Sodium intake combining cholinergic activation and noradrenaline into the lateral parabrachial nucleus.

The administration of cholinergic agonists like pilocarpine intraperitoneally (i.p.) or carbachol intracerebroventricularly (i.c.v.) induces water, but non significant hypertonic NaCl intake. These treatments also produce pressor responses, which may inhibit sodium intake. Noradrenaline (NOR) acting on α2-adrenoceptors in the lateral parabrachial nucleus (LPBN) deactivates inhibitory mechanisms increasing fluid depletion-induced sodium intake. In the present study, we investigated: (1) water and 1.8% NaCl intake in rats treated with pilocarpine i.p. or carbachol i.c.v. combined with NOR into the LPBN; (2) if inhibitory signals from cardiovascular receptors are blocked by NOR in the LPBN. Male Holtzman rats with stainless steel guide-cannulas implanted in the lateral ventricle and bilaterally in the LPBN were used. Bilateral injections of NOR (80nmol/0.2µl) into the LPBN decreased water intake (0.8±0.3, vs. saline (SAL): 2.9±0.3ml/180min) induced by pilocarpine (1mg/kg of body weight) i.p., without changing 1.8% NaCl intake (0.8±2.4, vs. SAL: 0.5±0.3ml/180min). Prazosin (1mg/kg of body weight) i.p. blocked pressor responses and increased water and 1.8% NaCl intake (6.3±1.7 and 14.7±3.5ml/180min, respectively) in rats treated with pilocarpine combined with NOR into the LPBN. Prazosin i.p. also increased 1.8% NaCl intake in rats treated with carbachol i.c.v combined with NOR into the LPBN. The results suggest that different signals inhibit sodium intake in rats treated with cholinergic agonists, among them those produced by increases of arterial pressure that are not efficiently deactivated by NOR acting in the LPBN.

Role of α2-adrenoceptors in the lateral parabrachial nucleus in the control of body fluid homeostasis.

Central α2-adrenoceptors and the pontine lateral parabrachial nucleus (LPBN) are involved in the control of sodium and water intake. Bilateral injections of moxonidine (α2-adrenergic/imidazoline receptor agonist) or noradrenaline into the LPBN strongly increases 0.3 M NaCl intake induced by a combined treatment of furosemide plus captopril. Injection of moxonidine into the LPBN also increases hypertonic NaCl and water intake and reduces oxytocin secretion, urinary sodium, and water excreted by cell-dehydrated rats, causing a positive sodium and water balance, which suggests that moxonidine injected into the LPBN deactivates mechanisms that restrain body fluid volume expansion. Pretreatment with specific α2-adrenoceptor antagonists injected into the LPBN abolishes the behavioral and renal effects of moxonidine or noradrenaline injected into the same area, suggesting that these effects depend on activation of LPBN α2-adrenoceptors. In fluid-depleted rats, the palatability of sodium is reduced by ingestion of hypertonic NaCl, limiting intake. However, in rats treated with moxonidine injected into the LPBN, the NaCl palatability remains high, even after ingestion of significant amounts of 0.3 M NaCl. The changes in behavioral and renal responses produced by activation of α2-adrenoceptors in the LPBN are probably a consequence of reduction of oxytocin secretion and blockade of inhibitory signals that affect sodium palatability. In this review, a model is proposed to show how activation of α2-adrenoceptors in the LPBN may affect palatability and, consequently, ingestion of sodium as well as renal sodium excretion.

Role of α2-adrenoceptors in the lateral parabrachial nucleus in the control of body fluid homeostasis

