Abstracts

Rationale: Infantile spasms (IS)–a catastrophic developmental encephalopathy of infancy–are often refractory to antiepileptic treatments. The ketogenic diet (KD) has emerged as an alternative treatment for this intractable population. We previously showed in the triple-hit model of IS that the KD induced intracerebral acidosis accounted for seizure reduction and accompanying neurodevelopmental improvements in the model (PMID: 34734183). Since the acid sensing ion channel (ASIC) 1a is responsible for all acid evoked currents in physiologically relevant pH ranges (5.0-7.4) in the brain, we assessed whether ASIC1a mediates the anti-convulsant effects of the KD._x000D_
Methods: We used the triple-hit IS model to assess spasm frequency in CRISPR ASIC1a knockout (KO) rat pups. Rats were artificially reared in the “pup-in-cup” setup and fed the KD (4:1 fats:carbs+protein) or a control-milk diet (CM; 1.7:1). We used the zero-magnesium (0-Mg) acute slice seizure model to assess the effect of acidosis on epileptiform activity and the role of ASIC1a, through extracellular field potential recordings in somatosensory cortical brain slices of postnatal day 7 (when intracerebral acidosis is observed in the model) pups treated with or without the KD. The slices were exposed to 0-Mg aCSF either at physiological pH (7.4) or low pH (6.8; as induced by KD in our model). The frequency and amplitude of the epileptiform discharges were analyzed. _x000D_
Results: Compared to wildtype (WT) controls, spasm frequency increased by 53.89% in ASIC1a KO pups fed the CM (n=4; P=0.02). Further, whereas the KD suppressed the number of spasms in WT pups, it had no effect on spasm frequency in ASIC1a KO pups; indicating that the spasms suppressing effects of the KD are mediated by ASIC1a (n=4; P=0.422).  We did not observe any sustained spontaneous activity in the cortical slices prior to the induction of epileptiform activity using 0-Mg. Lowering the pH to 6.8 led to an overall reduction in the epileptiform activity. In naïve pups (n=2), we observed a 92.50% reduction in the interictal spiking (IIS) frequency with a decrease in peak spiking (PS) amplitude of 26.73%. In control pups fed the KD (n=2) there was an 87.66% reduction in IIS frequency and 62.97% in PS amplitude. In IS pups, the reduction in epileptiform activity caused by the switch to low pH was more pronounced in KD (n=2) pups compared to CM (n=2) (IIS: KDL=100% vs CML=76.86%; PS amplitude: KDL=100% vs CML=36.38%). In addition to the IIS, IS pups also displayed sustained bursting activity when exposed to 0-Mg, and this behaviour was abolished with the switch to low pH. No bursting was observed in control animals fed either diet. _x000D_
Conclusions: Our behavioural data indicates that the KD-induced intracerebral acidosis acts via ASIC1a to produce its antiseizure effects. Our electrophysiological studies show heightened excitability in IS pups exposed to 0-Mg, which is attenuated by low pH. Further studies are needed to examine whether antiseizure effects in the model are dependent on ASIC1a. _x000D_
Funding: Canadian Institute of Health Research