Pharmacological plasticity of GABA_A receptors at dentate gyrus synapses in a rat model of temporal lobe epilepsy

Abstract

In the lithium-pilocarpine model (Li-pilocarpine) of temporal lobe epilepsy, GABA A receptormediated inhibitory postsynaptic currents (GABA A IPSCs) were recorded in dentate gyrus granule cells (GCs) from adult rat hippocampal slices. The properties of GABA A IPSCs were compared before and after superfusion of modulators in control conditions (Li-saline rats) and in Li-pilocarpine rats 24-48 h and 3-5 months (epileptic rats) after status epilepticus (SE). The mean peak amplitude of GABA A IPSCs increased by about 40% over Li-saline values in GCs 24-48 h after SE and remained higher in epileptic rats. In Li-pilocarpine rats, studied at 24-48 h after SE, diazepam (1 µm) lost 65% of its effectiveness at increasing the half-decay time (T 50% ) of GABA A miniature IPSCs (mIPSCs). Diazepam had no effects on mIPSC T 50% in epileptic rats. The benzodiazepine ligand flumazenil (1 µM), acting as an antagonist in Li-saline rats, exhibited a potent inverse agonistic effect on GABA A mIPSCs of GCs from Li-pilocarpine rats 24-48 h and 3-5 months after SE. The neurosteroid allopregnanolone (100 nM), which considerably prolonged GABA A mIPSCs in Li-saline rats, totally lost its effect in rats studied 24-48 h after SE. However, this decrease in effectiveness was transient and was totally restored in epileptic rats. In addition to the up-regulation in the number of receptors at individual GC synapses, we propose that these 'epileptic' GABA A receptors possess benzodiazepine binding sites with altered allosteric properties. The failure of benzodiazepine and neurosteroid to potentiate inhibition early after SE may be a critical factor in the development of epileptogenesis and occurrence of seizures.

Alterations of GABAA-Receptor Function and Allosteric Modulation During Development of Status Epilepticus

“…In the chronic epilepsy model, multiple subunit mRNA and/or protein changes have been observed days or weeks after SE (Brooks-Kayal et al 1998;Peng et al 2004). It is interesting to note that the alterations of phasic current sensitivity to diazepam and zinc during acute seizures in the present study were similarly observed in epileptic rats (Buhl et al 1996;Cohen et al 2003;Leroy et al 2004). As mentioned earlier, the alterations of allosteric modulation during acute seizures may result from short-term changes in GABA A receptors, whereas those that occur once epilepsy is established may result from long-term changes in GABA A receptors.…”

“…Diazepam consistently augmented mIPSC amplitude in the present study, which is in contrast with several previous reports that benzodiazepines prolonged the mIPSC decay but did not enhance amplitudes in normal animals (Cohen et al 2003;Leroy et al 2004;Poncer et al 1996). Diazepam modulation of mIPSC amplitude may be age dependent; potentiation of GABA-evoked currents (Zhang et al 2004) and mIPSC amplitudes (Cohen et al 2000;Defazio and Hablitz 1998;Hajos et al 2000;Nusser et al 1997;Perrais and Ropert 1999) has been observed in juvenile neurons including hippocampal dentate granule cells.…”

“…This finding is consistent with our previous report using isolated, dissociated dentate granule cells that diazepam sensitivity was reduced after prolonged seizures (Kapur and Macdonald 1997), supporting the idea that a reduction in allosteric modulation of GABA A receptors by diazepam may contribute to the rapid diazepam pharmacoresistance observed in behavioral studies (Jones et al 2002;Walton and Treiman 1988). Although the underlying cellular and molecular changes of GABA A receptors in the current study may be different from those that occur during epilepsy, the pharmacoresistance to diazepam was also observed in epileptic rats weeks or months after SE (Cohen et al 2003;Leroy et al 2004).…”

“…It was shown that GABA A -receptor-mediated inhibition was reduced in the epileptic rats (Isokawa 1996;Kobayashi and Buckmaster 2003). Several other studies, interestingly, demonstrated that mIPSC amplitude was enhanced in these animals (Cohen et al 2003;Gavrilovici et al 2006;Leroy et al 2004;Nusser et al 1998;Sun et al 2007). In contrast to the possible mechanisms for the acute changes we observed, alterations of GABAergic phasic inhibition during epilepsy are likely related to long-term changes in GABA A -receptor expression and/or function such as regulation of receptor gene expression and degradation or synthesis of multiple receptor subunit proteins.…”

“…Based on these studies in recombinant receptors, it is expected that benzodiazepines and zinc may exert different effects on GABAergic phasic inhibition. Acute alterations of GABA A -receptor function and/or pharmacology have been observed shortly after SE onset (Goodkin et al 2005;Kapur and Macdonald 1997;Naylor et al 2005) and after development of epilepsy weeks or months after SE (Cohen et al 2003;Leroy et al 2004;Peng et al 2004;Zhang et al 2004). However, the alterations of GABA Areceptor function and allosteric modulation during development of SE are still poorly understood.…”