Caffeine decreases intermittent hypoxia in preterm infants nearing term-equivalent age.

OBJECTIVE: To determine whether intermittent hypoxia (IH) persisting after 36 weeks postmenstrual age (PMA) can be attenuated using caffeine doses sufficient to maintain caffeine concentrations >20 µg ml-1. STUDY DESIGN: Twenty-seven infants born <32 weeks were started on caffeine citrate at 10 mg kg-1 day-1 when clinical caffeine was discontinued. At 36 weeks PMA, the dose was increased to 14 or 20 mg kg-1 day-1 divided twice a day (BID) to compensate for progressively increasing caffeine metabolism. Caffeine concentrations were measured weekly. The extent of IH derived from continuous pulse oximetry was compared to data from 53 control infants. RESULT: The mean (s.d.) gestational age of enrolled infants was 27.9±2 weeks. Median caffeine levels were >20 µg ml-1 on study caffeine doses. IH was significantly attenuated through 38 weeks PMA compared with the control group. CONCLUSION: Caffeine doses of 14 to 20 mg kg-1 day-1 were sufficient to maintain caffeine concentrations >20 µg ml-1 and reduce IH in preterm infants at 36 to 38 weeks PMA.

Resting oxygen consumption of premature infants covered with a plastic thermal blanket.

Premature infants in single-wall incubators covered with "thermal blankets" made of plastic packing material have large reductions in insensible water loss (IWL) compared with naked infants. We postulated that such reductions inevaporative heat loss would not result in decreases in caloric expenditure if body temperature were maintained by a servocontrolled heat source. Using an open-circuit technique, we measured oxygen consumption (VO2), carbon dioxide production (VCO2), heart rate (HR), respiratory rate (RR), and abdominal skin (Tabd), cheek, thigh, rectal, incubator air, wall, and room air temperatures in ten infants less than 37 weeks gestational age and from 2 to 24 days of age both naked and covered with a plastic thermal blanket. Tabd temperature was maintained between 36.2 and 36.8 C and rectal temperature between 36.8 and 37.2 C in each environment by manual or automatic servocontrol. A "resting state" was defined by using a combination of subjective and objective criteria. The mean values of VO2 during the "resting state" were 7.31 and 7.59 cc/kg of body weight per minute for naked and covered infants, respectively. There were no significant differences between mean values of VCO2, respiratory quotient, HR, RR, abdominal, cheek, thigh, or rectal temperatures in the two environments. Operant temperatures averaged 0.5 C lower when the infants were covered. These data support the hypothesis that decreases in insensible water loss do not necessarily imply reductions in caloric requirements in infants where Tabd is maintained by servocontrol.

Spontaneous arousals during quiet sleep in piglets: a visual and wavelet-based analysis.

STUDY OBJECTIVES: The objectives of the study were to characterize spontaneous arousals during NREM sleep in piglets and to compare two methods of identifying these events: a "visual" technique using spectral analysis and an automated technique using wavelets. Our goal was to understand the benefits and limits of these methods when applied to sleep in human infants. DESIGN: Arousals were identified by evaluating rapid changes in EEG low frequency activity, blood pressure (BP), and heart rate (HR). A cortical arousal was defined as a rapid decrease in EEG low frequency activity. An autonomic arousal was defined by a transient increase in heart rate or a transient change in mean arterial BP (MAP). SETTING: Laboratory study in sleeping and awake piglets. PARTICIPANTS: Five 1-2 week old piglets. INTERVENTIONS: Chronically instrumented with a femoral arterial line, EEG, EOG, EMG electrodes, and a micro-dialysis probe with its tip located in the rostral ventral medulla. Artificial CSF (aCSF) was dialyzed into the RVM throughout the experiments Measurements: For the visual analysis, the average delta power (0.5-4 Hz) for each 5-second epoch was determined using spectral analysis. MAP and HR were analyzed in 1-second bins. Video images were analyzed for body movements and eye openings. Transient changes in blood pressure, HR, and delta power were then visually identified. For the wavelet analysis, a quantitative, automated technique with a defined "wakefulness threshold" was used to identify rapid decreases in EEG low frequency activity and the rate of change of MAP. RESULTS: Using the visual method, 117 episodes associated with stereotypical hemodynamic, EEG, and behavioral changes (startle) were identified. Seventy five events occurred in isolation or were first in a series of "multiple" events, 41 "multiple" events were defined as events occurring <20 seconds following a previous event. Eighteen events were associated with the termination of apnea. In isolated events or those occurring first in a series, the onset of changes in HR and BP clearly preceded the decrease in EEG amplitude and delta power. Using wavelet analysis, 73 EEG arousals and 115 MAP transients were identified independently; 62% of the EEG events were associated with a transient change in MAP and HR, and in these cases the onset of the hemodynamic events preceded EEG arousals. EEG arousals and MAP transients, however, also occurred alone and not associated with a stereotypical pattern of a startle, changes in MAP and HR and the EEG. CONCLUSIONS: Many of these spontaneous arousals represent integrated EEG, hemodynamic, and behavioral processes similar to arousal phenomena described in adult rats and human infants, but the pattern of spontaneous arousals appears to be more heterogeneous than has been described for arousals induced by exogenous stimuli. Both the visual and wavelet analysis identified these events, but the wavelet technique has the potential advantage of better time resolution and automation of the analysis.

