Influence of housing conditions from weaning to adulthood on the ventilatory, thermoregulatory, and endocrine responses to hypoxia of adult female rats

Housing conditions affect animal physiology. We previously showed that the hypoxic ventilatory and thermoregulatory responses to hypoxia of adult male rats housed in triads during the juvenile period (postnatal day 21 to adulthood) were significantly reduced compared with animals housed in pairs. Because sex hormones influence development and responsiveness to environmental stressors, this study investigated the impact of housing on the respiratory and thermoregulatory physiology of female rats. Since neonatal stress attenuates the hypoxic ventilatory response (HVR) of female rats at adulthood, experiments were performed both on "control" (undisturbed) animals and rats subjected to neonatal maternal separation (NMS; 3 h/day, postnatal days 3-12). At adulthood, ventilatory activity was measured by whole body plethysmography under normoxic and hypoxic conditions [fraction of inspired oxygen (Fi(O(2))) = 0.12; 20 min]. The ventilatory and body temperature responses to hypoxia of female rats raised in triads were reduced compared with rats housed in pairs. Housing female rats in triads did not affect basal or hypoxic plasma corticosterone levels but did increase levels of estradiol significantly. We conclude that modest changes in housing conditions (pairs vs. triads) from weaning to adulthood does influence basic homeostatic functions such as temperature and respiratory regulation. Triad housing can reverse the manifestations of respiratory instability at adulthood induced by stressful neonatal treatments. This should raise awareness of the benefits of increasing social interactions in clinical settings but also caution researchers of the potential impact of such subtle changes on experimental protocols and interpretation of results.     

Plasticity in respiratory motor control: intermittent hypoxia and hypercapnia activate opposing serotonergic and noradrenergic modulatory systems.

Experimental results consistently show that the respiratory control system is plastic, such that environmental factors and experience can modify its performance. Such plasticity may represent basic neurobiological principles of learning and memory, whereby intermittent sensory stimulation produces long-term alterations (i.e. facilitation or depression) in synaptic transmission depending on the timing and intensity of the stimulation. In this review, we propose that intermittent chemosensory stimulation produces long-term changes in respiratory motor output via specific neuromodulatory systems. This concept is based on recent data suggesting that intermittent hypoxia produces a net long-term facilitation of respiratory output via the serotonergic system, whereas intermittent hypercapnia produces a net long-term depression by a mechanism associated with the noradrenergic system. There is suggestive evidence that, although both respiratory stimuli activate both modulatory systems, the balance is different. Thus, these opposing modulatory influences on respiratory motor control may provide a 'push-pull' system, preventing unchecked and inappropriate fluctuations in ventilatory drive.     

Respiratory chemoreceptor function in vertebrates comparative and evolutionary aspects

The sensing of blood gas tensions and/or pH is an evolutionarilyconserved, homeostatic mechanism, observable in almost all speciesstudied from invertebrates to man. In vertebrates, a shift fromthe peripheral O₂-oriented sensing in fish, to the central CO₂/pHsensing in most tetrapods reflects the specific behavioral requirementsof these two groups whereby, in teleost fish, a highly O₂-orientedcontrol of breathing matches the ever-changing and low oxygenlevels in water, whilst the transition to air-breathing increasedthe importance of acid–base regulation and O₂-relateddrive, although retained, became relatively less important.The South American lungfish and tetrapods are probably sistergroups, a conclusion backed up by many similar features of respiratorycontrol. For example, the relative roles of peripheral and centralchemoreceptors are present both in the lungfish and in landvertebrates. In both groups, the central CO₂/pH receptors dominatethe ventilatory response to hypercarbia (60–80%), whilethe peripheral CO₂/pH receptors account for 20–30%. Somebasic components of respiratory control have changed littleduring evolution. This review presents studies that reflectthe current trends in the field of chemoreceptor function, andseveral laboratories are involved. An exhaustive review on theprevious literature, however, is beyond the intended scope ofthe article. Rather, we present examples of current trends inrespiratory function in vertebrates, ranging from fish to humans,and focus on both O₂ sensing and CO₂ sensing. As well, we considerthe impact of chronic levels of hypoxia—a physiologicalcondition in fish and in land vertebrates resident at high elevationsor suffering from one of the many cardiorespiratory diseasestates that predispose an animal to impaired ventilation orcardiac output. This provides a basis for a comparative physiologythat is informative about the evolution of respiratory functionsin vertebrates and about human disease. Currently, most detailis known for mammals, for which molecular biology and respiratoryphysiology have combined in the discovery of the mechanismsunderlying the responses of respiratory chemoreceptors. Ourreview includes new data on nonmammalian vertebrates, whichstresses that some chemoreceptor sites are of ancient origin.     

Anisotropic dynamics of the JE-2147-HIV protease complex: drug resistance and thermodynamic binding mode examined in a 1.09 A structure

The structure of HIV protease (HIV Pr) bound to JE-2147 (also named AG1776 or KNI-764) is determined here to 1.09 A resolution. This highest-resolution structure for HIV Pr allows refinement of anisotropic displacement parameters (ADPs) for all atoms. Clustering based on the directional information in ADPs defines two sets of subdomains such that within each set, subdomains undergo similar anisotropic motion. These sets are (a) the core of monomer A grouped with both substrate-binding flaps and (b) the core of monomer B coupled to both catalytic aspartates (25A/B). The four-stranded beta-sheet (1-4 A/B and 95-99 A/B) that forms a significant part of the dimer interface exhibits large anisotropic amplitudes that differ from those of the other sets of subdomains. JE-2147 is shown here to be a picomolar inhibitor (K(i) = 41 +/- 18 pM). The structure is used to interpret the mechanism of association of JE-2147, a second-generation inhibitor for which binding is enthalpically driven, with respect to first-generation inhibitors for which binding is predominantly entropically driven [Velazquez-Campoy, A., et al. (2001) Arch. Biochem. Biophys. 390, 169-175]. Relative to the entropically driven inhibitor complexes, the JE-2147-HIV Pr complex exhibits an approximately 0.5 A movement of the substrate flaps in toward the substrate, suggesting a more compatible enthalpically driven association. Domains of the protease identified by clustering of ADPs also suggest a model of enthalpy-entropy compensation for all HIV Pr inhibitors in which dynamic coupling of the flaps is offset by an increased level of motion of the beta-sheet domain of the dimer interface (1-4 A/B and 95-99 A/B).     

