Ventral pontine catecholaminergic pathway mediates the vasopressin response to splanchnic osmostimulation in conscious rats

State-dependent plasticity in vasopressin neurones: dehydration-induced changes in activity patterning

Moderate dehydration impairs concentration and co-ordination, whereas severe dehydration can cause seizures, brain damage or death. To slow the progression of dehydration until body fluids can be replenished by drinking, the increased body fluid osmolality associated with dehydration increases vasopressin (antidiuretic hormone) secretion from the posterior pituitary gland. Increased vasopressin secretion reduces water loss in the urine by promoting water reabsorption in the collecting ducts of the kidney. Vasopressin secretion is largely determined by action potential discharge in vasopressin neurones, and depends on both the rate and pattern of discharge. Vasopressin neurone activity depends on intrinsic and extrinsic mechanisms. We review recent advances in our understanding of the physiological regulation of vasopressin neurone activity patterning and the mechanisms by which this is altered to cope with the increased secretory demands of dehydration.

Locus Coeruleus Lesions Decrease Oxytocin and Vasopressin Release Induced by Hemorrhage

The role of the noradrenergic nucleus Locus Coeruleus (LC) on hemorrhage-induced vasopressin (AVP) and oxytocin (OT) secretion was examined. Rats with LC lesion were submitted to three 1-min hemorrhage sessions at 5-min intervals; 15% of the total blood volume was withdrawn in each session. OT and AVP were measured in plasma, paraventricular (PVN) and supraoptic (SON) nuclei and in posterior pituitary (PP). LC Lesion did not affect basal plasma AVP or OT levels, but partly blocked the increase in plasma AVP and OT induced by hemorrhage. Hemorrhage produced decreases in content of AVP and OT in the PVN and SON and increased levels in the PP. These responses were attenuated in the lesioned group, but only in the PVN and PP. Data suggest a stimulatory role of the inputs from LC to PVN neurons on hemorrhage-induced OT and AVP secretion and that, this pathway is critical in the hypo-volemic neuroendocrine reflex.

Lesions of the dorsal noradrenergic bundle augment the renin response to blood loss but do not alter hypothalamic Fos expression

The goal of this study was to determine if the dorsal noradrenergic bundle (DNAB) plays an essential role in mediating increased plasma renin activity (PRA) and hypothalamic activation, as indicated by increased Fos expression, in response to a small volume blood loss in unanesthetized animals. Male Sprague-Dawley rats were prepared with bilateral 6-hydroxydopamine or sham lesions of the dorsal noradrenergic bundle. In both groups of animals, blood pressure decreased by only 10-15 mmHg following hemorrhage (10 ml/kg over 15 min). Plasma renin activity increased similarly in both groups after 5 ml/kg blood loss, but showed a significantly greater increase after 10 ml/kg blood loss in animals with 6-hydroxydopamine lesions than in those with sham lesions (increase of 13.8 +/- 2.0 ng/ml/h versus 8.4 +/- 1.2 ng/ml/h; P < 0.025). There were numerous Fos-immunoreactive cell nuclei in the supraoptic nucleus (SON) and parvicellular paraventricular hypothalamic nucleus (PVN) of hemorrhaged animals. The number of Fos-positive neurons did not differ between groups, indicating that the dorsal noradrenergic bundle does not convey the primary drive for supraoptic and paraventricular nucleus activation during blood loss. However, one or more of the forebrain regions innervated by the dorsal noradrenergic bundle may attenuate the sympathetic outflow that initiates renin release in response to hemorrhage.

Hypothalamic neuropeptide release after experimental subarachnoid hemorrhage: in vivo microdialysis study

OBJECTIVES

As evidence exists about independent regulation of peripheral and central release of the vasoactive and natriuretic neuropeptides arginine-vasopressin (AVP) and oxytocin (OXT), we investigated their release patterns following subarachnoid hemorrhage (SAH).

MATERIALS AND METHODS

After injection of 0.1 ml arterial blood or saline into the great cistern of 33 Wistar rats, AVP and OXT levels were measured in blood and by microdialysis in the hypothalamic supraoptic (SON) and paraventricular nucleus (PVN). For statistical analysis, the analysis of variance (ANOVA) was used with Tukey HSD post hoc ANOVA tests to determine specific group differences.

RESULTS

Plasma AVP and OXT peaked 2 h after SAH (P < 0.05), and normalized at 4 h. In the SON, both AVP and OXT peaked 4 h after SAH (P < 0.05). In the PVN, AVP increased in both groups (P < 0.05), while no OXT release occurred. By the sham group, any effect of experimental procedure was excluded.

CONCLUSIONS

The SAH-specific central neuropeptide release, which exceeded peripheral release and continued longer, may contribute to pathophysiological events following SAH.

