Rationale: Respiratory dysfunction—particularly central apnea accompanied by cardiac abnormalities—has been proposed as a major contributor to Sudden Unexpected Death in Epilepsy (SUDEP). Apnea in SUDEP is frequently characterized by prolonged hypercapnia and severe hypoventilation. In this pilot study, our aim was to investigate the hypercapnic ventilatory response and activation of neuropeptide-expressing neurons in a chronic epilepsy model characterized by tonic-clonic seizures. We focused on the retrotrapezoid nucleus (RTN), a key chemoreceptive site involved in CO₂ detection and respiratory drive. Methods: Adult Wistar rats were treated with kainic acid (KA; ~5 months post-injection, exhibiting spontaneous seizures) or saline (controls). Ventilatory frequency (fB) and heart rate (HR) were measured interictally using photoplethysmography before, during, and after a 1-hour exposure to acute hypercapnia (AH: 10% CO₂). Data were analyzed via two-way ANOVA followed by Fisher’s LSD post hoc test. After the AH or exposure to room air (RA: 0% CO₂), brainstem tissue was collected for in situ hybridization targeting preprogalanin (ppGal) and immunohistochemistry for c-Fos (a marker of neuronal activation), tyrosine hydroxylase (TH), and Phox2b. RTN neurons were identified as Phox2b⁺/TH⁻, and galaninergic RTN neurons were defined as ppGal⁺/Phox2b⁺/TH⁻. Results: KA-treated rats showed significantly attenuated ventilatory and HR responses to CO₂ compared to controls (fB: p < 0.01; HR: p < 0.0001), with minimal change from baseline. Notably, the baseline activation of galaninergic RTN neurons was substantially higher in epileptic animals (KA: 32.6±5%) compared to healthy controls (RA: 11.2±2%). This elevated activation was further amplified following hypercapnic exposure in epileptic rats (KA+AH: 59.2±6%) compared to healthy rats (RA+AH: 33.9±5%), suggesting a hyper-responsive phenotype. Conclusions: These findings demonstrate that the hypercapnic ventilatory response is impaired in KA-treated rats with tonic-clonic seizures, despite increased activation of galaninergic RTN neurons. This may reflect either (1) a compensatory upregulation of chemoreceptive activity in response to suppressed respiratory output, or (2) a direct inhibitory effect of galanin contributing to the blunted ventilatory drive. Such maladaptive plasticity could underlie respiratory dysfunction and heightened SUDEP risk in epilepsy. Given the RTN’s critical role in respiratory homeostasis and the prolonged modulatory effects of neuropeptides like galanin, our results suggest a novel mechanism for seizure-induced apnea. Further studies are warranted to explore the broader RTN and chemoreceptor network to better understand this response and guide development of neuropeptide-based therapeutic strategies.
Dereli et al. (Wed,) studied this question.