Quantitative evaluation of central-type benzodiazepine receptors with [(125)I] Iomazenil in experimental epileptogenesis. I. The rat kainate model of temporal lobe epilepsy

Ex vivo characterization of neuroinflammatory and neuroreceptor changes during epileptogenesis using candidate positron emission tomography biomarkers

OBJECTIVE

Identification of patients at risk of developing epilepsy before the first spontaneous seizure may promote the development of preventive treatment providing opportunity to stop or slow down the disease.

METHODS

As development of novel radiotracers and on-site setup of existing radiotracers is highly time-consuming and expensive, we used dual-centre in vitro autoradiography as an approach to characterize the potential of innovative radiotracers in the context of epilepsy development. Using brain slices from the same group of rats, we aimed to characterise the evolution of neuroinflammation and expression of inhibitory and excitatory neuroreceptors during epileptogenesis using translational positron emission tomography (PET) tracers; ¹⁸ F-flumazenil (¹⁸ F-FMZ; GABA_A receptor), ¹⁸ F-FPEB (metabotropic glutamate receptor 5; mGluR5), ¹⁸ F-flutriciclamide (translocator protein; TSPO, microglia activation) and ¹⁸ F-deprenyl (monoamine oxidase B, astroglia activation). Autoradiography images from selected time points after pilocarpine-induced status epilepticus (SE; baseline, 24 and 48 hours, 5, 10 and 15 days and 6 and 12-14 weeks after SE) were normalized to a calibration curve, co-registered to an MRI-based 2D region-of-interest atlas, and activity concentration (Bq/mm² ) was calculated.

RESULTS

In epileptogenesis-associated brain regions, ¹⁸ F-FMZ and ¹⁸ F-FPEB showed an early decrease after SE. ¹⁸ F-FMZ decrease was maintained in the latent phase and further reduced in the chronic epileptic animals, while ¹⁸ F-FPEB signal recovered from day 10, reaching baseline levels in chronic epilepsy. ¹⁸ F-flutriciclamide showed an increase of activated microglia at 24 hours after SE, peaking at 5-15 days and decreasing during the chronic phase. On the other hand, ¹⁸ F-deprenyl autoradiography showed late astrogliosis, peaking in the chronic phase.

SIGNIFICANCE

Autoradiography revealed different evolution of the selected targets during epileptogenesis. Our results suggest an advantage of combined imaging of inter-related targets like glutamate and GABA_A receptors, or microglia and astrocyte activation, in order to identify important interactions, especially when using PET imaging for the evaluation of novel treatments.

Neuroimaging Biomarkers of Experimental Epileptogenesis and Refractory Epilepsy

This article provides an overview of neuroimaging biomarkers in experimental epileptogenesis and refractory epilepsy. Neuroimaging represents a gold standard and clinically translatable technique to identify neuropathological changes in epileptogenesis and longitudinally monitor its progression after a precipitating injury. Neuroimaging studies, along with molecular studies from animal models, have greatly improved our understanding of the neuropathology of epilepsy, such as the hallmark hippocampus sclerosis. Animal models are effective for differentiating the different stages of epileptogenesis. Neuroimaging in experimental epilepsy provides unique information about anatomic, functional, and metabolic alterations linked to epileptogenesis. Recently, several in vivo biomarkers for epileptogenesis have been investigated for characterizing neuronal loss, inflammation, blood-brain barrier alterations, changes in neurotransmitter density, neurovascular coupling, cerebral blood flow and volume, network connectivity, and metabolic activity in the brain. Magnetic resonance imaging (MRI) is a sensitive method for detecting structural and functional changes in the brain, especially to identify region-specific neuronal damage patterns in epilepsy. Positron emission tomography (PET) and single-photon emission computerized tomography are helpful to elucidate key functional alterations, especially in areas of brain metabolism and molecular patterns, and can help monitor pathology of epileptic disorders. Multimodal procedures such as PET-MRI integrated systems are desired for refractory epilepsy. Validated biomarkers are warranted for early identification of people at risk for epilepsy and monitoring of the progression of medical interventions.