Chloralose-urethane Anesthesia Research Articles

The Effects of Mutual Interaction of Orexin-A and Glucagon-Like Peptide-1 on Reflex Swallowing Induced by SLN Afferents in Rats.

(1) Background: Our previous studies revealed that orexin-A, an appetite-increasing peptide, suppressed reflex swallowing via the commissural part of the nucleus tractus solitarius (cNTS), and that glucagon-like peptide-1 (GLP-1), an appetite-reducing peptide, also suppressed reflex swallowing via the medial nucleus of the NTS (mNTS). In this study, we examined the mutual interaction between orexin-A and GLP-1 in reflex swallowing. (2) Methods: Sprague–Dawley rats under urethane–chloralose anesthesia were used. Swallowing was induced by electrical stimulation of the superior laryngeal nerve (SLN) and was identified by the electromyographic (EMG) signals obtained from the mylohyoid muscle. (3) Results: The injection of GLP-1 (20 pmol) into the mNTS reduced the swallowing frequency and extended the latency of the first swallow. These suppressive effects of GLP-1 were not observed after the fourth ventricular administration of orexin-A. After the injection of an orexin-1 receptor antagonist (SB334867) into the cNTS, an ineffective dose of GLP-1 (6 pmol) into the mNTS suppressed reflex swallowing. Similarly, the suppressive effects of orexin-A (1 nmol) were not observed after the injection of GLP-1 (6 pmol) into the mNTS. After the administration of a GLP-1 receptor antagonist (exendin-4(5-39)), an ineffective dose of orexin-A (0.3 nmol) suppressed reflex swallowing. (4) Conclusions: The presence of reciprocal inhibitory connections between GLP-1 receptive neurons and orexin-A receptive neurons in the NTS was strongly suggested.

Effects of intravenous metamizole on ongoing and evoked activity of dura-sensitive thalamic neurons in rats

Migraine and tension-type headache (TTH) are the most common forms of primary headaches. A general key mechanism underlying development of both the diseases is the trigeminal system activation associated with the ascending nociceptive transmission via the trigemino-thalamo-cortical pathway. The ventroposteromedial (VPM) nucleus is a key thalamic structure, receiving afferent inflow from the craniofacial region; it holds the third-order neurons responsible for conveying sensory information from the extra- and intracranial nociceptors to the cortex. The VPM is currently seen as a therapeutic target for various antimigraine medications, which is shown to reduce the VPM neuronal excitability. A non-opioid analgesic metamizole is widely used in some countries for acute treatment of migraine or TTH. However, the precise mechanisms underlying anticephalgic action of metamizole remain unclear. The objective of our study performed in the rat model of trigemino-durovascular nociception was to evaluate the effects of intravenously administered metamizole on ongoing and evoked firing of the dura-sensitive VPM neurons. The experiments were carried out on rats under urethane–chloralose anesthesia. Cumulative administration of metamizole (thrice-repeated intravenous infusion of 150mg/kg performed 30min apart) in 56% of cases produced a suppression of both the ongoing activity of the thalamic VPM neurons and their responses to dural electrical stimulation. Although the inhibitory effect was prevailing, a number of VPM neurons were indifferent to the administration of metamizole. These data suggest that one of the main components of neural mechanism underlying anticephalgic action of metamizole is suppression of the thalamo-cortical nociceptive transmission associated with trigemino-vascular activation.

The Impact of Four Different Classes of Anesthetics on the Mechanisms of Blood Pressure Regulation in Normotensive and Spontaneously Hypertensive Rats

