title: Brainstem serotonin deficiency in the neonatal period: autonomic dysregulation during mild cold stress.
abstract: Based on previous studies in adult animals, devoid of 5-HT neurones, showing altered thermoregulation in cold stress (4°C) and a reduced ventilatory response to CO₂, we hypothesized that neonatal mice lacking 60-70% of their 5-HT neurones (Pet-1(-/-)) would have: (1) a reduced thermogenic response to a mild drop in ambient temperature (TA), (2) reduced V(E) and heart rate (HR) responses to mild cooling that reflect this reduced thermogenic response, and (3) a reduced ventilatory response to CO₂ after postnatal day 12 (P12), when 5-HT neurones become chemosensitive in vitro. We first determined that a 60-70% loss of 5-HT-positive neurones results in a ~90% loss of 5-HT from the brainstems of Pet-1(-/-) animals. We then subjected Pet-1(-/-) and wild-type (WT) mice (N = 5) to mild environmental cooling (T(A) = 29°C) at ~P12. T(A) was initially held at 34°C for ~20 min, reduced to 29°C over 15 min and held for an additional 10 min at steady state, and then returned to 34°C. From 34°C to 29°C, there was a robust increase in V(O₂) in P12WT, but not Pet-1(-/-) animals (68±19.9% versus -16±8%, respectively; P = 0.002). On average, body temperature (T(B)) dropped 1.1°C more in Pet-1(-/-) compared to WT animals (P = 0.03). HR remained unchanged in WT but dropped 22±2.3% in Pet-1(-/-) animals (P = 0.01). Genotype had no effect on tail temperature (T(T)), either at 34°C or 29°C. After cooling, values for V(O₂) and HR of Pet-1(-/-) animals were no different to values predicted by Q₁₀ effects alone, while values of WT animals were greater than predicted. V(E) increased in WT with cooling, while it decreased in Pet-1(-/-) animals (P = 0.002). Still, Pet-1(-/-) animals hyperventilated relative to WT (increased V(E)/V(O₂)) irrespective of T(A) (P = 0.002). As tested in a separate group of pups, there was no difference in the ventilatory response to CO₂ between WT and Pet-1(-/-) animals, either at P5 or P15. We conclude that during neonatal life in mouse pups: (1) brainstem 5-HT is critical for the thermogenic response to a mild drop in environmental temperature probably via a sympathetically-mediated increase in brown fat metabolism; (2) reduced thermogenesis probably contributes to the reduced HR and V(O₂) observed with 5-HT deficiency; and (3) the presence of some brainstem 5-HT is sufficient for an appropriate ventilatory response to hypercapnia up until P15. Infants with reduced brainstem 5-HT could be prone to cardiovascular and respiratory abnormalities resulting from compromised thermogenesis.
authors.affiliation: Department of Physiology and Neurobiology, Dartmouth Medical School, Lebanon, NH 03756, USA. crumbings@gmail.com
authors.forename: Kevin J
authors.initials: KJ
authors.lastname: Cummings
authors.forename: Aihua
authors.initials: A
authors.lastname: Li
authors.forename: Eugene E
authors.initials: EE
authors.lastname: Nattie
mesh_terms: D000367:Age Factors; D000704:Analysis of Variance; D000818:Animals; D000831:Animals, Newborn; D001341:Autonomic Nervous System / Q000503:physiopathology*; D001833:Body Temperature Regulation / Q000235:genetics; D001933:Brain Stem / Q000254:growth & development / Q000378:metabolism*; D003080:Cold Temperature; D058639:Cold-Shock Response / Q000235:genetics; D005838:Genotype; D006339:Heart Rate; D006935:Hypercapnia / Q000378:metabolism / Q000503:physiopathology; D007035:Hypothermia / Q000235:genetics / Q000378:metabolism / Q000503:physiopathology*; D051379:Mice; D018345:Mice, Knockout; D010641:Phenotype; D012123:Pulmonary Ventilation; D012701:Serotonin / Q000172:deficiency*; D014157:Transcription Factors / Q000172:deficiency / Q000235:genetics
chemical_list: C484684:Fev protein, mouse; D014157:Transcription Factors; D012701:Serotonin