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

Take a deep breath and wake up: The protean role of serotonin preventing sudden death in infancy

Recordings from infants who died suddenly and unexpectedly demonstrate the occurrence of recurring apneas, ineffective gasping, and finally, failure to restore eupnea and arouse prior to death. Immunohistochemical and autoradiographic data demonstrate a constellation of serotonergic defects in the caudal raphe nuclei in infants who died of Sudden Infant Death Syndrome (SIDS). The purpose of this review is to synthesize what is known about adaptive responses of the infant to severely hypoxic conditions, which unleash a flood of neuromodulators that inhibit cardiorespiratory function, thermogenesis, and arousal and the emerging role of serotonin, which combats this cardiorespiratory inhibition to foster autoresuscitation, eupnea, and arousal to ensure survival following an hypoxic episode. The laryngeal and carotid body chemoreflexes are potent in newborns and infants, and both reflexes can induce apnea and bradycardia, which may be adaptive initially, but must be terminated if an infant is to survive. Serotonin has a unique ability to touch on each of the processes that may be required to recover from hypoxic reflex apnea: gasping, the restoration of heart rate and blood pressure, termination of apneas and, eventually, stimulation of eupnea and arousal. Recurrent apneic events, bradycardia, ineffective gasping and a failure to terminate apneas and restore eupnea are observed in animals harboring defects in the caudal serotonergic system models - all of these phenotypes are reminiscent of and compatible with the cardiorespiratory recordings made in infants who subsequently died of SIDS. The caudal serotonergic system provides an organized, multi-pronged defense against reflex cardiorespiratory inhibition and the hypoxia that accompanies prolonged apnea, bradycardia and hypotension, and any deficiency of caudal serotonergic function will increase the propensity for sudden unexplained infant death.

Prenatal intermittent hypoxia sensitizes the laryngeal chemoreflex, blocks serotoninergic shortening of the reflex, and reduces 5-HT3 receptor binding in the NTS in anesthetized rat pups

We tested the hypothesis that exposure to intermittent hypoxia (IH) during pregnancy would prolong the laryngeal chemoreflex (LCR) and diminish the capacity of serotonin (5-hydroxytryptamine; 5-HT) to terminate the LCR. Prenatal exposure to IH was associated with significant prolongation of the LCR in younger, anesthetized, postnatal day (P) rat pups age P8 to P16 compared to control, room air (RA)-exposed rat pups of the same age. Serotonin microinjected into the NTS shortened the LCR in rat pups exposed to RA during gestation, but 5-HT failed to shorten the LCR in rat pups exposed to prenatal IH. Given these observations, we tested the hypothesis that prenatal hypoxia would decrease binding to 5-HT3 receptors in the nucleus of the solitary tract (NTS) where 5-HT acts to shorten the LCR. Serotonin 3 receptor binding was reduced in younger rat pups exposed to IH compared to control, RA-exposed rat pups in the age range P8 to P12. Serotonin 3 receptor binding was similar in older animals (P18-P24) regardless of gas exposure during gestation. The failure of the 5-HT injected into the NTS to shorten the LCR was correlated with a developmental decrease in 5-HT3 receptor binding in the NTS associated with exposure to prenatal IH. In summary, prenatal IH sensitized reflex apnea and blunted processes that terminate reflex apneas in neonatal rat pups, processes that are essential to prevent death following apneas such as those seen in babies who died of SIDS.

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

NEW FINDINGS

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. Failure to terminate apnoea and arouse is likely to contribute to sudden infant death syndrome (SIDS). Serotonin is deficient in the brainstems of babies who have died of SIDS. We tested the hypothesis that activation of serotoninergic neurons in the caudal medullary raphe, some of which project to the nucleus of the solitary tract (NTS), would shorten the laryngeal chemoreflex (LCR). We studied anaesthetized neonatal rat pups between postnatal days 9 and 17. We injected 5-40 μl of water into the larynx to elicit the LCR and measured the duration of respiratory disruption. Microinjection of 50 nl of 100 μm AMPA into the caudal medullary raphe shortened the apnoeas (P < 0.001) and respiratory inhibition (P < 0.005) associated with the LCR. When 50 nl of 30 mm ondansetron, a 5-HT₃ antagonist, was microinjected bilaterally into the NTS, AMPA microinjected into the caudal raphe no longer shortened the LCR. After bilateral microinjection of vehicle into the NTS, AMPA microinjection into the caudal raphe significantly shortened the LCR. AMPA, a glutamate receptor agonist, may activate many neurons within the caudal raphe, but blocking the 5-HT₃ receptor-dependent responses in the NTS prevented the shortening of the LCR associated with AMPA microinjections into the caudal raphe. Thus, serotonin originating from neurons in the caudal raphe acts through a 5-HT₃ receptor located in the NTS to terminate or shorten the LCR. Serotonin is deficient in the brainstems of babies who have died of SIDS, and deficient serotonergic termination of apnoea is likely to be relevant to the pathogenesis of SIDS.

Serotonin in the solitary tract nucleus shortens the laryngeal chemoreflex in anaesthetized neonatal rats

What is the central question of this study? Failure to terminate apnoea and arouse is likely to contribute to sudden infant death syndrome (SIDS). Serotonin is deficient in the brainstems of babies who died of SIDS. Therefore, we tested the hypothesis that serotonin in the nucleus of the solitary tract (NTS) would shorten reflex apnoea. What is the main finding and its importance? Serotonin microinjected into the NTS shortened the apnoea and respiratory inhibition associated with the laryngeal chemoreflex. Moreover, this effect was achieved through a 5-HT3 receptor. This is a new insight that is likely to be relevant to the pathogenesis of SIDS. The laryngeal chemoreflex (LCR), an airway-protective reflex that causes apnoea and bradycardia, has long been suspected as an initiating event in the sudden infant death syndrome. Serotonin (5-HT) and 5-HT receptors may be deficient in the brainstems of babies who die of sudden infant death syndrome, and 5-HT seems to be important in terminating apnoeas directly or in causing arousals or as part of the process of autoresuscitation. We hypothesized that 5-HT in the brainstem would limit the duration of the LCR. We studied anaesthetized rat pups between 7 and 21 days of age and made microinjections into the cisterna magna or into the nucleus of the solitary tract (NTS). Focal, bilateral microinjections of 5-HT into the caudal NTS significantly shortened the LCR. The 5-HT1a receptor antagonist, WAY 100635, did not affect the LCR consistently, nor did a 5-HT2 receptor antagonist, ketanserin, alter the duration of the LCR. The 5-HT3 specific agonist, 1-(3-chlorophenyl)-biguanide, microinjected bilaterally into the caudal NTS significantly shortened the LCR. Thus, endogenous 5-HT released within the NTS may curtail the respiratory depression that is part of the LCR, and serotonergic shortening of the LCR may be attributed to activation of 5-HT3 receptors within the NTS. 5-HT3 receptors are expressed presynaptically on C fibre afferents of the superior laryngeal nerve, and serotonergic shortening of the LCR may be mediated presynaptically by enhanced activation of inhibitory interneurons within the NTS.