Head-to head comparison of mGlu1 and mGlu5 receptor activation in chronic treatment of absence epilepsy in WAG/Rij rats

Acute treatment with positive allosteric modulators (PAMs) of mGlu1 and mGlu5 metabotropic glutamate receptors (RO0711401 and VU0360172, respectively) reduces the incidence of spike-and wave discharges in the WAG/Rij rat model of absence epilepsy. However, from the therapeutic standpoint, it was important to establish whether tolerance developed to the action of these drugs. We administered either VU0360172 (3 mg/kg, s.c.) or RO0711401 (10 mg/kg, s.c.) to WAG/Rij rats twice daily for ten days. VU0360172 maintained its activity during the treatment, whereas rats developed tolerance to RO0711401 since the 3rd day of treatment and were still refractory to the drug two days after treatment withdrawal. In response to VU0360172, expression of mGlu5 receptors increased in the thalamus of WAG/Rij rats after 1 day of treatment, and remained elevated afterwards. VU0360172 also enhanced mGlu5 receptor expression in the cortex after 8 days of treatment without changing the expression of mGlu1a receptors. Treatment with RO0711401 enhanced the expression of both mGlu1a and mGlu5 receptors in the thalamus and cortex of WAG/Rij rats after 3-8 days of treatment. These data were different from those obtained in non-epileptic rats, in which repeated injections of RO0711401 and VU0360172 down-regulated the expression of mGlu1a and mGlu5 receptors. Levels of VU0360172 in the thalamus and cortex remained unaltered during the treatment, whereas levels of RO0711401 were reduced in the cortex at day 8 of treatment. These findings suggest that mGlu5 receptor PAMs are potential candidates for the treatment of absence epilepsy in humans.

Negative expectations facilitate mechanical hyperalgesia after high-frequency electrical stimulation of human skin

BACKGROUND

High-frequency electrical stimulation (HFS) of human skin induces not only an increased pain sensitivity in the conditioning area but also an increased pain sensitivity to mechanical punctate stimuli in the non-conditioned surrounding skin area. The aim of the present study was to investigate whether this heterotopically increased mechanical pain sensitivity can be facilitated through the induction of negative expectations.

METHODS

In two independent conditions [a nocebo (n = 15) and control condition (n = 15)], we applied mechanical pain stimuli before, directly after, 10 min and 20 min after HFS in the skin area surrounding the conditioning area, and measured the reported pain intensity [visual analogue scale (VAS)]. All subjects (of both conditions) received a written instruction about the HFS protocol, but only the instruction in the nocebo condition was extended by the following text (in Dutch): 'After the HFS, your skin will become more sensitive to the pinprick stimulation'.

RESULTS

Our results clearly show that induced expectations of increased mechanical pain sensitivity after HFS facilitates the reported pain intensity after HFS more than when no information is given.

CONCLUSIONS

This study shows for the first time that brain mechanisms, via the induction of negative expectations, can facilitate heterotopic mechanical hyperalgesia after HFS of human skin.

Potentiation of mGlu5 receptors with the novel enhancer, VU0360172, reduces spontaneous absence seizures in WAG/Rij rats

Absence epilepsy is generated by the cortico-thalamo-cortical network, which undergoes a finely tuned regulation by metabotropic glutamate (mGlu) receptors. We have shown previously that potentiation of mGlu1 receptors reduces spontaneous occurring spike and wave discharges (SWDs) in the WAG/Rij rat model of absence epilepsy, whereas activation of mGlu2/3 and mGlu4 receptors produces the opposite effect. Here, we have extended the study to mGlu5 receptors, which are known to be highly expressed within the cortico-thalamo-cortical network. We used presymptomatic and symptomatic WAG/Rij rats and aged-matched ACI rats. WAG/Rij rats showed a reduction in the mGlu5 receptor protein levels and in the mGlu5-receptor mediated stimulation of polyphosphoinositide hydrolysis in the ventrobasal thalamus, whereas the expression of mGlu5 receptors was increased in the somatosensory cortex. Interestingly, these changes preceded the onset of the epileptic phenotype, being already visible in pre-symptomatic WAG/Rij rats. SWDs in symptomatic WAG/Rij rats were not influenced by pharmacological blockade of mGlu5 receptors with MTEP (10 or 30 mg/kg, i.p.), but were significantly decreased by mGlu5 receptor potentiation with the novel enhancer, VU0360172 (3 or 10 mg/kg, s.c.), without affecting motor behaviour. The effect of VU0360172 was prevented by co-treatment with MTEP. These findings suggest that changes in mGlu5 receptors might lie at the core of the absence-seizure prone phenotype of WAG/Rij rats, and that mGlu5 receptor enhancers are potential candidates to the treatment of absence epilepsy. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

