Microinjections of melanin concentrating hormone into the nucleus tractus solitarius of the rat elicit depressor and bradycardic responses.

The presence of melanin-concentrating hormone (MCH) containing processes, projecting from the lateral hypothalamus to the medial nucleus tractus solitarius (mNTS), has been reported in the rat. It was hypothesized that MCH acting within the mNTS may modulate the central regulation of cardiovascular function. This hypothesis was tested in urethane-anesthetized, artificially ventilated, adult male Wistar rats. Microinjections (100 nl) of MCH (0.25, 0.5, 0.75, and 1 mM) into the mNTS of anesthetized rats elicited decreases in mean arterial pressure (20.4+/-1.6, 50.7+/-3.3, 35.7+/-2.8 and 30.0+/-2.6 mm Hg, respectively). The decreases in heart rate in response to these concentrations of MCH were 40.0+/-8.7, 90.0+/-13.0, 48.0+/-7.3 and 48.0+/-8.0 beats/min, respectively. Maximum cardiovascular responses were elicited by a 0.5 mM concentration of MCH. Cardiovascular responses to MCH were similar in unanesthetized mid-collicular decerebrate rats. Control microinjections of normal saline (100 nl) did not elicit any cardiovascular response. Ipsilateral or bilateral vagotomy significantly attenuated MCH-induced bradycardia. Prior microinjections of PMC-3881-PI (2 mM; MCH-1 receptor antagonist) into the mNTS blocked the cardiovascular responses to microinjections of MCH. Microinjection of MCH (0.5 mM) into the mNTS decreased efferent greater splanchnic nerve activity. Direct application of MCH (0.5 mM; 4 nl) to barosensitive nucleus tractus solitarius (NTS) neurons increased their firing rate. These results indicate that: 1) MCH microinjections into the mNTS activate MCH-1 receptors and excite barosensitive NTS neurons, causing a decrease in efferent sympathetic activity and blood pressure, and 2) MCH-induced bradycardia is mediated via the activation of the vagus nerves.

Cardiovascular actions of adrenocorticotropin microinjections into the nucleus tractus solitarius of the rat.

The presence of adrenocorticotropin (ACTH) containing cells and melanocortin (MC) receptors has been reported in the nucleus tractus solitarius (NTS) of the rat. The importance of the NTS in the regulation of cardiovascular function is also well established. Based on these reports, it was hypothesized that ACTH acting within the NTS may modulate the central regulation of cardiovascular function. To test this hypothesis, cardiovascular effects of ACTH in the NTS were investigated in intact urethane-anesthetized and unanesthetized decerebrate, artificially ventilated, adult male Wistar rats. Microinjections of ACTH (0, 0.5, 1, 2, and 4 mM) into the medial subnucleus of NTS (mNTS) elicited decreases in mean arterial pressure (MAP; 0+/-0, 24.4+/-3.5, 35.7+/-4.3, 44.5+/-5.8 and 53.7+/-5.6 mm Hg, respectively) and heart rate (HR; 0+/-0, 25.7+/-5.3, 35.5+/-6.4, 47.5+/-12.1 and 55.0+/-5.6 beats/min, respectively). The onset and duration of the responses to microinjections of ACTH (0.5-4 mM) were 5-10 s and 45-120 s, respectively. Control microinjections of artificial cerebrospinal fluid (aCSF) did not elicit any response. The volume of all microinjections was 100 nl. The concentrations of ACTH that elicited depressor and bradycardic responses when microinjected into the mNTS (e.g. 1 or 2 mM, 100 nl), did not elicit a response when injected i.v. (n=5) or i.c.v. (n=2) indicating that there was no leakage of the drug from the injection site in the mNTS. Microinjections of MC3/4 receptor antagonists (acetyl-[Nle(4), Asp(5), d-2-Nal(7), Lys(10)]-cyclo-alpha-MSH amide, fragments 4-10 (SHU9119) and agouti-related protein (83-132) amide) into the mNTS blocked the responses to ACTH. Microinjections of ACTH (2 mM) into the mNTS decreased efferent greater splanchnic nerve activity. Bilateral vagotomy significantly attenuated ACTH-induced bradycardia. These results indicated that: 1) microinjections of ACTH into the mNTS elicited depressor and bradycardic responses, 2) these responses were mediated via MC3/4 receptors, 3) the depressor effects were mediated via a decrease in the activity of the sympathetic nervous system, and 4) the bradycardic responses were vagally mediated.

