Erythropoietin, a putative neurotransmitter during hypoxia, is produced in RVLM neurons and activates them in neonatal Wistar rats

Naoki Oshima,  Hiroshi Onimaru,  Akira Yamagata,  Seigo Itoh,  Hidehito Matsubara,  Toshihiko Imakiire,  Yasuhiro Nishida,  and Hiroo Kumagai.
Am J Physiol Regul Integr Comp Physiol. 2018 May 1

Hypoxia is a lack of oxygen reaching certain areas of the body. Hypoxia causes increases in the sympathetic response. Erythropoietin (EPO) is a glycoprotein cytokine that has been observed in many areas of the body, especially during hypoxia. EPO has also been shown to be present in the brain. This study looks at the role of EPO in the RVLM during hypoxia and the responses it causes.

To test the effects of EPO on RVLM neurons, medulla preparations were given a solution of EPO. EPO was shown to depolarize the RVLM neurons and increase the action potential frequency. The RVLM neurons were then given a solution of EPO and tetrodotoxin (TTX), which is a neurotoxin. The neurons still showed depolarization which suggests that EPO depolarizes the RVLM neurons.

RVLM neurons project to the mediolateral cell column (IML), where they connect with the sympathetic preganglionic neurons. To test if the activity in the RVLM, which was caused by EPO, was being transmitted to the IML neurons for sympathetic response, the RVLM area were given an EPO solution. Changes in the membrane potential (MP) and IML were then observed. Using a tungsten electrode at the TH-2 level of the spine, the results showed that the EPO solution caused a depolarization of the IML neurons. Latency of the neural signal was then studied by looking at the time from when the EPO solution was given, to the time changes occurred in the membrane potentials of the IML neurons. This time was show to be about 26.8 seconds. When the RVLM was given a solution of soluble erythropoietin receptors (SEPOR), which is an EPO antagonist, the latency was about 25 seconds. This shows that effects on the RVLM neurons are transmitted to IML neurons. 

Lucifer yellow, which is a lithium salt florescent dye, was used to test the depolarization of neurons after given a solution of EPO.  After the medulla preparation was given a solution of EPO, six neurons in the RVLM that showed depolarization were examined for erythropoietin receptor florescence. All six of the neurons were shown to exhibit EPOR activity.

To test if EPO is produced in the RVLM during hypoxia, two groups were created. Two sections of the medulla containing the RVLM were either given a gas high in nitrogen or a gas high in oxygen for 30 minutes. Afterwards they measured the levels of EPO mRNAs in the RVLMs using polymerase chain reaction (PCR). The levels of EPO mRNA was shown to increase in the RVLM areas of the hypoxic group when compared to the group given oxygen, which suggests that EPO production is increased during hypoxic conditions.

In conclusion, this study showed the presence and role of EPO within the RVLM. Hypoxia was shown to increase EPO production, which lead to EPO stimulating the RVLM neurons. This study provides some ideas of the mechanisms behind hypertension induced by hypoxia.

Paul M