Abstracts

Rationale: Sudden unexpected death in epilepsy (SUDEP) is the leading cause of epilepsy-related mortality and is typically attributed to respiratory arrest. Seizures induce an acute surge in extracellular adenosine, which inhibits neuronal activity and contributes to seizure cessation. Deficits in the adenosinergic system can cause a failure of seizure termination and subsequent status epilepticus; however, excessive adenosine signaling suppresses breathing and has been implicated in SUDEP pathophysiology. A weakness of adenosinergic explanations of SUDEP is that seizure-induced adenosine surging has been documented in the cortex, hippocampus, and thalamus, but never in the brainstem, where breathing is controlled. Understanding forebrain and brainstem adenosine dynamics may provide novel insights into adenosine-based therapeutics for seizures, SUDEP, and refractory status epilepticus.


Methods: Monitoring of real-time changes in extracellular adenosine concentrations was achieved using G protein-coupled receptor-activation based (GRAB) adenosine sensors. Adult, male, C57BL6 mice underwent the adenosine GRAB sensor viral transduction and optic fiber implantation in the hippocampus and dorsal raphe nucleus (DRN). An additional cohort of mice underwent implantation of cortical EEG electrodes in addition to instrumentation for fiber photometry. Mice received 5 injections of kainic acid (5 mg/kg) at 10-minute intervals with concomitant recording of adenosine signaling, EEG, and breathing via whole-body plethysmography.


Results: We observed that kainic acid-induced status epilepticus results in high amplitude, punctate adenosine surges (minutes in duration) superimposed on a slow, but lasting surge in adenosine concentrations (hours in duration). We found that kainic acid treatment caused adenosine surging in the DRN in addition to the hippocampus. The punctate spikes in adenosine signaling were precipitated by brief, high-frequency bursts of epileptiform activity. Seizures causing the strongest surges in adenosine displayed a characteristic spectral profile characterized by attenuation in low-frequency neuronal activity and increases in high-frequency activity. Critically, as status epilepticus progressed, subsequent surges in adenosine signaling became less effective in terminating seizure activity, representing a failure of the adenosine system. Lastly, status epilepticus induced overall respiratory suppression.


Conclusions: Our data demonstrate, for the first time, that: (1) seizures evoke adenosine surging in the brainstem in addition to the forebrain, an essential assumption of adenosinergic explanations of SUDEP, (2) the progressive failure of adenosine surging to quell epileptiform activity during the active stage of status epilepticus, and (3) the progressive respiratory decline associated with a seizure-induced increase in brainstem adenosine tone. Improving our understanding of the effects of peri-ictal adenosine signaling may help reduce the incidence of seizure-induced death.


Funding: NIH NINDS (D.B.: NS065957, NS103740, NS NS127846; BP: F32NS117792), NIH-funded Rutgers INSPIRE IRACDA Postdoctoral Program (BP: #GM093854).