Astrocytic GABA Accumulation in Experimental Temporal Lobe Epilepsy

Abstract

An imbalance of excitation and inhibition has been associated with the pathophysiology of epilepsy. Loss of GABAergic interneurons and/or synaptic inhibition has been shown in various epilepsy models and in human epilepsy. Despite this loss, several studies reported preserved or increased tonic GABAA receptor-mediated currents in epilepsy, raising the question of the source of the inhibitory transmitter. We used the unilateral intracortical kainate mouse model of temporal lobe epilepsy (TLE) with hippocampal sclerosis (HS) to answer this question. In our model we observed profound loss of interneurons in the sclerotic hippocampal CA1 region and dentate gyrus already 5 days after epilepsy induction. Consistent with the literature, the absence of interneurons caused no reduction of tonic inhibition of CA1 pyramidal neurons. In dentate granule cells the inhibitory currents were even increased in epileptic tissue. Intriguingly, immunostaining of brain sections from epileptic mice with antibodies against GABA revealed strong and progressive accumulation of the neurotransmitter in reactive astrocytes. Pharmacological inhibition of the astrocytic GABA transporter GAT3 did not affect tonic inhibition in the sclerotic hippocampus, suggesting that this transporter is not responsible for astrocytic GABA accumulation or release. Immunostaining further indicated that both decarboxylation of glutamate and putrescine degradation accounted for the increased GABA levels in reactive astrocytes. Together, our data provide evidence that the preserved tonic inhibitory currents in the epileptic brain are mediated by GABA overproduction and release from astrocytes. A deeper understanding of the underlying mechanisms may lead to new strategies for antiepileptic drug therapy.

Astrocytic Ca2+ Signaling in Epilepsy

“…Bestrophin-1 channels are Ca 2+ activated anion channels, and increased GABA release could hence be a downstream effect of increased Ca 2+ signaling in reactive astrocytes (Lee et al, 2010). In support of this conjecture is the finding of an accumulation of GABA in reactive astrocytes in a model of mTLE (Müller et al, 2020). Potentially, this is a protective aspect of reactive astrocytes to curb epileptiform activity in this pathological tissue.…”

Astrocyte Role in Temporal Lobe Epilepsy and Development of Mossy Fiber Sprouting

“…The major cause of epilepsy has been suggested to be a shift in the neural network of the brain toward excitation, straying away from the normally balanced excitatory-inhibitory state ( Sloviter, 1994 ; Maglóczky and Freund, 2005 ; Fritschy, 2008 ; Tóth et al, 2010 ; Huusko et al, 2015 ). This hypothesis however has previously struggled to explain the intermittent nature of epileptic seizures, that is, those experiencing epilepsy are not in a constant state of excitatory seizing ( Muller et al, 2020 ). A possible explanation for the relatively rare seizure event is a compensatory mechanism that is capable of restoring the balance of excitation–inhibition within the brain for at least a portion of time ( Walker and Kullmann, 2012 ; Pavlov and Walker, 2013 ; Staley, 2015 ).…”

“…Further, astrocyte-specific overexpression of Best1 within a Best1 KO mouse restores tonic inhibition and seizure susceptibility, strengthening the idea that astrocytic Best1 contributes to tonic inhibition by GABA release which suppresses seizure susceptibility ( Pandit et al, 2020 ). Muller et al showed a loss of hippocampal interneurons, thought to be responsible for a large portion of tonic inhibition, in a rodent model of TLE, yet interestingly found tonic inhibition levels to be the same between kainate-injected mice and control mice within the CA1 sector of the hippocampus ( Glykys and Mody, 2007 ; Muller et al, 2020 ). However, within dentate granule cells, tonic currents were found to be higher in kainate-injected mice than controls, indicating preserved as well as increased GABA levels in the epileptic hippocampus despite GABAergic interneuron loss ( Muller et al, 2020 ).…”

“…Muller et al showed a loss of hippocampal interneurons, thought to be responsible for a large portion of tonic inhibition, in a rodent model of TLE, yet interestingly found tonic inhibition levels to be the same between kainate-injected mice and control mice within the CA1 sector of the hippocampus ( Glykys and Mody, 2007 ; Muller et al, 2020 ). However, within dentate granule cells, tonic currents were found to be higher in kainate-injected mice than controls, indicating preserved as well as increased GABA levels in the epileptic hippocampus despite GABAergic interneuron loss ( Muller et al, 2020 ). In this study, astrocytes were found to display pronounced GABA accumulation and, similar to Pandit et al’s (2020) finding, suggested GABA accumulation through synthesis rather than extracellular uptake ( Muller et al, 2020 ).…”

Two-Photon Imaging to Unravel the Pathomechanisms Associated with Epileptic Seizures: A Review

“…This led to the understanding that the role of astrocytes goes far beyond its previously delineated supportive role in maintaining local structures and metabolism. This finding has revolutionized our understanding of epilepsy and has resulted in the concept that "astrocytic transformation" plays a major role in its pathophysiology [80].…”