Hypodopaminergic state of the nigrostriatal pathway drives compulsive alcohol use.

The neurobiological mechanisms underlying compulsive alcohol use, a cardinal feature of alcohol use disorder, remain elusive. The key modulator of motivational processes, dopamine (DA), is suspected to play an important role in this pathology, but its exact role remains to be determined. Here, we found that rats expressing compulsive-like alcohol use, operationalized as punishment-resistant self-administration, showed a decrease in DA levels restricted to the dorsolateral territories of the striatum, the main output structure of the nigrostriatal DA pathway. We then causally demonstrated that chemogenetic-induced selective hypodopaminergia of this pathway resulted in compulsive-like alcohol self-administration in otherwise resilient rats, accompanied by the emergence of alcohol withdrawal-like motivational impairments (i.e., impaired motivation for a natural reinforcer). Finally, the use of the monoamine stabilizer OSU6162, previously reported to correct hypodopaminergic states, transiently decreased compulsive-like alcohol self-administration in vulnerable rats. These results suggest a potential critical role of tonic nigrostriatal hypodopaminergic states in alcohol addiction and provide new insights into our understanding of the neurobiological mechanisms underlying compulsive alcohol use.

Early reduced dopaminergic tone mediated by D3 receptor and dopamine transporter in absence epileptogenesis.

OBJECTIVE: In Genetic Absence Epilepsy Rats From Strasbourg (GAERSs), epileptogenesis takes place during brain maturation and correlates with increased mRNA expression of D3 dopamine receptors (D3R). Whether these alterations are the consequence of seizure repetition or contribute to the development of epilepsy remains to be clarified. Here, we addressed the involvement of the dopaminergic system in epilepsy onset in GAERSs. METHODS: Experiments were performed using rats at different stages of brain maturation from three strains according to their increasing propensity to develop absence seizures: nonepileptic control rats (NECs), Wistar Hannover rats, and GAERSs. Changes in dopaminergic neurotransmission were investigated using different behavioral and neurochemical approaches: autoradiography of D3R and dopamine transporter, single photon emission computed tomographic imaging, acute and chronic drug effects on seizure recordings (dopaminergic agonists and antagonists), quinpirole-induced yawns and dopamine synaptosomal uptake, microdialysis, brain tissue monoamines, and brain-derived neurotrophic factor quantification. RESULTS: Autoradiography revealed an increased expression of D3R in 14-day-old GAERSs, before absence seizure onset, that persists in adulthood, as compared to age-matched NECs. This was confirmed by increased yawns, a marker of D3R activity, and increased seizures when animals were injected with quinpirole at low doses to activate D3R. We also observed a concomitant increase in the expression and activity of the dopamine transporter in GAERSs before seizure onset, consistent with both lowered dopamine basal level and increased phasic responses. SIGNIFICANCE: Our data show that the dopaminergic system is persistently altered in GAERSs, which may contribute not only to behavioral comorbidities but also as an etiopathogenic factor in the development of epilepsy. The data suggest that an imbalanced dopaminergic tone may contribute to absence epilepsy development and seizure onset, as its reversion by a chronic treatment with a dopamine stabilizer significantly suppressed epileptogenesis. Our data suggest a potential new target for antiepileptic therapies and/or improvement of quality of life of epileptic patients.

Assessing Susceptibility to Epilepsy in Three Rat Strains Using Brain Metabolic Profiling Based on HRMAS NMR Spectroscopy and Chemometrics.

The possibility that a metabolomic approach can inform about the pathophysiology of a given form of epilepsy was addressed. Using chemometric analyses of HRMAS NMR data, we compared several brain structures in three rat strains with different susceptibilities to absence epilepsy: Genetic Absence Epilepsy Rats from Strasbourg (GAERS), Non Epileptic Control rats (NEC), and Wistar rats. Two ages were investigated: 14 days postnatal (P14) before the onset of seizures and 5 month old adults with fully developed seizures (Adults). The relative concentrations of 19 metabolites were assessed using (1)H HRMAS NMR experiments. Univariate and multivariate analyses including multiblock models were used to identify the most discriminant metabolites. A strain-dependent evolution of glutamate, glutamine, scyllo-inositol, alanine, and glutathione was highlighted during cerebral maturation. In Adults, data from somatosensory and motor cortices allowed discrimination between GAERS and NEC rats with higher levels of scyllo-inositol, taurine, and phosphoethanolamine in NEC. This epileptic metabolic phenotype was in accordance with current pathophysiological hypothesis of absence epilepsy (i.e., seizure-generating and control networks) and putative resistance of NEC rats and was observed before seizure onset. This methodology could be very efficient in a clinical context.

Involvement of the thalamic parafascicular nucleus in mesial temporal lobe epilepsy.

