Pharmacologic independence of subfornical organ receptors mediating drinking

Water Drinking Behavior Associated with Aversive Arousal in Rats: An Integrative Approach

Cholinergic muscarinic stimulation of vast areas of the limbic brain induced a well-documented polydipsia in laboratory rats. This excessive water-drinking behavior has not received any convincing biological and physiological interpretation for the last 50 years. This review offers such an interpretation and suggests that cholinergically induced drinking response, mostly by carbachol, is associated with activation of the ascending mesolimbic cholinergic system that serves for initiation of emotional aversive arousal of the organism. The ascending cholinergic system originates from the laterodorsal tegmental nucleus, has a diffuse nature, and affects numerous subcortical limbic structures. It is proposed that the carbachol-induced drinking response is related to the state of anxiety and does not serve the regulation of thirst. Instead, the response is anxiety-induced polydipsia that might occur as a soothing procedure that decreases the aversiveness of the negative emotional state induced by carbachol. It is concluded that carbachol-induced water-drinking behavior is a rewarding process that contributes to alleviating the feeling of anxiety by bringing some relief from the cholinergically induced aversive state, and it is a homologue to anxiety-driven polydipsia in humans.

Brain Angiotensinergic Regulation of the Immune System: Implications for Cardiovascular and Neuroendocrine Responses

OBJECTIVE

The Renin-Angiotensin-Aldosterone System (RAAS) plays a major role in the regulation of cardiovascular functions, water and electrolytic balance, and hormonal responses. We perform a review of the literature, aiming at providing the current concepts regarding the angiotensin interaction with the immune system in the brain and the related implications for cardiovascular and neuroendocrine responses.

METHODS

Appropriate keywords and MeSH terms were identified and searched in Pubmed. Finally, references of original articles and reviews were examined.

RESULTS

Angiotensin II (ANG II), beside stimulating aldosterone, vasopressin and CRH-ACTH release, sodium and water retention, thirst, and sympathetic nerve activity, exerts its effects on the immune system via the Angiotensin Type 1 Receptor (AT 1R) that is located in the brain, pituitary, adrenal gland, and kidney. Several actions are triggered by the binding of circulating ANG II to AT 1R into the circumventricular organs that lack the Blood-Brain-Barrier (BBB). Furthermore, the BBB becomes permeable during chronic hypertension thereby ANG II may also access brain nuclei controlling cardiovascular functions. Subfornical organ, organum vasculosum lamina terminalis, area postrema, paraventricular nucleus, septal nuclei, amygdala, nucleus of the solitary tract and retroventral lateral medulla oblongata are the brain structures that mediate the actions of ANG II since they are provided with a high concentration of AT 1R. ANG II induces also T-lymphocyte activation and vascular infiltration of leukocytes and, moreover, oxidative stress stimulating inflammatory responses via inhibition of endothelial progenitor cells and stimulation of inflammatory and microglial cells facilitating the development of hypertension.

CONCLUSION

Besides the well-known mechanisms by which RAAS activation can lead to the development of hypertension, the interactions between ANG II and the immune system at the brain level can play a significant role.

Muscarinic M2 receptor promotes vasopressin synthesis in mice supraoptic nuclei

Muscarinic acetylcholine receptors have been suggested to be implicated in arginine-vasopressin secretion because intracerebroventricular muscarinic agonist administration induces arginine-vasopressin release into the circulation. Although which subtype is involved in the regulation of arginine-vasopressin secretion is unclear, M₂ receptors have been reported to be highly expressed in the hypothalamus. In the present study, M₂ receptor-knockout mice were used to elucidate whether M₂ receptor regulates arginine-vasopressin synthesis in the paraventricular nuclei and supraoptic nuclei of the hypothalamus. The number of arginine-vasopressin-immunoreactive neurons in M₂ receptor-knockout mice was significantly decreased in the supraoptic nuclei, but not in the paraventricular nuclei compared with wild-type mice. Plasma arginine-vasopressin level in M₂ receptor-knockout mice was also significantly lower than in the wild-type mice. Urinary volume and frequency as well as water intake in M₂ receptor-knockout mice were significantly higher than those in wild-type mice. The V₂ vasopressin receptor expression in kidneys of M₂ receptor-knockout mice was comparable with that of wild-type mice, and increased urination in M₂ receptor-knockout mice was significantly decreased by administration of desmopressin, a specific V₂ receptor agonist, suggesting that V₂ receptors in the kidneys of M₂ receptor-knockout mice are intact. These results suggest that M₂ receptors promote arginine-vasopressin synthesis in the supraoptic nuclei and play a role in the regulation and maintenance of body fluid.

