Endocrine Abstracts (2003) 6 P50

The oxytocin response to increased plasma sodium following ovariectomy and oestrogen replacement

DE Hartley1, SL Dickson2 & ML Forsling1

1Neuroendocine Lab, King's College London, London, UK; 2Department of Physiology, Cambridge University, Cambridge, UK.

The effect of ovarian steroids on the vasopressin response to an osmotic stimulus in the rat has been extensively studied. Oxytocin also influences salt and water balance in the rat, but the effect of oestrogen on the response of the hormone to altered plasma sodium has not been established. Observations have been performed on rats ovariectomised under anaesthesia with and without oestrogen replacement to determine whether the response was affected by reproductive status. Fos-protein expression in the supraoptic nucleus was additionally used as an indicator of neurohypophysial function. Intact rats at oestrus were also studied. Two weeks after ovariectomy arterial and venous cannulae were implanted under anaesthesia and 48h later the animals were infused with 0.15MNaCl for 30min and 0.050ml/min followed by 1.5MNaCl for 90min. Blood samples were taken at 1, 45 and 90min after the start of the hypertonic infusion for the determination of packed cell volume, plasma oxytocin and osmolality. After a minimum of 48h recovery, animals were given a second hypertonic challenge (5.5ml/kg 1.5MNaCl at 0.225ml/min) and the anaesthetised and perfused with 4% paraformaldehyde fixative for brain collection. Sections were processed for the immunocytochemical detection of Fos-protein. The number of Fos positive cells in the intact and oestrogen-replaced group was not significantly different. While the Fos positive cells in the ovariectomised group was significantly lower (p<0.05, t-test) than the intact group, the circulating concentrations of oxytocin were similar in all groups after 90min hypertonic saline infusion, being 41.6±3.0pmol/l in the ovariectomised rats compared to 48.4±2.6 and 38.6±1.4pmol/l respectively in the intact and oestrogen replaced rats. Fos protein expression reflects the response of both oxytocinergic and vasopressinergic neurones. The failure to see an effect on the oxytocin response may reflect the fact that oxytocinergic cells do not express oestrogen receptor-alpha and relatively few express oestrogen receptor-beta.

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