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The solitary tract nucleus (NTS), the primary gustatory relay, predominantly ipsilaterally projects to the pontine parabrachial nucleus (PB), the secondary relay. But, Williams et al. (1996) indicated a significant contralateral projection. The present study examined a pathway of the contralateral projection and a distribution of axon terminals in the contralateral PB with an anterograde neuronal tracing method using wheat germ agglutinin-conjugated horseradish peroxidase. After recording taste responses in the rostral portion of the NTS with a glass pipette containing a tracer solution, the tracer was injected electrophoretically. Following tracer injection we found anterograde labelings in the bilateral PB. Labeled axons connecting the bilateral PB were observed in the rostral medullary velum. Anterograde labelings in the contralateral PB were distributed symmetrically to those in the ipsilateral PB. However, they were quite sparse compared with dense labelings in the ipsilateral side. Further, in the ipsilateral PB very dense labelings were seen in areas abutting dorsally and ventrally to the brachium conjunctivum, where gustatory neurons have been reported to be located, but such denser labelings were not found in a diffuse distribution of the labelings in the contralateral side. The findings indicate that NTS gustatory neurons contralaterally project to the PB through the rostral medullary velum and suggest that the contralateral projection is not significant in gustatory information processing. [Jpn J Physiol 54 Suppl:S176 (2004)]
