Rationale: Children with Down syndrome (DS, trisomy of chromosome 21) have an increased risk of infantile spasms (IS) (incidence: 2%-10% vs. ~0.03% in the general population). IS further impair cognitive function and exacerbate neurodevelopmental delays in children with DS. To investigate the pathophysiology of IS in DS, we used the non-mosaic and most complete genetic mouse model of DS extant, TcMAC21, that carries human chromosome 21 containing 93% of protein-coding genes (Kazuki et al., 2020).
Methods: TcMAC21 mice and their euploid littermates on postnatal day (P) 15-18 were implanted with electroencephalographic (EEG) electrode and recorded 3 days after surgery. Following 1-h baseline recording, the GABA
B receptor agonists γ-butyrolactone (GBL, 100-200 mg/kg, i.p.) or baclofen (BAC, 8 mg/kg, i.p.) were administered to induce IS (Cortez et al., 2009). Whole-cell current clamp recordings were performed in layer V pyramidal neurons to determine basic membrane properties and excitability, in brain slices prepared from P13-P19 euploid and trisomic mice. Separate voltage-clamp experiments were performed to assess the excitatory-to-inhibitory (E-I) ratio. A bipolar stimulating electrode was placed in layer II/III to evoked synaptic currents. Neurons were clamped at -70 mV or 0 mV for excitatory and inhibitory postsynaptic currents (EPSCs, IPSCs), respectively. The E-I ratio was assessed at 1x, 1.25x, 1.5x, 1.75x, and 2x threshold (T) intensity.
Results: During 1-h baseline recording, spasm-like behavior or hypsarrhythmia-like aberrant EEG activity were not detected in either the euploid or TcMAC21 mice.
Administration GBL or BAC caused brief, repetitive extensor spasms in TcMAC21 mice (n=4) but rarely in euploid mice (n=6). On EEG, GBL or BAC reduced background EEG amplitude with emergence of rhythmic, sharp-and-slow wave activity in euploid mice or high-amplitude burst-like (epileptiform) events in TcMAC21 mice. The burst-like EEG events in TcMAC21 mice were associated with behavioral spasms. Basic membrane properties (resting membrane potential, input resistance, action-potential threshold, rheobase, input-output relationship) of layer V pyramidal neurons were not different between TcMAC21 mice (n=20 cells from 7 mice) and euploid controls (n=18 cells from 6 mice). However, EPSCs evoked at various intensities (1x, 1.25x, 1.5x, 1.75x and 2x T) were significantly larger in TcMAC21 mice (n=21 cells from 8 mice) than euploid controls (n=16 cells from 6 mice) (p < 0.05, t-test), while IPSCs were similar between the two groups, leading to increased E-I ratio.