Increased spinal monoamine concentrations after chronic thoracic dorsal rhizotomy in goats

Inaugural Review Prize 2023: The exercise hyperpnoea dilemma: A 21st-century perspective

During mild or moderate exercise, alveolar ventilation increases in direct proportion to metabolic rate, regulating arterial CO2 pressure near resting levels. Mechanisms giving rise to the hyperpnoea of exercise are unsettled despite over a century of investigation. In the past three decades, neuroscience has advanced tremendously, raising optimism that the 'exercise hyperpnoea dilemma' can finally be solved. In this review, new perspectives are offered in the hope of stimulating original ideas based on modern neuroscience methods and current understanding. We first describe the ventilatory control system and the challenge exercise places upon blood-gas regulation. We highlight relevant system properties, including feedforward, feedback and adaptive (i.e., plasticity) control of breathing. We then elaborate a seldom explored hypothesis that the exercise ventilatory response continuously adapts (learns and relearns) throughout life and ponder if the memory 'engram' encoding the feedforward exercise ventilatory stimulus could reside within the cerebellum. Our hypotheses are based on accumulating evidence supporting the cerebellum's role in motor learning and the numerous direct and indirect projections from deep cerebellar nuclei to brainstem respiratory neurons. We propose that cerebellar learning may be obligatory for the accurate and adjustable exercise hyperpnoea capable of tracking changes in life conditions/experiences, and that learning arises from specific cerebellar microcircuits that can be interrogated using powerful techniques such as optogenetics and chemogenetics. Although this review is speculative, we consider it essential to reframe our perspective if we are to solve the till-now intractable exercise hyperpnoea dilemma.

Raphe gene expression changes implicate immune-related functions in ventilatory plasticity following carotid body denervation in rats

The regulation of blood gases in mammals requires precise feedback mechanisms including chemoreceptor feedback from the carotid bodies. Carotid body denervation (CBD) leads to immediate hypoventilation (increased PaCO₂) in adult rats, but over a period of days and weeks ventilation normalizes due in part to central (brain) mechanisms. Here, we tested the hypothesis that functional ventilatory recovery following CBD correlated with significant shifts in medullary raphe gene expression of molecules/pathways associated with known or novel forms of neuroplasticity. Tissue punches were obtained from snap frozen brainstems collected from rats 1-2days or 14-15days post-sham or post-bilateral CBD surgery (verified by physiologic measurements), and subjected to mRNA sequencing to identify, quantify, and statistically compare gene expression level differences among these groups of rats. We found the greatest number of gene expression changes acutely after CBD (154 genes), with fewer changes in the weeks after CBD (69-80 genes) and the fewest changes in expression among the time control groups (39 genes). Little or no changes were observed for multiple genes associated with serotonin- or glutamate receptor-dependent forms of neuroplasticity. However, an unbiased assessment of gene expression changes using a bioinformatics pathway analysis highlighted multiple changes in gene expression in signaling pathways associated with immune function. These included several growth factors and cytokines associated with peripheral and innate immune systems. Thus, these medullary raphe gene expression data support a role for immune-related signaling pathways in the functional restoration of blood gas control after CBD, but little or no role for serotonin- or glutamate receptor-mediated plasticity.

Changes in glutamate receptor subunits within the medulla in goats after section of the carotid sinus nerves

The mechanisms which contribute to the time-dependent recovery of resting ventilation and the ventilatory CO2 chemoreflex after carotid body denervation (CBD) are poorly understood. Herein we tested the hypothesis that there are time-dependent changes in the expression of specific AMPA, NMDA, and/or neurokinin-1 (NK1R) receptors within respiratory-related brain stem nuclei acutely or chronically after CBD in adult goats. Brain stem tissues were collected acutely (5 days) or chronically (30 days) after sham or bilateral CBD, immunostained with antibodies targeting AMPA (GluA1 or GluA2), NMDA (GluN1), or NK-1 receptors, and optical density (OD) compared. Physiological measurement confirmed categorization of each group and showed ventilatory effects consistent with bilateral CBD (Miller et al. J Appl Physiol 115: 1088-1098, 2013). Acutely after CBD, GluA1 OD was unchanged or slightly increased, but GluA2 and GluN1 OD were reduced 15-30% within the nucleus tractus solitarius (NTS) and in other medullary respiratory nuclei. Chronically after CBD, GluA1 was reduced (P < 0.05) within the caudal NTS and in other nuclei, but there was significant recovery of GluA2 and GluN1 OD. NK1 OD was not significantly different from control after CBD. We conclude that the initial decrease in GluA2 and GluN1 after CBD likely contributes to hypoventilation and the reduced CO2 chemoreflex. The partial recovery of ventilation and the CO2 chemoreflex after CBD parallel a time-dependent return of these receptors to near control levels but likely depend upon additional initiating and maintenance factors for neuroplasticity.