Non-NMDA glutamatergic receptors modulate acetylcholine release in the rat subfornical organ area

The present study was designed to examine whether glutamatergic receptor mechanisms modulate the release of acetylcholine (ACh) in the region of the subfornical organ (SFO) using intracerebral microdialysis methods in freely moving rats. Perfusion of either non-N-methyl-d-aspartate (NMDA) agonist quisqualic acid (QA, 50 microM) or kainic acid (KA, 50 microM) through the microdialysis probe significantly enhanced the ACh release in the SFO area. Local perfusion of the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 and 50 microM) did not change the basal release of ACh. CNQX (10 microM) administered together with either QA (50 microM) or KA (50 microM) in the SFO area antagonized the stimulant effect of the agonists on the ACh release. In urethane-anesthetized rats, repetitive electrical stimulation (500 microA, 10 Hz) of the medial septum (MS) significantly increased dialysate ACh concentrations in the region of the SFO. The increase in the ACh release elicited by the MS stimulation was significantly potentiated by perfusion of QA (50 microM), and the QA-induced potentiation was prevented by CNQX (10 microM) treated together with QA. These results show that the glutamatergic synaptic inputs enhance the ACh release in the SFO area through non-NMDA receptors. The data further suggest that the septal cholinergic inputs to the SFO area are potentiated by non-NMDA receptor mechanisms.

The vagus nerve and thirst

This paper reviews the experiments, which demonstrate conclusively the involvement of the abdominal vagus nerve in normal expression of most aspects of thirst in rats, by Gerard P. Smith and his colleagues published between 1975 and 1984. The nature of that vagal contribution differs with the type of primary thirst signal. Thus, there is no clear or unitary answer concerning whether the contribution of the vagus nerve is purely sensory, or some general tonic action within the central nervous system. Subsequent studies using cFos mapping of intracellular dehydration in conjunction with vagotomy and/or hepatic manipulations are also reviewed and further illustrate the involvement of abdominal information, both in the initiation as well as the termination of drinking. Many of the questions that were raised by Smith during these pioneering studies remain unaddressed and unanswered.

Diversity of the muscarinic and nicotinic responses of subfornical organ neurons in rat slice preparations

Recently we found that subfornical organ (SFO) neurons were activated through nicotinic receptors as well as muscarinc. In this study, the preferential responses of single SFO neurons to both muscarine and nicotine were examined by using rat slice preparations, and current and voltage clamp recordings. When the amplitudes of the depolarizations and inward currents by muscarine and nicotine in single SFO neurons were compared, some neurons showed a higher sensitivity to muscarine than to nicotine while others showed vice versa. These data indicate that acetylcholine activates SFO neurons preferentially through either muscarinic or nicotinic receptors and suggest a diversity of cholinergic responses in the SFO.

Activation of cholinergic pathways from the septum to the subfornical organ area under hypovolemic condition in rats

In urethane-anesthetized rats, extracellular concentrations of acetylcholine (ACh) in the region of the subfornical organ (SFO) in response to electrical stimulation of the septum and hypovolemia were monitored with in vivo microdialysis methods. Repetitive electrical stimulation (500 microA, 5-20 Hz) of the medial septum significantly increased dialysate ACh concentrations in the region of the SFO. Non-hypotensive hypovolemia caused by subcutaneous administration of polyethylene glycol (PEG, 30%, 5 ml) elicited significantly increased ACh levels in the region of the SFO. The hypovolemia-induced ACh release was attenuated by microinjection of the local anesthetic lidocaine (2%, 0.2 microl) into the medial septum. These results suggest that septal cholinergic pathways to the SFO area may be activated under the hypovolemic condition.

