Serotonin therapies for opioid-induced disordered swallow and respiratory depression

Opioids are well-known to cause respiratory depression, but despite clinical evidence of dysphagia, the effects of opioids on swallow excitability and motor pattern are unknown. We tested the effects of the clinically relevant opioid buprenorphine on pharyngeal swallow and respiratory drive in male and female rats. We also evaluated the utility of 5-HT1A agonists (8-OH-DPAT and buspirone) to improve swallowing and breathing following buprenorphine administration. Experiments were performed on 44 freely breathing Sprague-Dawley rats anesthetized with sodium pentobarbital. Bipolar fine wire electrodes were inserted into the mylohyoid, thyroarytenoid, posterior cricoarytenoid, thyropharyngeus, and diaphragm muscles to measure electromyographic (EMG) activity of swallowing and breathing. We evaluated the hypotheses that swallowing varies by stimulus, opioids depress swallowing and breathing, and that 5-HT1A agonists improve these depressions. Our results largely confirmed the following hypotheses: 1) swallow-related EMG activity was larger during swallows elicited by esophageal distension plus oral water infusion than by either stimulus alone. 2) Buprenorphine depressed swallow in both sexes, but females were more susceptible to total swallow suppression. 3) Female animals were also more vulnerable to opioid-induced respiratory depression. 4) 8-OH-DPAT rescued breathing following buprenorphine-induced respiratory arrest, and pretreatment with the partial 5-HT1A agonist buspirone prevented buprenorphine-induced respiratory arrest in female animals. 5) 8-OH-DPAT enhanced mylohyoid and thyropharyngeus EMG amplitude during swallow but did not restore excitability of the swallow pattern generator following total suppression by buprenorphine. Our results highlight sex-specific and behavior-specific effects of buprenorphine and provide preclinical evidence of a 5HT1A agonist for the treatment of respiratory depression and dysphagia.NEW & NOTEWORTHY This is the first study, to our knowledge, to evaluate sex-specific effects of opioid administration on pharyngeal swallow. We expand on a small but growing number of studies that report a lower threshold for opioid-induced respiratory depression in females compared with males, and we are the first to produce this effect with the partial μ-opioid-receptor agonist buprenorphine. This is the first demonstration, to our knowledge, that activation of 5-HT1A receptors can improve swallow and breathing outcomes following systemic buprenorphine administration.

Acute gastrointestinal permeability after traumatic brain injury in mice precedes a bloom in Akkermansia muciniphila supported by intestinal hypoxia

Traumatic brain injury (TBI) increases gastrointestinal morbidity and associated mortality. Clinical and preclinical studies implicate gut dysbiosis as a consequence of TBI and an amplifier of brain damage. However, little is known about the association of gut dysbiosis with structural and functional changes of the gastrointestinal tract after an isolated TBI. To assess gastrointestinal dysfunction, mice received a controlled cortical impact or sham brain injury and intestinal permeability was assessed at 4 h, 8 h, 1 d, and 3 d after injury by oral administration of 4 kDa FITC Dextran prior to euthanasia. Quantification of serum fluorescence revealed an acute, short-lived increase in permeability 4 h after TBI. Despite transient intestinal dysfunction, no overt morphological changes were evident in the ileum or colon across timepoints from 4 h to 4 wks post-injury. To elucidate the timeline of microbiome changes after TBI, 16 s gene sequencing was performed on DNA extracted from fecal samples collected prior to and over the first month after TBI. Differential abundance analysis revealed that the phylum Verrucomicrobiota was increased at 1, 2, and 3 d after TBI. The Verrucomicrobiota species was identified by qPCR as Akkermansia muciniphila, an obligate anaerobe that resides in the intestinal mucus bilayer and produces short chain fatty acids (e.g. butyrate) utilized by intestinal epithelial cells. We postulated that TBI promotes intestinal changes favorable for the bloom of A. muciniphila. Consistent with this premise, the relative area of mucus-producing goblet cells in the medial colon was significantly increased at 1 d after injury, while colon hypoxia was significantly increased at 3 d. Our findings reveal acute gastrointestinal functional changes coupled with an increase of beneficial bacteria suggesting a potential compensatory response to systemic stress after TBI.

