Modeling Effects of Variable preBötzinger Complex Network Topology and Cellular Properties on Opioid-Induced Respiratory Depression and Recovery

The preBötzinger complex (preBötC), located in the medulla, is the essential rhythm-generating neural network for breathing. The actions of opioids on this network impair its ability to generate robust, rhythmic output, contributing to life-threatening opioid-induced respiratory depression (OIRD). The occurrence of OIRD varies across individuals and internal and external states, increasing the risk of opioid use, yet the mechanisms of this variability are largely unknown. In this study, we utilize a computational model of the preBötC to perform several in silico experiments exploring how differences in network topology and the intrinsic properties of preBötC neurons influence the sensitivity of the network rhythm to opioids. We find that rhythms produced by preBötC networks in silico exhibit variable responses to simulated opioids, similar to the preBötC network in vitro. This variability is primarily due to random differences in network topology and can be manipulated by imposed changes in network connectivity and intrinsic neuronal properties. Our results identify features of the preBötC network that may regulate its susceptibility to opioids.

Purinergic signaling mediates neuroglial interactions to modulate sighs

Sighs prevent the collapse of alveoli in the lungs, initiate arousal under hypoxic conditions, and are an expression of sadness and relief. Sighs are periodically superimposed on normal breaths, known as eupnea. Implicated in the generation of these rhythmic behaviors is the preBötzinger complex (preBötC). Our experimental evidence suggests that purinergic signaling is necessary to generate spontaneous and hypoxia-induced sighs in a mouse model. Our results demonstrate that driving calcium increases in astrocytes through pharmacological methods robustly increases sigh, but not eupnea, frequency. Calcium imaging of preBötC slices corroborates this finding with an increase in astrocytic calcium upon application of sigh modulators, increasing intracellular calcium through g-protein signaling. Moreover, photo-activation of preBötC astrocytes is sufficient to elicit sigh activity, and this response is blocked with purinergic antagonists. We conclude that sighs are modulated through neuron-glia coupling in the preBötC network, where the distinct modulatory responses of neurons and glia allow for both rhythms to be independently regulated.

Inspiratory rhythm generation is stabilized by Ih

Cellular and network properties must be capable of generating rhythmic activity that is both flexible and stable. This is particularly important for breathing, a rhythmic behavior that dynamically adapts to environmental, behavioral, and metabolic changes from the first to the last breath. The pre-Bötzinger complex (preBötC), located within the ventral medulla, is responsible for producing rhythmic inspiration. Its cellular properties must be tunable, flexible as well as stabilizing. Here, we explore the role of the hyperpolarization-activated, nonselective cation current (I_h) for stabilizing PreBötC activity during opioid exposure and reduced excitatory synaptic transmission. Introducing I_h into an in silico preBötC network predicts that loss of this depolarizing current should significantly slow the inspiratory rhythm. By contrast, in vitro and in vivo experiments revealed that the loss of I_h minimally affected breathing frequency, but destabilized rhythmogenesis through the generation of incompletely synchronized bursts (burstlets). Associated with the loss of I_h was an increased susceptibility of breathing to opioid-induced respiratory depression or weakened excitatory synaptic interactions, a paradoxical depolarization at the cellular level, and the suppression of tonic spiking. Tonic spiking activity is generated by nonrhythmic excitatory and inhibitory preBötC neurons, of which a large percentage express I_h. Together, our results suggest that I_h is important for maintaining tonic spiking, stabilizing inspiratory rhythmogenesis, and protecting breathing against perturbations or changes in network state.NEW & NOTEWORTHY The I_h current plays multiple roles within the preBötC. This current is important for promoting intrinsic tonic spiking activity in excitatory and inhibitory neurons and for preserving rhythmic function during conditions that dampen network excitability, such as in the context of opioid-induced respiratory depression. We therefore propose that the I_h current expands the dynamic range of rhythmogenesis, buffers the preBötC against network perturbations, and stabilizes rhythmogenesis by preventing the generation of unsynchronized bursts.

