Article #1

Marina, N., Abdala, A., Korsak, A., Simms, A., Allen, A., Paton, J., and Gourine, A., 2011. Control of sympathetic vasomotor tone by catecholaminergic C1 neurons of the rostral ventrolateral medulla oblongata. Cardiovascular Research, 91(4), pp.703-710.

Introduction

Scientists of Researchers of the European Society of Cardiology from University College London, University of Bristol, and the University of Melbourne conducted research to explain the sympathetic activation in patients with sleep apnoea by reevaluating the role of specific catecholaminergic C1 neurons in the rostral ventrolateral medulla (RVLM) on both resting sympathetic vasomotor tone and arterial blood pressure (ABP).

Methods

Sympathoexcitatory C1 neurons were silenced using a lentiviral vector—the application of an insect peptide allatostatin—to express the inhibitory Drosophila allatostatin receptors (AlstR). When allatostatin peptide and AlstR bind, it results in the reversible inhibition of C1 neurons. The in vivo model involved anesthetized artificially ventilated rats with denervated peripheral chemoreceptors and arterial baroreceptors, where femoral arteries were cannulated and ABP, PCO2, PO2, and pH were measured. Implanted bipolar silver electrodes were utilized to measure both tracheal pressure and blood pressure to monitor left renal nerve sympathetic nerve activity (SNA). The in situ model involved perfused working heart brainstem-spinal cord preparations placed in a recording chamber; a cannula was inserted in the descending aorta to measure thoracic sympathetic nerve (tSNA) and phrenic nerve activity. The scientists, furthermore, utilized EGFP fluorescence for confocal images to detect allatostatin receptor distribution.

Results

The in vivo inhibition of about half of the C1 neurons caused ~50% decrease in renal SNA and a significant reduction in ABP by ~25mmHg. This suggests that C1 neurons play a vital role in maintaining resting sympathetic vasomotor tone. Meanwhile, in situ, there was a significant fall in perfusion pressure by 6.3mmHg, reduction in Hering-Traube pressure waves amplitude, and a 29% reduction in respiratory-related tSNA bursts. Moreover, autoradiographs confirm successful targeting and secure allatostatin binding as C1 neurons transduce with AlstR-EGFP. C1 neurons, however, are not responsible for increased SNA when CO2 levels rise, and their inhibition did not affect hypercapnia.

Physiological Relevance

Patients with sleep apnoea—irregularity in breathing pattern during sleep—experience systemic hypoxia and hypercapnia episodes resulting in increased sympathetic tone associated with a higher risk of cardiovascular disease.

Fig. 1 Neural regulation of blood pressure

Constriction and dilation of blood vessels controlled by vascular smooth muscle cells aid in the control of vasomotor tone. C1 neurons control sympathetic activity linked to cardiovascular function by releasing catecholamine. Catecholamine signals the sympathetic preganglionic neurons in the spinal cord—which activates the sympathetic nervous system, resulting in increased respiratory rate, heart rate, blood pressure, and blood glucose levels. The binding of catecholamine to beta-1 adrenoreceptors activates stimulatory G-proteins and promotes ATP conversion into cAMP using adenylate cyclase. cAMP then activates protein kinases—activating the funny channel and L-type calcium channels. This results in an increase in If currents, an influx of Ca2+, and fewer opened K+ channels—eventually increasing the chance of depolarization and reaching an action potential. Pacemaker potentials occur more frequently, increasing preload and stroke volume. There is a positive chronotropic, inotropic, and dromotropic effect. This increased sympathetic activity causes vasoconstriction, increasing blood pressure, and potentially hypertension. The clinical concern stems from this hypertension which deteriorates the blood vessel walls over time.

However, during C1 neuron inhibition, the opening of G-protein-coupled inward-rectifying potassium channels led to hyperpolarization resulting in difficulty in firing action potentials, which reduced ABP. The paper concludes that medullary catecholaminergic C1 neurons are vital in regulating resting sympathetic vasomotor tone and ABP. Yet, C1 neurons do not seem to control sympathoexcitation invoked by central actions of CO2.

Reviewed by: A. Lee, N. Shanavaz

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Article #2

Marina, N., Abdala, A., Trapp, S., Li, A., Nattie, E., Hewinson, J., Smith, J., Paton, J., and Gourine, A., 2010. Essential Role of Phox2b-Expressing Ventrolateral Brainstem Neurons in the Chemosensory Control of Inspiration and Expiration. Journal of Neuroscience, 30(37), pp.12466-12473.

Introduction

The following paper conveys a behavioral and cognitive study conducted by scientists from University College London, Imperial College London, University of Bristol, Dartmouth Medical School, and National Institutes of Health Maryland. This research holds the purpose of unveiling the chemosensory role of Phox2b-expressing neurons of the retrotrapezoid nucleus (RTN) located in the ventrolateral brainstem in regulating respiratory activities.

