MGluR5 mediates the interaction between late-LTP, network activity, and learning

Abstract

Hippocampal synaptic plasticity and learning are strongly regulated by metabotropic glutamate receptors (mGluRs) and particularly by mGluR5. Here, we investigated the mechanisms underlying mGluR5-modulation of these phenomena. Prolonged pharmacological blockade of mGluR5 with MPEP produced a profound impairment of spatial memory. Effects were associated with 1) a reduction of mGluR1a-expression in the dentate gyrus; 2) impaired dentate gyrus LTP; 3) enhanced CA1-LTP and 4) suppressed theta (5-10 Hz) and gamma (30-100 Hz) oscillations in the dentate gyrus. Allosteric potentiation of mGluR1 after mGluR5 blockade significantly ameliorated dentate gyrus LTP, as well as suppression of gamma oscillatory activity. CA3-lesioning prevented MPEP effects on CA1-LTP, suggesting that plasticity levels in CA1 are driven by mGluR5-dependent synaptic and network activity in the dentate gyrus. These data support the hypothesis that prolonged mGluR5-inactivation causes altered hippocampal LTP levels and network activity, which is mediated in part by impaired mGluR1-expression in the dentate gyrus. The consequence is impairment of long-term learning.

Figure 1. Prolonged mGluR5 antagonism in vivo inhibits working and reference memory performance.

A, B. MPEP was given daily (1.8 µg, i.c.v.), 30 min prior to testing of learning performance in an 8-arm radial maze where only 4 arms were baited with food. By the third trial day a significant impairment of both reference (A) and working memory (B) was apparent. Asterices denote statistical significance (p<0.05). Data are represented as mean±S.E.M.

Figure 2. Prolonged mGluR5 antagonism results in reduced expression of mGluR1 in the dentate gyrus, but not in CA1 region.

Expression of mGluR5, mGluR2/3 and NR2A remain unchanged both in the dentate gyrus and CA1. A, B. Western blot analysis of NR2A, mGluR5, mGluR1 and mGluR2/3 receptors in the dentate gyrus (A) and in the CA1 region (B). Each lane shows receptor expression in individual animals from control and treated groups. C, D. Densitometric analysis of NR2A, mGluR5, mGluR1 and mGluR2/3 expression are shown in the dentate gyrus (C) and in CA1 (D), following acute or prolonged treatment with MPEP. Each individual value was normalized by the expression of ß-actin. Values are mean±S.E.M of six individual determinations. Asterix denotes statistically significant difference (p<0.05).

Figure 3. An impairment of LTP occurred after prolonged mGluR5 antagonism is partly mediated by mGluR1 in the dentate gyrus.

A, B. HFT induces robust LTP of PS amplitude (A) and fEPSP slope (B) in controls treated with vehicle for three days (open circles). Prolonged application of MPEP (filled circles) results in a significant impairment of LTP induced 30 min after the final injection, compared to vehicle-injected controls. Potentiation of mGluR1 by Ro67-4853 after treatment with MPEP (grey diamonds) led to a significant recovery of late-LTP. Data are represented as mean±S.E.M. C. Analogues represent fEPSP responses during pre-HFT baseline, 5 min post-HFT and 24h post-HFT period, recorded after treatment with (i) vehicle, (ii) MPEP, and (iii) MPEP and Ro67-4853. Scale bars: vertical 2 mV, horizontal 5 ms.

Figure 4. Prolonged mGluR5 antagonism in vivo enhances late-LTP in the CA1 region in vitro.

A. Prolonged in vivo treatment with MPEP results in an enhancement of late-LTP in the CA1 region in vitro, when compared with controls. B. Analogues represent (i) pre-HFT, (ii) 5 min post-HFT and (ii) 4h post-HFT, at the timepoints noted, in vehicle and MPEP-treated animals. For controls: vertical bar: 2 mV, horizontal bar: 5 ms. For MPEP: vertical bar: 1 mV, horizontal bar: 5 ms. Data are represented as mean±S.E.M.

Figure 5. Induction phase of LTP is enhanced in CA3-lesioned animals after prolonged MPEP treatment.

A. A transverse section through the rat brain at the level of ca. 3.1–3.3 mm posterior to bregma, demonstrating the lesioning of the hippocampal CA3 region as a result of kainate injection. B. Daily administration of MPEP for three days in CA3-lesioned rats resulted in an enhanced induction of LTP in CA1 region in comparison with CA3-lesioned animals that were treated with vehicle. Data are represented as mean±S.E.M. C. Analogues represent (i) pre-HFT, (ii) 5 min post-HFT and (iii) 24h post-HFT, in CA3-lesioned animals following treatment either with vehicle or MPEP. Vertical bar: 2 mV, horizontal bar: 5 msec.

Figure 6. Hippocampal network activity is altered in animals treated with an mGluR5 antagonist.

A, B. Relative theta (5–10 Hz, A) and gamma (30–100 Hz, B) power in the dentate gyrus prolonged treatment with vehicle (open circles), MPEP alone (filled squares) or MPEP with Ro67-4853 (filled triangles). Note that potentiation of mGluR1 with Ro67-4853 partially prevented the suppression of gamma oscillations, which was caused by prolonged MPEP treatment. The values represent averaged data for five 4-s epochs selected after test-pulses at each time-point and normalised to pre-injection values (mean±S.E.M.). Curve-fits are plotted based on distance weighted least squares for time-points after HFT.