Effect of sinus denervation and vagotomy on c-fos expression in the nucleus tractus solitarius after exposure to C02

Exposure to hypercapnia and electrical stimulation of the carotid sinus nerve (CSN) has been shown to induce c-fos expression in several brain stem regions including the nucleus tractus solitarius (NTS). To test whether the labeled neurons were activated directly by hypercapnia or secondarily via the carotid bodies (sinus nerve), adult rats were exposed to either air or 14–16% CO₂ for 1 h. Experiments were done on eight groups: (1) exposure to air, (2) exposure to CO₂, (3) chronic CSN denervation/CO₂, (4) chronic unilateral CSN denervation/CO₂, (5) chronic sham CSN denervation/CO₂, (6) anesthetized/CO₂, (7) anesthetized and acute vagotomy/CO₂, and (8) premedicated with morphine, 10 mg s.c., 20 min before exposure to CO₂. After exposure to CO₂ or air the rats were anesthetized, perfused with 4% paraformaldehyde and the brains processed for immunohistochemical staining for c-fos protein using the PAP (i.e. peroxidase anti-peroxidase) technique. Labeled neurons in the area of the NTS in every second 50-μm section were counted and their position plotted using a microscope and camera lucida attachment. Rats exposed to CO₂ had a significantly greater number of labeled neurons in the NTS than those exposed to air. Other interventions, such as CSN denervation, surgery, anesthesia, vagotomy or injection of morphine did not significantly affect the level of c-fos expression in rats exposed to hypercapnia, indicative of central stimulation rather than secondary peripheral input. These responsive neurons may be part of a widespread central chemoreceptive complex. Received: 27 March 1995/Received after revision and accepted: 21 November 1995