Epileptic seizures typically result in delayed neuronal loss secondary to the initial damage and an up-regulation in connexin43 (Cx43). This study investigated the role of Cx43 gap junctions in lesion spread and cell loss following epileptiform activity. Epileptiform injury in hippocampal slice cultures was induced by 48 h exposure to 100 μM bicuculline methochloride (BMC). During the 24h recovery period following BMC treatment, lesion spread was observed in the CA1. A Cx43 mimetic peptide, applied during either the BMC treatment or recovery periods, produced concentration- and exposure time-dependent neuroprotection, as measured by propidium iodide uptake at the end of the recovery period. During the BMC period, peptide concentrations between 5 and 50 μM (sufficient to block hemichannels) had a protective effect while a substantial gap junction blockade with 500 μM peptide exacerbated the lesion. By contrast, all doses applied during the recovery period protected the CA1 region from further damage. The results indicate that while the slices are undergoing excessive neuronal firing and epileptic stress, gap junction communication appears to be essential for tissue survival but hemichannel opening may be damaging. Following epileptiform insult, however, gap junction communication plays a crucial role in the spread of neuronal damage. The findings from this study identify gap junction communication as a potential therapeutic target for epilepsy.
"In this case, higher concentrations of mimetic peptides are presumed to be required in order to block both GJ channels as well as hemichannels (Ebihara, 2003; Ebihara et al., 2003). In a slice culture model wherein added bicuculline caused epileptiform activity, low concentrations of a Cx43 mimetic peptide (amino acid sequence VDCFLSRPTEKT) targeting the extracellular loop two of Cx43, mainly blocked Cx43 hemichannels and prevented seizure-induced neuronal death (Yoon et al., 2010). Higher doses of the peptide, which inhibited GJs in addition to the hemichannels, increased the severity of the seizure-induced lesion. "
[Show abstract][Hide abstract] ABSTRACT: Enhanced gap junctional communication (GJC) between neurons is considered a major factor underlying the neuronal synchrony driving seizure activity. In addition, the hippocampal sharp wave ripple complexes, associated with learning and seizures, are diminished by GJC blocking agents. Although gap junctional blocking drugs inhibit experimental seizures, they all have other nonspecific actions. Besides interneuronal GJC between dendrites, inter-axonal and inter-glial GJC is also considered important for seizure generation. Interestingly, in most studies of cerebral tissue from animal seizure models and from human patients with epilepsy, there is up-regulation of glial, but not neuronal gap junctional mRNA and protein. Significant changes in the expression and post-translational modification of the astrocytic connexin Cx43, and Panx1 were observed in an in vitro Co++ seizure model, further supporting a role for glia in seizure-genesis, although the reasons for this remain unclear. Further suggesting an involvement of astrocytic GJC in epilepsy, is the fact that the expression of astrocytic Cx mRNAs (Cxs 30 and 43) is several fold higher than that of neuronal Cx mRNAs (Cxs 36 and 45), and the number of glial cells outnumber neuronal cells in mammalian hippocampal and cortical tissue. Pannexin expression is also increased in both animal and human epileptic tissues. Specific Cx43 mimetic peptides, Gap 27 and SLS, inhibit the docking of astrocytic connexin Cx43 proteins from forming intercellular gap junctions, diminishing spontaneous seizures. Besides GJs, Cx membrane hemichannels in glia and Panx membrane channels in neurons and glia are also inhibited by gap junctional pharmacological blockers. Although there is no doubt that connexin-based gap junctions and hemichannels, and pannexin-based membrane channels are related to epilepsy, the specific details of how they are involved and how we can modulate their function for therapeutic purposes remain to be elucidated.
Frontiers in Physiology 05/2014; 5:172. DOI:10.3389/fphys.2014.00172 · 3.53 Impact Factor
"This effect was seen at concentrations that inhibit hemichannel opening but not gap junctional coupling (O'Carroll et al., 2008). We have previously demonstrated that Peptide5 is neuroprotective in a bicuculline methochloride (BMC) induced hippocampal slice culture model of epilepsy (Yoon et al., 2010). Similar to the results seen in the ex vivo SCI study, concentrations that inhibit hemichannels were protective during the BMC insult, while concentrations that would prevent gap junctional communication exacerbated the cell death. "
[Show abstract][Hide abstract] ABSTRACT: Connexin43 (Cx43) is a gap junction protein up-regulated after spinal cord injury and is involved in the on-going spread of secondary tissue damage. To test whether a connexin43 mimetic peptide (Peptide5) reduces inflammation and tissue damage and improves function in an in vivo model of spinal cord injury, rats were subjected to a 10g, 12.5mm weight drop injury at the vertebral level T10 using a MASCIS impactor. Vehicle or connexin43 mimetic peptide was delivered directly to the lesion via intrathecal catheter and osmotic mini-pump for up to 24h after injury. Treatment with Peptide5 led to significant improvements in hindlimb function as assessed using the Basso-Beattie-Bresnahan scale. Peptide5 caused a reduction in Cx43 protein, increased Cx43 phosphorylation and decreased levels of TNF-α and IL-1β as assessed by Western blotting. Immunohistochemistry of tissue sections 5 weeks after injury showed reductions in astrocytosis and activated microglia as well as an increase in motor neuron survival. These results show that administration of a connexin mimetic peptide reduces secondary tissue damage after spinal cord injury by reducing gliosis and cytokine release and indicate the clinical potential for mimetic peptides in the treatment of spinal cord patients.
Neuroscience Research 02/2013; 75(3). DOI:10.1016/j.neures.2013.01.004 · 1.94 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Astrocytes express a plethora of ion channels, neurotransmitter receptors and transporters and thus are endowed with the machinery to sense and respond to neuronal activity. Recent studies have implicated astrocytes in important physiological roles in the CNS, such as synchronization of neuronal firing, ion homeostasis, neurotransmitter uptake, glucose metabolism and regulation of the vascular tone. Astrocytes are abundantly coupled through gap junctions allowing them to redistribute elevated K(+) concentration from sites of excessive neuronal activity. Growing evidence now suggests that dysfunctional astrocytes are crucial players in epilepsy. Investigation of specimens from patients with pharmacoresistant temporal lobe epilepsy and epilepsy models revealed alterations in expression, localization and function of astroglial K(+) and water channels, entailing impaired K(+) buffering. Moreover, malfunction of glutamate transporters and the astrocytic glutamate-converting enzyme, glutamine synthetase, as observed in epileptic tissue suggested that astrocyte dysfunction is causative of hyperexcitation, seizure spread and neurotoxicity. Accordingly, dysfunctional astrocytes should be considered as promising targets for new therapeutic strategies. In this review, we will summarize current knowledge of astrocyte dysfunction in temporal lobe epilepsy and discuss putative mechanisms underlying these alterations.
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