Chronic central serotonin depletion attenuates ventilation and body temperature in young but not adult Tph2 knockout rats

Abstract

Genetic deletion of brain serotonin (5-HT) neurons in mice leads to ventilatory deficits and increased neonatal mortality during development. However, it is unclear if the loss of the 5-HT neurons or the loss of the neurochemical 5-HT led to the observed physiologic deficits. Herein, we generated a mutant rat model with constitutive central nervous system (CNS) 5-HT depletion by mutation of the tryptophan hydroxylase 2 (Tph2) gene in dark agouti (DA(Tph2-/-)) rats. DA(Tph2-/-) rats lacked TPH immunoreactivity and brain 5-HT but retain dopa decarboxylase-expressing raphe neurons. Mutant rats were also smaller, had relatively high mortality (∼50%), and compared with controls had reduced room air ventilation and body temperatures at specific postnatal ages. In adult rats, breathing at rest and hypoxic and hypercapnic chemoreflexes were unaltered in adult male and female DA(Tph2-/-) rats. Body temperature was also maintained in adult DA(Tph2-/-) rats exposed to 4°C, indicating unaltered ventilatory and/or thermoregulatory control mechanisms. Finally, DA(Tph2-/-) rats treated with the 5-HT precursor 5-hydroxytryptophan (5-HTP) partially restored CNS 5-HT and showed increased ventilation (P < 0.05) at a developmental age when it was otherwise attenuated in the mutants. We conclude that constitutive CNS production of 5-HT is critically important to fundamental homeostatic control systems for breathing and temperature during postnatal development in the rat.

Active sleep unmasks apnea and delayed arousal in infant rat pups lacking central serotonin

“…Our study confirms the findings of Kaplan and colleagues, who first described apnea in TPH2Ϫ/Ϫ pups (22). Our novel finding is that 5-HT deficiency leads to apnea in AS but not in QS.…”

“…Our data indicate that a loss of central 5-HT leads to profound hypoventilation in infant rats across all states. Our findings extend those of Kaplan and colleagues, who first reported the reduced ventilation of TPH2Ϫ/Ϫ rat pups (22). Importantly, the present data indicate that the reduced breathing of TPH2Ϫ/Ϫ pups is not secondary to reduced metabolic drive; rather, they are hypoventilating, indicated by a considerably reduced V I-to-V CO 2 ratio.…”

“…As we recorded breathing and sleep patterns in unanesthetized WT and TPH2Ϫ/Ϫ pups~30 min after surgery for EMG implantation with isoflurane anesthesia, it is possible that the altered sleep architecture demonstrated by TPH2Ϫ/Ϫ pups is related to the residual effects of isoflurane. However, we note that the respiratory variables and HR we report for TPH2Ϫ/Ϫ pups are close to those reported previously for pups tested without prior anesthesia (9,22). With regard to the mechanism(s), cholinergic neurons of the laterodorsal (LDT) and pedunculopontine (PPT) tegmentii send projections to, and activate, glutamatergic neurons with the pontine reticular formation to initiate and maintain AS (5).…”

“…The generation of the TPH2Ϫ/Ϫ mutant rat lines on a Dark Agouti background has been described previously (22). Briefly, the rat TPH2 locus was targeted with zinc finger nucleases to generate a short (10 bp) deletion in exon 7.…”

“…Defects in serotonin (5-hydroxytryptamine; 5-HT) neurons including reduced 5-HT and its biosynthetic enzyme tryptophan hydroxylase 2 (TPH2) are consistently identified in SIDS cases (14,38). 5-HT-deficient neonatal rodents, like future SIDS cases, display more apnea than controls (10,22), suggesting a potential pathophysiological link between serotonergic defects and apnea in SIDS cases.…”