Gender comparisons of control of breathing and metabolism in conscious mice exposed to cold

Enhanced non-eupneic breathing following hypoxic, hypercapnic or hypoxic-hypercapnic gas challenges in conscious mice

C57BL6 mice display non-eupneic breathing and spontaneous apneas during wakefulness and sleep as well as markedly disordered breathing following cessation of a hypoxic challenge. We examined whether (1) C57BL6 mice display marked non-eupneic breathing following hypercapnic or hypoxic-hypercapnic challenges, and (2) compared the post-hypoxia changes in non-eupneic breathing of C57BL6 mice to those of B6AF1 (57BL6 dam × A/J sire) and Swiss-Webster mice, which display different ventilatory responses than C57BL6 mice. C57BL6 mice displayed marked increases in respiratory frequency and non-eupneic breathing upon return to room-air after hypoxic (10% O2, 90% N2), hypercapnic (5% CO2, 21% O2 and 74% N2) and hypoxic-hypercapnic (10% O2, 5% CO2 and 85% N2) challenges. B6AF1 mice displayed less tachypnea and reduced non-eupneic breathing post-hypoxia, whereas Swiss-Webster mice displayed robust tachypnea with minimal increases in non-eupneic breathing post-hypoxia. These studies demonstrate that non-eupneic breathing increases after physiologically-relevant hypoxic-hypercapnic challenge in C57BL6 mice and suggest that further studies with these and B6AF1 and Swiss-Webster mice will help define the genetics of non-eupneic breathing.

Hypoxia-induced ventilatory responses in conscious mice: gender differences in ventilatory roll-off and facilitation

The aim of this study was to compare the ventilatory responses of C57BL6 female and male mice during a 15 min exposure to a hypoxic-hypercapnic (H-H) or a hypoxic (10% O(2), 90% N(2)) challenge and subsequent return to room air. The ventilatory responses to H-H were similar in males and females whereas there were pronounced gender differences in the ventilatory responses during and following hypoxic challenge. In males, the hypoxic response included initial increases in minute volume via increases in tidal volume and frequency of breathing. These responses declined substantially (roll-off) during hypoxic exposure. Upon return to room-air, relatively sustained increases in these ventilatory parameters (short-term potentiation) were observed. In females, the initial responses to hypoxia were similar to those in males whereas roll-off was greater and post-hypoxia facilitation was smaller than in males. The marked differences in ventilatory roll-off and post-hypoxia facilitation between female and male C57BL6 mice provide evidence that gender is of vital importance to ventilatory control.

M(1)/M(3) and M(2)/M(4) muscarinic receptor double-knockout mice present distinct respiratory phenotypes

We investigated the role of muscarinic acetylcholine receptors in the control of breathing. Baseline breathing at rest and ventilatory responses to brief exposures to hypoxia (10% O(2)) and hypercapnia (3% and 5% CO(2)), measured by whole-body plethysmography in partially restrained animals, were compared in mice lacking either M(1) and M(3) or M(2) and M(4) muscarinic receptors, and in wild-type matched controls. M(1/3)R double-knockout mice showed at rest an elevated ventilation (V (E)) due to a large (57%) increase in tidal volume (V(T)). Chemosensory ventilatory responses were unaltered. M(2/4)R double-knockout mice were agitated and showed elevated V (E) and breathing frequency (f(R)) at rest when partially restrained, but unaltered V (E) and low f(R) when recorded unrestrained. Chemosensory ventilatory responses were unaltered. The results suggest that M(1) and M(3) receptors are involved in the control of tidal volume, while M(2) and M(4) receptors may be involved in the control of breathing frequency at rest and response to stress.

Acetazolamide affects breathing differently in ICR and C57 mice

Acetazolamide (ACZ) administration was compared on ventilation in outbred male ICR Swiss Webster (ICR) and inbred C57BL/6J (C57) mice, used in development of transgenic strains. We hypothesized that in both strains ACZ would affect breathing similarly. Mice received intraperitoneally vehicle and the next week ACZ (40 mg/kg), and were exposed to air for 90 min, followed by 5-min exposure to 10% O(2), air for 15 min, and to 5 min of 5% CO(2) in O(2). Ventilation was evaluated using plethysmography. ACZ stimulated ventilation in both stains exposed to air. C57 mice minimally increased frequency and tidal volume, whereas ICR mice markedly increased frequency. Strain differences in the ventilatory pattern in response to hypoxia and hypercapnia occurred. ACZ-treated ICR mice decreased hypoxic responsiveness to 50% of vehicle values, whereas ACZ had no effect in C57 mice. ACZ decreased hypercapnic ventilatory responsiveness in both strains. Differential effects of ACZ breathing in these two strains suggest that genetic factors modulate its effect on breathing.

Increased ventilation and CO2 chemosensitivity in acetylcholinesterase knockout mice

To investigate the effects of a permanent excess of acetylcholine (AChE) on respiration, breathing and chemosensitivity were analyzed from birth to adulthood in mice lacking the AChE gene (AChE-/-), in heterozygotes, and in control wild-type (AChE+/+) littermates. Breathing at rest and ventilatory responses to brief exposures to hypoxia (10% O2) and hypercapnia (3-5% CO2) were measured by whole-body plethysmography. At rest AChE-/- mice show larger tidal volumes (VT, + 96% in adults), overall ventilation (VE, + 70%), and mean inspiratory flow (+270%) than wild-type mice, with no change in breathing frequency (fR). AChE-/- mice have a slightly blunted response to hypoxia, but increased VE and fR responses to hypercapnia. Heterozygous animals present no consistent alterations of breathing at rest and chemosensitivity is normal. Adult AChE-/- mice have an increased VE/VO2 and a marginally higher normalized VO2. The results suggest that the hyperventilation and altered chemosensitivity in AChE-/- mice largely reflect alterations of central respiratory control.