Influence of hypergravity on hypothalamic vasopressin and oxytocin neurons in rats

This study was aimed to evaluate the reaction of the vasopressin (VP) and oxytocin (OT) neurons of the supraoptic nucleus (SON) in rats to single or repeated hypergravity (HG). Special attention was paid to the tyrosine hydroxylase (TH) expression in VP neurons as a marker of the neuron activation. Rats were revolved in a centrifuge with overloading 2G for 5 days or 34 days as well as for 34 days plus 5 days with an interval of 39 days between two rotations. Control rats were kept in a centrifuge room. Radioimmununoassay, quantitative and semi-quantitative immunocytochemistry and in situ hybridization were used to evaluate: a) VP concentration in the pituitary posterior lobe (PL) and in plasma; b) the number of VP-, OT- and TH-immunoreactive neurons in the SON; c) the optic density of VP-, OT- and TH-immunoreactive materials in cell bodies (SON) and distal axons (PL), d) the optic density of VP and OT mRNAs signals (S35) in the whole SON on microfilms. According to our data, VP neurons were strongly activated during HG (5 days or 34 days) that was manifested in the functional hypertrophy of the neurons, greatly increased concentrations of VP mRNA in the SON and VP in plasma, the onset of the TH expression. The neurons showed initially (5 days) the functional insufficiency (VP release > VP synthesis) followed by their adaptation (subsequent 29 days) to the increased need in VP (VP release < VP synthesis). No reaction of VP neurons was observed to repeated HG. In contrast to VP neurons, OT neurons did not react to short-term HG or showed functional depression after the long-term treatment.