Depressor Nerve Research Articles

Myelinated fiber distribution in the aortic depressor nerve of male and female spontaneously hypertensive rats (728.22)

Aortic depressor nerve (ADN) is an afferent nerve that contains myelinated (fast conducting) and unmyelinated (slow conduction) fibers that send information from aortic mechanoreceptors to central nervous system [1]. Previous studies form our laboratory [2,3] in spontaneously hypertensive rats (SHR) have demonstrated morphological alterations on the ADN in SHR, using male and female animals indistinctively. In this study we investigated the ADN fiber distribution comparing male and female adult (20 weeks old) SHR. After the (AP) and heart rate (HR) recordings, together with the ADN spontaneous activity, the left ADN were prepared for light microscopy and morphometry. Myelinated fiber distributions were compared between proximal and distal segments in the same gender and between genders. Myelinated axon and G ratio distributions were also investigated. Myelinated fiber diameter range from 1 to 5 µm, with peak (~40% of fibers) at 2 µm while myelinated axon size range from 0.5 to 4 µm, with peak (~50% of axons) at 1.5 µm. No differences were observed between segments or genders. This indicates that gender does not influence on the ADN myelinated fiber size, allowing the use of male and females for morphological studies.[1] Krieger and Marseillan, Am J Physiol, 1963:205; [2] Fazan et. al, J Auton Nerv Syst 77, 1999:133; [3] Fazan et. al, Am J Physiol Heart Circ Physiol 280,2001:H1560.Grant Funding Source: FAPESP, CAPES, CNPq and FAEPA

Dietary NaCl alters several sympathoexcitatory and sympathoinhibitory reflexes: dependence on the ventral lamina terminalis (683.4)

Elevated dietary NaCl intake enhances both sympathoexcitatory and sympathoinhibitory responses evoked from the rostral ventrolateral medulla. These enhanced responses are prevented by prior lesion of the LT. The purpose of the present study was to test whether increased dietary NaCl intake enhanced a number of sympathetic reflexes dependent on neurotransmission in the rostral ventrolateral medulla and whether this was prevented by LT lesion. Male Sprague‐Dawley rats were fed 0.1% (low) or 4.0% (high) NaCl for 14‐18 days. Electrical stimulation of sciatic afferent nerves (1ms, 1‐20Hz, 500 uA) produced greater increases in renal SNA (5 Hz: 125±9% vs 187±23%, n=6/group) and mean ABP (5Hz: 14±2mmHg vs 28±2mmHg, n=6/group) of rats ingesting high versus low NaCl, respectively. LT lesion prevented the enhanced responses of renal SNA (low: 129±13% vs high: 144±13%, n=3/group) and mean ABP (low: 17±2mmHg vs high: 14±5mmHg, n=5/group). Stimulation of the aortic depressor nerve (1ms, 1‐20 Hz, 500 uA) produced greater decreases in renal SNA (low: ‐53±10% vs high: ‐74±7%, n=6/group) and mean ABP (low: ‐16±3mmHg vs high: ‐32±3mmHg, n=6/group) of rats fed low versus high NaCl diets. Again, LT lesion prevented the enhanced responses of renal SNA (low: ‐58±10% vs high: ‐59±6%, n=3/group) and mean ABP (low: ‐18±5mmHg vs high: ‐15±3mmHg, n=3/group). Similar findings were observed for additional reflexes including activation of vagal afferents and intracerebroventricular infusion of 1M NaCl. These findings suggest dietary salt activates LT neurons to alter the responsiveness of hindbrain neurons and enhance sympathetic reflexes.Grant Funding Source: Supported by NIH R01HL113270 and AHA Established Investigator Award

Nitric oxide stimulates glutamatergic synaptic inputs to baroreceptor neurons through potentiation of Cav2.2-mediated Ca2+ currents

