OSMOTIC RESPONSIVENESS AND CROSS-TALK INVOLVING OXYTOCIN, BUT NOT VASOPRESSIN OR AMINO-ACIDS, BETWEEN THE SUPRAOPTIC NUCLEI IN VIRGIN AND LACTATING RATS

Lactation is associated with complex changes of the hypothalamo-neuroh ypophysial system, and oxytocin released within the hypothalamic supra optic (SON) and paraventricular nuclei may serve as a signal of commun ication between the magnocellular nuclei in lactating rats. In the fir st study, the intranuclear and peripheral release patterns of oxytocin and vasopressin in response to intraperitoneal hypertonic saline were studied in virgin and lactating rats to determine if the reduced osmo responsiveness of the oxytocinergic and vasopressinergic systems durin g lactation is reflected by reduced release not only into blood, but a lso within the SON. Simultaneous microdialysis was performed within th e SON and the jugular vein before and up to 6 hr after peripheral osmo tic stimulation (3.0 M NaCl, 0.6 ml/100 gm body weight, i.p.). There w as an immediate increase in secretion of both oxytocin and vasopressin into blood, whereas peptide release within the SON was delayed and pe aked after 4-5 hr. Peripheral release of both peptides was significant ly reduced in lactating animals, whereas within the, SON release of ox ytocin, but not vasopressin, was significantly reduced during lactatio n. In the second study, cross talk between the SONs-another phenomenon which seems to be characteristic for lactation-was studied. Microdial ysis of one SON with hypertonic perfusion medium (with 1 M NaCl) signi ficantly increased the release of oxytocin, vasopressin, and various a mino acids (aspartate, glutamate, serine, glutamine, gamma amino butyr ic acid, and arginine) within the ipsilateral SON. In contrast to virg in female and male animals, this unilateral stimulation of the SON res ulted in a transiently increased release of oxytocin in the contralate ral SON of lactating rats. The release of vasopressin and amino acids within the contralateral SON of lactating rats remained unchanged, ind icating specific activation of contralateral oxytocinergic neurons.