Most anesthetics induce characteristic hemodynamic changes leading to blood pressure (BP) reduction but the role of renin-angiotensin system (RAS), sympathetic nervous system (SNS) and nitric oxide (NO) synthesis in this BP reduction is unknown. We therefore studied the influence of four widely used anesthetics - pentobarbital (P), isoflurane (ISO), ketamine-xylazine (KX) and chloralose-urethane (CU) - on the participation of these vasoactive systems in BP maintenance. BP effects elicited by the acute sequential blockade of RAS (captopril), SNS (pentolinium) and NO synthase (L-NAME) were compared in conscious and anesthetized Wistar or spontaneously hypertensive rats (SHR). Except for pentobarbital all studied anesthetics evidenced by diminished BP responses to pentolinium. The absolute pentolinium-induced BP changes were always greater in SHR than Wistar rats. KX anesthesia eliminated BP response to pentolinium and considerably enhanced BP response to NO synthase inhibition in SHR. In both rat strains the anesthesia with ISO or CU augmented BP response to captopril, decreased BP response to pentolinium and attenuated BP response to NO synthase inhibition. In conclusion, pentobarbital anesthesia had a modest influence on BP level and its maintenance by the above vasoactive systems. Isoflurane and chloralose-urethane anesthesia may be used in cardiovascular experiments if substantial BP decrease due to altered contribution of RAS, SNS and NO to BP regulation does not interfere with the respective research aim. Major BP reduction (namely in SHR) due to a complete SNS absence is a major drawback of ketamine-xylazine anesthesia.

Modulation of Baroreflex Sensitivity by Endogenous Angiotensins in Lamb Solitary Tract Nucleus

We and others find that within the nucleus of the solitary tract (NTS) of the rat endogenous Ang II attenuates while Ang‐(1–7) facilitates the baroreflex sensitivity (BRS) in the control of heart rate. It is not known whether this same relationship exists in the sheep NTS. Therefore, we performed microinjection studies in 6 wk old sheep (n = 5) under chloralose‐urethane anesthesia (35/750 mg/kg) which maintains both increases and decreases in BRS for heart rate control. To assess the effects of blockade of either the AT1 or Ang‐(1–7) (AT7) receptors, we tested BRS before and after bilateral NTS injection of the AT1 antagonist candesartan (CV, 200 pmol/200 nl at 4 injection sites) and retested the reflex again after the AT7 antagonist D‐Ala7‐Ang‐(1–7) (D‐Ala, 10 pmol/200 nl at 4 injection sites). BRS was tested using various doses of phenylephrine to determine the slope of the relationship between changes in mean arterial pressure (MAP) and pulse interval as ms/mm Hg. Resting BRS was 0.3 ± 0.2 ms/mm Hg and MAP was 81 ± 7 mm Hg. NTS injection of CV increased BRS to a new baseline of 1.0 ± 0.2 ms/mm Hg, (p<0.05). From this elevated level D‐Ala significantly reduced BRS back to baseline. There was no effect of either agent on MAP. These data suggest that baseline baroreceptor reflex control of heart rate in lambs is regulated by Ang II and Ang‐(1–7) in the NTS in an opposite manner. Support: HD‐047584, HD‐017644, HL‐51952.

Is urethane–chloralose anaesthesia appropriate for pharmacokinetic–pharmacodynamic assessment? Studies with carvedilol

Introduction The aim of the work was to establish the impact of urethane–chloralose anaesthesia on pharmacokinetic–pharmacodynamic (PK–PD) properties of carvedilol in control rats and l-NAME hypertensive animals. Methods Male Wistar Rats were randomly divided into: control ( n = 12) with tap water to drink and L-NAME rats ( n = 12) with L-NAME solution (40 mg/kg/day) to drink for 2 weeks. Effects of carvedilol (1 mg kg − 1 , i.v.) on blood pressure and heart rate were recorded during 3 h in conscious and urethane (500 mg kg − 1 , i.p.) – chloralose (50 mg kg − 1 , i.p.) anaesthetized rats. Carvedilol plasma pharmacokinetics was studied by means of traditional blood sampling. PK–PD modeling of carvedilol was made by means of an effect compartment model. Results Neither urethane–chloralose nor L-NAME modified estimation of pharmacokinetic parameters of carvedilol. Although urethane–chloralose did not modify potency of carvedilol comparing with awake animals in control and hypertensive group, maximal negative chronotropic response was significantly greater in anaesthetized L-NAME rats in comparison to awake animals. Conversely, anaesthesia did not modify maximal chronotropic response to carvedilol in control rats. Whilst no differences were found in the estimated potency of carvedilol hypotensive response comparing control and L-NAME rats in both awake and anaesthetized conditions, maximal hypotensive effect of carvedilol was significantly greater in anaesthetized control and L-NAME animals in comparison to conscious rats. L-NAME rats showed a greater maximal hypotensive response comparing to control group. Discussion Urethane–chloralose anaesthesia is an acceptable experimental condition for the evaluation of PK–PD properties of carvedilol, considering that it does not affect the potency of carvedilol for its chronotropic and hypotensive effect. Conclusions obtained from urethane–chloralose anaesthetized animals, regarding the impact of l-NAME treatment on PK–PD properties of carvedilol, did not differ from those obtained from conscious animals. Anaesthesia did not modify pharmacokinetic behaviour of carvedilol in both normotensive and L-NAME hypertensive rats.

