Paroxysmal Discharges in Tissue Slices From Pediatric Epilepsy Surgery Patients: Critical Role of GABAB Receptors in the Generation of Ictal Activity.

Simon Levinson,Conny H Tran,Brett Viker,Michael S Levine,Gary W Mathern,Harry V Vinters,Joshua Barry,Carlos Cepeda

doi:10.3389/fncel.2020.00054

Abstract

In the present study, we characterized the effects of bath application of the proconvulsant drug 4-aminopyridine (4-AP) alone or in combination with GABAA and/or GABAB receptor antagonists, in cortical dysplasia (CD type I and CD type IIa/b), tuberous sclerosis complex (TSC), and non-CD cortical tissue samples from pediatric epilepsy surgery patients. Whole-cell patch clamp recordings in current and voltage clamp modes were obtained from cortical pyramidal neurons (CPNs), interneurons, and balloon/giant cells. In pyramidal neurons, bath application of 4-AP produced an increase in spontaneous synaptic activity as well as rhythmic membrane oscillations. In current clamp mode, these oscillations were generally depolarizing or biphasic and were accompanied by increased membrane conductance. In interneurons, membrane oscillations were consistently depolarizing and accompanied by bursts of action potentials. In a subset of balloon/giant cells from CD type IIb and TSC cases, respectively, 4-AP induced very low-amplitude, slow membrane oscillations that echoed the rhythmic oscillations from pyramidal neurons and interneurons. Bicuculline reduced the amplitude of membrane oscillations induced by 4-AP, indicating that they were mediated principally by GABAA receptors. 4-AP alone or in combination with bicuculline increased cortical excitability but did not induce seizure-like discharges. Ictal activity was observed in pyramidal neurons and interneurons from CD and TSC cases only when phaclofen, a GABAB receptor antagonist, was added to the 4-AP and bicuculline solution. These results emphasize the critical and permissive role of GABAB receptors in the transition to an ictal state in pediatric CD tissue and highlight the importance of these receptors as a potential therapeutic target in pediatric epilepsy.

Highlights

There are two major GABA receptor subtypes in the central nervous system, type A and type B (Olsen and Sieghart, 2009)
We found that a subset of balloon/giant cells from cortical dysplasia (CD) type IIb and tuberous sclerosis complex (TSC) cases can sense K+ surges induced by 4-AP
Studies in human brain tissue have demonstrated the exquisite sensitivity of CD tissue samples to 4-AP, a K+-channel blocker that increases neurotransmitter release by prolonging action potential duration (Buckle and Haas, 1982; Avoli et al, 2003). 4-AP produces neuronal synchrony manifested by rhythmic membrane oscillations caused by synchronous GABA release from interneurons and possibly by increases in gap junctional currents and/or permeability (Traub et al, 1995; Traub et al, 2001; Szente et al, 2002)

Summary

Introduction

There are two major GABA receptor subtypes in the central nervous system, type A (including the A-ρ subfamily) and type B (Olsen and Sieghart, 2009). The ionotropic GABAA receptor mediates fast inhibitory neurotransmission and is preferentially localized in postsynaptic membranes. When the neurotransmitter binds the multimeric GABAA receptor, it allosterically opens a chloride ion channel leading, in most cases, to membrane hyperpolarization (Olsen and Sieghart, 2009). GABAB receptors function via multiple mechanisms including inwardly rectifying K+ channels, voltage-gated Ca2+ channels, and adenylyl cyclase, all of which result in either reduced neurotransmitter release or hyperpolarization of the neuron (Newberry and Nicoll, 1984; Thompson and Gahwiler, 1992; Bowery et al, 2002; Bettler et al, 2004; Frangaj and Fan, 2018). Reduced function of GABAA receptors is traditionally thought to contribute to a breakdown in inhibitory neurotransmission. The role of GABAB receptors in epileptogenesis, especially in humans, is less well understood

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