Protective role for type-1 metabotropic glutamate receptors against spike and wave discharges in the WAG/Rij rat model of absence epilepsy

Eight-month old WAG/Rij rats, which developed spontaneous occurring absence seizures, showed a reduced function of mGlu1 metabotropic glutamate receptors in the thalamus, as assessed by in vivo measurements of DHPG-stimulated polyphosphoinositide hydrolysis, in the presence of the mGlu5 antagonist MPEP as compared to age-matched non-epileptic control rats. These symptomatic 8-month old WAG/Rij rats also showed lower levels of thalamic mGlu1α receptors than age-matched controls and 2-month old (pre-symptomatic) WAG/Rij rats, as detected by immunoblotting. Immunohistochemical and in situ hybridization analysis indicated that the reduced expression of mGlu1 receptors found in symptomatic WAG/Rij rats was confined to an area of the thalamus that excluded the ventroposterolateral nucleus. No mGlu1 receptor mRNA was detected in the reticular thalamic nucleus. Pharmacological manipulation of mGlu1 receptors had a strong impact on absence seizures in WAG/Rij rats. Systemic treatment with the mGlu1 receptor enhancer SYN119, corresponding to compound RO0711401, reduced spontaneous spike and wave discharges spike-wave discharges (SWDs) in epileptic rats. Subcutaneous doses of 10 mg/kg of SYN119 only reduced the incidence of SWDs, whereas higher doses (30 mg/kg) also reduced the mean duration of SWDs. In contrast, treatment with the non-competitive mGlu1 receptor antagonist, JNJ16259685 (2.5 and 5 mg/kg, i.p.) increased the incidence of SWDs. These data suggest that absence epilepsy might be associated with a reduction of mGlu1 receptors in the thalamus, and that compounds that amplify the activity of mGlu1 receptors might be developed as novel anti-absence drugs. This article is part of a Special Issue entitled 'Trends in neuropharmacology: in memory of Erminio Costa'.

On-off intermittency in time series of spontaneous paroxysmal activity in rats with genetic absence epilepsy

In the present paper we consider the on-off intermittency phenomena observed in time series of spontaneous paroxysmal activity in rats with genetic absence epilepsy. The method to register and analyze the electroencephalogram with the help of continuous wavelet transform is also suggested.

An effective correlation dimension and burst suppression ratio of the EEG in rat. Correlation with sevoflurane induced anaesthetic depth

BACKGROUND AND OBJECTIVE

Anaesthesiologists need parameters that measure the depth of anaesthesia. In the context of this need, the present study investigated in rats how two variables from the electroencephalogram, the burst suppression ratio and effective correlation dimension correlated with a measure of anaesthetic depth as measured in the strength of a noxious withdrawal reflex.

METHODS

Eight rats were exposed to different inspiratory concentrations of sevoflurane, each rat in two separate experiments. In the first experiment, spontaneously breathing animals could move freely and no painful stimuli were applied. In the second experiment, in mechanically ventilated restrained anaesthetized rats, the withdrawal reflex was measured every 80 s. In both experiments the electroencephalogram was continuously recorded. The concentration in the effector compartment was estimated using a first order two compartment model. Correlation dimension was computed following the Grassberger/Procaccia/Takens approach with optimized parameter settings to achieve maximum sensitivity to anaesthetic drug effects and enable real-time computation. The Hill, equation was fitted to the data, describing the effect as a function of sevoflurane concentration.

RESULTS

Good correlations of Depth of Anaesthesia with correlation dimension as well as burst suppression ratio were established in both types of experiments. Arousal by noxious stimuli decreased burst suppression ratio and increased correlation dimension. The effective sevoflurane concentration associated with 50% of the maximum effect (C50) was higher in experiment II (stimulation) than in experiment I (no stimulation): i.e. for correlation dimension 2.18% vs. 0.60% and for burst suppression ratio 3.07% vs. 1.73%. The slope factors were: gammaCD = 4.15 vs. gammaCD = 1.73 and gammaBSR = 5.2 vs. gammaBSR = 5.4. Correlation dimension and burst suppression ratio both correlated with the strength of the withdrawal reflex with correlation coefficients of 0.46 and 0.66 respectively (P < 0.001).

CONCLUSIONS

Both correlation dimension and burst suppression ratio are related to anaesthetic depth and are affected by noxious stimuli. The relationship between anaesthetic depth and burst suppression ratio is confirmed and the potential of correlation dimension is demonstrated.