High-dose Norepinephrine Induces Disruption of Myocardial Extracellular Matrix and Left Ventricular Dilatation and Dysfunction in a Novel Feline Model

Intravenous norepinephrine (NE) at a dose of 1-6 microg/kg/minute can induce increased extracellular matrix (ECM) and hypertrophic cardiomyopathy. This study aimed to investigate the effects of a higher dose of NE on cardiac remodeling. After intraperitoneal urethane-chloralose anesthesia, 7 cats (3.03 +/- 0.58 kg) received intravenous infusion of NE 30 microg/kg/minute for 3 hours. Aortic blood pressure and heart rate (HR) were measured by polygraphy at 0, 5, 15, 30, 60, 90, 120, and 180 minutes. Left ventricular size and ejection fraction (EF) were measured by M-mode echocardiography before and after NE administration. Histopathology was performed by hematoxylin-eosin, silver impregnation, and Sirius red staining. Activity of matrix metalloproteinases (MMP) in the left ventricle was measured by zymography. Mean blood pressure (mmHg) increased from 139 +/- 20 to 198 +/- 19, 187 +/- 23, and 166 +/- 16 at 15, 30, and 60 minutes, respectively, during NE infusion. HR (beats/minute) decreased from 214 +/- 10 to 158 +/- 28 at 15 minutes and then recovered gradually. The left ventricles showed significant dilatation (end-diastolic diameter: from 1.20 +/- 0.18 to 1.58 +/- 0.23cm, p=0.003; end-systolic diameter: from 0.62 +/- 0.23 to 1.35 +/- 0.29cm, p=0.002) and hypokinesia (EF: from 86.2 +/- 5.2 to 33.1 +/- 16.5%, p < 0.001). Histopathology revealed that left ventricular myocytes were elongated, wavy, and fragmented, while collagen fibers were overstretched, straightened, and disrupted. MMP-9 activity was significantly elevated (p = 0.003 vs. control), while MMP-2 activity was unchanged. High-dose NE increases MMP-9 activity and causes ECM disruption, left ventricular dilatation and dysfunction.

Perturbation of intraparenchymal arterioles during K+-induced cortical spreading depression in cats