Pioglitazone restores mitochondrial function but does not spare cortical tissue following mild brain contusion

Pioglitazone interacts through the mitochondrial protein mitoNEET to improve brain bioenergetics following traumatic brain injury. To provide broader evidence regarding the therapeutic effects of pioglitazone after traumatic brain injury, the current study is focused on immediate and delayed therapy in a model of mild brain contusion. To assess pioglitazone therapy on mitochondrial bioenergetics in cortex and hippocampus, we use a technique to isolate subpopulations of total, glia-enriched and synaptic mitochondria. Pioglitazone treatment was initially administered at either 0.25, 3, 12 or 24 h following mild controlled cortical impact. At 48 h post-injury, ipsilateral cortex and hippocampus were dissected and mitochondrial fractions were isolated. Maximal mitochondrial respiration injury-induced deficits were observed in total and synaptic fractions, and 0.25 h pioglitazone treatment following mild controlled cortical impact was able to restore respiration to sham levels. While there are no injury-induced deficits in hippocampal fractions, we do find that 3 h pioglitazone treatment after mild controlled cortical impact can significantly increase maximal mitochondrial bioenergetics compared to vehicle-treated mild controlled cortical impact group. However, delayed pioglitazone treatment initiated at either 3 or 24 h after mild brain contusion does not improve spared cortical tissue. We demonstrate that synaptic mitochondrial deficits following mild focal brain contusion can be restored with early initiation of pioglitazone treatment. Further investigation is needed to determine functional improvements with pioglitazone beyond that of overt cortical tissue sparing following mild contusion traumatic brain injury.

Deglutition and the Regulation of the Swallow Motor Pattern

Despite centuries of investigation, questions and controversies remain regarding the fundamental genesis and motor pattern of swallow. Two significant topics include inspiratory muscle activity during swallow (Schluckatmung, i.e., "swallow-breath") and anatomical boundaries of the swallow pattern generator. We discuss the long history of reports regarding the presence or absence of Schluckatmung and the possible advantages of and neural basis for such activity, leading to current theories and novel experimental directions.

Clinically relevant mitochondrial-targeted therapy improves chronic outcomes after traumatic brain injury

Pioglitazone, an FDA-approved compound, has been shown to target the novel mitochondrial protein mitoNEET and produce short-term neuroprotection and functional benefits following traumatic brain injury. To expand on these findings, we now investigate the dose- and time-dependent effects of pioglitazone administration on mitochondrial function after experimental traumatic brain injury. We then hypothesize that optimal pioglitazone dosing will lead to ongoing neuroprotection and cognitive benefits that are dependent on pioglitazone-mitoNEET signalling pathways. We show that delayed intervention is significantly more effective than early intervention at improving acute mitochondrial bioenergetics in the brain after traumatic brain injury. In corroboration, we demonstrate that mitoNEET is more heavily expressed, especially near the cortical contusion, in the 18 h following traumatic brain injury. To explore whether these findings relate to ongoing pathological and behavioural outcomes, mice received controlled cortical impact followed by initiation of pioglitazone treatment at either 3 or 18 h post-injury. Mice with treatment initiation at 18 h post-injury exhibited significantly improved behaviour and tissue sparing compared to mice with pioglitazone initiated at 3 h post-injury. Further using mitoNEET knockout mice, we show that this therapeutic effect is dependent on mitoNEET. Finally, we demonstrate that delayed pioglitazone treatment improves serial motor and cognitive performance in conjunction with attenuated brain atrophy after traumatic brain injury. This study illustrates that mitoNEET is the critical target for delayed pioglitazone intervention after traumatic brain injury, mitochondrial-targeting is highly time-dependent after injury and there is an extended therapeutic window to effectively treat mitochondrial dysfunction after brain injury.

Silencing long ascending propriospinal neurons after spinal cord injury improves hindlimb stepping in the adult rat

Long ascending propriospinal neurons (LAPNs) are a subpopulation of spinal cord interneurons that directly connect the lumbar and cervical enlargements. Previously we showed, in uninjured animals, that conditionally silencing LAPNs disrupted left-right coordination of the hindlimbs and forelimbs in a context-dependent manner, demonstrating that LAPNs secure alternation of the fore- and hindlimb pairs during overground stepping. Given the ventrolateral location of LAPN axons in the spinal cord white matter, many likely remain intact following incomplete, contusive, thoracic spinal cord injury (SCI), suggesting a potential role in the recovery of stepping. Thus, we hypothesized that silencing LAPNs after SCI would disrupt recovered locomotion. Instead, we found that silencing spared LAPNs post-SCI improved locomotor function, including paw placement order and timing, and a decrease in the number of dorsal steps. Silencing also restored left-right hindlimb coordination and normalized spatiotemporal features of gait such as stance and swing time. However, hindlimb-forelimb coordination was not restored. These data indicate that the temporal information carried between the spinal enlargements by the spared LAPNs post-SCI is detrimental to recovered hindlimb locomotor function. These findings are an illustration of a post-SCI neuroanatomical-functional paradox and have implications for the development of neuronal- and axonal-protective therapeutic strategies and the clinical study/implementation of neuromodulation strategies.

