Estrogen Effects on Neural Activation after Short‐term Vs. Long‐term Furosemide Treatment in Rats

Estrogen influences body Na + balance, but the central pathways involved in these effects are currently unknown. Studies using immunohistochemical labeling of the fos protein to study central areas involved in body fluid balance report no effect of Na + loss in male rats. However, standard methods of producing Na + loss are of comparatively long duration, and it is possible central activation is not sustained. Accordingly, our objective was to assess the effect of estrogen on activation in response to short‐term and long‐term Na + depletion using furosemide, a diuretic‐natriuretic. Adult female rats were ovariectomized, allowed to recover for 7 days, and then given estradiol benzoate (EB; 10 μg/0.1 ml oil, sc) or oil vehicle (OIL; 0.1 ml, sc). Rats were injected with furosemide (2 X 5 mg/kg, sc) by one of two protocols. For the short term protocol, rats were perfused with paraformaldehyde 2 hrs after furosemide; for the long‐term protocol, rats were perfused 18 hrs after furosemide. Brains were removed and cut into 40μ sections and then immunolabeled for fos (Santa Cruz; 1:30,000). Fos immunolabeling was apparent in the supraoptic nucleus of the hypothalamus after short‐term, but not long‐term, Na + depletion. Fos labeling was not present in the paraventricular hypothalamic nucleus with either protocol, but was present after both protocols in forebrain circumventricular organs (CVOs). Estrogen did not affect fos immunolabeling in either protocol. Thus, independent of estrogen, acute Na + loss produces transient neural activation in some central areas involved in body Na + regulation, and sustained activation in CVOs, which monitor circulating hormones and electrolytes. Supported by OCAST HR12‐196