Contributions of the pre-Bötzinger complex and the Kölliker-fuse nuclei to respiratory rhythm and pattern generation in awake and sleeping goats

We investigated in three groups of awake and sleeping goats whether there are differences in ventilatory responses after injections of Ibotenic acid (IA, glutamate receptor agonist and neurotoxin) into the pre-Bötzinger complex (preBötC), lateral parabrachial (LPBN), medial (MPBN) parabrachial, or Kölliker-Fuse nuclei (KFN). In one group, within minutes after bilateral injection of 10μl IA (50mM) into the preBötC, there was a 10-fold increase in breathing frequency, but 1.5h later, the goats succumbed to terminal apnea. These data are consistent with findings in reduced preparations that the preBötC is critical to sustaining normal breathing. In a second group, increasing volumes (0.5-10μl) of IA injected at weekly intervals into the preBötC elicited a near-dose-dependent tachypnea and irregular breathing that lasted at least 5h. There were apneas restricted to wakefulness, but none were terminal. Postmortem histology revealed that the preBötC was 90% destroyed, but there was a 25-40% above normal number of neurons in the presumed parafacial respiratory group that may have contributed to maintenance of arterial blood gas homeostasis. In a third group, bilateral injections (1 and 10μl) of IA into the LPBN, MPBN, or KFN did not significantly increase breathing in any group, and there were no terminal apneas. However, 3-5h after the injections into the KFN, breathing frequency was decreased and the three-phase eupneic breathing pattern was eliminated. Between 10 and 15h after the injections, the eupneic breathing pattern was not consistently restored to normal, breathing frequency remained attenuated, and there were apneas during wakefulness. Our findings during wakefulness and NREM sleep warrant concluding that (a) the preBötC is a primary site of respiratory rhythm generation; (b) the preBötC and the KFN are determinants of respiratory pattern generation; (c) after IA-induced lesions, there is time-dependent plasticity within the respiratory control network; and (d) ventilatory control mechanisms are state dependent.

Changes in neurochemicals within the ventrolateral medullary respiratory column in awake goats after carotid body denervation

A current and major unanswered question is why the highly sensitive central CO2/H(+) chemoreceptors do not prevent hypoventilation-induced hypercapnia following carotid body denervation (CBD). Because perturbations involving the carotid bodies affect central neuromodulator and/or neurotransmitter levels within the respiratory network, we tested the hypothesis that after CBD there is an increase in inhibitory and/or a decrease in excitatory neurochemicals within the ventrolateral medullary column (VMC) in awake goats. Microtubules for chronic use were implanted bilaterally in the VMC within or near the pre-Bötzinger Complex (preBötC) through which mock cerebrospinal fluid (mCSF) was dialyzed. Effluent mCSF was collected and analyzed for neurochemical content. The goats hypoventilated (peak +22.3 ± 3.4 mmHg PaCO2) and exhibited a reduced CO2 chemoreflex (nadir, 34.8 ± 7.4% of control ΔVE/ΔPaCO2) after CBD with significant but limited recovery over 30 days post-CBD. After CBD, GABA and glycine were above pre-CBD levels (266 ± 29% and 189 ± 25% of pre-CBD; P < 0.05), and glutamine and dopamine were significantly below pre-CBD levels (P < 0.05). Serotonin, substance P, and epinephrine were variable but not significantly (P > 0.05) different from control after CBD. Analyses of brainstem tissues collected 30 days after CBD exhibited 1) a midline raphe-specific reduction (P < 0.05) in the percentage of tryptophan hydroxylase-expressing neurons, and 2) a reduction (P < 0.05) in serotonin transporter density in five medullary respiratory nuclei. We conclude that after CBD, an increase in inhibitory neurotransmitters and a decrease in excitatory neuromodulation within the VMC/preBötC likely contribute to the hypoventilation and attenuated ventilatory CO2 chemoreflex.

