Catecholaminergic nucleus tractus solitarius lesions in anesthetized rats alter baroreflexes differently with age.

To explore whether catecholaminergic lesions produced chemically in the nucleus tractus solitarius (NTS) would alter the baroreflex impairment that normally occurs with age, we compared baroreflex responses in 3- and 14-month-old rats given bilateral microinjections of 6-hydroxydopamine (6-OHDA) or its vehicle into the NTS. After 2 weeks, basal pressures were unaffected but heart rates were lower in 14- than in 3-month-old rats. Sympathetic nerve inhibition elicited reflexly during intravenous infusions of sodium nitroprusside was reduced in 3-month-old rats, but increased in 14-month-old rats. Depressor, bradycardiac, and sympatho-inhibitory responses to afferent aortic nerve stimulation were likewise reduced in 3-month-old rats and oppositely enhanced in 14-month-old rats. Enhanced bradycardia probably does not depend on increased parasympathetic sensitivity because responses to vagal stimulation did not differ between age groups. Although underlying mechanisms remain uncertain, by showing that baroreflex responses become altered differently in 3- and 14-month-old rats by 6-OHDA these results suggest that the baroreflex changes normally occurring with age may be due, at least in part, to modified catecholaminergic mechanisms in the NTS.     

Arterial baroreflex inhibition by midbrain periaqueductal grey in anaesthetized rats

Midbrain periaqueductal grey (PAG) provokes the defense reaction when stimulated. The present study was conducted to determine whether, and how, the PAG produces baroreflex inhibition, a feature characterizing the hypothalamic defense reaction. In chloralose-urethane anaesthetized rats, baroreflex vagal bradycardia and baroreflex hypotension were provoked by aortic depressor nerve stimulation. When the PAG was electrically stimulated baroreflex vagal bradycardia was remarkably suppressed; suppression of baroreflex hypotension was observed following bilateral vagotomy. In contrast, chemical stimulation of the PAG by d,l-homocysteic acid microinjection markedly suppressed baroreflex vagal bradycardia but only minimally suppressed baroreflex hypotension. These findings suggest that whereas overall PAG stimulation inhibits not only cardiac but also vascular components of baroreflexes, inhibition of the latter component either depends largely on activation of passing fibers or requires recruitment of a larger number of PAG cell bodies. PAG inhibition of baroreflex vagal bradycardia was not affected following spinal cord transection at C1, indicating that the inhibition was exclusively central in origin and not due to peripheral, prejunctional inhibition of vagal acetylcholine release by increased cardiac sympathetic nerve activities. The PAG inhibition of baroreflexes was greatly attenuated following electrolytic as well as chemical destruction of the parabrachial region. On the other hand, when the PAG was extensively lesioned, baroreflex inhibition produced by hypothalamic defense area stimulation was markedly diminished. PAG excitation thus causes powerful inhibition of arterial baroreflexes which is mediated by the parabrachial region; the PAG also mediates a major fraction of hypothalamic inhibition of the baroreflexes.     

Nociceptive afferents selectively modulate the cardiac component of the peripheral chemoreceptor reflex via actions within the solitary tract nucleus

Our previous findings showed that the nucleus of the solitary tract (NTS) mediated part of the tachycardia evoked during somatic noxious stimulation. Here, we investigated the interaction between somatic nociceptor- and peripheral chemoreceptor-evoked cardiac changes. We sought to determine whether this interaction occurred within the NTS, the primary site of termination of chemoreceptor afferents. In a working heart-brainstem preparation of rat, mechanical noxious activation of a forelimb evoked a tachycardia of 17.5+/-3 (mean+/-S.E.M.) b.p.m., whereas sodium cyanide (7-30 microg) stimulation of peripheral chemoreceptors produced a sub-maximal bradycardia of -140+/-15 b.p.m. During nociceptor stimulation the sodium cyanide-evoked bradycardia was attenuated to -42.6+/-12 b.p.m. but could be prevented by a multiple bilateral NTS microinjection of bicuculline (i.e. -173+/-18 b.p.m.). Furthermore, the activity of NTS neurones responding to peripheral chemoreceptor stimulation increased from 2.8+/-1.3 to 9.4+/-1.9 Hz during sodium cyanide injection (n=7; P<0.01). The latter response was attenuated reversibly to 2.9+/-0.9 Hz during simultaneous stimulation of the brachial nerve. Pressure ejection of bicuculline abolished this inhibitory action of brachial-nerve stimulation on the chemoreceptor-evoked excitatory synaptic response. We conclude that somatic noxious stimulation attenuates the chemoreceptor reflex-evoked bradycardia via a GABA(A)ergic mechanism in the NTS.     