Activation of muscarinic receptors in rat subfornical organ neurones

Cholinergic muscarinic inputs to subfornical organ (SFO) neurones in rats were studied using histochemical, molecular-biological and electrophysiological techniques. Neurones in the medial septum and the diagonal band (MS-DBB) were retrogradely labelled by a tracer wheat germ agglutinin-conjugated horseradish peroxidase-colloidal gold complex injected into the SFO. Some in the MS-DBB were double-labelled by choline acetyltransferase (ChAT) antibody. Many ChAT-immunoreactive fibres were observed in the SFO. M3 muscarinic receptor subtype-like immunoreactivity, detected using a polyclonal antiserum, was observed in the SFO. In slice preparations, muscarine induced inward currents in a dose-related manner. The inward currents were suppressed by the relatively M3 muscarinic receptor selective antagonist 4-diphenylacetoxy-N-methylpiredine methiodide. In the whole-cell current mode, muscarine depolarized the membrane with increased frequency of action potentials. Reverse transcriptase-polymerase chain reaction showed the presence of M2-M5 receptor mRNA in the SFO tissues. These results suggest that the SFO receives cholinergic muscarinic synaptic inputs from the MS-DBB. Acetylcholine postsynaptically activates and depolarizes neurones in the SFO partly through specific muscarinic receptors, including M3 receptor subtypes.

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.

Muscarinic modulation of GABAergic transmission to neurons in the rat subfornical organ

Cholinergic actions on subfornical organ (SFO) neurons in rat slice preparations were studied by using whole cell voltage- and current-clamp recordings. In the voltage-clamp recordings, carbachol and muscarine decreased the frequency of GABAergic inhibitory postsynaptic currents (IPSCs) in a dose-dependent manner, with no effect on the amplitudes or the time constants of miniature IPSCs. Meanwhile, carbachol did not influence the amplitude of the outward currents induced by GABA. Furthermore, carbachol and muscarine also elicited inward currents in a TTX-containing solution. From the current-voltage relationship, the reversal potential was estimated to be −7.1 mV. These carbachol-induced responses were antagonized by atropine. In the current-clamp recordings, carbachol depolarized the membrane with increased frequency of action potentials. These observations suggest that acetylcholine suppresses GABA release through muscarinic receptors located on the presynaptic terminals. Acetylcholine also directly affects the postsynaptic membrane through muscarinic receptors, by opening nonselective cation channels. A combination of these presynaptic and postsynaptic actions may enhance activation of SFO neurons by acetylcholine.

Effect of NMDA-induced lesion of the subfornical organ on the angiotensin II binding sites density and acetylcholinesterase or NADPH-diphorase activities in the lamina terminalis of the rat brain

