mGluR5 positive allosteric modulators facilitate both hippocampal LTP and LTD and enhance spatial learning

Abstract

Highly selective positive allosteric modulators (PAMs) of metabotropic glutamate receptor subtype 5 (mGluR5) have emerged as a potential approach to treat positive symptoms associated with schizophrenia. mGluR5 plays an important role in both long-term potentiation (LTP) and long-term depression (LTD), suggesting that mGluR5 PAMs may also have utility in improving impaired cognitive function. However, if mGluR5 PAMs shift the balance of LTP and LTD or induce a state in which afferent activity induces lasting changes in synaptic function that are not appropriate for a given pattern of activity, this could disrupt rather than enhance cognitive function. We determined the effect of selective mGluR5 PAMs on the induction of LTP and LTD at the Schaffer collateral-CA1 synapse in the hippocampus. mGluR5-selective PAMs significantly enhanced threshold theta-burst stimulation (TBS)-induced LTP. In addition, mGluR5 PAMs enhanced both DHPG-induced LTD and LTD induced by the delivery of paired-pulse low-frequency stimulation. Selective potentiation of mGluR5 had no effect on LTP induced by suprathreshold TBS or saturated LTP. The finding that potentiation of mGluR5-mediated responses to stimulation of glutamatergic afferents enhances both LTP and LTD and supports the hypothesis that the activation of mGluR5 by endogenous glutamate contributes to both forms of plasticity. Furthermore, two systemically active mGluR5 PAMs enhanced performance in the Morris water maze, a measure of hippocampus-dependent spatial learning. Discovery of small molecules that enhance both LTP and LTD in an activity-appropriate manner shows a unique action on synaptic plasticity that may provide a novel approach for the treatment of impaired cognitive function.

Figure 1. VU-29 potentiates DHPG induced increases in PI hydrolysis in rat hippocampal slices

A. 500nM VU-29 induced a significant leftward shift in the concentration response curve of DHPG-induced PI hydrolysis in rat hippocampal slices. In the presence of VU-29 the EC₅₀ of DHPG was 4.5 ± 1 µM, compared with 8 ± 1.6 µM in the absence of VU-29. Additionally, the maximum response was enhanced to by 38 ± 13 %. B. 3 µM DHPG increased baseline PI hydrolysis, which was significantly enhanced by pre-incubation of 5 µM VU-29 but not altered by 100 µM 5MPEP in rat hippocampal slices. 5MPEP blocked the potentiation caused by VU-29. (n = 5, experiments performed in triplicate). Error bars represent S.E.M. *** p<0.0001, ** p<0.001.

Figure 2. The mGluR5 allosteric potentiator VU-29 facilitates the induction of LTP in area CA1 of the hippocampus

A. Four trains of 10 Hz TBS induced a long lasting potentiation of the fEPSP slope at the SC-CA1 synapse. B. Threshold TBS induced a slight potentiation of the fEPSP slope. C. A 30 min incubation of 500 nM VU-29 did not alter the baseline initial fEPSP slope recorded in the rat hippocampal CA1 region (n = 8). D. In control slices, threshold TBS induced a slight potentiation of fEPSP. In the presence of VU-29 (20 min pre-incubation) the same stimulation induced a significant potentiation (n = 12; p<0.05). Error bars represent S.E.M.

Figure 3. Potentiation of LTP by VU-29 is blocked by mGluR5 neutral allosteric modulator 5MPEP

A. Pre-incubation of 100 µM 5MPEP completely inhibited VU-29-facilitated TBS-induced LTP back to vehicle control (n = 8; p<.0.05). B. 10 Hz TBS induced LTP was not altered by the pre-incubation of 100 µM 5MPEP (n = 8; p>0.05). C. Threshold TBS induced LTP was not altered by pre-incubation of 100 µM 5MPEP (n = 5; p>0.05). Error bars represent S.E.M.