Contraction-sensitive skeletal muscle afferents inhibit arterial baroreceptor signalling in the nucleus of the solitary tract: role of intrinsic GABA interneurons

Arterial baroreceptor and skeletal muscle receptor afferents relay sensory information to the nucleus of the solitary tract (NTS) during exercise. Previous studies have suggested that skeletal muscle afferent input inhibits baroreflex function; however, detailed information on the role of muscle afferents and GABAergic mechanisms in the NTS is limited. Furthermore, identification of specific afferent modalities that activate GABAergic neurons in the NTS remains unknown. In the present study, we examined the neuroanatomical and physiological interactions between spinal dorsal horn cells that transmit contraction-sensitive input from skeletal muscle and GABAergic interneurons in the NTS. Biotinylated dextran amine (BDA, 10%, 25-100 nL) microinjection into dorsal horn of the cervical spinal cord was combined with glutamate decarboxylase (GAD) immunohistochemistry to visualize the nature of the relationship of BDA-labeled fibers in the NTS with GAD immunoreactivity (GAD-ir). BDA-labeled axons and terminal processes were localized in the medial, commissural, dorsomedial and dorsolateral subdivisions of the caudal NTS. Moreover, BDA-labeled fibers were observed in close proximity to GAD-ir structures throughout these regions of the NTS.The physiological interaction between skeletal muscle receptor and arterial baroreceptor afferents was investigated using an arterially perfused, decerebrate rat preparation. Activation of skeletal muscle afferents by electrically evoked twitch contraction of the forelimb attenuated baroreflex responsiveness (BR, calculated as the ratio of changes in heart rate to systemic pressure) from -1.5+/-0.3 bpm.mm Hg(-1) to -0.1+/-0.1 bpm.mm Hg(-1) (control versus contraction, P<0.05, n=15). However, forelimb contraction failed to inhibit the reflex bradycardia evoked by activation of peripheral chemoreceptor afferents, indicating a reflex-specific action. Bilateral microinjection of bicuculline methiodide (BIC, 10 microM, 40-60 nL) into the caudal NTS restored baroreflex responsiveness during contraction (-1.6+/-0.2 versus -0.1+/-0.1 versus -1.5+/-0.2 bpm.mmHg(-1), control versus contraction versus contraction+BIC P<0.05, n=8). We conclude that activation of ascending spinal neurons from the cervical dorsal horn by contraction-sensitive skeletal muscle afferents selectively inhibits arterial baroreceptor signaling in the NTS via activation of a GABAergic mechanism.     

Effects of the head-down tilt posture on postnatal development of the aortic baroreflex in the rabbit

To clarify the effects of the head-down tilt (HDT) posture on the postnatal development of the aortic baroreflex, we raised rabbits from 3-4 weeks of age for 34-36 d in this posture, which simulates the headward shift of body fluid in space, and examined the structural and functional characteristics of the aortic nerves and baroreflex responses. The rabbits were divided into 3 different groups: 20 degrees HDT, environmental control (EC), and vivarium control (VIV). Left aortic nerve activity (ANA) at basal arterial pressure was defined as 100%, and the maximum ANA at maximal increase in arterial pressure was 265+/-103% (mean+/-SD, n=5) in the HDT, 482+/-170% (n=4) in the EC, and 677+/-491% (n=4) in the VIV groups. The minimum ANA at maximal decrease in arterial pressure was 67.3+/-13.8% in the HDT, 40.1+/-10.2% in the EC, and 38.6+/-13.8% in the VIV groups. The maximal change of ANA in response to maximal change in arterial pressure in the HDT group was significantly less than that of either the EC or VIV group (p<0.05). The average baroreflex gain in the HDT group was significantly less than in the VIV group (p<0.05). The number of unmyelinated fibers in the left aortic nerve in the HDT group was significantly less than that of each control group (p<0.05). Thus the development of the aortic baroreflex system is apparently depressed by the HDT posture, suggesting that similar consequences will be observed in mammals that develop in space.     