Nitric oxide (NO) increases glutamate release to the second-order neurons in the nucleus tractus solitarius (NTS). N-type Ca(2+) channel is essential for triggering glutamate release at synaptic terminals. In this study, we determined the role of Cav2.2 subunit in NO-induced increase in glutamate synaptic inputs to NTS neurons. The second-order NTS neurons and nodose ganglionic (NG) neurons were identified by applying DiA, a fluorescent lipophilic tracer, on aortic depressor nerve in rats. NO donor DEA/NO significantly increased tractus solitarius (TS)-evoked excitatory postsynaptic currents (EPSCs) in second-order NTS neurons, an effect was abolished by pretreatment of slice with ODQ, an inhibitor for soluble isoform of guanylyl cyclase. DEA/NO decreased the paired-pulse ratio of TS-evoked EPSCs, while increased the frequency, but not the amplitude, of miniature EPSCs in second-order NTS neurons. Furthermore, DEA/NO significantly increased Ba(2+) currents in identified baroreceptor NG neurons. However, DEA/NO had little effect on the Ba(2+) currents in the presence of specific N-type Ca(2+) blocker ω-conotoxin GVIA. In addition, immunocytochemistry staining revealed that Cav2.2 subunit immunoreactivates were colocalized with DiA-labeled baroreceptor nerve terminals in the NTS. Collectively, these findings suggest that NO stimulates glutamatergic synaptic inputs to second-order NTS neurons through augmentation of Cav2.2-mediated N-type Ca(2+) currents.

Acute Experimental Diabetes and Aortic Depressor Nerve Ultrastructural Morphometry: Effects of Insulin Treatment

Previous results from our laboratory showed myelinated fiber alterations in the aortic depressor nerve (ADN) from rats rendered diabetic for 15 days (short term). The ultrastructure and the acute effect of insulin treatment, particularly in the unmyelinated fibres of this nerve, were not yet investigated in this experimental model, being the aim of this work. Wistar rats received a single intravenous injection of streptozotocin (STZ – 40 mg/Kg) 15 days (N=7) before the experiments. Control animals (N=7) received vehicle (citrate buffer) and treated animals (n=6) received a subcutaneous injection of insulin (10 IU/Kg) on a daily basis beginning 3 days after the STZ injection. Under pentobarbital anaesthesia the ADNs were isolated and had their spontaneous activity recorded. Afterwards, proximal and distal segments of the nerves were prepared for transmission electron microscopy studies. Morphometry of the myelinated and unmyelinated fibres was carried out with the aid of computer software. Nerves from diabetic animals showed some unmyelinated fiber enlargement with signs of swelling and degeneration, and the Schwann cells mitochondria presented enlargement and clustering formation. On treated animals, an important thickening of the Schwann cells’ basement membrane was present and some myelinated fibers showed thin myelin sheaths. The endoneural blood vessels ultrastrucutre was preserved in all nerves from the three experimental groups. Myelinated axon area and diameter was significantly larger on insulin treated animals, compared to both, controls and acute diabetic groups, on both segments. No other differences were observed between groups for the myelinated or unmyelinated fibers morphometric parameters. Investigation of experimental models of diabetes in early or acute stage of the disease provide relevant information on the physiopathological mechanisms involved in the alterations present in patients with chronic diabetes and its complications. We suggest that insulin treatment alone might have a role on the morphological alterations present in diabetic neuropathy.

Differential Contribution of Afferent and Central Pathways to the Development of Baroreflex Dysfunction in Chronic Kidney Disease

The effects of chronic kidney disease on baroreflex control of renal sympathetic nerve activity (RSNA) and deficits in afferent and central components of the baroreflex were studied in juvenile and adult male Lewis Polycystic Kidney (LPK) and control Lewis rats under anesthesia (n=35). Blood pressure (BP), heart rate (HR), aortic depressor nerve activity (ADNA), and RSNA were determined after pharmacological manipulation of BP. Responses to ADN stimulation (4.0 V, 2.0 ms, 1-24 Hz) were determined, and the aortic arch was collected for histomorphometry. In juvenile LPK versus age-matched Lewis rats, gain of RSNA (-1.5±0.2 versus -2.8±0.2%/mm Hg; P<0.05) and ADNA (2.5±0.3 versus 5.0±0.6%/mm Hg; P<0.05), but not HR barocurves, were reduced. BP, HR, and RSNA responses to ADN stimulation were normal or enhanced in juvenile LPK. In adult LPK versus age-matched Lewis, the gain and range of RSNA (gain: -1.2±0.1 versus -2.2±0.2%/mm Hg, range: 62±8 versus 98±7%) and HR (gain: -0.7±0.1 versus -3.5±0.7 bpm/mm Hg, range: 44±8 versus 111±19 bpm) barocurves were reduced (P<0.05). The gain and range of the ADNA barocurves were also reduced in adult LPK versus Lewis [1.5±0.4 versus 5.2±1.1 (%/mm Hg) and 133±35 versus 365±61 (%) P<0.05] and correlated with aortic arch vascular remodeling. BP, HR, and RSNA responses to ADN stimulation were significantly reduced in adult LPK. Our data demonstrate a deficit in the afferent component of the baroreflex that precedes the development of impaired central regulation of RSNA and HR in chronic kidney disease, and that progressive impairment of both components is associated with marked dysfunction of the baroreflex pathway.

