RELEASE OF VASOPRESSIN FROM SUPRAOPTIC NEURONS WITHIN THE MEDIAN-EMINENCE IN-VIVO - A COMBINED MICRODIALYSIS AND PUSH-PULL PERFUSION STUDY IN THE RAT

The present study was designed to investigate whether or not arginine vasopressin (AVP) is released from magnocellular neurons within the me dian eminence (ME) in vivo. Urethane-anesthetized adult male Wistar ra ts were equipped with a microdialysis probe aimed at the supraoptic (S ON) or paraventricular nucleus (PVN), a push-pull perfusion probe rest ing in the ME, and a blood microdialysis probe within the jugular vein . Dialysis of the SON (but not the PVN) with Ringer's solution contain ing 56 mmol l(-1) K+ resulted in an increase in AVP release within the ME (to 492 +/- 192% of release during basal conditions, P < 0.05) and into blood (to 138 +/- 9%, P < 0.01) whereby the release probably occ urred from axonal swellings and nerve terminals of supraoptic neurons which project through the internal zone of the ME to the posterior pit uitary. The calculated amount of AVP released into the extracellular f luid of the ME was high enough (approximately 1 pg/mu l) to hypothesiz e that the neuropeptide could enter the portal blood capillaries in ph ysiologically relevant concentrations. Taken together, the present stu dy indicates that activation of magnocellular neurons is accompanied b y release of AVP within the median eminence. We assume that AVP releas ed in this way might mediate a communication between the hypothalamic- neurohypophysial system and the hypothalamic-pituitary-adrenal axis in response to selected stressful stimuli.