Forced swimming stimulates the expression of vasopressin and oxytocin in magnocellular neurons of the rat hypothalamic paraventricular nucleus

Previous studies have shown that a 10-min forced swimming session triggers the release of both vasopressin and oxytocin into the extracellular fluid o f the hypothalamic paraventricular (PVN) and supraoptic nuclei (SON) in rat s. At the same time oxytocin, but not vasopressin, was released from the ax on terminals into the blood. Here we combined forced swimming with in situ hybridization to investigate whether (i) the stressor-induced release of va sopressin and oxytocin within the PVN originates from parvo- or magnocellul ar neurons of the nucleus, and (ii) central release with or without concomi tant peripheral secretion is followed by changes in the synthesis of vasopr essin and/or oxytocin. Adult male Wistar rats were killed 2, 4 or 8 h after a 10-min forced swimming session and their brains processed for in situ hy bridization using S-35-labelled oligonucleotide probes. As measured on phot o-emulsion-coated slides, cellular vasopressin mRNA concentration increased in magnocellular PVN neurons 2 and 4 h after swimming (P < 0.05). Similarl y, oxytocin mRNA concentration was significantly increased in magnocellular neurons of the PVN at 2 and 8 h (P < 0.05). We failed to observe significa nt effects on vasopressin and oxytocin mRNA levels in the parvocellular PVN and in the SON. Taken together with results from previous studies, our dat a suggest that magnocellular neurons are the predominant source of vasopres sin and oxytocin released within PVN in response to forced swimming. Furthe rmore, in the case of vasopressin, central release in the absence of periph eral secretion is followed by increased mRNA levels, implying a refill of d epleted somato-dendritic vasopressin stores. Within the SON, however, mRNA levels are poor indicators of the secretory activity of magnocellular neuro ns during stress.