Central resetting of baroreflex in the spontaneously hypertensive rat.

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.

Peripheral versus central cardiorespiratory effects of morphine.

The administration of morphine sulfate (MS, 2 mg/kg) into the right atrium in decerebrate rats, produced a dramatic bradycardia, apnea, and a slight transient biphasic blood pressure response within 1 sec. The bradycardia was slowly restored to control levels within 8-10 min and was prevented by pretreatment with atropine. Apnea (7.3 +/- 2.2 sec) was followed by a period of rapid shallow breathing. Acute tolerance to these effects developed to subsequent doses of morphine. In paralyzed artificially ventilated animals, morphine produced: (1) cessation of phrenic nerve (PN) activity which was followed by bursts of shortened duration; and (2) concomitant excitation of the recurrent laryngeal nerve (RLN), this activity exhibited a continuous discharge which was asynchronous with that of phrenic nerve. These actions of morphine were qualitatively similar to those observed with phenyldiguanide (PDG) and were found to be reflexogenic, i.e. emanating from the lungs via stimulation of pulmonary C-fibers. Bilaterally vagotomized animals only responded to morphine with respiratory depression, characterized by a severe decrease in respiratory rate and minute ventilation. On the other hand, animals with intact vagi showed stimulation of rate and minute ventilation following a brief period of apnea. They were less sensitive than vagotomized animals to the depressant action during the first 8 min after administration of morphine. All the effects of morphine were blocked by pretreatment with naloxone (50-400 micrograms/kg, i.v.). These results indicate that the initial cardiorespiratory effects of morphine are due to a peripheral reflex action arising from the stimulation of opiate receptors associated with pulmonary C-fibers, e.g. J-fibers.

Hypertensive response following stimulation of opiate receptors in the caudal ventrolateral medulla.

In urethane-anesthetized rats, vasodepressor neuron pools were located bilaterally in and adjacent to the A1 area of the ventrolateral medulla by injecting the neuroexcitatory amino acid, L-glutamate. Ventrolateral vasodepressor areas included the caudalateral part of the nucleus reticularis gigantocellularis, the rostrolateral part of the nucleus reticularis ventralis, and the dorsal nucleus reticularis lateralis. In the ventrolateral vasodepressor areas L-glutamate elicited a transient fall in blood pressure (BP) and heart rate (HR). The opiate agonist (D-ala2-met5)-enkephalinamide (DAME) was used to stimulate opiate receptors in vasodepressor sites, identified with L-glutamate. In these sites, bilateral injections (0.1 microliter/site) of DAME caused a dose-related (2.5-500.0 ng) increase in blood pressure and heart rate, as well as exaggeration of the response to occlusion of the carotid. The effects of DAME on blood pressure were completely abolished by alpha-adrenergic blockade (phentolamine, 2 mg/kg, i.v.) and all effects of DAME were reversed by the administration of naloxone HCl (1 mg/kg, i.v.). Naloxone reversal was accompanied by an unexpected "rebound" hypertension. Saline had no significant effects when injected, or administered intravenously, in the absence or presence of DAME. It was concluded that stimulation of opiate receptors in the ventrolateral vasodepressor areas activated sympathetic outflow. An enkephalinergic system in this area of the brain stem may serve to modulate blood pressure, heart rate and cardiovascular reflexes.

Cardiovascular response to injections of enkephalin in the pressor area of the ventrolateral medulla.

