Jesse Dostal

Post-sigh breathing behavior and spontaneous pauses in the C57BL/6J (B6) mouse.

The purpose was to examine sighs and spontaneous pauses in regard to the stability of resting breathing in the B6 strain, compared to the A/J strain. A 5-HT1A receptor agonist (buspirone) and a chromosomal substitution strain (B6a1) were used to further alter breathing patterning. Ten-minute recordings of room air breathing were collected from unanaesthetized B6, A/J, and B6a1 mice. Despite no differences between strains in the magnitude and incidence of sighs, post-sigh apneas, the variation for duration of expiration (Te) after sighs, and the number of spontaneous pauses were greater in the B6, while Shannon Entropy (nonlinear metrics) for Te after sighs was lower in B6, compared to the other strains. Buspirone and chromosomal substitution eliminated post-sigh apneas and decreased spontaneous pauses. A greater irregularity and the lower complexity of post-sigh breathing in B6 are reversed by elements on A/J chromosome 1 and by increased 5-HT1A serotonergic tone.

Effects of buspirone on posthypoxic ventilatory behavior in the C57BL/6J and A/J mouse strains

Buspirone, a partial agonist of the serotonergic 5-HT1A receptor, improves breathing irregularities in humans with Rett syndrome or brain stem injury. The purpose of this study was to examine whether buspirone alters posthypoxic ventilatory behavior in C57BL/6J (B6) and A/J mouse strains. Measurements of ventilatory behavior were collected from unanesthetized adult male mice (n=6 for each strain) using the plethysmographic method. Mice were given intraperitoneal injections of vehicle or several doses of buspirone and exposed to 2 min of hypoxia (10% O2) followed by rapid reoxygenation (100% O2). Twenty minutes later, mice were tested for hypercapnic response (8% CO(2)-92% O2). On a separate day, mice were injected with the 5-HT1A receptor antagonist 4-iodo-N-{2-[4-(methoxyphenyl)-1-piperazinyl] ethyl}-N-2-pyridinylbenzamide (p-MPPI) before the injection of buspirone, and measurements were repeated. In separate studies, arterial blood-gas analysis was performed for each strain (n=12 in B6 and 10 in A/J) with buspirone or vehicle. In both strains, buspirone stimulated ventilation at rest. In the B6 mice, the hypoxic response was unchanged, but the response to hypercapnia was reduced with buspirone (5 mg/kg; P<0.05). With reoxygenation, vehicle-treated B6 exhibited periodic breathing and greater variation in ventilation compared with A/J (P<0.01). In B6 animals, >or=3 mg/kg of buspirone reduced variation and prevented the occurrence of posthypoxic periodic breathing. Both effects were reversed by p-MPPI. Treatment effect of buspirone was not explained by a difference in resting arterial blood gases. We conclude that buspirone improves posthypoxic ventilatory irregularities in the B6 mouse through its agonist effects on the 5-HT1A receptor.

Domperidone and ventilatory behavior: Sprague-Dawley versus Brown Norway rats.

Domperidone, a dopamine D(2) receptor antagonist, is a tool for uncovering the tonic and dynamic effects of the peripheral dopaminergic system in unanesthestized animals. The hypothesis was that domperidone effects would vary between strains of the same species. Ventilatory behavior -- frequency and minute ventilation -- was measured by the plethysmographic method in unrestrained adult male Sprague-Dawley (SD: n=8) and Brown Norway (BN: n=8) rats before, during and after rapid transition to 100% O(2) after 5 min of 13% O(2)/3% CO(2). Tests were done 60 min after intraperitoneal injection of either vehicle (0.1% lactic acid in saline) or a dose of domperidone (0.1, 0.5, 1.0, or 5.0mg/kg) dissolved in vehicle, each on a separate day. Resting frequency and minute ventilation (mean+/-standard deviation) decreased after domperidone in the BN strain (e.g. 94.63/min+/-4.99 versus 87.37/min+/-9.59, p=0.42; 77.3 ml/min+/-9.25 versus 62.13 ml/min+/-11.5, p=0.019, respectively), but did not change in the SD. With increasing doses of domperidone the ventilatory response to hypoxia and reoxygenation became similar owing to a decrease in frequency and minute ventilation in the SD. At a dose altering SD hypoxic responses, the hypercapnic ventilatory response was not significantly affected. In conclusion, breathing frequency and minute ventilation over a challenge with hypoxia and reoxygenation differ with domperidone depending upon genetic background. We speculate that hypoxic ventilatory responses may be differently configured even among strains of the same species.

Identification of novel mouse genes conferring posthypoxic pauses

Although central to the susceptibility of adult diseases characterized by abnormal rhythmogenesis, characterizing the genes involved is a challenge. We took advantage of the C57BL/6J (B6) trait of hypoxia-induced periodic breathing and its absence in the C57BL/6J-Chr 1(A/J)/NaJ chromosome substitution strain to test the feasibility of gene discovery for this abnormality. Beginning with a genetic and phenotypic analysis of an intercross study between these strains, we discovered three quantitative trait loci (QTLs) on mouse chromosome 1, with phenotypic effects. Fine-mapping reduced the genomic intervals and gene content, and the introgression of one QTL region back onto the C57BL/6J-Chr 1(A/J)/NaJ restored the trait. mRNA expression of non-synonymous genes in the introgressed region in the medulla and pons found evidence for differential expression of three genes, the highest of which was apolipoprotein A2, a lipase regulator; the apo a2 peptide fragment (THEQLTPLVR), highly expressed in the liver, was expressed in low amounts in the medulla but did not correlate with trait expression. This work directly demonstrates the impact of elements on mouse chromosome 1 in respiratory rhythmogenesis.

Post-hypoxic Unstable Breathing in the C57BL/6J Mouse: Effects of Acetazolamide

We examined whether acetazolamide (ACZ), a carbonic anhydrase inhibitor, would alter post-hypoxic ventilatory behavior and periodic breathing in the C57BL/6J (B6) mouse. Experiments were performed with unanaesthetized, awake adult male B6 mice (n = 5, 2.5 months old, 21.3 +/- 1.5 g, mean +/- SD) and ventilatory behavior was measured using a flow through body plethysmography. Mice were given an intraperitoneal injection of either vehicle or ACZ (40 mg/kg) and one hour later exposed to 1 min of 8% O2-balance N2 (poikilocapnic hypoxia) or 12%-O2, 3% CO2-balance N2 (non-poikilocapnic hypoxia) followed by rapid reoxygenation (100% O2) of 5 minutes. One minute after reoxygenation, ACZ-treated animals exhibited post-hypoxic frequency decline (p < 0.05), a lower coefficient of variability for frequency (p < 0.001) and no tendency towards periodic breathing (p < 0.05), as compared to vehicle-treated animals. ACZ improves unstable breathing in the B6 model of periodic breathing, despite producing post-hypoxic frequency decline. Our speculation is that periodic breathing occurs through pathways independent of the A5 pontine area.