Dynamic Rhythmogenic Network States Drive Differential Opioid Responses in the In Vitro Respiratory Network

Death from opioid overdose is typically caused by opioid-induced respiratory depression (OIRD). A particularly dangerous characteristic of OIRD is its apparent unpredictability. The respiratory consequences of opioids can be surprisingly inconsistent, even within the same individual. Despite significant clinical implications, most studies have focused on average dose-r esponses rather than individual variation, and there remains little insight into the etiology of this apparent unpredictability. The preBötzinger complex (preBötC) in the ventral medulla is an important site for generating the respiratory rhythm and OIRD. Here, using male and female C57-Bl6 mice in vitro, we demonstrate that the preBötC can assume different network states depending on the excitability of the preBötC and the intrinsic membrane properties of preBötC neurons. These network states predict the functional consequences of opioids in the preBötC, and depending on network state, respiratory rhythmogenesis can be either stabilized or suppressed by opioids. We hypothesize that the dynamic nature of preBötC rhythmogenic properties, required to endow breathing with remarkable flexibility, also plays a key role in the dangerous unpredictability of OIRD.SIGNIFICANCE STATEMENT Opioids can cause unpredictable, life-threatening suppression of breathing. This apparent unpredictability makes clinical management of opioids difficult while also making it challenging to define the underlying mechanisms of OIRD. Here, we find in brainstem slices that the preBötC, an opioid-sensitive subregion of the brainstem, has an optimal configuration of cellular and network properties that results in a maximally stable breathing rhythm. These properties are dynamic, and the state of each individual preBötC network relative to the optimal configuration of the network predicts how vulnerable rhythmogenesis is to the effects of opioids. These insights establish a framework for understanding how endogenous and exogenous modulation of the rhythmogenic state of the preBötC can increase or decrease the risk of OIRD.

Dual mechanisms of opioid-induced respiratory depression in the inspiratory rhythm-generating network

The analgesic utility of opioid-based drugs is limited by the life-threatening risk of respiratory depression. Opioid-induced respiratory depression (OIRD), mediated by the μ-opioid receptor (MOR), is characterized by a pronounced decrease in the frequency and regularity of the inspiratory rhythm, which originates from the medullary preBötzinger Complex (preBötC). To unravel the cellular- and network-level consequences of MOR activation in the preBötC, MOR-expressing neurons were optogenetically identified and manipulated in transgenic mice in vitro and in vivo. Based on these results, a model of OIRD was developed in silico. We conclude that hyperpolarization of MOR-expressing preBötC neurons alone does not phenocopy OIRD. Instead, the effects of MOR activation are twofold: (1) pre-inspiratory spiking is reduced and (2) excitatory synaptic transmission is suppressed, thereby disrupting network-driven rhythmogenesis. These dual mechanisms of opioid action act synergistically to make the normally robust inspiratory rhythm-generating network particularly prone to collapse when challenged with exogenous opioids.

Continuous, multidimensional coding of 3D complex tactile stimuli by primary sensory neurons of the vibrissal system

Across all sensory modalities, first-stage sensory neurons are an information bottleneck: they must convey all information available for an animal to perceive and act in its environment. Our understanding of coding properties of primary sensory neurons in the auditory and visual systems has been aided by the use of increasingly complex, naturalistic stimulus sets. By comparison, encoding properties of primary somatosensory afferents are poorly understood. Here, we use the rodent whisker system to examine how tactile information is represented in primary sensory neurons of the trigeminal ganglion (Vg). Vg neurons have long been thought to segregate into functional classes associated with separate streams of information processing. However, this view is based on Vg responses to restricted stimulus sets which potentially underreport the coding capabilities of these neurons. In contrast, the current study records Vg responses to complex three-dimensional (3D) stimulation while quantifying the complete 3D whisker shape and mechanics, thereby beginning to reveal their full representational capabilities. The results show that individual Vg neurons simultaneously represent multiple mechanical features of a stimulus, do not preferentially encode principal components of the stimuli, and represent continuous and tiled variations of all available mechanical information. These results directly contrast with proposed codes in which subpopulations of Vg neurons encode select stimulus features. Instead, individual Vg neurons likely overcome the information bottleneck by encoding large regions of a complex sensory space. This proposed tiled and multidimensional representation at the Vg directly constrains the computations performed by more central neurons of the vibrissotrigeminal pathway.

