The trigeminocardiac and diving reflexes are among the most powerful autonomic reflexes. Electrical or mechanical stimulation of the trigeminal nerve evokes a dramatic decrease in heart rate in animals including man, and has often been termed the `trigeminocardiac reflex'in the clinical literature, and `trigeminal depressor responses'in animal studies. A subset of the trigeminocardiac reflex is the diving reflex. Stimulation of the diving reflex by exposing the nasal mucosa to water or air-borne chemical irritants evokes a pronounced bradycardia with heart rate decreasing up to 51% upon a single facial submersion. However an exaggerated diving reflex has been implicated in sudden infant death syndrome (SIDS). SIDS is the leading cause of death among infants who are 1 month to 1 year old. One of the highest risk factors for SIDS is cigarette smoking and while tobacco smoke contains a number of deleterious agents in addition to nicotine, nicotine is of particular concern in relation to SIDS due to its ability to cross the placenta and concentrate in the fetus, and prenatal nicotine exposure impairs the ability of newborn animals to resuscitate from cardiorespiratory challenges. This project will directly test the hypotheses that activation of sensory neurons in the trigeminocardiac and diving reflexes excites cardiac vagal neurons, these reflex pathways are endogenously and differentially modulated by nicotinic and muscarinic cholinergic receptors, determine where in this reflex pathway this modulation occurs, and finally whether fetal exposure to nicotine exaggerates this excitation of cardiac vagal neurons. To test these hypotheses we will utilize techniques that are quite novel to this field. Two such new approaches include the identification of the synaptic terminals of sensory neurons originating in the nasal mucosa using lentivirus expression of enhanced yellow fluorescent protein (eYFP), as well as the expression of channelrhodopsin-2 (ChR2), a light activated cation channel, to selectively photoactivate the fibers and synaptic terminals of these nasal sensory neurons.
Project Narrative - Public Health Relevance Statement This work will address hypotheses fundamental to understanding the cellular basis and mechanisms by which nicotinic and muscarinic cholinergic receptors modulate the diving reflex within the brainstem, and will also suggest which receptors and processes are altered by fetal exposure to nicotine that increases the risk of cardiorespiratory diseases such as sudden infant death syndrome (SIDS).
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