Limited changes in locomotor recovery and unaffected white matter sparing after spinal cord contusion at different times of day

The circadian gene expression rhythmicity drives diurnal oscillations of physiological processes that may determine the injury response. While outcomes of various acute injuries are affected by the time of day at which the original insult occurred, such influences on recovery after spinal cord injury (SCI) are unknown. We report that mice receiving moderate, T9 contusive SCI at ZT0 (zeitgeber time 0, time of lights on) and ZT12 (time of lights off) showed similar hindlimb function recovery in the Basso mouse scale (BMS) over a 6 week post-injury period. In an independent study, no significant differences in BMS were observed after SCI at ZT18 vs. ZT6. However, the ladder walking test revealed modestly improved performance for ZT18 vs. ZT6 mice at week 6 after injury. Consistent with those minor effects on functional recovery, terminal histological analysis revealed no significant differences in white matter sparing at the injury epicenter. Likewise, blood-spinal cord barrier disruption and neuroinflammation appeared similar when analyzed at 1 week post injury at ZT6 or ZT18. Therefore, locomotor recovery after thoracic contusive SCI is not substantively modulated by the time of day at which the neurotrauma occurred.

Long ascending propriospinal neurons provide flexible, context-specific control of interlimb coordination

Within the cervical and lumbar spinal enlargements, central pattern generator (CPG) circuitry produces the rhythmic output necessary for limb coordination during locomotion. Long propriospinal neurons that inter-connect these CPGs are thought to secure hindlimb-forelimb coordination, ensuring that diagonal limb pairs move synchronously while the ipsilateral limb pairs move out-of-phase during stepping. Here, we show that silencing long ascending propriospinal neurons (LAPNs) that inter-connect the lumbar and cervical CPGs disrupts left-right limb coupling of each limb pair in the adult rat during overground locomotion on a high-friction surface. These perturbations occurred independent of the locomotor rhythm, intralimb coordination, and speed-dependent (or any other) principal features of locomotion. Strikingly, the functional consequences of silencing LAPNs are highly context-dependent; the phenotype was not expressed during swimming, treadmill stepping, exploratory locomotion, or walking on an uncoated, slick surface. These data reveal surprising flexibility and context-dependence in the control of interlimb coordination during locomotion.

Improved locomotor recovery after contusive spinal cord injury in Bmal1-/- mice is associated with protection of the blood spinal cord barrier

The transcription factor BMAL1/ARNTL is a non-redundant component of the clock pathway that regulates circadian oscillations of gene expression. Loss of BMAL1 perturbs organismal homeostasis and usually exacerbates pathological responses to many types of insults by enhancing oxidative stress and inflammation. Surprisingly, we observed improved locomotor recovery and spinal cord white matter sparing in Bmal1-/- mice after T9 contusive spinal cord injury (SCI). While acute loss of neurons and oligodendrocytes was unaffected, Bmal1 deficiency reduced the chronic loss of oligodendrocytes at the injury epicenter 6 weeks post SCI. At 3 days post-injury (dpi), decreased expression of genes associated with cell proliferation, neuroinflammation and disruption of the blood spinal cord barrier (BSCB) was also observed. Moreover, intraspinal extravasation of fibrinogen and immunoglobulins was decreased acutely at dpi 1 and subacutely at dpi 7. Subacute decrease of hemoglobin deposition was also observed. Finally, subacutely reduced levels of the leukocyte marker CD45 and even greater reduction of the pro-inflammatory macrophage receptor CD36 suggest not only lower numbers of those cells but also their reduced inflammatory potential. These data indicate that Bmal1 deficiency improves SCI outcome, in part by reducing BSCB disruption and hemorrhage decreasing cytotoxic neuroinflammation and attenuating the chronic loss of oligodendrocytes.

