Angiotensin and baroreflex control of the circulation

There is a close association between the location of angiotensin (Ang) receptors and many important brain nuclei involved in the regulation of the cardiovascular system. The present review encompasses the physiological role of Ang II in the brainstem, particularly in relation to its influence on baroreflex control of the heart and kidney. Activation of AT1 receptors in the brainstem by fourth ventricle (4V) administration to conscious rabbits or local administration of Ang II into the rostral ventrolateral medulla (RVLM) of anesthetized rabbits acutely increases renal sympathetic nerve activity (RSNA) and RSNA baroreflex responses. Administration of the Ang antagonist Sarile into the RVLM of anesthetized rabbits blocked the effects of Ang II on the RSNA baroreflex, indicating that the RVLM is the major site of sympathoexcitatory action of Ang II given into the cerebrospinal fluid surrounding the brainstem. However, in conscious animals, blockade of endogenous Ang receptors in the brainstem by the 4V AT1 receptor antagonist losartan resulted in sympathoexcitation, suggesting an overall greater activity of endogenous Ang II within the sympathoinhibitory pathways. However, the RSNA response to airjet stress in conscious rabbits was markedly attenuated. While we found no effect of acute central Ang on heart rate baroreflexes, chronic 4V infusion inhibited the baroreflex and chronic losartan increased baroreflex gain. Thus, brainstem Ang II acutely alters sympathetic responses to specific afferent inputs thus forming part of a potentially important mechanism for the integration of autonomic response patterns. The sympathoexcitatory AT1 receptors appear to be activated during stress, surgery and anesthesia.

Dissociation between the circulating renin-angiotensin system and angiotensin II receptors in central losartan-induced hypertension

Losartan, an AT1 angiotensin II (ANG II) receptor non-peptide antagonist, induces an increase in mean arterial pressure (MAP) when injected intracerebroventricularly (icv) into rats. The present study investigated possible effector mechanisms of the increase in MAP induced by icv losartan in unanesthetized rats. Male Holtzman rats (280-300 g, N = 6/group) with a cannula implanted into the anterior ventral third ventricle received an icv injection of losartan (90 µg/2 µl) that induced a typical peak pressor response within 5 min. In one group of animals, this response to icv losartan was completely reduced from 18 ± 1 to 4 ± 2 mmHg by intravenous (iv) injection of losartan (2.5-10 mg/kg), and in another group, it was partially reduced from 18 ± 3 to 11 ± 2 mmHg by iv prazosin (0.1-1.0 mg/kg), an alpha1-adrenergic antagonist (P<0.05). Captopril (10 mg/kg), a converting enzyme inhibitor, injected iv in a third group inhibited the pressor response to icv losartan from 24 ± 3 to 7 ± 2 mmHg (P<0.05). Propranolol (10 mg/kg), a ß-adrenoceptor antagonist, injected iv in a fourth group did not alter the pressor response to icv losartan. Plasma renin activity and serum angiotensin-converting enzyme activity were not altered by icv losartan in other animals. The results suggest that the pressor effect of icv losartan depends on angiotensinergic and alpha1-adrenoceptor activation, but not on increased circulating ANG II.

Angiotensin-(1-7) improves the post-ischemic function in isolated perfused rat hearts

We evaluated the effects of angiotensin-(1-7) (Ang-(1-7)) on post-ischemic function in isolated hearts from adult male Wistar rats perfused according to the Langendorff technique. Local ischemia was induced by coronary ligation for 15 min. After ischemia, hearts were reperfused for 30 min. Addition of angiotensin II (Ang II) (0.20 nM, N = 10) or Ang-(1-7) (0.22 nM, N = 10) to the Krebs-Ringer perfusion solution (KRS) before the occlusion did not modify diastolic or systolic tension, heart rate or coronary flow (basal values for Ang-(1-7)-treated hearts: 0.72 ± 0.08 g, 10.50 ± 0.66 g, 216 ± 9 bpm, 5.78 ± 0.60 ml/min, respectively). During the period of occlusion, the coronary flow, heart rate and systolic tension decreased (values for Ang-(1-7)-treated hearts: 2.83 ± 0.24 ml/min, 186 ± 7 bpm, 6.95 ± 0.45 g, respectively). During reperfusion a further decrease in systolic tension was observed in control (4.95 ± 0.60 g) and Ang II-treated hearts (4.35 ± 0.62 g). However, in isolated hearts perfused with KRS containing Ang-(1-7) the further reduction of systolic tension during the reperfusion period was prevented (7.37 ± 0.68 g). The effect of Ang-(1-7) on the systolic tension was blocked by the selective Ang-(1-7) antagonist A-779 (2 nM, N = 9), by the bradykinin B2 antagonist HOE 140 (100 nM, N = 10), and by indomethacin pretreatment (5 mg/kg, ip, N = 8). Pretreatment with L-NAME (30 mg/kg, ip, N = 8) did not change the effect of Ang-(1-7) on systolic tension (6.85 ± 0.61 g). These results show that Ang-(1-7) at low concentration (0.22 nM) improves myocardial function (systolic tension) in ischemia/reperfusion through a receptor-mediated mechanism involving release of bradykinin and prostaglandins.