Central α2-adrenoceptors and the pontine lateral parabrachial nucleus (LPBN) are involved in the control of sodium and water intake. Bilateral injections of moxonidine (α2-adrenergic/imidazoline receptor agonist) or noradrenaline into the LPBN strongly increases 0.3 M NaCl intake induced by a combined treatment of furosemide plus captopril. Injection of moxonidine into the LPBN also increases hypertonic NaCl and water intake and reduces oxytocin secretion, urinary sodium, and water excreted by cell-dehydrated rats, causing a positive sodium and water balance, which suggests that moxonidine injected into the LPBN deactivates mechanisms that restrain body fluid volume expansion. Pretreatment with specific α2-adrenoceptor antagonists injected into the LPBN abolishes the behavioral and renal effects of moxonidine or noradrenaline injected into the same area, suggesting that these effects depend on activation of LPBN α2-adrenoceptors. In fluid-depleted rats, the palatability of sodium is reduced by ingestion of hypertonic NaCl, limiting intake. However, in rats treated with moxonidine injected into the LPBN, the NaCl palatability remains high, even after ingestion of significant amounts of 0.3 M NaCl. The changes in behavioral and renal responses produced by activation of α2-adrenoceptors in the LPBN are probably a consequence of reduction of oxytocin secretion and blockade of inhibitory signals that affect sodium palatability. In this review, a model is proposed to show how activation of α2-adrenoceptors in the LPBN may affect palatability and, consequently, ingestion of sodium as well as renal sodium excretion.

Angiotensinergic and cholinergic receptors of the subfornical organ mediate sodium intake induced by GABAergic activation of the lateral parabrachial nucleus.

Bilateral injections of the GABA(A) agonist muscimol into the lateral parabrachial nucleus (LPBN) induce 0.3 M NaCl and water intake in satiated and normovolemic rats, a response reduced by intracerebroventricular (icv) administration of losartan or atropine (angiotensinergic type 1 (AT1) and cholinergic muscarinic receptor antagonists, respectively). In the present study, we investigated the effects of the injections of losartan or atropine into the subfornical organ (SFO) on 0.3M NaCl and water intake induced by injections of muscimol into the LPBN. In addition, using intracellular calcium measurement, we also tested the sensitivity of SFO-cultured cells to angiotensin II (ANG II) and carbachol (cholinergic agonist). In male Holtzman rats with cannulas implanted bilaterally into the LPBN and into the SFO, injections of losartan (1 µg/0.1 µl) or atropine (2 nmol/0.1 µl) into the SFO almost abolished 0.3M NaCl and water intake induced by muscimol (0.5 nmol/0.2 µl) injected into the LPBN. In about 30% of the cultured cells of the SFO, carbachol and ANG II increased intracellular calcium concentration ([Ca²âº](i)). Three distinct cell populations were found in the SFO, i.e., cells activated by either ANG II (25%) or carbachol (2.6%) or by both stimuli (2.3%). The results suggest that the activation of angiotensinergic and cholinergic mechanisms in the SFO is important for NaCl and water intake induced by the deactivation of LPBN inhibitory mechanisms with muscimol injections. They also show that there are cells in the SFO activated by both angiotensinergic and cholinergic stimuli, perhaps those involved in the responses to muscimol into the LPBN.

Facilitation of sodium intake by combining noradrenaline into the lateral parabrachial nucleus with prazosin peripherally.

Injections of noradrenaline into the lateral parabrachial nucleus (LPBN) increase arterial pressure and 1.8% NaCl intake and decrease water intake in rats treated with the diuretic furosemide (FURO) combined with a low dose of the angiotensin converting enzyme inhibitor captopril (CAP). In the present study, we investigated the influence of the pressor response elicited by noradrenaline injected into the LPBN on FURO+CAP-induced water and 1.8% NaCl intake. Male Holtzman rats with bilateral stainless steel guide-cannulas implanted into LPBN were used. Bilateral injections of noradrenaline (40 nmol/0.2 µl) into the LPBN increased FURO+CAP-induced 1.8% NaCl intake (12.2±3.5, vs., saline: 4.2±0.8 ml/180 min), reduced water intake and strongly increased arterial pressure (50±7, vs. saline: 1±1 mmHg). The blockade of the α1 adrenoceptors with the prazosin injected intraperitoneally abolished the pressor response and increased 1.8% NaCl and water intake in rats treated with FURO+CAP combined with noradrenaline injected into the LPBN. The deactivation of baro and perhaps volume receptors due to the cardiovascular effects of prazosin is a mechanism that may facilitate water and NaCl intake in rats treated with FURO+CAP combined with noradrenaline injected into the LPBN. Therefore, the activation of α2 adrenoceptors with noradrenaline injected into the LPBN, at least in dose tested, may not completely remove the inhibitory signals produced by the activation of the cardiovascular receptors, particularly the signals that result from the extra activation of these receptors with the increase of arterial pressure.

