Conversion of Testosterone to Estradiol is Necessary for the Development of Long‐term Facilitation in Adult Male Rats

Abstract

Phrenic long-term facilitation (pLTF) and ventilatory long-term facilitation (vLTF) are well-defined models of respiratory neuroplasticity. In each model, acute intermittent hypoxia (AIH) stimulates persistent increases in respiratory neural drive or minute ventilation respectively. Recent findings from our laboratory indicate that sex hormones are necessary for the induction of respiratory neuroplasticity in both male and female rats. For example, AIH induced pLTF and vLTF only develop during the proestrus phase of the estrous cycle in female rats, notable for high levels of circulating 17β-estradiol, the most neuroactive form of estrogen. Removal of the ovaries (ovariectomy, OVX), the primary source of circulating sex hormones in females, also eliminates the development of pLTF and vLTF. Plasticity can be restored in OVX females with estradiol supplementation supporting a key role for estradiol signaling in development of respiratory plasticity. Testosterone, the principle sex hormone in males, is also important for development of respiratory plasticity. Male rats demonstrate reduced pLTF magnitude with aging, in correlation with reduced circulating testosterone levels, and removal of the gonads (gonadectomy, GDX) eliminates AIH-induced pLTF. Testosterone replacement is sufficient to restore pLTF in GDX male rats, but requires the activity of aromatase, the enzyme responsible for converting testosterone to estradiol, suggesting that estradiol may also play a key role in development of respiratory neuroplasticity in male rats. It is unknown whether conversion of testosterone to estradiol is necessary for development of AIH-induced plasticity in gonadally-intact male rats. To address this question, we used systemic injections of Letrozole, an aromatase inhibitor, as a first step towards determining the necessity of testosterone conversion to estradiol for development of AIH-induced respiratory neuroplasticity in gonadally-intact adult male rats. We hypothesized that aromatization of testosterone to estradiol would be necessary for induction of both AIH-induced vLTF and pLTF. Letrozole (1mg/kg), or a vehicle control, was administered subcutaneously over four days. On day three, vLTF was measured via whole-body plethysmography. On the fourth day, pLTF was quantified via phrenic nerve recordings. Researchers were blinded to treatment for all experiments and data analysis. Preliminary findings suggest that daily Letrozole injections impaired development of AIH-induced vLTF and pLTF, supporting our hypothesis. These data provide further evidence that estradiol plays a critical role in the development of respiratory neuroplasticity in both males and females.