A dynamical model for generating synthetic data to quantify active tactile sensing behavior in the rat

As it becomes possible to simulate increasingly complex neural networks, it becomes correspondingly important to model the sensory information that animals actively acquire: the biomechanics of sensory acquisition directly determines the sensory input and therefore neural processing. Here, we exploit the tractable mechanics of the well-studied rodent vibrissal ("whisker") system to present a model that can simulate the signals acquired by a full sensor array actively sampling the environment. Rodents actively "whisk" ∼60 vibrissae (whiskers) to obtain tactile information, and this system is therefore ideal to study closed-loop sensorimotor processing. The simulation framework presented here, WHISKiT Physics, incorporates realistic morphology of the rat whisker array to predict the time-varying mechanical signals generated at each whisker base during sensory acquisition. Single-whisker dynamics were optimized based on experimental data and then validated against free tip oscillations and dynamic responses to collisions. The model is then extrapolated to include all whiskers in the array, incorporating each whisker's individual geometry. Simulation examples in laboratory and natural environments demonstrate that WHISKiT Physics can predict input signals during various behaviors, currently impossible in the biological animal. In one exemplary use of the model, the results suggest that active whisking increases in-plane whisker bending compared to passive stimulation and that principal component analysis can reveal the relative contributions of whisker identity and mechanics at each whisker base to the vibrissotactile response. These results highlight how interactions between array morphology and individual whisker geometry and dynamics shape the signals that the brain must process.

The Pathophysiology of Rett Syndrome With a Focus on Breathing Dysfunctions

Rett syndrome (RTT), an X-chromosome-linked neurological disorder, is characterized by serious pathophysiology, including breathing and feeding dysfunctions, and alteration of cardiorespiratory coupling, a consequence of multiple interrelated disturbances in the genetic and homeostatic regulation of central and peripheral neuronal networks, redox state, and control of inflammation. Characteristic breath-holds, obstructive sleep apnea, and aerophagia result in intermittent hypoxia, which, combined with mitochondrial dysfunction, causes oxidative stress-an important driver of the clinical presentation of RTT.

The Cellular and Mechanical Basis for Response Characteristics of Identified Primary Afferents in the Rat Vibrissal System

Compared to our understanding of the response properties of receptors in the auditory and visual systems, we have only a limited understanding of the mechanoreceptor responses that underlie tactile sensation. Here, we exploit the stereotyped morphology of the rat vibrissal (whisker) array to investigate coding and transduction properties of identified primary tactile afferents. We performed in vivo intra-axonal recording and labeling experiments to quantify response characteristics of four different types of identified mechanoreceptors in the vibrissal follicle: ring-sinus Merkel; lanceolate; clublike; and rete-ridge collar Merkel. Of these types, only ring-sinus Merkel endings exhibited slowly adapting properties. A weak inverse relationship between response magnitude and onset response latency was found across all types. All afferents exhibited strong "angular tuning," i.e., their response magnitude and latency depended on the whisker's deflection angle. Although previous studies suggested that this tuning should be aligned with the angular location of the mechanoreceptor in the follicle, such alignment was observed only for Merkel afferents; angular tuning of the other afferent types showed no clear alignment with mechanoreceptor location. Biomechanical modeling suggested that this tuning difference might be explained by mechanoreceptors' differential sensitivity to the force directed along the whisker length. Electron microscopic investigations of Merkel endings and lanceolate endings at the level of the ring sinus revealed unique anatomical features that may promote these differential sensitivities. The present study systematically integrates biomechanical principles with the anatomical and morphological characterization of primary afferent endings to describe the physical and cellular processing that shapes the neural representation of touch.

Decoupling kinematics and mechanics reveals coding properties of trigeminal ganglion neurons in the rat vibrissal system

Tactile information available to the rat vibrissal system begins as external forces that cause whisker deformations, which in turn excite mechanoreceptors in the follicle. Despite the fundamental mechanical origin of tactile information, primary sensory neurons in the trigeminal ganglion (Vg) have often been described as encoding the kinematics (geometry) of object contact. Here we aimed to determine the extent to which Vg neurons encode the kinematics vs. mechanics of contact. We used models of whisker bending to quantify mechanical signals (forces and moments) at the whisker base while simultaneously monitoring whisker kinematics and recording single Vg units in both anesthetized rats and awake, body restrained rats. We employed a novel manual stimulation technique to deflect whiskers in a way that decouples kinematics from mechanics, and used Generalized Linear Models (GLMs) to show that Vg neurons more directly encode mechanical signals when the whisker is deflected in this decoupled stimulus space.

DOI:http://dx.doi.org/10.7554/eLife.13969.001

Animals must gather sensory information from the world around them and act on that information. Specialized sensory cells convert physical information from the environment into electrical signals that the brain can interpret. In the case of hearing, this physical information consists of changes in air pressure, and for vision, it is patterns of light bouncing off of objects.

Rodents rely heavily on touch information from their whiskers to explore their world. When a whisker touches an object, it deforms and bends. The first neurons to respond to whisker touch – so called primary sensory neurons – represent contact between the whisker and the object in the form of electrical signals, but exactly how they do this is unclear.

