Non-phasic discharge and osmoresponsiveness of cat magnocellular neuroendocrine cells

Cardiovascular changes induced by central hypertonic saline are accompanied by glutamate release in awake rats

To elucidate neurochemical mechanisms responsible for cardiovascular responses induced by central salt loading, we directly perfused the paraventricular nucleus (PVN) of the hypothalamus region with hypertonic saline (0.3 or 0.45 M) by using an in vivo brain microdialysis technique. We then measured the extracellular concentrations of glutamate in the PVN region in conscious rats along with the blood pressure and heart rate. Blood pressure, heart rate, and glutamate levels were increased by perfusion of 0.45 M saline; however, they did not change by perfusion of 0.3 M saline. Next, we examined the possible involvement of glutamate in the cardiovascular responses induced by hypertonic saline. Dizocilpine, a noncompetitive antagonist of the N-methyl-D-aspartate (NMDA) receptor, attenuated the increases of blood pressure and heart rate, although 6-cyano-7-nitroquinoxaline-2,3-dione, an antagonist of the non-NMDA receptor, did not affect the blood pressure and heart rate. Our results show that local perfusion of the hypothalamic PVN region with hypertonic saline elicits a local release of glutamate, which may act via NMDA-type glutamate receptors to produce cardiovascular responses.

Release of angiotensin in the paraventricular nucleus in response to hyperosmotic stimulation in conscious rats: a microdialysis study

Angiotensin peptides are thought to act as neurotransmitters or neuromodulators in central osmoregulation. We tested the hypothesis that angiotensin peptides are released in the paraventricular nucleus (PVN) of the hypothalamus upon local osmotic stimulation. Brain microdialysis and radioimmunoassay (RIA) techniques were used to measure the release of immunoreactive angiotensin II (irANG II) in the PVN following direct stimulation of this area with hyperosmotic solutions. In conscious rats, perfusion of the PVN with 0.3 M and 0.6 M NaCl in artificial cerebrospinal fluid (aCSF) elicited concentration-dependent increases in irANG II release to 5.52 +/- 0.53, (P < 0.01, n = 8) and 9.01 +/- 1.03 pg/100 microliters, (P < 0.001, n = 7), respectively, from basal values of 3.04 +/- 0.46 pg/100 microliters. Local perfusion of the PVN with 1.2 M glucose in aCSF also resulted in an increased release of irANG II from 3.07 +/- 0.87 to 6.24 +/- 0.45 pg/100 microliters (P < 0.05, n = 5). Fractionization of angiotensin peptides by HPLC followed by RIA revealed that ANG II (1-8) and ANG III (2-8) were released in similar amounts in the perfusate collected during 0.6 M NaCl stimulation (4.79 +/- 0.69 and 3.45 +/- 0.76 pg/100 microliters, respectively). Our results show that both, ANG II and ANG III are released in the PVN in response to local hyperosmotic stimulation. They support the concept that angiotensin peptides in the PVN are involved as neurotransmitters in central osmotic control.

Responses of neurons in the rat anterior hypothalamic periventricular zone to osmotic stimulation and angiotensin II in vitro

Extracellular recordings were made in vitro from neurons in slices of the rat periventricular zone surrounding the anterior portion of the third ventricle. Spontaneous unit activity of these neurons was decreased in 39.5% of the neurons tested, increased in 32.9%, and biphasically changed (i.e., excitation-inhibition) in 10.5%, in response to bath application of hypertonic artificial cerebrospinal fluid (aCSF) containing elevated levels of NaCl (330 mOsm). Similar responses were observed following bath application of hypertonic aCSF prepared with additional mannitol. Angiotensin II in the perfusion medium dramatically increased spontaneous unit activity in 73.7% of neurons tested, while, unexpectedly, 5.3% were inhibited. These results suggest that some neurons in the periventricular zone are themselves osmosensitive, and may be involved in the regulation of water balance.

Neuronal responses of the anterior commissural nucleus to osmotic stimulation and angiotensin II in hypothalamic slices in the rat

The firing rate and pattern of activity of neurons in the anterior commissural nucleus (ACN), which was rich in oxytocin-containing neurons, were studied electrophysiologically in hypothalamic slices. Extracellular recording showed that most ACN neurons exhibited irregular or regular continuous spontaneous unit activity. Other neurons showed short burst patterns of activity or were silent. The majority of ACN neurons were activated by bath application of angiotensin II, and a substantial number of them showed inhibitory or excitatory responses to hypertonic bathing medium. These results indicate that magnocellular neurosecretory neurons in the ACN may participate in the regulation of water balance.