Intracerebroventricular carbachol induces FOS immunoreactivity in lamina terminalis neurons projecting to the supraoptic nucleus

The organum vasculosum of the lamina terminalis and subfornical organ: regulation of thirst

Thirst and water intake are regulated by the organum vasculosum of the lamina terminalis (OVLT) and subfornical organ (SFO), located around the anteroventral third ventricle, which plays a critical role in sensing dynamic changes in sodium and water balance in body fluids. Meanwhile, neural circuits involved in thirst regulation and intracellular mechanisms underlying the osmosensitive function of OVLT and SFO are reviewed. Having specific Nax channels in the glial cells and other channels (such as TRPV1 and TRPV4), the OVLT and SFO detect the increased Na+ concentration or hyperosmolality to orchestrate osmotic stimuli to the insular and cingulate cortex to evoke thirst. Meanwhile, the osmotic stimuli are relayed to the supraoptic nucleus (SON) and paraventricular nucleus of the hypothalamus (PVN) via direct neural projections or the median preoptic nucleus (MnPO) to promote the secretion of vasopressin which plays a vital role in the regulation of body fluid homeostasis. Importantly, the vital role of OVLT in sleep-arousal regulation is discussed, where vasopressin is proposed as the mediator in the regulation when OVLT senses osmotic stimuli.

Intravenous CDP-choline activates neurons in supraoptic and paraventricular nuclei and induces hormone secretion

The aim of the present study was to assess the effects of intravenous (i.v.) cytidine-5'-diphosphate (CDP)-choline administration on the activation of oxytocin and vasopressin neurons in the supraoptic (SON) and paraventricular nuclei (PVN), using the immunohistochemical identification of c-Fos expression as a marker of neuronal activation and to correlate this with the plasma hormone levels. Rats were catheterized under sevofluorane anesthesia and experiments were conducted 24h later. Blood samples were withdrawn from arterial catheter at 2, 5, 10, 20, 40 and 60 min after CDP-choline (0.5, 1.0 and 2.0 g/kg; i.v.) or saline (1.0 ml/kg; i.v.) for the measurement of plasma oxytocin and vasopressin levels by radioimmunoassay. Animals were sacrificed 90 min after CDP-choline administration for dual immunohistochemistry which was performed on paraformaldehyde-fixed vibratome sections. Dual immunohistochemistry for c-Fos and oxytocin or vasopressin revealed that CDP-choline activates these neurons in a dose-dependent manner. Light microscopic analyses showed that, about 41%, 75% or 87% of the oxytocin neurons and about 18%, 46% or 82% of the vasopressin neurons in SON express c-Fos, thus activated, by the dosages of 0.5, 1.0 or 2.0 g/kg CDP-choline, respectively. Increases in c-Fos expression were about 29%, 62% or 81% for the oxytocin neurons and about 38%, 70% or 78% for the vasopressin neurons in PVN with the dosages of 0.5, 1.0 or 2.0 g/kg CDP-choline, respectively. When compared to the control groups (8% and 7% oxytocin or 2% and 5% vasopressin neuronal activation in SON or PVN, respectively), these increases were found to be statistically significant (p<0.05). In the PVN most of the magnocellular neurons were activated while less number of parvocellular neurons expressed c-Fos in response to CDP-choline challenge. In correlation with c-Fos data, CDP-choline increased plasma oxytocin and vasopressin levels both dose- and time-dependently. Results of the present study suggested that peripheral administration of CDP-choline is able to increase plasma oxytocin and vasopressin levels while activating the respective neurons.

