Secretion of Vasopressin, Oxytocin, and Two Neurophysins from Rat Hypothalamo-Neurohypophyseal Explants in Organ Culture

Abstract

Secretion of the neurohypophyseal principles arginine vasopressin, oxytocin, vasopressin-neurophysin, and oxytocin-neurophysin by hypothalamic-neurohypophyseal explants in organ culture was studied. The viability of explant cultures was tested on day 6 by their ability to incorporate ³H-amino acid into acid-precipitable protein, specifically vasopressin-neurophysin. The explants released all four neurosecretory products in a controlled manner during 8 successive days of culture. In addition, these substances were all present in neural lobes and hypothalami at the end of the culture period. Although the tissue retained a 1:1 molar ratio between the hormone and corresponding neurophysin, the levels of neurophysins in culture media were much higher than the levels of hormones. Through immunoprecipitation techniques we determined that both arginine vasopressin and oxytocin were unstable in culture media containing explants or in medium containing 10% heat-inactivated fetal calf serum. This suggests that neurophysins are more stable indicators of secretion by explants than are the corresponding hormones. An increased release of arginine vasopressin, oxytocin, vasopressin-neurophysin, and oxytocin-neurophysin occurred in response to elevated K⁺ and to increased sodium ions in the medium, and these four substances were released in different molecular proportions. The results indicate that explant cultures may be useful for comparisons of secretion of the two neurophysins and, to a more limited extent, of the two hormones in response to such stimuli.

Responsiveness of Oxytocin-Producing Neurons to Acute Salt Loading in Rats: Comparisons with Vasopressin-Producing Neurons

“…Our observation that acute increases of P""m are associat ed with increases in [OT-RNP] is consistent with previous findings from chronic and in vitro studies [9,13,18,21]. Furthermore, it seems clear that oxytocin-producing neu rons of rats are even more responsive with respect to the amounts of neurophysin released to small and moderate increses in Posm than vasopressin-producing neurons.…”

“…For os molality increases of 9-28 mosm/kg H :0 induced in LE rats over 40 min, the rate of increase of [OT-RNP] (21.6 fmol ml"' mosm ' kg ') was almost 3 times the rate of in crease of [VP-RNP] (8.3 fmol ml 1 mosm 1 kg '). These in vivo observations are also in accordance with our in vitro findings [21]. Since OT-RNP and VP-RNP appear to have similar half-lives and volumes of distribution [18], it is un likely that these factors account for the differences seen.…”

“…This responsiveness could be solely a consequence of the common evolutionary origin of oxyto cinergic and vasopressinergic neurons from vasotocinergic neurons, or suggest that oxytocin, like vasopressin, might play a role in osmoregulation. Recently, we have found that rat hypothalamo-neurohypophysial explants in organ cul ture display a greater tendency to release oxytocin and oxy tocin-associated neurophysin (OT-RNP) than vasopressin and vasopressin-associated neurophysin (VP-RNP) when osmolality of the incubation medium is raised [21]. Further more, recent observations by Edwards and La Rochelle [10] suggested that oxytocin is responsible for several renal ef fects during the initial stages of dehydration (1-3 h) in ho mozygous Brattleboro (DI) rats.…”

“…Neurophysin re lease has been found to provide an excellent index of activ ity for vasopressinergic and oxytocinergic neurons [5,6,17,21].…”

Expression and plasticity of glutamate receptors in the supraoptic nucleus of the hypothalamus

“…Magnocellular neuroendocrine cells (MNCs) of the supraoptic nucleus (SON) of the hypothalamus produce the peptide hormones vasopressin (VP) and oxytocin (OT) and their specific carrier proteins, the neurophysins (NP; Stern et al, 1986). Both hormones, which are secreted into the peripheral circulation, act at distant organs to regulate body water and salt, cardiovascular function, as well as parturition and lactation (Brimble et al, 1988;Hatton, 1990;Huang and Dellmann, 1996;McDonald et al, 1976;Sjöquist et al, 1999;Soares et al, 1999;Swanson and Mogenson, 1981;Wakerley et al, 1978).…”