Stimulation of carotid body chemoreceptors by saline saturated with 100% CO2 elicited an increase in mean arterial pressure, respiratory rate, tidal volume, and minute ventilation (VE). Microinjections of L-glutamate into a midline area 0.5-0.75 mm caudal and 0.3-0.5 mm deep with respect to the calamus scriptorius increased VE. Histological examination showed that the site was located in the commissural nucleus of the nucleus tractus solitarii (NTS). The presence of excitatory amino acid receptors [N-methyl-D-aspartic acid (NMDA); kainate, quisqualate/alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) and trans 1-amino-cyclopentane-trans-1,3-dicarboxylic acid (ACPD)] in this area was demonstrated by microinjections of appropriate agonists. Simultaneous blockade of NMDA and non-NMDA receptors by combined injections of DL-2-aminophosphonoheptanoate (AP-7; 1 nmol) and 6,7-dinitro-quinoxaline-2,3-dione (DNQX; 1 nmol) abolished the responses to stimulation of carotid body on either side. Combined injections of AP-7 and DNQX did not produce a nonspecific depression of neurons because the responses to another agonist, carbachol, remained unaltered. Inhibition of the neurons in the aforementioned area with microinjections of muscimol (which hyperpolarizes neuronal cell bodies but not fibers of passage) also abolished the responses to subsequent carotid body stimulation on either side.(ABSTRACT TRUNCATED AT 250 WORDS)

Carotid and aortic chemoreceptor function was studied in normal Wistar rats. Sodium cyanide, lobeline HCl, and doxapram HCl in the doses of 2-400 mug/kg injected into the external carotid artery stimulated respiration significantly. Injections of the drugs into the ascending aorta produced less effects which were abolished after section of the carotid sinus nerves. The drugs produced a significant increase in the carotid sinus nerve activity but failed to do so in the aortic depressor or recurrent laryngeal nerves. These results indicate that the carotid chemoreceptor function in Wistar rats in normal while functional aortic chemoreceptors are absent in them.

The mechanism of cardiovascular responses to chemical stimulation of the hypothalamic arcuate nucleus (ARCN) was studied in urethane-anesthetized adult male Wistar rats. At the baseline mean arterial pressure (BLMAP) close to normal, ARCN stimulation elicited decreases in MAP and sympathetic nerve activity (SNA). The decreases in MAP elicited by ARCN stimulation were attenuated by either gamma-aminobutyric acid (GABA), neuropeptide Y (NPY), or beta-endorphin receptor blockade in the ipsilateral hypothalamic paraventricular nucleus (PVN). Combined blockade of GABA-A, NPY1 and opioid receptors in the ipsilateral PVN converted the decreases in MAP and SNA to increases in these variables. Conversion of inhibitory effects on the MAP and SNA to excitatory effects following ARCN stimulation was also observed when the BLMAP was decreased to below normal levels by an infusion of sodium nitroprusside. The pressor and tachycardic responses to ARCN stimulation at below normal BLMAP were attenuated by blockade of melanocortin 3/4 (MC3/4) receptors in the ipsilateral PVN. Unilateral blockade of GABA-A receptors in the ARCN increased the BLMAP and heart rate (HR) revealing tonic inhibition of the excitatory neurons in the ARCN. ARCN stimulation elicited tachycardia regardless of the level of BLMAP. ARCN neurons projecting to the PVN were immunoreactive for glutamic acid decarboxylase 67 (GAD67), NPY, and beta-endorphin. These results indicated that: 1) at normal BLMAP, decreases in MAP and SNA induced by ARCN stimulation were mediated via GABA-A, NPY1 and opioid receptors in the PVN, 2) lowering of BLMAP converted decreases in MAP following ARCN stimulation to increases in MAP, and 3) at below normal BLMAP, increases in MAP and HR induced by ARCN stimulation were mediated via MC3/4 receptors in the PVN. These results provide a base for future studies to explore the role of ARCN in cardiovascular diseases.