Figure 4. The NMDA receptor antagonist, D-AP5, and the Src-family kinase inhibitor, PP 1, block VU-29-facilitated LTP of fEPSP in rat hippocampal CA1 region

A. 50 µM D-AP5 (n = 8) completely blocked TBS-induced LTP in the presence of VU-29 (p>0.05 compared to vehicle treated slices). B. Bar graph depicting percent LTP induced by threshold TBS stimulation in the presence of VU-29 and 50 µM D-AP5 (n = 8) or 20 µM PP 1 (n = 7). Error bars represent S.E.M.

Figure 5. VU-29 potentiates chemically induced mGluR-LTD in area CA1 of the rat hippocampus

A. Field EPSPs are reduced upon addition of the group I mGluR agonist, DHPG, in a concentration dependent manner. B. In control slices, 25 µM DHPG induced a modest level of LTD that was significantly enhanced in slices pre-incubated with 1 µM VU-29 (n = 4; p<0.05), while having no effect on acute depression (n = 4–6; p>0.05). C. Pre-incubation of the slice with 100 µM 5MPEP selectively inhibits the ability of VU-29 (1 µM) to enhance DHPG-LTD (n = 3–4; p<0.05) while having no effect on DHPG-LTD by itself (n = 3; p>0.05 compared to DHPG alone). D. Pre-incubation of the slice with 20 µM of the MEK inhibitor, U0126, inhibits the ability of VU-29 (1 µM) to enhance DHPG-LTD (n = 3–4, p<0.05). Error bars represent S.E.M.

Figure 6. Selective enhancement of mGluR5 receptor signaling significantly increases stimulus induced mGluR-LTD

A. Paired-pulse low frequency stimulation induces mGluR-dependent LTD that was significantly enhanced following pre-incubation of the slice with 1 µM VU-29 (n = 6–7; p<0.05). The enhancement of PP-LFS induced LTD is independent of NMDA receptor activation and selective enhancement of mGluR5 receptor signaling has no effect on NMDA receptor-dependent LTD. B. Incubation of the slice with 50 µM D-AP5 did not affect the ability of 1 µM VU-29 to potentiate PP-LFS-induced LTD (n=5; p<0.01). C. Low frequency stimulation induces NMDA receptor-dependent LTD that is not enhanced in the presence of 1 µM VU-29 (n = 5–6; p>0.05). Error bars represent S.E.M.

Figure 7. VU-29-facilitated LTP shares similar mechanisms as TBS induced LTP in area CA1 of the hippocampus

A. 500 nM VU-29 did not alter LTP induced by a suprathreshold TBS protocol that induces robust LTP. In control slices, a 4X 100 Hz TBS induced robust LTP. The same stimulation in the presence of VU-29 yielded a potentiation that was not significantly different from control (n = 5; p>0.05). B. 500nM VU-29 did not alter the induction of LTP by a suprathreshold TBS protocol in slices in which LTP was previously fully saturated. LTP was induced by 4 trains of 10 Hz TBS. After 30 minutes, the slices were incubated with 500 nM VU-29, followed by another 4 trains of 10 Hz TBS, which did not overcome the potentiation induced by the first 4 trains of TBS (n = 8; p>0.05). Error bars represent S.E.M.

Figure 8. The systemically active mGluR5 PAMs CDPPB and ADX47273 enhance performance in the Morris water maze

A. CDPPB (10 mg/kg) decreases latency to reach platform (A1), decreases mean number of days to reach criteria (≤ 15 sec to reach platform; p003C;0.05) (A2), and increases time spent in target quadrant during probe trial (A3) (n = 12–13; p<0.05). B. ADX47273 (10 mg/kg) also decreases latency to reach platform (B1), decreases mean number of days to reach criteria (≤ 15 sec to reach platform; p<0.05) (B2), and increases time spent in target quadrant during probe trial (B3) (n = 11–12; p<0.05).