Medullary Neurons Research Articles

Endomorphin-2 (Endo2) and substance P (SubP) co-application attenuates SubP-induced excitation and alters frequency plasticity in neonatal rat in vitro preparations

Substance P (SubP) and endomorphin-2 (Endo2) are co-localized presynaptically in vesicles of neurons adjacent to inspiratory rhythm-generating pre-Botzinger Complex (preBotC) neurons but the effects of co-released SubP and Endo2 on respiratory motor control are not known. To address this question, SubP alone or a combination of SubP and Endo2 (SubP/Endo2) were bath-applied in a sustained (15-min) or intermittent (5-min application, 5-min washout, x3) pattern at 10–100 nM to neonatal rat brainstem-spinal cord preparations. During neuropeptide application, SubP/Endo2 co-applications generally attenuated SubP-induced increases in burst frequency and decreases in burst amplitude. With respect to frequency plasticity (long-lasting increase in burst frequency 60 min post-neuropeptide application), SubP-induced frequency plasticity was increased with sustained SubP/Endo2 co-applications at 20 and 100 nM. Intermittent SubP/Endo2 co-applications tended to decrease the level of frequency plasticity induced by intermittent SubP alone applications. SubP/Endo2 co-applications revealed potentially new functions for neurokinin-1 (NK1R) and mu-opioid (MOR) receptors on respiratory rhythm-generating medullary neurons.

Ticagrelor-related dyspnea beyond adenosine: Insights into retrotrapezoid hyperactivity

Ticagrelor, a P2Y12 receptor antagonist, has been demonstrated to induce dyspnea, which is not associated with cardiac or pulmonary alterations, or metabolic disturbances. The attribution of ticagrelor-related dyspnea to excess adenosine has been widely proposed, yet is not supported by experimental data. In this paper, we put forth a novel hypothesis that the hyperactivity of the retrotrapezoid nucleus, a group of ventral medullary neurons involved in respiratory modulation, is the underlying cause of ticagrelor-related dyspnea. This hypothesis offers a theoretical resolution to the discrepancies and controversies present in previous theories.

Genetic identification of medullary neurons underlying congenital hypoventilation.

Mutations in the transcription factors encoded by PHOX2B or LBX1 correlate with congenital central hypoventilation disorders. These conditions are typically characterized by pronounced hypoventilation, central apnea, and diminished chemoreflexes, particularly to abnormally high levels of arterial PCO2. The dysfunctional neurons causing these respiratory disorders are largely unknown. Here, we show that distinct, and previously undescribed, sets of medullary neurons coexpressing both transcription factors (dB2 neurons) account for specific respiratory functions and phenotypes seen in congenital hypoventilation. By combining intersectional chemogenetics, intersectional labeling, lineage tracing, and conditional mutagenesis, we uncovered subgroups of dB2 neurons with key functions in (i) respiratory tidal volumes, (ii) the hypercarbic reflex, (iii) neonatal respiratory stability, and (iv) neonatal survival. These data provide functional evidence for the critical role of distinct medullary dB2 neurons in neonatal respiratory physiology. In summary, our work identifies distinct subgroups of dB2 neurons regulating breathing homeostasis, dysfunction of which causes respiratory phenotypes associated with congenital hypoventilation.

Respiration and Cough in Pompe Disease: Results from a neuromechanical computational model of the respiratory control system