Hepatic denervation does not affect plasma vasopressin response to intragastric hypertonic saline in conscious rats

Peripheral osmoreceptors monitor dietary NaCl and modify central nervous system and renal sympathetic nervous system activity accordingly. Experimental evidence suggests that these responses are dependent on the hepatic nerves. Peripheral osmoreceptors also modify arginine vasopressin (AVP) secretion. However, although hepatic denervation reportedly blunts activation of both supraoptic and paraventricular hypothalamic neurons after intraportal NaCl infusion, the role of the hepatic nerves in the AVP release has not been directly examined. The present study tests the hypothesis that the hepatic nerves modify AVP release in response to intragastric NaCl infusion. Wistar-Kyoto rats (WKY) received either hepatic denervation or a sham operation. Intragastric NaCl infusion significantly elevated plasma AVP in both sham-operated WKY and hepatic-denervated WKY, and the responses were not different between these groups. Second, previous studies suggest that both AVP secretion and baroreflexes are blunted in spontaneously hypertensive rats (SHR), deficits that contribute to the observed hypertension in SHR. We hypothesized that SHR also have a blunted peripheral osmoreceptor reflex and that this contributes to NaCl-sensitive hypertension. In contrast to our prediction, in SHR intragastric NaCl infusion induced an increase in plasma AVP that was similar to that in the WKY groups. Thus, although hepatic osmoreceptors are important for chronic regulation of arterial pressure, renal sympathetic nervous system activity, and the activity of hypothalamic neurons, they do not appear to influence plasma AVP concentration in response to intragastric NaCl.

Cardiovascular, endocrine, and body fluid-electrolyte responses to salt loading in mRen-2 transgenic rats

We previously demonstrated that mRen-2 transgenic [Tg(+)] rats are sensitive to chronic high NaCl intake, showing increased arterial pressure and vasopressin (VP) secretion. In this study, we determined the effect of a chronic osmotic challenge, 4 days of drinking 2% NaCl, on direct arterial blood pressure, heart rate, fluid-electrolyte balance, circadian rhythm of mean arterial pressure (MAP), and changes in plasma VP and catecholamines. Under baseline conditions, male Tg(+) rats showed a significant shift in the peak in circadian MAP into the light portion of the day-night cycle. Substitution of 2% NaCl for drinking water caused a rapid increase in MAP, 20 +/- 5 mmHg in Tg(+) rats within 6 h. Whereas the amplitude of circadian MAP fluctuations increased in salt-loaded Tg(+) rats, there was no significant change in the circadian timing of peak MAP with salt loading. Tg(+) rats showed exaggerated osmotic-induced increases in plasma VP, norepinephrine (NE), and epinephrine (Epi) compared with Tg(-) rats. Plasma NE and Epi were increased two- and fourfold, respectively, in the hypertensive rats with no significant change in the Tg(-) rats. Intravenous administration of a VP antagonist did not alter arterial pressure in either Tg(+) or Tg(-) rats. Tg(+) and Tg(-) rats showed a positive sodium balance with no significant difference observed between the groups. Tg(+) rats showed a significant increase in salt consumption, plasma sodium, osmolality, and hematocrit, accompanied by a negative water balance. We conclude that Tg(+) rats are sensitive to acute and chronic osmotic stimuli in terms of blood pressure, fluid-electrolyte balance, and plasma VP and catecholamines. Whereas elevated plasma VP does not contribute to the hypertensive response, increased sympathetic drive may mediate the salt-induced blood pressure changes in this model.

Central c-Fos expression in neonatal and adult rats after subcutaneous injection of hypertonic saline

Centrally-mediated responses to plasma hyperosmolality include compensatory drinking and pituitary secretion of vasopressin and oxytocin in both adult and neonatal rats. However, the anorexia that is produced by plasma hyperosmolality in adult rats is not evident in neonates, perhaps due to functional immaturity of osmoresponsive hindbrain circuits. To examine this possibility, the present study compared treatment-induced brain expression of the immediate-early gene product c-Fos as a marker of neural activation in adult and two-day-old rats after subcutaneous injection of 2 M NaCl (0.1 ml/10 g body weight). This treatment produced marked hypernatremia in adult and two-day-old rats without altering plasma volume. Several brain regions (including components of the lamina terminalis, the paraventricular and supraoptic nuclei of the hypothalamus, and the area postrema) were activated to express c-Fos similarly in adult and two-day-old rats after 2 M NaCl injection, consistent with previous reports implicating a subset of these regions in osmotically-stimulated drinking and neurohypophyseal secretion. In contrast, other areas of the brain that were activated to express c-Fos in adult rats after 2 M NaCl injection were not activated in neonates: these areas included the central nucleus of the amygdala, the parabrachial nucleus and catecholamine cell groups within the caudal medulla. This study demonstrates that certain brain regions that are osmoresponsive in adult rats (as defined by induced c-Fos expression) are not osmoresponsive in two-day-old rats. When considered in the context of known differences between the osmoregulatory capacities of adult and neonatal rats, our results are consistent with the idea that osmoresponsive forebrain centres are primarily involved in osmotically-stimulated compensatory drinking and neurohypophyseal secretion, whereas osmoresponsive regions of the hindbrain are important for concomitant inhibition of feeding and gastric emptying.