Lesion of the commissural nucleus of the solitary tract/A2 noradrenergic neurons facilitates the activation of angiotensinergic mechanisms in response to hemorrhage.

In the present study, we investigated the effects of lesions of A2 neurons of the commissural nucleus of the solitary tract (cNTS) alone or combined with the blockade of angiotensinergic mechanisms on the recovery of arterial pressure (AP) to hemorrhage in conscious rats. Male Holtzman rats (280-320g) received an injection of anti-dopamine-beta-hydroxylase-saporin (12.6ng/60nl; cNTS/A2-lesion, n=28) or immunoglobulin G (IgG)-saporin (12.6ng/60nl, sham, n=24) into the cNTS and 15-21days later had a stainless steel cannula implanted in the lateral ventricle. After 6days, rats were submitted to hemorrhage (four blood withdrawals, 2ml/300g of body weight every 10min). Both cNTS/A2-lesioned and sham rats had similar hypotension to hemorrhage (-62±7 and -73±7mmHg, respectively), however cNTS/A2-lesioned rats rapidly recovered from hypotension (-5±3mmHg at 30min), whereas sham rats did not completely recover until the end of the recording (-20±3mmHg at 60min). Losartan (angiotensin type 1 receptor antagonist) injected intracerebroventricularly (100µg/1µl) or intravenously (i.v.) (10mg/kg of body weight) impaired the recovery of AP in cNTS/A2-lesioned rats (-24±6 and -35±7mmHg at 30min, respectively). In sham rats, only i.v. losartan affected the recovery of AP (-39±6mmHg at 60min). The results suggest that lesion of the A2 neurons in the cNTS facilitates the activation of the angiotensinergic pressor mechanisms in response to hemorrhage.

Importance of central AT1 receptors for sodium intake induced by GABAergic activation of the lateral parabrachial nucleus.

The blockade of the inhibitory mechanisms for sodium intake with GABAergic activation in the lateral parabrachial nucleus (LPBN) induces strong ingestion of water and hypertonic NaCl in satiated and normovolemic rats. A question that remains is if the activity of facilitatory mechanisms, like angiotensin II, is necessary for sodium and water intake induced by muscimol (GABA(A) receptor agonist) injected into the LPBN. Therefore, in the present study, we investigated the effects of the blockade of angiotensinergic AT(1) receptors with losartan injected i.c.v. on 0.3 M NaCl and water intake induced by muscimol injected into the LPBN in satiated and normovolemic rats. Male Holtzman rats with stainless steel cannulas implanted bilaterally into the LPBN and unilaterally into the lateral ventricle were used. Bilateral injections of muscimol (0.5 nmol/0.2 µl) into the LPBN combined with i.c.v. injection of vehicle induced 0.3 M NaCl (31.7 ± 1.8 ml/240 min, vs. saline: 0.4 ± 0.3 ml/240 min) and water intake (21.5 ± 1.9 ml/240 min, vs. saline: 0.8 ± 0.2 ml/240 min). Losartan (50 and 100 µg/1.0 µl) injected i.c.v. reduced the effects of LPBN-muscimol on 0.3 M NaCl (18.9 ± 1.9 and 9.9 ± 1.7 ml/240 min, respectively) and water intake (9.8 ± 1.7 and 5.1 ± 1.1 ml/240 min, respectively). The results suggest that the activation of central AT(1) angiotensinergic receptors is essential for hypertonic NaCl and water intake induced by the blockade of the inhibitory mechanisms with muscimol injected into the LPBN in satiated and normovolemic rats.

Inhibition of central angiotensin II-induced pressor responses by hydrogen peroxide.

