Mechanisms underlying regulation of respiratory pattern by nicotine in preBötzinger complex

Cholinergic neurotransmission plays a role in regulation of respiratory pattern. Nicotine from cigarette smoke affects respiration and is a risk factor for sudden infant death syndrome (SIDS) and sleep-disordered breathing. The cellular and synaptic mechanisms underlying this regulation are not understood. Using a medullary slice preparation from neonatal rat that contains the preBötzinger Complex (preBötC), the hypothesized site for respiratory rhythm generation, and generates respiratory-related rhythm in vitro, we examined the effects of nicotine on excitatory neurotransmission affecting inspiratory neurons in preBötC and on the respiratory-related motor activity from hypoglossal nerve (XIIn). Microinjection of nicotine into preBötC increased respiratory frequency and decreased the amplitude of inspiratory bursts, whereas when injected into XII nucleus induced a tonic activity and an increase in amplitude but not in frequency of inspiratory bursts from XIIn. Bath application of nicotine (0.2--0.5 microM, approximately the arterial blood nicotine concentration immediately after smoking a cigarette) increased respiratory frequency up to 280% of control in a concentration-dependent manner. Nicotine decreased the amplitude to 82% and increased the duration to 124% of XIIn inspiratory bursts. In voltage-clamped preBötC inspiratory neurons (including neurons with pacemaker properties), nicotine induced a tonic inward current of -19.4 +/- 13.4 pA associated with an increase in baseline noise. Spontaneous excitatory postsynaptic currents (sEPSCs) present during the expiratory period increased in frequency to 176% and in amplitude to 117% of control values; the phasic inspiratory drive inward currents decreased in amplitude to 66% and in duration to 89% of control values. The effects of nicotine were blocked by mecamylamine (Meca). The inspiratory drive current and sEPSCs were completely eliminated by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) in the presence or absence of nicotine. In the presence of tetrodotoxin (TTX), low concentrations of nicotine did not induce any tonic current or any increase in baseline noise, nor affect the input resistance in inspiratory neurons. In this study, we demonstrated that nicotine increased respiratory frequency and regulated respiratory pattern by modulating the excitatory neurotransmission in preBötC. Activation of nicotinic acetylcholine receptors (nAChRs) enhanced the tonic excitatory synaptic input to inspiratory neurons including pacemaker neurons and at the same time, inhibited the phasic excitatory coupling between these neurons. These mechanisms may account for the cholinergic regulation of respiratory frequency and pattern.