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An in vitro brainstem preparation of the neonatal rat with intact right vagal (X) innervation of the right atrium, and intact medullary roots of the left X and glossopharyngeal (IX) nerves for stimulation was developed. The preparation was continuously superfused with artificial CSF at 25 degrees C. The electrical activity of the right atrium was recorded to determine the heart rate. Applications of atropine or propranolol to the superfusate did not alter the heart rate. Electrical stimulation (0.5 ms pulse, 20 Hz) of the left IX and X afferents elicited a reduction in the heart rate from 70.3 +/- 13.2 to 50.6 +/- 13.2 beats/min (mean +/- SD, p &lt; 0.05), which was abolished after division of the right X or application of atropine to the superfusing solution. A similar reflex bradycardia was seen in a preparation with intact left vagal-right atrium innervation during right IX and X afferent stimulation. Cervical spinal cord transection affected neither the baseline heart rate nor the magnitude of the reflex bradycardia. Longitudinal sectioning of the medulla oblongata in the mid-line down to the level of the posterior inferior cerebellar artery abolished the heart rate response. After bilateral cervical vagotomies, electrical stimulation (0.5 ms pulse, 20 Hz, up to 100 microA) of the ventrolateral medulla oblongata, lateral funiculus at C2 or intermediate nucleus of the spinal cord at Th1-4 did not affect the heart rate. These results indicate that the functions in the lower brainstem are preserved in this preparation, at least in regard to the generation of reflex bradycardia. The results also suggest that the laterality of cardiac vagal innervation and sympathetic innervation will develop during the postnatal period. This preparation may be useful for the study of the central neuronal network controlling the heart rate.

An In Vitro Brainstem-Heart Preparation of the Neonatal Rat with Intact Right Vagus Nerve.