Hypothalamic disconnection caudal to paraventricular nucleus affects cardiovascular and drinking responses to central angiotensin II and carbachol

The paraventricular nucleus of the hypothalamus (PVN) is an important area of the brain involved in the control of cardiovascular system and fluid-electrolyte balance. In the present study we evaluated the effects of hypothalamic disconnection (HD) caudal to PVN in the pressor and dipsogenic responses induced by intracerebroventricular (icv) injections of angiotensin II (ANG II) or carbachol (cholinergic agonist). Male Holtzman rats (280-320 g) with a stainless steel cannula implanted into the lateral ventricle and submitted to sham or HD surgery were used. HD (2 or 15 days) reduced the pressor responses to ANG II (50 ng/1μl) icv (8±3 and 11±3 mm Hg, respectively, vs. sham: 23±3 and 21±2 mm Hg) or carbachol (4 nmol/1 μl) icv (8±2 and 21±3 mm Hg, respectively, vs. sham: 33±3 and 33±3 mm Hg), without changing baseline arterial pressure. Acutely (2-4 days), HD also reduced water intake to icv ANG II (3.3±2.2 vs. sham: 14.2±3.0 ml/60 min) or carbachol (4.4±1.8 vs. sham: 11.4±1.6 ml/60 min); however, chronically (15-17 days), HD produced no change on ANG II- and carbachol-induced water intake, in spite of the increased daily water intake and urinary volume. The results suggest that medial projections caudal to PVN are important for pressor and dipsogenic responses to central angiotensinergic and cholinergic activation.

Glucocorticoid modulation of atrial natriuretic peptide, oxytocin, vasopressin and Fos expression in response to osmotic, angiotensinergic and cholinergic stimulation

The regulation of fluid and electrolyte homeostasis involves the participation of several neuropeptides and hormones that utilize hypothalamic cholinergic, alpha-adrenergic and angiotensinergic neurotransmitters and pathways. Additionally, it has been suggested that hypothalamus-pituitary-adrenal axis activity modulates hormonal responses to blood volume expansion. In the present study, we evaluated the effect of dexamethasone on atrial natriuretic peptide (ANP), oxytocin (OT) and vasopressin (AVP) responses to i.c.v. microinjections of 0.15 M and 0.30 M NaCl, angiotensin-II (ANG-II) and carbachol. We also evaluated the Fos protein immunoreactivity in the median preoptic (MnPO), paraventricular (PVN) and supraoptic (SON) nuclei. Male Wistar rats received an i.p. injection of dexamethasone (1 mg/kg) or vehicle (0.15 M NaCl) 2 h before the i.c.v. microinjections. Blood samples for plasma ANP, OT, AVP and corticosterone determinations were collected at 5 and 20 min after stimulus. Another set of rats was perfused 120 min after stimulation. A significant increase in plasma ANP, OT, AVP and corticosterone levels was observed at 5 and 20 min after each central stimulation compared with isotonic saline-injected group. Pre-treatment with dexamethasone decreased plasma corticosterone and OT levels, with no changes in the AVP secretion. On the other hand, dexamethasone induced a significant increase in plasma ANP levels. A significant increase in the number of Fos immunoreactive neurons was observed in the MnPO, PVN and SON after i.c.v. stimulations. Pre-treatment with dexamethasone induced a significant decrease in Fos immunoreactivity in these nuclei compared with the vehicle. These results indicate that central osmotic, cholinergic, and angiotensinergic stimuli activate MnPO, PVN and SON, with a subsequent OT, AVP, and ANP release. The present data also suggest that these responses are modulated by glucocorticoids.

The association of cardiovascular responses with brain c-fos expression after central carbachol in the near-term ovine fetus