During K+-induced cortical spreading depression (CSD) in the rat cerebral cortex, we previously observed repetitive wave-ring spread of oligemia/hypoperfusion followed by hyperemia/hyperperfusion. The oligemia/hypoperfusion occurred before upstream feeding arteriolar changes, suggesting that the capillary changes of the oligemia/hypoperfusion was of neuronal origin (Tomita M et al (2005) J CBF & M, in press). In this communication, we examined intraparenchymal arteriolar changes in the early phase of tissue oligemia. Six cats were used. Under -chloralose-urethan anesthesia, a skull window was opened in the parieto-temporal region. The dura was removed and the cerebral cortex was exposed. An optical fiber of 150 m in diameter was inserted into the brain tissue obliquely from 3 mm behind the skull window, so that the light source was positioned at the center of the ROI 1 mm below the pia. Intraparenchymal 20–40 m arterioles were both trans-illuminated in silhouette from below and epi-illuminated from above. They were monitored with a Canon video camera, and images were taken into a computer through a Scion frame grabber card (LG-3). When K+ was microinjected into the cortex at a site near the ROI, oligemic wave-ring spread was reproduced as reported previously (Tomita Y et al (2002) Neurosci Lett:322,157). We found that capillary flow in the oligemic area became very slow, with clustered RBCs passing through small vessels like ants marching. During oligemia/hypoperfusion, we noticed in all cases segmental constriction/dilatation of arteriole(s), forming a sausage-string shape (Fig. 1 and Fig. 2a) which had never been seen in other series of experiments. Chronological changes of arteriolar diameter were seen; for example, in 2 cases, a part of the arteriole suddenly contracted by 50% for about 1 to 1.5 sec and then dilated to 250% at 10 sec and remained dilated for more than 1 min (Fig. 2b). In the remaining cases, the time courses of constriction/dilatation of arteriole(s) were rather slow. In one case, the vascular dilatation occurred at a branching site and moved peripherally along the arteriole, like the movement of an egg swallowed by a snake. Despite these events, subtraction pictures revealed that the majority of the epi-cortical pial arteries did not change their diameter in this early phase. Based on these observations, we speculate that arterioles were perturbed by discrepant controls of high K+ concentration in the extracellular fluid, unknown feedback signals from oligemic tissue, and autonomic ganglionic regulation.

Sympathetic activity and myocardial damage after stimulation of dorsal medulla and vagotomy in a novel animal model

Many kinds of brain lesions cause sympathetic hyperexcitation and myocardial damage. A novel animal model was developed for the correlation of sympathetic excitation with ventricular dysfunction and myocardial damage. Six cats (3.23±0.26 kg) under intraperitoneal urethane-chloralose anesthesia were artificially ventilated and bilaterally vagotomized. They underwent an electrical stimulation of unilateral dorsal medulla for 180 min (monopolar square-wave pulses, 10 Hz, 10 V, 0.5 ms). Mean blood pressure, heart rate plasma concentration of norepinephrine and left ventricular size and ejection fraction were measured at 0, 5, 15, 30, 60, 90, 120 and 180 min. Mean blood pressure (mm Hg), heart rate (beats/min) and norepinephrine (pg/ml) increased abruptly from 128±15, 203±22 and 353±123 to 234±26, 240±13 and 4727±2159 at 5 min after electrical stimulation (all p<0.01). The left ventricles showed significant dilatation (end-diastolic diameter: from 1.35±0.13 to 1.84±0.21 cm, p=0.006; end-systolic diameter: from 0.65±0.20 to 1.54±0.24 cm, p=0.002) and hypokinesia (ejection fraction: from 88.9±6.4% to 37.3±8.7%, p<0.001). Cardiac pathology revealed myocardial hemorrhage, cardiomyocyte apoptosis and coagulative myocytolysis (contraction band necrosis), characterized by sarcoplasmic coagulation, granulation and disruption. In conclusions, the present experiment develops a novel animal model in which stimulation of the pressor area in the dorsal medulla in vagotomized cats produces sympathetic hyperexcitation accompanied with myocardial dysfunction and damage. This model may be applicable for studying protective effect of drugs on myocardial dysfunction and damage caused by sympathetic hyperexcitation occurring in brain diseases.

EFFECTS OF HALOTHANE AND URETHANE–CHLORALOSE ANAESTHESIA ON THE PRESSOR AND CEREBROVASCULAR RESPONSES TO 7–NITROINDAZOLE, AN INHIBITOR OF NITRIC OXIDE SYNTHASE

