Group 1 and 2 metabotropic glutamate receptors play differential roles in hippocampal long-term depression and long-term potentiation in freely moving rats

Abstract

This study examined the role of metabotropic glutamate receptors (mGluRs) in hippocampal long-term depression (LTD) in vivo. The group 1 mGluR antagonist (S)4-carboxyphenylglycine (4CPG), group 1/2 antagonist (RS)-alpha-methyl-4-carboxyphenylglycine (MCPG), and group 2 antagonists (RS)-alpha-methylserine-O-phos-phate monophenyl ester (MSOPPE) and (2S)-alpha-ethylglutamic acid (EGLU) were used. The NMDA receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid (AP5) was used to examine the NMDA receptor contribution to the observed LTD. Adult male Wistar rats underwent implantation of stimulating and recording electrodes into the Schaffer collaterals and CA1 stratum radiatum, respectively. After recovery of 5-7 d, the field EPSP was measured from freely moving animals. Drugs were applied either before or after 1 Hz low-frequency train (LFT) or 100 Hz stimulation via a cannula implanted in the lateral cerebral ventricle. Nine hundred pulses at 1 Hz produced an LTD that was marked and long-lasting. This LTD was completely inhibited by pre-LFT application of AP5. MCPG inhibited LTD from 2 hr post-LFT. 4CPG partially impaired LTD. MSOPPE and EGLU completely blocked induction of LTD, although short-term depression remained intact. MSOPPE did not block long-term potentiation (LTP) induced by 100 Hz stimulation, whereas 4CPG produced a significant inhibition. When MSOPPE was present, LTD could not be induced either before or after LTP induction, whereas LTD could be induced in an identical protocol in vehicle-injected animals. These results suggest a differential role for mGluRs in NMDA receptor-dependent hippocampal LTD in vivo. Group 1 mGluRs may play a role in both LTD and LTP, whereas group 2 mGluRs may be critically involved only in LTD induction.

Fig. 1.

LTD in the CA1 region in vivo is dependent on activation of both NMDA receptors and mGluRs.A, A low-frequency train (LFT) in the presence of a vehicle injection (n = 19) results in a robust LTD that persists for 24 hr. Previous injection of the NMDA receptor antagonist AP5 (20 mm/5 μl;n = 9) completely inhibits the induction of LTD, from t = 5 min post-LFT. B, Original analog traces showing evoked responses in the CA1 region at three time points: preinjection, t = 5 min, andt = 4 hr post-LFT, in (1) a vehicle-injected animal and (2) an animal injected with MCPG (200 mm/5 μl).C, MCPG (200 mm/5 μl;n = 8) significantly inhibits LTD, fromt = 120 min post-LFT, compared with vehicle-injected controls (n = 10). *p < 0.05, **p < 0.01, ***p < 0.001. Line breaks(//) indicate change in time scale.

Fig. 2.

LTD in the CA1 region in vivo is modulated by group 1 mGluRs. A, The group 1 mGluR antagonist 4CPG (20 mm/5 μl; n = 7) partially impairs LTD when compared with vehicle-injected controls (n = 12), from t = 120 min post-LFT. This effect was significantly different from baseline values.B, Original analog traces showing evoked responses in the CA1 region at three time points: preinjection,t = 5 min, and t = 4 hr post-LFT, in an animal injected with 4CPG (20 mm/5 μl).C, Dose–response curve for the antagonist effect of 4CPG (2–40 mm/5 μl) on LTD in the CA1 region. The values represent the magnitude of LTD observed at 4 hr post-LFT.D, Application of 4CPG (20 mm/5 μl;n = 5) 5 min after LFT results in a significant inhibition of LTD, from t = 135 min post-HFT compared with vehicle-injected controls (n = 5). *p < 0.05. Line breaks(//) indicate change in time scale.

Fig. 3.

LTP in the CA1 region in vivo is modulated by group 1 mGluRs. A, 4CPG (20 mm/5 μl; n = 5) significantly inhibits LTP, from t = 60 min after a high-frequency tetanus (HFT) compared with vehicle-injected controls (n = 6).B, Application of 4CPG (20 mm/5 μl;n = 5) 5 min after HFT results in a significant inhibition of LTP, from t = 45 min post-HFT compared with vehicle-injected controls (n = 5). *p < 0.05, **p < 0.01, ***p < 0.001. Line breaks(//) indicate change in time scale.

Fig. 4.

LTD in the CA1 region in vivocritically involves group 2 mGluRs. A, The group 2 mGluR antagonist MSOPPE (40 mm/5 μl;n = 12) completely blocks LTD, fromt = 90 min post-LFT when compared with vehicle-injected controls (n = 12).B, Original analog traces showing evoked responses in the CA1 region at three time points: preinjection,t = 5 min, and t = 4 hr post-LFT, in an animal injected with MSOPPE (40 mm/5 μl).C, Dose–response curve for the antagonist effect of MSOPPE (4–80 mm/5 μl) on LTD in the CA1 region. The values represent the magnitude of LTD observed at 4 hr post-LFT.D, The group 2 mGluR antagonist EGLU (n = 6) completely blocks LTD, fromt = 75 min post-LFT when compared with vehicle-injected controls (n = 11). *p < 0.05, **p < 0.01, ***p < 0.001. Line breaks(//) indicate change in time scale.

Fig. 5.

Group 1 and 2 mGluRs contribute to maintenance rather than initiation of LTD. A, Application of MSOPPE (40 mm/5 μl; n = 5) 5 min after LFT results in a significant inhibition of LTD, fromt = 105 min post-LFT compared with vehicle-injected controls (n = 5). B, Application of a subthreshold concentration of MSOPPE (20 mm/5 μl) in conjunction with 4CPG (20 mm/5 μl;n = 5) 5 min post-LFT results in a complete block of the 4CPG-resistant fraction of LTD, from t = 105 min compared with vehicle-injected controls (n = 5).

Fig. 6.

LTP induction does not depend on activation of group 2 mGluRs. A, The group 2 mGluR antagonist MSOPPE (n = 7) does not inhibit LTP in the CA1 region when compared with vehicle-injected controls (n = 6).B, Original analog traces showing evoked responses in the CA1 region at three time points: preinjection,t = 5 min, and t = 4 hr post-HFT, in an animal injected with MSOPPE (40 mm/5 μl). Line breaks (//) indicate change in time scale.

Fig. 7.

LTD but not LTP depends on activation of group 2 mGluRs. A, HFT in the presence of a vehicle injection (n = 5) results in a marked potentiation of fEPSP. LFT applied 1 hr after HFT causes evoked potentials to return to baseline values. MSOPPE (n = 5) applied before an identical protocol results in an LTP that cannot be reversed by application of LFT. B, Original analog traces showing evoked responses in the CA1 region at three time points: preinjection,t = 60 min, and t = 4 hr post-tetanus, in (1) an animal injected with MSOPPE (40 mm/5 μl) in which HFT is followed 1 hr later by LFT, and (2) an animal injected with MSOPPE (40 mm/5 μl) in which LFT is followed 1.5 hr later by HFT. C, Vehicle injection (n = 6) before LFT results in a marked depression of fEPSP. HFT applied 1.5 hr after LFT results in a reversal of depression to approximately pre-LFT values. MSOPPE (n = 6) applied before LFT results in a depression of LFT that recovers gradually. Application of HFT 1.5 hr after LFT produces a marked potentiation of fEPSP to LTP levels.Line breaks (//) indicate change in time scale.