Mesial temporal lobe epilepsy (MTLE) is characterized by focal seizures, associated with hippocampal sclerosis, and often resistance to antiepileptic drugs. The parafascicular nucleus (PF) of the thalamus is involved in the generation of physiological oscillatory rhythms. It receives excitatory inputs from the cortex and inhibitory inputs from the basal ganglia, a system implicated in the control of epileptic seizures. The aim of this study was to examine the involvement of the PF in the occurrence of hippocampal paroxysmal discharges (HPDs) in a chronic animal model of MTLE in male mice. We recorded the local field potential (LFP) and the extracellular and intracellular activity of hippocampal and PF neurons during spontaneous HPDs in vivo. The end of the HPDs was concomitant with a slow repolarization in hippocampal neurons leading to an electrical silence. In contrast, it was associated in the PF with a transient increase in the power of the 10-20 Hz band in LFPs and a depolarization of PF neurons resulting in a sustained firing. We tested the role of the PF in the control of HPDs by single 130 Hz electrical stimulation of this nucleus and bilateral intra-PF injection of NMDA and GABA(A) antagonist and agonist. High-frequency PF stimulation interrupted ongoing HPDs at an intensity devoid of behavioral effects. NMDA antagonist and GABA(A) agonist suppressed hippocampal discharges in a dose-dependent way, whereas NMDA agonist and GABA(A) antagonist increased HPDs. Altogether, these data suggest that the PF nucleus plays a role in the modulation of MTLE seizures.

Dentate gyrus and hilus transection blocks seizure propagation and granule cell dispersion in a mouse model for mesial temporal lobe epilepsy.

Epilepsy-associated changes of the anatomical organization of the dentate gyrus and hilus may play a critical role in the initiation and propagation of seizures in mesial temporal lobe epilepsy (MTLE). This study evaluated the role of longitudinal projections in the propagation of hippocampal paroxysmal discharges (HPD) in dorsal hippocampus by performing a selective transection in a mouse model for MTLE obtained by a single unilateral intrahippocampal injection of kainic acid (KA). Full transections of the dentate gyrus and hilus were performed in the transverse axis at 22 days after KA injection when spontaneous HPD were fully developed. They: (i) significantly reduced the occurrence of HPD; (ii) increased their duration at the KA injection site; (iii) abolished their spread along the longitudinal axis of the hippocampal formation and; (iv) limited granule cell dispersion (GCD) of the dentate gyrus posterior to the transection. These data suggest that: (i) longitudinal projections through the dentate gyrus and hilus are involved in HPD spread; (ii) distant hippocampal circuits participate in the generation and cessation of HPD and; (iii) GCD requires continuous HPD to develop, even when seizures are established. Our data reveal a critical role for longitudinal projections in the generation and spread of hippocampal seizures.

Identifying neural drivers with functional MRI: an electrophysiological validation.

Whether functional magnetic resonance imaging (fMRI) allows the identification of neural drivers remains an open question of particular importance to refine physiological and neuropsychological models of the brain, and/or to understand neurophysiopathology. Here, in a rat model of absence epilepsy showing spontaneous spike-and-wave discharges originating from the first somatosensory cortex (S1BF), we performed simultaneous electroencephalographic (EEG) and fMRI measurements, and subsequent intracerebral EEG (iEEG) recordings in regions strongly activated in fMRI (S1BF, thalamus, and striatum). fMRI connectivity was determined from fMRI time series directly and from hidden state variables using a measure of Granger causality and Dynamic Causal Modelling that relates synaptic activity to fMRI. fMRI connectivity was compared to directed functional coupling estimated from iEEG using asymmetry in generalised synchronisation metrics. The neural driver of spike-and-wave discharges was estimated in S1BF from iEEG, and from fMRI only when hemodynamic effects were explicitly removed. Functional connectivity analysis applied directly on fMRI signals failed because hemodynamics varied between regions, rendering temporal precedence irrelevant. This paper provides the first experimental substantiation of the theoretical possibility to improve interregional coupling estimation from hidden neural states of fMRI. As such, it has important implications for future studies on brain connectivity using functional neuroimaging.

Manipulating the epileptic brain using stimulation: a review of experimental and clinical studies.

Neurostimulation represents an interesting alternative therapy for patients resistant to drug treatment or who cannot benefit from resective surgery. Theoretically, neurostimulation allows the control of seizures to be tailored to the individual patient and specific form of epilepsy. Here, we review both experimental and clinical studies that have reported the possible control of epileptic seizures by means of different approaches using electrical stimulation (vagus nerve stimulation, deep brain stimulation and repetitive transcranial magnetic stimulation). The rationale for targeting specific areas that have thus far been considered (i.e., vagus nerve, cerebellum, anterior or centromedial thalamus, basal ganglia, cortex and temporal lobe) is addressed in the light of experimental data and clinical effectiveness in different models and forms of epilepsy. The type of seizures that can be considered for neurostimulation, as well as the optimal parameters such as stimulation frequency and modes of stimulation (chronic, continuous or adaptative), are discussed to determine the best candidates for such a therapeutic strategy. This review points out the need for improved knowledge of neural circuits that generate seizures and/or allow their propagation, as well as a better understanding of the mechanisms of action of neurostimulation.

