TNF-a Receptor 1 Activity Is Differentially Required for Phrenic Long-term Facilitation (pLTF) Over the Course of Motor Neuron Death in Adult Rats

Intrapleural injection of cholera toxin B fragment conjugated to saporin (CTB-SAP) selectively kills respiratory motor neurons, and mimics aspects of neuromuscular disorders and neurodegenerative diseases including phrenic motor neuron death, decreased phrenic motor output, and increased microglial cell number. Over the course of motor neuron loss, enhanced A2A receptor-dependent phrenic plasticity is observed early (7d), and then constrained 5-HT receptor-dependent phrenic plasticity is observed later (28d). In addition, COX-1/2-induced inflammation differentially impacts breathing and phrenic plasticity in CTB-SAP rats, but the underlying mechanism by which COX-1/2-induced inflammation impacts breathing and plasticity over the course of motor neuron death has not been studied. We observe that increased cervical neuroinflammatory gene expression ( e.g., tumor necrosis factor-a; TNF-a) is reduced back to control levels in the presence of ketoprofen in CTB-SAP rats, and it is known that TNF-a is involved in phrenic plasticity in naïve rats. Thus, we hypothesized that acute TNF-a receptor inhibition attenuates phrenic plasticity at 7d and enhances it at 28d TNF-a following CTB-SAP-induced neuropathology. Using anesthetized, paralyzed and ventilated CTB-SAP-treated adult Sprague Dawley rats, we studied acute intermittent hypoxia (AIH; 3, 5 min bouts of 10.5% O 2 ) induced pLTF following acute intrathecal C 4 delivery of a TNF-a receptor 1 inhibitor (soluble TNFR1; sTNFR1; 0.13 ug/ul) or vehicle (aCSF) to rats that received bilateral, intrapleural injections of: 1) CTB-SAP (25ug), or 2) un-conjugated CTB and SAP (control) in rats treated 7d or 28d prior. Surprisingly, preliminary results indicate that acute sTNFR1 delivery does not affect pLTF in 7d CTB-SAP-treated rats (n=5; i.e., pLTF still enhanced) but it enhanced pLTF in 28d CTB-SAP-treated rats (n=6; p<0.05) vs. controls (n=6 at 7d; n=7 at 28d) and vs. vehicle treated rats (controls n=6 for 7d and n=9 for 28d; CTB-SAP, n=4 for 7d and 28d each). These preliminary observations suggest that TNF-a does not contribute to pLTF in 7d CTB-SAP-treated rats, whereas it may undermine pLTF in 28d CTB-SAP-treated rats. Studies are underway to assess the cellular source of TNF-a and the impact of chronic sTNFR1 delivery on pLTF, diaphragm activity, and breathing. Since most patients with neuromuscular disorders or neurodegenerative disease develop respiratory insuffciency and ultimately become ventilator dependent, our long-range goal is to develop new strategies to enhance the functional capacity of spared motor neurons and reduce ventilatory compromise in motor neuron loss pathology. These novel strategies, if successful, will identify new therapeutic targets for future, translational studies to enhance breathing in patients with neuromuscular disorders. Supported by a Spinal Cord Injury and Disease Research grant. 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.