Water intake and activity of hypothalamoneurohypophysial system during osmotic and sodium stimulation in rats

Abstract

Intrajugular infusion (200 microliters/min for 10 min) of 0.85 M NaCl or 1.7 M mannitol in conscious adult male Sprague-Dawley rats increased plasma osmolality similarly and had an additive effect when combined. Plasma Na+ concentration, however, increased with infusion of 0.85 M NaCl, decreased with 1.7 M mannitol, and was not significantly altered by the combined solution. Irrespective of changes in plasma Na+ concentration, plasma vasopressin and oxytocin concentrations were elevated to a similar degree after independent infusion of 0.85 M NaCl or 1.7 M mannitol. With the combined infusion, the change in plasma vasopressin was additive but the change in oxytocin tended to be greater. Accordingly, glucose utilization increased throughout the hypothalamoneurohypophysial system after infusion of 0.85 M NaCl and 1.7 M mannitol. With the combined infusion, however, the change in glucose utilization in the paraventricular nucleus was additive but a synergistic effect occurred in the supraoptic nucleus and neural lobe. Drinking responses were similar in all groups receiving hypertonic solutions, with no additive effect after the combined stimulus. Although our results do not completely rule out the participation of cerebrospinal fluid sodium receptors, it is more likely that osmoreceptors regulate the activity of the hypothalamoneurohypophysial system and drinking behavior. Unlike the magnocellular system, however, drinking behavior seems to be negatively influenced by a stress component of the osmotic stimulation.

Vasopressin and oxytocin release evoked by NaCl loads are selectively blunted by area postrema lesions

“…Finally, in assessing the specificity of the disruption by APX of neurohypophysial hormone secretion in response to hypertonic NaCl solution, we also determined whether comparable impairments in VP and OT secretion would be seen when rats with APX were infused with a hyperosmotic solution that did not cause substantial hypernatremia. In the control animals the mixed hyperosmotic solution stimulated increases in P VP and P OT , as expected (24,27,43). It was therefore striking that, after treatment with the mixed hyperosmotic solution, rats with APX did not show blunted increases in P VP and P OT .…”

Central effects of various ligands on drinking behavior in eels acclimated to seawater

“…In mammals, it is well known that blood hyperosmolarity stimulates AVP secretion (Brimble et al, 1977) and drinking rate (Gilman, 1937;Olsson, 1972;Thrasher et al, 1980a,b;Kadekaro et al, 1995). On the other hand, in the seawater eel, blood hyperosmolarity decreases water intake (Ando et al, 2000a;Takei, 2000).…”

Translation of Salt Retention to Central Activation of the Sympathetic Nervous System in Hypertension

“…However, whether these osmoreceptors also mediate changes in sympathetic activity in response to changes in osmolality has not been established definitely. In particular, although it is known that brain receptors responding to changes in osmolality, rather than changes in Na per se , can trigger alterations in BP and sympathetic activity, 35,36 whether they reside in CVOs has not been investigated. Nevertheless, indirect evidence supporting a role for the OVLT and SFO includes anatomical viral tracing studies showing that the neuronal circuitry exists whereby these CVOs could influence the sympathetic nervous system, 37,38 Moreover, OVLT and SFO neurons identified as projecting to the PVN, 39 an important autonomic relay station, or polysynaptically to the kidney 40 express c‐fos following acute increases in osmolality.…”

“…Thirst and vasopressin release, pressor and sympathoexcitatory responses and excitation of dispersed osmosensitive neurons can be triggered by both increases in sodium concentration and hyperosmolar solutions of non‐electrolytes, suggesting that osmoreception forms the basis of the responses. However, it is important to note that responses to hypertonic NaCl solutions are quantitatively larger 35,36 , 45,46 . One explanation for these results is that depolarization of osmoreceptive neurons is mediated by increases in the opening probability of non‐selective cation channels and this signal could be amplified by increases in extracellular Na concentration either by directly increasing the permeability of the channels or by increasing the gradient for current flow, because Na is the major current‐carrying cation 46 .…”

“…However, it is important to note that responses to hypertonic NaCl solutions are quantitatively larger. 35,36,45,46 One explanation for these results is that depolarization of osmoreceptive neurons is mediated by increases in the opening probability of non-selective cation channels and this signal could be amplified by increases in extracellular Na concentration either by directly increasing the permeability of the channels or by increasing the gradient for current flow, because Na is the major current-carrying cation. 46 Alternatively, it has been proposed that both osmosensitive and sodium-sensitive cells exist in distinct sites.…”