One possibility is that primary sensory neurons encode the movement of the whisker itself. Whenever a whisker touches an object, the whisker is deflected in a particular direction by a particular amount and at a particular speed. These movement-related features are referred to as the “kinematic” properties of whisker-object contact. Alternatively, these whisker sensory neurons might be more concerned with the forces at the base of the whisker caused by object contact. These forces are the “mechanical” properties of whisker-object contact.

Bush, Schroeder et al. set out to determine whether the electrical response of these whisker sensory neurons mainly encode kinematic or mechanical information. However, these two types of information are often closely related to each other: put simply, small whisker movements tend to accompany small forces and vice versa. Bush, Schroeder et al. therefore devised a method to deliver touch stimuli to the whiskers in a way that separates kinematic from mechanical information. Mathematical models were then developed to compare how well the neurons represent each type of information. The models showed that whisker sensory neurons generally encode mechanical signals more directly than kinematic ones.

This information adds to our understanding of how animals learn about the world through their senses. However, the analysis of Bush, Schroeder et al. relies on the long-standing simplification that whisker motion is two-dimensional, whereas in reality whiskers move in three dimensions. Therefore, a future challenge is to examine how sensory neurons represent information about touch, such as the location or shape of an object, during three-dimensional whisker-object contact.

DOI:http://dx.doi.org/10.7554/eLife.13969.002

Conditional and unconditional estimation of multidimensional quality of life after hematopoietic stem cell transplantation: a longitudinal follow-up of 415 patients

Emerging literature suggests that quality of life (QOL) after bone marrow transplantation is relatively good but is accompanied in some patients by a variety of residual difficulties. The studies supporting this finding, however, have been somewhat limited in scale, scope, design, and analysis. We comprehensively measured changes in multidimensional QOL in a 4-year longitudinal follow-up of 415 adult patients who received hematopoietic stem cell transplants at Fred Hutchinson Cancer Research Center. Questionnaire packets containing 271 items were mailed annually posttransplantation to patients' homes. Standard methods of analysis yielded conditional estimates depending on compliance and survival, whereas new, likelihood-based methods generated unconditional estimates applicable to the full intent-to-treat population. Typical QOL levels generally remained high over the entire study period. Most QOL functioning significantly improved over 4 years, with the remainder showing no important decrement. Although isolated problem areas, such as sexual dissatisfaction, did emerge, the level of dysfunction for most physical and psychological scales remained below 30% of scale maxima. Broadly similar results were obtained for conditional estimation, which may contain an optimistic bias, and for unconditional estimation, which largely avoids the bias. Because concurrence was obtained between the 2 types of estimation, we conclude that most patients really do experience good levels of QOL in the 4 years after transplantation. Although some problems can be anticipated, typical patients can look forward to a QOL after transplantation that is broadly comparable to that of the normal population.

Physician participation in research surveys. A randomized study of inducements to return mailed research questionnaires

The authors randomly selected 400 physicians from a population of 1,545 practicing physicians providing follow-up care to patients who received bone marrow or blood stem cell transplants at the Fred Hutchinson Cancer Research Center to determine interest in receiving Internet-based transplant information. In a two-factor completely randomized factorial design, the 400 physicians were assigned to receive mailed surveys with either no compensation or a $5 check and either no follow-up call or a follow-up call 3 weeks after mailing. Overall, 51.5% of the physicians returned the mailed surveys. Comparison of logit models showed that inclusion of a $5 check in the mailer significantly (p = .016) increased the probability of returning the surveys (57.5% vs. 45.5%). In contrast, the telephone follow-up had no overall effect. The authors concluded a modest financial reward can significantly improve physician response rates to research surveys but a telephone follow-up may be inefficient and even ineffective.

Web site design and development issues: the Washington State Breast and Cervical Health Program Web Site Demonstration Project

PURPOSE/OBJECTIVES

To explore the development of a customized Web site to assist Breast and Cervical Health Program (BCHP) outreach staff in a community screening program and to evaluate the Internet knowledge and access issues and barriers of outreach staff during a two-year period using the Web site.

DESIGN

Knowledge, access issues and barriers, and descriptive questionnaires.

SETTINGS

Comprehensive cancer center in Seattle, WA, workshops, and presentations around the state.

SAMPLE

BCHP outreach workers, screening coordinators, and almost exclusively public health nurses from regional health districts and program-contracted clinics.

