A Novel Oxytocin Expressing Microglia Population in the Brain Subfornical Organ

The neurotransmitter oxytocin has been implicated in a wide range of physiological actions such as lactation, feeding behavior, metabolism, and cardiovascular regulation. The current dogma is that brain oxytocin synthesis and release is primarily restricted to the paraventricular nucleus of the hypothalamus (PVN) and supraoptic nucleus (SON). However, the subfornical organ (SFO), a sensory circumventricular organ, is also involved in the aforementioned physiological processes. Thus, we hypothesized that oxytocin positive cells are also present in the SFO. A combination of mRNA measurements and immunohistochemical approaches in mice were used to identify and characterize SFO oxytocin expression. Immunohistochemistry analysis of the SFO in adult C57Bl/6 mice revealed clear rostral to caudal oxytocin expression, albeit at lower levels than that seen in the PVN and SON. Whereas rostral SFO oxytocin expression appeared primarily on fibers, cell body localization of oxytocin was found in the medial and caudal SFO (medial: 4±1; caudal: 5±1 cells/section, n=8). Taking into consideration the role of oxytocin in pregnancy, lactation, and maternal bonding, we also investigated sex differences in SFO oxytocin expression and found no significant differences between male and female (diestrous) mice along the rostral to caudal axis of the SFO (e.g. medial: 3.0±1 vs. 4±1 cells/section, male vs. female, n=3–4, p>0.05). Real‐time PCR analysis of oxytocin in SFO micropunches confirmed transcript expression, suggestive of oxytocin production in this brain nucleus. Building upon this, we further characterized the cell identity of SFO oxytocin‐expressing cells. While oxytocin in the PVN and SON is localized to neurons, immunohistochemical analysis in the SFO demonstrated no co‐localization of oxytocin and the neuronal marker NeuN. This was corroborated using an antibody for the excitatory neuronal marker Ca2+/calmodulin‐dependent protein kinase II (CaMKII), as well as a GABAergic reporter mouse line (GAD65‐mCherry); SFO oxytocin did not co‐localize with CAMKII or GABA neurons. Similarly, we did not observe any co‐localization between oxytocin and glial fibrillary acidic protein (astrocytes) or nestin (neuronal precursors) in the SFO. In contrast, the vast majority of oxytocin positive cells in the SFO co‐localized with the microglial marker ionized calcium binding adaptor molecule 1 (Iba1; 86±15.2%, oxytocin/Iba1 co‐localized cells, n=8/2‐3 sections per animal; see Figure). Moreover, the majority of microglia in the SFO demonstrated oxytocin expression (88.2±19.8%, Iba1/oxytocin co‐localized cells, n=8/2‐3 sections per animal). Collectively, these findings indicate that: 1) In addition to the PVN and SON, oxytocin positive cells are also present in the SFO; and 2) SFO oxytocin expressing cells are microglia. Furthermore, these findings raise the possibility that this novel oxytocin microglial subtype in the SFO may contribute to a wide range of physiological processes ‐ from reproduction to fluid balance to cardiovascular regulation. Support or Funding Information HL116776 This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .