Abstracts

Rationale: Cerebellar dysfunction is a common clinical manifestation in patients with Dravet syndrome (DS), a severe form of pediatric epileptic encephalopathy. Consistent with this, a DS mouse model resulting from deletion of Scn1b, the gene encoding the voltage-gated sodium channel b1/b1B subunits, shows significant ataxia.  However, the pathophysiological mechanisms underlying cerebellar dysfunction associated with loss-of-function SCN1B DS mutations remain unclear.  Methods: Here, we investigated the neuronal excitability and synaptic function of Purkinje cells and interneurons in Scn1b null and wildtype (WT) acute cerebellar slices using the whole cell patch-clamp recording technique.    Results: In slices prepared from postnatal day (P)14-18 Scn1b null mice, both Purkinje cells and interneurons had significantly higher thresholds for action potential initiation and lower repetitive firing frequency compared to neurons in slices prepared from WT littermates.  In a subpopulation of Scn1b null Purkinje cells (16/26) and interneurons (5/7), high frequency repetitive firing could not be evoked. In contrast, high frequency repetitive firing could be evoked in all Purkinje cells and interneurons tested in WT slices. Scn1b null Purkinje cells also showed increased frequencies of spontaneous and miniature excitatory postsynaptic currents (sEPSCs and mEPSCs), but reduced frequencies of spontaneous and miniature postsynaptic inhibitory currents (sIPSCs and mIPSCs). There were no differences in the amplitudes of sEPSCs/mEPSCs or sIPSCs/mIPSCs between genotypes.  Moreover, paired-pulse evoked synaptic responses at parallel fiber-Purkinje cell or climbing fiber-Purkinje cell synapses showed altered short-term synaptic plasticity in Scn1b null slices. Scn1b deletion appears to strengthen paired-pulse facilitation (PPF) at the parallel fiber-Purkinje synapses as well as to promote PPF at the climbing fiber-Purkinje cell synapses by reversing the polarity of paired-pulse responses or by suppressing paired-pulse depression (PPD). Conclusions: Taken together, these data suggest that presynaptic mechanisms underlie the changes in spontaneous synaptic transmission in Scn1b null cerebella. We propose that reduced neuronal excitability and altered synaptic function in cerebellar GABAergic neurons contribute to cerebellar dysfunction associated with SCN1B-linked DS.    Funding: Supported by NIH grants R37-NS-076752 to LLI.