Different metabotropic glutamate receptors play opposite roles in synaptic plasticity of the rat medial vestibular nuclei

Abstract

In the medial vestibular nuclei (MVN) of rat brainstem slices, the role of group II and III metabotropic glutamate receptors (mGluRs) and of the subtypes of group I mGluRs: mGluR1, mGluR5, was investigated in basal synaptic transmission and in the induction and maintenance of long-term potentiation (LTP). We used selective antagonists and agonists for mGluRs and we analysed the field potentials evoked by vestibular afferent stimulation before and after high-frequency stimulation (HFS) to induce LTP. The group II and III mGluR antagonist, (R,S)-alpha-2-methyl-4sulphonophenylglycine (MSPG), induced LTP per se and caused a reduction of the paired-pulse facilitation (PPF) ratio indicating an enhancement of glutamate release. This suggests that group II and III mGluRs are activated under basal conditions to limit glutamate release. Both the group II and III mGluR selective antagonists, 2S-2-amino-2-(1S,2S-2-carboxycycloprop-1-yl)-3-(xanth-9-yl)propanoate (LY341495) and (R,S)-alpha-methylserine-O-phosphate (MSOP), induced LTP, and the selective agonists, (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate (APDC) and L(+)-2-amino-4-phosphonobutyric acid (L-AP4) depressed the field potentials and prevented HFS-LTP, with a prevailing contribution of group II mGluRs over that of group III mGluRs. The mGluR1 antagonist, 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt) prevented the full development and maintenance of HFS-LTP. By contrast, the mGluR5 antagonist, 2-methyl-6-phenylethynylpyridine (MPEP) induced LTP per se, which was impeded by CPCCOEt, and it had no effect on LTP once induced by HFS. The PPF analysis showed an enhancement of glutamate release during MPEP potentiation. The group I mGluR agonist, (R,S)-3,5-dihydroxyphenylglycine (DHPG) induced LTP per se, which was blocked by CPCCOEt. By contrast the mGluR5 agonist, (R,S)-2-chloro-5-hydroxypheylglycine (CHPG) prevented LTP elicited by HFS and DHPG as well. In conclusion vestibular LTP is inhibited by group II and III mGluRs during the early induction phase while it is facilitated by mGluR1 for achieving its full expression and consolidation. An additional inhibitory control is exerted by mGluR5 at the level of this facilitatory phase.

Figure 1. Recordings in the ventral part of the MVN

A, recording sites in the ventral portion of the MVN (filled dots) and stimulating zone (black area) are plotted on the diagram of a brainstem slice. B, typical vestibular field potentials recorded in the ventral portion of MVN in normal and Ca²⁺-free solution. C, single pulse response is superimposed onto a 3 ms paired-pulse stimulation to show how N1 peak negative voltage was calculated compared to the baseline (vertical lines). Abbreviations: D, descending vestibular nucleus; Md, medial vestibular nucleus: dorsal portion; Mv, medial vestibular nucleus: ventral portion; L, lateral vestibular nucleus; S, superior vestibular nucleus; R, recording electrode; St, stimulating electrode.

Figure 2. Effect of the antagonist for group II and group III mGluRs on the field potential N1 wave

In A, C and D the N1 wave amplitude was measured every 15 s, expressed as a percentage of the baseline and plotted as a function of time in single experiments (time course). In this and the following figures, the bars indicate the drug infusion period, and the arrow shows the HFS delivery time. A, time course of the MSPG effect. MSPG induces LTP which is not further enhanced by HFS. B, plot of mean percentage of the baseline PPF during potentiation induced by MSPG (n = 4). The first point represents the baseline PPF evaluated 5 min before the beginning of MSPG potentiation. Note the reduction of PPF ratio after LTP induction and its quick recovery to the control value. C, time course of the MSPG effect under AP-5 and after AP-5 washout. Note that, under AP-5, MSPG only provokes a transitory increase of the N1 wave. D, time course of the MSPG effect under CPCCOEt.

Figure 3. Effects of the selective antagonists and agonists for group II and group III mGluRs on the field potential N1 wave

A and B, time course of the LY341495 and MSOP effects, respectively. The inserts show averaged field potentials (10 sweeps) before drug infusion (1) and after drug washout, when potentiation is established (2). Note that (1) and (2) correspond to numbers indicated on the plots. C and D, time course of the APDC and l-AP4 effects, respectively. Note that HFS applied during the drug depressant effect is not able to induce LTP.

Figure 4. Effect of the selective antagonist for mGluR1 on the induction and maintenance of HFS-long term potentiation

A, time course of the effects of HFS under CPCCOEt infusion which lasted 10 min (•) and 15 min (○) after HFS. Note that potentiation developed fully when CPCCOEt washout began 10 min after HFS, and it faded away when the drug infusion lasted more then 15 min. The insert shows averaged field potentials (10 sweeps) before HFS (1), after HFS under CPCCOEt (2) and after drug washout (3) in the case of short-term CPCCOEt infusion. B, time course of the effect of CPCCOEt applied after HFS, when potentiation is induced. The insert shows averaged field potentials (10 sweeps) after HFS (1), during CPCCOEt infusion (2) and after drug washout (3).

Figure 5. Effect of the selective antagonist for mGluR5 on the field potential N1 wave and on HFS-LTP

A, time course of the MPEP effect. Potentiation induced by MPEP is not affected by HFS. B, effect of MPEP in the presence of CPCCOEt (○) and AP-5 (•). C, changes of PPF during potentiation induced by MPEP alone (•, n = 4) and MPEP under AP-5 (○, n = 4). The first point represents the baseline PPF evaluated 5 min before the beginning of MPEP potentiation. The insert shows the conditioning (1) and test (2) N1 waves (average of 10 sweeps) with inter-pulse intervals of 50 ms, before (control) and after induced potentiation (potentiation). D, time course of the effect of MPEP applied 10 min after HFS, when potentiation is induced.

Figure 6. Effect of the agonist for group I mGluRs on the field potential N1 wave

A, time course of the DHPG effect. DHPG induces LTP which is not further enhanced by HFS. B, time course of the effect of DHPG under AP-5 and after AP-5 washout. C, time course of the effect of DHPG under CPCCOEt and after CPCCOEt washout. Note that AP-5 and CPCCOEt fully block DHPG potentiation which is induced after AP-5 washout.

Figure 7. Effect of the selective agonist for mGluR5 on the induction of potentiation by HFS and DHPG

A, time course of the CHPG effect on the induction of potentiation by HFS. Note that under CHPG, HFS can induce a slight depression (▴) or have no effect (•). HFS induces LTP after drug washout. B and C, time course of the DHPG effect under CHPG: complete prevention of potentiation, confirmed by the induction of LTP after CHPG washout (B), induction of slight potentiation (C) which can fully develop at CHPG washout (•) or decline to nil (▴).