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Effects of 5‐HT2 and A2A Antagonists on pERK and pAKT Levels in the NTS and Spinal Cord of Rats Exposed to Chronic Sustained Hypoxia

Different patterns of hypoxic exposure can result in long‐term changes of ventilation and the hypoxic ventilatory response. For example, acute intermittent hypoxia (AIH) causes long term facilitation (LTF) of ventilation and phrenic nerve activity, with an increased acute HVR and persistent increase in ventilation in normoxia after AIH. Others have shown that LTF involves plasticity in serotonin 5HT2 neurotransmission (Gq pathway) and A2A adenosine neurotransmission (Gs pathway) in the spinal cord. Chronic sustained hypoxia (CSH) also increases the acute HVR and ventilation remains greater than control when normoxia is restored, which is called ventilatory acclimatization to hypoxia (VAH). We previously showed VAH involves plasticity in the nucleus tractus solitarius (NTS). Here we tested the hypothesis that Gq and Gs signaling play similar roles in LTF and VAH. Systemic inhibition of the Gq pathway (5‐HT2 antagonist Ketanserin, 1 mg/Kg per day) and the Gs pathway (A2A receptor antagonist MSX‐3, 0.5 mg/Kg per day) administered with subcutaneous osmotic pumps during 7 days of CSH (inspired PO2 = 70 mm Hg) decreased but did not eliminate VAH in male adult rats. This is different than previous reports from other laboratories showing LTF is abolished by blocking the Gq pathway with ketanserin but increased by blocking the Gs pathway with MSX‐3 administered before exposure to moderate AIH. To understand the site of action of Gq and Gs effects on VAH, we collected tissue from (1) the NTS, and (2) the spinal cord (between C3 and C5). We used Western blots of pERK and pAKT to measure Gq and Gs activation, respectively, in 4 groups of rats (n≥6 each) administered drugs or vehicle (DMSO) for 7 days as previously described above for ventilation studies: CON = DMSO in normoxia, CSH = DMSO in hypoxia, KET = ketanserin in CSH, MSX = MSX‐3 in CSH. CSH decreased pERK and pAKT in the spinal cord but had no effect on them in the NTS, indicating Gq and Gs pathways respond differently to CSH in the NTS versus spinal cord. In the NTS, pERK decreased with KET but not MSX during CSH. In the spinal cord, MSX during CSH restored pERK and pAKT to normoxic CON levels. Effects of KET on pERK and pAKT in the spinal cord during CSH are not available yet. The data indicate the Gq and Gs signaling with CSH is different than previously reported for AIH in the spinal cord. Also, Gq signaling in the NTS may contribute to VAH. Further study is necessary to determine how such differences in Gq and Gs signaling in the NTS and spinal cord depend on specific neurons in these locations.