5‐HT2B Receptor Expression in the Phrenic Motor Nucleus of Rats with Intrapleural CTB‐Saporin‐Induced Phrenic Motor Neuron Death

Abstract

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 and decreased phrenic motor output. In this model, respiratory plasticity (acute intermittent hypoxia induced phrenic long‐term facilitation; pLTF) is enhanced following 7 days (d) of CTB‐SAP compared to control treatment. In contrast, pLTF in 28d CTB‐SAP treated rats is comparable to pLTF in controls, suggesting that the underlying mechanisms of plasticity at both time points may differ. pLTF in naïve rats predominately requires 5‐HT2 receptors; however, there is an alternative mechanism for pLTF that requires A2A receptors. Our preliminary data suggest that A2A receptors are increased in the phrenic motor nucleus of 7d, not 28d, CTB‐SAP treated rats (Craig et al., 2017), suggesting that 7d CTB‐SAP rats utilize the alternative mechanism for enhanced pLTF. However, it remains unknown how CTB‐SAP induced phrenic motor neuron death affects 5‐HT2A/B receptor expression, and if 5‐HT2A/B receptors are utilized for pLTF in CTB‐SAP treated rats. Here, we tested the hypothesis that 5‐HT2B receptor expression in the phrenic motor nucleus is decreased in 7d CTB‐SAP treated rats, but is not different in 28d CTB‐SAP treated rats when compared to controls using immunohistochemistry. 5‐HT2B receptor expression was studied in Sprague Dawley rats 7 and 28 days following bilateral intrapleural injections of: 1) CTB‐SAP (25μg), or 2) un‐conjugated CTB and SAP (control). Our preliminary data indicate that following CTB‐SAP injection, total 5‐HT2B receptor positive pixels and C4 fractional area are decreased in the phrenic motor nucleus and non‐phrenic ventral horn at 7d, not 28d, vs. controls (n=3 for each group; p<0.05). This suggests 5‐HT2B receptors recover to control levels and are potentially utilized for pLTF in 28d CTB‐SAP treated rats. Future studies will be focused on whether 5‐HT2A receptor expression is similarly affected, whether 5‐HT2A/B receptor expression is impacted on other cell types (i.e., glial cells), and which 5‐HT receptor isoform is required for pLTF following CTB‐SAP‐induced phrenic motor neuron death. This project increases our understanding of the expression of receptors associated with respiratory plasticity and its implications for breathing following motor neuron death.Support or Funding InformationSupported by NIH K99/R00 HL119606This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.