Rationale: We have previously shown that intranasal (IN) administration of leptin augments the hypercapnic ventilatory response (HCVR) and attenuates inspiratory flow limitation, thereby improving obesity hypoventilation in mice with diet-induced obesity (DIO). We also demonstrated that the respiratory effects of leptin are mediated by neurons of the dorsomedial hypothalamus projecting to the dorsal and medullary raphe nuclei. We hypothesized that intranasal leptin augments HCVR via the 5-HT pathway, and that these effects would be abolished by the 5-HT 2 A/2C receptor blocker pimavanserin or by the elimination of 5-HT neurons projecting to the hypoglossal nucleus. Methods: We used 29 Sert-Flp male mice, in which FlpO recombinase is expressed under the serotonin transporter promoter with DIO (45-50 g of weight). In Experiment 1, we conducted a randomized crossover trial comparing a single dose of IN leptin (0.8 mg/kg) vs. vehicle and subcutaneous pimavanserin (3 mg/kg) vs. placebo in 15 mice (four treatment combinations: leptin + pimavanserin, leptin + placebo, vehicle + pimavanserin, vehicle + placebo), each administered one week apart. Minute ventilation (VE) was measured during quiet wakefulness at normocapnia and 8% CO 2 in a thermoneutral environment (28-30°C) 30 minutes after each treatment. HCVR was calculated as the slope of VE versus inspired CO 2 using linear least-squares regression. In Experiment 2, a separate group of DIO Sert-Flp mice received stereotactic injections into the hypoglossal nucleus of either retrograde AAV harboring Flp-dependent caspase (n = 7) or a retrograde AAV-GFP control virus (n = 7). Respiratory measurements were performed 4 weeks after viral transfection in a cross-over randomized fashion (leptin vs vehicle) one week apart. Results: In Experiment 1, leptin did not affect VE under normocapnia but increased minute ventilation under hypercapnic conditions in the absence of pimavanserin. Pimavanserin alone had no effect on ventilation at normocapnia but markedly attenuated the HCVR regardless of leptin treatment. The combination of leptin and pimavanserin significantly suppressed normocapnic breathing. In Experiment 2, control virus-treated mice showed abundant 5-HT + neurons in the medullary raphe (MR) and in the dorsal raphe (DR). Many 5-HT+ MR neuronal bodies were GFP+ suggesting monosynaptic projections to the hypoglossal nucleus. In contrast, a very few 5-HT+ DR neuronal bodies were GFP+, but we observed abundant GFP+ fibers enmeshing DR neurons Caspase treatment eliminated 5-HT + neurons in MR, but not in DR. As in Experiment 1, leptin did not affect baseline ventilation but increased hypercapnic ventilation and HCVR in mice treated with the control virus. Elimination of 5-HT + neurons projecting to the hypoglossal nucleus abolished leptin’s effects on HCVR. Conclusion: Intranasal leptin augments HCVR acting on the 5-HT+ MR neurons monosynaptically projecting to the hypoglossal nucleus. Grant support: R01 HL128970, R01 HL174409 This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.
Ruiz et al. (Fri,) studied this question.