Late Onset Pompe Disease (LOPD) is an incurable, rare progressive genetic disease. It is characterized by glycogen build-up, due to mutations in the gene encoding the lysosomal enzyme acid alpha glucosidase, throughout muscle and neural regions. This accumulation is associated with cell death and muscle fiber swelling, eventually leading to respiratory insuffciency and airway clearance (i.e. cough) deficiency. Impairments in effective airway clearance result from the inability to clear normal pulmonary volume. Emerging evidence indicates significant respiratory neuron dysfunction, altering pulmonary function in humans and animal models. It has been suggested that respiratory and airway clearance insuffciency observed in LOPD may be attributed to medullary motoneurons potentially associated with the respiratory and cough pattern generators (Fuller et al., 2013; DeRuisseau et al., 2009). However, the precise neural mechanisms that suppress breathing and cough are not fully understood. One possible target for LOPD respiratory insuffciencies may be reduced medullary inspiratory neuron activity within this network, which could result in decreased phrenic motoneuron output. Using a stochastic neural network simulator that consisted of discrete “integrate and fire” populations, we simulated a systematic decrease in three medullary inspiratory neuron populations: inspiratory driver (I-Driver), inspiratory decrementing (I-Dec), or inspiratory augmenting (I-Aug) neurons. We simulated breathing and three cough trials. By targeting these populations, we expected global decreased neural bursts, suppressed inspiratory motor drive, and/or suppressed expiratory motoneuron output. Simulations indicated that when the I-Dec motoneuron population decreased by 25%, it resulted in apnea and abolished cough. Reducing the I-Aug motoneuron population by 20% resulted in longer inspiratory phase durations (TI) and a decreased respiratory rate. During cough, there was a decrease in the duration and amplitudes of the expiratory motoneuronal activity associated with decreased cough receptor and pulmonary stretch receptor afferent feedback. When the I-Driver motoneuron population was reduced by 25%, TI decreased and I amplitude increased during respiration. During cough, lumbar and phrenic motoneuron discharge duration and amplitude decreased. These results are consistent with previously reported in vivo data. Overall, our simulation data support the plausibility of glycogen storage dysfunction impairing medullary neuronal and premotoneuronal excitability within the inspiratory network. Our results also suggest that some neurons may be more sensitive to glycogen storage than others within the brainstem inspiratory network. Supported by NHLBI L30HL165496, NIH T32 HL 134621 and 3OT2OD023854. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Exploring Serotonergic mechanisms in Opioid Induced Respiratory Depression

Opioids profoundly depress breathing which can be fatal in the case of overdose. Despite growing knowledge of the pontine and medullary respiratory control circuits that are inhibited by opioids, the circuit and cellular mechanisms of opioid induced respiratory depression are not fully understood. Serotonin neurons located in the caudal medullary raphe modulate breathing and possess mu opioid receptors. Therefore, we hypothesized that serotonin-producing raphe neurons contribute to opioid induced respiratory depression. To test this, we employed to experimental approaches: 1) arterially perfused in situ working heart brainstem preparations and 2) ex vivo acute brain slice electrophysiology to identify and characterize serotonin neuron inputs onto brainstem respiratory neurons. To specifically activate serotonin neurons, we used ePet-Cre::Ai32 mice which Cre-dependently express channelrhodopsin-2, a light-activated cation channel, in serotonin neurons. At the circuit level, using the in situ preparation, we found that activation of caudal medullary raphe serotonin neurons increased respiratory rate by decreasing inspiratory and expiratory duration. Fentanyl application blocked the respiratory effects induced by serotonin neuron activation which were subsequently restored using naloxone. These results suggest serotonin neurons in the caudal medullary raphe are inhibited by opioids. To characterize the serotonin-mediated mechanisms at the cellular level, we made acute brain slices containing dorsal raphe neurons and lateral parabrachial neurons from ePet-Cre::Ai32 mice. We verified that optical stimulation resulted in action potentials and serotonin release from dorsal raphe serotonin neurons. Optically evoked serotonin release caused 5HT1A receptor-mediated outward currents in dorsal raphe serotonin neurons. To locate and characterize serotonin neuron inputs onto lateral parabrachial neurons, we performed whole-cell voltage clamp recordings from lateral parabrachial neurons and delivered paired light pulses (10 ms duration, 50 ms interval). We observed optically evoked inhibitory post-synaptic currents (IPSCs) that were blocked by the GABA-A receptor antagonist gabazine (1 μM). Optically evoked IPSCs had long latency (> 5 ms) and high jitter (> 1 ms), indicating stimulation of serotonin terminals resulted in polysynaptic GABA release onto lateral parabrachial neurons. Future work will examine the influence of opioids on this raphe-parabrachial circuit. Uncovering the role of serotonin neurons in opioid induced respiratory depression may reveal targets for restoring breathing in the presence of opioids or other conditions where life-threatening apneas occur. This work was supported by the National Institutes of Health (NIDA) R01DA047978-05 grant to ESL. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

C2 Spinal Cord Hemisection Reduces Mitochondrial Volume Density in Phrenic Motor Neurons