Effects of caffeine on learning and memory in rats tested in the Morris water maze

We studied some of the characteristics of the improving effect of the non-specific adenosine receptor antagonist, caffeine, using an animal model of learning and memory. Groups of 12 adult male Wistar rats receiving caffeine (0.3-30 mg/kg, ip, in 0.1 ml/100 g body weight) administered 30 min before training, immediately after training, or 30 min before the test session were tested in the spatial version of the Morris water maze task. Post-training administration of caffeine improved memory retention at the doses of 0.3-10 mg/kg (the rats swam up to 600 cm less to find the platform in the test session, P<=0.05) but not at the dose of 30 mg/kg. Pre-test caffeine administration also caused a small increase in memory retrieval (the escape path of the rats was up to 500 cm shorter, P<=0.05). In contrast, pre-training caffeine administration did not alter the performance of the animals either in the training or in the test session. These data provide evidence that caffeine improves memory retention but not memory acquisition, explaining some discrepancies among reports in the literature.

Excitatory amino acid receptor blockade within the caudal pressor area and rostral ventrolateral medulla alters cardiovascular responses to nucleus raphe obscurus stimulation in rats

Pressor responses elicited by stimulation of the nucleus raphe obscurus (NRO) depend on the integrity of the rostral ventrolateral medulla (RVLM). Therefore, to test the participation of excitatory amino acid (EAA) receptors in the cardiovascular responses evoked by NRO stimulation (1 ms, 100 Hz, 40-70 µA, for 10 s), the EAA antagonist kynurenic acid (Kyn) was microinjected at different sites in the ventrolateral medullar surface (2.7 nmol/200 nl) of male Wistar rats (270-320 g, N = 39) and NRO stimulation was repeated. The effects of NRO stimulation were: hypertension (deltaMAP = +43 ± 1 mmHg, P<0.01), bradycardia (deltaHR = -30 ± 7 bpm, P<0.01) and apnea. Bilateral microinjection of Kyn into the RVLM, which did not change baseline parameters, almost abolished the bradycardia induced by NRO stimulation (deltaHR = -61 ± 3 before vs -2 ± 3 bpm after Kyn, P<0.01, N = 7). Unilateral microinjection of Kyn into the CVLM did not change baseline parameters or reduce the pressor response to NRO stimulation (deltaMAP = +46 ± 5 before vs +48 ± 5 mmHg after Kyn, N = 6). Kyn bilaterally microinjected into the caudal pressor area reduced blood pressure and heart rate and almost abolished the pressor response to NRO stimulation (deltaMAP = +46 ± 4 mmHg before vs +4 ± 2 mmHg after Kyn, P<0.01, N = 7). These results indicate that EAA receptors on the medullary ventrolateral surface play a role in the modulation of the cardiovascular responses induced by NRO stimulation, and also suggest that the RVLM participates in the modulation of heart rate responses and that the caudal pressor area modulates the pressor response following NRO stimulation.

14-3-3 proteins in apoptosis

The once obscure members of the 14-3-3 protein family play significant roles in the determination of cell fate. By inhibiting the pro-apoptotic BAD (Bcl-2-antagonist of cell death) and the transcription factor FKHRL-1, 14-3-3 displays important anti-apoptotic characteristics. To date, five points of interaction of 14-3-3 with the apoptotic machinery have been identified. How these interactions are regulated still remains a mystery.