Autocrine/paracrine modulation of baroreceptor activity after antidromic stimulation of aortic depressor nerve in vivo

Activation of the sensory nerve endings of non-myelinated C-fiber afferents evokes release of autocrine/paracrine factors that cause localized vasodilation, neurogenic inflammation, and modulation of sensory nerve activity. The aims of this study were to determine the effect of antidromic electrical stimulation on afferent baroreceptor activity in vivo, and investigate the role of endogenous prostanoids and hydrogen peroxide (H2O2) in mediating changes in nerve activity. Baroreceptor activity was recorded from the left aortic depressor nerve (ADN) in anesthetized rats before and after stimulating the ADN for brief (5–20 s) periods. The rostral end of the ADN was crushed or sectioned beforehand to prevent reflex changes in blood pressure. Antidromic stimulation of ADN using parameters that activate both myelinated A-fibers and non-myelinated C-fibers caused pronounced and long-lasting (> 1 min) inhibition of baroreceptor activity (n = 9, P < 0.05), with the magnitude and duration of inhibition dependent on the duration of the stimulation period (n = 5). Baroreceptor activity was only transiently inhibited after selective stimulation of A-fibers. The inhibition of activity after antidromic stimulation of A and C fibers was prolonged after administration of the cyclooxygenase inhibitor indomethacin (5 mg/kg, IV, n = 7) and abolished after administration of PEG-catalase (104 units/kg, IV, n = 7), an enzyme that catalyzes the decomposition of H2O2 to water and oxygen. The results demonstrate a long-lasting inhibition of baroreceptor activity after antidromic stimulation of ADN and suggest that endogenous prostanoids and H2O2 oppose and mediate the inhibition, respectively. These mechanisms may contribute to rapid baroreceptor resetting during acute hypertension and be engaged during chronic baroreceptor activation therapy in patients with hypertension.

Effect of simvastatin in the autonomic system is dependent on the increased gain/sensitivity of the baroreceptors

A number of mechanisms have been proposed to explain the pleiotropic effect of statin therapy to reduce sympathetic outflow in cardiovascular disease. We tested the hypothesis that statin treatment could improve baroreflex gain-sensitivity triggered by morphological adaptations in the mechanoreceptor site, thus reducing sympathetic activity, regardless of arterial pressure (AP) level reduction. Male spontaneously hypertensive rats (SHR) were divided into control (SHR, n = 8) and SHR-simvastatin (5 mg/kg/day, for 7 days) (SHR-S, n = 8). After treatment, AP, baroreflex sensitivity (BRS) in response to AP-induced changes, aortic depressor nerve activity, and spectral analyses of pulse interval (PI) and AP variabilities were performed. Internal and external carotids were prepared for morphoquantitative evaluation. Although AP was similar between groups, sympathetic modulation, represented by the low frequency band of PI (SHR: 6.84 ± 3.19 vs. SHR-S: 2.41 ± 0.96 msec2) and from systolic AP variability (SHR: 3.95 ± 0.36 vs. SHR-S: 2.86 ± 0.18 mmHg2), were reduced in treated animals. In parallel, simvastatin induced an increase of 26% and 21% in the number of elastic lamellae as well as a decrease of 9% and 25% in the carotid thickness in both, external and internal carotid, respectively. Moreover, improved baroreceptor function (SHR: 0.78 ± 0.03 vs. SHR-S: 1.06 ± 0.04% mv/mmHg) was observed in addition to a 115% increase in aortic depressor nerve activity in SHR-S rats. Therefore, our data suggest that the reduction of sympathetic outflow in hypertension by simvastatin treatment may be triggered by structural changes in the carotid arteries and increased BRS in response to an improvement of the baroreceptors discharge and consequently of the afferent pathway of the baroreflex arch.

