Disturbed chemical neurotransmission and sudden infant death syndrome--the peripheral arterial chemoreceptors as an example

Peripheral chemoreceptor activity in sleeping neonates exposed to warm environments

In neonates, it is often assumed that ventilatory control and heat stress interact. Thus the two factors have been implicated in various pathologies (apnoea, sudden infant death syndrome). However, little is known about the mechanisms of this interaction, and the influence of sleep is still debated. This study aimed at determining the influence of warm exposure on the decrease in ventilation during a hyperoxic test (HT), which is considered to be a measure of peripheral chemoreceptor activity. The test was performed in active (AS) and quiet sleep (QS) in 12 neonates exposed to thermoneutral or warm environments. The HT consisted of 30 s of inspired, 100% O(2). The ventilatory response was assessed in terms of a response time, defined as the time elapsing between HT onset and the first significant change in V(E). Our results show that, in both thermal conditions, the fall in V(E) was higher in AS than in QS. Warm exposure significantly enhanced the ventilatory response in AS (-27.5 +/- 8.7% vs. -38.3 +/- 8.8%, P < 0.01) but not in QS. A thermometabolic drive or inputs from thermoreceptors could be involved in the reinforcement of peripheral chemoreceptor activity in AS in warmer environments, which could contribute to an increasing risk of apnoea in neonates with altered chemoreceptor function. Since hypothalamic structures are involved in thermoregulatory, sleep processes and (probably) in respiratory control, it could well be the principal site where this interaction occurs.

Control of breathing in infants born to smoking mothers

OBJECTIVE

To determine whether infants born to smoking mothers have an abnormal respiratory drive and a blunted ventilatory response to hypoxia.

STUDY DESIGN

Sixty-four healthy infants, aged 2 to 24 months, were classified into smoking (n = 19) or non-smoking (n = 45) groups based on maternal smoking habits. Resting ventilation, lung function, and mouth pressure 100 milliseconds after an airway occlusion at the onset of inspiration (P(0.1 )) were measured. The ventilatory response to hypoxia was assessed in 15 infants (6 in the smoking group and 9 in the non-smoking group) while breathing 14% oxygen.

RESULTS

Respiratory drive (P(0.1 ) = 4.9 +/- 1.3 cm H(2 )O) was lower in infants in the smoking group compared with those in the non-smoking group (P(0.1 ) = 5.9 +/- 1.2 cm H(2 )O) (P <.05). The time to peak tidal expiratory flow (tPTEF) was also shorter (0.25 +/- 0.04 seconds vs 0.32 +/- 0.09 seconds, P <.05). Infants born to non-smoking mothers showed a significant ventilatory response to hypoxia and a 24.6% increase in P(0.1 ). Infants in the smoking group showed a blunted ventilatory response to hypoxia and no increase in P(0.1 ). A dose-response relationship existed between the number of cigarettes smoked by the mother (0, 1 to 10, >10 per day) and the results for P(0.1 ) and tPTEF. Paternal smoking had no influence on the infant's resting ventilation, respiratory drive, or ventilatory response to hypoxia.

CONCLUSIONS

Infants born to smoking mothers have a reduced drive to breathe and a blunted ventilatory response to hypoxia. These findings may contribute to the increased risk of sudden infant death syndrome in these infants.

Functional and developmental studies of the peripheral arterial chemoreceptors in rat: effects of nicotine and possible relation to sudden infant death syndrome

The drive on respiration mediated by the peripheral arterial chemoreceptors was assessed by the hyperoxic test in 3-day-old rat pups. They accounted for 22.5 +/- 8.8% during control conditions, but only for 6.9 +/- 10.0% after nicotine exposure, an effect counteracted by blockade of peripheral dopamine type 2 receptors (DA2Rs). Furthermore, nicotine reduced dopamine (DA) content and increased the expression of tyrosine hydroxylase (TH) in the carotid bodies, further suggesting that DA mediates the acute effect of nicotine on arterial chemoreceptor function. During postnatal development TH and DA2R mRNA levels in the carotid bodies decreased. Thus, nicotine from smoking may also interfere with the postnatal resetting of the oxygen sensitivity of the peripheral arterial chemoreceptors by increasing carotid body TH mRNA, as well as DA release in this period. Collectively these effects of nicotine on the peripheral arterial chemoreceptors may increase the vulnerability to hypoxic episodes and attenuate the protective chemoreflex response. These mechanisms may underlie the well-known relation between maternal smoking and sudden infant death syndrome.