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.

The relationship between hippocampal EEG theta activity and locomotor behaviour in freely moving rats: effects of vigabatrin

The relationship between hippocampal electroencephalogram (EEG) theta activity and locomotor speed in both spontaneous and forced walking conditions was studied in rats after vigabatrin injection (500 mg/kg i.p.). Vigabatrin increased the percentage of time that rats spent being immobile. During spontaneous walking in the open field, the speed of locomotion was increased by vigabatrin, while theta peak frequency was decreased. Vigabatrin also reduced the theta peak frequency during forced (speed controlled) walking. There was only a weak positive correlation (r=0.22) between theta peak frequency and locomotor speed for the saline condition. Furthermore, vigabatrin abolishes the weak relationship between speed of locomotion and theta peak frequency. Vigabatrin and saline did not differ in the slope of the regression line, but showed different offset points at the theta peak frequency axis. Thus, other factors than speed of locomotion seem to be involved in determination of the theta peak frequency.

Rat models of genetic absence epilepsy: what do EEG spike-wave discharges tell us about drug effects?

Electroencephalographic studies in the WAG/Rij rats of Nijmegen and genetic absence epileptic rats of Strasbourg (GAERS), two genetic models for human generalized absence epilepsy, illustrate the usefulness of drug-electroencephalogram (EEG) interaction studies. In the EEG of both types of rats, spontaneously occurring spike-wave discharges are present. For drug discovery, a model with predictive validity is imperative, and both the WAG/Rij and the GAERS models seem adequate. The present paper discusses effects on spike-wave discharges of various compounds that are clinically used. Not only new antiepileptic drugs, such as remacemide, loreclezole, lamotrigine, tiagabine, gabapentin, progabide and levetiracetam are evaluated, but also drugs used for other purposes, such as etomidate and fentanyl-fluanisone for anesthesia, opioidergic drugs and drugs used for strokes. It is shown that some new antiepileptic drugs, such as tiagabine, have spike-wave discharge-increasing properties, while other drugs are worth studying in clinical trials for antiabsence treatment. Furthermore, it is shown that many commonly used drugs such as analgesics, anesthetics and drugs to treat stroke generally enhance spike-wave discharges. It can be concluded that EEG monitoring is imperative for the discovery and development of potentially antiepileptic compounds and that genetic rat models such as the WAG/Rij or GAERS, to a large extent, can reliably predict clinical efficacy of various types of compounds as well as alert us of potentially adverse effects.

Effects of diazepam on auditory evoked potentials of rats elicited in a ten-tone paradigm

The effect of diazepam on sensory gating was studied in rats by measuring diazepam effects on auditory evoked potentials (AEPs) elicited in a ten-tone paradigm. Trains of 10 repetitive tone-pip stimuli were presented. Rats (n = 8) received 4 mg x kg(-1) diazepam subcutaneously or vehicle, counterbalanced over two sessions. Diazepam decreased the amplitude of the middle-latency P30 component and increased the amplitudes of the late-latency N60 and P67 components. The increase in the late-latency components might be due to a diazepam-induced decrease in arousal. Stimulus repetition decreased the amplitudes of the middle-latency N18 and P30 components in both conditions. This suggests that automated neuronal recovery functions underlying sensory gating remain intact with diazepam. In the vehicle condition, the amplitude of the late-latency P67 decreased with stimulus repetition, but not in the diazepam condition. This suggests a diazepam-induced decrease of behaviourally mediated habituation.

The influence of diazepam on the electroencephalogram-evoked potential interrelation in rats

Though being a sedative, diazepam increases beta-activity in the electroencephalogram (EEG). Diazepam also affects auditory evoked potentials (AEPs). We investigated if the effect of diazepam on AEPs could be ascribed to its beta-increasing effect. Eight rats received vehicle and diazepam counterbalanced. AEPs were ranked in ten categories according to the percentages of beta-activity in the pre-stimulus electroencephalogram (EEG). With vehicle, the P(29), N(51) and P(67) AEP components increased and cross-correlation coefficients (CCCs) declined with increasing beta-activity. With diazepam AEP components and CCCs remained unchanged. All AEPs with diazepam closely resembled the AEP of the lowest beta-category obtained with vehicle. We conclude that the effect of diazepam on AEPs can not be ascribed to its beta-enhancing effect. Diazepam disrupts the normal AEP-EEG relation such that diazepam-effects on AEPs seem to reflect the sedative effects of diazepam and not its beta-increasing effects.