Endogenous neurotrophins and plasticity following spinal deafferentation

Neurons intrinsic to the spinal cord dorsal horn receive input from various classes of long-distance projection systems. Two of the best known of these are primary afferent and descending monoaminergic axons. Together with intrinsic interneurons, activity in these axonal populations shapes the early part of the sensory experience before it is transmitted to supraspinal structures via ascending projection axons. Injury to dorsal roots, which contain the centrally projecting branches of primary afferent axons, results in their permanent disconnection from the spinal cord, as well as sensory dysfunction such as pain. In animals, experimental dorsal root injuries affecting a small number of roots produce dynamic behavioural changes, providing evidence for the now familiar concept that sensory processing at the level of the spinal cord is not hard-wired. Changes in behaviour following rhizotomy suggest changes in spinal sensory circuitry, and we and others have shown that the density of spinal serotonergic axons as well as processes of inhibitory interneurons increases following rhizotomy. Intact primary afferent axons are less apt to sprout into denervated territory. Recent work from our group has asked (1) what is the stimulus that induces sprouting of serotonergic (and other) axons and (2) what prevents spared primary afferent axons from occupying the territory of those lost to injury. This article will review the evidence that a single factor upregulated by dorsal root injury, brain-derived neurotrophic factor (BDNF), underpins both serotonergic sprouting and a lack of primary afferent plasticity. BDNF also differentially modulates some of the behavioural consequences of dorsal root injury: antagonizing endogenous BDNF improves spontaneous mechanosensory recovery but prevents recovery from rhizotomy-induced hypersensitivity to cold. These findings reinforce the notion that in disease states as complex and variable as spinal cord injury, single pharmacological interventions are unlikely to produce meaningful results. However, understanding the differences in capacity for plasticity among different systems, as well as their triggers, should allow for more patient-tailored therapies.

Contributions of central and peripheral chemoreceptors to the ventilatory response to CO2/H+

The major objective of this review is to evaluate existing information and reach conclusions regarding whether there is interaction between P(CO(2))/H(+) stimulation of carotid (peripheral) and intracranial (central) chemoreceptors. Interaction is defined as a ventilatory response to simultaneous changes in the degree of Pco2/H(+) stimulation of both chemoreceptors that is greater (hyperadditive) or less (hypoadditive) than the sum of the responses when stimulation of each set of chemoreceptors is individually altered. Simple summation of the simultaneous changes in stimuli results in no interaction (i.e., additive interaction). Knowledge of the nature of central/peripheral interaction is crucial for determining the physiological significance of newer models of ventilatory control based on recent neuroanatomic observations of the circuitry of key elements of the ventilatory control system. In this review, we will propose that these two sets of receptors are not functionally separate but rather that they are dependent on one another such that the sensitivity of the medullary chemoreceptors is critically determined by input from the peripheral chemoreceptors and possibly other breathing-related reflex afferents as well. The short format of this minireview demands that we be somewhat selective in developing our ideas. We will briefly discuss the limitations of experiments used to study CO(2)/H(+) sensitivity and interaction to date, traditional views of the relative contributions of peripheral and central chemoreceptors to CO(2)/H(+) sensitivity, the evidence for and against different types of interaction, and the effect of tonic carotid chemoreceptor afferent activity on central control mechanisms.

Layers of exercise hyperpnea: modulation and plasticity

Despite the fundamental biological significance of the ventilatory response to mild or moderate physical activity (the exercise hyperpnea), we still know remarkably little concerning its underlying mechanisms. Part of the difficulty in revealing those mechanisms may arise due to confusion between multiple mechanistic layers, each contributing to the impressive degree of regulation achieved. The primary, feedforward exercise stimulus or stimuli increase ventilation in approximate proportion to changes in metabolic rate. Chemoreceptor feedback then minimizes deviations from optimal blood gas regulation, most often preventing excessive hypocapnia in non-human mammals. Recent evidence has accumulated, suggesting that adaptive control strategies including modulation and plasticity may adjust the feedforward and/or feedback contributions when blood gas homeostasis proves inadequate. In this review, we present evidence from a goat model of exercise hyperpnea concerning the existence of modulation and plasticity, and specifically mechanisms known as short-term and long-term modulation of the exercise ventilatory response. Throughout the review, we consider available evidence concerning the relevance of these mechanisms to humans. Plasticity is a property only recently recognized in the neural system subserving respiratory control, and the application of these concepts to the exercise ventilatory response in humans is in its infancy. Modulation and plasticity may confer an ability of individuals to adapt their exercise ventilatory response so that it remains appropriate in the face of life-long changes in endogenous (e.g. development, aging, onset of disease) or exogenous (e.g. altitude, wearing a breathing apparatus during physical exertion) physiological conditions.