Methods

In vivo and in situ models were used to evaluate the functional importance of this neuron group for both resting respiratory activity and the CO2-evoked respiratory responses by reversibly inhibiting these neurons with an insect peptide allatostatin—following transduction where a lentiviral construct is used to express the G-protein-coupled Drosophila allatostatin receptor.

The PRSx8-AlstR-EGFP-LV was first introduced to target the Phox2b-expressing neurons in the ventrolateral brainstem. Next comes the introduction of allatostatin to the process, which silences the AlstR-EGFP-transduced Phox2b-expressing ventrolateral brainstem neurons by an acute inhibition.

Results

After the inhibition of Phox2b-expressing neurons, the scientists observed that in anesthetized rats in normocapnic conditions (in vivo), there is no indication of expiratory abdominal activity; whereas, in situ preparations, there was a minimum amplitude. Phrenic nerve discharge decreases in amplitude in both preparations; post-inspiratory activity reduces in situ. Nonetheless, systemic hypercapnia causes a rise in active expiration. In the absence of the peripheral chemoreceptor input, the inhibition of Phox2b-expressing neurons during hypercapnia removed the CO2-evoked abdominal expiratory activity in anesthetized rats and in situ preparations. Inspiratory responses from increased levels of CO2 were reduced by 28% in anesthetized rats with denervated carotid bodies; 60% in conscious rats with peripheral chemoreceptors intact.

Physiological Relevance

Fig. 2 Inhibition of AlstR-expressing Phox2b neurons

Phrenic nerve discharge amplitude decreases as Phox2b-expressing neurons are inhibited by allatostatin. Neurons then undergo hyperpolarization, meaning allatostatin is a leading factor to firing action potentials that result in increased sympathetic activity. This demonstrates that these neurons apply an excitatory drive to the respiratory network at rest.

In conclusion, the collected data showcases a recognizable function of the Phox2b-expressing neurons of the ventrolateral brainstem, including pH-sensitive RTN neurons. These neurons regulate the use of abdominal expiratory muscles during periods of enforced breathing (episodes of hypercapnia) and promote increased inspiratory motor activity as a result of increased CO2. More data from the study supports a strong dependence of central expiratory drive on Phox2b-expressing neurons located in the ventrolateral brain stem. This strengthens the hypothesis that these neurons contribute significantly to CO2-evoked increases in inspiratory activity and makes evident that by inhibiting the Phox2b-expressing neurons, allatostatin develops central apnea in AlstR-EGFP-transduced, anaesthetised rats.

The scientists proposed that a distinct component with chemosensitive elements—located in other areas of the brainstem and/or RTN neurons—that do not express Phox2b is responsible for the CO2 drive for inspiration. Therefore, this depicts an essential consolidative function of the Phox2b-expressing neurons in the formation and control of expiratory and inspiratory activities associated with the chemosensory control of breathing.

Bibliography

Anon, 2016. Spinal Cord & Brainstem: Surface and Sectional Anatomy. Duke Neurosciences – Lab 2: Spinal Cord & Brainstem: Surface and sectional anatomy. Available at: https://brain.oit.duke.edu/lab03/lab03.html [Accessed December 6, 2021].

Anon, 2019. Sleep apnoea. NHS choices. Available at: https://www.nhs.uk/conditions/sleep-apnoea/#:~:text=Sleep%20apnoea%20is%20when%20your,obstructive%20sleep%20apnoea%20(OSA) [Accessed December 6, 2021].

Anon, 2020. How covid-19 affects your lungs. Edward-Elmhurst Health. Available at: https://www.eehealth.org/about-us/newsroom/news/2020/06/how-covid-19-affects-your-lungs/ [Accessed December 6, 2021].

Anon, Sleep apnea: Causes, symptoms, tests & treatments. Cleveland Clinic. Available at: https://my.clevelandclinic.org/health/diseases/8718-sleep-apnea [Accessed December 6, 2021].

Duffin, J., 2004. Functional Organization of Respiratory Neurones: A brief review of current questions and speculations. The Physiological Society. Available at: https://physoc.onlinelibrary.wiley.com/doi/full/10.1113/expphysiol.2004.028027 [Accessed December 6, 2021].

Guyenet, P.G. et al., 2013. C1 neurons: The body’s emts. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology, 305(3).

Investigation, T.J.of C., 2012. NPARM in phox2b: Why some things just should not be expanded. JCI. Available at: https://www.jci.org/articles/view/63884/figure/1 [Accessed December 6, 2021].

Laurent, G.J. & Shapiro, S.D., 2006. Encyclopedia of respiratory medicine, Amsterdam: Elsevier – Academic Press.

Reece, J.B. & Campbell, N.A., 2011. Campbell Biology, Boston, MA: Pearson Learning Solutions. Villines, Z., 2021. Types of sleep apnea: Obstructive, Central, and complex J. Hilbert, ed. Medical

News Today. Available at: https://www.medicalnewstoday.com/articles/types-of-sleep-apnea [Accessed December 6, 2021].