The diaphragm muscle (DIAm) is the major inspiratory pump for breathing. Neural control of the DIAm during breathing involves the recruitment of fatigue resistant motor units comprising smaller phrenic motor neurons (PhMNs), which innervate type I and IIa fibers. More fatigable DIAm motor units comprise larger PhMNs that innervate type IIx/IIb fibers, which generate greater forces but are more fatigable. The fatigable motor units are only infrequently recruited for expulsive behaviors of the DIAm. Thus, there are marked differences in the activation history of DIAm motor units. Consistent with their higher activity levels, previous studies from our lab have shown that smaller PhMNs and type I and IIa DIAm fibers have greater mitochondrial volume density (MVD). PhMNs located in the cervical spinal cord (C3-C5) receive direct excitatory input from medullary premotor neurons, which is primarily ipsilateral. Thus, C2 spinal cord hemisection (C2SH) disrupts the neural pathway essential for breathing and significantly reduces DIAm activity on the same side. Therefore, we hypothesized that C2SH significantly reduces MVD of smaller PhMNs. In Sprague Dawley rats, PhMNs on both sides were retrogradely labeled by intrapleural injection of cholera toxin B (CTB) 3 days prior to C2SH (right side). In addition, DIAm EMG activity was recorded on both sides by inserting bipolar electrodes. Mitochondria were labeled by intraspinal injection of MitoTracker Red and imaged in identified PhMNs using 3D confocal microscopy. A series of optical slices (0.5 mm) of PhMNs were deconvolved to improve contrast, then thresholded for MitoTracker labeling followed by 3D reconstruction, providing detailed images of mitochondrial morphometry and distribution within PhMNs. In sham rats, the MVD of smaller (lower somal surface area) PhMNs was higher compared to larger PhMNs. C2SH reduced DIAm EMG activity on the ipsilateral side across a 14-day period, while activity on the contralateral side increased. After 14 days, C2SH reduced MVD by ~25% in smaller PhMNs on the ipsilateral side. We conclude that the reduced activity of smaller PhMNs induced by C2SH induced mitochondrial morphological changes, most likely due to reduced mitochondrial biogenesis. These results indicate that reduced DIAm activity associated with mechanical ventilation may impose mitochondrial remodeling that reduces mitochondrial oxidative capacity in PhMNs. These adaptations may disrupt mitochondrial function and impair weaning from mechanical ventilation. Research support by NIH grants HL166204, HL146114 and AG44615. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Cortical-brainstem circuitry attenuates physiological stress reactivity.

Exposure to stressful stimuli promotes multi-system biological responses to restore homeostasis. Catecholaminergic neurons in the rostral ventrolateral medulla (RVLM) facilitate sympathetic activity and promote physiological adaptations, including glycaemic mobilization and corticosterone release. While it is unclear how brain regions involved in the cognitive appraisal of stress regulate RVLM neural activity, recent studies found that the rodent ventromedial prefrontal cortex (vmPFC) mediates stress appraisal and physiological stress responses. Thus, a vmPFC-RVLM connection could represent a circuit mechanism linking stress appraisal and physiological reactivity. The current study investigated a direct vmPFC-RVLM circuit utilizing genetically encoded anterograde and retrograde tract tracers. Together, these studies found that stress-activated vmPFC neurons project to catecholaminergic neurons throughout the ventrolateral medulla in male and female rats. Next, we utilized optogenetic terminal stimulation to evoke vmPFC synaptic glutamate release in the RVLM. Photostimulating the vmPFC-RVLM circuit during restraint stress suppressed glycaemic stress responses in males, without altering the female response. However, circuit stimulation decreased corticosterone responses to stress in both sexes. Circuit stimulation did not modulate affective behaviour in either sex. Further analysis indicated that circuit stimulation preferentially activated non-catecholaminergic medullary neurons in both sexes. Additionally, vmPFC terminals targeted medullary inhibitory neurons. Thus, both male and female rats have a direct vmPFC projection to the RVLM that reduces endocrine stress responses, likely by recruiting local RVLM inhibitory neurons. Ultimately, the excitatory/inhibitory balance of vmPFC synapses in the RVLM may regulate stress reactivity and stress-related health outcomes. KEY POINTS: Glutamatergic efferents from the ventromedial prefrontal cortex target catecholaminergic neurons throughout the ventrolateral medulla. Partially segregated, stress-activated ventromedial prefrontal cortex populations innervate the rostral and caudal ventrolateral medulla. Stimulating ventromedial prefrontal cortex synapses in the rostral ventrolateral medulla decreases stress-induced glucocorticoid release in males and females. Stimulating ventromedial prefrontal cortex terminals in the rostral ventrolateral medulla preferentially activates non-catecholaminergic neurons. Ventromedial prefrontal cortex terminals target medullary inhibitory neurons.

