Perinatal exposure to nicotine causes deficits associated with a loss of nicotinic receptor function

Abstract

We investigated the role played by beta2-containing neuronal nicotinic receptors [nicotinic acetylcholine receptors (nAChRs)] in mediating nicotine's side effects in the fetus and newborn. Pregnant WT and mutant mice lacking the beta2 nAChR subunit were implanted with osmotic minipumps that delivered either water or a controlled dose of nicotine. Subsequently, we compared the development of the sympathoadrenal system and breathing and arousal reflexes of offspring shortly after birth, a period of increased vulnerability to nicotine exposure. Newborn WT pups exposed to nicotine exhibited all of the deficits associated with maternal tobacco and nicotine use, and linked to poor neonatal outcome: growth restriction, unstable breathing, and impaired arousal and catecholamine biosynthesis. Remarkably similar deficits were detected in pups lacking beta2-containing nAChRs. Loss-of-function of these nAChRs consequently reproduces with astonishing fidelity many of the abnormalities caused by perinatal nicotine exposure. We propose that the underlying mechanisms of nicotine's detrimental side effects on a range of crucial defensive reflexes involve loss of function of nAChR subtypes, possibly via activity-dependent desensitization.

Fig. 1.

Nicotine exposure restricts growth via β2-containing nAChRs. Nicotine-exposed WT pups were severely growth-restricted compared with WT controls (A). *, P < 0.0001. Nicotine-exposed mutants, however, were appropriately grown. Mutant as well as WT pups exposed to nicotine breathed more slowly at rest (B), suggesting that nicotine acts via non-β2-containing nAChRs to alter basal breathing rhythm. Con, control; Nic, nicotine-exposed. Error bars indicate SEM.

Fig. 2.

Nicotine exposure destabilizes breathing via β2-containing nAChRs. Pups were studied in a plethysmograph (A). Inspired PO₂ and PCO₂ varied cyclically (B). Respiratory rate (f; % baseline) during hypoxia (P1–P6) and recovery in air (R1–R6) is shown (C), as is breathing rhythm after 43 min (D–G). Control WT pups had a weak rate response (P1 and P2 in C) and never became apneic during recovery (D). Control mutants were tachypneic during hypoxia (P1 and P2 in C) and became apneic during recovery (E). Nicotine-exposed WT pups behaved like the mutant (C Lower; compare F and G). Error bars indicate SEM.

Fig. 3.

Nicotine depresses hypoxic arousal. Arousal (body movement, MVT) was pressure artifact (A). Nicotine reduced arousal in WT pups but had the reverse effect in mutants (B). *, P = 0.02 for genotype-by-treatment interaction; MVT is expressed as % of total test. The frequency of sighing (a brainstem reflex that stabilizes lung volume) was comparable across genotypes and treatment groups (C and D). Interaction P = 0.3. Nicotine acting at β2-containing nAChRs consequently depresses cortical, not brainstem, reflexes. The brisk arousal response of the nicotine-exposed mutant may reflect greater catecholamine activity (Fig. 4). Error bars indicate SEM.

Fig. 4.

Nicotine exposure reduces adrenal catecholamine biosynthesis. In control (water-exposed) pups, catecholamine content was lower in the mutant (KO). Nicotine exposure had opposing effects in WT and mutant (KO) pups, reducing levels in the former but elevating them in the latter (Right; *, †, and ¶, P < 0.05). Preservation of this reflex could have important survival benefits (nicotine-treated mutants). Also see Table 1. Error bars indicate SEM.