Abnormal hippocampal melatoninergic system: a potential link between absence epilepsy and depression-like behavior in WAG/Rij rats?

Absence epilepsy and depression are comorbid disorders, but the molecular link between the two disorders is unknown. Here, we examined the role of the melatoninergic system in the pathophysiology of spike and wave discharges (SWDs) and depression-like behaviour in the Wistar Albino Glaxo from Rijswijk (WAG/Rij) rat model of absence epilepsy. In WAG/Rij rats, SWD incidence was higher during the dark period of the light-dark cycle, in agreement with previous findings. However, neither pinealectomy nor melatonin administration had any effect on SWD incidence, suggesting that the melatoninergic system was not involved in the pathophysiology of absence-like seizures. Endogenous melatonin levels were lower in the hippocampus of WAG/Rij rats as compared to non-epileptic control rats, and this was associated with higher levels of melatonin receptors in the hippocampus, but not in the thalamus. In line with the reduced melatonin levels, cell density was lower in the hippocampus ofWAG/Rij rats and was further reduced by pinealectomy. As expected, WAG/Rij rats showed an increased depression-like behaviour in the sucrose preference and forced swim tests, as compared to non-epileptic controls. Pinealectomy abolished the difference between the two strains of rats by enhancing depression-like behaviour in non-epileptic controls. Melatonin replacement displayed a significant antidepressant-like effect in bothWAG/Rij and control rats. These findings suggest that a defect of hippocampal melatoninergic system may be one of the mechanisms underlying the depression-like phenotype inWAG/Rij rats and that activation of melatonin receptors might represent a valuable strategy in the treatment of depression associated with absence epilepsy

The Α2δ subunit and absence epilepsy: beyond calcium channels?

Spike-wave discharges, underlying absence seizures, are generated within a cortico-thalamo-cortical network that involves the somatosensory cortex, the reticular thalamic nucleus, and the ventrobasal thalamic nuclei. Activation of T-type voltage-sensitive calcium channels (VSCCs) contributes to the pathological oscillatory activity of this network, and some of the first-line drugs used in the treatment of absence epilepsy inhibit T-type calcium channels. The α2δ subunit is a component of high voltage-activated VSCCs (i.e., L-, N-, P/Q-, and R channels) and has also been found to be associated with T channels. The presence of the α2δ subunit facilitates VSCC activation. Hence, one expects that drugs that bind to, and inhibit the α2δ subunit, e.g. gabapentin and pregabalin, are protective against absence epilepsy and that mice lacking the α2δ subunit are resistant to evoked absence seizures. In contrast, gabapentin and pregabalin are not clinically useful and may even be detrimental in the treatment of absence epilepsy, and ducky mice lacking the α2δ subunit develop absence seizures. This suggests that the α2δ subunit displays functions that go beyond the regulation of VSCCs, and that these functions are involved in the regulation of the cortico-thalamo-cortical network. This viewpoint critically examines the role of the α2δ subunit in the pathophysiology of absence seizures focusing on the potential role of the α2δ ligands, thrombospondin

Pharmacological activation of mGlu5 receptors with the positive allosteric modulator, VU0360172 modulates thalamic GABAergic transmission