Axonal projection of the medullary expiratory neurons in the feline thoracic spinal cord

Expiratory neurons in the caudal ventral respiratory group extend descending axons to the lumbar and sacral spinal cord, and they possess axon collaterals, the distribution of which has been well-documented. Likewise, these expiratory neurons extend axons to the thoracic spinal cord and innervate thoracic expiratory motoneurons. These axons also give rise to collaterals, and their distribution may influence the strength of synaptic connectivity between the axons and the thoracic expiratory motoneurons. We investigated the distribution of axon collaterals in the thoracic spinal cord using a microstimulation technique. This study was performed on cats; one cat was used to make an anatomical atlas and six were used in the experiment. Extracellular spikes of expiratory neurons were recorded in artificially ventilated cats. The thoracic spinal gray matter was microstimulated from dorsal to ventral sites at 100-μm intervals using a glass-insulated tungsten microelectrode with a current of 150–250 μA. The stimulation tracks were made at 1 mm intervals along the spinal cord in segments Th9 to Th13, and the effective stimulating sites of antidromic activation in axon collaterals were systematically mapped. The effective stimulating sites in the contralateral thoracic spinal cord with expiratory neurons in the caudal ventral respiratory group (cVRG) occupied 14.4% of the total length of the thoracic spinal cord examined. The mean percentage of effective stimulating tracks per unit was 18.6 ± 4.4%. The distribution of axon collaterals of expiratory neurons in the feline thoracic spinal cord indeed resembled that reported in the upper lumbar spinal cord. We propose that a single medullary expiratory neuron exerts excitatory effects across multiple segments of the thoracic spinal cord via its collaterals.

Central Mechanisms of Thermoregulation and Fever in Mammals.

Thermoregulation is a fundamental homeostatic function in mammals mediated by the central nervous system. The framework of the central circuitry for thermoregulation lies in the hypothalamus and brainstem. The preoptic area (POA) of the hypothalamus integrates cutaneous and central thermosensory information into efferent control signals that regulate excitatory descending pathways through the dorsomedial hypothalamus (DMH) and rostral medullary raphe region (rMR). The cutaneous thermosensory feedforward signals are delivered to the POA by afferent pathways through the lateral parabrachial nucleus, while the central monitoring of body core temperature is primarily mediated by warm-sensitive neurons in the POA for negative feedback regulation. Prostaglandin E2, a pyrogenic mediator produced in response to infection, acts on the POA to trigger fever. Recent studies have revealed that this circuitry also functions for physiological responses to psychological stress and starvation. Master psychological stress signaling from the medial prefrontal cortex to the DMH has been discovered to drive a variety of physiological responses for stress coping, including hyperthermia. During starvation, hunger signaling from the hypothalamus was found to activate medullary reticular neurons, which then suppress thermogenic sympathetic outflows from the rMR for energy saving. This thermoregulatory circuit represents a fundamental mechanism of the central regulation for homeostasis.

Neural correlate of reduced respiratory chemosensitivity during chronic epilepsy.

While central autonomic, cardiac, and/or respiratory dysfunction underlies sudden unexpected death in epilepsy (SUDEP), the specific neural mechanisms that lead to SUDEP remain to be determined. In this study, we took advantage of single-cell neuronal Ca2+ imaging and intrahippocampal kainic acid (KA)-induced chronic epilepsy in mice to investigate progressive changes in key cardiorespiratory brainstem circuits during chronic epilepsy. Weeks after induction of status epilepticus (SE), when mice were experiencing recurrent spontaneous seizures (chronic epilepsy), we observed that the adaptive ventilatory responses to hypercapnia were reduced for 5 weeks after SE induction with its partial recovery at week 7. These changes were paralleled by alterations in the chemosensory responses of neurons in the retrotrapezoid nucleus (RTN). Neurons that displayed adapting responses to hypercapnia were less prevalent and exhibited smaller responses over weeks 3-5, whereas neurons that displayed graded responses to hypercapnia became more prevalent by week 7. Over the same period, chemosensory responses of the presympathetic rostral ventrolateral medullary (RVLM) neurons showed no change. Mice with chronic epilepsy showed enhanced sensitivity to seizures, which invade the RTN and possibly put the chemosensory circuits at further risk of impairment. Our findings establish a dysfunctional breathing phenotype with its RTN neuronal correlate in mice with chronic epilepsy and suggest that the assessment of respiratory chemosensitivity may have the potential for identifying people at risk of SUDEP.