The effects of a GABA(A) agonist in the rostral ventral medulla on sleep and breathing in newborn piglets.

STUDY OBJECTIVES: Abnormalities in the rostral ventral medulla (RVM) in human infants may contribute to the etiology of the sudden infant death syndrome (SIDS) or a subset of SIDS, by interfering with cardiorespiratory and arousal responses to physiological stimuli often encountered during sleep. The purpose of this study was to determine whether inhibition of groups of neurons in the RVM in newborn piglets would alter sleep and/or the sleep-modulation of breathing. We hypothesized that inhibition of neurons in the RVM would produce less wakefulness or increase the low frequency power (delta) during Quiet sleep. DESIGN: Unanesthetized piglets were studied in a whole-body plethysmograph. Artificial cerebral spinal fluid (aCSF) or the GABAA agonist, muscimol, was dialyzed into the RVM for 40 minutes after a control period consisting of aCSF dialysis. Sleep was analyzed using a combination of EEG spectral analysis and behavioral observations. SETTING: N/A. PARTICIPANTS: N/A. INTERVENTIONS: N/A. MEASUREMENTS AND RESULTS: Cardiorespiratory variables varied with state. Dialysis of neither aCSF nor muscimol into the RVM resulted in alterations in resting respiration, BP, HR, or VO2 or their modulation by state. Compared to control dialysis with aCSF, muscimol dialysis caused dramatic effects on sleep architecture. Sleep cycling was abolished in some experiments, whereas in others there were decreases in low-frequency EEG activity or delta power. The animals in which sleep cycling ceased continued in a perpetual state of drowsiness interspersed with periods of wakefulness. CONCLUSIONS: We conclude that dialysis of muscimol into the RVM has little effect on resting breathing, blood pressure, or heart rate or their modulation by state, but interferes with normal sleep architecture. We speculate that abnormalities in the ventral medulla may alter sleep cycling or interfere with arousal mechanisms, thus contributing to the etiology of at least a subset of SIDS.

Entrainment of respiration to rocking in premature infants: coherence analysis.

Vestibular influences on breathing pattern were investigated in 18 premature infants in the neonatal intensive care nursery. Respiratory abdominal movements were recorded while the babies were manually rocked at varying rates between 30 and 60 cycles/min (cpm). Coherence spectra were estimated between the respiratory and rocker signals, and their magnitudes were evaluated at the rocking frequency, with coherence spectra > 0.85 indicative of strong entrainment to rocking. At least one incident of entrainment was seen in 15 of 18 infants, with 2:1 ratios (2 breaths/rocker cycle) occurring at rocking frequencies of 30-40 cpm (8 of 18 subjects) and 1:1 entrainment at rates of 42-50 cpm (5 of 18 subjects). More complex synchronization was observed in three infants, with patterns consisting of alternans between 2:1 and 3:2 ratios (5:3 entrainment). Infants > 35 wk postconceptional age exhibited greater coherence to rocking than infants < 35 wk (P < 0.01), indicating a maturational change in the reflex may occur. Results show that the natural stimulation of rocking a newborn provides a phasic input to its respiratory pattern generator that is capable of resetting the system's oscillation and entraining its rhythm.

Effect of acute hypoxia on respiration and brain stem blood flow in the piglet.