Injections of angiotensin-converting enzyme 2 inhibitor MLN4760 into nucleus tractus solitarii reduce baroreceptor reflex sensitivity for heart rate control in rats

Injections of the angiotensin(1-7) [Ang(1-7)] antagonist [d-Ala7]-Ang(1-7) into the nucleus of the solitary tract (NTS) of Sprague-Dawley rats reduce baroreceptor reflex sensitivity (BRS) for control of heart rate by approximately 40%, whereas injections of the angiotensin II (Ang II) type 1 receptor antagonist candesartan increase BRS by 40% when reflex bradycardia is assessed. The enzyme angiotensin-converting enzyme 2 (ACE2) is known to convert Ang II to Ang(1-7). We report that ACE2 activity, as well as ACE and neprilysin activities, are present in plasma membrane fractions of the dorsomedial medulla of Sprague-Dawley rats. Moreover, we show that BRS for reflex bradycardia is attenuated (1.16 +/- 0.29 ms mmHg-1 before versus 0.33 +/- 0.11 ms mmHg-1 after; P < 0.05; n = 8) 30-60 min following injection of the selective ACE2 inhibitor MLN4760 (12 pmol in 120 nl) into the NTS. These findings support the concept that within the NTS, local synthesis of Ang(1-7) from Ang II is required for normal sensitivity for the baroreflex control of heart rate in response to increases in arterial pressure.     

Somatosensory and hypothalamic inhibitions of baroreflex vagal bradycardia in rats

Somatosensory and forebrain mechanisms inhibiting arterial baroreflexes were investigated in chloraloseurethane anesthetized and artificially ventilated rats. Electrical stimulation of the sciatic nerve (ScN) and the hypothalamic pressor area (HP) suppressed baroreflex vagal bradycardia (BVB) and hypotension provoked by electrical stimulation of the aortic depressor nerve (ADN). Suppression of BVB was more marked, but inhibitory potencies of ScN and HP were not different. These two inhibitions were considered to have a functional implication in common, since both were accompanied by increase in hindlimb vascular conductance. A variety of experiments were conducted to localize the target site of ScN and HP inhibitions of BVB. Either ScN or HP stimulations was without effect on antidromic compound spike potentials along ADN evoked by microstimulation of the nucleus tractus solitarius (NTS), precluding the possibility of these inhibitions being presynaptic. Both ScN and HP stimulation suppressed ADN-induced field potentials in the NA region which provoked vagal bradycardia upon microstimulation, but failed to affect ADN-induced responses, either field or unitary, in the NTS region. Antidromic unitary responses in the NA region to vagus cardiac branch stimulation were suppressed by ScN and HP stimulations in NTS-lesioned rats. Intracisternal bicuculline, a GABA antagonist, was found to abolish both ScN and HP inhibitions of BVB, while intracisternal muscimol, a GABA agonist, eliminated bradycardia. These findings suggest that somatosensory and forebrain inhibitions of BVB occur principally at the preganglionic cell level and are probably mediated by a GABAergic mechanism.     

Transient voltage-dependent potassium currents are reduced in NTS neurons isolated from renal wrap hypertensive rats

Whole cell patch-clamp measurements were made in neurons enzymatically dispersed from the nucleus of the solitary tract (NTS) to determine if alterations occur in voltage-dependent potassium channels from rats made hypertensive (HT) by unilateral nephrectomy/renal wrap for 4 wk. Some rats had the fluorescent tracer DiA applied to the aortic nerve before the experiment to identify NTS neurons receiving monosynaptic baroreceptor afferent inputs. Mean arterial pressure (MAP) was greater in 4-wk HT (165 +/- 5 mmHg, n = 26, P < 0.001) rats compared with normotensive (NT) rats (109 +/- 3 mmHg measured in 10 of 69 rats). Transient outward currents (TOCs) were observed in 67-82% of NTS neurons from NT and HT rats. At activation voltages from -10 to +10 mV, TOCs were significantly less in HT neurons compared with those observed in NT neurons (P < 0.001). There were no differences in the voltage-dependent activation kinetics, the voltage dependence of steady-state inactivation, and the rise and decay time constants of the TOCs comparing neurons isolated from NT and HT rats. The 4-aminopyridine-sensitive component of the TOC was significantly less in neurons from HT compared with NT rats (P < 0.001), whereas steady-state outward currents, whether or not sensitive to 4-aminopyridine or tetraethylammonium, were not different. Delayed excitation, studied under current clamp, was observed in 60-80% of NTS neurons from NT and HT rats and was not different comparing neurons from NT and HT rats. However, examination of the subset of NTS neurons exhibiting somatic DiA fluorescence revealed that DiA-labeled neurons from HT rats had a significantly shorter duration delayed excitation (n = 8 cells, P = 0.022) than DiA-labeled neurons from NT rats (n = 7 cells). Neurons with delayed excitation from HT rats had a significantly broader first action potential (AP) and a slower maximal downstroke velocity of repolarization compared with NT neurons with delayed excitation (P = 0.016 and P = 0.014, respectively). The number of APs in the first 200 ms of a sustained depolarization was greater in HT than NT neurons (P = 0.012). These results suggest that HT of 4-wk duration reduces TOCs in NTS neurons, and this contributes to reduced delayed excitation and increased AP responses to depolarizing inputs. Such changes could alter baroreflex function in hypertension.     