1. Neural angiotensinergic circuitry located in the lamina terminalis has been proposed to be involved in blood pressure regulation and fluid homeostasis. 2. ANG II binding sites have been described to be localized throughout the lamina terminalis including the subfornical organ (SFO), the median preoptic nucleus (MnPO), and the organum vasculosum lamina terminalis (OVLT). 3. The present experiment was designed to investigate the ANG II binding sites localization in the lamina terminalis. For this purpose, we have compared the ANG II binding sites, acetylcholinesterase, and NADPH-diaphorase distributions throughout the lamina terminalis. Additionally, we have studied the effect of the preferential lesion of SFO neuronal cell bodies by local injection of NMDA on the ANG II binding sites density in different areas of the lamina terminalis. 4. Male Wistar rats were anesthetized, immobilized in a stereotaxic apparatus, and 500 nl of saline or 250 nmol NMDA was injected into the SFO. 5. Animals were sacrificed 1 week later, the brain was removed, frozen, and sagittal 16 microm slices were cut in a cryostat. Alternate brain slices were incubated with [125I]-Sar1-ANG II for receptor autoradiography or histochemically stained for visualization of acetylcholinesterase and NADPH-diaphorase activities. Binding capacity was determined by computerized quantitative densitometry of autoradiograms. The intensity of histochemical reactions was measured as relative units obtained by computerized densitometry processing of the brain slices stained for either activity. 6. Acetylcholinesterase staining was mainly located in the SFO, with faint staining reaction in other areas of the lamina terminalis. NADPH-diaphorase staining was homogeneously distributed throughout the lamina terminalis. A significant positive correlation was observed between acetylcholinesterase and NADPH-diaphorase stainings in the SFO of control and NMDA-lesioned rats. 7. ANG II binding sites were localized throughout the lamina terminalis. A significant positive correlation was observed between the density of ANG II binding sites and the intensity of acetylcholinesterase or NADPH-diaphorase staining in the SFO of control and NMDA-lesioned rats. 8. The distribution of the NADPH-diaphorase staining was found to closely match the distribution of the ANG II binding sites in the lamina terminalis. 9. Neuronal lesion of the SFO caused significant reductions in the density of ANG II biding sites in the SFO (-68%) and the MnPO (-48%). No changes were observed either in the OVLT or outside the lamina terminalis in the superior colliculus. 10. The present results indicate the following: first, the presence of high levels of acetylcholinesterase staining in the SFO and of NADPH-diaphorase throughout the lamina terminalis; second, that ANG II binding sites in the SFO and possibly in the MnPO are localized in neuronal cell bodies; third, that SFO lesion did not affect the expression of ANG II binding sites in the OVLT, thus suggesting that these binding sites correspond to different angiotensinergic system: and finally, the existence of a striking correlation between the distribution of the ANG II binding sites and NADPH-diaphorase throughout the lamina terminalis, thus suggesting a interrelation between angiotensinergic and nitrergic systems in the lamina terminalis.

Octreotide-induced drinking, vasopressin, and pressure responses: role of central angiotensin and ACh

The involvement of central angiotensinergic and cholinergic mechanisms in the effects of the intracerebroventricularly injected somatostatin analog octreotide (Oct) on drinking, blood pressure, and vasopressin secretion in the rat was investigated. Intracerebroventricular Oct elicited prompt drinking lasting for 10 min. Water consumption depended on the dose of Oct (0.01, 0.1, and 0. 4 microgram). The drinking response to Oct was inhibited by pretreatments with the intracerebroventricularly injected angiotensin-converting enzyme inhibitor captopril, the AT(1)/AT(2) angiotensin receptor antagonist saralasin, the selective AT(1) receptor antagonist losartan, or the muscarinic cholinergic receptor antagonist atropine. The dipsogenic effect of Oct was not altered by prior subcutaneous injection of naloxone. Oct stimulated vasopressin secretion and enhanced blood pressure. These responses were also blocked by pretreatments with captopril or atropine. Previous reports indicate that the central angiotensinergic and cholinergic mechanisms stimulate drinking and vasopressin secretion independently. We suggest that somatostatin acting on sst2 or sst5 receptors modulates central angiotensinergic and cholinergic mechanisms involved in the regulation of fluid balance.

Drinking and Fos-immunoreactivity in rat brain induced by local injection of angiotensin I into the subfornical organ

Previous studies suggested that angiotensinergic stimulation in the subfornical organ (SFO) has effects on the anterior third ventricle (AV3V) region and the hypothalamus for dipsogenic response and vasopressin release. In this study, Angiotensin I (ANG I) was directly injected into the SFO and this stimulated drinking. Injection of ANG I into the SFO also induced Fos-immunoreactivity (Fos-ir) in the AV3V region and in the vasopressin neurons of the supraoptic and paraventricular nuclei (SON and PVN). Pretreatment of the SFO with either captopril, an ANG converting enzyme inhibitor, or losartan, an AT1 receptor antagonist, abolished both drinking and Fos-ir induced by ANG I. Water intake partially decreased ANG I-induced Fos-ir in the SON and PVN, but not in the other areas. These results indicate that there is an ANG converting system in the SFO and suggest that neurons in the AV3V region and vasopressin cells in the hypothalamus can be regulated by angiotensinergic components in the SFO.