Hydrogen peroxide (H(2)O(2)), important reactive oxygen species produced endogenously, may have different physiological actions. The superoxide anion (O(2)(·-)) is suggested to be part of the signaling mechanisms activated by angiotensin II (ANG II) and central virus-mediated overexpression of the enzyme superoxide dismutase (that dismutates O(2)(·-) to H(2)O(2)) reduces pressor and dipsogenic responses to central ANG II. Whether this result might reflect elevation of H(2)O(2) rather than depletion of O(2)(·-) has not been addressed. Here we investigated the effects of H(2)O(2) injected intracerebroventricularly (i.c.v.) or ATZ (3-amino-1,2,4-triazole, a catalase inhibitor) injected intravenously (i.v.) or i.c.v. on the pressor responses induced by i.c.v. injections of ANG II. Normotensive male Holtzman rats (280-320 g, n=5-13/group) with stainless steel cannulas implanted in the lateral ventricle were used. Prior injection of H(2)O(2) (5 µmol/1 µl) or ATZ (5 nmol/1 µl) i.c.v. almost abolished the pressor responses induced by ANG II (50 ng/1 µl) also injected i.c.v. (7 ± 3 and 5 ± 3 mm Hg, respectively, vs. control: 19 ± 4 mm Hg). Injection of ATZ (3.6 mmol/kg b.wt.) i.v. also reduced central ANG II-induced pressor responses. Injections of H(2)O(2) i.c.v. and ATZ i.c.v. or i.v. alone produced no effect on baseline arterial pressure. Central ANG II, H(2)O(2) or ATZ did not affect heart rate. The results show that central injections of H(2)O(2) and central or peripheral injections of ATZ reduced the pressor responses induced by i.c.v. ANG II, suggesting that exogenous or endogenous H(2)O(2) may inhibit central pressor mechanisms activated by ANG II.

Lateral parabrachial nucleus and central amygdala in the control of sodium intake.

The lateral parabrachial nucleus (LPBN) and the central nucleus of the amygdala (CeA) are important areas for the control of sodium appetite. In the present study we investigated the effects of bilateral lesions of the CeA on the facilitation of water and 0.3 M NaCl intake produced by the blockade of serotonergic mechanisms or activation of alpha(2)-adrenoceptors with bilateral injections of methysergide or moxonidine, respectively, into the LPBN. Male Holtzman rats (n=5-8) with bilateral sham or electrolytic lesions of the CeA (2 mA; 10 s) and stainless steel cannulas implanted bilaterally in the LPBN were used. In sham rats treated with the diuretic furosemide (10 mg/kg b.w.) combined with the angiotensin converting enzyme inhibitor captopril (5 mg/kg b.w) subcutaneously, bilateral injections of moxonidine (0.5 nmol) or methysergide (4 microg) into the LPBN increased 0.3 M NaCl intake (29.8+/-5.1 and 19.5+/-3.7 ml/2 h, respectively, versus vehicle: 8.3+/-1.4 ml/2 h) and water intake (17.9+/-3.7 and 23.3+/-2.8 ml/2 h, respectively, versus vehicle: 11.5+/-1.6 ml/2 h). Lesions of the CeA (5-18 days) abolished the increase in 0.3 M NaCl and water intake produced by bilateral injections of moxonidine (10.3+/-2.8 and 6.8+/-2.3 ml/2 h, respectively) and reduced the increase produced by methysergide (13.6+/-2.5 and 14.5+/-3.2 ml/2 h, respectively) into the LPBN. The present results show that the increase in water and 0.3 M NaCl intake produced by serotonergic blockade and alpha(2)-adrenergic activation in the LPBN depends on the integrity of the CeA, suggesting that facilitatory mechanisms present in the CeA are essential for the increase of water and hypertonic NaCl intake produced by the blockade of the inhibitory mechanisms of the LPBN.

Adrenergic mechanisms of the Kölliker-Fuse/A7 area on the control of water and sodium intake.