Electrophysiological properties of medullary neurons involved with sympatho-inhibitory component of baroreflex in rats submitted to chronic intermittent hypoxia

We evaluated whether or not CIH affect the sympatho-inhibitory pathway of the baroreflex. Male Wistar rats were submitted to CIH for 10 days while control rats were maintained in normoxia. Using the working heart-brainstem preparations we recorded respiratory and sympathetic activities as well as intracellular recordings of RVLM pre-sympathetic, CVLM and NTS neurons. Baroreflex was activated by stimulation of aortic depressor nerve (ADN) during inspiration or expiration. In control rats (n = 32), the inhibitory thoracic sympathetic nerve (tSN) reflex responses to ADN stimulation were more pronounced during expiration than during inspiration (P < 0.05). In CIH rats (n = 31) the magnitude of tSN (-87.4 ± 6 vs -71.5%; p < 0.05) inhibitory responses to ADN stimulation during expiration, but not inspiration, were larger than in controls. Also in CIH rats, RVLM and CVLM neurons that displayed expiratory modulation, exhibited larger inhibitory (-83 ± 5 vs -59 ± 3%; p < 0.05; n = 20) and excitatory (79 ± 3 vs 55 ± 4%; p < 0.05; n = 23) reflex responses, respectively, to ADN stimulation than in controls. In the presence of synaptic blockade, we did not observe any changes in the electrophysiological properties (excitability and input resistance) of RVLM and CVLM neurons of CIH and control rats. However, ADN stimulation during expiration produced a larger increase in the firing frequency of NTS neurons (109 ± 3 vs 35 ± 2%; p < 0.05; n = 25) in CIH than in control rats. These data indicate that the increased baroreflex sympatho-inhibitory responses during expiration of CIH rats are not related to changes in the intrinsic properties of RVLM pre-sympathetic and CVLM neurons, but are linked to changes in the synaptic transmission or electrophysiological properties of NTS neurons.

Plasma leptin inhibits the response of nucleus of the solitary tract neurons to aortic baroreceptor stimulation

Leptin receptors have been identified within the nucleus of the solitary tract (NTS) and leptin injections into the caudal NTS inhibit the baroreceptor reflex. However, whether plasma leptin alters the discharge of NTS neurons mediating aortic baroreceptor reflex activity is not known. A series of electrophysiological single unit recording experiments was done in the urethane-chloralose anesthetized, paralyzed and artificially ventilated Wistar and Zucker obese rat with either their neuroaxis intact or with mid-collicular transections. Single units in NTS antidromically activated by electrical stimulation of depressor sites in the caudal ventrolateral medulla (CVLM) were found to display a cardiac cycle-related rhythmicity. These units were tested for their responses to stimulation of the aortic depressor nerve (ADN) and intra-carotid injections of leptin (50-200ng/0.1ml). Of 63 single units tested in NTS, 33 were antidromically activated by stimulation of CVLM depressor sites and 18 of these single units responded with a decrease in discharge rate after intracarotid injections of leptin. Thirteen of these leptin responsive neurons (∼72%) were excited by ADN stimulation. Furthermore, the excitatory response of these single units to ADN stimulation was attenuated by about 50% after the intracarotid leptin injection. Intracarotid injections of leptin (200ng/0.1ml) in the Zucker obese rat did not alter the discharge rate of NTS-CVLM projecting neurons. These data suggest that leptin exerts a modulatory effect on brainstem neuronal circuits that control cardiovascular responses elicited during the reflex activation of arterial baroreceptors.

