AAG) and Ganetespib (STA9090) had no therapeutic effect. Although this is a negative result, it also has a meaningful exploration value for whether Hsp90 inhibitors have therapeutic effects on other epilepsy types.

Rebound burst firing in the reticular thalamus is not essential for pharmacological absence seizures in mice.

Intrinsic burst and rhythmic burst discharges (RBDs) are elicited by activation of T-type Ca(2+) channels in the thalamic reticular nucleus (TRN). TRN bursts are believed to be critical for generation and maintenance of thalamocortical oscillations, leading to the spike-and-wave discharges (SWDs), which are the hallmarks of absence seizures. We observed that the RBDs were completely abolished, whereas tonic firing was significantly increased, in TRN neurons from mice in which the gene for the T-type Ca(2+) channel, CaV3.3, was deleted (CaV3.3(-/-)). Contrary to expectations, there was an increased susceptibility to drug-induced SWDs both in CaV3.3(-/-) mice and in mice in which the CaV3.3 gene was silenced predominantly in the TRN. CaV3.3(-/-) mice also showed enhanced inhibitory synaptic drive onto TC neurons. Finally, a double knockout of both CaV3.3 and CaV3.2, which showed complete elimination of burst firing and RBDs in TRN neurons, also displayed enhanced drug-induced SWDs and absence seizures. On the other hand, tonic firing in the TRN was increased in these mice, suggesting that increased tonic firing in the TRN may be sufficient for drug-induced SWD generation in the absence of burst firing. These results call into question the role of burst firing in TRN neurons in the genesis of SWDs, calling for a rethinking of the mechanism for absence seizure induction.

Cortical alterations in a model for absence epilepsy and febrile seizures: in vivo findings in mice carrying a human GABA(A)R gamma2 subunit mutation.

Childhood absence epilepsy (CAE) is one of the most common forms of epilepsy among children. The study of a large Australian family demonstrated that a point mutation in the gene encoding the gamma2 subunit of the GABA(A) receptor (G2R43Q) leads to an autosomal dominantly inherited form of CAE and febrile seizures (FS). In a transgenic mouse model carrying the gamma2 (R43Q) mutation heterozygous animals recapitulate the human phenotype. In-vitro experiments indicated that this point mutation impairs cortical inhibition and thus increases the likelihood of seizures. Here, using whole-cell (WC) and extracellular (EC) recordings as well as voltage-sensitive dye imaging (VSDI), we systematically searched for an in vivo correlate of cortical alterations caused by the G2R43Q mutation, as suggested by the mentioned in vitro results. We measured spontaneous and whisker-evoked activity in the primary somatosensory cortex and ventral posteriomedial nucleus of the thalamus (VPM) before and after intraperitoneal injection of the ictogenic substance pentylenetetrazol (PTZ) in urethane-anesthetized G2R43Q mice and controls in a blinded setting. Compared to wildtype controls in G2R43Q mice after PTZ injection we found 1.) Increased cortical spontaneous activity in layer 2/3 and layer 5/6 pyramidal neurons (increased standard deviation of the mean membrane potential in WC recordings), 2.) Increased variance of stimulus evoked cortical responses in VSDI experiments. 3.) The cortical effects are not due to increased strength or precision of thalamic output. In summary our findings support the hypothesis of a cortical pathology in this mouse model of human genetic absence epilepsy. Further study is needed to characterize underlying molecular mechanisms.

Model-Informed Precision Dosing Guidance of Ethosuximide Developed from a Randomized Controlled Clinical Trial of Childhood Absence Epilepsy.

Ethosuximide was identified as the optimal option for new-onset childhood absence epilepsy (CAE) in a randomized, two-phase dose escalation comparative effectiveness trial of ethosuximide, lamotrigine, and valproic acid. However, 47% of ethosuximide initial monotherapy participants experienced short-term treatment failure. This study aimed to characterize the initial monotherapy ethosuximide exposure-response relationship and to propose model-informed precision dosing guidance. Dose titration occurred over a 16-20-week period until patients experienced seizure freedom or intolerable side effects. Subjects with initial monotherapy failure were randomized to one of the other two medications and dose escalation was repeated. A population pharmacokinetic model was created using plasma concentration data (n = 1,320), collected at 4-week intervals from 211 unique participants during both the initial and second monotherapy phases. A logistic regression analysis was performed on the initial monotherapy cohort (n = 103) with complete exposure-response data. Eighty-four participants achieved seizure freedom with a wide range of ethosuximide area under the curves (AUC) ranging from 420 to 2,420 µg·h/mL. AUC exposure estimates for achieving a 50% and 75% probability of seizure freedom were 1,027 and 1,489 µg·h/mL, respectively, whereas the corresponding cumulative frequency of intolerable adverse events was 11% and 16%. Monte Carlo Simulation indicated a daily dose of 40 and 55 mg/kg to achieve 50% and 75% probability of seizure freedom in the overall population, respectively. We identified the need for adjusted mg/kg dosing in different body weight cohorts. This ethosuximide proposed model-informed precision dosing guidance to achieve seizure freedom carries promise to optimize initial monotherapy success for patients with CAE.