Rho-kinase inhibition enhances axonal plasticity and attenuates cold hyperalgesia after dorsal rhizotomy

Dorsal rhizotomy results in primary deafferentation of the dorsal horn with concomitant sprouting of spared intraspinal monoaminergic axons. Because descending monoaminergic systems are thought to mitigate nociceptive transmission from the periphery and because dorsal rhizotomy can result in neuropathic pain, we sought to determine whether the rhizotomy-induced sprouting response could be further augmented. Because myelin-derived molecules mask endogenous plasticity of CNS axons and because myelin-inhibitory signaling occurs through the Rho-GTPase pathway, we inhibited Rho-pathway signaling after cervical dorsal rhizotomy in rats. An increase in the density of serotonergic- and tyrosine hydroxylase-positive fibers was seen in the dorsal horn 1 week after septuple rhizotomy, and axon density continued to increase for at least 1 month. One week after septuple rhizotomy, administration of intrathecal Y-27632, an antagonist of Rho-kinase (ROCK), increased the density of both fiber types over vehicle-treated controls. To examine behavioral effects of both cervical rhizotomy and ROCK inhibition, we examined responses to evoked pain: mechanical and thermal allodynia and cold hyperalgesia in the forepaw were examined after single, double, and quadruple rhizotomies of dorsal roots of the brachial plexus. The most notable behavioral outcome was the development of cold hyperalgesia in the affected forepaw after rhizotomies of the C7 and C8 dorsal roots. Application of Y-27632 both attenuated cold hyperalgesia and induced monoaminergic plasticity after C7/8 rhizotomy. Thus, inhibition of Rho-pathway signaling both promoted the sprouting of intact supraspinal monoaminergic fibers and alleviated pain after dorsal rhizotomy.

Plasticity in the control of breathing following sensory denervation

The purpose of this manuscript is to review the results of studies on the recovery or plasticity following a denervation- or lesion-induced change in breathing. Carotid body denervation (CBD), lung denervation (LD), cervical (CDR) and thoracic (TDR) dorsal rhizotomy, dorsal spinal column lesions, and lesions at pontine, medullary, and spinal sites all chronically alter breathing. The plasticity after these is highly variable, ranging from near complete recovery of the peripheral chemoreflex in rats after CBD to minimal recovery of the Hering-Breuer inflation reflex in ponies after LD. The degree of plasticity varies among the different functions of each pathway, and plasticity varies with the age of the animal when the lesion was made. In addition, plasticity after some lesions varies between species, and plasticity is greater in the awake than in the anesthetized state. Reinnervation is not a common mechanism of plasticity. There is evidence supporting two mechanisms of plasticity. One is through upregulation of an alternate sensory pathway, such as serotonin-mediated aortic chemoreception after CBD. The second is through upregulation on the efferent limb of a reflex, such as serotonin-mediated increased responsiveness of phrenic motoneurons after CDR, TDR, and spinal cord injury. Accordingly, numerous components of the ventilatory control system exhibit plasticity after denervation or lesion-induced changes in breathing; this plasticity is uniform neither in magnitude nor in underlying mechanisms. A major need in future research is to determine whether "reorganization" within the central nervous system contributes to plasticity following lesion-induced changes in breathing.

Neuroplasticity in respiratory motor control

Although recent evidence demonstrates considerable neuroplasticity in the respiratory control system, a comprehensive conceptual framework is lacking. Our goals in this review are to define plasticity (and related neural properties) as it pertains to respiratory control and to discuss potential sites, mechanisms, and known categories of respiratory plasticity. Respiratory plasticity is defined as a persistent change in the neural control system based on prior experience. Plasticity may involve structural and/or functional alterations (most commonly both) and can arise from multiple cellular/synaptic mechanisms at different sites in the respiratory control system. Respiratory neuroplasticity is critically dependent on the establishment of necessary preconditions, the stimulus paradigm, the balance between opposing modulatory systems, age, gender, and genetics. Respiratory plasticity can be induced by hypoxia, hypercapnia, exercise, injury, stress, and pharmacological interventions or conditioning and occurs during development as well as in adults. Developmental plasticity is induced by experiences (e.g., altered respiratory gases) during sensitive developmental periods, thereby altering mature respiratory control. The same experience later in life has little or no effect. In adults, neuromodulation plays a prominent role in several forms of respiratory plasticity. For example, serotonergic modulation is thought to initiate and/or maintain respiratory plasticity following intermittent hypoxia, repeated hypercapnic exercise, spinal sensory denervation, spinal cord injury, and at least some conditioned reflexes. Considerable work is necessary before we fully appreciate the biological significance of respiratory plasticity, its underlying cellular/molecular and network mechanisms, and the potential to harness respiratory plasticity as a therapeutic tool.

