Genomic Tryptophan Hydroxylase 2 (Tph2) Mutation in Rats Increases Mortality and Disrupts Eupneic Breathing During Postnatal Development

Abstract

Multiple genetically-modified mouse models which induce serotonergic (5-HT) system dysfunction or neuron deletion have major effects on growth and breathing control, particularly during development. However, it remains unclear what the effects of selective brain 5-HT loss compared to neuron loss/dysfunction are on eupneic breathing and ventilatory chemoreflexes during development or adulthood. Thus, we testes the hypothesis that Tph2 mutation in the Dark Agouti (DA) rat (DATph2-/-) would lead to failure of CNS 5-HT synthesis, reduced growth, increased apnea and post-natal mortality and altered chemoreflexes. Truncation mutations generated two mutant Tph2 lines (M2 and M3) of Dahl S (SS) rats. HPLC measurements of multiple CNS tissues showed both M2 and M3 DATph2-/- mutant lines failed to produce 5-HT despite normal levels of norepinephrine and dopamine. Wild type and DATph2-/- rats both showed robust expression of the 5-HT biosynthetic enzyme amino acid decarboxylase in the medullary raphe, but TPH2 immunoreactivity was only found in wilt type rats. Eupneic ventilation was reduced coincident with an increased estimated mortality 0-4 days and 10-14 days of age in DATph2-/- rats, and body weights lagged throughout development. Resting breathing frequency was also reduced (~14%) in adult DATph2-/- rats, but there was no measurable effect on the ventilatory response to hypercapnia in mutant rats. These data suggest that CNS 5-HT deficiency in rats leads to high mortality and severe ventilatory disruption during development but has little or no effect on breathing control as adults.