Changes in local brain stem perfusion that alter extracellular fluid Pco2 and/or [H+] near central chemoreceptors may contribute to the decrease in respiration observed during hypoxia after peripheral chemoreceptor denervation and to the delayed decrease observed during hypoxia in the newborn. In this study, we measured the changes in respiration and brain stem blood flow (BBF) during 2-4 min of hypoxic hypoxia in both intact and denervated piglets and calculated the changes in brain stem Pco2 and [H+] that would be expected to occur as a result of the changes in BBF. All animals were anesthetized, spontaneously breathing, and between 2 and 7 days of age. Respiratory and other variables were measured before and during hypoxia in all animals, and BBF (microspheres) was measured in a subgroup of intact and denervated animals at 0, 30, and 260 s and at 0 and 80 s, respectively. During hypoxia, minute ventilation increased and then decreased (biphasic response) in the intact animals but decreased only in the denervated animals. BBF increased in a near linear fashion, and calculated brain stem extracellular fluid Pco2 and [H+] decreased over the first 80 s both before and after denervation. We speculate that a rapid increase in BBF during acute hypoxia decreases brain stem extracellular fluid Pco2 and [H+], which, in turn, negatively modulate the increase in respiratory drive produced by peripheral chemoreceptor input to the central respiratory generator.

Muscimol dialysis in the rostral ventral medulla reduced the CO(2) response in awake and sleeping piglets.

Some victims of sudden infant death syndrome have arcuate nucleus abnormalities. The arcuate nucleus may be homologous with ventral medullary structures in the cat known to be involved in the control of breathing and the response to systemic hypercapnia. We refer to putative arcuate homologues in the piglet collectively as the rostral ventral medulla (RVM). We inhibited the RVM in awake and sleeping, chronically instrumented piglets by microdialysis of the GABA(A) receptor agonist muscimol. Muscimol dialysis (10 and 40 mM) had no effect on eupnea but caused a significant reduction in the response to hypercapnia during both wakefulness (34.8 +/- 8.7 and 30.7 +/- 10.1%, respectively) and sleep (36.7 +/- 6.7 and 49.5 +/- 8.9%, respectively). The effect of muscimol on the CO(2) response was entirely via a reduction in tidal volume and appeared to be greater during non-rapid-eye-movement sleep. We conclude that the piglet RVM contains neurons of importance in the response to systemic CO(2) during both wakefulness and non-rapid-eye-movement sleep. We hypothesize that dysfunction of homologous regions in the human infant could lead to impaired ability to respond to hypercapnia, particularly during sleep, which could potentially be involved in the pathogenesis of sudden infant death syndrome.

Effect of aortic balloon inflation on ventilation and brain stem blood flow in piglets.

Increases in brain stem blood flow (BBF) during hypoxia may decrease tissue PCO2/[H+], causing minute ventilation (VE) to decrease. To determine whether an increase in BBF, isolated from changes in arterial PO2 and PCO2, can affect respiration, we obstructed the thoracic aorta with a balloon in 31 intact and 24 peripherally chemobarodenervated, anesthetized, spontaneously breathing newborn piglets. Continuous measurements of cardiorespiratory variables were made before and during 2 min of aortic obstruction. Radiolabeled microspheres were used to measure BBF before and approximately 30 s after balloon inflation in eight intact and five denervated animals. After balloon inflation, there was a rapid increase in mean blood pressure in both the intact and denervated animals, followed within 10 s by a decrease in tidal volume and VE. In the intact animals, the decrease in VE after acute hypertension can be ascribed to a baroreceptor-mediated reflex. After peripheral chemobarodenervation, however, acute hypertension continued to produce a decrease in VE, which cannot be explained by baroreceptor stimulation. In these denervated animals, aortic balloon inflation was associated with an increase in BBF (13.1 +/- 2.7%; P less than 0.05). We speculate that the increase in BBF during hypoxia may contribute to the decrease in ventilation observed after carotid body denervation.

Modulation of hypoxic depressions of ventilatory activity in the newborn piglet by mesencephalic mechanisms.

In neonates, ventilatory responses to hypoxia are 'biphasic,' with an augmentation followed by a decline. The hypoxia-induced augmentations in ventilation are attenuated and the depressions are accentuated following denervation of the peripheral chemoreceptors. Piglets that were decerebrated at a rostral mesencephalic level exhibited these hypoxia-induced depressions. These depressions were lessened following transection through the caudal mesencephalon. Mesencephalic mechanisms play a fundamental role in the brainstem regulation of ventilatory responses to hypoxia.

