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

Abstract

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; 18F‐flumazenil (18F‐FMZ; GABAA receptor), 18F‐FPEB (metabotropic glutamate receptor 5; mGluR5), 18F‐flutriciclamide (translocator protein; TSPO, microglia activation) and 18F‐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, 18F‐FMZ and 18F‐FPEB showed an early decrease after SE. 18F‐FMZ decrease was maintained in the latent phase and further reduced in the chronic epileptic animals, while 18F‐FPEB signal recovered from day 10, reaching baseline levels in chronic epilepsy. 18F‐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, 18F‐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 GABAA receptors, or microglia and astrocyte activation, in order to identify important interactions, especially when using PET imaging for the evaluation of novel treatments.

99mTc-HMPAO SPECT imaging reveals brain hypoperfusion during status epilepticus

“…This early neuronal loss may be associated with the reduction of blood ow acutely after SE. In addition, we have previously shown early reduction in neurotransmitter receptor density and amino acid metabolism 24-48 hours after SE [4,5], which may be at least partially unchained by the hypo-perfusion-induced neuronal death as seen by 99m Tc-HMPAO SPECT results obtained from the present study.…”

“…Among animal models, the rat lithiumpilocarpine model is one of the most widely used. After pilocarpine-induced SE, rats show BBB impairment, neuroin ammation, neuronal loss, reactive gliosis (affecting both microglia and astroglia), axonal sprouting and neurotransmitter imbalance among other alterations, nally leading to the development of chronic epilepsy [4][5][6].…”

In vivo hippocampal cornu ammonis 1–3 glutamatergic abnormalities are associated with temporal lobe epilepsy surgery outcomes

“…2,4 In addition, it may have a role in epileptogenesis, as a sustained reduction in allosteric mGluR5 availability in bilateral hippocampi and amygdala was demonstrated in rats during the latent period following pilocarpine-induced status epilepticus. 5,6 Thus, evidence is emerging of a potential role for mGluR5 as a biomarker for epileptogenicity and as a target for pharmacological interventions to treat or prevent epilepsy. 7,8 Over the past years, positron emission tomography (PET) studies with [ 11 C]ABP688, a radioligand with high affinity for the allosteric transmembrane site of mGluR5, have provided in vivo information of whole-brain mGluR5 availability in healthy and diseased individuals.…”

“…Whereas mGluR5 is involved in physiological processes such as learning and memory, in epilepsy, it has been shown to participate in the induction of ictal events 2,4 . In addition, it may have a role in epileptogenesis, as a sustained reduction in allosteric mGluR5 availability in bilateral hippocampi and amygdala was demonstrated in rats during the latent period following pilocarpine‐induced status epilepticus 5,6 . Thus, evidence is emerging of a potential role for mGluR5 as a biomarker for epileptogenicity and as a target for pharmacological interventions to treat or prevent epilepsy 7,8 …”

Fingolimod as a Treatment in Neurologic Disorders Beyond Multiple Sclerosis

“…Furthermore, 30-40% of patients with epilepsy are partially or fully resistant to the therapy [63]. In recent years, research has focused on understanding the processes leading to epilepsy, i.e., epileptogenesis, to better understand the pathophysiology of this disorder and has identified neuroinflammation, neurotransmitter dysregulation, and neuronal loss, among others, as potential key actors in epilepsy pathogenesis [64][65][66][67][68].…”