Protective effects of SSRI, Citalopram in mutant APP and mutant Tau expressed dorsal raphe neurons in Alzheimer's disease

Depression is among the most common neuropsychiatric comorbidities in Alzheimer's disease (AD) and other Tauopathies. Apart from its anti-depressive and anxiolytic effects, selective serotonin reuptake inhibitor (SSRI) treatment also offers intracellular modifications that may help to improve neurogenesis, reduce amyloid burden & Tau pathologies, and neuroinflammation in AD. Despite its multifaceted impact in the brain, the exact physiological and molecular mechanism by which SSRIs such as Citalopram improve neurogenesis and synaptogenesis in dementia is poorly understood. In the current study, we investigated the protective role of SSRI, Citalopram, in serotonergic, medullary raphe neurons (RN46A-B14). RN46A-B14 cells were transfected with wild-type and mutant APP and Tau cDNAs for 24 h and then treated with 20 μM Cit for 24 h. We then assessed mRNA and protein levels of pTau, total Tau, serotonin related proteins such as TPH2, SERT, and 5HTR1a, synaptic proteins and the cytoskeletal structure. We also assessed cell survival, mitochondrial respiration and mitochondrial morphology. The mutant APP and Tau transfected cells showed increased levels of serotonin related proteins and mRNA, while the mRNA and protein levels of synaptic proteins were downregulated. Citalopram treatment significantly reduced pathologically pTau level along with the serotonin related protein levels. On the other hand, there was a significant increase in the mRNA and protein levels of synaptic genes and cytoskeletal structure in the treated groups. Further, Citalopram also improved cell survival, mitochondrial respiration and mitochondrial morphology in the treated cells that express mAPP and mTau. Taken together these findings suggest Citalopram could not only be a promising therapeutic drug for treating patients with depression, but also for AD patients.

Effects of aconitine on the respiratory activity of brainstem-spinal cord preparations isolated from newborn rats.

Aconitine is a sodium channel opener, but its effects on the respiratory center are not well understood. We investigated the dose-dependent effects of aconitine on central respiratory activity in brainstem-spinal cord preparations isolated from newborn rats. Bath application of 0.5-5μM aconitine caused an increase in respiratory rhythm and decrease in the inspiratory burst amplitude of the fourth cervical ventral root (C4). Separate application of aconitine revealed that medullary neurons were responsible for the respiratory rhythm increase, and neurons in both the medulla and spinal cord were involved in the decrease of C4 amplitude by aconitine. A local anesthetic, lidocaine (100μM), or a voltage-dependent sodium channel blocker, tetrodotoxin (0.1μM), partially antagonized the C4 amplitude decrease by aconitine. Tetrodotoxin treatment tentatively decreased the respiratory rhythm, but lidocaine tended to further increase the rhythm. Treatment with 100μM riluzole or 100μM flufenamic acid, which are known to inhibit respiratory pacemaker activity, did not reduce the respiratory rhythm enhanced by aconitine + lidocaine. The application of 1μM aconitine depolarized the preinspiratory, expiratory, and inspiratory motor neurons. The facilitated burst rhythm of inspiratory neurons after aconitine disappeared in a low Ca2+/high Mg2+ synaptic blockade solution. We showed the dose-dependent effects of aconitine on respiratory activity. The antagonists reversed the depressive effects of aconitine in different manners, possibly due to their actions on different sites of sodium channels. The burst-generating pacemaker properties of neurons may not be involved in the generation of the facilitated rhythm after aconitine treatment.

