Extended secondhand tobacco smoke exposure induces plasticity in nucleus tractus solitarius second-order lung afferent neurons in young guinea pigs

Infants and young children experiencing extended exposure to secondhand smoke (SHS) have an increased occurrence of asthma, as well as increased cough, wheeze, mucus production and airway hyper-reactivity. Plasticity in lung reflex pathways has been implicated in causing these symptoms, as have changes in substance P-related mechanisms. Using whole-cell voltage-clamp recordings and immunohistochemistry in brainstem slices containing anatomically identified second-order lung afferent nucleus tractus solitarius (NTS) neurons, we determined whether extended SHS exposure during the equivalent period of human childhood modified evoked or spontaneous excitatory synaptic transmission, and whether those modifications were altered by endogenous substance P. SHS exposure enhanced evoked synaptic transmission between sensory afferents and the NTS second-order neurons by eliminating synaptic depression of evoked excitatory postsynaptic currents (eEPSCs), an effect reversed by the neurokinin-1-receptor antagonist (SR140333). The recruitment of substance P in enhancing evoked synaptic transmission was further supported by an increased number of substance P-expressing lung afferent central terminals synapsing onto the second-order lung afferent neurons. SHS exposure did not change background spontaneous EPSCs. The data suggest that substance P in the NTS augments evoked synaptic transmission of lung sensory input following extended exposure to a pollutant. The mechanism may help to explain some of the exaggerated respiratory responses of children exposed to SHS.