Mechanisms underlying regulation of respiratory pattern by nicotine in preBotzinger complex

Cholinergic neurotransmission plays a role in regulation of respiratory pat tern. Nicotine from cigarette smoke affects respiration and is a risk facto r for sudden infant death syndrome (SIDS) and sleep-disordered breathing. T he cellular and synaptic mechanisms underlying this regulation are not unde rstood. Using a medullary slice preparation from neonatal rat that contains the preBotzinger Complex (preBotC), the hypothesized site for respiratory rhythm generation, and generates respiratory-related rhythm in vitro, we ex amined the effects of nicotine on excitatory neurotransmission affecting in spiratory neurons in preBotC and on the respiratory-related motor activity from hypoglossal nerve (XIIn). Microinjection of nicotine into preBotC incr eased respiratory frequency and decreased the amplitude of inspiratory burs ts, 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 muM, approximately the arterial bloo d nicotine concentration immediately after smoking a cigarette) increased r espiratory frequency up to 280% of control in a concentration-dependent man ner. Nicotine decreased the amplitude to 82% and increased the duration to 124% of XIIn inspiratory bursts. In voltage-clamped preBotC inspiratory neu rons (including neurons with pacemaker properties), nicotine induced a toni c inward current of -19.4 +/- 13.4 pA associated with an increase in baseli ne noise. Spontaneous excitatory postsynaptic currents (sEPSCs) present dur ing the expiratory period increased in frequency to 176% and in amplitude t o 117% of control values; the phasic inspiratory drive inward currents decr eased in amplitude to 66% and in duration to 89% of control values. The eff ects of nicotine were blocked by mecamylamine (Meca). The inspiratory drive current and sEPSCs were completely eliminated by 6-cyano-7-nitroquinoxalin e-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 to nic current or any increase in baseline noise, nor affect the input resista nce in inspiratory neurons. In this study, we demonstrated that nicotine in creased respiratory frequency and regulated respiratory pattern by modulati ng the excitatory neurotransmission in preBotC. Activation of nicotinic ace tylcholine receptors (nAChRs) enhanced the tonic excitatory synaptic input to inspiratory neurons including pacemaker neurons and at the same time, in hibited the phasic excitatory coupling between these neurons. These mechani sms may account for the cholinergic regulation of respiratory frequency and pattern.