We examined the effect of 7–nitroindazole (7NI), a reportedly relatively specific inhibitor of the neuronal isoform of nitric oxide synthase (nNOS), on mean arterial blood pressure and on cerebral blood flow in rats under three different types of anaesthesia: urethane–chloralose, halothane, or urethane preceded by induction of anaesthesia with halothane. In rats under urethane–chloralose anaesthesia, 7NI induced an increase in mean systemic arterial blood pressure. In contrast, halothane used for induction and maintenance of anaesthesia eliminated the 7NI–induced systemic pressor effect, while halothane used only for induction of anaesthesia greatly attenuated the 7NI–induced systemic pressor effect. Cerebral blood flow, as measured by Laser Doppler flowmetry, decreased significantly to 85–72% of baseline within 5–10 min after i.p. 7NI injection regardless of the type of anaesthesia. Blockade of the systemic pressor effect of 7NI by halothane but not of the reduction in cerebral blood flow produced by 7NI is consistent with prior evidence that: (1) the cerebral vasculature and the peripheral vasculature differ in the isoforms of NOS involved in maintaining vascular tone, with nNOS more important in the former and endothelial NOS (eNOS) in the latter; and (2) halothane interferes with eNOS–mediated vascular tone but not nNOS–mediated control of cerebral blood flow. The fact that 7NI yields a pressor effect that can be attenuated by halothane, as also true for isoform–non–selective NOS inhibitors, raises the possibility that 7NI may to some extent inhibit endothelial NO formation.

Spinal segments communicating resting sympathetic activity to postganglionic nerves of the stellate ganglion.

It has been shown earlier using sympathetic reflexes and anatomic techniques that preganglionic neurons controlling different effectors occupy wide and overlapping ranges of adjacent segments in the spinal cord (cardiac: T1-T7, vertebral: T2-T8). Because, however, the majority of preganglionic neurons are silent at resting states, the present study was designed to estimate the segmental map of subsets of these neurons including only those active at rest using simultaneous recordings from the inferior cardiac and vertebral nerves, under chloralose-urethan or urethan anesthesia. In 22 cats, thoracic white rami T1-T8 were cut in a sequential manner. Three-minute-long data segments were recorded between sectionings and analyzed in the frequency domain using the fast Fourier transform. We found that cardiac and vertebral active maps involved segments T3-T5 and T4-T8, respectively. In individual experiments, however, most of the power of rhythmic activity originated from only one or two segments and the dominant segments for the two nerves never overlapped. Moreover, the separation between dominant segments generating cardiac and vertebral nerve discharges was wider and the distribution of tonically active preganglionic neurons projecting to each nerve was narrower under urethan than chloralose-urethan anesthesia. We conclude that the proportion of active to quiescent preganglionic neurons regulating cardiac and vertebral nerve discharges varies from spinal segment to segment and that active neurons projecting to these nerves are nonoverlapping.

Increase of Gonadotropin-Releasing Hormone Concentration in Pituitary Portal Blood after Substance P Administration in Male Rats

Substance P (SP) affects gonadotropin release from the anterior pituitary gland. In the present study we tested whether SP exerts this effect through GnRH release into pituitary portal blood in intact male rats (INT), orchidectomized rats with s.c. chronically implanted empty Silastic capsule (ORCX), testosterone capsule (ORCX + T), and 17β-estradiol capsule (ORCX + E₂). The pituitary glands were exposed by the transpharyngeal approach under urethane-chloralose anesthesia. Then, the stalk portal vessels were cut and three 30-min portal blood samples were collected. Each first sample of blood was treated as a control before 0.2 ml injection of normal saline, 5 µg, or 25 µg of SP in 0.2 ml of normal saline into the internal carotid artery. GnRH concentration in the purified portal plasma were measured by RIA. Injection of SP into the internal carotid artery caused a significant increase in GnRH concentration in pituitary portal plasma only in INT rats. The higher dose of SP markedly increased GnRH concentration in the 1st blood sample (p < 0.001) and in the 2nd blood sample GnRH concentration was lower but still significant higher than prior SP injection (p < 0.05). The lower dose of SP increased GnRH concentration later, only in the 2nd portal blood sample after intracarotid SP injection (p < 0.001). Injection of normal saline had no effect on GnRH concentration in pituitary portal blood in INT rats. In ORCX, ORCX testosterone- and estrogen-implanted rats portal plasma GnRH concentrations were not changed significantly after injection of both doses of SP. These results indicate that SP stimulates GnRH release into pituitary portal blood and the influence of SP on GnRH neurons depends on the levels of circulating gonadal steroid hormones.

