Graded and dynamic reflex summation of myelinated and unmyelinated rat aortic baroreceptors

Low intensity stimulation of aortic baroreceptor afferent fibers as a potential therapeutic alternative for hypertension treatment

Carotid baroreceptor stimulation has been clinically explored for antihypertensive benefits, but neuromodulation of aortic baroreceptor afferents remains unexplored for potential translation into the clinic. Published studies have used supramaximal stimulations, which are unphysiological and energy inefficient. The objective of the present study was to identify optimal low-charge nerve stimulation parameters that would provide a clinically-relevant (20–30 mmHg) decrease in mean arterial pressure (MAP) in anesthetized spontaneously hypertensive rats. Stimulations of 20 s were delivered to the left aortic depressor nerve (ADN) of these rats using low ranges of pulse amplitudes (≤ 0.6 mA), widths (≤ 0.5 ms) and frequencies (≤ 5 Hz). We also assessed the effects of continuous (20 s) versus intermittent (5 s ON/3 s OFF and 5 s ON/3 s OFF for 20 s) stimulation on MAP, heart rate (HR), mesenteric (MVR) and femoral (FVR) vascular resistance using low (5 Hz) and high (15 Hz) frequencies. Lower pulse amplitudes (0.2 mA) produced 9 ± 2 to 18 ± 2 mmHg decreases in MAP. Higher pulse amplitudes (0.4 mA) produced a median MAP reduction of 28 ± 4 mmHg at 0.2 ms and 5 Hz, with no added benefit seen above 0.4 mA. Continuous and intermittent low frequency stimulation at 0.4 mA and 0.2 ms produced similar sustained decreases in MAP, HR, MVR and FVR. Continuous high frequency stimulation at 0.4 mA and 0.2 ms produced larger reductions in MAP, HR, MVR and FVR compared with all low frequency and/or intermittent high frequency stimulations. We conclude from these findings that “low intensity intermittent” electrical stimulation is an effective alternate way for neuromodulation of the aortic baroreceptor afferents and to evoke a required restoration of MAP levels in spontaneously hypertensive rats. This approach enables low energy consumption and markedly lowers the excessive decreases in MAP and hemodynamic disturbances elicited by continuous high-charge injection protocols.

Dedicated C-fiber vagal sensory afferent pathways to the paraventricular nucleus of the hypothalamus

The nucleus of the solitary tract (NTS) receives viscerosensory information from the vagus nerve to regulate diverse homeostatic reflex functions. The NTS projects to a wide network of other brain regions, including the paraventricular nucleus of the hypothalamus (PVN). Here we examined the synaptic characteristics of primary afferent pathways to PVN-projecting NTS neurons in rat brainstem slices.Expression of the Transient Receptor Potential Vanilloid receptor (TRPV1+ ) distinguishes C-fiber afferents within the solitary tract (ST) from A-fibers (TRPV1-). We used resiniferatoxin (RTX), a TRPV1 agonist, to differentiate the two. The variability in the latency (jitter) of evoked excitatory postsynaptic currents (ST-EPSCs) distinguished monosynaptic from polysynaptic ST-EPSCs. Rhodamine injected into PVN was retrogradely transported to identify PVN-projecting NTS neurons within brainstem slices. Graded shocks to the ST elicited all-or-none EPSCs in rhodamine-positive NTS neurons with latencies that had either low jitter (<200 µs - monosynaptic), high jitter (>200 µs - polysynaptic inputs) or both. RTX blocked ST-evoked TRPV1 + EPSCs whether mono- or polysynaptic. Most PVN-projecting NTS neurons (17/21 neurons) had at least one input polysynaptically connected to the ST. Compared to unlabeled NTS neurons, PVN-projecting NTS neurons were more likely to receive indirect inputs and be higher order. Surprisingly, sEPSC rates for PVN-projecting neurons were double that of unlabeled NTS neurons. The ST synaptic responses for PVN-projecting NTS neurons were either all TRPV1+ or all TRPV1-, including neurons that received both direct and indirect inputs. Overall, PVN-projecting NTS neurons received direct and indirect vagal afferent information with strict segregation regarding TRPV1 expression.

