Luxi Xia

Gestational nicotine exposure exaggerates hyperthermic enhancement of laryngeal chemoreflex in rat pups

Laryngeal chemoreflex (LCR) apnea occurs in infant mammals in response to water or other liquids in the laryngeal lumen and is suspected to contribute to some cases of the sudden infant death syndrome. We have previously found that the LCR is prolonged in neonatal piglets and rats that are warmed 1-3 degrees C above their normal body temperatures and that this prolongation is exaggerated in rat pups whose mothers have been exposed to cigarette smoke during pregnancy. We have therefore examined the effects on the LCR of combined prenatal nicotine exposure and brief postnatal hyperthermia. Nicotine was infused in pregnant rats via implanted osmotic minipumps (6.5mg/kg per day) from gestational day 3 (G3) until G21, the day of delivery. Control animals received saline infusions. On postnatal days 4-12, LCR apnea and respiratory disruption, elicited by intralaryngeal water, were measured with and without hyperthermia in anesthetized pups. The results indicate that prenatal exposure to nicotine significantly exaggerates the hyperthermic prolongation of the LCR.

An adenosine A2A antagonist injected in the NTS reverses thermal prolongation of the LCR in decerebrate piglets

Hyperthermia prolongs the laryngeal chemoreflex (LCR). Under normothermic conditions, adenosine antagonists shorten and adenosine A(2A) (Ad-A(2A)) agonists prolong the LCR. Therefore, we tested the hypothesis that SCH-58261, an Ad-A(2A) receptor antagonist, would prevent thermal prolongation of the LCR when injected unilaterally within the nucleus of the solitary tract (NTS). We studied decerebrate piglets aged 4-13 days. We elicited the LCR by injecting 0.1ml of water into the larynx and recorded integrated phrenic nerve activity. The laryngeal chemoreflex was prolonged when the body temperature of each piglet was raised approximately 2.5 degrees C, and SCH-58261 reversed the thermal prolongation of the LCR when injected into the NTS (n=13), but not when injected in the nucleus ambiguus (n=9). Injections of vehicle alone into the NTS did not alter the thermal prolongation of the LCR (n=9). We conclude that activation of adenosine receptors, perhaps located on GABAergic neurons in the NTS, contributes to thermal prolongation of the LCR.

GABAergic processes mediate thermal prolongation of the laryngeal reflex apnea in decerebrate piglets

We tested the hypotheses that elevated body temperature would prolong reflex apnea following electrical stimulation of the superior laryngeal nerve (SLN) in decerebrate neonatal piglets and that thermal prolongation of reflex apnea after stimulation of the SLN depended on GABAergic mechanisms. These studies were conducted in 13 decerebrate piglets (age 3-15 days). The SLN was stimulated at approximately 1.5 times the threshold stimulus level for 10 s starting at the beginning of inspiration. We measured the duration of the apnea and respiratory disruption that followed SLN stimulation. Elevating body temperature prolonged the duration of the apnea and respiratory disruption that followed SLN stimulation, and treatment with antagonists of gama-aminobutyric acid A-type (GABAA) receptors reversed the thermal prolongation of reflex apnea and the period of respiratory disruption even though body temperature remained elevated. We conclude that elevated body temperature enhances or amplifies GABAergic mechanisms that prolong the respiratory inhibition following electrical stimulation of the SLN.

Laryngeal water receptors are insensitive to body temperature in neonatal piglets.

Heat stress and the laryngeal chemoreflex (LCR) have both been implicated as possible contributors to the sudden infant death syndrome (SIDS). We recently reported that moderate hyperthermia, induced in decerebrate piglets by external heating, substantially prolonged the LCR elicited by injecting 0.1 ml of water into the larynx through a prepositioned transnasal catheter. To examine the question of whether hyperthermia influences the responses of laryngeal water receptors, we recorded single fiber action potentials in fine strands of the superior laryngeal nerve (SLN) in decerebrate piglets while the larynx was filled with water or isotonic saline. Water receptors, identified by their much brisker response to water than to saline, were studied with body temperature at 37.9+/-0.2 degrees C, after warming the animal to 40.6+/-0.2 degrees C and after cooling back to 37.7+/-0.3 degrees C. The results show no effect of body temperature change, in this range, on the responses of the laryngeal water receptors and thus suggest that the potentiation of the LCR by hyperthermia is mediated by a central action.

An adenosine A2A agonist injected in the nucleus of the solitary tract prolongs the laryngeal chemoreflex by a GABAergic mechanism in decerebrate piglets

Hyperthermic prolongation of the laryngeal chemoreflex (LCR) in decerebrate piglets is prevented or reversed by GABAA receptor antagonists and adenosine A2A (Ad‐A2A) receptor antagonists administered in the nucleus of the solitary tract (NTS). Therefore, we tested the hypothesis that enhanced GABAA activity and administration of the Ad‐A2A agonist, CGS‐21680, would prolong the LCR in normothermic conditions. We studied 46 decerebrate piglets ranging from 3 to 8 postnatal days of age. Focal injection into the NTS of 100 nl of 0.5 m nipecotic acid, a GABA reuptake inhibitor, significantly (P < 0.05) prolonged the LCR in normothermic conditions in 10 of 11 animals tested. Injecting 100 nl of 5–12.5 μm CGS‐21680 unilaterally or bilaterally into the NTS also prolonged the LCR in normothermic conditions (n= 15), but the effect was smaller than that of unilateral injection of nipecotic acid. Systemic administration of the GABAA receptor antagonist, bicuculline, prevented the CGS‐21680‐dependent prolongation of the LCR in normothermic animals (n= 11). We conclude that thermal prolongation of the LCR depends on a thermally sensitive process or set of neurons in the NTS, which, when activated by elevated brain temperature, enhances adenosinergic and GABAergic function in the region of the NTS. These results emphasize the importance of a thermally sensitive integrative site in the dorsal medulla that, along with sites in the ventral medulla, determine the response to laryngeal chemoreflex stimulation.