Abstract-Experiments were carried out in urethane-anesthetized, artificially ventilated, adult male Wistar rats. Microinjections (50 nL) of N-methyl-D-aspartic acid (1, 5, and 10 mmol/L), but not artificial cerebrospinal fluid, into the hypothalamic arcuate nucleus (ARCN) elicited increases in mean arterial pressure (5.7Ϯ0.5, 13.2Ϯ1.4, and 17.3Ϯ1.1 mm Hg, respectively) and heart rate (24.3Ϯ4. Key Words: blood pressure Ⅲ heart rate Ⅲ intrathecal injection Ⅲ microinjection Ⅲ N-methyl-D-aspartic acid Ⅲ sympathetic nerve activity T he hypothalamic arcuate nucleus (ARCN) is located bilaterally at the base of the third ventricle. Direct projections from the ARCN to the intermediolateral cell column of the spinal cord (IML), rostral ventrolateral medullary pressor area (RVLM), nucleus tractus solitarius, dorsal motor nucleus of the vagus, parabrachial nucleus, raphe nuclei, periaqueductal gray, and hypothalamic paraventricular nucleus (PVN) have been identified. 1-3 These reports suggest that the ARCN may be involved in the central regulation of cardiovascular function. There are very few studies in which the ARCN has been stimulated chemically to evaluate its role in cardiovascular or sympathetic nerve regulation. 4 -6 Therefore, a systematic study was carried out to investigate the cardiovascular effects of ARCN stimulation. Methods General ProceduresExperiments were done in adult male Wistar rats (Charles River Laboratories, Wilmington, MA) weighing 300 to 360 g (nϭ91). All of the animals were housed under controlled conditions with a 12:12-hour light-dark cycle. Food and water were available to the animals ad libitum. The experiments were performed according to the National Institutes of Health Guide for the Care and Use of Laboratory Animals and with the approval of the institutional animal care and use committee of the university.The general procedures have been described in detail elsewhere. 7 Briefly, the rats were anesthetized with urethane (1.2 to 1.4 g/kg; injected IV in divided doses). The absence of a pressor response and/or withdrawal of the limb in response to pinching of a hind paw indicated that the rats were properly anesthetized. The rats were artificially ventilated, and end-tidal CO 2 was maintained at 3.5% to 4.5%. Rectal temperature was maintained at 37.0Ϯ0.5°C. Blood pressure and heart rate (HR) were recorded by standard techniques. VagotomySilk sutures were placed loosely around the vagus nerves bilaterally for subsequent identification and sectioning of the nerves. Microinjections Into the ARCNDetails of microinjection procedure are mentioned elsewhere. 7 The coordinates for the ARCN were as follows: 3.6 to 3.8 mm caudal to the bregma, 0.1 to 0.2 mm lateral to the midline, and 9.8 to 10.2 mm deep from the dura. The same coordinates for the ARCN were used

1. The importance of the medullo-spinal neuronal pools in the regulation of cardiovascular function has been known for a long time. However, important groups of these neurons, interconnections between them and the neurotransmitters released at their projections have been identified with certainty only during the past two decades. 2. Some of the medullo-spinal neuronal pools mediating cardiovascular function include the nucleus tractus solitarius, caudal ventrolateral medullary depressor area, rostral ventrolateral medullary pressor area, nucleus ambiguus and intermediolateral cell column of the thoracolumbar spinal cord. Interactions between these selected neuronal groups and neurotransmitters in the pathways connecting them are discussed in the present short review.

Experiments were done in urethane-anesthetized, barodenervated, male Wistar rats. Chemical stimulation of the hypothalamic paraventricular nucleus (PVN) by unilateral microinjections of Nmethyl-D-aspartic acid (NMDA) elicited increases in mean arterial pressure (MAP) and greater splanchnic nerve activity (GSNA). The increases in the MAP and GSNA induced by chemical stimulation of the PVN were significantly exaggerated by bilateral microinjections of D-AP7 and NBQX (ionotropic glutamate receptor antagonists) into the medial subnucleus of the nucleus tractus solitarius (mNTS). These results were confirmed by single unit recordings; i.e., excitation of mNTS barosensitive neurons caused by chemical stimulation of the ipsilateral PVN was blocked by application of D-AP7 and NBQX to these neurons. Bilateral microinjections of D-AP7 and NBQX into the mNTS elicited pressor responses which were significantly attenuated by inhibition of PVN neurons by bilateral microinjections of muscimol. Unilateral microinjections of fluorogold into the mNTS resulted in bilateral retrograde labeling of the PVN neurons. Unilateral microinjections of biotinylated dextran amine into the PVN resulted in anterograde labeling of axons and terminals in the mNTS bilaterally and the labeled terminals exhibited vesicular glutamate transporter-2 immunoreactivity. These results indicated that; 1) a tonically active glutamatergic bilateral projection from the PVN to the mNTS exists, 2) bilateral blockade of ionotropic glutamate receptors in the mNTS exaggerates the increases in MAP and GSNA to the chemical stimulation of the PVN, and 3) this projection may serve as a restraint mechanism for excitatory cardiovascular effects of PVN stimulation.