The cardiovascular effects of the injection of an enkephalin analogue, [D-ala2-met5]enkephalinamide (DAME) into the pressor area of the rostral ventrolateral medulla were studied in urethane-anesthetized and decerebrate rats. The excitatory amino acid L-glutamate was used to identify the ventrolateral medulla. The pressor responses to L-glutamate were elicited from an area that included the nucleus reticularis gigantocellularis, the medial aspect of the nucleus reticularis parvocellularis and the dorsal portion of the nucleus reticularis lateralis. Injection (0.1 microliter volume) of DAME (2.5-500.0 ng/site) into the ventrolateral medulla elicited a dose-related decrease in arterial blood pressure and heart rate and attenuated the carotid occlusion response (COR). Control injections (0.1-0.2 microliter vol) of saline into the same area failed to produce any response. The specificity of this opiate response was tested with naloxone HCl, an opiate antagonist, which prevented, as well as reversed, the action of DAME both by intravenous (i.v.) administration and by injection into the ventrolateral medulla. It was concluded that the ventrolateral medulla plays a role in the generation of vasomotor tone and that stimulation of opiate receptors in this area by an enkephalin analogue produced hypotension, bradycardia and modification of cardiovascular reflexes.

Vasopressor and depressor areas in the rat medulla. Identification by microinjection of L-glutamate.

L-Glutamate (in 0.1 microliter of 0.9% NaCl) was injected via multibarrelled glass micropipettes into the ventrolateral medulla of urethane-anesthetized rats. This area, explored stereotaxically, extended from 2.2 mm rostral (near trapezoid bodies) to 0.4 mm caudal, with respect to the obex, 2.4 mm lateral to the mid-line on each side and 2.4 mm deep from the ventral surface of the medulla. Two types of responses were elicited by injection of L-glutamate. One type was a dose-related increase in arterial pressure and heart rate; these responses were elicited from the lateral portion of nucleus reticularis gigantocellularis, the medial aspect of nucleus reticularis parvocellularis and the dorsal-lateral reticular nucleus. The second type of response was a dose-related fall in arterial pressure with no change in heart rate; this response was localized caudal to pressure areas in the caudal ventrolateral part of nucleus reticularis gigantocellularis, ventrolateral nucleus reticularis ventralis, nucleus ambiguous and the A1 region. Glutamic acid diethylester (GDEE), an antagonist of L-glutamate, blocked all the cardiovascular effects of L-glutamate. These results indicate the presence of receptors for glutamate in the pressor and depressor areas. Glutamic acid diethylester caused a fall in blood pressure when injected on its own into pressor sites suggesting the existence of a glutaminergic input to the pressor sites. Inhibition of neuronal activity in pressor sites produced by microinjection of muscimol (a potent neuroinhibitory analogue of GABA) caused a decrease in blood pressure. On the other hand, pressor responses resulted following similar inhibition in the depressor sites. These results indicate that the pressor and depressor sites identified in the ventral medulla of the rat may have an important role to play in central cardiovascular regulation.

Carotid baroreflex in the rat: role of glutamate receptors in the medial subnucleus of the solitary tract.

Experiments were done in urethane-anesthetized adult male Wistar rats to investigate the role of glutamate receptors in the medial subnucleus of the solitary tract (mNTS) in mediating the carotid sinus baroreflex responses. The carotid sinus on one side was isolated from the general circulation and perfused with a warm perfusion fluid (37 degrees C; pH 7.4) saturated with 100% oxygen. The carotid sinus was then connected to an apparatus that permitted application of pressure increments (20-100 mm Hg) to stimulate specifically baroreceptors. The mNTS ipsilateral to the isolated carotid sinus was identified by microinjections (100 nL) of L-glutamate (5 mM). The stereotaxic coordinates for mNTS were: 0.5-0.6 mm rostral to the calamus scriptorius, 0.5-0.6 mm lateral to the midline, and 0.5-0.6 mm deep from the dorsal medullary surface. Microinjections of either D(-)-2-amino-7-phosphono-heptanoic acid, which is an N-methyl-D-aspartic acid (NMDA) receptor antagonist (5 mM) or 2,3-dioxo-6-nitro-1,2,3,4-tetrahydro-benzo[f]quinoxaline-7-sulfonamide disodium (a non-NMDA receptor antagonist; 2 mM) significantly attenuated the depressor responses elicited by carotid baroreceptor stimulation. Simultaneous blockade of NMDA and non-NMDA receptors in the ipsilateral mNTS completely abolished the depressor responses to carotid baroceptor stimulation. Microinjections of either (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA; 50 mM) or (RS)-alpha-cyclopropyl-4-phosphono-phenyl-glycine (CPPG; 80 mM) did not alter baroreflex responses. AIDA blocked group I and III while CPPG blocked all three groups of metabotropic glutamate receptors (mGLURs). These results suggest that ionotropic glutamate receptors, but not mGLURs, in the mNTS mediate the reflex depressor responses to carotid baroreceptor stimulation in the rat.