The blockade of serotoninergic receptors with methysergide or the activation of alpha(2)-adrenoceptors with moxonidine into the lateral parabrachial nucleus (LPBN) increases water and 0.3 M NaCl intake in rats treated with furosemide (FURO) combined with captopril (CAP). In the present study we investigated the effects of bilateral injections of noradrenaline (the endogenous neurotransmitter for alpha-adrenoceptors) alone or combined with the alpha(2)-adrenoceptor antagonist RX 821002 into the LPBN or into the rostral portion of the Kölliker-Fuse nucleus that includes also the A7 area (KF/A7 area) on FURO+CAP-induced water and 0.3 M NaCl intake. Male Holtzman rats with bilateral stainless steel guide-cannulas implanted into KF/A7 area or LPBN were used. FURO+CAP-induced 0.3 M NaCl intake strongly increased after bilateral injections of noradrenaline (80 or 160 nmol/0.2 microl) into LPBN (26.5+/-5.9 and 20.7+/-2.0 ml/2 h versus saline: 4.4+/-0.9 ml/2 h) or into the KF/A7 area (31.5+/-6.1 and 25.9+/-4.7 ml/2 h versus saline: 7.2+/-1.6 ml/2 h). Water intake increased with noradrenaline injected in KF/A7 area, however, this treatment reduced 0.06 M sucrose intake, suggesting that the increase of water and NaCl intake is not related to non-specific effect. Bilateral injections of RX 821002 (160 nmol/0.2 microl) into LPBN or KF/A7 area abolished the effects of noradrenaline (160 nmol/0.2 microl) in the same areas on 0.3 M NaCl intake (7.5+/-2.5 and 9.8+/-4.4 ml/2 h, respectively). Moxonidine (0.5 nmol/0.2 microl) injected bilaterally into the KF/A7 area increased 0.3 M NaCl intake (39.5+/-6.3 ml/3 h) and water intake, while methysergide (4 microg/0.2 microl) into the KF/A7 area did not alter 0.3 M NaCl or water intake. The results suggest that alpha(2)-adrenoceptor activation is a common mechanism in the KF/A7 area and LPBN to facilitate sodium intake. However, the serotonergic mechanism is present in LPBN, not in the KF/A7 area.

Antidipsogenic effects of central adenosine-5'-triphosphate.

Besides other physiological functions, adenosine-5'-triphosphate (ATP) is also a neurotransmitter that acts on purinergic receptors. In spite of the presence of purinergic receptors in forebrain areas involved with fluid-electrolyte balance, the effect of ATP on water intake has not been investigated. Therefore, we studied the effects of intracerebroventricular (icv) injections of ATP (100, 200 and 300 nmol/microL) alone or combined with DPCPX or PPADS (P1 and P2 purinergic antagonists, respectively, 25 nmol/microL) on water intake induced by water deprivation. In addition, the effect of icv ATP was also tested on water intake induced by intragastric load of 12% NaCl (2 mL/rat), acute treatment with the diuretic/natriuretic furosemide (20 mg/kg), icv angiotensin II (50 ng/microL) or icv carbachol (a cholinergic agonist, 4 nmol/microL), on sodium depletion-induced 1.8% NaCl intake, and on food intake induced by food deprivation. Male Holtzman rats (280-320 g, N = 7-11) had cannulas implanted into the lateral ventricle. Icv ATP (300 nmol/microL) reduced water intake induced by water deprivation (13.1 +/- 1.9 vs saline: 19.0 +/- 1.4 mL/2 h; P < 0.05), an effect blocked by pre-treatment with PPADS, but not DPCPX. Icv ATP also reduced water intake induced by NaCl intragastric load (5.6 +/- 0.9 vs saline: 10.3 +/- 1.4 mL/2 h; P < 0.05), acute furosemide treatment (0.5 +/- 0.2 vs saline: 2.3 +/- 0.6 mL/15 min; P < 0.05), and icv angiotensin II (2.2 +/- 0.8 vs saline: 10.4 +/- 2.0 mL/2 h; P < 0.05), without changing icv carbachol-induced water intake, sodium depletion-induced 1.8% NaCl intake and food deprivation-induced food intake. These data suggest that central ATP, acting on purinergic P2 receptors, reduces water intake induced by intracellular and extracellular dehydration.

Water deprivation and the double- depletion hypothesis: common neural mechanisms underlie thirst and salt appetite.

Water deprivation-induced thirst is explained by the double-depletion hypothesis, which predicts that dehydration of the two major body fluid compartments, the extracellular and intracellular compartments, activates signals that combine centrally to induce water intake. However, sodium appetite is also elicited by water deprivation. In this brief review, we stress the importance of the water-depletion and partial extracellular fluid-repletion protocol which permits the distinction between sodium appetite and thirst. Consistent enhancement or a de novo production of sodium intake induced by deactivation of inhibitory nuclei (e.g., lateral parabrachial nucleus) or hormones (oxytocin, atrial natriuretic peptide), in water-deprived, extracellular-dehydrated or, contrary to tradition, intracellular-dehydrated rats, suggests that sodium appetite and thirst share more mechanisms than previously thought. Water deprivation has physiological and health effects in humans that might be related to the salt craving shown by our species.

