Carbenoxolone modifies spontaneous inhibitory and excitatory synaptic transmission in rat somatosensory cortex

Abstract

Gap junction (GJ) coupling between neocortical GABAergic interneurons plays a critical role in the synchronization of activity in cortical networks in physiological and pathophysiological states, e.g., seizures. Past studies have shown that GJ blockers exert anticonvulsant actions in both in vivo and in vitro models of epilepsy. However, the precise mechanisms underlying these antiepileptic effects have not been fully elucidated. This is due, in part, to a lack of information of the influence of GJ blockade on network activity in the absence of convulsant agents or enhanced neuronal excitation. One key question is whether GJ blockers act on excitatory or inhibitory systems, or both. To address this issue, we examined the effects of the GJ blocker carbenoxolone (CarbX, 150 μM) on spontaneous inhibitory postsynaptic currents (sIPSCs) and excitatory postsynaptic currents (sEPSCs) in acute slices of rat somatosensory cortex. Results showed that CarbX decreased the amplitude and frequency of sIPSCs by 30.2% and 25.7%, respectively. CarbX increased the mean frequency of sEPSCs by 24.1%, but had no effect on sEPSC amplitude. During blockade of GABA A -mediated events with picrotoxin (20 μM), CarbX induced only a small increase in sEPSC frequency that was not statistically different from control, indicating CarbX enhancement of sEPECs was secondary to the depression of synaptic inhibition. These findings suggest that in neocortex, blockade of GJs leads to an increase in spontaneous excitation by uncoupling GABAergic interneurons, and that electronic communication between inhibitory cells plays a significant role in regulating tonic synaptic excitation. Keywordscarbenoxolone; gap junction; neocortex; EPSC; IPSC; in vitro Neural communication through gap junctions (GJs) plays an important role in the generation and maintenance of synchronized neuronal network activity in both physiological and pathophysiological conditions, e.g., seizures [26]. Morphological and electrophysiological evidence indicate that electrotonic coupling between GABAergic interneurons exists widely in the cortex [6], and plays a critical role in the synchronization of network activity. Repetitive stimulation of hippocampal circuits induces GABAergic synchronization which is blocked by carbenoxolone (CarbX), a putative GJ uncoupler [9]. In hippocampus [2,23] and neocortex [3,13], 4-aminopyridine (4-AP) induces synchronized GABAergic network activity that is independent of ionotropic glutamate receptor activation, but is suppressed by GJ blockade. * Corresponding author: Tel: 718-270-3804; Fax: 718-270-2241 E-mail: lie.yang@downstate.edu Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which ...

Excitatory effects of gap junction blockers on cerebral cortex seizure‐like activity in rats and mice

“…The results of this study are consistent with recent findings showing that carbenoxolone (a broad-spectrum gap junction blocker (Gajda et al, 2005;Nilsen et al, 2006) causes paradoxical excitatory effects at the level of individual neurons within the cerebral cortex (Yang & Ling, 2007). In this study by Yang and Ling, an increase in excitatory postsynaptic potential amplitude was shown to be indirectly caused by uncoupling of (GABAergic) inhibitory interneurons.…”

“…Connexin36 gap junctions are restricted primarily to a subclass of interneurons, parvalbumin‐containing basket cells (Deans et al., 2001; Liu & Jones, 2003; Markram et al., 2004; Baude et al., 2007), which synapse onto pyramidal cells in the region of the soma or proximal dendrites (Deans et al., 2001; Liu & Jones, 2003; Markram et al., 2004). Blocking direct electrical communication within this network of cells by closing gap junctions is likely to have a disinhibitory effect on pyramidal cell activity, hence the excitatory effects observed at both the cellular level (Yang & Ling, 2007) and at the population level in the present study.…”

“…Less is known about the relationship between seizures and pharmacologic manipulation of gap junctions in the isolated cerebral cortex. One recent study has shown that carbenoxolone, a broad‐spectrum gap junction blocker, causes paradoxical excitatory effects at the level of individual neurons within the cerebral cortex (Yang & Ling, 2007). An excitatory effect of carbenoxolone has also been observed at the cellular level in the hippocampus (Jahromi et al., 2002).…”

Spontaneous activity in the developing gerbil auditory cortex in vivo involves GABAergic transmission

“…In thalamocortical slices, the connexin-36 specific gap junction blocker MFQ disrupted SOs by decreasing neuronal discharge in the gerbil ACx when acutely applied in slices (Kotak et al, 2007). Moreover, gap junction populations not only transmit information across various neuronal and glial populations during development, but also interconnect inhibitory GABAergic interneurons (Yang and Ling, 2007; Tovar et al, 2009). Therefore, we were curious to know whether GABAergic activity can be manipulated in vivo and in separate brain slice experiments, whether gap junction manipulation can affect inhibitory synapse function when SOs are prominent.…”

Bridging the Gap – Understanding the Role of Gap Junctions in Seizures

“…The prediction is that connexin36 gap junction blockade will tend to have a pro-seizure effect and there is accumulating experimental evidence to support this hypothesis. Yang and Ling (2007) have shown an increase in excitatory post synaptic potential amplitude following uncoupling of (GABAergic) inhibitory interneurons with carbenoxolone. Carbenoxolone is a broad spectrum gap junction blocker (Gajda et al 2005;Nilsen et al 2006) and would have likely blocked all gap junctions in this study.…”