Respiratory modulation of pre- and postganglionic lumbar vasomotor sympathetic neurons in the rat.

The respiratory contribution to the activity of lumbar vasomotor pre- and postganglionic neurons was compared with previously reported respiratory patterns in the activity of neurons in the rostral ventrolateral medulla (RVLM-SE). Two patterns of respiratory modulation were observed characterized by (1) a depression of activity during inspiration and a postinspiratory peak (n = 19), and (2) a peak of activity during inspiration (n = 3). These patterns were similar to those previously reported in RVLM-SE neurons. The latency from the phrenic burst to the onset of respiratory modulation was consistent with the conduction time from the RVLM to the sympathetic chain. This suggests that respiratory modulation observed in lumbar sympathetic activity originates, in part, in RVLM-SE neurons.

Effects of adenosine and xanthine derivatives on breathing during acute hypoxia in the anesthetized newborn piglet.

Neonates of animals and humans exhibit a paradoxical ventilatory response to hypoxia characterized by an initial increase in minute ventilation followed by a late, sustained decrease. Exogenous adenosine analogues cause respiratory depression, and the xanthine derivative aminophylline, a competitive inhibitor of adenosine receptors, decreases the amount of hypoxic ventilatory depression in the newborn piglet. Other xanthine derivative such as enprofylline are weak adenosine antagonists. The purpose of this report is to test the hypothesis that enprofylline would not reverse ventilatory depression caused by hypoxia, supporting the suggestion that adenosine contributes to hypoxic ventilatory depression. To confirm the weak adenosine antagonism of enprofylline, L-N6-(phenylisopropyl)adenosine (PIA) was administered to six newborn piglets until respiratory depression was achieved. Either aminophylline or enprofylline was then administered. Aminophylline, but not enprofylline, reversed the respiratory depression caused by PIA. In seven additional piglets, respiratory depression was first produced by 10% oxygen breathing and the ability of saline, aminophylline, and enprofylline to reverse the decrease in ventilation was evaluated. The administration of either saline or enprofylline produced little change in minute ventilation (9.8% +/- 3.7% and -11.7% +/- 7.7%, respectively), whereas aminophylline consistently produced an increase (43.5% +/- 7.3% [P less than 0.001]). Both aminophylline and enprofylline increased heart rate (P less than 0.01), whereas saline produced no significant change. Blood pressure was increased by enprofylline but not by aminophylline or saline. These findings suggest that, in the anesthetized newborn piglet, adenosine contributes to ventilatory depression caused by hypoxia.

Aminophylline reduces hypoxic ventilatory depression without increasing catecholamines.

Aminophylline reduces hypoxic ventilatory depression in newborn piglets and can enhance the release of catecholamines (CATs), which in turn may stimulate ventilation. To determine if the effect of aminophylline on ventilation was due to the release of CATs, we measured plasma CATs and ventilation in two groups of spontaneously breathing newborn piglets less than 4 days old, treated with either aminophylline (n = 7) or normal saline solution (n = 6) during both normoxia and hypoxia. The piglets were anesthetized with ketamine and xylazine and intubated, and the femoral artery was catheterized. Epinephrine and norepinephrine were measured before and 30 minutes after treatment with aminophylline (15 mg/kg) or normal saline. The animals were exposed to 10% oxygen and the CATs again measured after 5 minutes of hypoxia. Respiratory rate, expiratory flow integrated to minute ventilation (VE), heart rate, and blood pressure were continuously recorded. CATs were assayed by high-pressure liquid chromatography with electrochemical detection. Treatment with aminophylline during normoxia was associated with an increase in tidal volume. During hypoxia, treatment with aminophylline prevented a fall in VE and respiratory rate seen in the normal saline group. Epinephrine and norepinephrine increased during hypoxia, but there was no difference between the groups at 5 minutes. In our model the increase in CATs observed during hypoxia was not enhanced by aminophylline. This is consistent with the hypothesis that some mechanism other than catecholamine release is responsible for the effect of aminophylline in reducing neonatal hypoxic respiratory depression.

Teaching residents in the neonatal intensive care unit: a non-traditional approach.

