Category: Tissue, system and organ physiology Epileptiform Activity Triggers Long-Term Plasticity of GABAB Receptor Signalling in the Developing Rat Hippocampus Abbreviated title: Long-Term Plasticity of GABAB Receptor Signalling

GABAB receptor (GABABR)-mediated presynaptic inhibition regulates neurotransmitter release from synaptic terminals. In the neonatal hippocampus, GABABR activation reduces GABA release and terminates spontaneous network discharges called Giant Depolarising Potentials (GDPs). Blocking GABABRs transforms GDPs into longer epileptiform discharges. Thus, GABABR-mediated presynaptic inhibition of GABA release (GABA auto-inhibition) controls both spontaneous network activity and excitability in the developing hippocampus. Here we show that extensive release of endogenous GABA during epileptiform activity impairs GABA auto-inhibition, but not GABABR-mediated inhibition of glutamate release, leading to hyperexcitability of the neonatal hippocampal network. Paired-pulse depression of GABA release (PPD) and heterosynaptic depression of glutamate release were used to monitor the efficacy of presynaptic GABABRs-mediated inhibition in slices. PPD, but not heterosynaptic depression, was dramatically reduced after potassium (K)-induced ictal-like discharges (ILDs), suggesting a selective impairment of GABABR-dependent presynaptic inhibition of GABAergic terminals. Impairing GABA auto-inhibition induced a 44% increase in GDP width and the appearance of pathological network discharges. Preventing GABA-induced activation of GABABRs during ILDs avoided PPD loss and most modifications of the network activity. In contrast, a partial block of GABABRs induced network discharges strikingly similar to those observed after K-driven ILDs. Finally, neither loss of GABA auto-inhibition nor network hyperexcitability could be observed following synchronous release of endogenous GABA in physiological conditions (during GDPs at 1 Hz). Thus, epileptiform activity was instrumental to impair GABABR-dependent presynaptic inhibition of GABAergic terminals. In conclusion, our results indicate that endogenous GABA released during epileptiform activity can reduce GABA auto-inhibition and trigger pathological network discharges in the newborn rat hippocampus. Such functional impairment may play a role in acute post-seizure plasticity. in se rm -0 03 80 22 1, v er si on 1 13 M ay 2 00 9 INTRODUCTION The occurrence of seizures during development strongly increases the probability of developing chronic epilepsy at adult age (Holmes & Ben-Ari, 1998). This suggests that plastic changes occurring during and after seizures underlay subsequent remodelling. Ictal discharges are stereotyped epileptic events involving the synchronous activation of thousands of neurons in the cortex and hippocampus. Long-term changes associated with ictal activity have been thoroughly characterised in the developing brain, and include neuronal hyperexcitability (Villeneuve et al., 2000;Bender et al., 2003), glutamatergic axonal sprouting (Bender et al., 2003;Holmes et al., 1998) and modifications of ionotropic γ-aminobutyric acid (GABA)-receptor mediated synaptic transmission (Chen et al., 1999;Khalilov et al., 2003). Impaired GABAB receptor (GABABR)-mediated signalling has been observed after epileptic activity in the adult hippocampus (Mangan & Lothman, 1996;Haas et al., 1996;Chandler et al., 2003;Wu & Leung, 1997). In the developing hippocampus, however, it is not clear whether GABABR-mediated signalling is functional after seizures. GABABRs play a crucial role in controlling the physiological activity of the immature hippocampal network. GABAA currents, which mediate most of the inhibitory drive in the adult, are excitatory during the first postnatal week of life (Cherubini et al., 1991). Consequently, the synchronous activation of hippocampal interneurons results in excitatory network discharges called Giant Depolarising Potentials (GDPs) (Ben-Ari et al., 1989). In these peculiar conditions, GABABR activation is essential to limit network excitability. Endogenous GABA released during GDPs activates GABABRs, thus reducing further GABA release from interneurons and leading to GDP termination (McLean et al., 1996). Consistent with this, the pharmacological block of GABABRs progressively increases GDP width, transforming GDPs into epileptiform discharges (McLean et al., 1996;Tosetti et al., 2004). Similar effects can also be observed after the functional impairment of GABABR-mediated presynaptic inhibition of GABA release (GABA auto-inhibition) by a prolonged application of baclofen, a selective GABABR agonist (Tosetti et al., 2004). This observation raises the interesting possibility that a sustained release of endogenous GABA may trigger a functional loss of GABA auto-inhibition, leading to hyperexcitability and pathological network activity. Ictal discharges, in se rm -0 03 80 22 1, v er si on 1 13 M ay 2 00 9 during which both GABA and glutamate are continuously released for several seconds (Velazquez & Carlen, 1999;Kohling et al., 2000;Rutecki et al., 1985), are one possible context for such loss to occur. Here we report that extensive endogenous GABA release during ictal-like discharges impairs GABA auto-inhibition in the newborn rat hippocampus. Such impairment contributes to the network hyperexcitability observed following the epileptiform discharges. These data thus reveal a novel role of GABABR-mediated signalling in acute post-seizure plasticity of the developing hippocampus. in se rm -0 03 80 22 1, v er si on 1 13 M ay 2 00 9