NEURONS IN COMMISSURAL NUCLEUS-TRACTUS-SOLITARII REQUIRED FOR FULL EXPRESSION OF THE PULMONARY C-FIBER REFLEX IN RAT

1. The pulmonary C fibre reflex, triggered by activating pulmonary C fibre endings in the lung, consists of rapid shallow breathing (which may be preceded by apnoea), bradycardia, and hypotension. The purpose of this work was to identify proximal synapses in this reflex. From pilot data, we hypothesized that neurones in a discrete region of the commissural nucleus in the nucleus tractus solitarii (NTS) are required for full expression of the pulmonary C fibre reflex. Studies were carried out in urethane-anaesthetized, unilaterally vagotomized, spontaneously breathing rats, in which diaphragm electromyogram, arterial pressure, and blood gases were measured. Phenyldiguanide (PDG) was injected in the right atrium to elicit the pulmonary C fibre reflex. Unilateral NTS injections were made through multibarrelled pipettes containing DL-homocysteic acid (DLH) to mimic the reflex, cobalt chloride to reversibly impair the reflex, and/or dye to mark the injection sites. 2. PDG (5-16-mu-g kg-1) injected in the right atrium of twenty-six rats produced the classic pulmonary C fibre reflex: a vagally mediated, rapid onset of rapid shallow breathing, bradycardia and hypotension. 3. Injection of DLH (3-12 nl of 20 mM for a total of 60-240 pmol) in the dorsomedial aspect of the commissural nucleus of the NTS in thirty rats mimicked the pulmonary C fibre reflex, producing rapid shallow breathing, hypotension, and a slight bradycardia. 4. Interruption of neuronal transmission by injecting cobalt chloride (15-30 nl of 100 mM) in the site where DLH produced rapid shallow breathing, reversibly impaired the rapid shallow breathing and bradycardia produced by right atrial injections of PDG in fifteen rats. The commissural region where DLH produced rapid shallow breathing and cobalt impaired the pulmonary C fibre reflex extended from 720-1100-mu-m caudal to the obex, 30-200-mu-m lateral to mid-line, and 200-600-mu-m ventral to the dorsal surface of the brain stem within the NTS. 5. Taken together, the results suggest that neurones within a discrete region in the dorsomedial commissural nucleus in caudal NTS are required for full expression of the pulmonary C fibre reflex.