Epilepsy is a diverse set of chronic neurological disorders characterized by seizures.
Epilepsy affects individuals of all ages, races, social classes, geographic regions
and nationalities (1-3). It is among the most common serious neurological conditions.
In developed countries, epilepsy has a prevalence of approximately 1% (4,5). Each
year, 24 per 100,000 people suffer from epilepsy in Europe and 53 per 100,000 in North
America (5-7). In developing countries, the incidence is even higher, with a rate
of up to 190 per 100,000 individuals (8,9). Furthermore, epilepsy can be considered
a malignant condition because sudden death in individuals with epilepsy is estimated
to be at least 20 times higher than in the general population (10,11).
The capacity to replicate human epilepsy in animal models is an important tool for
experimental study. Animal studies have contributed significantly to the understanding
of the biological basis of epileptogenesis and have provided evidence for the possible
mechanisms of action of antiepileptic drugs. However, the relevance of animal models
in human epilepsy research depends on how closely the model mimics the human condition
(12). These models have provided important information on the brain and the behavioral
mechanisms that could be involved in the etiology, pathophysiology and electrophysiological
events and their correlations with synaptic interactions. However, the belief that
the etiology of epilepsy can be traced to synaptic connections does not take into
account the fact that the strength of synaptic interactions may change based on the
intra- and extracellular ionic equilibrium.
The mechanisms involved in the intra- and extracellular regulation of ionic levels
are usually ignored; however, it has been shown that neuronal and glial activities
are intrinsically modulated by the ionic gradients through their cellular membranes.
These gradients depend on the complex interaction of mechanisms related to ionic homeostatic
regulation, such as the Na/K ATPase, cotransporters and exchanger enzymes. Furthermore,
paroxysmal discharges are accompanied by significant changes in the intra- and extracellular
ionic concentrations, which challenge the homeostatic equilibrium regulated by these
mechanisms. Focally induced cortical seizures are preceded by small reductions in
[Ca++]o that become intense during paroxysms (13,14). Posterior investigations (15,16)
have clearly demonstrated that hippocampal slices exposed to low [Ca++]o are able
to sustain non-synaptic epileptiform activity. Genetically epileptic baboons exhibited
such significant drops in their [Ca++]o levels that all synaptic transmissions must
have been blocked. However, the researchers did not observe any transmission disruptions
in the course of the seizures (17). Overall, these data disprove the widely held belief
that epileptic seizures are exclusively generated by the imbalance between excitation
and inhibition.
Simultaneous findings showed that changes in the chloride transmembrane gradient might
also occur and are able to modulate the activation of the gamma-aminobutyric acid
A (GABAA) receptors. These findings suggest that hyperpolarization or depolarization
may occur in a manner dependent on intracellular chloride accumulation (18-21). The
cation-chloride cotransporters and Cl-/HCO-
3 exchanger were identified as the main regulators of the intracellular chloride concentration
(22). In the mature brain, the low [Cl-]I level is associated with a Cl- Nernst potential
that is more negative than the transmembrane potential; this results in Cl- influx
and a hyperpolarizing effect when GABAA receptors are activated. In contrast, in the
immature brain, the high intracellular Cl- and positive Cl- Nernst potential relative
to the transmembrane potential cause a Cl- efflux and a depolarizing inward current.
Pathophysiological conditions, such as neuronal injures and the inflammatory state,
may also resemble the immature brain because of a decrease in the potassium chloride
transporter KCC2 (23-26).
Based on this information, it is not difficult to surmise that changes in the extracellular
concentration may also be accompanied by changes in the equilibrium of non-synaptic
mechanisms. The extracellular K+ accumulation, which is always associated with intense
neuronal firing, induces Cl- intrusion through the cotransporters and, in turn, reinforces
the increased excitation.
Because the synaptic circuit is part of a system in which non-synaptic mechanisms
control ionic homeostasis, it is difficult to ignore the effect non-synaptic mechanisms
have on seizure sustainment and progression. Therefore, our group has sought to investigate
the effects that changes in non-synaptic mechanisms have on different experimental
models of epilepsy. Due to the complexity, the first step of our investigation was
to develop a computational model to understand the dynamics of the main mechanisms
(27). The computational model has been extensively used in our group as an indispensable
tool to guide our analysis of the electrophysiological data. Simulations representing
the histological changes observed in the hippocampal slices are processed to understand
how the changes in ionic homeostasis may change the induced epileptiform activity
(28). Our preliminary results show that despite the cellular death associated with
the experimental models, the non-synaptic mechanisms are able to compensate for the
loss and enhance the epileptiform activity sustained by the neuronal tissue. These
promising first results open up new possibilities for understanding seizure disruption.
It is also becoming clear that the mechanisms and conditions that disrupt and sustain
seizures are highly complex. The evidence that non-synaptic mechanisms are able to
modulate the function of the synaptic circuit indicates that the problem is even more
complex than we suspected.
The simulations show that the typically intense ionic changes of the sites to which
the paroxysmal neuronal population is recruited are able to reduce the corresponding
transmembrane gradients of the ions to such a level that synaptic function is depressed.
Because the main anti-epileptic drugs target synaptic functioning, no effect would
be expected when the synapses are depressed. Therefore, these drugs would not act
during the ictal period, nor would they act in epilepsies where the triggering condition
is characterized by changes in ionic homeostasis, such as the intracellular Cl- accumulation
typical of the immature brain, brain injury and brain inflammation Figure 1.
Finally, we believe that this is the first step in a long scientific journey that
will trigger new research and debates. Thus, it is crucial to promote scientific collaboration
to investigate non-synaptic mechanisms of epilepsies and to discover promising drugs
that act non-synaptically. This new and exciting possibility for epilepsy research
makes us reflect on this quote by Galileo Galilei: The Bible shows the way to go to
heaven, not the way the heavens go.
ACKNOWLEGDMENTS
This work was supported by the following Brazilian agencies: Fundação de Amparo à
Pesquisa do Estado de Minas Gerais (FAPEMIG), Fundação de Amparo à Pesquisa de São
Paulo (FAPESP), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq),
Programa Nacional de Cooperação Acadêmica (Procad)/Coordenação de Aperfeiçoamento
de Pessoal de Nível Superior (CAPES) and Institutos Nacionais de Ciência e Tecnologia
(INCT) of Translational Neuroscience (Ministério Da Ciência e Tecnologia (MCT)/(Conselho
Nacional de Desenvolvimento Científico e Tecnológico CNPq/Fundação de Amparo à Pesquisa
de São Paulo (FAPESP).