Cannabidiol activation of vagal afferent neurons requires TRPA1

Vagal afferent neurons abundantly express excitatory transient receptor potential (TRP) channels, which strongly influence afferent signaling. Cannabinoids have been identified as direct agonists of TRP channels, including TRPA1 and TRPV1, suggesting that exogenous cannabinoids may influence vagal signaling via TRP channel activation. The diverse therapeutic effects of electrical vagus nerve stimulation also result from administration of the nonpsychotropic cannabinoid, cannabidiol (CBD); however, the direct effects of CBD on vagal afferent signaling remain unknown. We investigated actions of CBD on vagal afferent neurons, using calcium imaging and electrophysiology. CBD produced strong excitatory effects in neurons expressing TRPA1. CBD responses were prevented by removal of bath calcium, ruthenium red, and the TRPA1 antagonist A967079, but not the TRPV1 antagonist SB366791, suggesting an essential role for TRPA1. These pharmacological experiments were confirmed using genetic knockouts where TRPA1 KO mice lacked CBD responses, whereas TRPV1 knockout (KO) mice exhibited CBD-induced activation. We also characterized CBD-provoked inward currents at resting potentials in vagal afferents expressing TRPA1 that were absent in TRPA1 KO mice, but persisted in TRPV1 KO mice. CBD also inhibited voltage-activated sodium conductances in A-fiber, but not in C-fiber afferents. To simulate adaptation, resulting from chronic cannabis use, we administered cannabis extract vapor daily for 3 wk. Cannabis exposure reduced the magnitude of CBD responses, likely due to a loss of TRPA1 signaling. Together, these findings detail a novel excitatory action of CBD at vagal afferent neurons, which requires TRPA1 and may contribute to the vagal mimetic effects of CBD and adaptation following chronic cannabis use.NEW & NOTEWORTHY CBD usage has increased with its legalization. The clinical efficacy of CBD has been demonstrated for conditions including some forms of epilepsy, depression, and anxiety that are also treatable by vagus nerve stimulation. We found CBD exhibited direct excitatory effects on vagal afferent neurons that required TRPA1, were augmented by TRPV1, and attenuated following chronic cannabis vapor exposure. These effects may contribute to vagal mimetic effects of CBD and adaptation after chronic cannabis use.

Laterality Influences Central Integration of Baroreceptor Afferent Input in Male and Female Sprague Dawley Rats

We explored the effects of baroreceptor afferents laterality and sexual dimorphism on the expression of cardiovascular reflex responses to baroreflex activation in Sprague Dawley (SD) rats. Under urethane anesthesia, rats of either sex (total n = 18) were instrumented for left, right and bilateral aortic depressor nerve (ADN) stimulation (1–40 Hz, 0.2 ms, 0.4 mA for 20 s) and measurement of mean arterial pressure (MAP), heart rate (HR) and mesenteric (MVR) and femoral (FVR) vascular resistance. Female rats were matched for the diestrus phase of the estrus cycle. Left, right and bilateral ADN stimulation evoked frequency-dependent drops in MAP, HR, and MVR, and increases in FVR. Irrespective of sex, left and bilateral ADN stimulation as compared to right-sided stimulation mediated greater reflex reductions in MAP, HR, and MVR but not in FVR. In males, reflex bradycardic responses were greater in response to bilateral stimulation relative to both left- and right-sided stimulation. In females, left ADN stimulation evoked the largest increase in FVR. Left and bilateral ADN stimulations evoked greater reductions in MAP and MVR while left-sided stimulation produced larger increases in FVR in females compared with males. All other reflex responses to ADN stimulation were relatively comparable between males and females. These results show a differential baroreflex processing of afferent neurotransmission promoted by left versus right baroreceptor afferent inputs and sexual dimorphism in the expression of baroreflex responses in rats of either sex. Collectively, these data add to our understanding of physiological mechanisms pertaining to baroreflex control in both males and females.

Distinct Calcium Sources Define Compartmentalized Synaptic Signaling Domains

Nervous system communication relies on neurotransmitter release for synaptic transmission between neurons. Neurotransmitter is contained within vesicles in presynaptic terminals and intraterminal calcium governs the fundamental step of their release into the synaptic cleft. Despite a common dependence on calcium, synaptic transmission and its modulation varies highly across the nervous system. The precise mechanisms that underlie this heterogeneity, however, remain unclear. The present review highlights recent data that reveal vesicles sourced from separate pools define discrete modes of release. A rich diversity of regulatory machinery may further distinguish the different forms of vesicle release, including presynaptic proteins involved in trafficking, alignment, and exocytosis. These multiple vesicle release mechanisms and vesicle pools likely depend on the arrangement of vesicles in relation to specific calcium entry pathways that create compartmentalized spheres of calcium influence (i.e., domains). This diversity permits release specialization. This review details examples of how individual neurons rely on multiple calcium sources and unique regulatory schemes to provide differential release and discrete modulation of neurotransmitter release from specific vesicle pools-as part of network signal integration.

Neuropeptide Y-mediated sex- and afferent-specific neurotransmissions contribute to sexual dimorphism of baroreflex afferent function

Background

Molecular and cellular mechanisms of neuropeptide-Y (NPY)-mediated gender-difference in blood pressure (BP) regulation are largely unknown.