Effect of intracerebroventricularly injected insulin on urinary sodium excretion by cerebroventricular streptozotocin-treated rats

Recent evidence suggests that insulin may influence many brain functions. It is known that intracerebroventricular (icv) injection of nondiabetogenic doses of streptozotocin (STZ) can damage insulin receptor signal transduction. In the present study, we examined the functional damage to the brain insulin receptors on central mechanisms regulating glomerular filtration rate and urinary sodium excretion, over four periods of 30 min, in response to 3 µl insulin or 0.15 NaCl (vehicle) injected icv in STZ-treated freely moving Wistar-Hannover rats (250-300 g). The icv cannula site was visually confirmed by 2% Evans blue infusion. Centrally administered insulin (42.0 ng/µl) increased the urinary output of sodium (from 855.6 ± 85.1 to 2055 ± 310.6 delta%/min; N = 11) and potassium (from 460.4 ± 100 to 669 ± 60.8 delta%/min; N = 11). The urinary sodium excretion response to icv insulin microinjection was markedly attenuated by previous central STZ (100 µg/3 µl) administration (from 628 ± 45.8 to 617 ± 87.6 delta%/min; N = 5) or by icv injection of a dopamine antagonist, haloperidol (4 µg/3 µl) (from 498 ± 39.4 to 517 ± 73.2 delta%/min; N = 5). Additionally, insulin-induced natriuresis occurred by increased post-proximal tubule sodium rejection, despite an unchanged glomerular filtration rate. Excluding the possibility of a direct action of STZ on central insulin receptor-carrying neurons, the current data suggest that the insulin-sensitive response may be processed through dopaminergic D1 receptors containing neuronal pathways.

Melatonin and N-acetyl-serotonin cross the red blood cell membrane and evoke calcium mobilization in malarial parasites

The duration of the intraerythrocytic cycle of Plasmodium is a key factor in the pathogenicity of this parasite. The simultaneous attack of the host red blood cells by the parasites depends on the synchronicity of their development. Unraveling the signals at the basis of this synchronicity represents a challenging biological question and may be very important to develop alternative strategies for therapeutic approaches. Recently, we reported that the synchrony of Plasmodium is modulated by melatonin, a host hormone that is synthesized only during the dark phases. Here we report that N-acetyl-serotonin, a melatonin precursor, also releases Ca2+ from isolated P. chabaudi parasites at micro- and nanomolar concentrations and that the release is blocked by 250 mM luzindole, an antagonist of melatonin receptors, and 20 mM U73122, a phospholipase C inhibitor. On the basis of confocal microscopy, we also report the ability of 0.1 µM melatonin and 0.1 µM N-acetyl-serotonin to cross the red blood cell membrane and to mobilize intracellular calcium in parasites previously loaded with the fluorescent calcium indicator Fluo-3 AM. The present data represent a step forward into the understanding of the signal transduction process in the host-parasite relationship by supporting the idea that the host hormone melatonin and N-acetyl-serotonin generate IP3 and therefore mobilize intracellular Ca2+ in Plasmodium inside red blood cells.

Effect of naloxone on food competition aggression in food-restricted high and low aggression pigeons (Columba livia)

We determined the effect of the opiate receptor antagonist naloxone on aggression, emotion, feeder control, and eating behavior in high and low aggression female pigeons maintained at 80% of their normal weight and exposed to food competition interactions. Pigeons were divided into pairs by previously ranked high aggression (total time spent in offensive aggression exceeding 60 s/5 min; N = 6 pairs) and low aggression females (time spent in offensive aggression less than 10 s/5 min; N = 6 pairs). A pigeon in each pair received an sc dose of naloxone (1 mg kg-1 ml saline-1) and the other animal received the vehicle. Trials (10 min) were performed 30 min after the naloxone/vehicle administration. The naloxone group of high aggression pigeons showed lower scores of total time spent in offensive aggression (control: 98.6 ± 12.0; naloxone: 46.8 ± 6.6 s; P < 0.05) and higher scores of time spent in emotional responses (control: 3.5 ± 0.6; naloxone: 10.8 ± 2.4 s; P < 0.05) than controls. The other behaviors scored, feeder control and eating behavior, were not affected in this group. The naloxone group of low aggression pigeons, however, showed higher scores of offensive aggression than their controls (5.3 ± 1.3; naloxone: 28.7 ± 8.0 s; P < 0.05). The present results suggest that opiate receptor mechanisms are implicated in offensive aggression responses in high and low aggression pigeons. However, as reported for brain 5-hydroxytryptamine manipulation and GABA-A-benzodiazepine receptor manipulation, the effect of the opiate receptor antagonist on food competition aggression in pigeons was related to their pretreatment level of aggression.

