Ventilation and the Response to Hypercapnia after Morphine in Opioid-naive and Opioid-tolerant Rats.

BACKGROUND: Opioid-related deaths are a leading cause of accidental death, with most occurring in patients receiving chronic pain therapy. Respiratory arrest is the usual cause of death, but mechanisms increasing that risk with increased length of treatment remain unclear. Repeated administration produces tolerance to opioid analgesia, prompting increased dosing, but depression of ventilation may not gain tolerance to the same degree. This study addresses differences in the degree to which chronic morphine (1) produces tolerance to ventilatory depression versus analgesia and (2) alters the magnitude and time course of ventilatory depression. METHODS: Juvenile rats received subcutaneous morphine for 3 days (n = 116) or vehicle control (n = 119) and were then tested on day 4 following one of a range of morphine doses for (a) analgesia by paw withdraw from heat or (b) respiratory parameters by plethysmography-respirometry. RESULTS: Rats receiving chronic morphine showed significant tolerance to morphine sedation and analgesia (five times increased ED50). When sedation was achieved for all animals in a dose group (lowest effective doses: opioid-tolerant, 15 mg/kg; opioid-naive, 3 mg/kg), the opioid-tolerant showed similar magnitudes of depressed ventilation (-41.4 ± 7.0%, mean ± SD) and hypercapnic response (-80.9 ± 15.7%) as found for morphine-naive (-35.5 ± 16.9% and -67.7 ± 15.1%, respectively). Ventilation recovered due to tidal volume without recovery of respiratory rate or hypercapnic sensitivity and more slowly in morphine-tolerant. CONCLUSIONS: In rats, gaining tolerance to morphine analgesia does not reduce ventilatory depression effects when sedated and may inhibit recovery of ventilation.

Blast exposure causes early and persistent aberrant phospho- and cleaved-tau expression in a murine model of mild blast-induced traumatic brain injury.

Mild traumatic brain injury (mTBI) is considered the 'signature injury' of combat veterans that have served during the wars in Iraq and Afghanistan. This prevalence of mTBI is due in part to the common exposure to high explosive blasts in combat zones. In addition to the threats of blunt impact trauma caused by flying objects and the head itself being propelled against objects, the primary blast overpressure (BOP) generated by high explosives is capable of injuring the brain. Compared to other means of causing TBI, the pathophysiology of mild-to-moderate BOP is less well understood. To study the consequences of BOP exposure in mice, we employed a well-established approach using a compressed gas-driven shock tube that recapitulates battlefield-relevant open-field BOP. We found that 24 hours post-blast a single mild BOP provoked elevation of multiple phospho- and cleaved-tau species in neurons, as well as elevating manganese superoxide-dismutase (MnSOD or SOD2) levels, a cellular response to oxidative stress. In hippocampus, aberrant tau species persisted for at least 30 days post-exposure, while SOD2 levels returned to sham control levels. These findings suggest that elevated phospho- and cleaved-tau species may be among the initiating pathologic processes induced by mild blast exposure. These findings may have important implications for efforts to prevent blast-induced insults to the brain from progressing into long-term neurodegenerative disease processes.

CO2 relaxation of the rat lung parenchymal strip.

Evidence from liquid-filled rat lungs supported the presence of CO2-dependent, active relaxation of parenchyma under normoxia by unknown mechanisms (Emery et al., 2007). This response may improve matching of alveolar ventilation (V˙A) to perfusion (Q˙) by increasing compliance and V˙A in overperfused (high CO2) regions, and decrease V˙A in underperfused regions. Here, we have more directly studied CO2-dependent parenchymal relaxation and tested a hypothesized role for actin-myosin interaction in this effect. Lung parenchymal strips (∼1.5mm×1.5mm×15mm) from 16 rats were alternately exposed to normoxic hypocapnia ( [Formula: see text] ) or hypercapnia ( [Formula: see text] ). Seven specimens were used to construct length-tension curves, and nine were tested with and without the myosin blocker 2,3-butanedione monoxime (BDM). The results demonstrate substantial, reversible CO2-dependent changes in parenchyma strip recoil (up to 23%) and BDM eliminates this effect, supporting a potentially important role for parenchymal myosin in V˙A/Q˙ matching.

Sodium nitrite mitigates ventilator-induced lung injury in rats.

