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. 2018 May 29;23(9):2533-2540.
doi: 10.1016/j.celrep.2018.04.108.

Early Seizures Prematurely Unsilence Auditory Synapses to Disrupt Thalamocortical Critical Period Plasticity

Hongyu Sun  1 , Anne E Takesian  2 , Ting Ting Wang  3 , Jocelyn J Lippman-Bell  4 , Takao K Hensch  5 , Frances E Jensen  6
Affiliations

Early Seizures Prematurely Unsilence Auditory Synapses to Disrupt Thalamocortical Critical Period Plasticity

Hongyu Sun et al. Cell Rep. .

Abstract

Heightened neural excitability in infancy and childhood results in increased susceptibility to seizures. Such early-life seizures are associated with language deficits and autism that can result from aberrant development of the auditory cortex. Here, we show that early-life seizures disrupt a critical period (CP) for tonotopic map plasticity in primary auditory cortex (A1). We show that this CP is characterized by a prevalence of "silent," NMDA-receptor (NMDAR)-only, glutamate receptor synapses in auditory cortex that become "unsilenced" due to activity-dependent AMPA receptor (AMPAR) insertion. Induction of seizures prior to this CP occludes tonotopic map plasticity by prematurely unsilencing NMDAR-only synapses. Further, brief treatment with the AMPAR antagonist NBQX following seizures, prior to the CP, prevents synapse unsilencing and permits subsequent A1 plasticity. These findings reveal that early-life seizures modify CP regulators and suggest that therapeutic targets for early post-seizure treatment can rescue CP plasticity.

Keywords: AMPA receptor; NBQX; NMDA receptor; auditory cortex; autism; development; epilepsy; neurodevelopmental disorders; silent synapses; tonotopic plasticity.

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Conflict of interest statement

DECLARATION OF INTERESTS

The authors declare that they have no competing interests.