METHOD

Web site development was based on continuous input from sample. Detailed descriptions of computer and Internet resources and opinions about the use and usefulness of the BCHP Web site came from a 1996 evaluation and 1998 follow-up conducted using mailed and online Web questionnaires. "Hits" to the Web site were monitored monthly.

MAIN RESEARCH VARIABLES

Computer and Internet resources were used along with monthly Web site traffic and opinions about the use and usefulness of the BCHP Web site in the outreach program.

FINDINGS

Use of the BCHP Web site has risen steadily over two years to reach a stable plateau. User evaluations show a marked increase in the adoption of the Internet as a working tool. Users believe the Internet is becoming increasingly important to their work. More training and familiarization with the Web is needed.

CONCLUSIONS

The Web is an efficient medium for improving communication and providing easy access to resources within the BCHP program.

IMPLICATIONS FOR NURSING PRACTICE

Public health programs with meager resources can benefit from the relatively inexpensive use of customized and versatile Web sites.

Quality of life of 125 adults surviving 6-18 years after bone marrow transplantation

BACKGROUND

Recent studies examining the medical and psychosocial sequelae of bone marrow transplantation have reported most survivors do relatively well while a smaller group continues to experience less than optimal quality of life (QOL). Many of these studies are limited by small sample sizes, limited scope, and focus on a narrow (1-4 year) window of survival.

METHODS

The descriptive survey examined the QOL, late medical complications, psychological distress, demands of long-term recovery, and health perceptions of 125 adults surviving 6-18 (mean 10) years after marrow transplantation. Seven wide-ranging tests covering 271 items were completed on average in 90 min. Two tests were developed by the authors specifically for assessing QOL in this population.

RESULTS

74% of long-term survivors of bone marrow transplantation reported their current QOL was the same or better than before transplantation, 80% rated their current health status and QOL as good to excellent, and 88% said the benefits of transplantation outweighed the side effects. Ten years or more post-transplantation, long-term survivors continued to experience a moderate incidence of lingering complications and demands, including emotional and sexual dysfunction, fatigue, eye problems, sleep disturbance, general pain and cognitive dysfunction. However, the severity or degree of distress attributed to those complications was, for most survivors, consistently low. Nearly all were back to work or school. Only 5% rated both their QOL and health status as poor. Long-term survivors demonstrated good mood and low psychological distress compared to cancer and population norms, and had the same perceptions as the general population of their current health and expectation of future health. Demands attributed to long-term survival appeared to impose little hardship. The most frequently cited demand of recovery was the perceived lack of social support as time went on.

CONCLUSIONS

Almost all long-term survivors were leading full and meaningful lives. Persistent complications were, on the whole, dismissed as relatively trivial and the overwhelming majority viewed themselves as cured and well.

Samarium-153-EDTMP in bone metastases of hormone refractory prostate carcinoma: a phase I/II trial

Samarium-153-ethylenediaminetetramethylene phosphoric acid (EDTMP), a bone-seeking radiopharmaceutical, was given to prostate cancer patients in a dose escalation protocol for pain palliation to determine the maximally tolerated dose. Fifty-two patients with hormone refractory prostate cancer with bony metastases were treated with doses beginning at 0.5 mCi/kg (18.5 MBq/kg), escalating in 0.5-mCi (18.5 MBq) increments to 3.0 mCi/kg (111 MBq/kg). Pain response after treatment was assessed as well as hematologic and serum chemistry parameters. Pain palliation with a mean duration of 2.6 mo was present in 74% of the patients. Toxicity was exclusively hematologic at the highest dose levels. No infectious or bleeding complications occurred, with 45 of the 52 (86%) patients demonstrating complete hematologic recovery. Patients receiving higher doses had significantly greater reductions in serum prostate specific antigen and serum prostatic acid phosphatase levels. The patients receiving greater doses also showed a trend toward improved survival.

Distressing near-death experiences

Most reported near-death experiences include profound feelings of peace, joy, and cosmic unity. Less familiar are the reports following close brushes with death of experiences that are partially or entirely unpleasant, frightening, or frankly hellish. While little is known about the antecedents or aftereffects of these distressing experiences, there appear to be three distinct types, involving (1) phenomenology similar to peaceful near-death experiences but interpreted as unpleasant, (2) a sense of nonexistence or eternal void, or (3) graphic hellish landscapes and entities. While the first type may eventually convert to a typical peaceful experience, the relationship of all three types to prototypical near-death experiences merits further study. The effect of the distressing experience in the lives of individuals deserves exploration, as the psychological impact may be profound and long-lasting.