Effects of Hyperosmolality on Hypothalamic Astrocytic Area, mRNA Expression and Glutamate Balance In Vitro

During prolonged dehydration, body fluid homeostasis is challenged by extracellular fluid (ECF) hyperosmolality, which induce important functional changes in the hypothalamus, in parallel with other effector responses, such as the activation of the local renin-angiotensin system (RAS). Therefore, in the present study we investigated the role of sodium-driven ECF hyperosmolality on glial fibrillary acid protein (GFAP) immunoreactivity and protein expression, membrane capacitance, mRNA expression of RAS components and glutamate balance in cultured hypothalamic astrocytes. Our data show that hypothalamic astrocytes respond to increased hyperosmolality with a similar decrease in GFAP expression and membrane capacitance, indicative of reduced cellular area. Hyperosmolality also downregulates the transcript levels of angiotensinogen and both angiotensin-converting enzymes, whereas upregulates type 1a angiotensin II receptor mRNA. Incubation with hypertonic solution also decreases the immunoreactivity to the membrane glutamate/aspartate transporter (GLAST) as well as tritiated-aspartate uptake by astrocytes. This latter effect is completely restored to basal levels when astrocytes previously exposed to hypertonicity are incubated under isotonic conditions. Together with a direct effect on two important local signaling systems (glutamate and RAS), these synaptic rearrangements driven by astrocytes may accomplish for a coordinated increase in the excitatory drive onto the hypothalamic neurosecretory system, ultimately culminating with increased AVP release in response to hyperosmolality.