Cholinergic mechanisms subserving cardiovascular function in the medulla and spinal cord.

This investigation was designed to study the role of muscarinic receptor subtypes in the cardiovascular responses elicited by microinjection of cholinergic agonists in the intermediate portion of the NTS, the VLPA and VLDA areas and the IML of the spinal cord. Microinjections of L-glutamate (1.77 nmol in 20-50 nl in 0.9% sodium chloride solution) were used to identify these areas. Bilateral microinjections (0.02-2 nmol/site) of a potent M2 muscarinic receptor agonist, CD, but not those of a relatively selective M1 receptor agonist (McNA343; 3 nmol/site), into the intermediate portion of NTS and the VLDA induced depressor and bradycardic responses. In the VLPA these agonists elicited pressor and tachycardic effects while in the IML at T1-T3 only increase in HR was observed. Previous microinjections of a selective competitive M2 receptor antagonist (AFDX-116; 0.8-1.6 nmol/site), but not those of a potent selective M1 receptor antagonist (PZ; 1.5-2.0 nmol/site), into these areas blocked the effects of CD. These results indicate that the muscarinic receptors of M2 type may play a part in the regulation of cardiovascular function in the above-mentioned cardiovascular areas in a yet unidentified manner.

Spinal cord lactate concentration during chemical stimulation of the nucleus tractus solitarii in anesthetized rats.

This study was conducted to determine the mechanism of spinal cord blood flow (SCBF) decrease following the nucleus tractus solitarii (NTS) activation. In urethane-anesthetized, paralyzed and artificially ventilated rats, neurons in the NTS were chemically stimulated by microinjection of L-glutamate (1.7 nmol; 50 nl) and the lactate concentration, one of indicators of local neuronal metabolism, in the spinal cord was monitored in real time using an enzyme electrode. Before the chemical stimulation study, the responses of the enzyme electrode and its specificity were tested in vitro and in vivo. The electrode responded to step changes in lactate concentration and a calibration plot and regression line were obtained in vitro. The lactate concentration was significantly (P < 0.01) increased during induced apnea in vivo (n = 8). The lactate concentration in the spinal cord was not significantly changed by chemical stimulation of the NTS when arterial blood pressure (ABP) remained above the lower limit of spinal cord autoregulation (n = 21). When chemical stimulation of the NTS decreased ABP to below the lower limit of autoregulation (n = 18), the lactate concentration in the spinal cord was significantly (P < 0.01) increased. This may only be due to hypotensive effects because the lactate concentration was also significantly (P < 0.01) increased when the ABP was passively decreased below the lower limit of autoregulation by controlled hemorrhage in intact (n = 11) and sinoaortic denervated rats (n = 10). Intravenous lactate injection produced no significant increase in the current from the enzyme electrode in the spinal cord (n = 4). Using the electrode with inactivated enzyme solution, the current from the electrode did not change with the increase in lactate in the spinal cord. These findings indicate that the enzyme electrode can detect rapid changes of lactate, a product of anaerobic metabolism. These results also indicate that the spinal cord vasoconstrictor response elicited by chemical stimulation of the NTS, which was performed above the lower limit of spinal cord autoregulation in our previous study, may be due to neurogenic regulatory mechanism, but not to the secondary effects of changes in metabolism.

Blockade of cholinergic receptors in the C1 area abolishes hypertensive response to opiates in the A1 area of the ventrolateral medulla.