Autonomic regulation of organ vascular resistances during hypoxemia in the cat

This study aimed to dissect the roles played by the autonomic interoreceptors, the carotid bodies (cbs) and the aortic bodies (abs) on the vascular resistances of several organs in anesthetized, paralyzed, artificially ventilated cats challenged by systemic hypoxemia. Two 15 min challenges stimulated each of 5 animals in two different groups: (1) in the intact group hypoxic hypoxia (10% O2 in N2; HH) stimulated both abs and cbs, increasing neural output to the nucleus tractus solitarius (NTS); (2) in this group carbon monoxide hypoxia (30% O2 in N2 with the addition of CO; COH) stimulated only the abs, increasing neural output to the NTS. (3) In the second group in which their bilateral aortic depressor nerves had been transected only the cbs increased neural output to the NTS during the HH challenge; (4) in this aortic body resected group during COH neither abs nor cbs increased neural traffic to the NTS. CO and 10% O2 reduced Hb saturation to the same level. With the use of radiolabeled microspheres blood flow was measured in a variety of organs. Organ vascular resistance was calculated by dividing the aortic pressure by that organ's blood flow. The spleen and pancreas revealed a vasoconstriction in the face of systemic hypoxemia, thought to be sympathetic nervous system (SNS)-mediated. The adrenals and the eyes vasodilated only when cbs were stimulated. Vasodilation in the heart and diaphragm showed no effect of chemoreceptor stimulated increase in SNS output. Different chemoreceptor involvement had different effects on the organs.

Exercise training normalizes the blunted central component of the baroreflex in rats with heart failure: role of the PVN

Exercise training (ExT) normalizes the increased sympathetic outflow in chronic heart failure (HF). The underlying mechanisms are not clearly understood. We hypothesized that ExT normalized the blunted central component of the baroreflex control of renal sympathetic nerve activity (RSNA) in HF. Four groups of rats [sham operated (sham)-sedentary (Sed), sham-ExT, HF-Sed, and HF-ExT] were used. HF was induced by left coronary artery ligation, and ExT consisted of 3 wk of treadmill running. In anesthetized rats, the decrease in RSNA in response to aortic depressor nerve stimulation (5-40 Hz) in the HF-Sed group was significantly lower than that in the sham-Sed group (-37 ± 7% vs. -63 ± 8% at 40 Hz, P < 0.05). In the HF-ExT group, responses in RSNA, mean arterial pressure (MAP), and heart rate (HR) were not significantly different from those in the sham-Sed or sham-ExT groups. ExT normalized blunted RSNA, MAP, and HR responses to bicuculline microinjections into the paraventricular nucleus (PVN) in rats with HF. Activation of the PVN by blockade of GABA receptors with bicuculline in normal control rats blunted the centrally component of the baroreflex arc. GABAA-α1 and -β1 receptor protein expression were significantly lower (by 48% and 30%) in the HF-Sed group, but ExT normalized this difference between the HF and sham groups. These data suggest that one mechanism by which ExT alleviates elevated sympathetic outflow in HF may be through normalization of central integrative mechanisms, perhaps via improving the inhibitory GABAergic mechanism within the PVN, on the baroreflex arc.

Continuous and intermittent electrical stimulation of the aortic depressor nerve in conscious rats: time course of the hemodynamic responses

We investigated the time course of the hemodynamic responses to continuous (CONT) or intermittent (ON/OFF) electrical stimulation of the aortic depressor nerve (ADN) in conscious rats. Rats were implanted with electrodes around the ADN and a catheter into the femoral artery. On the next day the baseline systolic (SAP), diastolic (DAP), mean arterial pressure (MAP) and heart rate (HR) were recorded during 30 minutes and the rats were randomly assigned to CONT (0.5 mA; 0.25 ms; 30 Hz) or ON/OFF (5s ON/3s OFF; 0.5 mA; 0.25 ms; 30 Hz;) stimulation during 60 min. Changes were greater under CONT as compared to ON/OFF stimulation (SAP: −21 ± 4 vs −10 ± 2 %; DAP: −23 ± 4 vs −14 ± 3 %; MAP: −22 ± 4 vs −12 ± 3 %). In addition, under CONT stimulation the HR was reduced within the first 5 min. The hypotensive and bradycardic responses were maintained over the entire period of stimulation in the ON/OFF protocol. Nevertheless, as compared to the first 5 min, CONT stimulation showed a rapid transitory increase in DAP (72 ± 3 vs 64 ± 3 mmHg) and MAP (87 ± 3 vs 78 ± 4 mmHg). In conclusion, CONT stimulation of the ADN elicits greater hemodynamic responses as compared to the ON/OFF protocol. This finding indicates that the CONT stimulation is more effective to reduce the arterial pressure during long period of electric baroreflex activation.Financial support: FAPESP, CNPq and CAPES.