The Mitochondria-Associated ER Membranes Are Novel Subcellular Locations Enriched for Inflammatory-Responsive MicroRNAs

The mitochondria-associated endoplasmic reticulum (ER) membranes (MAMs) are specific ER domains that contact the mitochondria and function to facilitate communication between ER and mitochondria. Disruption of contact between the mitochondria and ER is associated with a variety of pathophysiological conditions including neurodegenerative diseases. Considering the many cellular functions of MAMs, we hypothesized that MAMs play an important role in regulating microRNA (miRNA) activity linked to its unique location between mitochondria and ER. Here we present new findings from human and rat brains indicating that the MAMs are subcellular sites enriched for specific miRNAs. We employed subcellular fractionation and TaqMan® RT-qPCR miRNA analysis to quantify miRNA levels in subcellular fractions isolated from male rat brains and six human brain samples. We found that MAMs contain a substantial number of miRNAs and the profile differs significantly from that of cytosolic, mitochondria, or ER. Interestingly, MAMs are particularly enriched in inflammatory-responsive miRNAs, including miR-146a, miR-142-3p, and miR-142-5p in both human and rat brains; miR-223 MAM enrichment was observed only in human brain samples. Further, mitochondrial uncoupling or traumatic brain injury in male rats resulted in the alteration of inflammatory miRNA enrichment in the isolated subcellular fractions. These observations demonstrate that miRNAs are distributed differentially in organelles and may re-distribute between organelles and the cytosol in response to cellular stress and metabolic demands.

Sex-specific vagal and spinal modulation of swallow and its coordination with breathing

Swallow-breathing coordination is influenced by changes in lung volume, which is modulated by feedback from both vagal and spinal sensory afferents. The purpose of this study was to manipulate feedback from these afferents, with and without a simultaneous mechanical challenge (chest compression), in order to assess the influence of each sensory pathway on swallow in rats. We hypothesized that manipulation of afferent feedback would shift the occurrence of swallow toward the inspiratory phase of breathing. Afferent feedback was perturbed by lidocaine nebulization, extra-thoracic vagotomy, or lidocaine administration to the pleural space in sodium pentobarbital anesthetized rats (N = 43). These different afferent perturbations were performed both in control conditions (no chest compression), and with chest compression. Manipulating pulmonary stretch receptor-mediated volume feedback in male animals decreased swallow occurrence. Female rats appear to rely more on spinal afferent feedback, as swallow occurrence shifted to late expiration with chest compression and vagotomy or lidocaine injections. Results suggest that sex-specific mechanisms modulate swallow-breathing coordination, and that vagal feedback is inhibitory to swallow-related muscles, while spinal feedback from pleural afferents has excitatory effects. This study supports the theory that a balance of vagal and spinal afferent feedback is necessary to maintain an optimal swallow pattern and swallow-breathing coordination.

Novel imaging biomarkers for mapping the impact of mild mitochondrial uncoupling in the outer retina in vivo

PURPOSE

To test the hypothesis that imaging biomarkers are useful for evaluating in vivo rod photoreceptor cell responses to a mitochondrial protonophore.

METHODS

Intraperitoneal injections of either the mitochondrial uncoupler 2,4 dinitrophenol (DNP) or saline were given to mice with either higher [129S6/eVTac (S6)] or lower [C57BL/6J (B6)] mitochondrial reserve capacities and were studied in dark or light. We measured: (i) the external limiting membrane-retinal pigment epithelium region thickness (ELM-RPE; OCT), which decreases substantially with upregulation of a pH-sensitive water removal co-transporter on the apical portion of the RPE, and (ii) the outer retina R1 (= 1/(spin lattice relaxation time (T1), an MRI parameter proportional to oxygen / free radical content.

RESULTS

In darkness, baseline rod energy production and consumption are relatively high compared to that in light, and additional metabolic stimulation with DNP provoked thinning of the ELM-RPE region compared to saline injection in S6 mice; ELM-RPE thickness was unresponsive to DNP in B6 mice. Also, dark-adapted S6 mice given DNP showed a decrease in outer retina R1 values compared to saline injection in the inferior retina. In dark-adapted B6 mice, transretinal R1 values were unresponsive to DNP in superior and inferior regions. In light, with its relatively lower basal rod energy production and consumption, DNP caused ELM-RPE thinning in both S6 and B6 mice.

CONCLUSIONS

The present results raise the possibility of non-invasively evaluating the mouse rod mitochondrial energy ecosystem using new DNP-assisted OCT and MRI in vivo assays.

