Comparison of volume expansion-induced fos expression among proestrus, diestrus, and ovariectomized female rats

During pregnancy there is a decrease in the osmotic threshold for vasopressin release and a 40% increase in plasma volume suggesting that reproductive hormones may alter neural regulation of volume. Previous studies from this laboratory show increased fos expression in oxytocinergic cells of the supraoptic nucleus (SON) of male rats following isotonic volume expansion. The present study examined fos expression following intravenous volume expansion of proestrus (n=4), diestrus (n=5), and ovariectomized (n=3) females. Rats were infused with isotonic saline equal to 10% of their body weight for 10 minutes followed by a maintenance infusion of 0.5 ml/min. Control animals (proestrus, n=3; diestrus, n=4) received 2 hour infusions of isotonic saline at 0.01 ml/min. The different experimental groups exhibited similar increases in central venous pressure in response to volume expansion. Fos expression (Oncogene, AB-5) was quantified. Volume expansion increased fos in the SON of both proestrus (2.8 ± 0.8 to 16.3 ± 7.6; p<0.05) and diestrus (5.1 ± 3.1 to 12.0 to 1.5). Fos expression in the SON of ovariectomized females (38.5 ± 9.5) was significantly greater than either proestrus or diestrus females while qualitatively similar to numbers of male rats. The results suggest that ovarian hormones can attenuate the activation of SON neurons in response to volume expansion.