Michael S. Hoffman

Spinal Adenosine A2a Receptor Activation Elicits Long-Lasting Phrenic Motor Facilitation

Acute intermittent hypoxia elicits a form of spinal, brain-derived neurotrophic factor (BDNF)-dependent respiratory plasticity known as phrenic long-term facilitation. Ligands that activate Gs-protein-coupled receptors, such as the adenosine 2a receptor, mimic the effects of neurotrophins in vitro by transactivating their high-affinity receptor tyrosine kinases, the Trk receptors. Thus, we hypothesized that A2a receptor agonists would elicit phrenic long-term facilitation by mimicking the effects of BDNF on TrkB receptors. Here we demonstrate that spinal A2a receptor agonists transactivate TrkB receptors in the rat cervical spinal cord near phrenic motoneurons, thus inducing long-lasting (hours) phrenic motor facilitation. A2a receptor activation increased phosphorylation and new synthesis of an immature TrkB protein, induced TrkB signaling through Akt, and strengthened synaptic pathways to phrenic motoneurons. RNA interference targeting TrkB mRNA demonstrated that new TrkB protein synthesis is necessary for A2a-induced phrenic motor facilitation. A2a receptor activation also increased breathing in unanesthetized rats, and improved breathing in rats with cervical spinal injuries. Thus, small, highly permeable drugs (such as adenosine receptor agonists) that transactivate TrkB receptors may provide an effective therapeutic strategy in the treatment of patients with ventilatory control disorders, such as obstructive sleep apnea, or respiratory insufficiency after spinal injury or during neurodegenerative diseases.

Repetitive Intermittent Hypoxia Induces Respiratory and Somatic Motor Recovery after Chronic Cervical Spinal Injury

Spinal injury disrupts connections between the brain and spinal cord, causing life-long paralysis. Most spinal injuries are incomplete, leaving spared neural pathways to motor neurons that initiate and coordinate movement. One therapeutic strategy to induce functional motor recovery is to harness plasticity in these spared neural pathways. Chronic intermittent hypoxia (CIH) (72 episodes per night, 7 nights) increases synaptic strength in crossed spinal synaptic pathways to phrenic motoneurons below a C2 spinal hemisection. However, CIH also causes morbidity (e.g., high blood pressure, hippocampal apoptosis), rendering it unsuitable as a therapeutic approach to chronic spinal injury. Less severe protocols of repetitive acute intermittent hypoxia may elicit plasticity without associated morbidity. Here we demonstrate that daily acute intermittent hypoxia (dAIH; 10 episodes per day, 7 d) induces motor plasticity in respiratory and nonrespiratory motor behaviors without evidence for associated morbidity. dAIH induces plasticity in spared, spinal pathways to respiratory and nonrespiratory motor neurons, improving respiratory and nonrespiratory (forelimb) motor function in rats with chronic cervical injuries. Functional improvements were persistent and were mirrored by neurochemical changes in proteins that contribute to respiratory motor plasticity after intermittent hypoxia (BDNF and TrkB) within both respiratory and nonrespiratory motor nuclei. Collectively, these studies demonstrate that repetitive acute intermittent hypoxia may be an effective and non-invasive means of improving function in multiple motor systems after chronic spinal injury.

Spinal 5-HT7 receptor activation induces long-lasting phrenic motor facilitation.

Non‐technical summary  Acute intermittent hypoxia elicits a serotonin‐dependent form of respiratory plasticity known as phrenic long‐term facilitation via synthesis of brain‐derived neurotrophic factor (BDNF) and activation of its receptor TrkB. Recently we demonstrated that spinal activation of Gs protein‐coupled adenosine A2A receptors ‘trans‐activates’ the BDNF receptor, TrkB, and induces phrenic motor facilitation (PMF). Therefore we hypothesized that perhaps other Gs protein‐coupled serotonin receptors elicit PMF, specifically serotonin receptor type‐7 (5‐HT7) which are expressed in phrenic motor neurons and underlie multiple forms of spinal respiratory plasticity. We demonstrate that spinal 5‐HT7 receptor activation in anaesthetized rats elicits long‐lasting PMF. 5‐HT7 agonist‐induced PMF appears to require TrkB receptor activity and/or new synthesis of TrkB protein, in addition to activity of the signalling molecule PI3K. A more complete understanding of signalling mechanisms involved in PMF may guide development of novel therapeutic strategies to treat ventilatory control disorders.