Targeted deletion of neurokinin-1 receptor expressing nucleus tractus solitarii neurons precludes somatosensory depression of arterial baroreceptor-heart rate reflex

Neurokinin-1 receptor (NK1-R) expressing neurons are densely distributed throughout the nucleus tractus solitarii (NTS). However, their fundamental role in arterial baroreflex function remains debated. Previously, our group has shown that activation of contraction-sensitive somatic afferents evoke substance P (SP) release in the NTS and resets the arterial baroreflex via activation of a GABAergic NTS circuit. Based on these findings, we hypothesized that modulation of arterial baroreflex function by somatic afferents is mediated by NK1-R dependent inhibition of barosensitive NTS circuits. In the present study, SP-conjugated saporin toxin (SP-SAP) was used to ablate NK1-R expressing NTS neurons. Contraction-sensitive somatic afferents were activated by electrically-evoked muscle contraction and the arterial baroreceptor-heart rate reflex was assessed by constructing reflex curves using a decerebrate, arterially-perfused preparation. Baseline baroreflex sensitivity was significantly attenuated in SP-SAP-treated rats compared with control rats receiving either unconjugated SAP or vehicle. Muscle contraction significantly attenuated baroslope in SAP and vehicle-treated animals and shifted the baroreflex curves to higher systemic pressure. In contrast, somatic afferent stimulation failed to alter baroslope or shift the baroreflex curves in SP-SAP-treated animals. Moreover, when reflex sensitivity was partially restored in SP-SAP animals, somatic stimulation failed to attenuate baroreflex bradycardia. In contrast, SP-SAP and somatic stimulation failed to blunt the reflex bradycardia evoked by the peripheral chemoreflex. Immunohistochemistry revealed that pretreatment with SP-SAP significantly reduced the number of NK1-R expressing neurons in the caudal NTS, while sparing NK1-R expressing neurons rostral to the injection site. This was accompanied by a significant reduction in the number of glutamic acid decarboxylase (GAD67) expressing neurons at equivalent levels of the NTS. These findings indicate that immunolesioning of NK1-R expressing NTS neurons selectively abolishes the depressive effect of somatosensory input on arterial baroreceptor-heart rate reflex function.     

Vagal reflex actions of atrial natriuretic peptide survive physiological but not pathological cardiac hypertrophy in rat

Atrial natriuretic peptide (ANP) enhances cardiac vagal baroreflexes in normotensive animals. In spontaneously hypertensive rats (SHRs) this effect of ANP was absent. The reflex actions of ANP were preserved if hypertrophy was completely prevented in SHRs. However even a small amount of cardiac hypertrophy, with no hypertension, in SHRs was accompanied by a loss of the reflex bradycardic actions of ANP. In the present study, we investigated whether pathophysiological cardiac hypertrophy, induced by one-kidney, one-clip renovascular hypertension (1K-1C; n = 6), or physiological cardiac hypertrophy induced by chronic spontaneous, wheel-running exercise training (n = 7), similarly prevented vagal reflex actions of ANP. Cardiac baroreceptor-activated bradycardia was measured during rapid ramp increases ( approximately 5 s) in blood pressure after bolus doses of methoxamine or vehicle in conscious, chronically instrumented rats during infusions of ANP (50 pmol kg(-1) min(-1)). Compared with uninephrectomised control rats (n = 10), rats with 1K-1C had cardiac hypertrophy (approximately 55% increase in left ventricle:body weight (LV:BW) ratio; P < 0.05) and blunted vagal baroreflex gain (-0.93 +/- 0.18 versus-0.50 +/- 0.13 beats min(-1) mmHg(-1); P < 0.05). ANP did not augment baroreflex function in 1K-1C. Compared with their sedentary controls (n = 7), exercise-trained rats with cardiac hypertrophy ( approximately 20% increase LV:BW ratio; P < 0.05) also had blunted ramp baroreflex bradycardia (-1.28 +/- 0.23 versus-0.57 +/- 0.09 beats min(-1) mmHg(-1); P < 0.05). In contrast, ANP more than doubled baroreflex bradycardia in exercise-trained rats (P < 0.05). The aetiology of cardiac hypertrophy therefore influenced whether ANP retained its vagal baroreflex enhancing properties.     

