Effects of subtype-selective group I mGluR antagonists on synchronous activity induced by 4-aminopyridine/CGP 55845 in adult guinea pig hippocampal slices

Abstract

Co-application of the convulsant 4-aminopyridine (4-AP) and the GABA B receptor antagonist CGP 55845 to adult guinea pig hippocampal slices elicits giant GABA-mediated postsynaptic potentials (GPSPs) and epileptiform discharges. Here we tested the effects of the group I metabotropic glutamate receptor (mGluR) subtype-selective antagonists LY 367385 (mGlu1, 100 µM), MPEP (mGlu5, 10 µM), and MTEP (mGlu5, 500 nM) on this synchronous activity. Electrophysiological field recordings were performed in the CA3 region of hippocampal slices from adult guinea pigs. The mGlu5 receptor antagonists increased GPSP rate, but the mGlu1 receptor antagonist did not. This ability of mGlu5 receptor antagonists to increase the rate of GPSPs indicates that enough endogenous glutamate is released under these conditions to activate group I mGluR; nevertheless, co-application of a mGlu1 receptor antagonist (LY 367385 or JNJ 16259685) and MPEP did not decrease pre-existing epileptiform activity. Furthermore, co-application of LY 367385 and MPEP did not prevent the emergence of epileptiform activity. When ionotropic glutamate receptor (iGluR) antagonists were present, neither MPEP nor the group I mGluR agonist DHPG changed GPSP rate, suggesting that pyramidal cell-to-interneuron iGluR-mediated synaptic connections are involved in the rate change mechanism. In contrast to the lack of effect of group I mGluR antagonists on epileptiform activity in the 4-AP/CGP 55845 model, group I mGluR antagonists blocked the emergence of longer epileptiform events and decreased the overall amount of synchronous activity in the GABA A antagonist/4-AP model. In conclusion, in the 4-AP/CGP 55845 model, enough glutamate was released to activate group I mGluRs and affect GPSP rate via mGlu5 receptors; however, this group I mGluR activation was not required for the generation of the epileptiform activity.

Persistent ictal-like activity in rat entorhinal/perirhinal cortex following washout of 4-aminopyridine

“…The solution in the holding chamber where the slices were stored after cutting (Salah and Perkins 2008) contained (in mM) 125 NaCl, 25 NaHCO 3 , 2.5 KCl, 1.6 MgCl 2 , 2.0 CaCl 2 , 11 D-glucose. The 1.6 mM Mg 2+ /2.0 mM Ca 2+ extracellular recording solution contained (in mM) 130 NaCl, 24 NaHCO 3 , 2.5 NaH 2 PO 4 , 3.5 KCl, 1.6 MgSO 4 , 2.0 CaCl 2 , 10 D-glucose.…”

“…The ionotropic glutamate receptor antagonists used were the AMPA/kainate receptor antagonist 2,3-dioxo6-nitro-1,2,3,4,-tetrahydrobenzo[f] quinoxaline-7-sulfonamide disodium salt (NBQX, 10 μM) and the NMDA receptor antagonist D-(−)-2-amino-5-phosphonopentanoic acid (D-AP5, 50 μM). The group I mGluR antagonists used were ( S )-(+)-α-amino-4-carboxy-2-methylbenzeneacetic acid (LY 367385, 100 μM; Salah and Perkins, 2008), which preferentially blocks the mGlu1 receptor subtype, and 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP, 10 μM; Salah and Perkins, 2008), which preferentially blocks the mGlu5 receptor subtype. The general protein synthesis inhibitor cycloheximide (60 μM) was used in some experiments.…”

“…4-AP was considered to be completely washed out at 30 minutes of wash, based on data presented here (see Fig. 2 and the NMDA receptor antagonist effect on induction data), and additionally on control experiments in which giant GABA-mediated postsynaptic potentials (GPSPs), and the discharge deflections directly following each GPSP (as recorded in Salah and Perkins 2008) are completely gone by 23 ± 2 minutes of washing out the 4-AP (n=3, Salah and Perkins, unpublished data). Percent time spent in SE + ictal-like events after 4-AP washout was measured for the period encompassing 30–50 min of 4-AP washout.…”

GABAergic synchronization in the limbic system and its role in the generation of epileptiform activity

“…This type of glutamatergic-independent synchronous activity can be recorded from practically any limbic structures maintained in the brain slice (Avoli et al, 1996aBenini et al, 2003;Sudbury and Avoli, 2007;Panuccio et al, 2009) as well as in the isolated guinea pig brain (Uva et al, 2009). It has also been reported that the rate of occurrence of these GABAergic spikes are not influenced by modulating metabotropic glutamate receptor activity (Salah and Perkins, 2008).…”

Brain extracellular space, hyaluronan, and the prevention of epileptic seizures

“…This mechanism is unlikely in the Has3 KO mice, however, because the location of axon terminals of recurrent excitatory connections in CA1 is in the basal dendrites in SO (Deuchars and Thomson, 1996), not in the pyramidal cell layer where the reduced ECS volume fraction is localized (Arranz et al, 2014). Since the requirement is for ionotropic and not metabotropic glutamate receptors, presumably the glutamatergic transmission is needed for the trigger and/or synchronization mechanisms rather than to provide a sustained depolarization of the cell (see discussion in Salah and Perkins, 2008). …”