Glutamatergic neurotransmission modulates hypoxia-induced hyperventilation but not anapyrexia.

The interaction between pulmonary ventilation (V E) and body temperature (Tb) is essential for O2 delivery to match metabolic rate under varying states of metabolic demand. Hypoxia causes hyperventilation and anapyrexia (a regulated drop in Tb), but the neurotransmitters responsible for this interaction are not well known. Since L-glutamate is released centrally in response to peripheral chemoreceptor stimulation and glutamatergic receptors are spread in the central nervous system we tested the hypothesis that central L-glutamate mediates the ventilatory and thermal responses to hypoxia. We measured V E and Tb in 40 adult male Wistar rats (270 to 300 g) before and after intracerebroventricular injection of kynurenic acid (KYN, an ionotropic glutamatergic receptor antagonist), alpha-methyl-4-carboxyphenylglycine (MCPG, a metabotropic glutamatergic receptor antagonist) or vehicle (saline), followed by a 1-h period of hypoxia (7% inspired O2) or normoxia (humidified room air). Under normoxia, KYN (N = 5) or MCPG (N = 8) treatment did not affect V E or Tb compared to saline (N = 6). KYN and MCPG injection caused a decrease in hypoxia-induced hyperventilation (595 +/- 49 for KYN, N = 7 and 525 +/- 84 ml kg-1 min-1 for MCPG, N = 6; P < 0.05) but did not affect anapyrexia (35.3 +/- 0.2 for KYN and 34.7 +/- 0.4 masculine C for MCPG) compared to saline (912 +/- 110 ml kg-1 min-1 and 34.8 +/- 0.2 masculine C, N = 8). We conclude that glutamatergic receptors are involved in hypoxic hyperventilation but do not affect anapyrexia, indicating that L-glutamate is not a common mediator of this interaction.

Antagonism of adenosine A(1) receptors in the NTS does not affect the chemoreflex in awake rats.

The possible involvement of adenosine A(1) receptors in neurotransmission of the sympathoexcitatory component of the chemoreflex in the nucleus tractus solitarii (NTS) of awake rats was evaluated. Unilateral microinjection of increasing doses of adenosine (0.01, 0.06, 0.12, 1.25, 2.5, and 5.0 nmol/50 nl) into the lateral aspect of the commissural NTS produced a long-lasting increase in baseline mean arterial pressure (MAP) and no changes in baseline heart rate (HR). Microinjection of adenosine at 1.25 nmol/50 nl (ED(50)) into the NTS (n = 9) produced a significant increase in baseline MAP (119 +/- 3, 122 +/- 4, and 117 +/- 4 mmHg at 30 s, 1 min, and 2 min, respectively) compared with control (102 +/- 3 mmHg) but no significant changes after previous microinjection of 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), an adenosine A(1) receptor antagonist (107 +/- 3, 107 +/- 3, and 106 +/- 3 mmHg at 30 s, 1 min, and 2 min, respectively) compared with control (102 +/- 3 mmHg). Microinjection of adenosine before and after DPCPX into the same site of the lateral commissural NTS produced no changes in baseline HR. In another group of rats (n = 8), microinjection of DPCPX (0.285 nmol/50 nl) into lateral and midline aspects of the commissural NTS produced no significant changes in pressor (+46 +/- 4 vs. +47 +/- 2 mmHg) or bradycardic responses (-216 +/- 9 vs. -226 +/- 12 beats/min) to chemoreflex activation with intravenous potassium cyanide compared with control responses. These data show that microinjection of adenosine into the NTS produced a small and long-lasting pressor response by activating A(1) receptors and that blockade of these receptors produced no changes in cardiovascular responses to chemoreflex activation. We conclude that adenosine A(1) receptors are not involved in processing of the chemoreflex afferents at the NTS level.