Previous studies have shown that injection of the mGlu5 receptor positive allosteric modulator (PAM) VU0360172 into either the thalamus or somatosensory cortex markedly reduces the frequency of spike-and-wave discharges (SWDs) in the WAG/Rij model of absence epilepsy. Here we have investigated the effects of VU0360172 on GABA transport in the thalamus and somatosensory cortex, as possible modes of action underlying the suppression of SWDs. Systemic VU0360172 injections increase GABA uptake in thalamic synaptosomes from epileptic WAG/Rij rats. Consistent with this observation, VU0360172 could also enhance thalamic GAT-1 protein expression, depending on the dosing regimen. This increase in GAT-1 expression was also observed in the thalamus from non-epileptic rats (presymptomatic WAG/Rij and Wistar) and appeared to occur selectively in neurons. The tonic GABAA receptor current present in ventrobasal thalamocortical neurons was significantly reduced by VU0360172 consistent with changes in GAT-1 and GABA uptake. The in vivo effects of VU0360172 (reduction in tonic GABA current and increase in GAT-1 expression) could be reproduced in vitro by treating thalamic slices with VU0360172 for at least 1 hour and appeared to be dependent on the activation of PLC. Thus, the effects of VU0360172 do not require an intact thalamocortical circuit. In the somatosensory cortex, VU0360172 reduced GABA uptake but did not cause significant changes in GAT-1 protein levels. These findings reveal a novel mechanism of regulation mediated by mGlu5 receptors, which could underlie the powerful anti-absence effect of mGlu5 receptor enhancers in animal models

The anti-absence effect of mGlu5 receptor amplification with VU0360172 is maintained during and after antiepileptogenesis

Ethosuximide (ETX) has become the drug of choice in the treatment of patients with absence seizures taking into account both its efficacy, tolerability and antiepileptogenic properties. However, 47% of subjects treated with ETX failed in therapy, and most antiepileptic drugs have cognitive side effects. VU0360172, a positive allosteric modulator (PAM) of mGluR5, acutely and chronically administered decreased seizures dose dependently in rats of the WAG/Rij strain, a genetic absence model. Here it is investigated whether anti-epileptogenesis induced by ETX alters the sensitivity of VU0360172 as an anti-absence drug, and cognition is affected during and after chronic ETX treatment. Method: Male WAG/Rij rats were chronically treated with ETX for 4 months. EEG’s were recorded during and after treatment as well as challenged with VU0360172. Rats were also periodically exposed to a cue discrimination learning task in a Y-maze. mGlu5 receptors were quantified with Western Blot. Results: Antiepileptogenesis was successfully induced by ETX and VU0360172 showed a time and dose dependent anti-absence action. However, chronic ETX treated rats showed a decrease in absences both during and after the end treatment, without clear time and dose related effects. The decrease of sensitivity for VU0360172 was not accompanied by a change in mGluR5 expression in cortex and thalamus. Chronic ETX enhanced motivation to collect sucrose pallets and this was followed by an increase in cued discrimination learning. It is concluded that VU0360172 keeps its antiabsence effects after chronic treatment. Moreover, its differential effects in the two groups cannot be explained by a simple receptor down regulation suggesting a more downstream interaction between ETX and mGluR5. The cognitive enhancing effects of ETX, as found at the end of the experiment might be mediated to the antidepressant action of ETX as expressed by an increase in the rewarding properties of sucrose pallets

Alterations in the α2 δ ligand, thrombospondin-1, in a rat model of spontaneous absence epilepsy and in patients with idiopathic/genetic generalized epilepsies

Objectives Thrombospondins, which are known to interact with the α2δ subunit of voltage-sensitive calcium channels to stimulate the formation of excitatory synapses, have recently been implicated in the process of epileptogenesis. No studies have been so far performed on thrombospondins in models of absence epilepsy. We examined whether expression of the gene encoding for thrombospondin-1 was altered in the brain of WAG/Rij rats, which model absence epilepsy in humans. In addition, we examined the frequency of genetic variants of THBS1 in a large cohort of children affected by idiopathic/genetic generalized epilepsies (IGE/GGEs). Methods We measured the transcripts of thrombospondin-1 and α2δ subunit, and protein levels of α2δ, Rab3A, and the vesicular glutamate transporter, VGLUT1, in the somatosensory cortex and ventrobasal thalamus of presymptomatic and symptomatic WAG/Rij rats and in two control strains by real-time polymerase chain reaction (PCR) and immunoblotting. We examined the genetic variants of THBS1 and CACNA2D1 in two independent cohorts of patients affected by IGE/GGE recruited through the Genetic Commission of the Italian League Against Epilepsy (LICE) and the EuroEPINOMICS-CoGIE Consortium. Results Thrombospondin-1 messenger RNA (mRNA) levels were largely reduced in the ventrobasal thalamus of both presymptomatic and symptomatic WAG/Rij rats, whereas levels in the somatosensory cortex were unchanged. VGLUT1 protein levels were also reduced in the ventrobasal thalamus of WAG/Rij rats. Genetic variants of THBS1 were significantly more frequent in patients affected by IGE/GGE than in nonepileptic controls, whereas the frequency of CACNA2D1 was unchanged. Significance These findings suggest that thrombospondin-1 may have a role in the pathogenesis of IGE/GGEs