Environmental enrichment improves behavioral outcome in the AY-9944 model of childhood atypical absence epilepsy.

Atypical absence seizures are drug resistant in the majority of children with Lennox-Gastaut syndrome and herald a poor neurodevelopmental outcome. Here we studied the effects of environmental enrichment, enriched housing conditions designed to stimulate sensory and motor systems in the brain, on behavioral outcome in mice treated with the cholesterol biosynthesis inhibitor AY-9944 (AY), a clinically relevant model of atypical absence epilepsy. Beginning at postnatal day (P) 2, C3H mice were treated with AY (7.5 mg/kg) every 6 days until P20 and then weaned into enriched or standard cages. After 30 days (∼P50), AY mice from the enriched housing condition exhibited less behavioral hyperactivity and anxiety, improved olfactory recognition, and spatial learning, but no significant reduction in the number of ictal discharges in comparison with their non-enriched cohorts. The beneficial effects of environmental enrichment in AY model were in some behavioral tests gender-specific in favor of males suggesting that other, possibly hormonally mediated mechanisms, may interact with the therapeutic effects of enrichment. Taken together, these data provide a starting point to derive clinical occupational therapies for improving behavioral outcome in cases of intractable childhood seizures.

N-acetylcysteine aggravates seizures while improving depressive-like and cognitive impairment comorbidities in the WAG/Rij rat model of absence epilepsy.

N-acetylcysteine (NAC) is an antioxidant with some demonstrated efficacy in a range of neuropsychiatric disorders. NAC has shown anticonvulsant effects in animal models. NAC effects on absence seizures are still not uncovered, and considering its clinical use as a mucolytic in patients with lung diseases, people with epilepsy are also likely to be exposed to the drug. Therefore, we aimed to study the effects of NAC on absence seizures in the WAG/Rij rat model of absence epilepsy with neuropsychiatric comorbidities. The effects of NAC chronic treatment in WAG/Rij rats were evaluated on: absence seizures at 15 and 30 days by EEG recordings and animal behaviour at 30 days on neuropsychiatric comorbidities. Furthermore, the mechanism of action of NAC was evaluated by analysing brain expression levels of some possible key targets: the excitatory amino acid transporter 2, cystine-glutamate antiporter, metabotropic glutamate receptor 2, the mechanistic target of rapamycin and p70S6K as well as levels of total glutathione. Our results demonstrate that in WAG/Rij rats, NAC treatment significantly increased the number and duration of SWDs, aggravating absence epilepsy while ameliorating neuropsychiatric comorbidities. NAC treatment was linked to an increase in brain mGlu2 receptor expression with this being likely responsible for the observed absence seizure-promoting effects. In conclusion, while confirming the positive effects on animal behaviour induced by NAC also in epileptic animals, we report the aggravating effects of NAC on absence seizures which could have some serious consequences for epilepsy patients with the possible wider use of NAC in clinical therapeutics.

Effect of intracranial administration of ethosuximide in rats with spontaneous or pentylenetetrazol-induced spike-wave discharges.

PURPOSE: Generalized absence seizures are characterized by bilateral spike-wave discharges (SWDs), particularly in the frontoparietal cortical region. In WAG/Rij and GAERS rats with absence epilepsy, recent evidence indicates that SWDs arise first from the lateral somatosensory cortex (LSC), that is, the cortical focus theory. To further understand the cortical role in SWD generation, two epileptic rat models were assessed. METHODS: Two models, Long-Evans rats with spontaneous SWDs and Wistar rats with low-dose pentylenetetrazol-induced SWDs (20 mg/kg, i.p.), were administered intracortical or intrathalamic ethosuximide (ESM) or saline. Electroencephalographic recordings were analyzed before and after intracranial microinfusion to evaluate onset, frequency, and duration of SWDs. KEY FINDINGS: In both epileptic rat models, ESM in the LSC significantly reduced SWD number, shortened SWD duration, and delayed SWD onset compared to saline. By contrast, ESM in the medial somatosensory cortex had little effect compared to saline. Intrathalamic infusion of ESM only delayed SWD onset. SIGNIFICANCE: These findings suggest that the LSC may be essential for the occurrence of SWDs. Our data support the cortical focus theory for the generation of absence seizures.

