Time- and age-dependent effects of serotonin on gasping and autoresuscitation in neonatal mice

Abstract

The role of brain stem serotonin (5-hydroxytryptamine, 5-HT) in autoresuscitation in neonatal life is unclear. We hypothesized that a specific loss of 5-HT would compromise gasping and autoresuscitation mainly in the second postnatal week and that acute restoration of 5-HT would reverse the defects. We exposed postnatal day (P)4–5, P8–9, and P11–12 tryptophan-hydroxylase-2 knockout ( TPH2−/−) and wild-type littermates (WT) to 10 episodes of anoxia (97% N2, 3% CO2), measuring survival, gasp latency, gasp frequency ( fB), and the time required to restore eupnea and heart rate. We also tested P8–9 TPH2−/− mice after restoring 5-HT with a single injection of 5-hydroxytryptophan (5-HTP) 1–2 h before testing or with multiple injections beginning 24 h before testing. At P4–5 and P8–9, but not at P11–12, gasp latency and the recovery of eupnea were delayed ∼2- to 3-fold in TPH2−/− pups compared with WT ( P < 0.001). At all ages, TPH2−/− pups displayed reduced gasp fB (∼20–30%; P < 0.001) and delayed heart rate recovery (∼60%; P = 0.002) compared with WT littermates. TPH2−/− survival was reduced compared with WT ( P < 0.001), especially at P8–9 and P11–12 ( P = 0.004). Whereas 1–2 h of 5-HTP treatment improved the gasp latency and fB of P8–9 TPH2−/− pups, improved cardiorespiratory recovery and survival required 24 h of treatment. Our data suggest that 5-HT operates over a long time span (∼24 h) to improve survival during episodic severe hypoxia. Early in development (P4–9), 5-HT is critical for both respiratory and cardiovascular components of autoresuscitation; later (P11–12), it is critical mainly for cardiovascular components. Nevertheless, the effect of 5-HT deficiency on survival is most striking from P8 to P12.