1. Biot’s respiration named after Camille Biot is an abnormal ataxic breathing pattern, where rapid series of shallow respiration are followed by a period of apnea. In order to known how AMP kinase, knock down leads to Biot respiration in mice; it is necessary to known how AMP kinase affects respiratory rate. AMP kinase is a metabolic sensor and is activated in response to stretching of the lung alveolar cells and other stressors. During respiration, the lung goes through cycles of expansion and contraction (Budinger et al., 2008). When the alveolar cells stretch during expansion, the 5′ AMP-activated protein kinase (AMPK) in the cells are activated. Dystroglygan a receptor in the basal membrane of lung epithelium is important in the activation of AMP kinase (Budinger et al., 2008).
Respiratory acidosis can also induce the expression of cyclic AMP. Rapid shallow breathing, increases the blood CO2 and decreases blood O2 and PH. Hypoxia activates AMP protein kinases in the pulmonary artery and carotid body (Hardie & Ashford, 2014). AMP kinase activation is key to regulating ion channels in these two sites. Activation of respiratory receptors in the carotid bodies can be blocked by using AMP protein kinase antagonist. Carotid body is a small organ in the carotid artery. AMP kinase causes depolarization of Type 1 cells in the carotid artery by triggering Ca2+ influx and also causes the secretion of neurotransmitter that excite the sensory neurons (Gozal, 2016). AMP kinase is a key mediator of hypoxia response coupling in lungs. The enzyme is critical to conveying hypoxia message to carotid artery (Gozal, 2016). The carotid artery secretes neurotransmitters that act on the afferent sensory neurons and increases the depth of respiratory rate (Gozal, 2016). AMP kinase is an acute sensor of oxygen in the body. Mice that is deficient in this enzyme has alerted breathing pattern (Mahmoud et al., 2013).
Like other cells in the body, cells of the ventilatory system also respond to metabolic stress. The carotid body and pulmonary arteries have oxygen sensing cells that are vital in maintaining homeostatic mechanism in the body. They alter respiratory rate, based on the cellular and blood PO2 levels. AMP kinase in these cells links hypoxia to Ca2+ signaling. While systemic arteries dilate in response to hypoxia; pulmonary arteries constrict in response to hypoxia. This response of pulmonary arteries is due to inhibition of K+ ion channels. (Prabhakar & Semenza, 2012)
Deficiency of AMPK-α1 and AMPK-α2 created by knockout of the gene in mice, abolish ventilator response. Lack of AMP kinase can cause ventilator depression and this can be lead to respiratory failure, when the animal is anaesthetized. Mice lacking AMP kinase show hypoventilation and apnea during hypoxia. AMPK-α1 is more important than AMPK-α2 in bringing about this effect. Though carotid bodies of these mice were sensitive to hypoxia, they failed to initiate the neural response cause by the activation of dorsal and ventral brain nuclei. AMP kinase is essential for coordination of hypoxia response with the brain stem in mice. (Mahmoud et al., 2016)
2. Breathing is a rhythmic motor behavior and is required to be alive. preBotzinger complex is a region in the medulla that is hypothesized to contain the signaling circuits that regulates breathing pattern (Kam, Worrell, Janczewski, Cui, & Feldman, 2013). This assumption was drawn from studies that were conducted in vitro and in vivo in a number of experimental animals. These studies were able to demonstrate that the rhythm of breathing arose from a complex that lied on the ventro- medial aspect of the medulla and was named preBotzinger complex (Kam, Worrell, Janczewski, Cui, & Feldman, 2013). Complete destruction of this complex, causes disturbances in breathing pattern and sometimes a complete arrest of respiration. The Pre-Botzinger complex is made up of a network of heterogeneous neurons that are interconnected through synaptic units. These synaptic connections are mediated by the glutamate neurotransmitter. Neurons that have rhythm making properties were identified in this complex. One set of neurons generate current by activating sodium channels and the other set generated current by activating Ca2+ channel. According to a group of researchers, these two pace making neurons are responsible for the two different rhythmic signals generated in this complex in different ages and environment. (Smith, Abdala, Borgmann, Rybak, & Paton, 2013)
