The long‐term impact of ovariectomy on ventilation and expression of phrenic long‐term facilitation in female rats

NEW FINDINGS: What is the central question of this study? Would ovariectomy cause prolonged changes in ventilation and sustained loss of acute, intermittent hypoxia‐induced neuroplasticity or would these outcomes be restored with time? What is the main finding and its importance? Our main findings demonstrate that ovariectomy elicits minimal alteration in overall breathing function but impairs acute, intermittent hypoxia‐induced plasticity for ≤ 12 weeks. ABSTRACT: Sex hormones are necessary to enable respiratory neuroplasticity, including phrenic long‐term facilitation (pLTF), a form of respiratory motor plasticity elicited by acute, intermittent hypoxia (AIH). Female rats exhibit a progressive increase in phrenic nerve amplitude after AIH characteristic of pLTF only during pro‐oestrus, the stage of the oestrous cycle notable for elevated circulating oestradiol levels. Removal of the ovaries [ovariectomy (OVX)], the primary source of circulating oestradiol, also eliminates AIH‐induced pLTF after 1 week. Ovariectomy is used routinely as a model to examine the impact of sex hormones on CNS structure and function, but the long‐term impact of OVX is rarely examined. Extra‐ovarian sites of oestradiol synthesis, including multiple CNS sites, have been identified and might possess the capacity to restore oestradiol levels, in part, over time, impacting respiratory function and the expression of respiratory neuroplasticity. We examined both ventilation in awake, freely behaving female rats, using barometric plethysmography, and the expression of AIH‐induced pLTF in anaesthetized, ventilated female rats 2 and 12 weeks after OVX and compared them with age‐matched ovarian‐intact female rats. Our findings indicate that chronic OVX had little impact on baseline breathing or in the response to respiratory challenge (10% O(2), 5% CO(2), balance N(2)) during plethysmography. However, OVX rats at both 2 and 12 weeks demonstrated a persistent loss of AIH‐induced pLTF relative to control animals (P < 0.01), suggesting that other sources of oestradiol synthesis were insufficient to restore pLTF. These data are consistent with our previous work indicating that oestradiol plays a key role in expression of AIH‐induced respiratory neuroplasticity.