Perioral myoclonia with absences and myoclonic status aggravated by oxcarbazepine.

Perioral myoclonia with absences belongs to the "idiopathic generalised epilepsy syndromes in development", currently not yet cited in the ILAE classification. This epilepsy syndrome is associated with a seizure type that appears to be specific. Here, we report polygraphic recordings of this seizure type in a young boy, previously misdiagnosed with focal epilepsy. EEG and clinical features were useful to differentiate diagnosis of his seizures from other absence or myoclonic seizures. Interestingly, some seizures were associated with neck myoclonia. Home video recording of myoclonic status aggravated by inappropriate treatment is also presented. [Published with video sequences].

Veratridine induced absence like-seizure in the freely moving rats: a study correlating the behavioural findings with the electrophysiological activities.

OBJECTIVES: Veratridine was characterized previously as an experimental model of epilepsy in vitro. The aim of this preliminary investigation is to identify the pattern of seizure induced by this model in vivo. MATERIAL AND METHODS: Veratridine (200 µg/kg) was administered intraperitoneally to male Sprague-Dawley rats and the electrical activity of the brain was recorded as surface electroencephalogram (EEG). RESULTS: The animals developed behavioral effects manifested as grooming, masticatory movements, facial automatism and wet dog shakes (WDSs). There were episodes of complete quiescent periods for 2-5 minutes before the animals presumed activity which were repeated every 15-20 minutes. The seizure activity during this silent activity showed fast frequency signals in the surface EEG correlating with absence seizure. The WDS behaviour was associated with electrical spikes on the EEG. When the rats were pre-treated with 200mg/kg ethosuximide (ETX), EEG recordings did not display the same fast frequency signal as that observed in animals receiving veratridine only. The number and duration of WDSs were not altered by ETX (200-400 mg/kg). CONCLUSION: Veratridine produced an absence like-seizure activity in the surface EEG, sensitive to ETX and correlates with its behavioural effects.

Intra-amygdaloid injection of kainic acid in rats with genetic absence epilepsy: the relationship of typical absence epilepsy and temporal lobe epilepsy.

We showed previously that genetic absence epilepsy rats from Strasbourg (GAERS) resist secondary generalization of focal limbic seizures after electrical kindling. We now investigate the effect of intra-amygdaloid injection of kainic acid, as another model of temporal lobe epilepsy, focusing on epileptogenesis, spike-and-wave discharges (SWDs), and the transition from basal to SWD states in GAERS. The EEG was recorded from the hippocampus and cortex of adult GAERS and Wistar rats before kainic acid injections into the basolateral amygdala and for 3 months thereafter. EEG and video recordings monitored SWDs and convulsive seizures. We analyzed spectral changes of the EEG during kainic acid-induced status epilepticus, SWDs, for 10 s before (silent period) and for 2 s before (transition period) SWDs. After the injection of kainic acid, all animals experienced convulsive seizures for at least 3 h. The first convulsive seizure was significantly delayed in GAERS compared with Wistar rats. SWDs and increases in power of the delta, alpha, and beta frequency ranges during the transition period disappeared after the kainic acid injection for 1-3 d and gradually reappeared. Power increases in the delta and alpha ranges were significantly correlated with the number of SWDs, in the beta and alpha ranges with their mean duration. Neo-Timm's staining at the end of experiments demonstrated that mossy fiber sprouting in GAERS is less pronounced than in Wistar rats. Our findings show that mechanisms underlying absence epilepsy and temporal lobe epilepsy interact with each other, although a site of this interaction remains to be defined.

Monoamine variability in the chronic model of atypical absence seizures.