Because of the increasing constraints on the amount of time pediatric residents may train in the neonatal intensive care unit (NICU), concerns have been raised about the adequacy of their exposure to acute emergencies in the delivery room and their hands-on experience with sick neonates. Importantly, there are also concerns about the consistency and quality of supervision of PL-1 residents by second- and third-year residents, who themselves may not have had sufficient training in the NICU. To address these concerns, we have instituted an educational plan that links an experienced neonatal nurse practitioner (NNP) one-on-one with a PL-1 resident in a collaborative team. This plan differs from the traditional resident-to-resident supervisory model. An anonymous survey of our residents (n = 14) indicates enthusiastic endorsement of this new educational model. NNPs as first-line teachers in the NICU provide a new approach for residency training programs.

The New Hampshire Perinatal Program: twenty years of perinatal outreach education.

OBJECTIVE: To describe 20 years of regional outreach education by the New Hampshire Perinatal Program, its interaction with all 26 community hospitals in the state with maternity services and an additional four in adjoining Vermont. STUDY DESIGN: This paper describes educational initiatives responsive to the needs of perinatal physicians and nurses. The core of the program is the transport conference held annually at each referring hospital in which maternal-fetal and infant referrals are discussed. There are additional community hospital-based programs, programs at convenient locations in the region and medical center conferences and skills programs. RESULTS: The program annually awards 10,000 continuing medical education credits (CME) and nursing contact hours. Evaluation and feedback from all participants is encouraged. New Hampshire has one of the lowest perinatal mortality rates in the county, which reflects in part the accomplishments of the program. CONCLUSION: Perinatal outreach education is a shared responsibility of providers in both the academic center and community hospitals and is necessary to ensure optimal care for mothers and infants.

Noninvasive blood pressure measurement in the neonate.

Measurements of blood pressure are important in the care of the critically ill neonate. Although direct measurement of mean arterial pressure remains the standard, noninvasive methods provide clinically useful estimations of pressure. Clinicians are often faced with situations where measurements of pressure obtained by direct and indirect methods do not agree. Although the explanation for this is not entirely clear, at least two factors are important: first, direct techniques measure pressure while indirect (noninvasive) methods measure flow; second, neither direct or indirect measurements are straightforward and both are subject to error. Of the noninvasive methods, the oscillometric and Doppler devices appear to be most clinically useful in the newborn intensive care unit.

Developmental profiles of infant EEG: overlap with transient cortical circuits.

OBJECTIVE: To quantify spectral power in frequency specific bands and commonly observed types of bursting activities in the EEG during early human development. METHODS: An extensive archive of EEG data from human infants from 35 to 52 weeks postmenstrual age obtained in a prior multi-center study was analyzed using power spectrum analyses and a high frequency burst detection algorithm. RESULTS: Low frequency power increased with age; however, high frequency power decreased from 35 to 45 weeks. This unexpected decrease was largely attributable to a rapid decline in the number of high frequency bursts. CONCLUSIONS: The decline in high frequency bursting activity overlaps with a developmental shift in GABA's actions on neurons from depolarizing to hyperpolarizing and the dissolution of the gap junction circuitry of the cortical subplate. SIGNIFICANCE: We postulate that quantitative characterization of features of the EEG unique to early development provide indices for tracking changes in specific neurophysiologic mechanisms that are critical for normal development of brain function.

Reversible blunting of arousal from sleep in response to intermittent hypoxia in the developing rat.

Arousal is an important survival mechanism when infants are confronted with hypoxia during sleep. Many sudden infant death syndrome (SIDS) infants are exposed to repeated episodes of hypoxia before death and have impaired arousal mechanisms. We hypothesized that repeated exposures to hypoxia would cause a progressive blunting of arousal, and that a reversal of this process would occur if the hypoxia was terminated at the time of arousal. P5 (postnatal age of 5 days), P15, and P25 rat pups were exposed to either eight trials of hypoxia (3 min 5% O(2) alternating with room air) (group A), or three hypoxia trials as in group A, followed by five trials in which hypoxia was terminated at arousal (group B). In both groups A and B, latency increased over the first four trials of hypoxia, but reversed in group B animals during trials 5-8. Progressive arousal blunting was more pronounced in the older pups. The effects of intermittent hypoxia on heart rate also depended on age. In the older pups, heart rate increased with each hypoxia exposure. In the P5 pups, however, heart rate decreased during hypoxia and did not return to baseline between exposures, resulting in a progressive fall of baseline values over successive hypoxia exposures. In the group B animals, heart rate changes during trials 1-4 also reversed during trials 5-8. We conclude that exposure to repeated episodes of hypoxia can cause progressive blunting of arousal, which is reversible by altering the exposure times to hypoxia and the period of recovery between hypoxia exposures.

