“Can You Hear Me Now?” AMPA Receptor-Mediated Tonotopy Disruption by Early Life Seizures

Commentary

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

Sun H, Takesian AE, Wang TT, Lippman-Bell JJ, Hensch TK, Jensen FE. Cell Rep 2018;23:2533–2540.

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.

This elegant study by Sun et al. cleverly puts together several pieces of the autism–epilepsy puzzle. First, there is a mutual relationship between autism spectrum disorder (ASD) and epilepsy: About 40% of children with ASD also have epilepsy. Children with a special epilepsy syndrome, infantile spasms, may develop ASD in up to 35% of cases, although a recent meta-analysis specifies a pooled estimate ~20% for ASD in infantile spasms (1). Focal seizures and Dravet syndrome generally have even higher ASD association rates (>40%). In children who develop epilepsy with tuberous sclerosis, this risk rises up to 50% (2). Concurrent intellectual disability increases ASD association by about 5 times compared to general population and the age under 18 years increases the association even more (~13 times; 1). Second, children with ASD have impairments in the acoustic processing that may significantly contribute to their poor language development, probably due to impaired auditory cortex development (3). Auditory neurofeedback may improve the ASD outcome (4). This impairment was replicated in mouse models of autism, which also indicated that deficiencies in the auditory cortex are likely following severe impacts that may happen during a critical developmental period of thalamocortical connectivity represented by connections from basal ventral medial geniculate nucleus (MGN) to the auditory cortex during postnatal days (P) 12–15 (5). In humans, this auditory critical period lasts up to 12 years of age (6). Finally, in the auditory cortex, impairments may affect tonotopic map plasticity, which is supported by activation of NMDA receptor-containing synapses—or likely by unsilencing NMDA receptor-containing synapses by activation (in the widest meaning of the word) of AMPA receptors in those synapses (7). The major idea of the paper is: What if the language-related autistic traits in children (or mice) with epilepsy are due to deteriorating impact of early life seizures that may erroneously mimic critical developmental period for the auditory cortex—and once we know the mechanism, is there anything we can do about it?

The authors applied several different techniques to prove their points. In the first part, they used (almost) horizontal brain sections preserving MGN connections to the auditory cortex. Stimulation of the latero-medial axis of the MGN elicited topological cortical responses assessed optically by voltage-sensitive dye. Thus, a geometrical profile (topography) of a maximal response (an equivalent to tonotopy) was obtained. One group of mice was injected with saline prior to the critical period (P9–11), and the other group received three injections of convulsant pentylenetetrazole (PTZ). If a 7 kHz tone was played during the critical period, it affected cortical tonotopy in saline-injected mice but not in the mice with prior PTZ seizures. This finding indicates that the seizures have the power to alter critical period or to shift tonotopy development. Then, to determine the role of silent synapses, the authors used whole cell current recordings in the auditory cortex with cells clamped at −60 mV while stimulating at approximately 50% efficacy in the MGN. Then the cell was clamped at +40 mV (removing the Mg²+ blockade from NMDA receptors) and MGN was stimulated with the same intensity. The difference in response failure rate indicated proportion of silent synapses. This difference was relatively large (>30%) when recorded during a critical period (P12–P15) but substantially smaller (~2%) if recorded after the critical period (P16–21). This finding indicates increase in activation NMDA receptor-containing synapses at P16–21 as AMPA receptors were inserted/activated there during activity ongoing within the critical period. If PTZ seizures (P9–11) were brought into this experiment, there were larger spontaneous excitatory currents at 24 hours after the last seizure (P12) because of an increase in AMPA receptor-mediated EPSCs. Further, the P12 failure rate at +40 mV holding potential dramatically decreased. This finding indicates that the silent synapses were already activated. Finally, if AMPA receptor antagonist was injected into mice 1 hour after PTZ seizures, it was able to normalize both amplitude and frequency of enlarged EPSCs. AMPA receptor antagonist also significantly enhanced failure rate in the stimulated whole cell currents. This indicates that the activation of silent NMDA-containing synapses due to early life seizure activity could have been prevented by AMPA receptor blockade.

Here, the authors demonstrate that insertion/activation of AMPA receptor to previously silent NMDA synapses may, in fact, bypass the critical period in development of the auditory cortex, impair cortical tonotopy (or an equivalent thereof) and, with far-reaching human extension, potentially contribute to the poor language skills in children with ASD. The authors showed one possible mechanism of how the excessive neuronal activity can affect critical period in the development of the auditory cortex. Critical periods of development involve extreme sensitivity to modifying events (6). For different CNS systems and functions, they occur in different ontogeny periods. Modifying events during critical periods may alter the system permanently; in other times, the system is just primed and the priming may be recalled later (8). Frequently, critical periods for various systems and functions in the CNS are associated with periods of heighten plasticity, a phenomenon that often (but not exclusively) involves NMDA receptor-mediated synaptic transmission modifiable by hyperexcitability (9). However, other plastic changes may be involved, such as plasticity/development of GABA concentrations, chloride transporters, norepinephrine, or developmental loss of the GluN2B subunit of the NMDA receptor (9–12). The importance of critical period plasticity rests in prevention of developmental elimination of silent synapses. Thus, PTZ-induced seizures in this study or in strychnine-induced hyperexcitation (9) are effective in either closing down the window or keeping it open, again depending on the CNS system and timing of the impact.

There are some minor shortcomings of the study. First, it is unclear what the next step is. This was barely suggested in the discussion, pointing to a proof-of-principle rescue of the auditory critical period (and also social behavior; 13) by the AMPA receptor antagonist after PTZ seizures. More expanded discussion on a potential use of talampanel as an add-on therapy in seizure syndromes with high ASD comorbidity would be extremely interesting. Then, “limbic seizures” are a misnomer; PTZ induces generalized clonic seizures with the same semiology as the seizures that generalize from a limbic focus, as clearly shown by many (14). This, of course, does not contradict substantial involvement of thalamocortical circuitry in the symptomatology of seizures with limbic origin (15). Finally, several four-group designs in the study cry for use of a 2-way ANOVA that may significantly expand the horizons of the analysis.

In conclusion, this was a very well-designed translational study showing that silencing AMPA receptors after neonatal seizures may help in maintaining the critical period for correct development of auditory cortex. Indeed, future experiments and clinical studies demonstrating that this treatment will also preserve correct auditory cortex function and, in extension, will prevent occurrence of language impairments in children with ASD associated with childhood epilepsy syndromes are more than desirable.