Circulating gonadal hormones modulate serotonergic innervation of the phrenic motor nucleus

Acute Intermittent Hypoxia (AIH) is a respiratory stimulus which triggers a form of respiratory neuroplasticity known as phrenic long-term facilitation (pLTF). The cellular mechanisms to pLTF depend, in part, on the severity of the hypoxia during each of the AIH exposures. Moderate AIH (mAIH, 35-55mmHG PaO2) and severe AIH (sAIH, 25-30mmHG PaO2), work through unique cell signaling pathways to achieve similar forms of pLTF. When administered in a 3 by 5 minute protocol, mAIH produces pLTF through a serotonin (5HT) dependent pathway referred to as the “Q-pathway”. Episodic release of 5HT during AIH and the subsequent activation of 5HT receptors (5HT2A and 5HT2B) on phrenic motor neurons elicits downstream cellular signaling that ultimately facilitates pLTF. Thus, differences in serotonergic signalling may greatly impact the capacity for mAIH to induce respiratory neuroplasticity. Prior studies have shown that changes in circulating gonadal hormones may influence 5HT fiber density in other CNS centers. In the hypothalamus of female rats, levels of 5HT rise and fall alongside the principal gonadal hormone (estrogen) during the normal estrus cycle. Based on our own preliminary findings, we hypothesized that the presence of serotonergic boutons in the ventral cervical spinal cord would be inversely related to levels of circulating gonadal hormone levels in both males and females. We used 3D immunohistochemistry (IHC) to quantify 5HT-containing boutons from ventral raphe neurons in the ventral cervical spinal cord of young male and female Sprague-Dawley rats with different physiological levels of circulating gonadal hormones. A clear and significant difference in 5HT bouton numbers was revealed in female rats; high circulating estrogen was associated with low 5HT bouton numbers when compared with female rats with low circulating estrogen following removal of the ovaries (p< 0.0071). However, consistent with prior reports, no statistical differences in 5HT bouton numbers were seen in female rats across stages of the estrous cycle. A similar relationship between 5HT boutons and circulating hormones was seen in males where high circulating testosterone was associated with low 5HT bouton numbers compared with male rats with low circulating testosterone following castration (p< 0.0001). Ongoing studies are underway to: (1) identify serotonin boutons near retrogradely labeled phrenic motor neurons specifically, and (2) determine whether expression of 5HT receptors is influenced by changes in circulating gonadal hormones. This research was supported by NIH NHLBI R01HL146477 and the University of Minnesota Division of Physical Therapy and Rehabilitation Science. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.