Abstracts

Rationale: Understanding how dysfunctional brain networks give rise to seizures and impact their propagation and termination are central goals in epilepsy research and can inform the development of targeted treatments of seizures and of the comorbidities associated with epilepsy. Here, we investigate excitatory ventral hippocampal (VH) networks before and week after repeated hippocampal seizures (kindling), a procedure that results in chronic network changes, with simultaneous electrophysiology and functional MRI with optogenetic stimulation in rats. We directly image brain-wide network dynamics of single seizures to reveal focal and, for the first time, focal to bilateral tonic-clonic seizures. We also conduct tests for anxiety and depression, two common epilepsy-related comorbidities. Finally, we report on the relationship between hippocampal networks and resulting seizure dynamics based on within-subject data.  Methods: Twenty-five Sprague Dawley rats were injected with AAV-CAMKII-ChR2 and then implanted with an optrode in the VH1,2 and an electrode in the ipsilateral medial prefrontal cortex (mPFC). Twelve rats were optogenetically-kindled (12 non-kindled controls). All rats were scanned before and after treatment using CBV-fMRI with optogenetic modulation of VH neurons, and >10 weeks later in a subset of rats, we imaged optogenetically-induced seizures using BOLD-fMRI (2-4 seizures per rat, n=5,7). All imaging was performed on a 7T small animal MRI system with simultaneous electrophysiological recordings.  Results: Widespread increases in excitatory VH network activity were detected following kindling (7 ROIs were significantly increased after FWE-correction at p<0.05, 6/7 ROIs were ipsilateral and ranged from 8-56% increase in volume activated compared to controls, n=12,12). The largest change was detected in the mPFC (6.4-fold increase in LFP bandpower and 56% in activated volume following kindling). Because the VH-prefrontal cortical circuit has been implicated in anxiety and depression3, we tested and found that kindled rats were more anxious (n=7,8, p=0.046). For seizure imaging, seizure duration in kindled rats was 30.7+-8.4s (p=0.001) longer than in non-kindled and 15.5+-2.2 more ROIs were activated (p<0.001). Propagation network analysis indicated activity remained ipsilateral in controls while activity in kindled rats propagated gradually to cortex bilaterally. mPFC was more rapidly activated. Mediodorsal thalamus, a region implicated in seizure generalization4, was active only in kindled rats. Next, we investigated the linear relationship between the VH network and seizure networks and found that 95% of the variability was explained in controls compared to 77% in kindled rats (p<0.001, p=0.05, n=7,5). Finally, we investigated the positive predictive value of VH network activity for seizure involvement and found that in ROIs that were consistently activated it was 0.86+-0.08 in controls (3 ROIs, n=7) and 0.89+-0.07 in kindled rats (14 ROIs, n=5), suggesting that assessing VH activity reliably informs on seizure pathways.  Conclusions: In conclusion, these results reveal the brain-wide excitatory VH networks that underlie hippocampal seizure propagation and the long-term impact of repeated seizures on these networks. 1Neuroimage 2015; 107;229-241 2Neuroimage 2015; 123;173-1843Neuron 2010; 65(2);257-2694Epilepsia 2007; 49(2);256-268  Funding: This work was supported by NIH/NINDS R01NS087159, NIH/NIA R01NS091461, NIH/NINDS RF1AG047666, and NIH/NIMH RF1MH114227.