The Enlightened Brain: Novel Imaging Methods Focus on Epileptic Networks at Multiple Scales

Abstract

Epilepsy research is rapidly adopting novel fluorescence optical imaging methods to tackle unresolved questions on the cellular and circuit mechanisms of seizure generation and evolution. State of the art two-photon microscopy and wide-field fluorescence imaging can record the activity in epileptic networks at multiple scales, from neuronal microcircuits to brain-wide networks. These approaches exploit transgenic and viral technologies to target genetically encoded calcium and voltage sensitive indicators to subclasses of neurons, and achieve genetic specificity, spatial resolution and scalability that can complement electrophysiological recordings from awake animal models of epilepsy. Two-photon microscopy is well suited to study single neuron dynamics during interictal and ictal events, and highlight the differences between the activity of excitatory and inhibitory neuronal classes in the focus and propagation zone. In contrast, wide-field fluorescence imaging provides mesoscopic recordings from the entire cortical surface, necessary to investigate seizure propagation pathways, and how the unfolding of epileptic events depends on the topology of brain-wide functional connectivity. Answering these questions will inform pre-clinical studies attempting to suppress seizures with gene therapy, optogenetic or chemogenetic strategies. Dissecting which network nodes outside the seizure onset zone are important for seizure generation, propagation and termination can be used to optimize current and future evaluation methods to identify an optimal surgical strategy.

Functional brain imaging in larval zebrafish for characterising the effects of seizurogenic compounds acting via a range of pharmacological mechanisms

“…Whilst this process is often exploited for anti‐epileptic drug development (Kandratavicius et al, 2014), this can also occur through the inadvertent action of chemicals, including drugs (Ruffmann et al, 2006). The exact mechanisms that lead to seizures are not fully understood (Scharfman, 2007) and a major challenge is that, to understand the mechanistic basis of seizure generation and identify novel drug targets, studies of brain networks linking local and global scales are crucial (Rossi et al, 2018; Staley, 2015). Importantly, and in contrast with other current animal models, functional brain imaging in the larval zebrafish allows this and also provides a more direct indicator of neural activity than, for example, behavioural assessments of convulsions (Winter et al, 2008).…”

The Heated Relationship Between Neural Activity and Seizures

“…Multicellular calcium imaging (MCI), primarily via 2-photon (2P) microscopes, has been applied to study the activity of neurons and its relationship to seizures. 4 In contrast to single-unit electrophysiological recordings, which still retain certain, especially temporal, advantages, this approach can often monitor the activity of a larger number of neurons at once, and more precisely identify the types of neurons being monitored, both of which increase the strength of the conclusions. To date, however, these studies, at least those that have been performed in intact animals and not brain slices, have mostly imaged ictal activity generated by application of chemoconvulsants such as 4-aminopyridine (4-AP, but see the study by Meyer et al 5 ).…”

Two-Photon Imaging to Unravel the Pathomechanisms Associated with Epileptic Seizures: A Review

“…When probing molecular environments wherein intensity-based measurements alone are often insufficient, fluorescence lifetime-based measurements may be capable of yielding additional data [26]. Finally, the development of a wide range of fluorescent markers has made it possible to study various aspects of the pathophysiology of epilepsy, such as neurotransmitter levels [27,28], metabolism [28,29], pathological neuronal activity [27], and hypersynchronous network firing [30].…”