5‐Hydroxytryptophan, a precursor for serotonin synthesis, reduces seizure‐induced respiratory arrest

Abstract

Summary Objective The DBA/1 mouse is a relevant animal model of sudden unexpected death in epilepsy (SUDEP), as it exhibits seizure-induced respiratory arrest (S-IRA) evoked by acoustic stimulation, followed by cardiac arrhythmia and death. Defects in serotonergic neurotransmission may contribute to S-IRA. The tryptophan hydroxylase-2 (TPH2) enzyme converts L-tryptophan to 5-hydroxytryptophan (5-HTP), a precursor for central nervous system (CNS) serotonin (5-HT) synthesis; and DBA/1 mice have a polymorphism that decreases TPH2 activity. We, therefore, hypothesized that supplementation with 5-HTP may bypass TPH2 and suppress S-IRA in DBA/1 mice. Methods TPH2 expression was examined by Western blot in the brainstem of DBA/1 and C57BL/6J mice both with and without acoustic stimulation. Changes in breathing and cardiac electrical activity in DBA/1 and C57BL/6J mice that incurred sudden death during generalized seizures evoked by pentylenetetrazole (PTZ) were studied by plethysmography and electrocardiography. The effect of 5-HTP administration on seizure-induced mortality evoked by acoustic stimulation or by PTZ was investigated in DBA/1 mice. Results Repetitive acoustic stimulation resulted in reduced TPH2 protein in the brainstem of DBA/1 mice as compared with C57BL/6J mice. S-IRA evoked by acoustic stimulation in DBA/1 mice was significantly reduced by 5-HTP. Following S-IRA, cardiac electrical activity could be detected for minutes before terminal asystole and death in both DBA/1 and C57BL/6J mice after PTZ treatment. The incidence of S-IRA by PTZ administration was greater in DBA/1 than in C57BL/6J mice, and administration of 5-HTP also significantly reduced S-IRA by PTZ in DBA/1 mice. Significance Our data suggest that S-IRA is the primary event leading to death incurred in most DBA/1 and some C57BL/6J mice during PTZ-evoked seizures. Suppression of S-IRA by 5-HTP suggests that 5-HT transmission contributes to the pathophysiology of S-IRA, and that 5-HTP, an over-the-counter supplement available for human consumption, may be clinically useful in preventing SUDEP.

The effect of atomoxetine, a selective norepinephrine reuptake inhibitor, on respiratory arrest and cardiorespiratory function in the DBA/1 mouse model of SUDEP

“…Other studies have also demonstrated that S-IRA is the primary event causing sudden death following evoked generalized seizures in most cases (Buchanan et al, 2014; Faingold et al, 2010; Zhang et al, 2016). Therefore, S-IRA is a likely contributor to some SUDEP cases.…”

“…DBA/1 mice were originally purchased from Envigo (Indianapolis, IN) and were housed and bred in the animal facility at Massachusetts General Hospital under a temperature- and humidity-controlled environment (12-h light/dark cycle), provided with rodent food and water ad libitum . DBA/1 mice were “primed” by daily exposure to acoustic stimulation using an electric bell (96 dB SPL, UC4-150, Zhejiang People’s Electronics, China) for 3–4 days, starting from postnatal day 26–28, to establish consistent susceptibility to audiogenic seizures (AGSz) and S-IRA (Zhang et al, 2016). The same acoustic stimulus parameters were used in priming and induction of AGSz.…”

“…ECG recordings were performed in anesthetized DBA/1 mice as previously described (Zhang et al, 2016). Briefly, three subdermal needle electrodes (Model F-E2, Grass Instruments, Warwick, RI) were inserted into the skin across the thorax, and cardiac electrical activity was recorded using a P511 AC amplifier (Grass Technologies, Warwick, RI) with the following parameters: gain 20,000; bandpass filter 0.3–3,000 Hz.…”

“…Clinical and animal studies demonstrate that seizure-induced respiratory arrest (S-IRA) is the primary event leading to death after generalized tonic-clonic seizures in many cases (Bateman et al, 2008; Blum, 2009; Bravo et al, 2015; Buchanan et al, 2014; Faingold et al, 2010; Langan et al, 2000; Pezzella et al, 2009; Ryvlin et al, 2013; So et al, 2000; Zhang et al, 2016), although cardiac dysfunction may also contribute to seizure-induced sudden death (Frasier et al, 2016; Kalume et al, 2013). Previous studies implicate serotonergic and adenosinergic neurotransmission in the pathogenesis of S-IRA in animal models of SUDEP, including the DBA/1 mouse (Feng and Faingold, 2015; Richerson et al, 2016).…”

Serotonergic agents act on 5-HT 3 receptors in the brain to block seizure-induced respiratory arrest in the DBA/1 mouse model of SUDEP

“…The operational criteria for S-IRA were defined by the appearance of a deep respiratory gasp and relaxation of the pinnae determined visually, which were invariant indicators in previous studies that S-IRA had begun and death was imminent [8]. Although S-IRA was behaviorally determined by qualitative (visual) breathing assessment, our previous quantitative studies using plethysmography indicated that S-IRA leads to terminal asystole and death in DBA/1 mice [32]. Resuscitation was accomplished by placing the outflow polyethylene tube (4.4 mm external diam.)…”