SUMMARY The role of central nervous system in the resetting of baroreflex was investigated in 5-month-old spontaneously hypertensive rats (SHR) of Okamoto strain. Age-matched Wistar-Kyoto (WKY) rats were used as normotensive controls. The aortic nerves, which in the rat, contain few or no chemoreceptor fibers, were stimulated electrically using a wide range of stimulus frequencies. The depressor responses (expressed as percent decrease in blood pressure as compared to its blood pressure value prior to aortic nerve stimulation) produced by these stimulations were significantly smaller in SHR than those in WKY. In another series of experiments, changes in the efferent limb of the baroreflex arc (i.e., greater splanchnic nerve activity) in response to stimulation of the baroreceptor afferents in the aortic nerve were recorded. Inhibition of the greater splanchnic nerve activity due to aortic nerve stimulation was found to be significantly smaller in SHR than in the WKY. Control sympathetic nerve activity was greater in SHR than in WKY. These results suggest that the central bulbospinal nervous system may be another site for resetting of baroreflex in hypertension. (Hypertension 5:346-352, 1983 Methods Forty male, 5-month old, (SHR) of Okamoto strain and 35 age-matched Wistar-Kyoto (WKY) rats were obtained from Taconic Farms (New York) and maintained under standard laboratory conditions for 1 week prior to experimentation. During this time, blood pressures were monitored periodically by tail-cuff method 19 to verify the normotensive or hypertensive status of the animals. Blood Pressure Responses in Decerebrate RatsThese experiments were carried out in 15 SHR and 10 WKY. The details of decerebration procedure have been described elsewhere. 20 Briefly, external and internal carotid arteries were ligated under halothane anesthesia. The animal was then fixed in a stereotaxic instrument, and bilateral parietal craniotomy was performed. The cortex was aspirated to visualize the colliculi, and a midcollicular transection was performed. The cranial cavity was then carefully packed with oxidized cellulose. Anesthesia was discontinued, and the animal was fixed in a supine position. Body temperature was monitored and maintained at 37°C by a temperature controller (Bailey Instruments, BAT8). These preparations were stable for at least 10 to 12 hours, and their cardiovascular and respiratory parameters were comparable to those of conscious restrained animals. 20Throughout the duration of the experiment, the animals urinated spontaneously periodically and no distension of the bladder was observed. 346

Arakawa H, Chitravanshi VC, Sapru HN. The hypothalamic arcuate nucleus: a new site of cardiovascular action of angiotensin-(1-12) and angiotensin II. Am J Physiol Heart Circ Physiol 300: H951-H960, 2011. First published December 24, 2010 doi:10.1152/ajpheart.01144.2010.-The hypothalamic arcuate nucleus (ARCN) has been reported to play a significant role in cardiovascular regulation. It has been hypothesized that the ARCN may be one of the sites of cardiovascular actions of angiotensins (ANGs). Experiments were carried out in urethaneanesthetized, artificially ventilated, adult male Wistar rats. The ARCN was identified by microinjections of N-methyl-D-aspartic acid (NMDA; 10 mM). Microinjections (50 nl) of ANG-(1-12) (1 mM) into the ARCN elicited increases in mean arterial pressure (MAP), heart rate (HR), and greater splanchnic nerve activity (GSNA). The tachycardic responses to ANG-(1-12) were attenuated by bilateral vagotomy. The cardiovascular responses elicited by ANG-(1-12) were attenuated by microinjections of ANG II type 1 receptor (AT1R) antagonists but not ANG type 2 receptor (AT2R) antagonist. Combined inhibition of ANG-converting enzyme (ACE) and chymase in the ARCN abolished ANG-(1-12)-induced responses. Microinjections of ANG II (1 mM) into the ARCN also increased MAP and HR. Inhibition of ARCN by microinjections of muscimol (1 mM) attenuated the pressor and tachycardic responses to intravenously administered ANG-(1-12) and ANG II (300 pmol/kg each). These results indicated that 1) microinjections of ANG-(1-12) into the ARCN elicited increases in MAP, HR, and GSNA; 2) HR responses were mediated via both sympathetic and vagus nerves; 3) AT1Rs, but not AT2Rs, in the ARCN mediated ANG-(1-12)-induced responses; 4) both ACE and chymase were needed to convert ANG-(1-12) to ANG II in the ARCN; and 5) ARCN plays a role in mediating the cardiovascular responses to circulating ANGs. blood pressure; heart rate; microinjection; N-methyl-D-aspartic acid; sympathetic nerve activity THE HYPOTHALAMIC ARCUATE NUCLEUS (ARCN) may play a significant role in cardiovascular regulation (10, 38). Consistent with this notion, we (31) have recently reported that chemical stimulation of the ARCN elicited increases in mean arterial pressure (MAP), heart rate (HR), and sympathetic nerve activity (SNA). These reports have provided a basis for investigations on different neurotransmitters and neuromodulators in the ARCN that may play a role in the regulation of cardiovascular function in normal and pathological states.A new endogenous angiotensin (ANG), ANG-(1-12), has recently been identified (30,44). Intravenous administration of this peptide has been reported to elicit an immediate pressor response in the rat, and this effect was blocked by prior administration of an ANG-converting enzyme (ACE) inhibitor or an ANG II type 1 receptor (AT 1 R) antagonist (30). These data indicated that in the periphery, ANG-(1-12) exerts its actions via a rapid conversion to ANG II (30). High concentrations of ANG-(1-12) have been reported in th...