Discharge patterns of somatosensitive neurons in the nucleus tractus solitarius of the cat

Encoding of sensory information by nucleus of the solitary tract (NTS) neurons is incompletely understood. Using extracellular single-unit recording in alpha-chloralose-urethane anesthetized cats, we have examined the discharge characteristics of NTS neurons to activation of somatic Adelta and C fiber afferents by skeletal muscle contraction evoked by electrical stimulation of lower lumbar/upper sacral ventral roots. Generally, somatic afferent stimulation evoked two distinct firing patterns. The first population (36/43 cells) increased their firing rate to brief somatic stimuli. A subset (21/27 cells) exhibited a rapid decay of their firing rate during sustained somatic stimulation. Peak instantaneous firing frequency (F(p)) increased proportionally with the intensity of somatic stimulation (105+/-4 vs. 119+/-4 vs. 139+/-4 Hz, 10, 20 and 40 Hz, respectively, P<0.0001), whereas steady-state firing frequency (F(ss)) was not altered (25+/-2 vs. 27+/-2 vs. 27+/-2 Hz, 10, 20 and 40 Hz, respectively, P=0.72). Two indices were derived to quantify the decay properties. The decay rate constant (obtained from exponential curve fitting) was not altered by stimulation frequency (461+/-10 vs. 442+/-14 vs. 429+/-26 ms, 10, 20 and 40 Hz, respectively, P=0.415), nor was the decay index (derived to express the percent reduction in firing rate with respect to the initial peak firing rate; 76+/-2 vs. 77+/-2 vs. 81+/-2%, 10, 20 and 40 Hz, respectively, P=0.187). In contrast, the second population (seven of 43 cells) decreased their firing rate to stimulation. Of the NTS neurons tested for barosensitivity (29/36), none responded to pressure stimulation. These results have identified a population of somatosensitive NTS neurons that exhibit rapid firing rate decay properties during sustained stimulation. However, this population could faithfully encode phasic excitation during rhythmic somatosensory input. These results are discussed in relation to the role of somatosensory input on baroreflex function.     

Novel modified Ussing chamber for the study of absorption and secretion in human endoscopic biopsies.

The aim of this study was to design and evaluate a modified Ussing chamber, that makes use of constant air suction (modified Ussing air suction chamber, MUAS) for fixation of biopsy specimens. Standard size forceps biopsies were taken from the descending part of duodenum from patients undergoing endoscopy. Short circuit current (SCC) and conductance (G) were measured during basal conditions and after addition of different sugars and secretagogues. Histologic examination was performed to determine the degree of tissue damage after study in the chamber. Basal SCC was 54.7 +/- 4.3 microA x cm(-2) and G was 58.7 +/- 4.7 mS x cm(-2) (mean +/- SEM, n=48) and steady values of these parameters were observed for at least 2 h. Reproducible and steady responses in SCC were obtained with D-glucose (SCCmax=172 +/- 22.1 microA x cm(-2); EC50=6.9 +/- 0.7 mM, n=5) and D-galactose (SCCmax=233 +/- 55.7 microA x cm(-2); EC50=9.2 +/- 0.7 mM, n=3), and secretory responses with 5-hydroxytryptamine, 100 microM (DeltaSCC= 16.1 +/- 3.8 microA x cm(-2), n=10), histamine, 100 microM (DeltaSCC=24.0 +/- 4.1 microA x cm(-2), n=10) and prostaglandin E2, 1 microM (DeltaSCC=30.3 +/- 5.4 microA x cm(-2), n=6). Experimental biopsy specimens showed intact surface epithelium by histologic examination and did not differ from controls apart from minor indications of edge damage. No difference in basal electrical parameters and D-glucose fluxes were found between Helicobacter pylori positive and negative patients. Our data suggests that the MUAS chamber represents a promising alternative approach to measure transport processes in intestinal endoscopic biopsies.     