Central cholinergic mechanisms play important roles in the control of cardiovascular responses. However, in utero development of brain cholinergic mechanism in regulation of arterial pressure before birth is largely unknown. This study investigated cardiovascular responses to central application of carbachol in fetuses and determined functional development of the central cholinergic systems controlling fetal pressor responses in utero. Chronically prepared near-term ovine fetuses (90% gestation) received an injection of carbachol intracerebroventricularly (i.c.v.). Fetal cardiovascular responses were measured, and the brains were used for c-fos mapping studies. In response to carbachol injection i.c.v., fetal systolic, diastolic, and mean arterial pressure (MAP) immediately increased, accompanied by a bradycardia. The maximum increase of MAP was at 30 min after the i.c.v. injection of carbachol and lasted 90 min. Associated with the pressor response, the neuronal activity marked with c-fos was enhanced significantly in the fetal anterior third ventricle (AV3V) region (including the median preoptic nucleus and organum vasculosum of the lamina terminalis) in the forebrain, and in the area postrema, lateral parabrachial nucleus, nucleus tractus solitary, and rostral ventrolateral medulla in the hindbrain. These results indicate that the central cholinergic mechanism is functional in the control of fetal blood pressure at the last third of gestation, and the central AV3V region and hindbrain have been intact relatively during in utero development in sheep at 90% gestational stage.

Brain muscarinic receptor subtypes mediating water intake and Fos following cerebroventricular administration of bethanecol in rats

RATIONALE

The brain regions and receptor subtypes involved in water intake following central cholinergic stimulation have been incompletely characterized.

OBJECTIVES

To examine whether drinking and brain Fos-immunoreactivity (ir) induced in rats by central administration of bethanecol is reversed by either the preferential M1 antagonist pirenzepine, the M3 antagonist 4-DAMP, or their combination.

METHODS

Male Sprague-Dawley rats were surgically implanted with cerebroventricular cannulae. The muscarinic agonist, bethanecol was used as the dipsogenic agent. Either nonselective (atropine) or selective muscarinic receptor antagonists were injected together with bethanecol to determine blockade of drinking. In parallel studies, Fos-ir was assessed in discrete brain regions.

RESULTS

Bethanecol-induced drinking was completely blocked by atropine or by a combination of pirenzepine and 4-DAMP; these latter antagonists alone produced sub-total inhibition of drinking. In contrast, water intake induced by angiotensin II was unaffected by combination of pirenzepine and 4-DAMP. Fos-ir was induced by bethanecol in many brain regions previously implicated in body fluid regulation, including subfornical organ and the magnocellular supraoptic and paraventricular hypothalamic nuclei. Induced Fos-ir was substantially but not completely prevented by co-injection of either pirenzepine or 4-DAMP, but their combination did not seem markedly more effective than either alone.

CONCLUSIONS

Drinking induced by brain muscarinic receptor stimulation seems to proceed by a combination of M1 and M3 receptor subtypes. Drinking induced by angiotensin II occurs independently of this mechanism. Fos-ir induced in fluid-related brain regions by bethanecol either uses additional receptor type(s) or is less easily blocked than drinking behavior.

Functional relationship between subfornical organ cholinergic stimulation and cellular activation in the hypothalamus and AV3V region

The subfornical organ (SFO) has been suggested to be important for water intake and secretion of vasopressin (AVP). However, the role of the SFO cholinergic mechanism in the control of body fluid regulation is not clear. This study determined the effects of local cholinergic stimulation in the SFO produced by administration of physostigmine on drinking and cellular excitation in the anterior third ventricle (AV3V) region and in the supraoptic and paraventricular nuclei (SON and PVN). The results showed that injection of physostigmine into the SFO induced water intake and c-fos expression in the AV3V area as well as in the AVP containing neurons in the hypothalamus. Pretreatment of the SFO with mecamylamine, a nicotinic receptor antagonist, had no effect on physostigmine induced behavioral and c-fos responses. The muscarinic receptor blocker atropine, however, abolished both drinking and cellular activation after injection of physostigmine into the SFO. Immunostaining experiments demonstrated positive acetyltransferase (ChAT) in the SFO. Intensive ChAT immunoreactivity was located in the cholinergic fibers in the SFO. Together, the results indicate that SFO cholinergic mechanisms are important in co-operation with the AV3V and hypothalamic neurons in the control of thirst and AVP-mediated body fluid homeostasis.