Enhancement of GABA _A -current run-down in the hippocampus occurs at the first spontaneous seizure in a model of temporal lobe epilepsy

Abstract

Refractory temporal lobe epilepsy (TLE) is associated with a dysfunction of inhibitory signaling mediated by GABA A receptors. In particular, the use-dependent decrease (run-down) of the currents (I GABA ) evoked by the repetitive activation of GABA A receptors is markedly enhanced in hippocampal and cortical neurons of TLE patients. Understanding the role of I GABA run-down in the disease, and its mechanisms, may allow development of medical alternatives to surgical resection, but such mechanistic insights are difficult to pursue in surgical human tissue. Therefore, we have used an animal model (pilocarpine-treated rats) to identify when and where the increase in I GABA run-down occurs in the natural history of epilepsy. We found: (i) that the increased run-down occurs in the hippocampus at the time of the first spontaneous seizure (i.e., when the diagnosis of epilepsy is made), and then extends to the neocortex and remains constant in the course of the disease; (ii) that the phenomenon is strictly correlated with the occurrence of spontaneous seizures, because it is not observed in animals that do not become epileptic. Furthermore, initial exploration of the molecular mechanism disclosed a relative increase in α4-, relative to α1-containing GABA A receptors, occurring at the same time when the increased run-down appears, suggesting that alterations in the molecular composition of the GABA receptors may be responsible for the occurrence of the increased run-down. These observations disclose research opportunities in the field of epileptogenesis that may lead to a better understanding of the mechanism whereby a previously normal tissue becomes epileptic.GABA A receptor | pilocarpine rat | Xenopus oocytes

Seizure-related regulation of GABAA receptors in spontaneously epileptic rats

“…Supporting evidence for this idea comes from studies demonstrating that repetitive pharmacological activation of GABA A Rs isolated from epileptic tissue produces a characteristic ‘rundown’ of GABA A R-mediated currents independently of changes in receptor affinity or membrane potential (Palma et al, 2007). Coincidentally, a switch in receptor assembly altering the proportion of α 4 /α 1 subunits incorporated into GABA A Rs can be detected at the same time that the ‘rundown’ effect is observed (Mazzuferi et al, 2010). The mechanism(s) contributing to a decrease in the stability of receptors at the plasma membrane of epileptic animals is unknown but alterations in the clustering of gephyrin, the main scaffolding protein required for proper anchoring of receptors to synaptic sites, might contribute to the differential presence of GABA A R at the cell surface.…”

“…Receptors present in granule cells from dentate gyrus (DG) of chronically epileptic animals show increased GABA A R-mediated current density, zinc blockade and clonazepam augmentation (Gibbs et al, 1997; Leroy et al, 2004). GABA A Rs isolated from epileptic tissue and ectopically expressed in Xenopus oocytes show a characteristic ‘rundown’ of GABA A R-dependent currents that is independent of changes in membrane potential or receptor affinity (Palma et al, 2007; Mazzuferi et al, 2010). This ‘rundown’ effect can be detected immediately after the first spontaneous seizure and appears to be linked to alterations in GABA A R assembly (Palma et al, 2007; Mazzuferi et al, 2010).…”

“…GABA A Rs isolated from epileptic tissue and ectopically expressed in Xenopus oocytes show a characteristic ‘rundown’ of GABA A R-dependent currents that is independent of changes in membrane potential or receptor affinity (Palma et al, 2007; Mazzuferi et al, 2010). This ‘rundown’ effect can be detected immediately after the first spontaneous seizure and appears to be linked to alterations in GABA A R assembly (Palma et al, 2007; Mazzuferi et al, 2010). Altered subunit expression directly impacts channel assembly and changes the cellular distribution of GABA A R promoting the impairment of inhibitory neurotransmission (Brooks-Kayal et al, 1998; Peng et al, 2004; Lund et al, 2008).…”

GABAA currents are decreased by IL-1β in epileptogenic tissue of patients with temporal lobe epilepsy: implications for ictogenesis

“…Glass electrodes (3-4 MΩ) were filled with 140 mM CsMethanesulfonate, 10 mM HEPES, 0.5 mM EGTA, and 2 mM Mg-ATP, 0.3 mM Na 3 -GTP, 2 mM MgCl 2 . I GABA currents were recorded at 0 mV to avoid spurious contributions of mechanosensitive Na + currents (Mazzuferi et al, 2010;Ragozzino et al, 2005), with glutamatergic currents at their reversal potential. Under these experimental conditions, with inactivated voltage-gated channels, cells were stable and healthy for at least 1 h. Spontaneous inhibitory postsynaptic currents were present but did not affect the quantification of GABA-induced currents.…”

Alterations in hippocampal myelin and oligodendrocyte precursor cells during epileptogenesis

“…The rats in the control group received an equivalent volume of 9% saline. To evaluate the dynamic changes in myelin and OPCs, the rats were sacrificed at various time points after SC reflecting the different phases of the natural trajectory of epilepsy: the acute phase of epileptogenesis was defined as the first 24 h after acute SC; the latent phase consisted of the following five days; the chronic phase was defined as the two weeks after the first spontaneous recurrent seizure (SRS); and the late chronic phase was defined as the two months after the first SRS (Mazzuferi et al, 2010). According to previous descriptions, the threshold discharge for the first spontaneous electrographic seizure was set at a frequency of 45 Hz, an amplitude of 42 baseline and a duration of 410 s (Goffin et al, 2007).…”