Bradycardia in serotonin-deficient Pet-1-/- mice: influence of respiratory dysfunction and hyperthermia over the first 2 postnatal weeks.
Neonatal rodents deficient in medullary serotonin neurons have respiratory instability and enhanced spontaneous bradycardias. This study asks if, in Pet-1(-/-) mice over development: 1) the respiratory instability leads to hypoxia; 2) greater bradycardia is related to the degree of hypoxia or concomitant hypopnea; and 3) hyperthermia exacerbates bradycardias. Pet-1(+/+), Pet-1(+/-), and Pet-1(-/-) mice [postnatal days (P) 4-5, P11-12, P14-15] were held at normal body temperature (T(b)) and were then made 2 degrees C hypo- and hyperthermic. Using a pneumotach-mask system with ECG, we measured heart rate, metabolic rate (Vo(2)), and ventilation. We also calculated indexes for apnea-induced hypoxia (total hypoxia: apnea incidence x O(2) consumed during apnea = microl.g(-1).min(-1)) and bradycardia (total bradycardia: bradycardia incidence x magnitude = beats missed/min). Resting heart rate was significantly lower in all Pet-1(-/-) animals, irrespective of T(b). At P4-5, Pet-1(-/-) animals had approximately four- to eightfold greater total bradycardia (P < 0.001), owing to an approximately two- to threefold increase in bradycardia magnitude and a near doubling in bradycardia incidence. Pet-1(-/-) animals had a significantly reduced Vo(2) at all T(b); thus there was no genotype effect on total hypoxia. At P11-12, total bradycardia was nearly threefold greater in hyperthermic Pet-1(-/-) animals compared with controls (P < 0.01). In both genotypes, bradycardia magnitude was positively related to the degree of hypopnea (P = 0.02), but there was no genotype effect on degree of hypopnea or total hypoxia. At P14-15, genotype had no effect on total bradycardia, but Pet-1(-/-) animals had up to seven times more total hypoxia (P < 0.001), owing to longer and more frequent apneas and a normalized Vo(2). We infer from these data that 1) Pet-1(-/-) neonates are probably not hypoxic from respiratory dysfunction until P14-15; 2) neither apnea-related hypoxia nor greater hypopnea contribute to the enhanced bradycardias of Pet-1(-/-) neonates from approximately P4 to approximately P12; and 3) an enhancement of a temperature-sensitive reflex may contribute to the greater bradycardia in hyperthermic Pet-1(-/-) animals at approximately P12.
Cummings
Kevin J
KJ
Dartmouth Medical School, 1 Medical Center Drive, Lebanon, NH 03766, USA.
Li
Aihua
A
Deneris
Evan S
ES
Nattie
Eugene E
EE
D000818:Animals; D000831:Animals, Newborn; D001049:Apnea / Q000235:genetics / Q000473:pathology / Q000503:physiopathology*; D049628:Body Size / Q000502:physiology; D001919:Bradycardia / Q000235:genetics / Q000473:pathology / Q000503:physiopathology*; D001933:Brain Stem / Q000002:abnormalities; D004195:Disease Models, Animal; D005260:Female; D005334:Fever / Q000235:genetics / Q000473:pathology / Q000503:physiopathology*; D005838:Genotype; D006339:Heart Rate / Q000502:physiology; D006801:Humans; D007223:Infant; D008297:Male; D008636:Mesencephalon / Q000002:abnormalities; D051379:Mice; D008817:Mice, Mutant Strains; D012131:Respiratory Insufficiency / Q000235:genetics / Q000473:pathology / Q000503:physiopathology*; D012701:Serotonin / Q000172:deficiency*; D013398:Sudden Infant Death; D014157:Transcription Factors / Q000235:genetics* / Q000378:metabolism
C484684:Fev protein, mouse; D014157:Transcription Factors; D012701:Serotonin