Effects of polytherapy compared with monotherapy in antiepileptic drugs: an animal study

Although monotherapy in epilepsy treatment is frequently advocated, this is not based on studies with equal drug loads. This study was performed to investigate the experimental background of polytherapy with standardized drug loads. Dose-dependent effects on grip strength, accelerod performance, and spontaneous behavior of rats was used to study the effect of combining valproate and ethosuximide. The potency of the drugs (combination) was obtained by fitting the sigmoid Emax equation to the data. Drug interaction was assessed using the isobologram method and quantified by comparing equivalent drug loads with their 95% confidence intervals. We found that the effects of valproate and ethosuximide combine in a simple additive way in the grip strength experiment as well as in the accelerod experiment. In the behavioral studies, however, a higher drug load of the combination was needed to obtain the same amount of sedation, signifying infra-additivity. Infra-additivity of sedative effects is an important finding because this is by far the most frequent side effect mentioned in human studies. However, assessment of the therapeutic effect of the combination will have to be completed before a preference for mono- or polytherapy, based on the balance of adverse effects and efficacy, can be expressed.

Diazepam biphasically modulates [3H]TBOB binding to the convulsant site of the GABAA receptor complex

Interactions of GABA, bicuculline methochloride and diazepam with [3H]TBOB binding to rat brain membranes were evaluated in vitro. GABA displaced [3H]TBOB binding with and IC50 of 4 microM and a slope factor near unity. The competitive GABA antagonist bicuculline methochloride shifted the displacement curve of GABA parallelly to the right, indicating that the interaction of GABA with [3H]TBOB binding is of an allosteric nature. In the presence of GABA, diazepam displaced the binding of [3H]TBOB according to a two-site model: a high affinity site with an IC50 of about 50 nM and a lower affinity site with an IC50 of about 30 microM. Bicuculline methochloride abolished the nanomolar displacement by diazepam and increased the micromolar IC50 value. These results indicate that the interaction of the high affinity diazepam site with the [3H]TBOB binding site is totally GABA dependent and that the low affinity effect of diazepam on [3H]TBOB binding is at least partially GABA dependent. It is likely that the low affinity potency of diazepam to displace [3H]TBOB binding has physiological relevance.

Effects of lamotrigine on absence seizures in rats

Lamotrigine, a novel antiepileptic drug thought to inhibit the excitatory neurotransmitter release and indicated for convulsive seizures, was tested for its efficacy in modulating non-convulsive seizures of the absence type of epilepsy. This was done in a rat model for absence epilepsy: the WAG/Rij strain of rats. In side-effect free doses, lamotrigine did not inhibit absence seizures in these animals. Only in a high dose a reduction of spike-wave activity was noticed, but in that dose side-effects such as abnormal locomotion, were also seen. These findings underline the differential pharmacological profile of convulsive and non-convulsive generalized epilepsy.

Interconvertible kinetic states of t-butylbicycloorthobenzoate binding sites of the gamma-aminobutyric acidA ionophores

The kinetics of t-[3H]butylbicycloorthobenzoate (TBOB) binding to the convulsant sites of the gamma-aminobutyric acidA (GABAA) receptor-ionophore complex were examined in synaptosomal membrane preparations of rat brain. On and off rates of TBOB binding were accelerated by 1 microM GABA and decelerated by 1 microM bicuculline methochloride, a GABAA antagonist. The presence of GABA and bicuculline methochloride created rapid and slow phases of dissociation, respectively. The three groups of rate constants distinguished for the dissociation of 4 nM and 30 nM [3H]TBOB represent multi-affinity states of the convulsant sites depending on the presence of GABA or bicuculline methochloride. Apparent association rate constants do not obey the equation k(app) = k(off) + k(on) [TBOB] without assuming interconvertibility of the kinetic states during binding. Avermectin B1a (AVM B1a), a chloride channel opening agent, accelerated the association and dissociation of TBOB and resulted in a biphasic effect on TBOB binding, i.e., enhancement at low concentrations (EC50 7.8 nM) followed by displacement at high concentrations (IC50 6.3 microM) of AVM B1a. AVM B1a resulted in similar biphasic effects on t-[35S]butylbicyclophosphorothionate binding. DIDS, an isothiocyanatostilbene derivative with irreversible anion channel blocking effect, selectively inhibited basal [3H]TBOB binding (IC50 125 microM DIDS) leaving the enhancement by AVM B1a unaffected.

The GABAA receptor complex in relation to epilepsy. Reversal of [3H]TBOB inhibition: a prediction of proconvulsive properties?