Sex-related correspondence between mechanical hypersensitivity and the discharge of medullary pain control neurons in neuropathic rats

Here we studied whether the sex-related difference in mechanical hypersensitivity induced by neuropathy is associated with the discharge rate of medullary pain control neurons. We performed experiments in male and female rats with spared nerve injury (SNI) model of peripheral neuropathy. Mechanical hypersensitivity was assessed behaviorally by monofilaments. Discharge rates of pain-control neurons were determined using in vivo single unit recordings under light anesthesia. Recording targets were two medullary nuclei involved in descending pain control: the rostral ventromedial medulla (RVM) and the medullary dorsal reticular nucleus (DRt). Based on the response to peripheral noxious stimulus, neurons were classified as pronociceptive RVM ON-like or DRt neurons, or antinociceptive RVM OFF-like neurons. Behavioral results indicated that the mechanical hypersensitivity induced by SNI was significantly stronger in females than males. The ongoing discharge rates of pronociceptive RVM ON-like neurons were higher and those of antinociceptive RVM OFF-like neurons lower in SNI females than SNI males. Ongoing discharge rates of pronociceptive DRt neurons were not significantly different between SNI females and males. The results suggest that a sex difference in the discharge rate of pain control neurons in the RVM but not DRt may contribute to the maintenance of stronger neuropathic hypersensitivity in females.

Opioid suppression of an excitatory pontomedullary respiratory circuit by convergent mechanisms.

Opioids depress breathing by inhibition of interconnected respiratory nuclei in the pons and medulla. Mu opioid receptor (MOR) agonists directly hyperpolarize a population of neurons in the dorsolateral pons, particularly the Kölliker-Fuse (KF) nucleus, that are key mediators of opioid-induced respiratory depression. However, the projection target and synaptic connections of MOR-expressing KF neurons are unknown. Here, we used retrograde labeling and brain slice electrophysiology to determine that MOR-expressing KF neurons project to respiratory nuclei in the ventrolateral medulla, including the preBötzinger complex (preBötC) and rostral ventral respiratory group (rVRG). These medullary-projecting, MOR-expressing dorsolateral pontine neurons express FoxP2 and are distinct from calcitonin gene-related peptide-expressing lateral parabrachial neurons. Furthermore, dorsolateral pontine neurons release glutamate onto excitatory preBötC and rVRG neurons via monosynaptic projections, which is inhibited by presynaptic opioid receptors. Surprisingly, the majority of excitatory preBötC and rVRG neurons receiving MOR-sensitive glutamatergic synaptic input from the dorsolateral pons are themselves hyperpolarized by opioids, suggesting a selective opioid-sensitive circuit from the KF to the ventrolateral medulla. Opioids inhibit this excitatory pontomedullary respiratory circuit by three distinct mechanisms-somatodendritic MORs on dorsolateral pontine and ventrolateral medullary neurons and presynaptic MORs on dorsolateral pontine neuron terminals in the ventrolateral medulla-all of which could contribute to opioid-induced respiratory depression.

Physical Training Chronically Stimulates the Motor Neuron Cell Nucleus in the Ts65Dn Mouse, a Model of Down Syndrome.

Down syndrome (DS) is a genetically-based disease based on the trisomy of chromosome 21 (Hsa21). DS is characterized by intellectual disability in association with several pathological traits among which early aging and altered motor coordination are prominent. Physical training or passive exercise were found to be useful in counteracting motor impairment in DS subjects. In this study we used the Ts65Dn mouse, a widely accepted animal model of DS, to investigate the ultrastructural architecture of the medullary motor neuron cell nucleus taken as marker of the cell functional state. Using transmission electron microscopy, ultrastructural morphometry, and immunocytochemistry we carried out a detailed investigation of possible trisomy-related alteration(s) of nuclear constituents, which are known to vary their amount and distribution as a function of nuclear activity, as well as the effect of adapted physical training upon them. Results demonstrated that trisomy per se affects nuclear constituents to a limited extent; however, adapted physical training is able to chronically stimulate pre-mRNA transcription and processing activity in motor neuron nuclei of trisomic mice, although to a lesser extent than in their euploid mates. These findings are a step towards understanding the mechanisms underlying the positive effect of physical activity in DS.

Scaling down synaptic strength results in changes to the pattern of fictive breathing in a model of the brainstem respiratory network: medullary and pontine sources