Spinal adenosine A2A receptor inhibition enhances phrenic long term facilitation following acute intermittent hypoxia

Phrenic long term facilitation (pLTF) is a form of respiratory plasticity induced by acute intermittent hypoxia. pLTF requires spinal serotonin receptor activation, new BDNF synthesis and TrkB receptor activation. Spinal adenosine 2A (A2A) receptor activation also elicits phrenic motor facilitation, but by a distinct mechanism involving new TrkB synthesis. Because extracellular adenosine increases during hypoxia, we hypothesized that A2A receptor activation contributes to acute intermittent hypoxia (AIH)‐induced pLTF. A selective A2A receptor antagonist (MSX‐3, 8 μg kg−1, 12 μl) was administered intrathecally (C4) to anaesthetized, vagotomized and ventilated male Sprague–Dawley rats before AIH (three 5 min episodes, 11% O2). Contrary to our hypothesis, pLTF was greater in MSX‐3 versus vehicle (aCSF) treated rats (97 ± 6%vs. 49 ± 4% at 60 min post‐AIH, respectively; P < 0.05). MSX‐3 and aCSF treated rats did not exhibit facilitation without AIH (time controls; 7 ± 5% and 9 ± 9%, respectively; P > 0.05). A second A2A receptor antagonist (ZM2412385, 7 μg kg−1, 7 μl) enhanced pLTF (85 ± 11%, P < 0.05), but an adenosine A1 receptor antagonist (DPCPX, 3 μg kg−1, 10 μl) had no effect (51%± 8%, P > 0.05), indicating specific A2A receptor effects. Intrathecal methysergide (306 μg kg−1, 15μl) blocked AIH‐induced pLTF in both MSX‐3 and aCSF treated rats, confirming that enhanced pLTF is serotonin dependent. Intravenous MSX‐3 (140 μg kg−1, 1 ml) enhanced both phrenic (104 ± 7%vs. 57 ± 5%, P < 0.05) and hypoglossal LTF (46 ± 13%vs. 28 ± 10%; P < 0.05). In conclusion, A2A receptors constrain the expression of serotonin‐dependent phrenic and hypoglossal LTF following AIH. A2A receptor antagonists (such as caffeine) may exert beneficial therapeutic effects by enhancing the capacity for AIH‐induced respiratory plasticity.

Spinal Plasticity following Intermittent Hypoxia: Implications for Spinal Injury

Plasticity is a fundamental property of the neural system controlling breathing. One frequently studied model of respiratory plasticity is long‐term facilitation of phrenic motor output (pLTF) following acute intermittent hypoxia (AIH). pLTF arises from spinal plasticity, increasing respiratory motor output through a mechanism that requires new synthesis of brain‐derived neurotrophic factor, activation of its high‐affinity receptor, tropomyosin‐related kinase B, and extracellular‐related kinase mitogen‐activated protein kinase signaling in or near phrenic motor neurons. Because intermittent hypoxia induces spinal plasticity, we are exploring the potential to harness repetitive AIH as a means of inducing functional recovery in conditions causing respiratory insufficiency, such as cervical spinal injury. Because repetitive AIH induces phenotypic plasticity in respiratory motor neurons, it may restore respiratory motor function in patients with incomplete spinal injury.

Respiratory plasticity following intermittent hypoxia: roles of protein phosphatases and reactive oxygen species

Plasticity is an important property of the respiratory control system. One of the best-studied models of respiratory plasticity is pLTF (phrenic long-term facilitation). pLTF is a progressive increase in phrenic motor output lasting several hours following acute exposure to intermittent hypoxia. Similar to many other forms of neuroplasticity, pLTF is pattern-sensitive; it is induced by intermittent, but not sustained hypoxia of similar cumulative duration. Our understanding of the cellular/synaptic mechanisms underlying pLTF has increased considerably in recent years. Here, we review accumulating evidence suggesting that the pattern-sensitivity of pLTF arises substantially from differential reactive oxygen species formation and subsequent protein phosphatase inhibition during intermittent compared with sustained hypoxia in or near phrenic motor neurons. A detailed understanding of the cellular/synaptic mechanisms of pLTF may provide the rationale for new pharmacological approaches in the treatment of severe ventilatory control disorders, such as obstructive sleep apnoea and respiratory insufficiency either following spinal cord injury or during neurodegenerative diseases such as amyotrophic lateral sclerosis.