Activation of 5-HT1A receptors in medullary raphé disrupts sleep and decreases shivering during cooling in the conscious piglet.

Activation of 5-HT1A receptors in the medullary raphé decreases sympathetically mediated brown adipose tissue (BAT) thermogenesis and peripheral vasoconstriction when previously activated with leptin, LPS, prostaglandins, or cooling. It is not known whether shivering is also modulated by medullary raphé 5-HT1A receptors. We previously showed in conscious piglets that activation of 5-HT1A receptors with (+/-)-8-hydroxy-2-(dipropylamino)-tetralin (8-OH-DPAT) in the paragigantocellularis lateralis (PGCL), a medullary region lateral to the raphé that contains substantial numbers of 5-HT neurons, eliminates rapid eye movement (REM) sleep and decreases shivering in a cold environment, but does not attenuate peripheral vasoconstriction. Hoffman JM, Brown JW, Sirlin EA, Benoit AM, Gill WH, Harris MB, Darnall RA. Am J Physiol Regul Integr Comp Physiol 293: R518-R527, 2007. We hypothesized that, during cooling, activation of 5-HT1A receptors in the medullary raphé would also eliminate REM sleep and, in contrast to activation of 5-HT1A receptors in the PGCL, would attenuate both shivering and peripheral vasoconstriction. In a continuously cool environment, dialysis of 8-OH-DPAT into the medullary raphé resulted in alternating brief periods of non-REM sleep and wakefulness and eliminated REM sleep, as observed when 8-OH-DPAT is dialyzed into the PGCL. Moreover, both shivering and peripheral vasoconstriction were significantly attenuated after 8-OH-DPAT dialysis into the medullary raphé. The effects of 8-OH-DPAT were prevented after dialysis of the selective 5-HT1A receptor antagonist WAY-100635. We conclude that, during cooling, exogenous activation of 5-HT1A receptors in the medullary raphé decreases both shivering and peripheral vasoconstriction. Our data are consistent with the hypothesis that neurons expressing 5-HT1A receptors in the medullary raphé facilitate spinal motor circuits involved in shivering, as well as sympathetic stimulation of other thermoregulatory effector mechanisms.

Activation of 5-HT1A receptors in the paragigantocellularis lateralis decreases shivering during cooling in the conscious piglet.

Activation of 5-HT(1A) receptors in the medullary raphé decreases sympathetic outflow to thermoregulatory mechanisms, including brown adipose tissue (BAT), thermogenesis, and peripheral vasoconstriction when these mechanisms are previously activated with leptin, prostaglandins, or cooling. These same mechanisms are also inhibited during rapid eye movement (REM) sleep. It is not known whether shivering is also modulated by medullary raphé neurons. We previously showed in the conscious piglet that activation of 5-HT(1A) receptors with 8-OH-DPAT (DPAT) in the paragigantocellularis lateralis (PGCL), a medullary region lateral to the midline raphé that contains 5-HT neurons, decreases heart rate, body temperature and muscle activity during non-rapid eye movement (NREM) sleep. We therefore hypothesized that activation of 5-HT(1A) receptors in the PGCL would also attenuate shivering and peripheral vasoconstriction during cooling. During REM sleep in a cool environment, shivering, carbon dioxide production, and body temperature decreased, and ear capillary blood flow and ear skin temperature increased. Shivering associated with rapid cooling was attenuated after dialysis of DPAT into the PGCL. In animals maintained in a continuously cool environment, dialysis of DPAT into the PGCL attenuated shivering and decreased body temperature, but there were no significant increases in ear capillary blood flow or ear skin temperature. We conclude that both naturally occurring REM sleep and exogenous activation of 5-HT(1A) receptors in the PGCL are associated with a suspension of shivering during cooling. Our data are consistent with the hypothesis that 5-HT neurons in the PGCL facilitate oscillating spinal motor circuits involved in shivering but are less involved in modulating sympathetically mediated thermoregulatory mechanisms.