Comparison of electrical thresholds of intradental nerves and jaw-opening reflex in the cat

In experimental animals the jaw-opening reflex in response to stimulation of pulp nerves has been used as a nociceptive reflex. However, there seems to be only scanty information about the amount and types of pulp nerve fibres that mediate the reflex. In the present work on 8 anesthetized cats electrical thresholds of single functional pulp nerve units were compared to the thresholds of jaw-opening reflex. Monopolar cathodal current pulses were applied to each canine tooth. Reflex responses of the digastric muscle were recorded. The inferior alveolar nerve of the left side in 3 cats was exposed for nerve dissection and responses of pulp nerve units coming from the lower left canine tooth were recorded. The mean threshold of the jaw-opening reflex with 10 ms pulses was 5.9 +/- 3.0 (SD) microA. Below or at the level only part of the fast conducting pulp nerve units could be activated. Thresholds of A- (n = 32) and C- (n = 24) fibres were 9.9 +/- 5.7 and 37.4 +/- 14.5 (SD) microA respectively. Nerves of the periodontal tissues (20 units recorded) were not activated with current pulses of up to 200 microA applied to the tooth. Consequently, at threshold level the jaw-opening reflex in response to the present type of stimulation is mediated by the fast conducting intradental nerve units.     

Acidosis inhibits oxidative phosphorylation in contracting human skeletal muscle in vivo

Somatic noxious stimulation can evoke profound cardiovascular responses by altering activity in the autonomic nervous system. This noxious stimulation attenuates the cardiac vagal baroreflex, a key cardiovascular homeostatic reflex. This attenuation is mediated via NK₁ receptors expressed on GABAergic interneurones within the nucleus of the solitary tract (NTS). We have investigated the effect of noxious stimulation and exogenous substance P (SP) on the sympathetic component of the baroreflex. We recorded from the sympathetic chain in a decerebrate, artificially perfused rat preparation. Noxious hindlimb pinch was without effect on the sympathetic baroreflex although the cardiac vagal baroreflex gain was decreased (56 %, P < 0.01). Bilateral NTS microinjection of SP (500 fmol) produced a similar selective attenuation of the cardiac vagal baroreflex gain (62 %, P < 0.005) without effect on the sympathetic baroreflex. Recordings from the cardiac sympathetic and vagal nerves confirmed the selectivity of the SP inhibition. Control experiments using a GABA_A receptor agonist, isoguvacine, indicated that both components of the baroreflex (parasympathetic and sympathetic) could be blocked from the NTS injection site. The NTS microinjection of a NK₁ antagonist (CP-99,994) in vivo attenuated the tachycardic response to hindlimb pinch. Our data suggest that noxious pinch releases SP within the NTS to selectively attenuate the cardiac vagal, but not the sympathetic, component of the baroreflex. This selective withdrawal of the cardiac vagal baroreflex seems to underlie the pinch-evoked tachycardia seen in vivo . Further, these findings confirm that baroreflex sympathetic and parasympathetic pathways diverge, and can be independently controlled, within the NTS.     

Activation of brainstem endorphinergic neurons causes cardiovascular depression and facilitates baroreflex bradycardia

The effects of electrical stimulation of the arcuate nucleus on blood pressure, heart rate and baroreflex sensitivity were studied in urethane-anesthetized Sprague-Dawley rats. Stimulation of the mid-anterior parts of the arcuate nucleus at 80 Hz, 0.8 ms and 50-200 microA caused a biphasic, depressor/pressor, response and moderate bradycardia. Intravenous administration of a vasopressin V1-receptor antagonist eliminated the pressor component and unmasked a pure depressor response. This depressor response could be inhibited by naltrexone, 2 mg/kg i.v., by an antiserum against beta-endorphin, 100 nl injected directly into the ipsilateral nucleus tractus solitarii, or by deafferentation of the dorsal vagal complex (nucleus tractus solitarii and dorsal vagal nucleus) by an ipsilateral, dorsolateral knife-cut of the medulla oblongata. Stimulation of the arcuate nucleus at currents of 20-40 microA did not influence basal blood pressure or heart rate but potentiated the reflex bradycardia induced by phenylephrine, and this effect was completely blocked by naltrexone. It is concluded that a beta-endorphin-containing pathway projecting from the arcuate nucleus to the ipsilateral dorsal vagal complex is involved in depressor cardiovascular regulation and in the facilitation of baroreflex bradycardia.     

The reflex bradycardia during brain ischemia in the rabbit.

