Breathing patterns after mid-cervical spinal contusion in rats.

Respiratory failure is the leading cause of death after cervical spinal injury. We hypothesized that incomplete cervical spinal injuries would alter respiratory pattern and initiate plasticity in the neural control of breathing. Further, we hypothesized that the severity of cervical spinal contusion would correlate with changes in breathing pattern. Fourteen days after C4-C5 contusions, respiratory frequency and tidal volume were measured in unanesthetized Sprague Dawley rats in a whole body plethysmograph. Phrenic motor output was monitored in the same rats which were anesthetized, vagotomized, paralyzed and ventilated to eliminate and/or control sensory feedback that could alter breathing patterns. The extent of spinal injury was approximated histologically by measurements of the injury-induced cyst area in transverse sections; cysts ranged from 2 to 28% of spinal cross-sectional area, and had a unilateral bias. In unanesthetized rats, the severity of spinal injury correlated negatively with tidal volume (R(2)=0.85; p<0.001) and positively with breathing frequency (R(2)=0.65; p<0.05). Thus, the severity of C4-C5 spinal contusion dictates post-injury breathing pattern. In anesthetized rats, phrenic burst amplitude was decreased on the side of injury, and burst frequency correlated negatively with contusion size (R(2)=0.51; p<0.05). A strong correlation between unanesthetized breathing pattern and the pattern of phrenic bursts in anesthetized, vagotomized and ventilated rats suggests that changes in respiratory motor output after spinal injury reflect, at least in part, intrinsic neural mechanisms of CNS plasticity initiated by injury.

5-HT2 receptors modulate excitatory neurotransmission to cardiac vagal neurons within the nucleus ambiguus evoked during and after hypoxia.

To examine the role of 5-HT2 receptors in the central cardiorespiratory network, and in particular the respiratory modulation of parasympathetic activity to the heart, we used an in vitro medullary slice that allowed simultaneous examination of rhythmic inspiratory-related activity recorded from hypoglossal rootlet and excitatory inspiratory-related neurotransmission to cardioinhibitory vagal neurons (CVNs) within the nucleus ambiguus (NA). Focal application of ketanserin, a 5-HT2 receptor antagonist, did not significantly alter the frequency of spontaneous excitatory postsynaptic excitatory currents (EPSCs) in CVNs in control conditions. However, ketanserin diminished spontaneous excitatory neurotransmission to CVNs during hypoxia. The inhibitory action of ketanserin was on 5-HT3 mediated EPSCs during hypoxia since these responses were blocked by the 5-HT3 receptor antagonist ondansetron. In addition, a robust inspiratory-related excitatory neurotransmission was recruited during recovery from hypoxia. Focal application of ketanserin during this posthypoxia period evoked a significant augmentation of the frequency of inspiratory-related, but not spontaneous EPSCs in CVNs. This excitatory effect of ketanserin was prevented by application of the purinergic receptor blocker pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS). These results demonstrate 5-HT2 receptors differentially modulate excitatory neurotransmission to CVNs during and after hypoxia. Activation of 5-HT2 receptors acts to maintain excitatory neurotransmission to CVNs during hypoxia, likely via presynaptic facilitation of 5-HT3 receptor-mediated neurotransmission to CVNs. However, activation of 5HT2 receptors diminishes the subsequent inspiratory-related excitatory neurotransmission to CVNs that is recruited during the recovery from hypoxia likely exerting an inhibitory action on inspiratory-related purinergic signaling.

Reactive oxygen species and respiratory plasticity following intermittent hypoxia.

The neural network controlling breathing exhibits plasticity in response to environmental or physiological challenges. For example, while hypoxia initiates rapid and robust increases in respiratory motor output to defend against hypoxemia, it also triggers persistent changes, or plasticity, in chemosensory neurons and integrative pathways that transmit brainstem respiratory activity to respiratory motor neurons. Frequently studied models of hypoxia-induced respiratory plasticity include: (1) carotid chemosensory plasticity and metaplasticity induced by chronic intermittent hypoxia (CIH), and (2) acute intermittent hypoxia (AIH) induced phrenic long-term facilitation (pLTF) in naïve and CIH preconditioned rats. These forms of plasticity share some mechanistic elements, although they differ in anatomical location and the requirement for CIH preconditioning. Both forms of plasticity require serotonin receptor activation and formation of reactive oxygen species (ROS). While the cellular sources and targets of ROS are not well known, recent evidence suggests that ROS modify the balance of protein phosphatase and kinase activities, shifting the balance towards net phosphorylation and favoring cellular reactions that induce and/or maintain plasticity. Here, we review possible sources of ROS, and the impact of ROS on phosphorylation events relevant to respiratory plasticity.

Serotonin-induced in vitro long-term facilitation exhibits differential pattern sensitivity in cervical and thoracic inspiratory motor output.