Spinal Cord Injury Scarring and Inflammation: Therapies Targeting Glial and Inflammatory Responses

Deficits in neuronal function are a hallmark of spinal cord injury (SCI) and therapeutic efforts are often focused on central nervous system (CNS) axon regeneration. However, secondary injury responses by astrocytes, microglia, pericytes, endothelial cells, Schwann cells, fibroblasts, meningeal cells, and other glia not only potentiate SCI damage but also facilitate endogenous repair. Due to their profound impact on the progression of SCI, glial cells and modification of the glial scar are focuses of SCI therapeutic research. Within and around the glial scar, cells deposit extracellular matrix (ECM) proteins that affect axon growth such as chondroitin sulfate proteoglycans (CSPGs), laminin, collagen, and fibronectin. This dense deposition of material, i.e., the fibrotic scar, is another barrier to endogenous repair and is a target of SCI therapies. Infiltrating neutrophils and monocytes are recruited to the injury site through glial chemokine and cytokine release and subsequent upregulation of chemotactic cellular adhesion molecules and selectins on endothelial cells. These peripheral immune cells, along with endogenous microglia, drive a robust inflammatory response to injury with heterogeneous reparative and pathological properties and are targeted for therapeutic modification. Here, we review the role of glial and inflammatory cells after SCI and the therapeutic strategies that aim to replace, dampen, or alter their activity to modulate SCI scarring and inflammation and improve injury outcomes.

Repeated Closed Head Injury in Mice Results in Sustained Motor and Memory Deficits and Chronic Cellular Changes

Millions of mild traumatic brain injuries (TBIs) occur every year in the United States, with many people subject to multiple head injuries that can lead to chronic behavioral dysfunction. We previously reported that mild TBI induced using closed head injuries (CHI) repeated at 24h intervals produced more acute neuron death and glial reactivity than a single CHI, and increasing the length of time between injuries to 48h reduced the cumulative acute effects of repeated CHI. To determine whether repeated CHI is associated with behavioral dysfunction or persistent cellular damage, mice receiving either five CHI at 24h intervals, five CHI at 48h intervals, or five sham injuries at 24h intervals were evaluated across a 10 week period after injury. Animals with repeated CHI exhibited motor coordination and memory deficits, but not gait abnormalities when compared to sham animals. At 10wks post-injury, no notable neuron loss or glial reactivity was observed in the cortex, hippocampus, or corpus callosum. Argyrophilic axons were found in the pyramidal tract of some injured animals, but neither silver stain accumulation nor inflammatory responses in the injury groups were statistically different from the sham group in this region. However, argyrophilic axons, microgliosis and astrogliosis were significantly increased within the optic tract of injured animals. Repeated mild CHI also resulted in microgliosis and a loss of neurofilament protein 200 in the optic nerve. Lengthening the inter-injury interval from 24h to 48h did not effectively reduce these behavioral or cellular responses. These results suggest that repeated mild CHI results in persistent behavioral dysfunction and chronic pathological changes within the visual system, neither of which was significantly attenuated by lengthening the inter-injury interval from 24h to 48h.

The TNFα-Transgenic Rat: Hippocampal Synaptic Integrity, Cognition, Function, and Post-Ischemic Cell Loss

The cytokine, tumor necrosis factor α (TNFα), is a key regulator of neuroinflammation linked to numerous neurodegenerative conditions and diseases. The present study used transgenic rats that overexpress a murine TNFα gene, under the control of its own promoter, to investigate the impact of chronically elevated TNFα on hippocampal synaptic function. Neuronal viability and cognitive recovery in TNFα Tg rats were also determined following an ischemic insult arising from reversible middle cerebral artery occlusion (MCAO). Basal CA3-CA1 synaptic strength, recorded in acute brain slices, was not significantly different between eight-week-old TNFα Tg rats and non-Tg rats. In contrast, slices from TNFα Tg rats showed significantly greater levels of long-term potentiation (LTP) in response to 100 Hz stimulation, suggesting that synaptic networks may be hyperexcitable in the context of elevated TNFα. Cognitive and motor deficits (assessed on the Morris Water Maze and Rotarod task, respectively) were present in TNFα Tg rats in the absence of significant differences in the loss of cortical and hippocampal neurons. TNF overexpression exacerbated MCAO-dependent deficits on the rotarod, but ameliorated cortical neuron loss in response to MCAO.