Chronic cervical spinal sensory denervation reveals ineffective spinal pathways to phrenic motoneurons in the rat

We hypothesized that pretreatment with chronic cervical dorsal rhizotomy (CDR; C(3)-C(6)) would reveal ineffective crossed spinal pathways to phrenic motoneurons. Anesthetized CDR (1 week post-rhizotomy) and control rats were spinally hemisected at C(2), and phrenic potentials were evoked by stimulating the ventrolateral funiculus contralateral and rostral to hemisection. Phrenic potentials contralateral to the stimulating electrode were evoked at lower stimulus currents (CDR=640 +/- 46 microA; control=900 +/- 50 microA; P<0.05) and potential amplitude was significantly greater in CDR versus control rats (P<0.05). The serotonin receptor antagonist methysergide (4 mg/kg, i.v.) had no effect on the crossed phrenic potential amplitude (91+/-17% of control at 800 microA; P>0.05). Thus, CDR enhances crossed phrenic pathways but serotonin receptor activation is not necessary to maintain this effect.

Altered respiratory motor drive after spinal cord injury: supraspinal and bilateral effects of a unilateral lesion

Because some bulbospinal respiratory premotor neurons have bilateral projections to the phrenic nuclei, we investigated whether changes in contralateral phrenic motoneuron function would occur after unilateral axotomy via C(2) hemisection. Phrenic neurograms were recorded under baseline conditions and during hypercapnic and hypoxic challenge in C(2) hemisected, normal, and sham-operated rats at 1 and 2 months after injury. The rats were anesthetized, vagotomized, and mechanically ventilated. No group differences were seen in contralateral neurograms at 1 month after injury. At 2 months, however, there was a statistically significant decrease in respiratory rate (RR) at normocapnia, an elevated RR during hypoxia, and an attenuated increase in phrenic neurogram amplitude during hypercapnia in the C(2)-hemisected animals. To test whether C(2) hemisection had induced a supraspinal change in respiratory motor drive, we recorded ipsilateral and contralateral hypoglossal neurograms during hypercapnia. As with the phrenic motor function data, no change in hypoglossal output was evident until 2 months had elapsed when hypoglossal amplitudes were significantly decreased bilaterally. Last, the influence of serotonin-containing neurons on the injury-induced change in phrenic motoneuron function was examined in rats treated with the serotonin neurotoxin, 5,7-dihydroxytryptamine. Pretreatment with 5,7-dihydroxytryptamine prevented the effects of C(2) hemisection on contralateral phrenic neurogram amplitude and normalized the change in RR during hypoxia. The results of this study show novel neuroplastic changes in segmental and brainstem respiratory motor output after C(2) hemisection that coincided with the spontaneous recovery of some ipsilateral phrenic function. Some of these effects may be modulated by serotonin-containing neurons.

Effects of phrenicotomy and exercise on hypoxia-induced changes in phrenic motor output

To investigate models of plasticity in respiratory motor output, we determined the effects of chronic unilateral phrenicotomy and/or exercise on time-dependent responses to episodic hypoxia in the contralateral phrenic nerve. Anesthetized (urethane), ventilated, and vagotomized rats were presented with three, 5-min episodes of isocapnic hypoxia (11% O(2)), separated by 5 min of hyperoxia (50% O(2)). Integrated phrenic (and hypoglossal) nerve discharge were recorded before and during each hypoxic episode, for the first 5 min after the first hypoxic episode, and at 30 and 60 min after the final episode. Of 36 rats, one-half were sedentary while the other one-half had free access to a running wheel; each of these groups was split into three subgroups: 1) unoperated, 2) chronic left phrenicotomy (27-37 days), and 3) sham operated. Neither unilateral phrenicotomy nor running wheel activity influenced the short-term hypoxic phrenic response (during hypoxia) or long-term facilitation (posthypoxia). Posthypoxia frequency decline was exaggerated in phrenicotomized-sedentary rats relative to unoperated-sedentary rats (change in burst frequency = -23+/-4 vs. -11 +/-5 bursts/min, respectively; 5 min posthypoxia; P<0.05), an effect that was eliminated by spontaneous exercise. The results indicate that neither voluntary running nor unilateral phrenicotomy has major effects on time-dependent hypoxic phrenic responses, with the exception of an unexpected effect of phrenicotomy on posthypoxia frequency decline in sedentary rats.