Conversion from testosterone to oestradiol is required to modulate respiratory long-term facilitation in male rats

Ventilation and phrenic output following high cervical spinal hemisection in male vs. female rats

Female sex hormones influence the neural control of breathing and may impact neurologic recovery from spinal cord injury. We hypothesized that respiratory recovery after C2 spinal hemisection (C2HS) differs between males and females and is blunted by prior ovariectomy (OVX) in females. Inspiratory tidal volume (VT), frequency (fR), and ventilation (VE) were quantified during quiet breathing (baseline) and 7% CO2 challenge before and after C2HS in unanesthetized adult rats via plethysmography. Baseline breathing was similarly altered in all rats (reduced VT, elevated fR) but during hypercapnia females had relatively higher VT (i.e. compared to pre-injury) than male or OVX rats (p<0.05). Phrenic neurograms recorded in anesthetized rats indicated that normalized burst amplitude recorded ipsilateral to C2HS (i.e. the crossed phrenic phenomenon) is greater in females during respiratory challenge (p<0.05 vs. male and OVX). We conclude that sex differences in recovery of VT and phrenic output are present at 2 weeks post-C2HS. These differences are consistent with the hypothesis that ovarian sex hormones influence respiratory recovery after cervical spinal cord injury.

Impact of intermittent hypoxia on long-term facilitation of minute ventilation and heart rate variability in men and women: do sex differences exist?

Following exposure to intermittent hypoxia, respiratory motor activity and sympathetic nervous system activity may persist above baseline levels for over an hour. The present investigation was designed to determine whether sustained increases in minute ventilation and sympathovagal (S/V) balance, in addition to sustained depression of parasympathetic nervous system activity (PNSA), were greater in men compared with women following exposure to intermittent hypoxia. Fifteen healthy men and women matched for age, race, and body mass index were exposed to eight 4-min episodes of hypoxia during sustained hypercapnia followed by a 15-min end-recovery period. The magnitude of the increase in minute ventilation during the end-recovery period, compared with baseline, was similar in men and women (men, 1.52 +/- 0.03; women, 1.57 +/- 0.02 fraction of baseline; P < 0.0001). In contrast, depression of PNSA and increases in S/V balance were evident during the end-recovery period, compared with baseline, in men (PNSA, 0.66 +/- 0.06 fraction of baseline, P < 0.0001; S/V balance, 2.8 +/- 0.7 fraction of baseline, P < 0.03) but not in women (PNSA, 1.27 +/- 0.19 fraction of baseline, P = 0.3; S/V balance, 1.8 +/- 0.6 fraction of baseline, P = 0.2). We conclude that a sustained increase in minute ventilation, which is indicative of long-term facilitation, is evident in both men and women following exposure to intermittent hypoxia and that this response is independent of sex. In contrast, sustained alterations in autonomic nervous system activity were evident in men but not in women.

Is there a link between intermittent hypoxia-induced respiratory plasticity and obstructive sleep apnoea?

Although neuroplasticity is an important property of the respiratory motor control system, its existence has been appreciated only in recent years and, as a result, its functional significance is not completely understood. The most frequently studied models of respiratory plasticity is respiratory long-term facilitation (LTF) following acute intermittent hypoxia and enhanced LTF following chronic intermittent hypoxia. Since intermittent hypoxia is a prominent feature of sleep-disordered breathing, LTF and/or enhanced LTF may compensate for factors that predispose to sleep-disordered breathing, particularly during obstructive sleep apnoea (OSA). Long-term facilitation has been studied most frequently in rats, and exhibits interesting properties consistent with a role in stabilizing breathing during sleep. Specifically, LTF: (1) is prominent in upper airway respiratory motor activity, suggesting that it stabilizes upper airways and maintains airway patency; (2) is most prominent during sleep in unanaesthetized rats; and (3) exhibits sexual dimorphism (greatest in young male and middle-aged female rats; smallest in middle-aged male and young female rats). Although these features are consistent with the hypothesis that upper airway LTF minimizes the prevalence of OSA in humans, there is little direct evidence for such an effect. Here we review advances in our understanding of LTF and its underlying mechanisms and present evidence concerning a potential role for LTF in maintaining upper airway patency, stabilizing breathing and preventing OSA in humans. Regardless of the relationship between LTF and OSA, a detailed understanding of cellular and synaptic mechanisms that underlie LTF may guide the development of new drugs to regulate upper airway tone, thereby offsetting the tendency for upper airway collapse characteristic of heavy snoring and OSA.