A MIDLINE AREA IN THE NUCLEUS COMMISSURALIS OF NTS MEDIATES THE PHRENIC-NERVE RESPONSES TO CAROTID CHEMORECEPTOR STIMULATION

The carotid body chemoreceptor afferents have been reported to project to a discrete area located in the nucleus commissuralis of nucleus tr actus solitarius [A. Vardhan et al., Am. J. Physiol., 264 (1993) R41-R 50]. The afore-mentioned study was done in spontaneously breathing rat s and the afferents and efferents located in the chest wall and the re spiratory tract of these animals were intact. In order to exclude the role, if any, of these afferents and efferents, in the present experim ents respiratory changes were monitored by recording the phrenic nerve activity instead of tracheal airflow. Experiments were carried out in pentobarbital-anesthetized, bilaterally vagotomized, paralyzed and ar tificially ventilated rats with a pneumothorax. The carotid body chemo receptors were stimulated with tracheal administration of nitrogen for 7-10 s. The chemoreceptor stimulation induced an increase in the freq uency and amplitude of phrenic nerve bursts. A decrease in the duratio n of inspiratory (T-I), expiratory (T-E) and total cycles (T-TOT) was observed in the phrenic nerve activity. Inhibition of neuronal cell bo dies by microinjections of muscimol (140 pmol/20 nl) into a discrete a rea in the commissural subnucleus of the nucleus tractus solitarius (c oordinates in mm: 0.3 rostral to 0.5 caudal, 0 to 0.5 lateral and 0.3 to 0.5 deep with respect to the calamus scriptorius), attenuated the p hrenic nerve responses to the carotid body stimulation. On the other h and, control injections of saline (0.9%) into this site did not alter the phrenic nerve responses to the carotid body stimulation. Microinje ctions of bicuculline methiodide (a GABA(A) receptor antagonist) rever sed the actions of muscimol. These results further confirm that the ca rotid body chemoreceptors project to a discrete area in the commissura l subnucleus of the nucleus tractus solitarius. This site is tentative ly called chemoreceptor projection site (CPS).