PURPOSE: We studied the variability of the slow-spike-and-wave discharges (SSWDs) derived from AY-9944 (AY) treatment during brain development of Long-Evans hooded (LEh) rats. METHODS: Although all LEh rats received the standard dose of AY (7.5 mg/kg), we have observed an intersubject variability of the total SSWD duration at postnatal day (P) 55. Therefore, we set out to investigate the underlying brain levels of norepinephrine (NE), dopamine (DA), and serotonin (5-HT) and its metabolite (5-HIAA), as determined by high-performance liquid chromatography (HPLC) analyses from four different brain regions: thalamus (Th), frontoparietal cortex (Cx), hippocampus (Hp), and brainstem (Bs). RESULTS: All brains were obtained after two baseline electrocorticographic (ECoG) recordings with characteristic chronic, recurrent, bilaterally synchronous 4-6 Hz SSWD, at P 55 (336.25 +/- 97.23 s/h) and P60 (494.50 +/- 150.36 s) (r = 0.951, r(2) = 0.904, p < 0.005, Pearson product). The thalamic NE levels and the brainstem NE, DA, and 5HT levels were all significantly correlated with baseline SSWD duration at P55 and P60 (p < 0.01, Pearson product). CONCLUSION: Our data indicate that brain monoamine levels may determine the intersubject variability of SSWD duration in AY rats with chronic atypical absence seizures.

Use of a new antiepileptic drug or an old one as first drug for treatment of absence epilepsy.

Treatment of absence epilepsy requires understanding the efficacy and side effects of several drugs, one of which first became available more than 50 years ago. Methods for drug development and procedures for evaluating their safety and efficacy over that time have changed dramatically. Observational studies of the efficacy of ethosuximide, a drug developed in the 1950s, reported complete seizure control in 40-60% of patients. Valproic acid, a drug with a broad spectrum of effect, showed robust efficacy as well for control of absence seizures. Because side effects limit use in some patients, newer drugs were evaluated in patients with absence seizures. Of drugs becoming available in the last 15 years, lamotrigine has some effect in absence seizures. Although older and newer drugs presently are used without the rigorous underpinnings of the highest quality of evidence, our analysis found that ethosuximide, valproate, and lamotrigine are effective in the treatment of absence seizures, with ethosuximide quite possibly being the first drug of choice.

Oxcarbazepine, not its active metabolite, potentiates GABAA activation and aggravates absence seizures.

PURPOSE: Studies in genetic absence epileptic rats from Strasbourg (GAERS) indicate that enhancement of gamma aminobutyric acid (GABA(A)) receptor activity is a critical mechanism in the aggravation of seizures by carbamazepine (CBZ). We examined whether structural analogs of CBZ, oxcarbazepine (OXC), and its active metabolite, monohydroxy derivative (MHD), also potentiate GABA(A) receptor current and aggravate seizures. METHODS: In vitro studies in Xenopus oocytes compared the three drugs' effect on GABA(A) receptor currents. In vivo studies compared seizure activity in GAERS after intraperitoneal drug administration. RESULTS: OXC potentiated GABA(A) receptor current and aggravated seizures in GAERS, similarly to the effect of CBZ. Conversely, MHD showed only a minor potentiation of GABA(A) receptor current and did not aggravate seizures. DISCUSSION: A hydroxyl group at the C-10 position on the CBZ tricyclic structure in MHD reduces GABA(A) receptor potentiation and seizure aggravation. Reports of the aggravation of absence seizures in patients taking OXC may result from circulating unmetabolized OXC rather than MHD.

GABA receptor proteins within lipid rafts in the AY-9944 model of atypical absence seizures.

PURPOSE: The inhibition of cholesterol synthesis with AY-9944 (AY) results in chronic recurrent atypical absence seizures in rodents. We hypothesized that cholesterol inhibition during the course of creating the AY model of atypical absence seizures results in an alteration of the entry of gamma-aminobutyric acid (GABA)(A) and GABA(B) receptors into lipid rafts that contributes to epileptogenesis in this model. METHODS: The cholesterol synthesis inhibitor AY (7.5 mg/kg) was administered on postnatal day (P) 2, P8, P14, and P20 in Long-Evans hooded rats. The incorporation of GABA(A) and GABA(B) receptor proteins into lipid rafts of the brain was then determined. RESULTS: AY produced a shift of both GABA(A) and GABA(B) receptors in the examined detergent-resistant membranes (DRMs) and the soluble fractions. The percentage of the GABA(A) and GABA(B) receptors that shifted out of the DRMs varied between 17% and 50%, but the proportion of receptors in DRMs were decreased to levels around that of P5 animals or even lower. The shift observed in the AY-treated versus control animals was statistically significant (p < 0.01) for both GABA(A) and GABA(B) receptors. CONCLUSION: Cholesterol synthesis inhibition during rat brain development that is induced by AY leads to chronic atypical absence seizures and is associated with an alteration of GABA(A) and GABA(B) receptor proteins within lipid rafts. These data suggest a novel avenue of investigation into the epileptogenesis of experimental chronic atypical absence seizures.