Changes in regional vascular resistance in response to microinjection of L-glutamate into different antero-posterior coordinates of the RVLM in awake rats.

Changes in mean arterial pressure (MAP) and in regional vascular resistance (RVR, hindquarter, mesenteric and renal) induced by microinjection of L-glutamate into three different antero-posterior coordinates of the rostral ventrolateral medulla (RVLM) [1,200-1,600 (microm (n=10), 1,601-2,000 microm (n=12) and 2,001-2,500 microm (n=6) rostral to the obex] were investigated in unanesthetized rats. Guide cannulas directed towards the RVLM were implanted 4 days prior to the experiments. Doppler probes were implanted around the superior mesenteric, inferior abdominal aorta and left renal arteries and a catheter was inserted into the femoral artery and vein 1 day prior to the experiments. Insertion of the injector into the RVLM produced an increase in baseline MAP, which was back to control levels 2 min later, when L-glutamate was microinjected. Microinjection of L-glutamate (1 nmol/30 nl) into the three antero-posterior coordinates of the RVLM produced an increase in MAP associated with a similar increase in hindquarter, mesenteric and renal vascular resistance, which were back to control 1 min later. Saline into the RVLM produced negligible effects on MAP and RVR. These findings suggest that the sympathetic vasomotor neurons involved in the regulation of the regional vascular resistance in rats are not topographically distributed in the antero-posterior coordinates of the RVLM. However, the experimental methods used to evaluate the topographic distribution of sympatho-vasomotor neurons in the RVLM and the measurement of the regional blood flow may not be precise enough to detect any possible differences.

Changes in regional vascular resistance in response to microinjection of L-glutamate into different antero-posterior coordinates of the RVLM in awake rats.

Changes in mean arterial pressure (MAP) and in regional vascular resistance (RVR, hindquarter, mesenteric and renal) induced by microinjection of L.-glutamate into three different antero-posterior coordinates of the rostral ventrolateral medulla (RVLM) [1,200-1,600 microm (n=10), 1,601-2,000 microm (n=12) and 2,001-2,500 microm (n=6) rostral to the obex] were investigated in unanesthetized rats. Guide cannulas directed towards the RVLM were implanted 4 days prior to the experiments. Doppler probes were implanted around the superior mesenteric, inferior abdominal aorta and left renal arteries and a catheter was inserted into the femoral artery and vein 1 day prior to the experiments. Insertion of the injector into the RVLM produced an increase in baseline MAP, which was back to control levels 2 min later, when L-glutamate was microinjected. Microinjection of L-glutamate (1 nmol/30 nl) into the three antero-posterior coordinates of the RVLM produced an increase in MAP associated with a similar increase in hindquarter, mesenteric and renal vascular resistance, which were back to control 1 min later. Saline into the RVLM produced negligible effects on MAP and RVR. These findings suggest that the sympathetic vasomotor neurons involved in the regulation of the regional vascular resistance in rats are not topographically distributed in the antero-posterior coordinates of the RVLM. However, the experimental methods used to evaluate the topographic distribution of sympatho-vasomotor neurons in the RVLM and the measurement of the regional blood flow may not be precise enough to detect any possible differences.

Hemodynamic responses to electrical stimulation of the aortic depressor nerve in awake rats.

Changes in mean arterial pressure (MAP), heart rate (HR), and vascular resistance (hindquarter and mesenteric territories) in response to electrical stimulation (ES) of the aortic depressor nerve (ADN) were evaluated in conscious freely moving rats. Platinum electrodes were implanted into the ADN of all rats studied, and some of these animals were also implanted with miniaturized Doppler probes around the superior mesenteric artery and inferior abdominal aorta (hindquarter). In both groups, the femoral artery and vein were catheterized one day before the experiments. In the first group of rats (n = 7), the control ES of the ADN in the range from 0.5 to 3.0 V (50 Hz, 10 ms) produced bradycardia and hypotension in an intensity-dependent manner, and treatment with methylatropine (intravenously) blocked the bradycardia but produced no significant changes in the hypotensive response. In a second group (n = 6), ES of the ADN was performed with the intensity fixed at 3 V and the frequency of the stimuli varying from 10 to 50 Hz. In this group, the hypotensive response was frequency dependent, whereas the bradycardic response was not. In a third group of rats (n = 6), ES of the ADN (2.5 V) produced hypotension (-35 +/- 4 mmHg), minor changes in the mesenteric (+5 +/- 14%), and vasodilation in hindquarter (-32 +/- 6%) vascular beds. The data show that 1) ES of the ADN produces a fall in pressure, bradycardia, vasodilation in the hindquarter, and no changes in the mesenteric vascular resistance, 2) methylatropine blocked the bradycardia and produced no effect on the hypotensive response to ES of the ADN, and 3) the baroreceptor afferent fibers involved in the hypotensive response to ES of ADN are sensitive to the variation of the frequency of the stimuli, whereas the fibers involved in the bradycardic response are not.

