Circadian Thermoregulation in Suckling Rabbit Pups

Abstract

The rabbit pup is well suited to track the age-dependent development of periodic thermoregulation during the suckling period. Since the litters are regularly nursed once per day for a total of 3 to 4 min, an exogenous, metabolic, nonphotic periodic variable is supposed to have an impact on the 24-h rhythm of body temperature. The authors monitored the course of core body temperature during the suckling period of 20 pups by means of a transmitter implanted intraperitoneally on day 3 postpartum. The 24-h mean rose from an average of 37.8+/-0.3 degrees C on day 4 of life to 39.5+/-0.2 degrees C at weaning on day 27, for 2 out of 20 pups, and day 28, for 18 out of 20 pups. In constant dim illumination, the pups exhibited a 24-h rhythm even on postnatal day 4, which consolidated around days 5 to 7. The rhythm consisted of a significant anticipatory rise of 0.4 to 0.6 degrees C above the respective 24-h mean commencing 2.5 to 3.5 h prior to nursing. Milk intake was followed by a further increase of temperature for an additional 0.3 to 0.6 degrees C. Then the temperature dropped for 1.2 to 1.5 degrees C within 1 to 3 h and returned to average 3 to 5 h later. During a 48-h fast, the rhythm continued to exist, though in a modified shape: the anticipatory component persisted almost unchanged; a further elevation of temperature, however, did not occur. Thus, the anticipatory component apparently is generated endogenously and the second surge represents an exogenous suckling-induced, thermogenic peak. When maternal nursing was advanced for 15 min/day for a total of 5 h, the temperature rhythm of the pups followed the shift of the zeitgeber in parallel. These data confirm the assumption that a circadian rhythm exists during the first postnatal days of the rabbit and that this rhythm is entrained by the 24-h nursing rhythm. The authors suggest that the biological significance of a feeding entrainable oscillator (FEO) in the rabbit might be to activate the pups prior to the periodic nursing visit of the rabbit doe. Thus, the pups are prepared to quantitatively use the one and only short nursing episode per day for maximal milk ingestion.

Endogenous clock gene expression in the suprachiasmatic nuclei of previsual newborn rabbits is entrained by nursing

“…We also found clear diurnal rhythmicity in body temperature consistent with previous reports for somewhat older pups (Jilge et al, 2000, including a marked rise starting about 2 h before the scheduled nursing time.…”

“…Here, we extend our findings on the diurnal patterns of expression of clock genes in the SCN of the previsual newborn rabbit (Caldelas et al, 2007) by showing for the first time in this model that these represent endogenous rhythms, that the mother's daily nursing visit entrains the expression of three of these genes (Per1, Per2, and Bmal1), and in parallel with the entrainment of pups' daily rhythm in body temperature (Jilge et al, 2000. …”

“…Thus, a 6-h shift on P3 in the time of nursing resulted in a corresponding shift in the pattern of expression of both sets of measures by P7 (see Caldelas et al 2007 for a comparison with the pattern of gene expression in nonshifted \control" pups on P7). In a methodological improvement on previous studies (Jilge et al, 2000Caldelas et al, 2007;Caba et al, 2008), this occurred even after ensuring complete separation of mothers and young between nursing visits to prevent possible vibrational cues associated with the mother's restlessness in From postnatal days 1 to 3 (P3) pups were nursed at 09:30 h and from P4 to P7 (P7) nursing was delayed 6-15:30 h. anticipation of nursing from triggering anticipatory responses in the pups. As nursing occurred in dim red light, we consider it most unlikely that the pups were entrained even partially by this weak photic cue.…”

“…To compare findings on gene expression in the SCN with a more general physiological measure known to be under circadian control (Jilge et al, 2000, we recorded the daily rhythm of core body temperature in an additional group of animals comprising 12 pups from nine litters. Each litter contained eight pups including one or maximally two experimental animals.…”

“…in aseptic conditions with a telemetric thermal sensor (Jonah 1 Ingestible Core Temperature Capsule, VitalSense System, MiniMitter Respironics Co., OR). Sensors had a volume of 1.19 mL and weighed 1.6 g, and so were considerably smaller than those used in previous studies of rabbit pups (Jilge et al, 2000. They measured temperature every 15 s from which the data logger calculated 1 min means.…”