Abstracts

Rationale: Sudden Unexpected Death in Epilepsy (SUDEP) is a leading cause of mortality in patients with epilepsy and is estimated to account for up to 17% of all deaths in this population. Although the pathophysiological mechanisms that lead to SUDEP remain poorly understood, data implicate cardiorespiratory dysfunction following a generalized tonic-clonic seizure (GTCS) as a possible underlying cause.  Serotonin (5-hydroxytryptamine; 5-HT) is a neurotransmitter that has diverse functions within the central nervous system, including: arousal, circadian rhythms, central chemoreception, and respiratory control. In addition, it acts as an anticonvulsant and reduces seizure risk in a variety of epileptic animal models, including preventing respiratory arrest following an audiogenic seizure in one mouse strain. Methods: We studied a transgenic mouse in which transcription of the X-linked 5-HT_2C receptor is blocked. Previous studies indicate that 5-HT_2C-null mice die prematurely and some deaths were witnessed following a GTCS. Anecdotal reports suggest that postictal respiratory arrest is the cause of death; however, cardiorespiratory monitoring was not performed and the epileptic profile of these mice remains poorly understood. 5-HT_2C mutant and wild type (WT) littermates were implanted with silver-wire recording leads bilaterally over frontal and parietal cortex and two reference electrodes over the olfactory bulbs as previously described in our laboratory. Electroencephalogram (EEG) data were collected, amplified, and analyzed using a PowerLab 16/35 system and LabChart Pro software from AD Instruments Inc. Results: We observed that 39.18% of our male 5-HT_2C-nullmice (29/74) died prematurely compared to only 1.20% of male WT mice (1/83; p < 0.001). However, premature mortality was not as high in female 5-HT_2C mutant mice. We observed premature death in 5.80% of female hemizygous (4/69) and 9.52% of female 5-HT_2C-null (6/63) compared with 5.26% of female WT mice (1/19). In almost every case of an unwitnessed death, mutant mice were found with hindlimb extension suggesting a GTCS seizure occurred immediately prior to death. Neither of the two WT mice that died prematurely had hindlimb extension and their deaths were attributable to non-epileptic causes. Using simultaneous video-EEG recordings, we found that male and female 5-HT_2C-null mice and 5-HT_2C hemizygous female mice showed three different spontaneous seizure phenotypes. First, we observed spike-wave discharges (SWDs) that were similar to seizures seen in absence epileptic mouse models. The second seizure phenotype was a generalized nonconvulsive seizure which showed little behavioral activity. The final seizure type was a GTCS, in which death was recorded in one animal. We were able to provoke audiogenic seizures in 5-HT_2C-null mice and these seizures were always fatal. However, the EEG recording showed no aberrant cortical activity, which suggests these seizures were subcortical, and possibly in the brainstem. Conclusions: These data demonstrate that 5-HT_2C mutant mice have a novel SUDEP phenotype that is adult-onset, as opposed to current genetic SUDEP models which have a high mortality rate in the first few weeks of life. In addition, these mice have a complex epileptic phenotype with four different adult-onset seizures observed. Moreover, although seizures were observed in female hemizygous and 5-HT_2C-null mutant mice, premature mortality in these mice was much lower than male 5-HT_2C-null littermates. Future experiments will investigate the role of 5-HT_2C receptors in central nervous system excitability in forebrain and brainstem regions and define the mechanisms leading to seizure-induced death in these mice. Funding: NINDS U01 NS090340 and F32 NS105329