Opiate receptor stimulation by microinjections of a delta-receptor agonist, D-Ala2-D-Leu5-enkephalin (DADLE) into the caudal depressor (A1) area of the ventrolateral medulla produced a hypertensive response which was prevented as well as reversed by the blockade of cholinergic receptors in the rostral pressor (C1) area. These results suggest that the hypertensive responses to opiates in the A1 area are mediated via cholinergic mechanisms in the rostral C1 area of the ventrolateral medulla and acetylcholine may be the neurotransmitter released in the ventrolateral pressor area.

Cardiovascular responses to chemical stimulation of the lateral tegmental field and adjacent medullary reticular formation in the rat.

Relatively few studies have been done to characterize cardiovascular responses to the chemical stimulation of sites located in the medullary lateral tegmental field (LTF) and most of them have been carried out in anesthetized animals. Our experiments were carried out in decerebrated, artificially ventilated, adult male Wistar rats. In the LTF, two types of cardiovascular responses were elicited. One type consisted of pressor responses accompanied by bradycardia. Such responses were elicited from a region 0.4 mm caudal to 0.8 mm rostral to the calamus scriptorius (CS); maximum responses were elicited from a site 0.6 mm rostral to the CS, 1.2 mm lateral to the midline and 1.2 mm deep from the dorsal medullary surface. Another type consisted of pressor responses without any change in heart rate; such responses were elicited from a region 1-1.6 mm rostral to the CS. Nucleus ambiguus (nAmb) and dorsal motor nucleus of the vagus (nDMX) and the reticular formation surrounding these areas were the main sites from which bradycardia (accompanied by either no or small changes in BP) was elicited. In the nAmb, maximum bradycardia was elicited from a site 0.6 mm rostral to the CS, 1.8 mm lateral to the midline and 2.4 mm deep from the dorsal medullary surface. In the nDMX, most prominent bradycardic responses were elicited at 0-0.6 mm rostral to the CS, and 0.6 mm lateral to the midline and 1 mm deep from the dorsal medullary surface. Cardiovascular effects elicited from sites in other well-known areas, such as the rostral ventrolateral medullary pressor area (RVLM) and caudal ventrolateral medullary depressor area (CVLM), and the nucleus tractus solitarius (nTS) were also included for comparison of different responses. These results are expected to prove useful in studies in which the microinjection technique is used to characterize cardiovascular responses.

Microinjections of glycine into the pre-Bötzinger complex inhibit phrenic nerve activity in the rat.

Microinjections of L-glutamate were used to identify the pre-Bötzinger complex in urethane-anesthetized, immobilized, bilaterally vagotomized, artificially ventilated, adult male Wistar rats. Unilateral microinjections (20-30 nl) of L-glutamate into the pre-Bötzinger complex on either side elicited a bilateral continuous phrenic nerve discharge superimposed on which was an increase in burst-frequency. Neurokinin-1 receptor immunoreactivity in the semi-compact region of the nucleus ambiguus and the area immediately ventral to it indicated that the site of microinjections was in the general region of pre-Bötzinger complex. Unilateral microinjections of glycine into the pre-Bötzinger complex caused an inhibition of phrenic nerve activity bilaterally in a concentration-dependent manner. At lower concentrations (1 and 3 mM) phrenic nerve burst-frequency as well as burst-amplitude were decreased. At higher concentrations (6 mM), complete bilateral cessation of phrenic nerve activity was observed. The effects of glycine were prevented by a prior microinjection of strychnine (0.5 mM) into the pre-Bötzinger complex. The specificity of strychnine as an antagonist for glycine receptors was established by its lack effect on GABA(A) receptors; muscimol was used as a GABA(A) receptor agonist. Unilateral microinjections of muscimol (0.01 and 0.1 mM) into previously identified pre-Bötzinger complex also caused a bilateral decrease in phrenic nerve burst-frequency and burst-amplitude. At higher concentrations (0.3 and 1 mM) muscimol microinjections into the pre-Bötzinger elicited a complete bilateral cessation of phrenic nerve activity. The effects of muscimol were not altered by prior microinjections of strychnine (0.5 mM) at the same site. These results demonstrate pharmacologically the presence of glycine receptors in the pre-Bötzinger complex. The role of these receptors in the regulation of respiration remains to be elucidated.

