Date Published: May 1, 2018
Publisher: American Physiological Society
Author(s): Naoki Oshima, Hiroshi Onimaru, Akira Yamagata, Seigo Itoh, Hidehito Matsubara, Toshihiko Imakiire, Yasuhiro Nishida, Hiroo Kumagai.
Recent studies indicate that erythropoietin (EPO) is present in many areas of the brain and is active in the restoration of impaired neurons. In this study, we examined the presence of EPO and its role in bulbospinal neurons in the rostral ventrolateral medulla (RVLM). Hypoxia is often accompanied by a high blood pressure (BP). We hypothesized that EPO is produced in response to hypoxia in RVLM neurons and then activates them. To investigate whether RVLM neurons are sensitive to EPO, we examined the changes in the membrane potentials (MPs) of bulbospinal RVLM neurons using the whole cell patch-clamp technique during superfusion with EPO. A brainstem-spinal cord preparation was used for the experiments. EPO depolarized the RVLM neurons, and soluble erythropoietin receptor (SEPOR), an antagonist of EPO, hyperpolarized them. Furthermore, hypoxia-depolarized RVLM neurons were significantly hyperpolarized by SEPOR. In histological examinations, the EPO-depolarized RVLM neurons showed the presence of EPO receptor (EPOR). The RVLM neurons that possessed EPORs showed the presence of EPO and hypoxia-inducible factor (HIF)-2α. We also examined the levels of HIF-2α and EPO messenger RNA (mRNA) in the ventral sites of the medullas (containing RVLM areas) in response to hypoxia. The levels of HIF-2α and EPO mRNA in the hypoxia group were significantly greater than those in the control group. These results suggest that EPO is produced in response to hypoxia in RVLM neurons and causes a high BP via the stimulation of those neurons. EPO may be one of the neurotransmitters produced by RVLM neurons during hypoxia.
Erythropoietin (EPO) is a hematopoietic cytokine that is mainly produced in the kidneys and liver, particularly during hypoxia (11). Furthermore, EPO has been observed in other organs including the retina, muscle, gut, pancreas, gonads, uterus, and lung (5). Recently, the presence of EPO has also been shown in the brain (4). EPO in the brain is reportedly involved in the improvement of brain function after damage (1, 15, 22). EPO is regulated by hypoxia-inducible factor (HIF)-2 during hypoxia (6), and Chavez et al. (4) showed the expressions of HIF-2α and EPO in cultured cortical astrocytes. The presence of EPO in the brainstem has also been reported. Using brainstem-spinal cord preparations from mice, Soliz et al. (25) reported that EPO and EPOR are present in the mouse brainstem. They also showed that EPO controls ventilation and that an EPO antagonist suppressed the frequency of nerve activity at C4 (9).
In total 80 bulbospinal RVLM neurons were examined. MP and the frequency of action potential (FAP) of those neurons under aCSF were as follows: MP, −45.4 ± 0.5 mV and FAP, 0.2 ± 0.1 Hz.
In this study, we examined whether EPO affects the electrophysiological properties of bulbospinal RVLM neurons and showed that EPO depolarized these neurons (Fig. 1, A–C). Furthermore, since the RVLM neurons showed depolarization during superfusion with EPO dissolved in a TTX solution (Fig. 1, D and E), we thought that EPO activated the bulbospinal RVLM neurons themselves. SEPOR is a competitor of EPOR. Since EPO that has bound to SEPORs cannot bind to EPORs, SEPOR is thought to inhibit the effect of EPO (29). In this study, since SEPOR hyperpolarized the bulbospinal RVLM neurons (Fig. 2, A–C), EPO was thought to be present in the RVLM and to have stimulated the RVLM neurons.
No conflicts of interest, financial or otherwise, are declared by the authors.
N.O., H.O., Y.N., and H.K. conceived and designed research; N.O. performed experiments; N.O., A.Y., S.I., H.M., T.I., and H.K. analyzed data; N.O. and H.K. interpreted results of experiments; N.O. prepared figures; N.O. drafted manuscript; N.O. and H.K. edited and revised manuscript; N.O. and H.K. approved final version of manuscript.