Protective role for type 4 metabotropic glutamate receptors against ischemic brain damage

We examined the influence of type 4 metabotropic glutamate (mGlu4) receptors on ischemic brain damage using the permanent middle cerebral artery occlusion (MCAO) model in mice and the endothelin-1 (Et-1) model of transient focal ischemia in rats. Mice lacking mGlu4 receptors showed a 25% to 30% increase in infarct volume after MCAO as compared with wild-type littermates. In normal mice, systemic injection of the selective mGlu4 receptor enhancer, N-phenyl-7-(hydroxyimino)cyclopropa[b]chromen-1a-caboxamide (PHCCC; 10 mg/kg, subcutaneous, administered once 30 minutes before MCAO), reduced the extent of ischemic brain damage by 35% to 45%. The drug was inactive in mGlu4 receptor knockout mice. In the Et-1 model, PHCCC administered only once 20 minutes after ischemia reduced the infarct volume to a larger extent in the caudate/putamen than in the cerebral cortex. Ischemic rats treated with PHCCC showed a faster recovery of neuronal function, as shown by electrocorticographic recording and by a battery of specific tests, which assess sensorimotor deficits. These data indicate that activation of mGlu4 receptors limit the development of brain damage after permanent or transient focal ischemia. These findings are promising because selective mGlu4 receptor enhancers are under clinical development for the treatment of Parkinson's disease and other central nervous system disorders

WAG/Rij rats show a reduced expression of CB1 receptors in thalamic nuclei and respond to the CB1 receptor agonist, R(+)WIN55,212-2, with a reduced incidence of spike-wave discharges

P>Purpose: Genetically epileptic WAG/Rij rats develop spontaneous absence-like seizures after 3 months of age. We used WAG/Rij rats to examine whether absence seizures are associated with changes in the expression of type-1 cannabinoid (CB1) receptors. Methods: Receptor expression was examined by in situ hybridization and western blot analysis in various brain regions of "presymptomatic" 2-month old and "symptomatic" 8-month-old WAG/Rij rats relative to age-matched nonepileptic control rats. Furthermore, we examined whether pharmacologic activation of CB1 receptor affects absence seizures. We recorded spontaneous spike-wave discharges (SWDs) in 8-month old WAG/Rij rats systemically injected with the potent CB1 receptor agonist, R(+)WIN55,212-2 (3-12 mg/kg, s.c.), given alone or combined with the CB1 receptor antagonist/inverse agonist, AM251 (12 mg/kg, s.c.). Results: Data showed a reduction of CB1 receptor mRNA and protein levels in the reticular thalamic nucleus, and a reduction in CB1 receptor protein levels in ventral basal thalamic nuclei of 8-month-old WAG/Rij rats, as compared with age-matched ACI control rats. In vivo, R(+)WIN55,212-2 caused a dose-dependent reduction in the frequency of SWDs in the first 3 h after the injection. This was followed by a late increase in the mean SWD duration, which suggests a biphasic modulation of SWDs by CB1 receptor agonists. Both effects were reversed or attenuated when R(+)WIN55,212-2 was combined with AM251. Discussion: These data indicate that the development of absence seizures is associated with plastic modifications of CB1 receptors within the thalamic-cortical-thalamic network, and raise the interesting possibility that CB1 receptors are targeted by novel antiabsence drugs