CO2-induced c-Fos expression in brainstem preprotachykinin mRNA containing neurons

Tachykinin peptides are found in brainstem regions involved in central chemoreception and they may play a modulatory role in ventilatory response to hypercapnia. We determined whether tachykinin peptide containing neurons are activated by CO(2) by combining in situ hybridization and immunohistochemistry (IHH). Experiments were performed in 21-day-old rats exposed to 12% CO(2) for 1 h. c-Fos expression was identified by IHH on free floating sections (40 microm) that were mounted and then hybridized with anti-sense 35S labeled ribonucleotide probe of the rat preprotachykinin A (PPT-A) gene. Sections were analyzed for expression of the PPT-A gene, c-Fos protein and colocalization of PPT-A gene with c-Fos protein. Within the chemosensory region of the nucleus tractus solitarius (nTS), 19% of c-Fos positive cells expressed PPT-A mRNA after hypercapnic loading. In medullary raphe nuclei, 64% of c-Fos positive cells expressed the PPT-A gene after exposure to CO(2), while 21% of c-Fos labeled neurons in parapyramidal nuclei also expressed PPT-A mRNA. These results indicate that a subpopulation of CO(2) activated neurons within the nTS and in the parapyramidal and midline regions of the ventral aspect of the medulla oblongata express the PPT-A gene, suggesting that these are substance P- or neurokinin A-containing neurons. Furthermore, these peptides may play a role in modulation of respiratory and cardiovascular responses to changes in CO(2)/H(+) content of the extracellular fluid.