The alga Bryothamnion seaforthii contains carbohydrates with antinociceptive activity

Bryothamnion seaforthii, a red alga common to the Northeastern coast of Brazil, was used to prepare the protein fraction F0/60 by ammonium sulfate precipitation. The chromatography of F0/60 on DEAE-Sephadel column resulted in two lectin fractions, PI and PII, which have antinociceptive properties in rodents. We determined the antinociceptive activity of the PII fraction and of a carbohydrate-containing fraction (CF) in mice. The CF was prepared from the dried algae, after digestion with 100 mM sodium acetate, pH 6.0, containing 5 mM cysteine, EDTA and 0.4% papain, at 60ºC. A 10% cetylpyridinium chloride was added to the filtrate, and the precipitate was dissolved with 2 M NaCl:ethanol (100:15, v/v) followed by the carbohydrate precipitation with ethanol. The final precipitate, in acetone, was dried at 25ºC. The PII fraction markedly inhibited acetic acid-induced abdominal writhing after ip administration (control: 27.1 ± 2.20; PII 0.1 mg/kg: 5.5 ± 1.85; 1 mg/kg: 1.6 ± 0.72 writhes/20 min) and after oral administration (control: 32.0 ± 3.32; PII 0.1 mg/kg: 13.1 ± 2.50; 1 mg/kg: 9.4 ± 3.96 writhes/20 min). PII was also effective against both phases of pain induced by 1% formalin (control, ip: 48.2 ± 2.40 and 27.7 ± 2.56 s; PII: 1 mg/kg, ip: 34.3 ± 5.13 and 5.6 ± 2.14 s; control, po: 44.5 ± 3.52 and 25.6 ± 2.39 s; PII 5 mg/kg, po: 26.5 ± 4.67 and 15.3 ± 3.54 s for the 1st and 2nd phases, respectively) and in the hot-plate test. The CF (ip) also displayed significant antinociceptive properties in all tests but at higher doses (1 and 5 mg/kg, ip and po). Thus, CF at the dose of 5 mg/kg significantly inhibited writhes (ip: 7.1 ± 2.47 and po: 14.5 ± 2.40 writhes/20 min) as well as the 1st (po: 19.6 ± 1.74 s) and 2nd (po: 7.1 ± 2.24 s) phases of the formalin test compared to controls ip and po. The antinociceptive effects of both the PII and CF in the formalin and hot-plate tests were prevented at least partially by pretreatment with the opioid receptor antagonist naloxone (2 mg/kg, sc). Moreover, both fractions retained antinociceptive activity in the acetic acid-induced writhing test following heating, a procedure which abolished the hemagglutinating activity of the fraction, presumably due to lectins also present. Finally, both fractions also prolonged the barbiturate-induced sleeping time. These results indicate that carbohydrate molecules present in the PII (26.8% carbohydrate) and CF (21% of the alga dried weight) obtained from B. seaforthii display pronounced antinociceptive activity which is resistant to heat denaturation and is mediated by an opioid mechanism, as indicated by naloxone inhibition.

Hemodynamic and hormonal actions of adrenomedullin

Adrenomedullin, a 52-amino acid residue peptide, has numerous biological actions which are of potential importance to cardiovascular homeostasis, growth and development of cardiovascular tissues and bone, prevention of infection, and regulation of body fluid and electrolyte balance. Studies in man using intravenous infusion of the peptide have demonstrated that, at plasma levels detected after myocardial infarction or in heart failure, adrenomedullin reduces arterial pressure, increases heart rate and cardiac output, and activates the sympathetic and renin-angiotensin systems but suppresses aldosterone. The thresholds for these responses differ, being lower under some experimental circumstances for arterial pressure than for the other biological effects. Adrenomedullin administration inhibits the pressor and aldosterone-stimulating action of angiotensin II in man. By contrast, the pressor effect of norepinephrine is little altered by concomitant adrenomedullin administration. Although in the absence of a safe, specific antagonist of the actions of endogenous adrenomedullin it is difficult to be certain about the physiological and pathophysiological importance of this peptide in man, current evidence suggests that it serves to protect against cardiovascular overload and injury. Hope has been expressed that adrenomedullin or an agonist specific for adrenomedullin receptors might find a place in the treatment of cardiovascular disorders.

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 ± 0.4ºC for MCPG) compared to saline (912 ± 110 ml kg-1 min-1 and 34.8 ± 0.2ºC, 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.

