Developmental up-regulation of NMDA receptors in the prefrontal cortex and hippocampus of mGlu5 receptor knock-out mice

Abstract

mGlu5 metabotropic glutamate receptors are highly expressed and functional in the early postnatal life, and are known to positively modulate NMDA receptor function. Here, we examined the expression of NMDA receptor subunits and interneuron-related genes in the prefrontal cortex and hippocampus of mGlu5^-/- mice and wild-type littermates at three developmental time points (PND9, - 21, and - 75). We were surprised to find that expression of all NMDA receptor subunits was greatly enhanced in mGlu5^-/- mice at PND21. In contrast, at PND9, expression of the GluN2B subunit was enhanced, whereas expression of GluN2A and GluN2D subunits was reduced in both regions. These modifications were transient and disappeared in the adult life (PND75). Changes in the transcripts of interneuron-related genes (encoding parvalbumin, somatostatin, vasoactive intestinal peptide, reelin, and the two isoforms of glutamate decarboxylase) were also observed in mGlu5^-/- mice across postnatal development. For example, the transcript encoding parvalbumin was up-regulated in the prefrontal cortex of mGlu5^-/- mice at PND9 and PND21, whereas it was significantly reduced at PND75. These findings suggest that in mGlu5^-/- mice a transient overexpression of NMDA receptor subunits may compensate for the lack of the NMDA receptor partner, mGlu5. Interestingly, in mGlu5^-/- mice the behavioral response to the NMDA channel blocker, MK-801, was significantly increased at PND21, and largely reduced at PND75. The impact of adaptive changes in the expression of NMDA receptor subunits should be taken into account when mGlu5^-/- mice are used for developmental studies.

Keywords: Hippocampus MK-801; Interneuron related genes; Locomotor activity; NMDA receptor subunits; Prefrontal cortex.

Fig. 1

Influence of genetic deletion of mGlu5 receptors on the expression of interneuron-related genes in the prefrontal cortex and hippocampus across postnatal development. mRNA levels encoding interneuron-related proteins in the prefrontal cortex and hippocampus at PND9 -21 and -75 are shown in a and b, respectively. Values are means ± S.E.M. of 5–9 mice per group. * p< 0.05 vs. the corresponding values of wild-type littermates (Student’s t-test). a PND9: Pvalb, t₁₀ = 2.3; Reln, t₁₀ = 5.6; Vip t₁₀ = 5.6; PND21: GAD1, t₁₁ = 4; Pvalb, t₁₁ = 3.1; PND75: GAD2, t₅ = 2.8, Pvalb, t₅ = 3.2, Vip, t₅ = 2.6. b PND9: Pvalb, t₈ = 11.1; Reln, t₇ = 3.5; PND21: Vip, t₉ = 2.2; PND75: SSt, t₆ = 6.2

Fig. 2

Expression of NMDA receptor subunits in the prefrontal cortex of mGlu5^−/− mice and wild-type littermates at PND9 and PND21. mRNA and protein levels at PND9 and PND21 are shown in a, c and b, d, respectively. Values are means ± S.E.M. of 4–9 mice per group. *p < 0.05 (Student’s t-test). a Grin2A, t₁₀ = 3.8; Grin2C, t₉ = 2.8; b GluN1, t₈ = 3.4; GluN2A, t₉ = 9.4; GluN2B, t₈ = 2.8; GluN2D, t₉ = 9.4 c Grin1, t₁₂ = 4.6; Grin2A, t₁₃ = 2.5; Grin2B, t₁₂ = 3.4 d GluN1, t ₉ = 7; GluN2A, t₈ = 4.1; GluN2B, t₁₀ = 3.4; GluN2D, t₉ = 7. Uncropped western blots are shown

Fig. 3

Expression of NMDA receptor subunits in the hippocampus of mGlu5^−/− mice and wild-type littermates at PND9 and PND21. mRNA and protein levels at PND9 and PND21 are shown in a–d. Values are means ± S.E.M. of 4–9 mice per group. *p < 0.05 (Student’s t-test). a Grin2A, t₉ = 3.5; Grin2C, t ₉ = 2.5; b GluN2A, t₁₀ = 4; GluN2B, t₉ = 2.7; GluN2D, t₁₀ = 7; c Grin2C, t₁₁ = 2.5; d GluN1, t₁₀ = 2.8; GluN2B, t₉ = 5.2; GluN2D, t₁₀ = 5.2. Uncropped western blots are shown

Fig. 4

Expression of NMDA receptor subunits in the prefrontal cortex and hippocampus of mGlu5^−/− mice and wild-type littermates at PND75. mRNA and protein levels in the prefrontal cortex and hippocampus are shown in a–d. Values are means ± S.E.M. of 4–6 mice per group. Uncropped western blots are shown

Fig. 5

MK-801-induced hyperactivity in mGlu5^−/− mice and wild-type littermates at PND21 and PND75. Locomotor activity in response to 0.32 mg/kg or 0.64 mg/kg MK-801 at PND21 and in response to 0.32 mg/kg or 0.64 mg/kg MK-801 at PND75 is shown in a–d. Mice were habituated to the environment for 60 min prior to the i.p. injection of MK-801. Values are means ± S.E.M. of 10 wild-type and 3 mGlu5^−/− mice in a, 5 wild-type and 6 mGlu5^−/− mice in b, 7 wild-type and 5 mGlu5^−/− mice in c, 7 wild-type and 4 mGlu5^−/− mice in d. Open bars = habituation phase; closed bars = response to MK-801. *p < 0.05 vs. wild-type mice (One-way ANOVA for repeated measures). b, F_1,119 = 27.875; p < 0.001; c F_1,119 = 44.692; p < 0.001