The effect of dexmedetomidine on the balance of myocardial energy requirement and oxygen supply and demand.

The effect of the alpha(2)-adrenergic agonist dexmedetomidine on the balance between myocardial energy requirement and oxygen supply and demand was investigated in 16 open-chest dogs anesthetized with either chloralose/urethane (CU) or fentanyl/halothane (FH). Myocardial energy requirement (estimated from the pressure work index), blood flow and its transmural distribution (radioactive microspheres), as well as myocardial oxygen and lactate extraction, were measured before and after administration of dexmedetomidine in doses ranging from 0.1 to 10 micrograms/kg intravenously. Under CU anesthesia, dexmedetomidine decreased heart rate, arterial blood pressure, and cardiac output. During FH anesthesia, dexmedetomidine reduced heart rate and cardiac output whereas arterial blood pressure increased. Dexmedetomidine decreased myocardial energy requirement only during CU anesthesia; myocardial oxygen supply and demand decreased in parallel. At the (large) dose of 10 micrograms/kg, myocardial oxygen extraction increased during both types of anesthesia. Dexmedetomidine >/= 1 microgram/kg increased endocardial/epicardial blood flow ratio during FH anesthesia. These data indicate that dexmedetomidine >/= 1 microgram/kg reduces myocardial energy requirements, especially when baseline heart rate and blood pressure are increased. Dexmedetomidine preserves endocardial perfusion and reduces oxygen demand in parallel with oxygen supply and energy requirements.

Temporal relationship between the recovery of longitudinal reflex systems and motor recovery in chronic spinally-lesioned rats

This study was performed to determine whether a correlation exists in the functional recovery of the longitudinal reflex systems and recovery of quadrupedal locomotion in chronic spinally-lesioned rats. Partial spinal lesions sparing only the right ventral quadrant (VQ) or dorsal quadrant (DQ) of the spinal cord at the lower thoracic (T9-10) levels were performed aseptically under Nembutal anesthesia. Quadrupedal locomotion reappeared within 2-3 weeks after the initial operations in VQ rats. However, no recovery of hindlimb movement was observed in DQ rats. In terminal experiments with VQ and DQ rats, the right brachial plexus (C6-T1) was dissected and mounted on a bipolar stimulating electrode. Spino-bulbo-spinal and interlimb reflex potentials were recorded from bilateral hindlimb muscles under urethane-chloralose anesthesia and Nembutal anesthesia, respectively. In VQ rats, these longitudinal reflex potentials could not be evoked for several days post-operatively. However, the early threshold elevation for evoking the reflex potentials returned gradually to the normal range in accordance with locomotor recovery. In DQ rats, on the other hand, disappearance of the reflex potentials persisted over 1 month observation period. These results suggest that the functional recovery of these longitudinal reflex systems in the ventral quadrant parallels the quadrupedal locomotor recovery after partial spinal cord lesions in rats.

Neural responses of rat area postrema to stimuli producing nausea

To identify a neuron within the area postrema (AP) that participates in producing nausea, neural responses of the rat AP to noxious, excessive distension of the stomach were recorded electrophysiologically under urethane-chloralose anesthesia. There were two types of the neural responses; one is characterized by increasing the frequency of discharges responding to the stomach distension (excitatory type), while the other shows the opposite response (decreasing the frequency) to the same stimulation (inhibitory type). After this identification, the effect of LiCl or apomorphine superfused on the floor of IVth ventricle was examined to ascertain a convergence of afferents responding to chemical (LiCl or apomorphine) as well as mechanical noxious stimulation (stomach distension) on the same AP neuron. It was revealed that the rat AP involves multimodality neurons responsive to various emetic stimuli, so indicating their participation in producing nausea.

Intravascular platelet aggregation and spontaneous contrast.