We examined the responses of whole aortic nerve activity, aortic baroreceptor activity, heart rate (HR), and arterial pressure (AP) to brain ischemia sustained for approximately 30 s in anesthetized spontaneously breathing rabbits. The minimum values of HR observed during brain ischemia were 76 +/- 11 beats/min (mean +/- S.E., n = 14) before sectioning the bilateral aortic nerve (BAN), and 161 +/- 12 beats/min after sectioning the left aortic nerve (LAN), and 225 +/- 11 beats/min after sectioning the LAN and right aortic nerve (RAN). Averages for reflex fall in HR during BAN, LAN, and RAN activation were 140 +/- 9, 78 +/- 7, and 62 +/- 7 beats/min, respectively, by subtracting the HR fall responses to brain ischemia in the absence of aortic baroreceptor afferents from their control values. The heights of the integrated whole left and right aortic nerve activities in systole slightly increased during brain ischemia, whereas the brain ischemia remarkably increased those activities during the diastolic phases. The brain ischemia induced a hysteresis in the mean AP-aortic barorecept or activity relationship. These results suggest that the total activity of aortic nerve fibers would determine the bradycardia evoked by brain ischemia and that the difference between the relative contributions of LAN and RAN on the brain ischemia-induced reflex bradycardia would reflect the total impulse frequency of aortic myelinated and non-myelinated fibers.     

Functional changes in baroreceptor afferent, central and efferent components of the baroreflex circuitry in type 1 diabetic mice (OVE26)

Baroreflex control of heart rate (HR) is impaired in diabetes mellitus. We hypothesized that diabetes mellitus induced functional changes of neural components at multiple sites within the baroreflex arc. Type 1 diabetic mice (OVE26) and FVB control mice were anesthetized. Baroreflex-mediated HR responses to sodium nitroprusside- (SNP) and phenylephrine- (PE) induced mean arterial blood pressure (MAP) changes were measured. Baroreceptor function was characterized by measuring the percent (%) change of baseline integrated aortic depressor nerve activity (Int ADNA) in response to SNP- and PE-induced MAP changes. The HR responses to electrical stimulation of the left aortic depressor nerve (ADN) and the right vagus nerve were assessed. Compared with FVB control mice, we found in OVE26 mice that (1) baroreflex-mediated bradycardia and tachycardia were significantly reduced. (2) The baroreceptor afferent function in response to MAP increase did not differ, as assessed by the parameters of the logistic function curve. But, the inhibition of Int ADNA in response to MAP decrease was significantly attenuated. (3) The maximum amplitude of bradycardic responses to right vagal efferent stimulation was augmented. (4) In contrast, the maximum amplitude of bradycardic responses to left ADN stimulation was decreased. Since Int ADNA was preserved in response to MAP increase and HR responses to vagal efferent stimulation were augmented, we conclude that a deficit of the central mediation of baroreflex HR contributes to the overall attenuation of baroreflex sensitivity in OVE26 mice. The successful conduction of physiological experiments on the ADN in OVE26 mice may provide a foundation for the understanding of cellular and molecular mechanisms of diabetes-induced cardiac neuropathy.     

Hypoxia inhibits baroreflex vagal bradycardia via a central action in anaesthetized rats

It is known that arterial baroreflexes are suppressed in stressful conditions. The present study was designed to determine whether and how hypoxia affects arterial baroreflexes, especially the heart rate component, baroreflex vagal bradycardia. In chloralose-urethane-anaesthetized rats, baroreflex vagal bradycardia was evoked by electrical stimulation of the aortic depressor nerve, and the effect of 15 s inhalation of hypoxic gas (4% O2) was studied. Inhalation of hypoxic gas was found to inhibit baroreflex vagal bradycardia. The inhibition persisted after bilateral transection of the carotid sinus nerve. Cervical vagus nerves were cut bilaterally and their peripheral cut ends were stimulated to provoke vagal bradycardia of peripheral origin so as to determine whether hypoxia could inhibit vagal bradycardia by acting on a peripheral site. In contrast to baroreflex vagal bradycardia, the vagus-induced bradycardia was not affected by hypoxic gas inhalation. It is concluded that baroreflex vagal bradycardia is inhibited by hypoxia and the inhibition is largely mediated by its direct central action.     