Influence of melatonin on the development of functional nicotinic acetylcholine receptors in cultured chick retinal cells

The influence of melatonin on the developmental pattern of functional nicotinic acetylcholine receptors was investigated in embryonic 8-day-old chick retinal cells in culture. The functional response to acetylcholine was measured in cultured retina cells by microphysiometry. The maximal functional response to acetylcholine increased 2.7 times between the 4th and 5th day in vitro (DIV4, DIV5), while the Bmax value for [125I]-alpha-bungarotoxin was reduced. Despite the presence of alpha8-like immunoreactivity at DIV4, functional responses mediated by alpha-bungarotoxin-sensitive nicotinic acetylcholine receptors were observed only at DIV5. Mecamylamine (100 µM) was essentially without effect at DIV4 and DIV5, while dihydro-ß-erythroidine (10-100 µM) blocked the response to acetylcholine (3.0 nM-2.0 µM) only at DIV4, with no effect at DIV5. Inhibition of melatonin receptors with the antagonist luzindole, or melatonin synthesis by stimulation of D4 dopamine receptors blocked the appearance of the alpha-bungarotoxin-sensitive response at DIV5. Therefore, alpha-bungarotoxin-sensitive receptors were expressed in retinal cells as early as at DIV4, but they reacted to acetylcholine only after DIV5. The development of an alpha-bungarotoxin-sensitive response is dependent on the production of melatonin by the retinal culture. Melatonin, which is produced in a tonic manner by this culture, and is a key hormone in the temporal organization of vertebrates, also potentiates responses mediated by alpha-bungarotoxin-sensitive receptors in rat vas deferens and cerebellum. This common pattern of action on different cell models that express alpha-bungarotoxin-sensitive receptors probably reflects a more general mechanism of regulation of these receptors.

Activation of neural cholecystokinin-1 receptors induces relaxation of the isolated rat duodenum which is reduced by nitric oxide synthase inhibitors

Cholecystokinin (CCK) influences gastrointestinal motility, by acting on central and peripheral receptors. The aim of the present study was to determine whether CCK has any effect on isolated duodenum longitudinal muscle activity and to characterize the mechanisms involved. Isolated segments of the rat proximal duodenum were mounted for the recording of isometric contractions of longitudinal muscle in the presence of atropine and guanethidine. CCK-8S (EC50: 39; 95% CI: 4.1-152 nM) and cerulein (EC50: 58; 95% CI: 18-281 nM) induced a concentration-dependent and tetrodotoxin-sensitive relaxation. Nomeganitro-L-arginine (L-NOARG) reduced CCK-8S- and cerulein-induced relaxation (IC50: 5.2; 95% CI: 2.5-18 µM) in a concentration-dependent manner. The magnitude of 300 nM CCK-8S-induced relaxation was reduced by 100 µM L-NOARG from 73 ± 5.1 to 19 ± 3.5% in an L-arginine but not D-arginine preventable manner. The CCK-1 receptor antagonists proglumide, lorglumide and devazepide, but not the CCK-2 receptor antagonist L-365,260, antagonized CCK-8S-induced relaxation in a concentration-dependent manner. These findings suggest that CCK-8S and cerulein activate intrinsic nitrergic nerves acting on CCK-1 receptors in order to cause relaxation of the rat duodenum longitudinal muscle.

Prazosin blocks the glutamatergic effects of N-methyl-D-aspartic acid on lordosis behavior and luteinizing hormone secretion in the estrogen-primed female rat

We have observed that intracerebroventricular (icv) injection of selective N-methyl-D-aspartic acid (NMDA)-type glutamatergic receptor antagonists inhibits lordosis in ovariectomized (OVX), estrogen-primed rats receiving progesterone or luteinizing hormone-releasing hormone (LHRH). When NMDA was injected into OVX estrogen-primed rats, it induced a significant increase in lordosis. The interaction between LHRH and glutamate was previously explored by us and another groups. The noradrenergic systems have a functional role in the regulation of LHRH release. The purpose of the present study was to explore the interaction between glutamatergic and noradrenergic transmission. The action of prazosin, an alpha1- and alpha2b-noradrenergic antagonist, was studied here by injecting it icv (1.75 and 3.5 µg/6 µL) prior to NMDA administration (1 µg/2 µL) in OVX estrogen-primed Sprague-Dawley rats (240-270 g). Rats manually restrained were injected over a period of 2 min, and tested 1.5 h later. The enhancing effect induced by NMDA on the lordosis/mount ratio at high doses (67.06 ± 3.28, N = 28) when compared to saline controls (6 and 2 µL, 16.59 ± 3.20, N = 27) was abolished by prazosin administration (17.04 ± 5.52, N = 17, and 9.33 ± 3.21, N = 20, P < 0.001 for both doses). Plasma LH levels decreased significantly only with the higher dose of prazosin (1.99 ± 0.24 ng/mL, N = 18, compared to saline-NMDA effect, 5.96 ± 2.01 ng/mL, N = 13, P < 0.05). Behavioral effects seem to be more sensitive to the alpha-blockade than hormonal effects. These findings strongly suggest that the facilitatory effects of NMDA on both lordosis and LH secretion in this model are mediated by alpha-noradrenergic transmission.