NMDA as well as non-NMDA receptors mediate the neurotransmission of inspiratory drive to phrenic motoneurons in the adult rat.

The neurotransmission of bulbospinal respiratory drive is believed to involve primarily non-NMDA receptors located in the phrenic motonucleus (PMN). This conclusion is based on studies carried out mainly on in vitro brainstem-spinal cord preparations of the neonatal rat. The present study was undertaken to investigate the transmitter/receptor mechanisms in the PMN which are involved in the neurotransmission of inspiratory drive, using an in vivo adult rat model. Microinjections of glutamate, NMDA and AMPA into the PMN elicited an increase in the phrenic nerve (PN) background discharge. These injections did not alter significantly the frequency of spontaneously occurring PN bursts confirming that mechanisms responsible for respiratory rhythm reside in the supraspinal structures. Microinjections of an NMDA receptor blocker (AP-7), in concentrations that did not alter the responses to a non-NMDA receptor agonist (AMPA), reduced the PN amplitude significantly. Similarly, microinjections of a potent non-NMDA receptor blocker (NBQX), in concentrations that did not alter responses to NMDA, reduced the PN amplitude significantly. Sequential microinjections, within an interval of 5 min, of AP-7 and NBQX into the PMN, resulted in a dramatic reduction in the spontaneous PN bursts. The reduction of PN amplitude started immediately after the microinjection of AP-7 and NBQX, either alone or in combination, and reached a maximum within 5-10 min. These results indicate that, unlike in the neonatal rat, both NMDA and non-NMDA receptors located in the PMN play a significant role in the neurotransmission of the inspiratory drive in the adult rat.

Different patterns of respiratory and cardiovascular responses elicited by chemical stimulation of dorsal medulla in the rat.

Respiratory and cardiovascular responses to microinjections (10 nl) of L-glutamate (10 mM) into the dorsal medulla were studied in spontaneously breathing urethane-anesthetized, adult male Wistar rats. A total of 10 patterns of respiratory and cardiovascular responses were observed: (1) hypotension alone; (2) hypotension and bradycardia; (3) hypotension and apnea; (4) hypotension, bradycardia, and apnea; (5) apnea alone; (6) hypotension and fast and shallow breathing; (7) hypotension, bradycardia, and fast and shallow breathing; (8) fast and shallow breathing alone; (9) sighs; and (10) increase in BP and HR accompanied with fast and shallow breathing. The sites from which a combination of hypotension, bradycardia, and apnea was elicited, occupied a region in the medial subnucleus of nucleus tractus solitarius (nTS), the reticular formation just ventral to it, and the dorsal motor nucleus of vagus. The sites from which hypotension alone or a combination of hypotension and apnea were elicited occupied the margins of the medial subnucleus of nTS. The sites from which apnea alone was elicited were located in the ventrolateral part of nTS and the reticular formation just ventral to it. In the commissural subnucleus of nTS, the responses comparable to those elicited by peripheral chemoreceptor stimulation (i.e., increase in BP, HR, and respiratory rate) were located in a midline region just caudal to the calamus scriptorius, the sites from which sighs were elicited were located slightly lateral and deeper, the sites from which fast and shallow breathing were elicited were located in the dorsal portion, slightly lateral to the midline. These results are expected to prove useful in studies in which microinjection technique is used to identify transmitters/receptors involved in mediating respiratory and cardiovascular reflex responses.

Phrenic nerve responses to chemical stimulation of the subregions of ventral medullary respiratory neuronal group in the rat.