The purpose of this study was to determine why spontaneous contrast developed after general anesthesia in dogs. Twenty-seven dogs underwent echocardiography before and after pentobarbital or chloralose-urethane general anesthesia. The results showed that none of the 12 dogs receiving pentobarbital and 10 of 15 dogs receiving chloralose-urethane anesthesia developed contrast, in association with large platelet and platelet-neutrophil aggregates (P < 0.01); this effect could also be reproduced in vitro. The administration of adenosine diphosphate or antiplatelet antibody to nine dogs confirmed that intravascular platelet aggregation can cause ultrasonic contrast. The implications of these findings for patients with spontaneous contrast are discussed.

Peripheral nerve stimulation increases serotonin and dopamine metabolites in rat spinal cord

Extracellular serotonin (5-HT), dopamine (DA), and their metabolites, 5-hydroxyin-doleacetic acid (5-HIAA), 3,4-dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA), were assessed in the rat lumbar spinal cord (L3-4) by in vivo microdialysis with high performance liquid chromatography and electrochemical detection (HPLC-ECD). Under urethane-chloralose anesthesia, basal levels of 5-HT and DA in the dialysates were approximately 1.0–1.2 pg/22 μl sample, 5-HIAA, DOPAC, and HVA were constant at 322.6 ± 14.9, 8.6 ± 0.7, and 10.4 ± 0.4 pg/22 μl sample (mean ± SE), respectively. Local application of 100 mM KCl via the dialysis probe increased the 5-HT and DA. Peripheral nerve stimulation that selectively excited the large (A-beta) or small (A-delta) myelinated fibers increased the metabolites. Excitation of the A-beta fibers increased the levels of 5-HIAA to 138%, DOPAC to 155%, and HVA to 143% of the controls. Stimulation of the A-delta fibers increased 5-HIAA to 121%, DOPAC to 120%, and HVA to 124% of the controls. The results suggest that nonnociceptive peripheral nerve stimulation may activate the descending 5-HT and DA systems in the spinal cord.

Target site of inhibition mediated by midbrain periaqueductal gray matter of baroreflex vagal bradycardia.

1. Both electrical and chemical stimulation of the midbrain periaqueductal gray matter (PAG) inhibit baroreflex vagal bradycardia (BVB). The present study was designed to determine the target site of this inhibition about which little is known. Electrical stimulation of the PAG, in particular of its dorsal portion, markedly suppressed BVB provoked by electrical stimulation of the aortic depressor nerve (ADN; percentage of inhibition = 91.0 +/- 9.7%, mean +/- SD; n = 64). To identify the target site of the inhibition, several types of experiments were conducted in rats under chloralose-urethan anesthesia. 2. The inhibition was exclusively of central origin because inhibition of BVB by stimulation of the PAG was unchanged after transection of the spinal cord at the C1 level. According to Wall's method, we examined whether PAG stimulation affects BVB presynaptically by modulating the excitability of ADN terminals in the nucleus tractus solitarius (NTS). However, excitability changes of ADN terminals by the PAG stimulation were not demonstrated. 3. Vagal bradycardia evoked by microinjection of glutamate into the nucleus ambiguus (NA) region was markedly suppressed by the PAG (percentage of inhibition = 85.9 +/- 9.1%; n = 9), an indication that vagal cardiac preganglionic neurons at this site were subject to the inhibitory action of the PAG. Basal vagal tone due to ongoing preganglionic neuronal activity was also subject to inhibitory control by the PAG because basal heart rate was increased by stimulation of the PAG after either C1 transection or NTS lesion. 4. We found that PAG stimulation suppressed ADN-induced field potentials in the NA region (37.7 +/- 13.8% relative to the control; n = 9) but only slightly in the NTS region (95.8 +/- 15.2%; n = 16). In addition, unitary recordings revealed that ADN-evoked unitary responses of neurons in the NA region were suppressed by PAG stimulation, whereas NTS baroreceptor neurons, either ADN responsive or nonresponsive, were scarcely inhibited by PAG stimulation. 5. These findings suggest that the PAG inhibited BVB mainly at the vagal preganglionic cell level and not at the NTS interneuron level. The conclusion is in harmony with our previous reports that the target site of hypothalamic inhibition of BVB in rats is also the preganglionic neurons and that hypothalamic inhibition of BVB is mediated predominantly by the PAG.

