Forebrain-independent generation of hyperthermic convulsions in infant rats

A physiologically validated rat model of term birth asphyxia with seizure generation after, not during, brain hypoxia

OBJECTIVE

Birth asphyxia (BA) is often associated with seizures that may exacerbate the ensuing hypoxic-ischemic encephalopathy. In rodent models of BA, exposure to hypoxia is used to evoke seizures, that commence already during the insult. This is in stark contrast to clinical BA, in which seizures are typically seen upon recovery. Here, we introduce a term-equivalent rat model of BA, in which seizures are triggered after exposure to asphyxia.

METHODS

Postnatal day 11-12 male rat pups were exposed to steady asphyxia (15 min; air containing 5% O2  + 20% CO2 ) or to intermittent asphyxia (30 min; three 5 + 5-min cycles of 9% and 5% O2 at 20% CO2 ). Cortical activity and electrographic seizures were recorded in freely behaving animals. Simultaneous electrode measurements of intracortical pH, Po2 , and local field potentials (LFPs) were made under urethane anesthesia.

RESULTS

Both protocols decreased blood pH to <7.0 and brain pH from 7.3 to 6.7 and led to a fall in base excess by 20 mmol·L-1 . Electrographic seizures with convulsions spanning the entire Racine scale were triggered after intermittent but not steady asphyxia. In the presence of 20% CO2 , brain Po2 was only transiently affected by 9% ambient O2 but fell below detection level during the steps to 5% O2 , and LFP activity was nearly abolished. Post-asphyxia seizures were strongly suppressed when brain pH recovery was slowed down by 5% CO2 .

SIGNIFICANCE

The rate of brain pH recovery has a strong influence on post-asphyxia seizure propensity. The recurring hypoxic episodes during intermittent asphyxia promote neuronal excitability, which leads to seizures only after the suppressing effect of the hypercapnic acidosis is relieved. The present rodent model of BA is to our best knowledge the first one in which, consistent with clinical BA, behavioral and electrographic seizures are triggered after and not during the BA-mimicking insult.

Nest Carbon Dioxide Masks GABA-Dependent Seizure Susceptibility in the Naked Mole-Rat

African naked mole-rats were likely the first mammals to evolve eusociality, and thus required adaptations to conserve energy and tolerate the low oxygen (O₂) and high carbon dioxide (CO₂) of a densely populated fossorial nest. As hypercapnia is known to suppress neuronal activity, we studied whether naked mole-rats might demonstrate energy savings in GABAergic inhibition. Using whole-colony behavioral monitoring of captive naked mole-rats, we found a durable nest, characterized by high CO₂ levels, where all colony members spent the majority of their time. Analysis of the naked mole-rat genome revealed, uniquely among mammals, a histidine point variation in the neuronal potassium-chloride cotransporter 2 (KCC2). A histidine missense substitution mutation at this locus in the human ortholog of KCC2, found previously in patients with febrile seizures and epilepsy, has been demonstrated to diminish neuronal Cl^- extrusion capacity, and thus impairs GABAergic inhibition. Seizures were observed, without pharmacological intervention, in adult naked mole-rats exposed to a simulated hyperthermic surface environment, causing systemic hypocapnic alkalosis. Consistent with the diminished function of KCC2, adult naked mole-rats demonstrate a reduced efficacy of inhibition that manifests as triggering of seizures at room temperature by the GABA_A receptor (GABA_AR) positive allosteric modulator diazepam. These seizures are blocked in the presence of nest-like levels of CO₂ and likely to be mediated through GABA_AR activity, based on in vitro recordings. Thus, altered GABAergic inhibition adds to a growing list of adaptations in the naked mole-rat and provides a plausible proximate mechanism for nesting behavior, where a return to the colony nest restores GABA-mediated inhibition.

Apnea Associated with Brainstem Seizures in Cacna1a S218L Mice Is Caused by Medullary Spreading Depolarization

Seizure-related apnea is common and can be lethal. Its mechanisms however remain unclear and preventive strategies are lacking. We postulate that brainstem spreading depolarization (SD), previously associated with lethal seizures in animal models, initiates apnea upon invasion of brainstem respiratory centers. To study this, we assessed effects of brainstem seizures on brainstem function and respiration in male and female mice carrying a homozygous S218L missense mutation that leads to gain-of-function of voltage-gated Ca_V2.1 Ca²⁺ channels and high risk for fatal seizures. Recordings of brainstem DC potential and neuronal activity, cardiorespiratory activity and local tissue oxygen were performed in freely behaving animals. Brainstem SD occurred during all spontaneous fatal seizures and, unexpectedly, during a subset of nonfatal seizures. Seizure-related SDs in the ventrolateral medulla correlated with respiratory suppression. Seizures induced by stimulation of the inferior colliculus could evoke SD that spread in a rostrocaudal direction, preceding local tissue hypoxia and apnea, indicating that invasion of SD into medullary respiratory centers initiated apnea and hypoxia rather than vice versa Fatal outcome was prevented by timely resuscitation. Moreover, NMDA receptor antagonists MK-801 and memantine prevented seizure-related SD and apnea, which supports brainstem SD as a prerequisite for brainstem seizure-related apnea in this animal model and has translational value for developing strategies that prevent fatal ictal apnea.SIGNIFICANCE STATEMENT Apnea during and following seizures is common, but also likely implicated in sudden unexpected death in epilepsy (SUDEP). This underlines the need to understand mechanisms for potentially lethal seizure-related apnea. In the present work we show, in freely behaving SUDEP-prone transgenic mice, that apnea is induced when spontaneous brainstem seizure-related spreading depolarization (SD) reaches respiratory nuclei in the ventrolateral medulla. We show that brainstem seizure-related medullary SD is followed by local hypoxia and recovers during nonfatal seizures, but not during fatal events. NMDA receptor antagonists prevented medullary SD and apnea, which may be of translational value.