Phrenic nerve (PN) responses to unilateral microinjections of L-glutamate (L-Glu, 5 mM) or N-methyl-D-aspartic acid (NMDA, 1 mM) into different subregions of ventral respiratory neuronal group (VRG) were studied in urethane-anesthetized, immobilized, and artificially ventilated, adult male Wistar rats. A 50-nl volume of microinjection was used in all the subregions of VRG except in Pre-Bötzinger complex (Pre-BötC) where a 20-nl volume was used. Unilateral microinjections of L-Glu or NMDA into the Bötzinger complex (BötC) and caudal VRG (cVRG), caused a transient cessation of phrenic nerve (PN) activity. Expiratory neurons, abundant in BötC and cVRG, were excited by stimulation of cardiopulmonary receptors while their responses to carotid chemoreceptor stimulation were variable. Microinjections of L-Glu or NMDA into the Pre-BötC caused an increase in the PN background discharge (this response was unique to Pre-BötC) superimposed on which was an increase in the PN burst frequency. Microinjections of L-Glu or NMDA into the rostral VRG (rVRG) caused an increase in the frequency and amplitude of PN bursts. Inspiratory neurons, abundant in Pre-BötC and rVRG, were excited and inhibited by activation of carotid chemoreceptors and cardiopulmonary receptors, respectively. The coordinates for the location of different subregions of VRG were as follows (reference points are listed in parentheses). BötC: 1.6-2.6 mm rostral (calamus scriptorius), 1.7-2.7 mm lateral (midline), and 2.3-2.8 mm deep (dorsal surface of medulla); Pre-BötC: 1.4-1.6 mm rostral, 1. 8-2.5 mm lateral, and 2.3-2.8 mm deep; rVRG: 0.4-1.4 mm rostral, 1. 6-2.5 mm lateral, and 2.3-2.8 mm deep; and cVRG: 0.5 mm caudal to 0. 5 mm rostral, 1.0-2.2 mm lateral, and 2.1-2.6 mm deep. A detailed map of the subregions of VRG, functionally identified by L-Glu and NMDA-microinjections, has been presented. These data are likely to prove useful in future studies on respiratory reflex mechanisms.

Microinjections of carbachol into the phrenic motor nucleus inhibit phrenic nerve activity in the rat.

Microinjections (50 nl) of carbachol (2.5-500 microM) into the phrenic motor nucleus (PMN) of anesthetized rats caused a dose-dependent decrease in the phrenic nerve (PN) burst-amplitude. Prior microinjections of pirenzepine and methoctramine (1 mM, each) into the PMN, in separate experiments, significantly attenuated the carbachol-induced inhibition of PN activity. These results suggest that inhibition of PN activity induced by microinjections of carbachol into the PMN is mediated via M(1) and M(2) receptors. Since pirenzepine and methoctramine microinjections into the PMN did not alter the control PN activity, it was concluded that in anesthetized rats cholinergic inputs to the PMN, if any, are not tonically active. It is possible that muscarinic receptors in the PMN come into play only under specific conditions such as activation of a reflex mechanism which alters PN activity. These hypotheses remain to be tested.

Chemical stimulation of the ventrolateral medullary depressor area decreases ipsilateral cerebral blood flow in anesthetized rats.

In anesthetized (chloralose and urethane), paralyzed and artificially ventilated rats, the neurons in the ventrolateral medullary depressor area (VLDA) were chemically stimulated by microinjections of L-glutamate (2.5-5 nmole in 100 nl of 0.9% sodium chloride solution) and the cerebral blood flow (CBF) was determined using a combination of labeled microspheres (57Co, 113Sn and 46Sc). Unilateral chemical stimulation of the VLDA (n = 11) produced a significant (P less than 0.05) decrease in CBF of the cerebral cortex ipsilateral to the stimulated VLDA; the CBF was 41 +/- 5 (mean +/- S.E.M.) and 29 +/- 4 ml.min-1.(100 g)-1 before and during the chemical stimulation of VLDA. The decrease in CBF was not due to the decrease in arterial blood pressure (ABP) caused by the chemical stimulation of the VLDA because the CBF during the chemical stimulation of the VLDA was significantly smaller (P less than 0.01) than the CBF during controlled hemorrhagic hypotension (n = 10). In another group of rats (n = 6), moderate hypertension was induced by blood transfusion. Unilateral chemical stimulation of the VLDA in these rats decreased ABP but it remained within normotensive range. A significant (P less than 0.05) decrease in CBF (from 46 +/- 12 to 29 +/- 7 ml.min-1.(100 g)-1) and a significant (P less than 0.01) increase in cerebrovascular resistance (from 2.7 +/- 0.4 to 4.3 +/- 0.6 mmHg per [ml.min-1.(100 g)-1]) was observed in the ipsilateral cerebral cortex of these rats. Chemical stimulation of the VLDA did not affect the reactivity of the cerebral vessels to hypercapnea (n = 5).(ABSTRACT TRUNCATED AT 250 WORDS)

