Acute exercise-induced activation of Phox2b-expressing neurons of the retrotrapezoid nucleus in rats may involve the hypothalamus

Abstract

The rat retrotrapezoid nucleus (RTN) contains neurons that have a well-defined phenotype characterized by the presence of vesicular glutamate transporter 2 (VGLUT2) mRNA and a paired-like homeobox 2b (Phox2b)-immunoreactive (ir) nucleus and the absence of tyrosine hydroxylase (TH). These neurons are important to chemoreception. In the present study, we tested the hypothesis that the chemically-coded RTN neurons (ccRTN) (Phox2b(+)/TH(-)) are activated during an acute episode of running exercise. Since most RTN neurons are excited by the activation of perifornical and lateral hypothalamus (PeF/LH), a region that regulates breathing during exercise, we also tested the hypothesis that PeF/LH projections to RTN neurons contribute to their activation during acute exercise. In adult male Wistar rats that underwent an acute episode of treadmill exercise, there was a significant increase in c-Fos immunoreactive (c-Fos-ir) in PeF/LH neurons and RTN neurons that were Phox2b(+)TH(-) (p<0.05) compared to rats that did not exercise. Also the retrograde tracer Fluoro-Gold that was injected into RTN was detected in c-Fos-ir PeF/LH (p<0.05). In summary, the ccRTN neurons (Phox2b(+)TH(-)) are excited by running exercise. Thus, ccRTN neurons may contribute to both the chemical drive to breath and the feed-forward control of breathing associated with exercise.

Keywords: ANOVA; CPG; DMH; FG; Fluoro-Gold; PAG; PE; PFA; PVH; PeF/LH; Phox2b; RTN; RVLM; TH; VGLUT2; VLM; VMH; analysis of variance; breathing; c-Fos immunoreactive; c-Fos immunoreactivity; c-Fos-ir; ccRTN; central autonomic pathways; central pattern generator; chemically-coded retrotrapezoid nucleus neurons; dorsal medial hypothalamus; exercise; immunoreactive; ir; medulla oblongata; paired-like homeobox 2b; paraformaldehyde; paraventricular nucleus of the hypothalamus; partial pressure of arterial carbon dioxide; partial pressure of arterial oxygen; periaqueductal gray; perifornical and lateral hypothalamus; polyethylene; retrotrapezoid nucleus; rostral ventrolateral medulla; tyrosine hydroxylase; ventrolateral medulla; ventromedial hypothalamus; vesicular glutamate transporter 2.