Intermittent hypoxia induces 5-HT-dependent, pattern-sensitive long-term facilitation (LTF) of spinal respiratory motor output. We used a split-bath in vitro neonatal rat brainstem-spinal cord preparation to test whether: 1) intermittent spinal 5-HT exposure (without hypoxia) is sufficient to induce LTF in phrenic and intercostal inspiratory motor outputs; 2) LTF magnitude is greater in intercostal versus phrenic activity; and 3) phrenic and intercostal motor output exhibits differential pattern sensitivity to 5-HT application. With a barrier at spinal segment C1, 5-HT (5 muM) was applied episodically (3 min 5-HT, 5 min wash, x3) to the spinal cord (C2-L1) while recording inspiratory bursts in cervical (C4 or C5) and thoracic (T5 or T6) ventral roots. Episodic 5-HT application increased cervical and thoracic burst amplitudes to 136+/-22% and 150+/-22% of baseline, respectively, at 120 min post-drug (P<0.01). Continuous 5-HT application (5 muM, 9 min) had no effect on cervical burst amplitude at 120 min post-drug, but increased thoracic burst amplitude to 142+/-11% of baseline at 120 min post-drug (P<0.001). Methysergide pretreatment abolished both cervical and thoracic 5-HT-induced LTF. Quantitative reverse transcriptase-polymerase chain reaction and immunocytochemistry revealed that 5-HT(2A) and 5-HT(7) receptor subtypes (receptors known to influence LTF expression in adult rats) are expressed in ventral cervical and thoracic spinal cord with no differences in expression levels due to spinal segment or age. Thus, 5-HT is sufficient to induce spinal LTF in neonatal rats and differences in pattern sensitivity suggest heterogeneity in underlying mechanisms.

Orientation of the Ig domains of CD8 alpha beta relative to MHC class I.

The cell surface glycoprotein CD8 functions as a coreceptor with the TCR for interaction with MHC class I. The cocrystal structure of the CD8 alpha alpha-MHC complex showed that one CD8 Ig domain provided the majority of the contact with MHC class I and that residue R4 of that domain contacted the alpha2 domain of MHC class I. We previously showed by mutational analysis that this residue was critical for binding to MHC class I. To determine which of the Ig domains for the CD8 alpha beta heterodimer would make the most contact with class I MHC, we expressed single-chain or dimeric forms of CD8 on COS-7 cells and measured the adhesion of MHC class I positive cells. We found that when one of the R4 residues was mutated in a CD8 alpha alpha homodimer binding comparable to that of wild type was observed, whereas a double R4 mutant severely impaired binding. However, when mutant CD8 alpha (R4K) was coexpressed with wild-type CD8 beta, binding was not observed. These results support the model in which it is CD8 alpha, not CD8 beta, that is making the most of the contact with MHC class I, including the alpha 2 domain. In addition, they demonstrate that a single-chain form of CD8 alpha alpha can bind to MHC class I.

Acyclovir use and survival among human immunodeficiency virus-infected patients with CD4 cell counts of < 500/mm3. The Terry Beirn Community Programs for Clinical Research on AIDS (CPCRA).

To examine the relationship between acyclovir use and survival in AIDS, we performed a retrospective analysis of data collected through an observational cohort of the 17-site Community Program for Clinical Research on AIDS (CPCRA), under the sponsorship of the National Institute of Allergy and Infectious Diseases. Data were analyzed regarding 2,368 patients with CD4+ lymphocyte counts of < 500/mm3, and 7,836 follow-up visits were conducted from September 1990 to July 1994. Factors associated with use of acyclovir were studied by stratified analysis of variance and Mantel-Haenzel chi 2 tests. The association between acyclovir and survival was studied with use of the proportional hazards regression model. Individuals reporting acyclovir use were more likely to be white, male, and homosexual; to have a history of herpes simplex and zoster; and to have lower CD4+ T cell counts than those who did not. After adjustments for differences in baseline factors, acyclovir use was not associated with prolonged survival.

A comparison of zidovudine, didanosine, zalcitabine and no antiretroviral therapy in patients with advanced HIV disease.

Three nucleoside analogues, zidovudine (AZT), didanosine (ddI), and zalcitabine (ddC), are approved for use in the treatment of patients with HIV infection. This retrospective study compares the 3 drugs and examines the overall utility of antiretroviral therapy by way of comparisons to a no treatment (No Rx) group in patients with advanced HIV disease. Patients with advanced HIV disease were enrolled in didanosine (August 1989-December 1990) or zalcitabine (October 1990-February 1992) expanded access programmes; continued on zidovudine treatment despite fulfilling criteria for zidovudine failure or intolerance; or maintained on no antiretroviral treatment. Statistical analysis revealed that patients on nucleoside analogue therapy had fewer opportunistic infections (P = 0.001) than those who received no antiretroviral treatment. The Kaplan-Meier 12-month estimate of survival was significantly longer among patients who switched from zidovudine to zalcitabine but not among patients who switched to didanosine, when compared to the other 2 groups (P = 0.05).

Future therapies in the management of critically ill AIDS patients.

The advent of antiviral therapy for HIV infection and anti-infective agents for the treatment and prophylaxis of Pneumocystis carinii pneumonia has had a significant impact on the survival and quality of life of persons with AIDS and associated conditions. This article discusses zidovudine and other antiviral therapies for HIV infection, as well as some of the new treatment and prophylactic strategies to manage opportunistic infections which can ameliorate the course of advanced HIV infection in patients who may require critical care.

Zidovudine-induced leukocytoclastic vasculitis.

Cutaneous reactions, including acne, pruritus, urticaria, and nail dyschromia, have been associated with zidovudine use. Cutaneous leukocytoclastic vasculitis is characterized by distinctive patterns of dermal perivascular inflammation without visceral involvement. We treated two cases of cutaneous leukocytoclastic vasculitis associated with fever in patients with severe human immunodeficiency virus-related immunodeficiency in which symptoms resolved on withdrawal of zidovudine therapy and recurred after rechallenge with the drug. This unusual hypersensitivity reaction to zidovudine is probably related to the immune dysregulation and the propensity toward allergic phenomena found in human immunodeficiency virus infection.