BACKGROUND: Nitrite (NO2) is a physiologic source of nitric oxide and protects against ischemia-reperfusion injuries. We hypothesized that nitrite would be protective in a rat model of ventilator-induced lung injury and sought to determine if nitrite protection is mediated by enzymic catalytic reduction to nitric oxide. METHODS: Rats were anesthetized and mechanically ventilated. Group 1 had low tidal volume ventilation (LVT) (6 ml/kg and 2 cm H2O positive end-expiratory pressure; n=10); group 2 had high tidal volume ventilation (HVT) (2 h of 35 cm H2O inspiratory peak pressure and 0 cm H2O positive end-expiratory pressure; n=14); groups 3-5: HVT with sodium nitrite (NaNO2) pretreatment (0.25, 2.5, 25 µmol/kg IV; n=6-8); group 6: HVT+NaNO2+nitric oxide scavenger 2-(4-carboxyphenyl)-4,5dihydro-4,4,5,5-tetramethyl-1H-imidazolyl-1-oxy-3oxide(n=6); group 7: HVT+NaNO2+nitric oxide synthase inhibitor N-nitro-L-arginine methyl ester (n=7); and group 8: HVT+NaNO2+xanthine oxidoreductase inhibitor allopurinol (n=6). Injury assessment included physiologic measurements (gas exchange, lung compliance, lung edema formation, vascular perfusion pressures) with histologic and biochemical correlates of lung injury and protection. RESULTS: Injurious ventilation caused statistically significant injury in untreated animals. NaNO2 pretreatment mitigated the gas exchange deterioration, lung edema formation, and histologic injury with maximal protection at 2.5 µmol/kg. Decreasing nitric oxide bioavailability by nitric oxide scavenging, nitric oxide synthase inhibition, or xanthine oxidoreductase inhibition abolished the protection by NaNO2. CONCLUSIONS: Nitrite confers protection against ventilator-induced lung injury in rats. Catalytic reduction to nitric oxide and mitigation of ventilator-induced lung injury is dependent on both xanthine oxidoreductase and nitric oxide synthases.

Lung volume reduction by bronchoscopic administration of steam.

RATIONALE: At present, bronchoscopic approaches to lung volume reduction (LVR) create airway obstruction to achieve parenchyma collapse, avoiding many risks of surgical LVR. However, LVR by these methods is limited by temporary or incomplete collapse and/or residual atelectatic and scarred tissue volumes. Heat-induced ablation of lung tissue is currently under investigation as an alternative LVR methodology. OBJECTIVES: We hypothesized that bronchoscopic steam injection can produce safe and effective LVR, and explored potential mechanisms for the effects. METHODS: Steam treatments were applied bilaterally to six cranial lobe segments of large dogs. For series 1, 14 dogs received one of three target heat dose levels (1, 4, or 8 cal · ml⁻¹ segment volume), and then 3 months of follow-up including pulmonary function testing and monitoring for complications. For series 2, 12 dogs received a single target dose (4 cal · ml⁻¹) or sham, similar follow-up, and then assessment of lobar mass, volume, and blood flow. Vapor content of series 2 steam was 40% greater than for series 1 (similar heat dose) to attempt more peripheral heat delivery. MEASUREMENTS AND MAIN RESULTS: Nineteen of 20 treatment animals survived with minimal evidence of serious risks or reduced pulmonary function testing volumes, but 1 died from pneumothorax 5 days post-treatment. Postmortem processing of animals that survived as planned revealed obvious dose-dependent lobe reductions, additional evidence of risks, and blood flow reduction that occurred immediately post-treatment. CONCLUSIONS: Bronchoscopic administration of steam is a potentially safe means to achieve LVR, but substantial risks are present and further research is recommended.

CO2 relaxes parenchyma in the liquid-filled rat lung.

CO(2) regulation of lung compliance is currently explained by pH- and CO(2)-dependent changes in alveolar surface forces and bronchomotor tone. We hypothesized that in addition to, but independently of, those mechanisms, the parenchyma tissue responds to hypercapnia and hypocapnia by relaxing and contracting, respectively, thereby improving local matching of ventilation (Va) to perfusion (Q). Twenty adult rats were slowly ventilated with modified Krebs solution (rate = 3 min(-1), 37 degrees C, open chest) to produce unperfused living lung preparations free of intra-airway surface forces. The solution was gassed with 21% O(2), balance N(2), and CO(2) varied to produce alveolar hypocapnia (Pco(2) = 26.1 +/- 2.4 mmHg, pH = 7.56 +/- 0.04) or hypercapnia (Pco(2) = 55.0 +/- 2.3 mmHg, pH = 7.23 +/- 0.02). The results show that lung recoil, as indicated from airway pressure measured during a breathhold following a large volume inspiration, is reduced approximately 30% when exposed to hypercapnia vs. hypocapnia (P < 0.0001, paired t-test), but stress relaxation and flow-dependent airway resistance were unaltered. Increasing CO(2) from hypo- to hypercapnic levels caused a substantial, significant decrease in the quasi-static pressure-volume relationship, as measured after inspiration and expiration of several tidal volumes, but hysteresis was unaltered. Furthermore, addition of the glycolytic inhibitor NaF abolished CO(2) effects on lung recoil. The results suggest that lung parenchyma tissue relaxation, arising from active elements in response to increasing alveolar CO(2), is independent of (and apparently in parallel with) passive tissue elements and may actively contribute to Va/Q matching.

Axonal degeneration in the Trembler-j mouse demonstrated by stimulated single-fiber electromyography.