Figures

Figure 1
Figure 1. Early-Life Seizures Disrupt an Auditory Critical Period
(A) Experimental strategy to evaluate the effects of early-life seizures on auditory CP plasticity. P9–P11 mice were injected daily with PTZ (50 mg/kg, i.p.) or saline and then exposed to pulsed 7 kHz tones during a CP for tonotopic plasticity (P12–P15). Changes in thalamocortical connectivity were assessed in vitro at P15 using VSD imaging. (B) Schematic of an auditory thalamocortical slice indicating six thalamic (MGBv) stimulus positions and 18 regions along L4 in A1. Electrical stimulation along the tonotopic axis of the MGBv produced a caudal-to-rostral shift in maximal VSD activity along the A1 tonotopic axis in naive mice. The color of each A1 region indicates the MGBv position that produced the maximal response across all naive mice. Inset shows approximate frequency representations of MGBv stimulus sites (Hackett et al., 2011). (C) Sample A1 response (ΔF/F) to MGBv stimulation in a thalamocortical slice from a PTZ-treated mouse (P15). Scale bar, 500 µm. (D) PTZ-induced seizures prevent the change of thalamocortical topography by 7 kHz tone exposure. The degree of orderly topography was calculated by the linear relationship between the location of peak response (ΔF/F) in A1 to MGBv stimuli. Average topographic curves are shown for naive saline-injected mice raised in a standard acoustic environment (black), 7 kHz exposed saline-injected mice (gray), naive mice with previous PTZ-induced seizures (orange), and 7 kHz exposed mice with previous PTZ-induced seizures (red). (E) Comparison of topographic slopes shows a significant change following 7 kHz tone exposure in control mice injected with saline, but not in mice injected with PTZ (mean ± SEM; *p < 0.05, n.s. p > 0.05).
Figure 2
Figure 2. NMDAR-Only Silent Synapses in L4 Pyramidal Neurons in Naive Mouse A1 Decrease during Normal Development
(A) Schematic of stimulation and recording sites in thalamocortical slices. (B and C) Representative minimal MGBv-evoked eEPSCs at +40 mV (upper traces) and −60 mV (lower traces) from P12 (B) and P21 (C) mice. Successes and failures of individual EPSCs are shown as filled and open circles, respectively. (D–G) AMPARs mediate fast eEPSCs at −60 mV, and NMDARs mediate slow-decaying eEPSCs at +40 mV (n = 5) (D). Additional 20 µM NBQX in the ACSF abolished the eESPCs at −60 mV (E). 50 µMD-AP5 in the ACSF completely blocked the slow-decaying eEPSCs at +40 mV (F). Summary plot of individual eEPSC amplitudes during the time course of an experiment showing AMPAR- and NMDAR-mediated EPSCs at different holding potentials (G). (H and I) Failure rates at −60 mV and +40 mV holding potentials from P12–P15 (H) and P16–P21 (I) mice. (J) Summary of failure rate difference at −60 mV and +40 mV. (K) Fraction of calculated silent synapses shows a significant decrease in L4 pyramidal neurons during normal development. Error bars represent mean ± SEM. *p < 0.05, n.s. p > 0.05.
Figure 3
Figure 3. Early-Life Seizures Enhance AMPAR Function and Promote Thalamocortical Silent Synapse Loss in A1
(A) Sample AMPAR-mediated sEPSCs in L4 pyramidal neurons of A1 from a P12 mouse 24 hr post-PTZ seizures (red) and littermate controls (black). (B) Cumulative distribution showing significantly larger sEPSC amplitudes from post-PTZ mice compared to littermate controls (*p < 0.001). (C and D) PTZ seizures resulted in increased sEPSC amplitude (mean ± SEM; *p < 0.05) (C) but no alterations in frequency (D). (E) Representative MGBv-evoked minimal eEPSCs in L4 pyramidal cells from each group. (F and G) Shown in (F): typical peak amplitudes for 35 consecutive minimal eEPSC responses at −60 mV, showing that (G) minimal eEPSC amplitudes were increased after PTZ seizures (mean ± SEM; *p < 0.05). (H) Representative eEPSC traces at +40 mV (upper traces) and −60 mV (lower traces) from a P12 control mouse. Successes and failures of individual EPSCs are shown as filled and open circles, respectively. (I) Failure rates at −60 mV and +40 mV holding potentials for controls. *p < 0.05. (J) Representative eEPSC traces at +40 mV (upper traces) and −60 mV (lower traces) from a P12 mouse at 24 hr post-PTZ. (K) Failure rates at −60 mV and +40 mV holding potentials for post-PTZ mice. ns, not significant. (L) Failure rate differences are significantly reduced after PTZ seizures (mean ± SEM; *p < 0.05). (M) Fraction of calculated silent synapses shows a significant decrease in post-PTZ seizure mice compared to littermate controls (mean ± SEM; *p < 0.05).
Figure 4
Figure 4. NBQX Rescues Auditory CP Plasticity following Early-Life Seizures by Preventing Premature Synapse Unsilencing
(A) Top: experimental strategy to evaluate the effects of NBQX treatment on seizure-induced impairment of auditory CP plasticity. P9–P11 mice were injected daily with PTZ (50 mg/kg, i.p.) followed by saline or NBQX (20 mg/kg, i.p.) at 1 hr post-seizures and then were exposed to pulsed 7 kHz tone pips during a CP for tonotopic plasticity (P12–P15). Tone-evoked changes in thalamocortical topography were assessed using in vitro VSD imaging at P15. Bottom: topographic slopes (*p < 0.05). (B) Representative AMPAR-mediated sEPSCs in L4 pyramidal neurons of A1 from P12 mice 24 hr post-PTZ seizures treated with saline (red) or NBQX (blue). (C) Cumulative distribution showing significantly smaller sEPSC amplitudes from NBQX-treated post-PTZ mice compared to saline-treated post-PTZ mice. (D and E) Increased sEPSC amplitude (D) and frequency (E) induced by prior PTZ seizures were reversed by NBQX treatment (mean ± SEM; *p < 0.05). (F) Sample eEPSC traces at +40 mV (upper traces) and −60 mV (lower traces) from a P12 mouse 24 hr post-PTZ seizures. Successes and failures of individual EPSCs are shown as filled and open circles, respectively. (G) Failure rates at −60 mV and +40 mV for post-PTZ mice. (H) Sample eEPSC traces at +40 mV (upper traces) and −60 mV (lower traces) from a P12 NBQX-treated post-PTZ mouse. (I) Failure rates at −60 mV and +40 mV for NBQX-treated post-PTZ mice. (J) The fraction of calculated silent synapses was significantly increased in post-PTZ mice treated with NBQX versus saline (mean ± SEM; *p < 0.05).
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