Abnormalities of serotonergic neurotransmission in animal models of SUDEP

Abstract

Sudden unexpected death in epilepsy (SUDEP) is a devastating event, and both DBA/1 and DBA/2 mice have been shown to be relevant animal models for studying SUDEP. DBA mice exhibit seizure-induced respiratory arrest (S-IRA), leading to cardiac arrest and subsequent sudden death after generalized audiogenic seizures (AGSs). This sequence of terminal events is also observed in the majority of witnessed human SUDEP cases. Several pathophysiological mechanisms, including respiratory/cardiac dysfunction, have been proposed to contribute to human SUDEP. Several (but not all) selective serotonin (5-HT) reuptake inhibitors (SSRIs), including fluoxetine, can reversibly block S-IRA, and abnormal expression of 5-HT receptors is found in the brainstem of DBA mice. DBA mice, which do not initially show S-IRA, exhibit S-IRA after treatment with a nonselective 5-HT antagonist. These studies suggest that abnormalities of 5-HT neurotransmission are involved in the pathogenesis of S-IRA in DBA mice. Serotonergic (5-HT) transmission plays an important role in normal respiration, and DBA mice exhibiting S-IRA can be resuscitated using a rodent ventilator. It is important and interesting to know if fluoxetine blocks S-IRA in DBA mice by enhancing respiratory ventilation. To test this, the effects of breathing stimulants, doxapram, and 5,6,7,8-tetrahydropyrido[4,3-d]pyrimidine (PK-THPP) were compared with the effects of fluoxetine on S-IRA in DBA/1 mice. Although fluoxetine reduces the incidence of S-IRA in DBA/1 mice, as reported previously, the same dose of fluoxetine fails to enhance baseline respiratory ventilation in the absence of AGSs. Doxapram and PK-THPP augment the baseline ventilation in DBA/1 mice. However, these breathing stimulants are ineffective in preventing S-IRA in DBA/1 mice. These data suggest that fluoxetine blocks S-IRA in DBA/1 mice by cellular/molecular mechanisms other than enhancement of basal ventilation. Future research directions are also discussed. This article is part of a Special Issue entitled "Genetic and Reflex Epilepsies, Audiogenic Seizures and Strains: From Experimental Models to the Clinic".

Keywords: 5-HT receptors; DBA mice; Fluoxetine; Respiratory arrest; SSRI; Serotonin.

Figure 1. The SSRI fluoxetine reduces S-IRA in DBA/1 mice

Systemic administration of fluoxetine, an SSRI, decreased the incidence of S-IRA in DBA/1 mice at doses that are comparable those that were previously used to establish SSRIs as potential antidepressants in rodents [45]. White bars, vehicle control; black bars, the incidence of S-IRA 30 min after fluoxetine administration; gray bars, recovery (the animals became susceptible to S-IRA again after washout of the drug) of S-IRA 24–72 hr after fluoxetine. N, number of mice tested. * Significantly different from control at p < 0.05. From [31] with permission.

Figure 2. The abnormal expression of 5-HT_2B and 5-HT_2C receptors in the brainstem of DBA/2 mice

Compared with seizure-resistant C57BL/6J mice, the expression of 5-HT_2B receptors was elevated, and that of 5-HT_2C receptors was reduced in the caudal brainstem of DBA/2 mice. These results may partly explain the differential effect of mCPP, a 5-HT_2B/2C receptor agonist, on the incidence of S-IRA in DBA/1 and DBA/2 mice. The error bars represent SDs. * Significantly different from C57BL/6J mice at p < 0.05. From [42] with permission.

Figure 3. Fluoxetine did not increase basal breathing and ventilatory response to CO₂ in anesthetized and conscious DBA/1 mice

A, representative traces of minute ventilation (V_E), respiratory frequency (f_R) and tidal volume (V_T) from anesthetized DBA/1 mice treated with fluoxetine (30 mg/kg) or vehicle, in room air or exposure to air + 7% CO₂. Data were normalized to the average V_E, f_R or V_T baseline value. Traces between the two dotted lines indicate the exposure time to 7% CO₂ gas mixture. B, effects of fluoxetine on the normalized V_E, f_R and V_T in room air and in air + 7% CO₂ in anesthetized DBA/1 mice using nose-only plethysmography. C, effects of fluoxetine on the normalized V_E, f_R and V_T in room air and in air + 7% CO₂ in conscious DBA/1 mice using whole-body plethysmography. The error bars represent SEMs. * p<0.05; ** p<0.01: significantly different from corresponding vehicle control. From [41] with permission.

Figure 4. Doxapram and PK-THPP stimulate basal breathing and increase ventilatory response to CO₂ in anesthetized DBA/1 mice

A, representative traces of minute ventilation (V_E), respiratory frequency (f_R) and tidal volume (V_T) from anesthetized DBA/1 mice treated with doxapram (50 mg/kg), PK-THPP (10 mg/kg) or their corresponding vehicles, in room air or exposure to air + 7% CO₂ using nose-only plethysmography. Data were normalized to the average V_E, f_R or V_T baseline value. Traces between the two dotted lines indicate the exposure time to 7% CO₂ gas mixture. B, effects of doxapram and PK-THPP on the normalized V_E, f_R and V_T in room air and in air + 7% CO₂ in anesthetized DBA/1 mice. The error bars represent SEMs. * p<0.05; ** p<0.01: significantly different from corresponding controls. From [41] with permission.