A midline area in the nucleus commissuralis of NTS mediates the phrenic nerve responses to carotid chemoreceptor stimulation.

The carotid body chemoreceptor afferents have been reported to project to a discrete area located in the nucleus commissuralis of nucleus tractus solitarius [A. Vardhan et al., Am. J. Physiol., 264 (1993) R41-R50]. The afore-mentioned study was done in spontaneously breathing rats and the afferents and efferents located in the chest wall and the respiratory tract of these animals were intact. In order to exclude the role, if any, of these afferents and efferents, in the present experiments respiratory changes were monitored by recording the phrenic nerve activity instead of tracheal airflow. Experiments were carried out in pentobarbital-anesthetized, bilaterally vagotomized, paralyzed and artificially ventilated rats with a pneumothorax. The carotid body chemoreceptors were stimulated with tracheal administration of nitrogen for 7-10 s. The chemoreceptor stimulation induced an increase in the frequency and amplitude of phrenic nerve bursts. A decrease in the duration of inspiratory (T1), expiratory (TE) and total cycles (TTOT) was observed in the phrenic nerve activity. Inhibition of neuronal cell bodies by microinjections of muscimol (140 pmol/20 nl) into a discrete area in the commissural subnucleus of the nucleus tractus solitarius (coordinates in mm: 0.3 rostral to 0.5 caudal, 0 to 0.5 lateral and 0.3 to 0.5 deep with respect to the calamus scriptorius), attenuated the phrenic nerve responses to the carotid body stimulation. On the other hand, control injections of saline (0.9%) into this site did not alter the phrenic nerve response to the carotid body stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of alpha 1-adrenergic receptors in the intermediolateral column in mediating the pressor responses elicited by the stimulation of ventrolateral medullary pressor area.

Microinjections of alpha 1-adrenergic receptor agonists into the intermediolateral cell column of the spinal cord (IML) elicit sympathoexcitatory responses. This observation, together with the identification of projections of epinephrine-containing cells in the rostral ventrolateral medullary pressor area (VLPA) to the IML, has prompted speculation that epinephrine may mediate pressor responses to the stimulation of the VLPA. This hypothesis was tested in pentobarbital-anesthetized, artificially ventilated, male Wistar rats. A mesenteric arterial branch was cannulated for monitoring blood pressure. Pressor responses were elicited predominantly from T8-T10 by injections (1.7 nmol/20 nl) of L-glutamate into the IML; maximum pressor responses (29.3 +/- 4 mmHg) were elicited from T9. Pressor responses were also elicited by injections of epinephrine into the IML at T9; maximum pressor effect (16.3 +/- 1.2 mmHg) was elicited by a dose of 0.05 pmol/20 nl. This effect of epinephrine at T9 was blocked by prior injections of prazosin (a selective alpha 1-adrenergic receptor blocker; 0.125 pmol/20 nl) at the same site. Stimulation of the VLPA by unilateral microinjections of glutamate elicited pressor responses (56 +/- 12 mmHg). Bilateral injections of prazosin at T8-T10, in the dose (0.125 pmol) that blocked a maximally effective dose of epinephrine, did not block the pressor responses to subsequent injections of glutamate into the VLPA. On the other hand, bilateral microinjections of AP-7 (an NMDA receptor blocker; 1 nmol/20 nl), but not DNQX (10 pmol; a non-NMDA receptor blocker), into the IML at T8-T10 blocked the pressor effects of the subsequent injections of glutamate into the VLPA.(ABSTRACT TRUNCATED AT 250 WORDS)