Sensory Afferent Selective Role of P2 Receptors in the Nucleus Tractus Solitarii for Mediating the Cardiac Component of the Peripheral Chemoreceptor Reflex in Rats

We have assessed the functional role of type 2 purinergic (P2) receptors within the caudal aspect of the commissural nucleus tractus solitarii (NTS) in mediating the peripheral chemoreceptor reflex cardiorespiratory response in the arterially perfused in situ working heart-brainstem preparation of rats. Microinjection in NTS of either suramin (100 pmol) or pyrinoxalphosphate-6-azophenyl-2',4'-disulphonic acid tetrasodium salt (PPADS; 10 pmol) depressed the reflex bradycardia (by ≈50 %), but not the tachypnoea, following peripheral chemoreceptor stimulation. In contrast, the reflex bradycardia produced by stimulation of pharyngo-oesophageal receptors was unaffected. Furthermore, microinjections in NTS of the P2X receptor agonist α,β-methyleneadenosine 5'-triphosphate (10 pmol) evoked a bradycardia which was antagonized by suramin (100 pmol). This P2X agonist reversibly potentiated the peripheral chemoreceptor-evoked bradycardia. The effect of suramin was selective to purinergic receptors because the bradycardia evoked by microinjection of α,β-methyleneadenosine 5'-triphosphate was blocked while the bradycardic responses to microinjections of NMDA or non-NMDA receptor agonists were not affected. From whole-cell recordings, some NTS neurones received convergent excitatory synaptic inputs from both peripheral chemoreceptors and receptors at the pharyngo-oesophageal junction. The excitatory postsynaptic response evoked by chemoreceptor stimulation was depressed by suramin, but convergent excitatory inputs from pharyngo-oesophageal receptors were unperturbed. Our findings support the hypothesis that caudal commissural NTS P2 purinergic receptors play a role in the neurotransmission of the parasympathetic (bradycardic) component of the chemoreceptor reflex. This effect is highly selective in that the chemoreceptor afferent-evoked tachypnoea, as well as other visceral receptor-mediated reflex bradycardia, remain unaffected.     

Effects of hypothalamic stimulation on activity of dorsomedial medulla neurons that respond to subdiaphragmatic vagal stimulation

1. Effects of hypothalamic stimulation on activity of dorsomedial medulla neurons that responded to subdiaphragmatic vagal stimulation were investigated in urethan-anesthetized rats. 2. Extracellular recordings were made from 231 neurons in the nucleus of the tractus solitarius (NTS) that fired repetitively in response to single-pulse subdiaphragmatic vagal stimulation and from 320 neurons in the dorsal motor nucleus of the vagal nerve (DMV) that responded antidromically to subdiaphragmatic vagal stimulation. The mean latencies of responses to subdiaphragmatic vagal stimulation were 90.3 +/- 17.1 ms (mean +/- SD) for NTS neurons, and 90.8 +/- 11.2 ms for DMV neurons. This indicated that both afferent and efferent subdiaphragmatic vagal fibers were thin and unmyelinated and had a conduction velocity of approximately 1 m/s. 3. In extracellular recordings from 320 DMV neurons, marked inhibition preceded the antidromic response and subdiaphragmatic vagal stimulation evoked orthodromic spikes in only a few neurons. 4. Intracellular recordings from 66 DMV neurons revealed inhibitory postsynaptic potentials (IPSPs) before the antidromic responses. These IPSPs suppressed spontaneous firing and prevented excitatory postsynaptic potentials (EPSPs) from generating action potentials. 5. Stimulation in all hypothalamic loci studied, the ventromedial hypothalamic nucleus (VMH), the lateral hypothalamic area (LHA), and the paraventricular nucleus (PVN), induced responses with similar characteristics of excitation alone or excitation followed by inhibition in most NTS and DMV neurons. 6. No reciprocal effect of VMH and LHA stimulation was observed on NTS and DMV neurons. 7. Intracellular recordings from DMV neurons revealed monosynaptic EPSPs in response to stimulation of the VMH, the LHA, and the PVN. 8. PVN stimulation evoked significantly more responses in NTS and DMV neurons than VMH stimulation and more responses in DMV neurons than LHA stimulation. This suggests a difference in the number of connections between each hypothalamic site and the dorsomedial medulla. 9. The same dorsomedial medulla neurons were tested with VMH and LHA stimulation. The respective mean latencies of the antidromic and the orthodromic NTS neuron responses were 37.3 +/- 3.2 and 39.6 +/- 12.9 ms for VMH stimulation and 29.8 +/- 5.3 and 31.8 +/- 8.7 ms for LHA stimulation. The mean latencies of the orthodromic DMV neuron responses were 39.4 +/- 8.3 ms for VMH stimulation and 31.1 +/- 5.2 ms for LHA stimulation. The estimated conduction velocity from the VMH to the dorsomedial medulla was approximately 0.25 m/s and from the LHA it was approximately 0.33 m/s, which was significantly faster.(ABSTRACT TRUNCATED AT 400 WORDS)