Some treatments cause seizure aggravation in idiopathic epilepsies (especially absence epilepsy).

Seizure aggravation by antiepileptic drugs (AEDs) is a rare phenomenon, occurring mostly in generalized epilepsies treated with drugs that are more efficacious against partial seizures. Its frequency is greatly overestimated by doctors and especially by patients. There are many other reasons for seizures to deteriorate but they are often not considered. Seizure aggravation by AEDs is important to recognize but equally important not to overdiagnose. It can largely be prevented by accurate syndromic diagnosis and the treatment of generalized epilepsies with drugs that are effective against primary generalized seizures and avoiding those that are not.

Evaluation of the effects of liraglutide on the development of epilepsy and behavioural alterations in two animal models of epileptogenesis.

Liraglutide (LIR) is a novel long-lasting glucagon-like peptide-1 (GLP-1) analogue that facilitates insulin signalling and shows also neuroprotective properties in different brain disease models. In this study, we explored the potential antiepileptogenic effects of LIR in two different animal models; namely, the mouse intrahippocampal kainic acid (KA) model of temporal lobe epilepsy and the WAG/Rij rat model of absence epileptogenesis. Moreover, we evaluated LIR effects on comorbidities in various behavioural tests. Mice with kainate-induced epilepsy were treated with LIR (300 µg/kg/day s.c.) for 4 weeks after status epilepticus and then evaluated for drug effects on seizure development and behavioural alterations, whereas WAG/Rij rats were treated for 17 weeks (starting at 30 days of age, before seizure onset) with LIR (300 µg/kg/day s.c.) in order to investigate whether an early chronic treatment was able to reduce the development of absence seizures and related comorbidities. Our results indicate that LIR was effective in reducing the development of spontaneous seizures in kainate-induced epilepsy; moreover, in this model, it prevented memory impairment and related anxiety-like behaviour in the open field (OF) test while in the forced swimming test (FST), LIR displayed an apparent pro-depressant effect that was instead related to reduced endurance as confirmed by rotarod test. In contrast, LIR was unable to modify the epileptogenic process underlying the development of absence seizures in WAG/Rij rats while being antidepressant in the FST in this strain. Our results indicate that LIR may represent a promising novel treatment to prevent and treat the epileptogenic process and its associated behavioural and cognitive alterations in some models of convulsive epilepsy characterized by neurodegeneration, since LIR effects are likely secondary to its recognised neuroprotective properties.

Positive allosteric modulation of metabotropic glutamate 4 (mGlu4) receptors enhances spontaneous and evoked absence seizures.

Individual metabotropic glutamate (mGlu) receptor subtypes have been implicated in the pathophysiology of epileptic seizures, and are potential targets for novel antiepileptic drugs. Here, we examined the role of the mGlu4 receptor subtype in absence seizures using as models: (i) WAG/Rij rats, which develop spontaneous absence seizures after 2-3months of age; and (ii) mice treated with pentylentetrazole (PTZ, 30mg/kg, s.c.). Expression of mGlu4 receptors was enhanced in the reticular thalamic nucleus (RTN) of symptomatic WAG/Rij rats as compared with age-matched controls, as assessed by immunoblotting and immunohistochemistry. No changes were found in other regions of WAG/Rij rats including ventrobasal thalamic nuclei, somatosensory cortex, and hippocampus. Electron microscopy and in situ hybridization data suggested that mGlu4 receptors in the RTN are localized on excitatory cortical afferents. Systemic injection of the selective mGlu4 receptor positive allosteric modulator, N-phenyl-7-(hydroxyimino)cyclopropa[b]chromen1a-carboxamide (PHCCC, 10mg/kg, s.c.), substantially enhanced the number of spike-and-wave discharges (SWDs) in WAG/Rij rats. Injection of PHCCC also enhanced absence-like seizures in PTZ-treated mice, whereas it was totally inactive in mGlu4 receptor knockout mice, which were intrinsically resistant to PTZ-induced seizures, as expected. This data supports the hypothesis that activation of mGlu4 receptors participates in the generation of absence seizures which can be exacerbated with the use of a positive allosteric modulator.