A potassium current controls burst termination in rat neocortical neurons after GABA withdrawal

GABA withdrawal syndrome: GABAA receptor, synapse, neurobiological implications and analogies with other abstinences

The sudden interruption of the increase of the concentration of the gamma-aminobutyric acid (GABA), determines an increase in neuronal activity. GABA withdrawal (GW) is a heuristic analogy, with withdrawal symptoms developed by other GABA receptor-agonists such as alcohol, benzodiazepines, and neurosteroids. GW comprises a model of neuronal excitability validated by electroencephalogram (EEG) in which high-frequency and high-amplitude spike-wave complexes appear. In brain slices, GW was identified by increased firing synchronization of pyramidal neurons and by changes in the active properties of the neuronal membrane. GW induces pre- and postsynaptic changes: a decrease in GABA synthesis/release, and the decrease in the expression and composition of GABAA receptors associated with increased calcium entry into the cell. GW is an excellent bioassay for studying partial epilepsy, epilepsy refractory to drug treatment, and a model to reverse or prevent the generation of abstinences from different drugs.

Exaggeration of epileptic-like patterns by nicotine receptor activation during the GABA withdrawal syndrome

To understand how nicotinic cholinergic receptors may participate in epileptic seizures, we tested the effects of nicotine and of the competitive nicotinic antagonists dihydro-beta-erythroidine and alpha-bungarotoxin on synaptic paroxysmal depolarization shifts (PDSs) and intrinsic bursts of action potentials recorded in slices from rats presenting a cortical status epilepticus. This model named GABA-withdrawal syndrome (GWS) appears consecutive to the interruption of a prolonged intracortical GABA infusion. Effects of both nicotinic antagonists suggest a distinct involvement of alpha4-beta2 and alpha7 subunits in shaping individual PDSs and patterning repetitive bursts. On one hand, in GWS rats, an increase of PDS latency and prolongation of PDS and bursts were induced by nicotine and reduced by dihydro-beta-erythroidine, but not by alpha-bungarotoxin. The K+ blocker tetraethylammonium also increased duration without changing latency. Thus, dihydro-beta-erythroidine-sensitive receptors exert distinct controls on the presynaptic generation of PDS and on the process which terminates PDSs and bursts. On the other hand, alpha-bungarotoxin depolarized neurons and generated rhythmic discharges of clustered bursts. Clustered bursts were also observed in slices obtained from GWS rats treated with the acetylcholinesterase inhibitor eserine. We suggest that both dihydro-beta-erythroidine and alpha-bungarotoxin-sensitive sites control paroxysmic activities in GWS and could be involved in some human and animal epilepsies presenting mutations of nicotinic cholinergic receptors.

Reduction of intracellular calcium removal rate can explain changes in seizure dynamics: studies in neuronal network models

Complex partial seizures originating from mesial temporal structures are characterized by relatively short durations of organized rhythmic activity (ORA) of 5-8 Hz, typically lasting less than 60s. Previous investigations into seizure dynamics have revealed that this ORA undergoes a monotonic decline prior to seizure evolution into intermittent bursting and subsequent seizure termination. Large neural network models of simplified single-compartment neurons were employed to address the hypothesis that changes in the free intracellular calcium ([Ca2+]i) removal rate during network bursting can result in the alterations of rhythmic seizure activity similar to that observed in recordings from humans. Both exponential and linear models of decreasing calcium removal rates resulted in changes in the predominant frequency of network bursting very similar in frequency and time course to those seen in human intracranial recordings. This supports the concept that changes in [Ca2+]i removal can explain this important network behavior, while not excluding alternative hypotheses. Identifying potential mechanisms underlying the dynamic changes seen in epileptogenic activity in large neural networks can provide important insights into seizure evolution and termination. Model neural network ensembles are attractive systems to address these questions that are difficult to investigate in biological preparations.

Ethanol, GABA and epilepsy

Ethanol exerts its behavioral effects largely by interacting with receptors to brain neurotransmitters. The molecular mechanisms involving these interactions are still not well known since an ideal model for their study is currently unavailable. In addition, responses to alcohol may vary due to factors such as genetic predisposition, ethanol concentration consumed, and stimuli such as stress, socialization, etc. The chronic consumption of alcohol, similar to that of other drugs such as benzodiazepines and barbiturates, is linked to GABAergic neurotransmission. GABA is the predominant inhibitory neurotransmitter in the brain. In a context of substance abuse, these three drugs first cause a gratifying effect, later tolerance and finally, physical and psychological dependence. If consumption is interrupted abruptly, a withdrawal syndrome occurs. The Alcohol Withdrawal Syndrome (AWS) is a state of hyperexcitability characterized by anxiety, fear, muscular rigidity and tonic-clonic seizures with epileptiform-type characteristics. The epileptic seizures seen during AWS are often similar to those seen in experimental epilepsy models such as "kindling" or GABA Withdrawal Syndrome (GWS) models. A possible correlation between these models and AWS will allow for a better understanding of the cellular and molecular effects that alcohol exerts on the brain.

Voltage imaging of epileptiform activity in slices from rat piriform cortex: onset and propagation

The piriform cortex is a temporal lobe structure with a very high seizure susceptibility. To investigate the spatiotemporal characteristics of epileptiform activity, slices of piriform cortex were examined by imaging electrical activity with a voltage-sensitive fluorescent dye. Discharge activity was studied for different sites of stimulation and different planes of slicing along the anterior-posterior axis. Epileptiform behavior was elicited either by disinhibition with a gamma-aminobutyric acid-A receptor antagonist or by induction with a transient period of spontaneous bursting in low-chloride medium. Control activity recorded with fluorescent dye had the same pharmacological and temporal characteristics as control activity reported previously with microelectrodes. Simultaneous optical and extracellular microelectrode recordings of epileptiform discharges showed the same duration, latency, and all-or-none character as described previously with microelectrodes. Under all conditions examined, threshold electrical stimulation applied throughout the piriform cortex evoked all-or-none epileptiform discharges originating in a site that included the endopiriform nucleus, a previously identified site of discharge onset. In induced slices, but not disinhibited slices, the site of onset also included layer VI of the adjoining agranular insular cortex and perirhinal cortex, in slices from anterior and posterior piriform cortex, respectively. These locations had not been identified previously as sites of discharge onset. Thus like the endopiriform nucleus, the deep agranular insular cortex and perirhinal cortex have a very low seizure threshold. Additional subtle differences were noted between the induced and disinhibited models of epileptogenesis. Velocity was determined for discharges after onset, as they propagated outward to the overlying piriform cortex. Propagation in other directions was examined as well. In most cases, velocities were below that for action potential conduction, suggesting that recurrent excitation and/or ephaptic interactions play a role in discharge propagation. Future investigations of the cellular and organizational properties of regions identified in this study should help clarify the neurobiological basis of high seizure susceptibility.