Effects of chemical stimulation of the subfornical organ on metabolic activity of the hypothalamo-neurohypophysial system in rats

Topical stimulation of the subfornical organ (SFO) with angiotensin II (ANG II) or acetylcholine (ACh) increased blood pressure, water intake, and the activity of the hypothalamo-neurohypophysial system (HNS). The pressor and drinking responses were higher and the metabolic activity of the HNS, measured with the [14C]deoxyglucose (DG) method, was lower with ANG-II than with ACh. Glucose utilization in the neural lobe increased by 165% with ACh as opposed to 65% with ANG II. These results demonstrate the ANG II and ACh mobilize differentially the mechanisms increasing blood pressure, drinking responses, and the activity of the HNS.

Effect of agents which alter the Na transport on the sodium appetite in rats

The effect of ouabain (OUA), ethacrinic acid (EA) and diphenylhydantoin (DPH) on sodium appetite was evaluated in sodium-depleted rats. These animals were injected with either OUA (50 pg) or EA (150 ng) into the third ventricle (3V), fourth ventricle (4V), lateral ventricle (LV), or lateral hypothalamus (LH). Decreased Na appetite was observed only after injections either into the 3V or LV. DPH injection (270 ng) enhanced sodium appetite in mildly depleted rats. These effects seem to be specific for Na appetite since EA and OUA injections into the 3V did not alter food intake in food-deprived rats. The drugs did not impair gustatory inputs since the water-glucose preference was not altered after OUA, EA or DPH administration. The decrease in Na appetite induced by a 3-hour infusion of OUA (1 microliter/hr) was reversed by a 3-hour infusion of DPH (1 microliter/hr), while a pretreatment with DPH prevented the inhibitory effect of OUA. The data show that agents which inhibit cellular Na efflux such as OUA and EA decreased sodium appetite while DPH, which inhibits cellular Na influx, induced the opposite effect, i.e., enhanced Na appetite. The results are consistent with the hypothesis that the Na content of sensor cells located on or near the walls of the 3V could be the signal for the central control of Na appetite.

Anatomical evidence that neural circuits related to the subfornical organ contain angiotensin II

Bidirectional connections between the subfornical organ and the hypothalamus are reviewed, and emphasis is placed on recent evidence for the presence of angiotensin II in some of these pathways. Additionally, evidence is presented suggesting that this peptide may serve as a neurotransmitter or neuroendocrine factor in the efferent projections of cell groups receiving neural inputs from the subfornical organ. It appears that angiotensin II may serve as one of the chemical messengers at each link in multi-synaptic pathways related to the subfornical organ.

Fluid intake, distribution, and excretion during lateral ventricular infusions of carbachol in rats

Infusion of carbachol into the lateral ventricles of rats at rates of 400 or 2000 ng/h for 6 h produced dose-related natriuresis, kaliuresis, and water drinking but no consumption of hypertonic NaCl solution. Electrolyte excretion and water intake were maximal during the first 2 h, and no further increases occurred after 4 h. Sodium losses were estimated as 15-30% of total extracellular sodium. Continuous infusion of 2000 ng/h carbachol for 6 days produced a chronic increase of water intake but no increased consumption of saline. Sodium balances were negative during the first day of infusion, but gradually returned to normal over 6 days. Plasma volume, hematocrit, and plasma sodium and potassium concentrations were normal in carbachol-infused animals on the 6th day. Cholinergic stimulation of the brain thus appeared to interfere with the usual salt appetite following sodium loss.