Aortic depressor nerve morphometry in male and female adult spontaneously hypertensive rats.

Previous studies form our laboratory [1,2] in spontaneously hypertensive rats (SHR) have demonstrated morphological alterations on the aortic depressor nerve (ADN), known to contain mainly barorreceptor fibers. Nevertheless, those studies used male and female rats indistinctly. We are investigating if there are morphological differences related to gender in the ADN of adult rats. Male and female SHR aged 20‐week were anesthetized for arterial pressure (AP) and heart rate (HR) recordings, together with the ADN spontaneous activity. After the recordings, the left ADN were prepared for light microscopy and morphometry. Comparisons were made between proximal and distal segments in the same gender and between genders. Males are significantly heavier than females but no differences in AP and HR between genders were found. There was a tendency towards larger fascicle values on males compared to females. Interestingly, this tendency was inverted on the myelinated fibers and respective axons sizes. Nevertheless, no significant differences were observed between genders on the morphometric parameters. We conclude that adult males and females can be used indistinctly for morphometric studies of the ADN if physiological parameters are similar between genders.

Electrical stimulation of the aortic depressor nerve in conscious state elicits similar hemodynamic responses in heart failure and control rats

Heart Failure (HF) is characterized by attenuation of the baroreflex. The present study investigated the hypotensive and bradycardic responses produced by electrical stimulation of the aortic depressor nerve (ADN) in HF rats. HF was induced by left coronary artery ligation. Six weeks after ligation (n=11), or sham surgery (control rats; n=12), the animals were anesthetized for femoral artery and vein cannulation and implantation of bipolar electrodes for ADN stimulation. Twenty‐four hours later, electrical stimulation of the ADN was applied during 20s (5, 15, 30, 60 and 90 Hz; 1mA; 2ms duration). Baseline mean arterial pressure (105 ± 3 vs 100 ± 4 mmHg in HF) and heart rate (341±10 vs 324±13 bpm in HF) were similar between HF and control rats. Electrical stimulation of the ADN (5 to 90 Hz) elicited similar hypotensive (14±2 to 49±6 vs 10±3 to 41±3 mmHg in HF rats) and bradycardic (17±6 to 75±15 vs 21±6 to 67±9 bpm in HF rats) responses in both groups. Taken into account that HF is characterized by baroreflex attenuation, these findings indicate a derangement of the baroreceptor endings and/or the electrical stimulation overriding the baroreflex attenuation in HF rats.Financial Support: FAPESP, CNPq.

Fascicle morphometry of the recurrent laryngeal nerve in short term experimental diabetes.

Previous results from our laboratory [1] showed that 50% of the aortic depressor nerves (ADN), the afferent arm of the baroreflex, show alterations in early stages of induced diabetes in rats. The recurrent laryngeal nerves (RLN) are known to contain barorreceptor fibers as well and there are no reports on the morphology of the RLN in short term diabetes. Male Wistar rats received a single intravenous streptozotocin injection and control rats received vehicle. On the 15th day after the injections, rats were anesthetized for arterial pressure (AP) and heart rate (HR) recordings, prior to RLN removal. Proximal and distal segments of the right and left RLN were prepared for light microscopy morphometry, carried out with the aid of a computer software. Body weight increased significantly on the control s(309 ± 9 g) compared to the diabetics (200 ± 4 g). As expected, blood glucose levels were significantly high in the diabetic animals. Both the AP and HR were reduced in the diabetic group, attaining significant difference on the HR values. Fascicle morphometry parameters were not different between groups. We suggest that 15 days was not long enough to trigger detectable differences in the RLN fascicles. This difference with our previous findings in the ADN is probably due to the nerve size, since the ADN is much smaller than the RLN.

