The team built their study around a phenomenon called immediate postictal potentiation, or PIP, which they regard as a unique experimental model of the enhanced excitability that characterizes the developing nervous system. In adult animals, an electrical seizure is followed by several minutes of postictal depression, during which a second stimulus elicits either no seizure at all or a shorter one. In immature animals, the opposite occurs: after an electrical seizure, the brain enters a state of enhanced excitability in which a second stimulus elicits a longer seizure. The researchers exploited this reversal using a technique they developed in previous work. They applied pairs of identical stimuli to the sensorimotor cortex of 12-day-old rats, an age whose neocortical development is considered comparable to that of a full-term human newborn. The first, conditioning stimulus elicits an electrical seizure, known as an afterdischarge, which is recorded in the contralateral cortex. One minute later, a test stimulus assesses neuronal excitability following the initial seizure. If the test afterdischarge is longer than the conditioning one, that indicates postictal potentiation; if it is shorter, that indicates the adult-like postictal depression.

The experimental protocol was elegant in its simplicity. Each animal received two pairs of cortical stimuli separated by 10 minutes. The first pair served as a control and showed the typical PIP of immature animals, with the test afterdischarge longer than the conditioning one. Immediately after the end of the first test afterdischarge, a glutamate receptor antagonist was administered. Ten minutes later, the second pair of stimuli was applied. If PIP was absent in the second pair, meaning the test afterdischarge was either not present or shorter than the conditioning one, the researchers took that as evidence that the receptor targeted by the antagonist was involved in mediating PIP and, by extension, was an important factor in the hyperexcitability of the immature brain. The 12-day-old male Wistar rats were implanted with epidural electrodes under brief ether anesthesia, kept on a heating pad at 34 degrees Celsius throughout, and only animals demonstrating intact righting, placing, and suckling reflexes were used. Each animal was studied only once.

The most dramatic result came from ifenprodil, a selective antagonist of NMDA receptors that contain the GluN2B subunit. This drug did not merely suppress immediate PIP; it converted the condition to an adult-like postictal depression, with the test afterdischarge significantly shorter than the conditioning one after drug administration. The effect was even more pronounced than that of dizocilpine, also known as MK-801, a potent non-competitive NMDA receptor antagonist, which also abolished PIP and produced postictal depression in the second stimulus pair. Memantine, a low-affinity NMDA receptor antagonist, showed a similar but smaller effect, with only a tendency for the test afterdischarge to be shorter than the conditioning one at the higher dose, most likely reflecting its lower affinity and dwell time at the receptor. By contrast, PEAQX, an antagonist that preferentially targets NMDA receptors containing the GluN2A subunit, the dominant form in adult animals, was ineffective at abolishing PIP. MTEP, an antagonist of metabotropic glutamate receptor 5 used to indirectly probe the NMDA system, also had no effect.

This subunit-specific pattern makes biological sense given the developmental progression of NMDA receptors. NMDA receptors are tetramers composed of two obligatory GluN1 subunits plus two additional subunits drawn from a set that most often consists of GluN2A or GluN2B. In immature animals, GluN2B subunits predominate, but during the third postnatal week there is a progressive increase in GluN2A subunits, which predominate in adults. The two subunit types confer different functional properties. GluN2B-containing receptors have longer channel opening times, stronger calcium influx currents, and slower deactivation than GluN2A-containing receptors. These characteristics very likely contribute to the enhanced neuronal excitability of the immature brain. When ifenprodil blocked the GluN2B-containing receptors, only GluN2A-containing receptors remained active, mimicking the adult situation. As a consequence, PIP disappeared and was replaced by the postictal depression characteristic of mature animals.

The role of AMPA receptors proved more complicated but revealed an unexpected second layer of the phenomenon. Neither of the two AMPA receptor antagonists tested, the competitive antagonist NBQX or IEM 1460, had a significant effect on immediate PIP. In fact, after IEM 1460 administration, the second stimulus pair showed dramatic PIP similar to that seen in the first pair. However, the AMPA antagonists demonstrated an effect not seen with any of the NMDA antagonists. In control animals, the potentiation following the first stimulus pair lingers: 10 minutes after the first test afterdischarge, the second conditioning afterdischarge has not returned to baseline and is sometimes even longer than the first test one. This prolonged potentiation was selectively abolished by the AMPA antagonists. IEM 1460 was particularly effective, returning the second conditioning afterdischarge to baseline excitability while the immediate potentiation 1 minute after the second conditioning stimulus was preserved. NBQX showed a similar effect at the low dose of 7.5 mg/kg, while higher doses of 15 and 30 mg/kg profoundly depressed all afterdischarges, essentially eliminating all forms of potentiation.

The selectivity of IEM 1460 is significant because it targets AMPA receptors that lack a functional GluA2 subunit, a form that predominates in immature animals. Like NMDA receptors, AMPA receptors are heteromeric tetramers composed of combinations of four subunits, GluA1 through GluA4, and their subunit composition varies during development. The absence of a GluA2 subunit markedly alters the functional characteristics of the AMPA receptor, most significantly making it permeable to calcium ions, which may contribute to increased neuronal excitability in the immature brain. The authors note that their data are consistent with the established role of AMPA receptors in status epilepticus, and that prolonged postictal potentiation may underlie the increased susceptibility of neonates to recurrent seizures and status epilepticus.

The findings carry therapeutic implications that extend beyond the acute treatment of neonatal seizures. It is well established that neonatal seizures lead to a greater incidence of epilepsy and cognitive deficits later in life. Previous work on models of hypoxic seizures in 10-day-old rats, which mimic many long-term postictal sequelae, has implicated AMPA receptors in those lasting changes. Developing drugs that target AMPA receptors lacking the GluA2 subunit may therefore be useful not only for treating acute neonatal seizures, especially recurrent ones, but also for attenuating some of the detrimental long-term consequences of neonatal seizures. On the NMDA side, the GluN2B subunit, being uniquely predominant in the neonatal period, should be considered an attractive target for the design of drugs to treat neonatal seizures.

The path to the clinic is complicated by the serious side effects that have blocked the therapeutic potential of NMDA and AMPA receptor antagonists, including effects on memory and cognition. The only NMDA receptor antagonist currently used in patients is the low-affinity antagonist memantine, prescribed for Alzheimer’s disease, which does not exhibit serious side effects. The authors argue that their results demonstrate the importance of both NMDA and AMPA receptors in the high seizure susceptibility of the immature brain and underscore the necessity of finding new antagonists of these receptors without severe side effects. Because PIP appears to be a function of the unique structure of glutamate receptors in the immature nervous system, particularly NMDA receptors containing the GluN2B subunit, understanding its mechanisms could provide valuable insights into designing more effective treatments for the seizures of newborns, a population for whom current therapies remain largely ineffective.

Subject of Research: NMDA receptors play an important role in postictal potentiation in immature rats

Article Title: NMDA receptors play an important role in postictal potentiation in immature rats

Article References: Mares, P., Burchfiel, J. L., & Kubova, H. (2026). NMDA receptors play an important role in postictal potentiation in immature rats. Frontiers in Synaptic Neuroscience, 18, Article 1742164. https://doi.org/10.3389/fnsyn.2026.1742164

Image Credits: AI Generated

DOI: 10.3389/fnsyn.2026.1742164

Keywords: neonatal seizures, NMDA receptors, AMPA receptors, GluN2B subunit, postictal potentiation, epilepsy, developing brain, glutamate receptors, status epilepticus, synaptic neuroscience, NMDA, receptors