beta 2 nicotinic acetylcholine receptor subunit modulates protective responses to stress: A receptor basis for sleep-disordered breathing after nicotine exposure

Abstract

Nicotine exposure diminishes the protective breathing and arousal responses to stress (hypoxia). By exacerbating sleep-disordered breathing, this disturbance could underpin the well established association between smoking and the increased risk of sudden infant death syndrome. We show here that the protective responses to stress during sleep are partially regulated by particular nicotinic acetylcholine receptors (nAChRs). We compared responses of sleeping wild-type and mutant mice lacking the beta2 subunit of the nAChR to episodic hypoxia. Arousal from sleep was diminished, and breathing drives accentuated in mutant mice indicating that these protective responses are partially regulated by beta2-containing nAChRs. Brief exposure to nicotine significantly reduced breathing drives in sleeping wild-type mice, but had no effect in mutants. We propose that nicotine impairs breathing (and possibly arousal) responses to stress by disrupting functions normally regulated by beta2-containing, high-affinity nAChRs.

Figure 1

Nicotine diminishes the drive to breathe in sleeping wild-type mice. Ventilation (V_E) was measured by whole-body plethysmography (A) after a single i.p. injection of saline (○, morning studies) or nicotine (■, 0.5 mg⋅kg⁻¹, afternoon studies). The HVR (period of hypoxia = shaded panels) was then recorded at +13 min, (B, mice awake) and again 1 h after injection (asleep, C). Only HVRs elicited during sleep are shown (D and E). The downward displacement of the curve of wild-type mice signaled diminished ventilatory (breathing) drive after nicotine exposure (D; *, P = 0.015), an effect not evident in mutant mice (E; P = 0.8 nicotine vs. saline).

Figure 2

The arousal response to episodic hypoxia is attenuated in β2 mutant mice. Either 20 min (A) or 50 min (B) of episodic hypoxia were administered; arousal from sleep was defined by movement (MVT) artifact (C). The arousal response from mutants (□) was consistently lower than from wild-type mice (■) to both stimuli (*, P = 0.015).

Figure 3

Respiratory responses to hypoxia are accentuated in β2 mutant mice. Mean ventilatory responses to 20-min episodic hypoxia illustrate persistent facilitation during the first posthypoxic recovery cycle (A) in mutants (○) but not wild-type mice (●). Comparison of the first (B) and fifth (C) recovery periods during 50-min episodic hypoxia illustrates that, in wild-type mice, significant facilitation was only evident after exposure to five cycles of repetitive hypoxia (C). Note the greater HVR of the mutants (D).

Figure 4

Hypoxia increases central respiratory drive in β2 mutant mice. Sudden hyperoxia rapidly diminished breathing efforts (A and B; ○, mutants; ●, wild-type mice). The fall in ventilation (ΔV_E) measured peripheral drive, which was comparable for both groups of mice at rest (C), but after a period of hypoxia, was less in mutants (D). The persistent hyperpnea of mutants in O₂ (“afterdischarge,” B) probably originated centrally.

Figure 5

Agarose gel electrophoresis of the reverse transcriptase–PCR products from total RNA of murine carotid bodies. Transcripts for the nAChR subunits α3, α4, α5, α7, β2, and β4, and tyrosine hydroxylase (TH) were detected.