Prenatal nicotine exposure increases the strength of GABA_A receptor-mediated inhibition of respiratory rhythm in neonatal rats

Infants born to mothers that smoke while pregnant have a relatively high incidence of central respiratory control abnormalities. Recent studies have shown that prenatal nicotine exposure increases GABA release and the frequency of GABAergic currents, leading to an up-regulation of GABA_A receptors in central neurones. Activation of GABA_A receptors inhibits ventilatory activity, with intense activation causing apnoea. These observations lead us to hypothesize that prenatal nicotine exposure alters GABAergic control of respiratory motor pattern in the early neonatal period. Osmotic minipumps were implanted in pregnant Sprague-Dawley rats on the fifth day of gestation, and filled with nicotine (6 mg kg^-1 day^-1 2.5 μl h^-1) or physiological saline (2.5 μl h^-1). Brainstem-spinal cord preparations from 1 - to 3-day-old neonates were studied under in vitro conditions. Electrical activity was recorded from the fourth cervical ventral root (C4 VR), which contains the axons of phrenic motoneurones. Bath application of GABA_A receptor agonists muscimol (250 μm) or pentobarbital sodium (60 μm) to the brainstem led to consistent, reversible and significant reductions in C4 VR burst frequency. In saline-exposed animals, frequency (bursts min^-1) fell from 6.8 ± 0.4 to a nadir of 2.8 ± 0.5 with muscimol, and from 6.5 ± 0.3 to a nadir of 2.9 ± 0.3 for pentobarbital; in nicotine-exposed animals, frequency fell from 6.3 ± 0.4 to 1.0 ± 0.4 with muscimol and from 6.4 ± 0.2 to 1.7 ± 0.4 with pentobarbital (P < 0.05 in all cases). The decrease in C4 VR frequency was significantly greater in nicotine-exposed compared to saline-exposed preparations with both muscimol and pentobarbital (P < 0.001 for both). There were no changes in the amplitude of C4 VR bursts under any condition. The GABA_A receptor antagonist bicuculline methiodide (8 μm) did not change C4 VR frequency or amplitude in either group, although it was effective in reversing the effects of muscimol. These experiments demonstrate that prenatal nicotine exposure alters the GABAergic regulation of respiratory rhythm in a reduced preparation. The results may lead to a better understanding of the perturbed breathing pattern observed in neonates that are exposed to nicotine in uttero.