Deep layer somatosensory cortical neurons initiate spike-and-wave discharges in a genetic model of absence seizures.

Typical absence has long been considered as the prototypic form of generalized nonconvulsive epileptic seizures. Recent investigations in patients and animal models suggest that absence seizures could originate from restricted regions of the cerebral cortex. However, the cellular and local network processes of seizure initiation remain unknown. Here, we show that absence seizures in Genetic Absence Epilepsy Rats from Strasbourg, a well established genetic model of this disease, arise from the facial somatosensory cortex. Using in vivo intracellular recordings, we found that epileptic discharges are initiated in layer 5/6 neurons of this cortical region. These neurons, which show a distinctive hyperactivity associated with a membrane depolarization, lead the firing of distant cortical cells during the epileptic discharge. Consistent with their ictogenic properties, neurons from this "focus" exhibit interictal and preictal oscillations that are converted into epileptic pattern. These results confirm and extend the "focal hypothesis" of absence epilepsy and provide a cellular scenario for the initiation and generalization of absence seizures.

The control of seizures by the basal ganglia? A review of experimental data.

Over the past few years, the role of the basal ganglia in epilepsy has been widely debated, the debate being mainly based on experimental data obtained from several animal models. In the present review, the possibility that basal ganglia circuits can generate some forms of seizure or participate to their initiation will first be addressed. In the second part of this chapter, recent data suggesting the involvement of the basal ganglia in the control of seizures will be discussed. Although it is clear that basal ganglia circuits cannot generate seizures and are unlikely to be involved in their initiation, numerous experimental data have revealed that seizures modify the activity of this system. More recently, the collection of pharmacological and electrophysiological data in animal models of epilepsy has led to the emergence of the basal ganglia as a possible control circuit for the seizures. These experimental data have already led to initial clinical trials in epileptic patients. The preliminary clinical data encourage the further development of experimental research in chronic models of epilepsy to better determine the exact output circuits involved in seizure interruption, the mechanisms participating in seizure control and whether the same circuits are involved in the control of different types of seizures. These studies may allow the identification of crucial structures and the types of epilepsy likely to benefit from this new therapeutic approach.

Neural adaptation to responsive stimulation: a comparison of auditory and deep brain stimulation in a rat model of absence epilepsy.

BACKGROUND: Responsive deep brain stimulation (rDBS) has been recently proposed to block epileptic seizures at onset. Yet, long-term stability of brain responses to such kind of stimulation is not known. OBJECTIVE: To quantify the neural adaptation to repeated rDBS as measured by the changes of anti-epileptic efficacy of bilateral DBS of the substantia nigra pars reticulata (SNr) versus auditory stimulation, in a rat model of spontaneous recurrent absence seizures (GAERS). METHODS: Local field potentials (LFP) were recorded in freely moving animals during 1 h up to 24 h under automated responsive stimulations (SNr-DBS and auditory). Comparison of seizure features was used to characterise transient (repetition-suppression effect) and long-lasting (stability of anti-epileptic efficacy, i.e. ratio of successfully interrupted seizures) effects of responsive stimulations. RESULTS: SNr-DBS was more efficient than auditory stimulation in blocking seizures (97% vs. 52% of seizures interrupted, respectively). Sensitivity to minimal interstimulus interval was much stronger for SNr-DBS than for auditory stimulation. Anti-epileptic efficacy of SNr-DBS was remarkably stable during long-term (24 h) recordings. CONCLUSIONS: In the GAERS model, we demonstrated the superiority of SNr-DBS to suppress seizures, as compared to auditory stimulation. Importantly, we found no long-term habituation to rDBS. However, when seizure recurrence was frequent, rDBS lack anti-epileptic efficacy because responsive stimulations became too close (time interval < 40 s) suggesting the existence of a refractory period. This study thus motivates the use of automated rDBS in patients having transient seizures separated by sufficiently long intervals.

Is ictal dystonia associated with an inhibitory effect on seizure propagation in focal epilepsies?

