Enhancement of postsynaptic GABAA and extrasynaptic NMDA receptor-mediated responses in the barrel cortex of Mecp2-null mice

Abstract

Rett syndrome (RTT) is a neurodevelopmental disorder that results from mutations in the X-linked gene for methyl-CpG-binding protein 2 (MECP2). The underlying cellular mechanism for the sensory deficits in patients with RTT is largely unknown. This study used the Bird mouse model of RTT to investigate sensory thalamocortical synaptic transmission in the barrel cortex of Mecp2-null mice. Electrophysiological results showed an excitation/inhibition imbalance, biased toward inhibition, due to an increase in efficacy of postsynaptic GABAA receptors rather than alterations in inhibitory network and presynaptic release properties. Enhanced inhibition impaired the transmission of tonic sensory signals from the thalamus to the somatosensory cortex. Previous morphological studies showed an upregulation of NMDA receptors in the neocortex of both RTT patients and Mecp2-null mice at early ages [Blue ME, Naidu S, Johnston MV. Ann Neurol 45: 541-545, 1999; Blue ME, Kaufmann WE, Bressler J, Eyring C, O'Driscoll C, Naidu S, Johnston MV. Anat Rec (Hoboken) 294: 1624-1634, 2011]. Although AMPA and NMDA receptor-mediated excitatory synaptic transmission was not altered in the barrel cortex of Mecp2-null mice, extrasynaptic NMDA receptor-mediated responses increased markedly. These responses were blocked by memantine, suggesting that extrasynaptic NMDA receptors play an important role in the pathogenesis of RTT. The results suggest that enhancement of postsynaptic GABAA and extrasynaptic NMDA receptor-mediated responses may underlie impaired somatosensation and that pharmacological blockade of extrasynaptic NMDA receptors may have therapeutic value for RTT.

Keywords: Mecp2 gene; Rett syndrome; barrel cortex; excitation/inhibition balance.

Fig. 1.

Impaired temporal summation of thalamocortical excitatory transmission in Mecp2-null mice. A and B: whole cell recordings of layer 4 excitatory neurons (regular spiking, RS) show similar firing patterns in wild type (WT; A) and Mecp2-null (B) mice. C and D: stimulation of ventrobasal complex (VB) induces an excitatory postsynaptic potential (EPSP)-inhibitory postsynaptic potential (IPSP) sequence, in which the IPSP reversal at ∼−70 mV indicates mediation by GABA_A receptors. At resting membrane potential (−60 mV; top), EPSP ≥ IPSP in WT mice while EPSP < IPSP in Mecp2-null mice. E and F: temporal summation of thalamic tonic (repetitive) inputs is impaired in Mecp2-null brain slices. G: plot averaged peak amplitudes show temporal summation curves of EPSPs for WT and Mecp2-null mice. Error bars indicate SE. Note that in Mecp2-null mice subsequent EPSPs are much smaller than the first EPSP. The large SE error bars in Mecp2-null mice result from 2 factors: 1) The change in amplitudes of subsequent EPSPs in Mecp2-null mice is ∼80% of the first EPSP, while that for WT mice is only 20% in the normalized temporal summation curves. 2) Sample size for Mecp2-null mice is 7, while that for WT mice is 16. H and I: example records of paired-pulse depression in WT and Mecp2-null mice. H.P., holding potential. J: averaged paired-pulse ratio (PPR), an index for presynaptic release probability, is not significantly different in Mecp2-null brain slices compared with WT. EPSC, excitatory postsynaptic current.

Fig. 2.

Excitation/inhibition ratio in the thalamocortical pathway is decreased in Mecp2-null mice. A and B: voltage-clamping of layer 4 barrel neurons to the reversal potentials of glutamate receptors (∼0 mV) and of GABA_A receptors (∼−70 mV) isolates GABA_A receptor-mediated inhibitory postsynaptic current (IPSC) and AMPA receptor (AMPAR)-mediated EPSC separately. C: GABA_A receptor-mediated IPSC is blocked by picrotoxin (PTX; trace 1 before and trace 2 after application). D: AMPAR-mediated EPSC is blocked by DNQX (trace 1 before and trace 2 after application). E and F: example records of EPSCs and IPSCs from the same neurons from WT and Mecp2-null mice. G: ratio of excitation to inhibition (AMPA/GABA) is decreased in Mecp2-null slices compared with those in WT.

Fig. 3.

Excitatory and inhibitory inputs to layer IV of the barrel cortex do not change in Mecp2-null mice. A and B: representative recordings of multiple input index (MII) for EPSCs. C: there is no significant difference in MII for EPSCs between WT and Mecp2-null mice. D and E: example records of MII of IPSCs. F: MIIs of IPSCs are similar in WT and Mecp2-null mice. G and H: representative recordings of PPR of IPSCs. I: PPRs of IPSCs are similar for both groups of mice.

Fig. 4.

Postsynaptic AMPAR and NMDA receptor (NMDAR) responses are not altered in barrel cortex of Mecp2-null mice. A and B: representative recordings of AMPAR-mediated spontaneous EPSCs (sEPSCs). C: averaged amplitude of sEPSCs in Mecp2-null mice is similar to that in WT mice. D and E: representative recordings of NMDAR-mediated sEPSCs. F: there is no difference in the amplitude of sEPSCs between WT and Mecp2-null mice.

Fig. 5.

Upregulated response of postsynaptic GABA_A receptors in barrel cortex of Mecp2-null mice. A and B: representative recordings of GABA_A receptor-mediated spontaneous IPSCs (sIPSCs). C and D: averaged amplitude of sIPSCs is larger in Mecp2-null mice than in WT mice. E and F: representative recordings of GABA_A receptor-mediated miniature IPSCs (mIPSCs). G and H: averaged amplitude of mIPSCs in Mecp2-null mice is also larger than that for WT mice.

Fig. 6.

Extrasynaptic NMDAR-mediated response is upregulated in the barrel cortex of Mecp2-null mice. A and B: representative recordings show extrasynaptic NMDAR-mediated responses. In Mg²⁺-free ACSF with AMPA and GABA receptor antagonists and MK-801, an open-channel NMDAR blocker, VB stimulation with single pulses at 0.1 Hz induces postsynaptic NMDAR-mediated EPSCs (circles) that are gradually blocked. After that, stimulation with 5–10 pulses at 100 Hz evokes extrasynaptic NMDAR (eNMDAR)-mediated EPSCs (stars, indicated by arrows). Note that in Mecp2-null mice (B), the amplitude of eNMDAR-mediated EPSCs is higher than in WT mice (A). The eNMDAR antagonist memantine blocks eNMDAR-mediated EPSCs. C: the normalized eNMDAR-mediated EPSC is larger in Mecp2-null mice than in WT mice.