The Neonatal Niche

1. The Neonatal Niche Neonates face problems not faced by adults Unique to neonates because of their size, anatomy and physiology Neonates adopt their own strategies to cope with these unique problems Coping strategies involve both behavior and physiology

2. Thermoregulation in Altricial Neonates

3. Thermoregulation Homeothermy—the physiological and behavioral maintenance of a relatively constant internal body temperature (homeotherms typically show circadian fluctuations in temperature) Endothermy—physiological and behavioral thermoregulation, but body temperatures may fluctuate widely. Ectothermy—use only behavioral means to regulate temperature. Poikilothermy—the fluctuation of internal body temperature closely related to environmental temperature. Cold blooded Warm blooded poikilothermy ectothermy endothermy homeothermy

4. Two aspects of thermoregulation 1. Heat gain volume of thermogenic tissue basal metabolic rate shivering or other thermogenesis 2. Heat loss surface area insulation—fat + feathers or hair panting , sweating, etc

5. Receptors for Temperature Peripheral thermoreceptors are found in the skin Central thermoreceptors are found in the anterior hypothalamus These thermoreceptors are important for behavioral and physiological thermoregulation both in the short term and in the long term.

6. Short- and Long-term Thermoregulation Short term thermoregulation is predominantly regulated by autonomic and somatic motor activity • Autonomic • shivering when cold, sweating when hot • peripheral vasodilaton when hot, constriction when cold • piloerection and panting in non-human mammals • Somatic • seeking warm (e.g. sunny) or cool (e.g. shade or water) areas • minimizing or maximizing surface area

7. Short- and Long-term Thermoregulation Long term (e.g. seasonal or as adaptation to different climates) thermoregulation is predominantly regulated by hormonal regulation of metabolism. Thyrotrophin releasing hormone (TRH) is secreted from the hypothalamus to stimulate release of thyroid-stimulating hormone (TSH, also thyrotrophin), which stimulates release of thyroxine from the thyroid gland. Tissues respond with an increase in their basal metabolic rate.

8. Autonomic and hormonal responses to thermal challenges are mediated primarily by the medial preoptic area. Behavioral responses to thermal challenges are mediated principally by the lateral hypothalamus. Maintenance of thermal homeostasis is arguably the greatest factor influencing both energy balance (via calories spent for thermogenesis and the need for fat stores) and water and mineral balance [water lost via evaporation through the skin, metabolic processes necessary for homeothermy and (in many non-humans)], panting.

9. Mechanisms of Thermoregulation in Adult Mammals Thomas, K. (1994) Thermoregulation in neonates. Neonatal Network, 13, 15-22.

10. Special Problems Faced by Altricial Neonates 1. Sensory and Motor Immaturity 2. CNS Immaturity 3. Physiological Immaturity e.g. autonomic control, homeostatic regulation 4. Morphological Immaturity e.g., small size, allometric growth

11. Thermoregulatory Problems of Altricial Mammals 1. cannot shiver 2. has precarious energy balance 3. small size 4. cannot vasoconstrict 5. no hair 6. little body fat, thin skin

12. The Problem with Being Small Linear 1 2 3 4 5 Surface 1 4 9 16 25 Volume 1 8 27 64 125 S:V = linear dimension 2/3

13. Effects of Growth on Surface:Volume Ratios, Rat age wt.(g) vol.(cm3) area (cm2 ) s:v s:v, x adult adult 400 400 328.5 0.8:1 1.0 60 day 200 200 206 1.1 1.25 10 day 30 30 58 2:1 2.5 newborn 5 5 17.5 3.5:1 4.4

14. Brown fat accounts for 5 to 6 percent of the body weight of the newborn rabbit. It is concentrated, as shown in sections, around the neck and between the shoulder blades. Human infant at birth has a thin sheet of brown adipose tissue between the shoulder blades and around the neck, and small deposits behind the breastbone and along the spine. Dawkins, M.J.R. & Hull, D (1965) The production of heat by fat. Scientific American, 213, 62-67.

17. Transection at level 3 Bignall, K.E., Heggeness, F.W. & Palmer, J.E. (1975) Experimental Neurology, 49, 174-188.

19. Taylor, P.M. (1960) Oxygen consumption in new-born rats. Journal of Physiology, 154, 153-168.

22. Behavior of rat-shaped robots programmed to move randomly without sensory controls

23. Cold Hot 37o C Cold Hot 37o C

24. ThermoregulationSummary Problem: Cannot shiver to produce heat Solution: Brown adipose tissue (BAT) Problem: Precarious energy balance Solution: Thermoregulate over narrow range Shut down to conserve energy Tolerate lowered temperature Problem: Small size, large surface:volume ratio Solution: Clump to reduce surface:volume ratio Problem: Poor insulation Solution: Clump, nest insulation, thermotaxis Also: Ultrasound in response to isolation

25. ThermoregulationSummary Non-shivering thermogenesis (BAT thermogenesis) Epiphenomenon? Neonatal adaptation? Precursor to adult thermogenesis? Not for most mammals, but could be considered a precursor for hibernators Is huddling a social behavior?

27. Factors Contributing to Neonatal Vulnerability to Thermal Stress • Heat Production • The ability to increase metabolic rate in response to cold stress begins around 28—30 weeks postconceptional age. Postconceptionally older infants can increase heat production, but the response is weaker than in the adult. 4’16 • Limited stores of metabolic substrates (glucose, glycogen, fat, etc.). • Heat production needs met primarily through non-shivering thermogenesis; however, the amount of brown fat stores are inversely related to gestational age. • Heat production obligates oxygen consumption, challenging the immature cardiovascular and pulmonary systems. • Large surface-to-mass ratio and large surface heat loss relative to heat-producing ability result in high metabolic rate. • Large evaporative loss due to status of skin maturation; evaporative loss itself may exceed heat production abilities.11’23 • Shivering response not well developed. Cannot initiate increased tone and shivering to increase heat production. • Insulation • Limited layer of subcutaneous fat, limited development of muscle and other tissues that provide insulation.4 • Small body diameter results in thinner layer of still air boundary layer, reducing insulation through this mechanism. • Vasomotor Response • Competent abilities to regulate skin blood flow documented in infants weighing >1 kg; however vasaconstriction abilities are outmatched by propensity for heat loss.4 • Sudomotor Response • Sweat production observed in infants of 29 weeks gestational age; maturation of response enhanced by extrauterine development. Response is slower and less efficient than in older child or adult, and occurs at a higher environmental temperature.24 • Motor Tone and Activity • Lower postconceptional aged and ill infants prone to decreased motor tone and less activity, resulting in decreased heat production. Infant with poor tone cannot use flexion posture effectively to reduce surface area and hence heat loss. • Behavioral • Limited ability to effectively communicate thermal needs or thermal comfort to caregiver. • Cues are subtle and nonspecific. • Cannot use volitional actions such as altering clothing, increasing ambient temperature, using motor activity to increase heat production, drinking warm or cool beverages to modify temperature. Thomas, K. (1994) Thermoregulation in neonates. Neonatal Network, 13, 15-22.

28. Infant thermoregulation rostral inhibition (NOT hypothalamus) brainstem activation Blood glucose, Thermoreceptors 1. Brown adipose tissue (BAT; brown fat) Feedback Adult thermoregulation Thomas, K. (1994) Thermoregulation in neonates. Neonatal Network, 13, 15-22.