Opioids remain a primary vehicle for pain management prescribed by clinicians. However, opioid-induced respiratory depression (OIRD) can cause apneas and be fatal at elevated doses. Two sites of action for these opioids have been suggested in the brainstem, one in the pons and the other in the preBötzinger Complex of the medulla. Additionally, there is electrophysiological evidence suggesting that an inward-rectifying K + current is opened upon μ-opioid receptor (μOR) activation causing neuronal hyperpolarizations. Activation of μORs may cause depletion of glutamate release from excitatory neurons. Our previous simulation efforts have demonstrated the plausibility of presynaptic actions of opioids among inspiratory neurons of the medulla and pons in depressing inspiratory drive or eliciting apnea within specific and singular synaptic connections. Here, we tested the effects of diminishing the strength of excitatory synaptic currents among several different classes of respiratory neurons in a large-scale model of the brainstem respiratory network and whether it would replicate OIRD. We simulated the simultaneous actions of μOR on multiple presynaptic sites among the medullary inspiratory network. We began by scaling down the synaptic conductances of target populations of medullary inspiratory neurons. The core of the model is made up of neurons that produce the fundamental inspiratory rhythm (“I-Driver” neurons), as well as neurons that normally have an inspiratory decrementing (“I-Dec”) or inspiratory augmenting (“I-Aug”) firing pattern and various expiratory phase firing patterns (“E-Dec,” “E-Aug,” “E-Tonic”). I-Driver neurons are analogous to medullary preBötzinger Complex neurons that putatively express μORs. In our model, these neurons project back onto other I-Driver neurons to promote recurrent excitation but also project onto I-Aug and I-Dec populations. Decreasing the synaptic strength from I-Driver neurons to these populations collectively slows fictive breathing until extended apnea occurs. To further understand how this apnea develops we systematically scaled down the synaptic strengths of I-Driver to each of the other specific populations and found that the excitatory connection from I-Driver to I-Dec populations is most influential in producing apnea. However, we also found that excitatory drive from tonic, non-respiratory modulated (NRM) neurons in the pons to the medulla could influence this pattern by grading down synaptic strength. In addition to any direct hyperpolarizing effects of opioids on μOR-expressing neurons, these data support the plausibility that OIRD could arise from changes due to glutamate release. This work was supported by the National Institutes of Health grant 1R01HL155721-01, 1R01HL163008, and T32HL134621. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Parabrachial tachykinin1-expressing neurons involved in state-dependent breathing control

Breathing is regulated automatically by neural circuits in the medulla to maintain homeostasis, but breathing is also modified by behavior and emotion. Mice have rapid breathing patterns that are unique to the awake state and distinct from those driven by automatic reflexes. Activation of medullary neurons that control automatic breathing does not reproduce these rapid breathing patterns. By manipulating transcriptionally defined neurons in the parabrachial nucleus, we identify a subset of neurons that express the Tac1, but not Calca, gene that exerts potent and precise conditional control of breathing in the awake, but not anesthetized, state via projections to the ventral intermediate reticular zone of the medulla. Activating these neurons drives breathing to frequencies that match the physiological maximum through mechanisms that differ from those that underlie the automatic control of breathing. We postulate that this circuit is important for the integration of breathing with state-dependent behaviors and emotions.

Selective neurodegeneration generated by intravenous α-synuclein pre-formed fibril administration is not associated with endogenous α-synuclein levels in the rat brain.

Selective loss of discrete neuronal populations is a prominent feature of many neurodegenerative conditions, but the molecular basis of this is poorly understood. A central role of α-synuclein in the selective neurodegeneration of Parkinson's disease has been speculated, as its level of expression critically determines the propensity of this protein to misfold. To investigate whether the propensity of neuronal cell loss is associated with the level of endogenous α-synuclein expression, non-transgenic rats were given a single intravenous administration of α-synuclein pre-formed fibrils (PFFs) reversibly complexed with the rabies virus glycoprotein peptide (RVG9R). The number of surviving cells in different neuronal populations was systematically quantified using unbiased stereology. Our data demonstrated that a non-selective, transvascular delivery of α-synuclein PFFs led to a time-dependent loss of specific populations of midbrain (but not olfactory) dopaminergic neurons, medullary (but not pontine) cholinergic neurons, and brainstem serotonergic neurons. Contrary to the central role of endogenous α-synuclein expression in determining the seeding and aggregation propensity of pathological α-synuclein, we did not observe an association between the levels of α-synuclein expression in different regions of the rodent brain (although did not ascertain this at the individual cell level) and neurodegenerative propensity. The results from our study highlight the complexity of the neurodegenerative process generated by α-synuclein seeding. Further investigations are therefore required to elucidate the molecular basis of neurodegeneration driven by exogenous pathogenic α-synuclein spread.

Serotonin Transporter Imaging in Multiple System Atrophy and Parkinson's Disease.