Differential effects on sympathetic nerve activities elicited by activation of neurons in the pressor areas of dorsal and rostral ventrolateral medulla in cats

Changes of the nerve activity of the sympathetic renal and vertebral nerves were elicited by microinjection of sodium glutamate (50 nmol/100 nl) into the pressor areas of the dorsal (DM) and rostral ventrolateral medulla (RVLM) in cats under urethane-chloralose anesthesia. Animals were bilaterally vagotomized, artificially ventilated, and paralyzed with gallamine triethiodide. The vertebral nerve activity always increased when pressor responses were induced by DM or RVLM stimulation. However, the effects of medullary stimulation on the renal nerve activity were variable. Three types of renal nerve responses concomitant with the pressor responses were observed in either baroreceptor-intact or baroreceptor-denervated cats. They were: (1) augmentation (type I); (2) attenuation (type II); and (3) insignificant change (type III). Type I responses were often elicited by RVLM stimulation whereas type II responses were often elicited by DM stimulation. Findings suggested that neurons integrating these sympathetic nerve activities were not equally distributed in the pressor areas of DM and RVLM. This result supports the notion that neurons located in different pressor areas of the brainstem exert differential effects over different sympathetic nerve activities.

Sympathoadrenal excitation and inhibition by lower brainstem stimulation in cats

Effects of stimulation of brainstem sites on hemodynamics and plasma catecholamine levels were assessed in cats under chloralose-urethane anesthesia. Pressor areas of the dorsal medulla (DM) and ventrolateral medulla (VLM) and the depressor area of the paramedian reticular nucleus (PRN) were stimulated electrically using a monopolar electrode, or chemically using sodium glutamate microinjection. Plasma levels of norepinephrine (NE) and epinephrine (EPI) were measured in caval blood above the adrenal veins. Electrical stimulation of the DM and VLM produced increases in blood pressure and in plasma NE and EPI levels that were enhanced after acute vagotomies. The NE and EPI responses were attenuated after acute, bilateral adrenalectomies, confirming augmented adrenomedullary secretion, whereas the pressor responses were intact. Injection of sodium glutamate into the same pressor regions of the DM or VLM also produced pressor responses and elevated plasma catecholamine levels, indicating that the responses resulted from activation of neuronal perikarya. Stimulation of the PRN attenuated pressor and catecholamine responses during stimulation of the DM and VLM. The results indicate that pressor responses during stimulation of the DM and VLM are due at least partly to activation of perikarya in these regions, are associated with but not dependent on adrenomedullary activation, and are enhanced after vagotomy; and that neurons of the PRN exert inhibitory modulation of the pressor and adrenomedullary responses during stimulation of the VLM and DM.

Effect of hindlimb suspension on cardiovascular responses to sympathomimetics and lower body negative pressure

To determine whether hindlimb suspension is associated with the development of cardiovascular deconditioning, male rats were studied before and after undergoing one of three treatment conditions for 9 days: 1) cage control (n = 15, CON), 2) horizontal suspension (n = 15, HOZ), and 3) head-down suspension (n = 18, HDS). Testing included lower body negative pressure administered during chloralose-urethan anesthesia and graded doses of sympathomimetic agents (norepinephrine, phenylephrine, and tyramine) administered to conscious unrestrained animals. Both HDS and HOZ were associated with a small decrease in the hypotensive response to lower body negative pressure. The HOZ group, but not the HDS group, exhibited augmented reflex tachycardia. Furthermore, both HDS and HOZ groups manifested reduced pressor responses to phenylephrine after treatment. These reductions were associated with significantly attenuated increases in mesenteric vascular resistance. However, baroreflex control of heart rate was not altered by the treatment conditions. Collectively, these results indicate that 9 days of HDS in rats does not elicit hemodynamic response patterns generally associated with cardiovascular deconditioning induced by hypogravic conditions.