Abstracts

Rationale: Better mapping of the brain regions activated during seizures can lead to novel insights and new therapies. Genetically engineered mice combined with tissue clearing and advanced microscopy offer novel opportunity to map seizure-activated regions.Methods: We used mice expressing tetracyclin-inducible GFP under the control of activity-dependent promoter of immediate early gene cfos (a kind gift from Dr Mark Mayford, Reijmers et al, 2007). Mice were kept on a high doxycycline diet and then switched to a dox-free diet for 60 hr. A single tonic-clonic seizure was induced by administration of GABA-A receptor antagonist pentylenetetrazol (PTZ, 40 mg/kg). Animal perfusion and tissue clearing was carried out as described by Yang et al (2014). Imaging was performed on a Zeiss 780 microscope.Results: Mice kept off-dox for 24 hr did not have any GFP expression. In the 48 hr off-dox mice GFP expression was present in lateral geniculate nuclei and scattered neurons in CA1 region. In the 60 hr off-dox mice, in addition to the GFP expression in lateral geniculate nuclei and CA1 region, neurons in barrel cortex, motor cortex, auditory cortex, and visual cortex also expressed GFP. There was a further increase in GFP-expressing cells in all these areas along with the appearance of sporadic GFP expression in DGCs of 72 hr off-dox mice. In the subsequent studies mice kept off-dox for 60 hr, to have minimum background GFP expression in the limbic structures, were injected with PTZ. Sixty minutes after a single PTZ-evoked seizure, there was a marked increase in GFP expression in the CA3 and CA1 pyramidal neurons, and in the subiculum. GFP-expressing neurons were also present in the medial entorhinal cortex, cingulate gyrus, ectorhinal cortex, and striatum. In the subsequent studies, GFP expression was determined 30 or 120 min following a PTZ-induced seizure. Thirty min following a single PTZ seizure, increased GFP expression was restricted to the CA1 region. On the other hand, the areas of GFP expression 120 min post-PTZ seizure were similar to those observed 60 min post-PTZ seizure. Ongoing studies are utilizing similar techniques to map seizure activity in continuous hippocampal stimulation (CHS)-induced status epilepticus and frontal lobe focal seizures.Conclusions: We have successfully integrated clarity tissue clearing technique to map seizure-activated regions in the cfos-GFP mice with high signal to background ratio and better spatial resolution.