Stress-induced attenuation of the hypercapnic ventilatory response in awake rats.

To test the hypothesis that stress alters the performance of the respiratory control system, we compared the acute (20 min) responses to moderate hypoxia and hypercapnia of rats previously subjected to immobilization stress (90 min/day) with responses of control animals. Ventilatory measurements were performed on awake rats using whole body plethysmography. Under baseline conditions, there were no differences in minute ventilation between stressed and unstressed groups. Rats previously exposed to immobilization stress had a 45% lower ventilatory response to hypercapnia (inspiratory CO(2) fraction = 0.05) than controls. In contrast, stress exposure had no statistically significant effect on the ventilatory response to hypoxia (inspiratory O(2) fraction = 0.12). Stress-induced attenuation of the hypercapnic response was associated with reduced tidal volume and inspiratory flow increases; the frequency and timing components of the response were not different between groups. We conclude that previous exposure to a stressful condition that does not constitute a direct challenge to respiratory homeostasis can elicit persistent (> or =24 h) functional plasticity in the ventilatory control system.     

Neonatal maternal separation enhances phrenic responses to hypoxia and carotid sinus nerve stimulation in the adult anesthetized rat.

In awake animals, our laboratory recently showed that the hypoxic ventilatory response of adult male (but not female) rats previously subjected to neonatal maternal separation (NMS) is 25% greater than controls (Genest SE, Gulemetova R, Laforest S, Drolet G, and Kinkead R. J Physiol 554: 543-557, 2004). To begin mechanistic investigations of the effects of this neonatal stress on respiratory control development, we tested the hypothesis that, in male rats, NMS enhances central integration of carotid body chemoafferent signals. Experiments were performed on two groups of adult male rats. Pups subjected to NMS were placed in a temperature-controlled incubator 3 h/day from postnatal day 3 to postnatal day 12. Control pups were undisturbed. At adulthood (8-10 wk), rats were anesthetized (urethane; 1.6 g/kg), paralyzed, and ventilated with a hyperoxic gas mixture [inspired O2 fraction (Fi(O2)) = 0.5], and phrenic nerve activity was recorded. The first series of experiments aimed to demonstrate that NMS-related enhancement of the inspiratory motor output (phrenic) response to hypoxia occurs in anesthetized animals also. In this series, rats were exposed to moderate, followed by severe, isocapnic hypoxia (Fi(O2) = 0.12 and 0.08, respectively, 5 min each). NMS enhanced both the frequency and amplitude components of the phrenic response to hypoxia relative to controls, thereby validating the use of this approach. In a second series of experiments, NMS increased the amplitude (but not the frequency) response to unilateral carotid sinus nerve stimulation (stimulation frequency range: 0.5-33 Hz). We conclude that enhancement of central integration of carotid body afferent signal contributes to the larger hypoxic ventilatory response observed in NMS rats.     

Neurotensin: role in psychiatric and neurological diseases

Neurotensin (NT), an endogenous brain-gut peptide, has a close anatomical and functional relationship with the mesocorticolimbic and neostriatal dopamine system. Dysregulation of NT neurotransmission in this system has been hypothesized to be involved in the pathogenesis of schizophrenia. Additionally, NT containing circuits have been demonstrated to mediate some of the mechanisms of action of antipsychotic drugs, as well as the rewarding and/or sensitizing properties of drugs of abuse. NT receptors have been suggested to be novel targets for the treatment of psychoses or drug addiction.     

Are circulating catecholamines involved in the control of breathing by fishes?

Summary In this review, we have provided evidence that elevated levels of circulating catecholamines are not a significant factor in the control of ventilation in fishes, but rather that this critical physiological function is controlled primarily by the external and/or internal respiratory status. This view, which opposes the more traditional consensus (review: Randall and Taylor, 1991), is based upon numerous experimental observations, theoretical considerations, and a re-evaluation of previous studies.First, circulating catecholamine levels become elevated in fish only as a final survival strategy during severe stress, whereas hyperventilatory responses begin with very slight alterations in blood/water chemistry. Thus, except during extreme stress, plasma catecholamine levels and gill ventilation volume (V _w) do not co-vary. Second, with the notable exception of the European eel (Anguilla anguilla; Peyraud-Waitzenegger, 1979), experimental elevations of circulating catecholamine levels by injection/infusion of adrenalin or noradrenalin either do not alter ventilation in a physiologically significant manner or may occasionally even depress ventilation. Further, the sudden release of endogenous catecholamines during severe stress does not appear to modify the pre-existing hyperventilatory response. Third, although treatment of fish with selective adrenoceptor antagonists has yielded conflicting results to both support and reject a role for circulating catecholamines in the control of ventilation, it is nonetheless clear that such as experimental protocol cannot adequately differentiate between central and peripheral adrenergic phenomena.We suggest that there is no basis to support a role for circulating catecholamines in the regulation of breathing in fishes, and contend that discrepancies in the literature reflect the inherent difficulties associated with separating neural and humoral adrenergic phenomena.