Relationship between the distribution of the paired-like homeobox gene (Phox2b) expressing cells and blood vessels in the parafacial region of the ventral medulla of neonatal rats.

It has been reported that central chemoreceptor cells in the medulla are distributed in close apposition to capillary blood vessels in the medulla. Phox2b-expressing neurons in the retrotrapezoid nucleus (RTN) respond to high CO(2)/H(+) stimulation and have been suggested to play an important role in central chemoreception. In newborn rats, the RTN overlaps at least partially with the parafacial respiratory group (pFRG), which consists predominantly of preinspiratory neurons. In the present study, we visualized the blood vessels in the ventral medulla of newborn rats using a neurobiotin method and examined the relationship between the blood vessels and the location of Phox2b-immunoreactive (-ir) neurons. We showed that Phox2b-ir neurons in the parafacial region of the rostral ventral medulla tended to assemble around capillary blood vessels. We also confirmed that pFRG/preinspiratory neurons that were sensitive to hypercapnic stimulation in the presence of tetrodotoxin were Phox2b-ir neurons and were tightly apposed to the blood vessels along the longitudinal axis. Our findings suggested that the location of Phox2b-ir neurons, including preinspiratory neurons of the pFRG, matched their role as sensors of blood CO(2) concentration.