Central alpha-adrenergic agonists and need-induced 3% NaCl and water intake.

In the present study, noradrenaline (NOR, alpha-non-specific adrenergic agonist), clonidine (CLO, alpha 2), phenylephrine (PHE, alpha 1) or isoproterenol (ISO, beta-agonist) was injected in the medial septal area (MSA) of water-deprived, sodium-deplete or food-deprived rats. NOR (80, 160 nmol) inhibited the intake of 3% NaCl, water deprivation-induced and meal-associated water intake. Food deprivation-induced food intake and 10% sucrose intake were not altered by NOR. CLO (10, 20, 30, 40 nmol) inhibited (80-100% inhibition compared to control during 60 min) the intake of 3% NaCl, water deprivation-induced and meal-associated water intake. CLO had a weaker inhibition on food and 10% sucrose intake (30-50% less than the control during 60 and 15 min, respectively). PHE (160 nmol) inhibited 3% NaCl intake and 10% sucrose intake (30% less than the control for 15-30 min). ISO (160 nmol) did not alter water or 3% NaCl intake. NOR induced an increase, CLO and ISO induced a decrease, and PHE no alteration in mean arterial pressure. NOR did not alter water or 3% NaCl intake when injected unilaterally into the caudate nucleus. The results suggest that NOR injected in the MSA acts on alpha 2-adrenergic receptors inducing a specific inhibition of 3% NaCl and water intake.

Receptor-mediated effects of clonidine on need-induced 3% NaCl and water intake.

Clonidine combined with adrenergic antagonists were injected in the medial septal area in order to characterize the type of receptors involved with its inhibitory effect on 3% NaCl and water intake of sodium-depleted (furosemide + 24 h of removal of ambient sodium) and 30-h water-deprived rats, respectively. The inhibitory effect of clonidine (20 nmol) on need-induced water intake was reduced 50% by an 80-nmol dose of either idazoxan, yohimbine or prazosin. The inhibitory effect of clonidine (30 nmol) on need-induced 3% NaCl intake was completely antagonized by idazoxan (80, 160 nmol), not altered by yohimbine (40-160 nmol), and partially potentiated (40 nmol) or inhibited (160 nmol) by prazosin. Propranolol did not alter the effects of clonidine on either water (80 nmol) or 3% NaCl (40-160 nmol) intake. The results suggest that the inhibitory effects of clonidine on 3% NaCl and water intake are mediated by different types of alpha2-adrenergic receptors.

Effects of central alpha-adrenergic agonists on hormone-induced 3% NaCl and water intake.

Male rats received intracerebroventricular (ICV) renin (600 ng) or daily subcutaneous injections of deoxycorticosterone (5 mg) to induce 3% NaCl and water intake. Noradrenaline (NOR; 40-160 nmol) and clonidine (CLO; 5-20 nmol) injected ICV induced 70 to 100% inhibition of the intakes. Phenylephrine (PHE; 40-160 nmol) injected ICV induced 60 to 95% inhibition of the intakes. NOR and PHE induced a stronger inhibition on the 3% NaCl intake induced by renin than on the intake induced by deoxycorticosterone (DOC), and CLO did the opposite. CLO was always more effective than PHE to induce inhibition of the intakes. The results suggest that NOR inhibits hormone (angiotensin II, aldosterone)-induced NaCl intake by acting mainly on alpha 2-adrenergic receptors.