Both Parkinson's disease (PD) and multiple system atrophy (MSA) exhibit degeneration of brainstem serotoninergic nuclei, affecting multiple subcortical and cortical serotoninergic projections. In MSA, medullary serotoninergic neuron pathology is well documented, but serotonin system changes throughout the rest of the brain are less well characterized. To use serotonin transporter [11 C]3-amino-4-(2-dimethylaminomethyl-phenylsulfaryl)-benzonitrile positron emission tomography (PET) to compare serotoninergic innervation in patients with MSA and PD. We performed serotonin transporter PET imaging in 18 patients with MSA, 23 patients with PD, and 16 healthy controls to explore differences in brainstem, subcortical, and cortical regions of interest. Patients with MSA showed lower serotonin transporter distribution volume ratios compared with patients with PD in the medulla, raphe pontis, ventral striatum, limbic cortex, and thalamic regions, but no differences in the dorsal striatal, ventral anterior cingulate, or total cortical regions. Controls showed greater cortical serotonin transporter binding compared with PD or MSA groups but lower serotonin transporter binding in the striatum and other relevant basal ganglia regions. There were no regional differences in binding between patients with MSA-parkinsonian subtype (n=8) and patients with MSA-cerebellar subtype (n=10). Serotonin transporter distribution volume ratios in multiple different regions of interest showed an inverse correlation with the severity of Movement Disorders Society Unified Parkinson's Disease Rating Scale motor score in patients with MSA but not patients with PD. Brainstem and some forebrain subcortical region serotoninergic deficits are more severe in MSA compared with PD and show an MSA-specific correlation with the severity of motor impairments. © 2022 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Activating SIRT-1 Signalling with the Mitochondrial-CoQ10 Activator Solanesol Improves Neurobehavioral and Neurochemical Defects in Ouabain-Induced Experimental Model of Bipolar Disorder.

Bipolar disorder (BD) is a chronic mental illness characterized by mood fluctuations that range from depressive lows to manic highs. Several studies have linked the downregulation of SIRT-1 (silent mating type information regulation-2 homologs) signaling to the onset of BD and other neurological dysfunctions. This research aimed to look into the neuroprotective potential of Solanesol (SNL) in rats given ICV-Ouabain injections, focusing on its effect on SIRT-1 signaling activation in the brain. Ouabain, found in hypothalamic and medullary neurons, is an endogenous inhibitor of brain Na+/K+ ATPase. The inhibition of brain Na+/K+ ATPase by Ouabain may also result in changes in neurotransmission within the central nervous system. SNL is a Solanaceae family active phytoconstituent produced from the plant Nicotiana tabacum. SNL is used as a precursor for the production of CoQ10 (Coenzyme Q10), a powerful antioxidant and neuroprotective compound. In the current study, lithium (Li), an important mood stabilizer drug, was used as a control. This study looked at the neuroprotective potential of SNL at dosages of 40 and 80 mg/kg in ICV-OUA injections that caused BD-like neurobehavioral and neurochemical defects in Wistar rats. Wistar rats were placed into eight groups (n = 6) and administered 1 mM/0.5 µL ICV-OUA injections for three days. Neurochemical assessments were done in rat brain homogenates, CSF, and blood plasma samples at the end of the experiment protocol schedule. Long-term SNL and lithium administration have been shown to decrease the number of rearing and crossings and reduce time spent in the center, locomotor activities, and immobility time. Solansesol treatment gradually raises the amount of Na+/K+ ATPase, limiting the severity of behavioural symptoms. These findings also revealed that SNL increases the levels of SIRT-1 in CSF, blood plasma, and brain homogenate samples. Moreover, in rat brain homogenates and blood plasma samples, SNL modulates apoptotic markers such as Caspase-3, Bax (pro-apoptotic), and Bcl-2 (anti-apoptotic). Mitochondrial-ETC complex enzymes, including complex-I, II, IV, V, and CoQ10, were also restored following long-term SNL treatment. Furthermore, SNL lowered inflammatory cytokines (TNF-α, IL-1β) levels while restoring neurotransmitter levels (serotonin, dopamine, glutamate, and acetylcholine) and decreasing oxidative stress markers. Histological examinations also validated Solanesol’s protective effect. As a result, our findings suggest that SNL, as a SIRT-1 signalling activator, may be a promising therapeutic approach for BD-like neurological dysfunctions.