PURPOSE: In focal epilepsy, ictal version and ictal dystonia are thought to reflect seizure spread into the frontal eye field and the basal ganglia, respectively. Here we investigated whether the occurrence of dystonia during seizure evolution reflects mechanisms preventing secondary generalization. To this aim, the evolution of seizures in patients with focal epilepsies was compared as to whether concomitant (1) dystonia, (2) dystonia and version, or (3) version occurred. METHODS: Seizure evolutions of 79 patients characterized by either dystonia (n=29; 232 seizures), dystonia and head version in the same seizure evolution (n=9; 83 seizures) or head version (n=41; 330 seizures), were included in the study. RESULTS: The rate of secondary generalization was significant lower in seizures with ictal dystonia (8%, 6 of 72 seizures) compared to seizures with ictal dystonia and version (62%, 13 of 21 seizures, p<0.0001) or compared to seizures with version (95%, 82 of 86 seizures, p<0.0001). CONCLUSION: This study shows that seizures with unilateral ictal dystonia are less likely to generalize as compared to seizures associated with version. This effect is likely to reflect the involvement of inhibitory mechanisms related to the basal ganglia, which exert an inhibiting effect on secondary seizure generalization.

Controlling seizures is not controlling epilepsy: a parametric study of deep brain stimulation for epilepsy.

Pharmacological inhibition and high-frequency stimulation (HFS) of the substantia nigra pars reticulata (SNr) suppress seizures in different animal models of epilepsy. The aim of the present study was to determine the optimal parameters of HFS to control spontaneous seizures in a genetic model of absence epilepsy in the rat. Single SNr stimulation that was bilateral, bipolar and monophasic at 60 Hz frequency and with 60-micros pulse width was optimal. However, when used for repeated stimulations, long-term suppression did not occur and even the number of seizures increased. A delay of at least 60 s between stimulations was necessary to be fully effective. Although single HFS of the SNr can be used to suppress ongoing seizures, repeated HFS is ineffective and could even aggravate seizures in our model. Thus investigations of accurate stimulation procedures are still needed.

Short-term changes in bilateral hippocampal coherence precede epileptiform events.

The mesial temporal lobe epilepsy syndrome (MTLE) is the most common form of focal epilepsies. MTLE patients usually respond very little to pharmacological therapy and surgical resection of temporal brain areas is mandatory. Finding less invasive therapies than resection of the sclerotic hippocampus requires knowledge of the network structures and dynamics involved in seizure generation. Investigation of the time interval immediately preceding seizure onset would help in understanding the initiation mechanisms of the seizure proper and, thereby, possibly improve therapeutical options. Here, we employed the in vivo intrahippocampal kainate model in mice, which is characterized by unilateral histological changes, resembling hippocampal sclerosis observed in human MTLE, and recurrent focal seizures. In these epileptic mice, population spikes occurred during epileptiform events (EEs) in the ipsilateral, histologically changed hippocampus, but also concomitantly in the contralateral, intact hippocampus. We studied synchronization processes between the ipsilateral, sclerotic hippocampus and the contralateral hippocampus immediately preceding the onset of EEs. We show that coherence between the two hippocampi decreased consistently and reliably for all EEs at 8 to 12 s before their onset at high frequencies (>100 Hz), without changes in power in these bands. This early decoupling of the two hippocampi indicates the time range for cellular and network mechanisms leading to increased excitability and/or synchronicity in the tissue and thus ultimately to epileptic seizures.

Single-unit analysis of substantia nigra pars reticulata neurons in freely behaving rats with genetic absence epilepsy.

PURPOSE: The substantia nigra pars reticulata (SNpr) is assumed to be involved in the control of several kinds of epileptic seizures, an assumption based mostly on neuropharmacologic evidence. However, only very few neurophysiological recordings from the basal ganglia support neuropharmacologic data. We investigated the electrophysiologic activity of SNpr neurons in rats with genetic absence epilepsy. METHODS: Electrocorticography (ECoG) and multi-unit recordings using permanently implanted tetrodes were obtained in freely behaving rats. After spike sorting, auto- and cross-correlation analysis was used to detect oscillatory neuronal activities and synchronizations. RESULTS: During interictal periods, neither oscillation nor synchronization could be observed in the firing patterns of SNpr neurons. At the beginning of the absence seizure, the firing rate increased significantly. The SNpr neurons started firing in bursts of action potentials. Bursts were highly correlated to the spike-and-wave discharges (SWDs) in the ECoG, mainly after the spike component of the cortical spike-and-wave complex. Moreover, pairs of SNpr neurons tended to fire synchronously. Before the end of the seizure, the firing rate decreased progressively, and the burst-firing pattern ended at or before the end of the SWDs. Once the SWDs had stopped, the SNpr neurons resumed their basal firing pattern as before the seizure onset. CONCLUSIONS: These results provide electrophysiologic evidence that firing patterns and synchronization of SNpr neurons are in phase with the occurrence of SWDs. The findings support the concept that nigral control mechanisms are involved in modulating the propagation of an ongoing generalized seizure.