Prenatal and early, but not late, postnatal exposure of mice to sidestream tobacco smoke increases airway hyperresponsiveness later in life

Abstract

Background: Cigarette smoke exposure in utero and during early postnatal development increases the incidence of asthma and airway hyperresponsiveness (AHR) later in life, suggesting that a possible critical period of developmental sensitivity exists in the prenatal and early postnatal periods.

Objective: We investigated mechanisms of susceptibility during critical developmental periods to sidestream smoke (SS) exposure and evaluated the possible effects of SS on neural responses.

Methods: We exposed three different age groups of mice to either SS or filtered air (FA) for 10 consecutive days beginning on gestation day (GD) 7 by maternal exposure or beginning on postnatal day (PND) 2 or PND21 by direct inhalation. Lung function, airway substance P (SP) innervation, and nerve growth factor (NGF) levels in broncho alveolar lavage fluid were measured after a single SS exposure on PND59.

Results: Methacholine (MCh) dose response for lung resistance (R(L)) was significantly elevated, and dynamic pulmonary compliance (C(dyn)) was significantly decreased, in the GD7 and PND2 SS exposure groups compared with the FA groups after SS exposure on PND59. At the same time points, the percent area of SP nerve fibers in tracheal smooth muscle and the levels of NGF were significantly elevated. MCh dose-response curves for R(L) and C(dyn), SP nerve fiber density, and the level of NGF were not significantly changed in the PND21 exposure group after SS exposure on PND59.

Conclusions: These results suggest that a critical period of susceptibility to SS exposure exists in the prenatal and early postnatal period of development in mice that results in increased SP innervation, increased NGF levels in the airway, and enhanced MCh AHR later in life.

Keywords: airway innervation; asthma; muscarinic agonists; neurokinin receptor; neurotrophic factor.

Figure 1

Time line for SS exposure and experimental measurement. Prenatal exposures occurred between GD7 and GD16; early postnatal exposure occurred PNDs 2–11; late postnatal exposure occurred PNDs 21–30. All groups were exposed to SS on PND59. Pulmonary function testing was conducted immediately before and 16 hr after reexposure. Samples for all other evaluations (bronchoalveolar lavage and airway removal for immunocytochemistry) were collected 16 hr after exposure.

Figure 2

MCh dose responses of R_L and C_dyn in GD7 (A and D), PND2 (B and E), and PND21 (C and F) FA- or SS-exposed mice before SS exposure on PND59. Data are mean ± SE of 6 mice/group.

Figure 3

MCh dose responses of R_L and C_dyn in GD7 (A and D), PND2 (B and E), and PND21 (C and F) FA- or SS-exposed mice after SS exposure on PND59. Data are mean ± SE of six mice/group. *Significant difference in corresponding data between FA and SS animals (p ≤ 0.05).

Figure 4

Fluorescence photomicrographs of SP-immunoreactive NFD within tracheal smooth muscle in GD7 FA- (A) or SS-exposed (B), PND2 FA- (C) or SS-exposed (D), and PND21 FA- (E) or SS-exposed (F) mice after SS exposure on PND59. (A, C, and E) FA exposure in GD7, PND2, and PND21 groups: negative SP immunoreactivity in tracheal smooth muscle. (B and D) SS exposure in GD7 and PND2 groups: increased SP-immunoreactive nerve fibers in tracheal smooth muscle. (F) SS exposure in PND21: few SP-immunoreactive nerve fibers are present in tracheal smooth muscle.

Figure 5

Changes of SP NFD in tracheal smooth muscle in GD7 (A), PND2 (B), and PND21 (C) FA- or SS-exposed mice after SS exposure on PND59. Data are mean ± SE of 8/group. *Significant difference between FA- and SS-exposed mice (p ≤ 0.05).

Figure 6

Levels of NGF release in BALF in GD7 (A), PND2 (B), and PND21 (C) FA- or SS-exposed mice after SS exposure on PND59. Data are mean ± SE of 6/group. NGF was measured by ELISA. *Significant difference between FA- and SS-exposed mice (p ≤ 0.05).