The Trembler-j (Tr-j) mouse is a naturally occurring mutant with a point mutation in the peripheral myelin protein-22 gene causing severe peripheral nerve demyelination. It is a genetically homologous murine model for Charcot-Marie-Tooth disease type 1A (CMT 1A). Our prior pilot studies using stimulated single-fiber needle electromyograpy (SSFEMG) showed increased jitter in 60-day-old Tr-j mice compared to age-matched, wildtype animals. The aim of this study was to better elucidate the etiology of increased jitter in Tr-j mice and test the following hypotheses: (1) the increased jitter in Tr-j mice is due to turnover of endplates secondary to axonal degeneration with reinnervation and not to conduction block secondary to demyelination of motor nerve axons; and (2) aging Tr-j mice demonstrate increased jitter and fiber density compared with younger mutant mice due to progressive motor axon loss. SSFEMG studies performed on 60- and 140-day-old mice indicated that average mean consecutive difference (MCD) and fiber density estimates (FDE) were significantly increased in Tr-j mice at both ages compared to age-matched wildtypes. FDE also increased substantially in older mutant mice. Intraperitoneal neostigmine injections produced significant reductions in average MCD in Tr-j mice, suggesting that impaired neuromuscular transmission is an early pathologic feature in these mice and likely reflects distal axonal degeneration. Our findings corroborate our prior pilot study, although in a much larger number of animals across a wider age span. Our study also indicates that SSFEMG, performed in a serial fashion, is a useful, noninvasive method of detecting progressive axon loss in this murine model of CMT 1A. This technique may be a valuable tool to study the affects of genetic or pharmaceutical interventions in murine models of peripheral neuropathy. Muscle Nerve, 2007.

Serum testosterone and estradiol in sudden infant death.

OBJECTIVE: To test the hypothesis that among infants who die unexpectedly, testosterone and/or estradiol levels are elevated in those diagnosed with SIDS versus those with known causes of death (controls). STUDY DESIGN: Postmortem blood was collected and coded from infant autopsies, and serum was prepared and frozen until assayed for total testosterone and estradiol by fluoroimmunoassay. Subject information was then collected from the medical examiner's report. RESULTS: Testosterone, but not estradiol, was significantly higher in 127 SIDS cases versus 42 controls for both males (4.8 +/- 0.4 vs 2.2 +/- 0.4 nmol, respectively; P < .005) and females (2.4 +/- 0.2 vs 1.6 +/- 0.2 nmol, respectively; P < 0.03). CONCLUSIONS: Higher testosterone levels in infant victims of unexpected, unexplained death may indicate a role for testosterone or related steroids in SIDS. Further research is needed to understand the potential utility of testosterone as an indicator of SIDS risk.

Nerve conduction abnormalities in the trembler-j mouse: a model for Charcot-Marie-Tooth disease type 1A?

The trembler-j mouse is a spontaneously occurring, demyelinating mutant secondary to a point mutation involving a leucine for proline substitution in the first transmembrane domain of the peripheral-myelin protein-22 (PMP-22) gene. It is considered to be a model for Charcot-Marie-Tooth disease type 1A (CMT1A), largely based upon pathologic observations. However, functional studies demonstrating homology with CMT1A patients have not been documented. Sciatic nerve conduction was performed on 30 and 72-day-old wildtype and trembler-j mice in a blinded fashion. The findings in the mutants in both age groups were consistent with profound demyelination. Trembler-j mice appear to have a greater degree of motor nerve conduction slowing relative to human studies involving patients with PMP-22 gene duplication. Functionally, the trembler-j is a good murine model for CMT1A associated with an identical point mutation but may represent a more severe disease phenotype than CMT1A secondary to PMP-22 gene duplication.

Mechanisms of ventilation inhomogeneity during vital capacity breaths standing and supine.

Overall inhomogeneity of ventilation distribution, as measured by single-breath vital capacity (VC) washout (SBW) is known to be greater supine vs. standing. To establish the underlying mechanisms 13 healthy males performed VC SBW of 4% SF(6) and He, standing and supine, with or without a 10 sec breathhold (BH). Overall inhomogeneity, as indicated by normalized phase III slopes, was >50% greater supine (SF(6) 13.1 x 10(-3); He 10.7 x 10(-3) L(-1)) than standing (SF(6) 8.6 x 10(-3); He 6.4 x 10(-3) L(-1); P<0.001). The (SF(6)-He) slope, an index of intraacinar inhomogeneity, did not change with posture. Breathholding, assumed to eliminate convective dependent inhomogeneity within and/or between small lung units, produced twice as great reduction of inhomogeneity when supine vs. standing. After BH inhomogeneity remained significantly greater supine vs. standing. In conclusion, at least two events seem to underlie the increased inhomogeneity when supine: (1) a substantially increased convection dependent non-uniformity between well-separated lung regions; and (2) a somewhat increased convection dependent non-uniformity within and/or between peripherally located lung units.