[3H]-t-Butylbicycloorthobenzoate ([3H]TBOB), a convulsant, is known to label a binding site on the GABAA receptor complex. Bicuculline methochloride (bicuculline MCl), folic acid, pentazocine, naloxone, ethyl-beta-carboline-3-carboxylate (beta CCE) and Ro 5-4864 have (pro)convulsive properties in vivo. In the present study, we determined the extent to which these compounds modify the binding of [3H]TBOB in the presence of IC50 amounts of GABA (5 microM) or diazepam (50 microM). We found that the GABA antagonist bicuculline MCl reversed the inhibitory effect of GABA on [3H]TBOB binding completely, as was expected. Folic acid, pentazocine and naloxone also reversed the inhibitory effect of GABA on [3H]TBOB binding. This finding is compatible with the view that the proconvulsive effects of these compounds can be credited to a reduction of GABAergic action at the GABAA receptor complex. We suggest that the reversal of GABA's inhibition of [3H]TBOB binding is a sufficient (but not a necessary) condition to predict proconvulsive (side) effects of drugs. beta CCE and Ro 5-4864 modified [3H]TBOB binding in the presence of GABA in a biphasic fashion. A unique relation between beta CCE, Ro 5-4864 and the GABAA complex might exist. Bicuculline MCl reversed the inhibitory effect of diazepam on [3H]TBOB binding only partly. beta CCE did not reverse the inhibitory effect of diazepam on [3H]TBOB binding, neither did Ro 5-4864. The presence of a GABA-independent interaction between a low affinity benzodiazepine recognition site and the TBOB site is proposed.

Binding of Ro 5-4864 to brain membranes of rats with and without absence-like phenomena

Spontaneously occurring spike-wave discharges (SWD) in rats are used as a model for absence epilepsy in humans. In vitro, the binding parameters of 3H-Ro 5-4864, a ligand labelling the peripheral benzodiazepine receptor, were determined for brain membranes of WAG/Rij rats, an inbred strain showing SWD, and for ACI rats, an inbred strain showing no SWD. No difference in the Kd but a small difference in the Bmax values between the strains were found. Recently, other investigators reported a correlation between a decrease in affinity for 3H-Ro 5-4864 and the occurrence of SWD. Our results suggest however that it is doubtful that a change in Kd of the peripheral benzodiazepine receptor is causal in the etio-pathology of the spontaneous absence like phenomena in rats.

Involvement of NMDA receptors in non-convulsive epilepsy in WAG/Rij rats

The involvement of the NMDA receptor in spontaneous non-convulsive epilepsy was studied by intracerebroventricular injections of APH and NMDA in WAG/Rij rats. The WAG/Rij rat strain is recognized as an animal model for human absence epilepsy. EEG registrations showed that APH (5 nmol/5 microliters; 25 nmol/5 microliters; 50 nmol/5 microliters) causes a dose-dependent decrease in the number and mean duration of the spike-wave discharges, while NMDA (50 pmol/5 microliters; 500 pmol/5 microliters; 5 nmol/5 microliters) induces a dose-dependent increase in the number. The effects of NMDA (5 nmol/5 microliters) can be blocked completely by APH (50 nmol/5 microliters). These results suggest an involvement of the NMDA receptor in experimental non-convulsive epilepsy, observed in the WAG/Rij model.

Effects of GABA-ergic agents on spontaneous non-convulsive epilepsy, EEG and behaviour, in the WAG/RIJ inbred strain of rats

The effects of GABAergic agents on non-convulsive epilepsy were studied by intracerebroventricular injections of muscimol and bicuculline in WAG/Rij rats. The WAG/Rij rat strain is recognized as an animal model for human absence epilepsy. EEG registrations and behavioural observations showed that muscimol dose-dependently increased the non-convulsive absence epilepsy. Besides this, it induced EEG spikes and body twitches. Bicuculline induced spikes and body twitches as well but decreased the non-convulsive epilepsy. All effects of muscimol can be blocked by bicuculline and vice versa, which suggests that the observed effects are genuine GABAA effects. These results implicate that non-convulsive epilepsy can be caused by a GABAergic hyperfunction.

The neurotoxic effect of monosodium glutamate (MSG) on the retinal ganglion cells of the albino rat

Monosodium glutamate (MSG) administered postnatally to the albino rat causes extensive destruction of the retina. This MSG effect does not result in complete blindness. Ganglion cells surviving the MSG treatment are healthy and functional. Using retrogradely transported HRP and Nissl staining in whole mounted retinas, it was found that the ganglion cells left after MSG treatment are not smaller than those in controls, that these cells do not belong to one cell size group, and that no cells size group is selectively missed. The results explain why photic entrainment of MSG treated animals is still possible.