Gestational cigarette smoke exposure and hyperthermic enhancement of laryngeal chemoreflex in rat pups.

Laryngeal chemoreflex (LCR) apnea occurs in infant mammals of many species in response to water or other liquids in the laryngeal lumen. The apnea can last for many seconds, sometimes leading to dangerous hypoxemia, and has therefore been considered as a possible mechanism in the Sudden Infant Death Syndrome (SIDS). We have found recently that this reflex is markedly prolonged in decerebrate piglets and anesthetized rat pups that are warmed 1-3 degrees C above their normal body temperatures. We intermittently exposed pregnant rats to cigarette smoke and examined the LCR in their four- to fifteen-day-old offspring under general anesthesia, with and without whole body warming. During warming, pups of gestationally smoke-exposed dams had significantly longer LCR-induced respiratory disruption than similarly warmed control pups. The results may be significant for the pathogenesis and/or prevention of SIDS as maternal cigarette smoking during human pregnancy and heat stress in infants are known risk factors for SIDS.

Laryngeal reflex apnea in neonates: Effects of CO2 and the complex influence of hypoxia

We have examined influence of hypocapnia, mild hypercapnia and hypoxia on the durations of fictive apnea and respiratory disruption elicited by injection of 0.1ml of water into the laryngeal lumen-the laryngeal chemoreflex (LCR)-in 20 unanesthetized, decerebrate, vagotomized piglets aged 4-10 days that were paralyzed and ventilated with a constant frequency and tidal volume. The LCR was enhanced by hypocapnia and attenuated by hypercapnia as reported by others. The responses to laryngeal stimulation during hypoxia were varied and complex: some animals showed abbreviated responses during the tachypnea of early hypoxia, followed after 10-15min by more prolonged apnea and respiratory disruption accompanying the reduction in ventilatory activity that commonly occurs during sustained hypoxia in neonates. We speculate that this later hypoxic enhancement of the LCR may be due to accumulation of adenosine in the brain stem.

Elevated Body Temperature Exaggerates Laryngeal Chemoreflex Apnea in Decerebrate Piglets

We investigated the interaction between body temperature and the duration of the laryngeal chemoreflex (LCR) in decerebrate piglets. Elevating body temperature by approximately 2 degrees C prolongs the duration of the LCR and the length of apnea associated with the reflex. This thermal prolongation seems to arise within the nucleus of the solitary tract in the brainstem, and we believe the thermal effect is mediated by enhanced GABAergic neurotransmission.

Interleukin-1β and interleukin-6 enhance thermal prolongation of the LCR in decerebrate piglets

Thermal stress and prior upper respiratory tract infection are risk factors for the Sudden Infant Death Syndrome. The adverse effects of prior infection are likely mediated by interleukin-1β (IL-1β). Therefore, we examined the single and combined effects of IL-1β and elevated body temperature on the duration of the Laryngeal Chemoreflex (LCR) in decerebrate neonatal piglets ranging in age from post-natal day (P) 3 to P7. We examined the effects of intraperitoneal (I.P.) injections of 0.3mg/Kg IL-1β with or without I.P. 10mg/Kg indomethacin pretreatment on the duration of the LCR, and in the same animals we also examined the duration of the LCR when body temperature was elevated approximately 2°C. We found that IL-1β significantly increased the duration of the LCR even when body temperature was held constant. There was a significant multiplicative effect when elevated body temperature was combined with IL-1β treatment: prolongation of the LCR was significantly greater than the sum of independent thermal and IL-1β-induced prolongations of the LCR. The effects of IL-1β, but not elevated body temperature, were blocked by pretreatment with indomethacin alone. We also tested the interaction between IL-6 given directly into the nucleus of the solitary tract (NTS) bilaterally in 100ngm microinjections of 50μL and pretreatment with indomethacin. Here again, there was a multiplicative effect of IL-6 treatment and elevated body temperature, which significantly prolonged the LCR. The effect of IL-6 on the LCR, but not elevated body temperature, was blocked by pretreatment with indomethacin. We conclude that cytokines interact with elevated body temperature, probably through direct thermal effects on TRPV1 receptors expressed pre-synaptically in the NTS and through cytokine-dependent sensitization of the TRPV1 receptor. This sensitization is likely initiated by cyclo-oxygenase-2 dependent synthesis of prostaglandin E2, which is stimulated by elevated levels of IL-1β or IL-6. Inflammatory sensitization of the LCR coupled with thermal prolongation of the LCR may increase the propensity for apnea and Sudden Infant Death Syndrome.

Activation of serotonergic neurons in the medullary caudal raphe shortens the laryngeal chemoreflex in anaesthetized neonatal rats

What is the central question of this study? Does activation of serotonergic neurons in the caudal medullary raphe, some of which project to the nucleus of the solitary tract, shorten the laryngeal chemoreflex? What is the main finding and its importance? We found that serotonin originating from neurons in the caudal raphe acts through a 5‐HT3 receptor located in the nucleus of the solitary tract to terminate reflex apnoea. Failure or deficiency of this arousal‐related process is likely to be relevant to the pathogenesis of sudden infant death syndrome.