Leptin in nucleus of the solitary tract alters the cardiovascular responses to aortic baroreceptor activation

Recent data suggests that neurons expressing the long form of the leptin receptor form at least two distinct groups within the caudal nucleus of the solitary tract (NTS): a group within the lateral NTS (Slt) and one within the medial (Sm) and gelantinosa (Sg) NTS. Discrete injections of leptin into Sm and Sg, a region that receives chemoreceptor input, elicit increases in arterial pressure (AP) and renal sympathetic nerve activity (RSNA). However, the effect of microinjections of leptin into Slt, a region that receives baroreceptor input is unknown. Experiments were done in the urethane-chloralose anesthetized, paralyzed and artificially ventilated Wistar or Zucker obese rat to determine leptin's effect in Slt on heart rate (HR), AP and RSNA during electrical stimulation of the aortic depressor nerve (ADN). Depressor sites within Slt were first identified by the microinjection of l-glutamate (Glu; 0.25M; 10nl) followed by leptin microinjections. In the Wistar rat leptin microinjection (50ng; 20nl) into depressor sites within the lateral Slt elicited increases in HR and RSNA, but no changes in AP. Additionally, leptin injections into Slt prior to Glu injections at the same site or to stimulation of the ADN were found to attenuate the decreases in HR, AP and RSNA to both the Glu injection and ADN stimulation. In Zucker obese rats, leptin injections into NTS depressor sites did not elicit cardiovascular responses, nor altered the cardiovascular responses elicited by stimulation of ADN. Those data suggest that leptin acts at the level of NTS to alter the activity of neurons that mediate the cardiovascular responses to activation of the aortic baroreceptor reflex.

Ultrastructural Morphometry of the Aortic Depressor Nerves and Extrinsic Renal Nerves: Similarities and Differences between Mice and Rats

Anatomical and physiological aspects involving rats and mice have shown similarities and differences between these experimental animal models. In cardiovascular physiology research, rats are being substituted by mice since mice are more susceptible to genetic manipulation. Nevertheless, little is known about mice normal anatomy and/or physiology to allow the correct interpretation of altered responses on genetically manipulated animals. We compared morphometric ultrastructural parameters of the aortic depressor nerve (ADN) and extrinsic renal nerve (SRN) between Wistar rats and C57BL/6J mice. After spontaneous activity recordings together with the arterial pressure pulse, ADN and SRN were dissected and prepared for light and transmission electron microscopy. Morphometry was performed with an image analysis software and took into consideration the fascicle area and diameter, myelinated and unmyelinated fiber number, density, area and diameter, myelin sheath area and the g-ratio. Comparisons were made for the same nerve between mice and rats and differences were considered significant when p<0.05. Both nerves were large in rats compared to mice, as were the myelinated fibers. On the other hand, unmyelinated fibers density, size and distributions were not different between species. These results suggest physiological differences on the fast conduction fibers between species, with possible different functional role. Morphological comparisons of the quantitative composition of peripheral nerves in different species and strains are very rare. Our study contributes to a morphological understanding of important nerves related to cardiovascular reflexes in two animal models. It also presents, for the first time, ultrastructural morphometric characteristics of the ADN in mice. This data provide a morphological basis for further studies involving functional investigations for reflex regulation of circulation in experimental model of hypertension and to improved cardiovascular system knowledge.

Ultrastructural and morphometric alterations in the aortic depressor nerve of rats due to long term experimental diabetes: Effects of insulin treatment

Most of the reports about an altered baroreflex attribute this condition to the diabetic efferent neuropathy of the aortic depressor nerve (ADN) (afferent arm of the baroreflex less explored). We evaluated the ADN ultrastructural alterations caused by long term experimental diabetes and the effects of insulin treatment. Wistar rats (N=14) received a single intravenous injection of streptozotocin (40 mg/kg) 12 weeks before the experiment. Control animals (N=9) received vehicle (citrate buffer). Insulin treated animals (N=8) received a single subcutaneous injection of insulin daily. Under pentobarbital anesthesia the ADNs were isolated and had their spontaneous activity recorded. Afterwards, proximal and distal segments of the nerves were prepared for transmission electron microscopy study. Morphometry of the unmyelinated fibers was carried out with the aid of computer software. ADN of the diabetic animals showed axonal atrophy for myelinated fibers, with more pronounced alterations of the myelin sheath, such as myelin infolding and out folding, presence of myelin balls and very thin myelin sheath in relation to the axonal size, particularly for the small myelinated fibers becoming evident. No differences were observed in myelinated fiber number and their density, as well as on the fascicular area. Unmyelinated fiber number was significantly larger in the diabetic group while fiber diameter was significantly smaller compared to control. This result suggests axonal atrophy or, if associated to the larger number of fibers present in this group, could indicate fiber sprouting. These alterations were more evident in the distal segments of the nerves and were moderated by insulin treatment.