Studies in juvenile and adult rodents suggest that neurons expressing neurokinin-1 receptor, μ-opioid receptor, tyrosine kinase B receptor, somatostatin receptor and Type 2 glutamate transporter in the Pre-Botzinger complex (Smith, Abdala, Borgmann, Rybak, & Paton, 2013). All these neurons modulate respiratory rhythm. Substances that influence these receptors can influence breathing pattern. For example, opioids, substance P, brain derived neurotropic factor and somatostatin, can act on these receptors and modulate breathing pattern (Thoby-Brisson & Greer, 2008). While opioids slow the breathing rhythm, substance P can markedly elevate breathing rhythm. Of the neuronal receptors discussed above, neurokinin-1 receptor is expected to have an important role in rhythm generation (Thoby-Brisson & Greer, 2008). Destruction of neurokinin -1 receptor neurons in this complex, resulted in ataxic breathing pattern. There are close to 600 neurokinin-1 neurons in the preBotzinger complex and their functioning is important in maintaining a normal breathing pattern. These neurons are pace maker neurons of the ventilation system. (Schwarzacher, Rub, & Deller, 2010)
The neurokinin neurons in the complex contain glutamate receptors that respond to glutamate mediated synaptic inputs. These inputs are important in maintaining the signaling circuit in the complex. The respiratory rhythm fails when glutamate receptor antagonist is used to block the glutamate receptor in the complex (Kam, Worrell, Janczewski, Cui, & Feldman, 2013). Thus, the complex relies on the glutamic synaptic drive for its function. While glutamate signaling is responsible for rhythm generation; modulation of rhythm is governed by a synaptic network regulated by chlorine mediated inhibitory inputs (Janczewski, Tashima, Hsu, Cui, & Feldman, 2013). Though inhibitor neurotransmitters like GABA and glycine are not required for rhythm generation, their action will help to lower the frequency of rhythm (Kam, Worrell, Janczewski, Cui, & Feldman, 2013). The pre Botzinger complex neurons are derived from Dbx1 expressing precursor. Destruction of Dbx1-expressing precursors in neonatal mice, resulted in impaired respiratory rhythm. Though pre Botzinger is not the sole complex involved in generation of rhythm, as of now, it is the major contributor to this role. (Wang et al., 2014)
As mentioned above, the complex receives neuronal inputs from excitatory and inhibitor neurotransmitter. The level of these neurotransmitter vary during sleep and wakefulness. As acetyl choline neurotransmitter is important in maintaining motor during sleep, Muere et al., studied the effect of Atropine on the respiratory rhythm. Atropine is an inhibitor of Acetyl choline. While atropine administered at the region of preBotzinger complex, did not have a significant effect on breathing pattern at wakefulness, it significantly decreased the frequency of respiratory rhythm during REM sleep. (Muere et al., 2013)
3. The fundamental control of breathing is in the motor neurons of the brain stems. Motor neurons are myelinated neurons. The elaborate cell membrane of Schwann cells wraps around the axons to form the myelin sheath. The myelinated axon conducts signal in a saltatory manner and conducts signal at an accelerated phase. The Pre Botzinger neuronal signals are conveyed through the hypoglossal nerve. The signal generated in the brain motor neurons is conveyed to the muscles in the pharynx, larynx, diaphragm, intercostal and respiratory muscle.
Schwan cell myelinate peripheral nerves and help in the fast conduction of neuronal impulse and this helps to maintain the rhythm and frequency of respiration. The brain stem receives input signals from the afferent nerves arising from the chemoreceptors and mechanoreceptors in the lungs and carotid artery. The discharge of signal occurs at the fraction of a second. Such fast conduction is enabled through myelinated nerve fiber. Demyelinating diseases that affect the central and peripheral nerves can affect breathing. (Feldman, Del Negro, & Gray, 2013)
Gullian Barr Syndrome characterized by immune mediated demyelination of peripheral nerves can cause respiratory motor failure and asphyxia (Feldman, Del Negro, & Gray, 2013). The demyelination and subsequent neuronal loss, eventually results in paralysis of muscles that aid in respiration. Though the specific effect of demyelination of Pre Botzinger neurons is unknown, demyelination of neurons in the brain stem can disrupt breathing pattern and frequency. Genetic disorder that disrupt myelin synthesis by Schwann cells and Glial cells can disrupt synaptic transmission. The effect of myelination on neuron on respiration is not yet investigated and thus the effect of myelination on rhythm is not known. Individuals who incur damage to the upper spinal cord can survive only with assisted ventilation and there is currently no mechanism that can restore the ability to breath independently in such person. Mutant mice that is defective in Tst-l/Oct-6/SCIP transcription factor has defective myelination (Jacob, Lebrun-Julien, & Suter, 2011). Mice that were homozygous for Tst-l/Oct-6/SCIP defective mutation, have fatal breathing difficulty and thus could be an ideal animal model to study the effect of myelination on breathing (Jacob, Lebrun-Julien, & Suter, 2011).
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