Excitatory effects of parvalbumin-expressing interneurons maintain hippocampal epileptiform activity via synchronous afterdischarges.
Epileptic seizures are characterized by periods of hypersynchronous, hyperexcitability within brain networks. Most seizures involve two stages: an initial tonic phase, followed by a longer clonic phase that is characterized by rhythmic bouts of synchronized network activity called afterdischarges (ADs). Here we investigate the cellular and network mechanisms underlying hippocampal ADs in an effort to understand how they maintain seizure activity. Using in vitro hippocampal slice models from rats and mice, we performed electrophysiological recordings from CA3 pyramidal neurons to monitor network activity and changes in GABAergic signaling during epileptiform activity. First, we show that the highest synchrony occurs during clonic ADs, consistent with the idea that specific circuit dynamics underlie this phase of the epileptiform activity. We then show that ADs require intact GABAergic synaptic transmission, which becomes excitatory as a result of a transient collapse in the chloride (Cl(-)) reversal potential. The depolarizing effects of GABA are strongest at the soma of pyramidal neurons, which implicates somatic-targeting interneurons in AD activity. To test this, we used optogenetic techniques to selectively control the activity of somatic-targeting parvalbumin-expressing (PV(+)) interneurons. Channelrhodopsin-2-mediated activation of PV(+) interneurons during the clonic phase generated excitatory GABAergic responses in pyramidal neurons, which were sufficient to elicit and entrain synchronous AD activity across the network. Finally, archaerhodopsin-mediated selective silencing of PV(+) interneurons reduced the occurrence of ADs during the clonic phase. Therefore, we propose that activity-dependent Cl(-) accumulation subverts the actions of PV(+) interneurons to perpetuate rather than terminate pathological network hyperexcitability during the clonic phase of seizures.
Department of Pharmacology, University of Oxford, Oxford, OX1 3QT, United Kingdom.
Tommas J
TJ
Ellender
Department of Pharmacology, University of Oxford, Oxford, OX1 3QT, United Kingdom, Medical Research Council Receptor Biology Unit, Institute of Infectious Disease and Molecular Medicine, Faculty of Health Sciences, University of Cape Town, Cape Town, Western Cape, 7701, South Africa, and.
Joseph V
JV
Raimondo
Department of Pharmacology, University of Oxford, Oxford, OX1 3QT, United Kingdom.
Agnese
A
Irkle
Department of Pharmacology, University of Oxford, Oxford, OX1 3QT, United Kingdom, Medical Research Council Anatomical Neuropharmacology Unit, Department of Pharmacology, University of Oxford, Oxford, OX1 3TH, United Kingdom.
Karri P
KP
Lamsa
Department of Pharmacology, University of Oxford, Oxford, OX1 3QT, United Kingdom, colin.akerman@pharm.ox.ac.uk.
Colin J
CJ
Akerman
D000200:Action Potentials / Q000502:physiology; D000818:Animals; D056654:CA3 Region, Hippocampal / Q000503:physiopathology*; D059330:GABAergic Neurons / Q000502:physiology*; D007395:Interneurons / Q000502:physiology*; D051379:Mice; D008959:Models, Neurological; D062308:Optogenetics; D010320:Parvalbumins / Q000378:metabolism*; D017966:Pyramidal Cells / Q000503:physiopathology*; D051381:Rats; D012640:Seizures / Q000503:physiopathology*
D010320:Parvalbumins
GABAA; chloride; epilepsy; hippocampus; optogenetics; parvalbumin