Bionic Baroreceptor Corrects Postural Hypotension in Rats With Impaired Baroreceptor

Impairment of the arterial baroreflex causes orthostatic hypotension. Arterial baroreceptor sensitivity degrades with age. Thus, an impaired baroreceptor plays a pivotal role in orthostatic hypotension in most elderly patients. There is no effective treatment for orthostatic hypotension. The aims of this investigation were to develop a bionic baroreceptor (BBR) and to verify whether it corrects postural hypotension. The BBR consists of a pressure sensor, a regulator, and a neurostimulator. In 35 Sprague-Dawley rats, we vascularly and neurally isolated the baroreceptor regions and attached electrodes to the aortic depressor nerve for stimulation. To mimic impaired baroreceptors, we maintained intracarotid sinus pressure at 60 mm Hg during activation of the BBR. Native baroreflex was reproduced by matching intracarotid sinus pressure to the instantaneous pulsatile aortic pressure. The encoding rule for translating intracarotid sinus pressure into stimulation of the aortic depressor nerve was identified by a white noise technique and applied to the regulator. The open-loop arterial pressure response to intracarotid sinus pressure (n=7) and upright tilt-induced changes in arterial pressure (n=7) were compared between native baroreceptor and BBR conditions. The intracarotid sinus pressure-arterial pressure relationships were comparable. Compared with the absence of baroreflex, the BBR corrected tilt-induced hypotension as effectively as under native baroreceptor conditions (native, -39±5 mm Hg; BBR, -41±5 mm Hg; absence, -63±5 mm Hg; P<0.05). The BBR restores the pressure buffering function. Although this research demonstrated feasibility of the BBR, further research is needed to verify its long-term effect and safety in larger animal models and humans.

Hypobaric Intermittent Hypoxia Attenuates Hypoxia-induced Depressor Response

BackgroundHypobaric intermittent hypoxia (HIH) produces many favorable effects in the cardiovascular system such as anti-hypertensive effect. In this study, we showed that HIH significantly attenuated a depressor response induced by acute hypoxia.Methodology/Principal FindingsSprague-Dawley rats received HIH in a hypobaric chamber simulating an altitude of 5000 m. The artery blood pressure (ABP), heart rate (HR) and renal sympathetic nerve activity (RSNA) were recorded in anesthetized control rats and rats received HIH. The baseline ABP, HR and RSNA were not different between HIH and control rats. Acute hypoxia-induced decrease in ABP was significantly attenuated in HIH rat compared with control rats. However, acute hypoxia-induced increases in HR and RSNA were greater in HIH rat than in control rats. After removal of bilateral ascending depressor nerves, acute hypoxia-induced depressor and sympathoexcitatory responses were comparable in control and HIH rats. Furthermore, acute hypoxia-induced depressor and sympathoexcitatory responses did not differ between control and HIH groups after blocking ATP-dependent K+ channels by glibenclamide. The baroreflex function evaluated by intravenous injection of phenylephrine and sodium nitroprusside was markedly augmented in HIH rats compared with control rats. The pressor and sympathoexcitatory responses evoked by intravenous injection of cyanide potassium were also significantly greater in HIH rats than in control rats.Conclusions/SignificanceOur findings suggest that HIH suppresses acute hypoxia-induced depressor response through enhancement of baroreflex and chemoreflex function, which involves activation of ATP-dependent K+ channels. This study provides new information and underlying mechanism on the beneficiary effect of HIH on maintaining cardiovascular homeostasis.