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Department of Physiology, School of Medicine, University of Auckland, New Zealand.
1. An attempt has been made to test the hypothesis that in the nucleus of the tractus solitarius (NTS) in the rat, the most caudal region of synaptic terminals of the carotid sinus nerve, just caudal to the obex, represents mainly the site of synapse of chemoreceptor fibres from the carotid body. 2. Under halothane anaesthesia, the neurotoxin kainic acid was used to lesion this region and a second region, immediately rostral to obex, where terminals are thought to arise mainly from baroreceptor fibres of the carotid sinus nerve. 3. Measurements based on the distribution of fluorescent dye co-injected with the kainic acid showed that the two groups of 100 nl microinjections were centered 0.82 mm apart and that the injectate spread through mean distances of 0.57 mm (caudal microinjections) and 0.52 mm (rostral microinjections). Nissl staining was used to determine cellular degeneration. The caudal lesions mostly involved ventrolateral and commissural subnuclei of NTS and the rostral lesions involved lateral and dorsolateral subnuclei. 4. Ventilatory sensitivity to hypoxia was tested under light halothane anaesthesia, 1 day after lesioning. To enhance the responses, the contralateral carotid sinus nerve was sectioned prior to experiments. Caudal lesions reduced the ventilatory response to inspired oxygen (20.9-9.6% O2) by a mean of 67% and rostral lesions by 18% of the effect produced by carotid sinus nerve section on that side. Subsequent section of the carotid sinus nerve on the side of the NTS lesion confirmed that caudal lesions produced effects comparable to those of carotid body denervation; rostral lesions did not. 5. These results strongly support the hypothesis that chemoreceptor and baroreceptor afferent fibres in the carotid sinus nerve synapse at substantially separable sites in the nucleus of the tractus solitarius. The identification of the site in NTS caudal to the obex as the principal site of carotid chemoreceptor synapses places them close to but not upon respiratory premotor neurones of the same nucleus.
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