Fos-Tau-LacZ mice reveal sex differences in brainstem c-fos activation in response to mild carbon dioxide exposure

Abstract

There are sex differences in the neurochemistry of brainstem nuclei that participate in the control of breathing, as well as sex differences in respiratory responses to hypoxia. Central chemoreception refers to the detection within the brain of minute changes in carbon dioxide (CO 2 ) levels and the subsequent modulation of breathing. Putative central chemoreceptor sites are widespread and include cells located near the ventral surface of the brainstem in the retrotrapezoid nucleus (RTN), in the medullary midline raphe nuclei, and, more dorsally in the medulla, in the nucleus of the solitary tract and in the locus coeruleus at the pontomedullary junction, as well as in the fastigial nucleus of the cerebellum. In this study, we ask if the cells that respond to CO 2 differ between the sexes. We used a transgenic mouse with a c-fos promoter driven tau-lacZ reporter construct (FTL) to map the locations of cells in the mouse brainstem and cerebellum that responded to exposure of mice of both sexes to 5% CO 2 or room air (control). X-gal (5-bromo-4-chloro-3-indolyl-beta-Dgalactopyranoside) histochemical staining to detect the beta-galactosidase enzyme produced staining in the brains of mice of both sexes in all of the previously identified putative chemoreceptor sites, with the exception of the fastigial nucleus. Notably, the male RTN region contained significantly more x-gal labeled cells than the female RTN region. In addition to new observations regarding potential sex differences in the retrotrapezoid region, we found the FTL mouse to be a useful tool for identifying cells that respond to the exposure of the whole animal to relatively low concentrations of CO 2 .

Brainstem catecholaminergic neurones and breathing control during postnatal development in male and female rats

“…Studies by Niblock et al (2012) observed that female RTN neurones are activated after 10% CO 2 exposure, but not after 5% CO 2 exposure, in vivo. On the other hand, male RTN neurones are activated both when exposed to 5% or 10% CO 2 , indicating that there are sex differences in the CO 2 threshold required for the activation of the RTN neurones, with males having a lower threshold (Niblock et al 2010(Niblock et al , 2012, which might explain the differences in CO 2 responses that we found. Also, other chemosensitive regions, such as the locus coeruleus (LC), may be acting on this CO 2 response in a sex-dependent manner.…”

“…Assessment of the sensitivity of central chemoreceptors to CO 2 in vivo indicates that the CO 2 threshold for the activation of RTN neurones is higher in females than in male adult mice (Niblock et al . , ). Studies by Niblock et al .…”

“…The difference between the present study and the previous study by Holley et al (2012) might be related to difference in rat strains used (Wistar in the present study vs. Sprague-Dawley) or the difference in days considered for analysis; we considered juvenile rats to be aged 20-21 days, whereas Holley et al (2012) performed experiments in P30 animals. Assessment of the sensitivity of central chemoreceptors to CO 2 in vivo indicates that the CO 2 threshold for the activation of RTN neurones is higher in females than in male adult mice (Niblock et al 2010(Niblock et al , 2012. Studies by Niblock et al (2012) observed that female RTN neurones are activated after 10% CO 2 exposure, but not after 5% CO 2 exposure, in vivo.…”

Central Chemoreceptors: Locations and Functions

“…Studies in healthy human subjects using mechanical ventilation to induce hypocapnia have demonstrated that the apneic threshold is <5 mmHg below the normal, eupneic PaCO 2 (157, 227). And the addition of very small amounts of inspired CO 2 to sleeping subjects who exhibit periodic breathing stabilizes breathing (17, 191). Thus, central and peripheral CO 2 sensitivity are very important in determining apnea and periodic breathing (10, 108, 264).…”

“…It seems that some level of CO 2 is needed to maintain the normal interactions among the different types of respiratory neurons and to provide a ‘drive’ to the system, at least in NREM sleep and anesthesia. This has been demonstrated dramatically by Bruce Lindsey and his colleagues (191) using their unique approach in decerebrate, ventilated cats with simultaneous multi-array recording of neurons each with individual electrode depth adjustment in the regions of the medullary raphe nuclei, the ventral respiratory column, and the pontine respiratory group (Nuding et al, 2005 abst). Hyperventilation decreased end-tidal PCO 2 from 29 to 10 mm Hg and abolished rhythmic phrenic nerve activity.…”