Methods

Baroreceptor sensitivity (BRS) was evaluated by measuring the response of BP to phenylephrine/nitroprusside. Serum NPY concentration was determined using ELISA. The mRNA and protein expression of NPY receptors were assessed in tissue and single-cell by RT-PCR, immunoblot, and immunohistochemistry. NPY was injected into the nodose while arterial pressure was monitored. Electrophysiological recordings were performed on nodose neurons from rats by patch-clamp technique.

Results

The BRS was higher in female than male and ovariectomized rats, while serum NPY concentration was similar among groups. The sex-difference was detected in Y₁R, not Y₂R protein expression, however, both were upregulated upon ovariectomy and canceled by estrogen replacement. Immunostaining confirmed Y₁R and Y₂R expression in myelinated and unmyelinated afferents. Single-cell PCR demonstrated that Y₁R expression/distribution was identical between A- and C-types, whereas, expressed level of Y₂R was ∼15 and ∼7 folds higher in Ah- and C-types than A-types despite similar distribution. Activation of Y₁R in nodose elevated BP, while activation of Y₂R did the opposite. Activation of Y₁R did not alter action potential duration (APD) of A-types, but activation of Y₂R- and Y₁R/Y₂R in Ah- and C-types frequency-dependently prolonged APD. N-type I_Ca was reduced in A-, Ah- and C-types when either Y₁R, Y₂R, or both were activated. The sex-difference in Y₁R expression was also observed in NTS.

Conclusions

Sex- and afferent-specific expression of Neuropeptide-Y receptors in baroreflex afferent pathway may contribute to sexual-dimorphic neurocontrol of BP regulation.

Viscerosensory input drives angiotensin II type 1A receptor-expressing neurons in the solitary tract nucleus

Homeostatic regulation of visceral organ function requires integrated processing of neural and neurohormonal sensory signals. The nucleus of the solitary tract (NTS) is the primary sensory nucleus for cranial visceral sensory afferents. Angiotensin II (ANG II) is known to modulate peripheral visceral reflexes, in part, by activating ANG II type 1A receptors (AT_1AR) in the NTS. AT_1AR-expressing NTS neurons occur throughout the NTS with a defined subnuclear distribution, and most of these neurons are depolarized by ANG II. In this study we determined whether AT_1AR-expressing NTS neurons receive direct visceral sensory input, and whether this input is modulated by ANG II. Using AT_1AR-GFP mice to make targeted whole cell recordings from AT_1AR-expressing NTS neurons, we demonstrate that two-thirds (37 of 56) of AT_1AR-expressing neurons receive direct excitatory, visceral sensory input. In half of the neurons tested (4 of 8) the excitatory visceral sensory input was significantly reduced by application of the transient receptor potential vallinoid type 1 receptor agonist, capsaicin, indicating AT_1AR-expressing neurons can receive either C- or A-fiber-mediated input. Application of ANG II to a subset of second-order AT_1AR-expressing neurons did not affect spontaneous, evoked, or asynchronous glutamate release from visceral sensory afferents. Thus it is unlikely that AT_1AR-expressing viscerosensory neurons terminate on AT_1AR-expressing NTS neurons. Our data suggest that ANG II is likely to modulate multiple visceral sensory modalities by altering the excitability of second-order AT_1AR-expressing NTS neurons.

Tonic aortic depressor nerve stimulation does not impede baroreflex dynamic characteristics concomitantly mediated by the stimulated nerve

Although electrical activation of the carotid sinus baroreflex (baroreflex activation therapy) is being explored as a device therapy for resistant hypertension, possible effects on baroreflex dynamic characteristics of interaction between electrical stimulation and pressure inputs are not fully elucidated. To examine whether the electrical stimulation of the baroreceptor afferent nerve impedes normal short-term arterial pressure (AP) regulation mediated by the stimulated nerve, we electrically stimulated the right aortic depressor nerve (ADN) while estimating the baroreflex dynamic characteristics by imposing pressure inputs to the isolated baroreceptor region of the right ADN in nine anesthetized rats. A Gaussian white noise signal with a mean of 120 mmHg and standard deviation of 20 mmHg was used for the pressure perturbation. A tonic ADN stimulation (2 or 5 Hz, 10 V, 0.1-ms pulse width) decreased mean sympathetic nerve activity (367.0 ± 70.9 vs. 247.3 ± 47.2 arbitrary units, P < 0.01) and mean AP (98.4 ± 7.8 vs. 89.2 ± 4.5 mmHg, P < 0.01) during dynamic pressure perturbation. The ADN stimulation did not affect the slope of dynamic gain in the neural arc transfer function from pressure perturbation to sympathetic nerve activity (16.9 ± 1.0 vs. 14.7 ± 1.6 dB/decade, not significant). These results indicate that electrical stimulation of the baroreceptor afferent nerve does not significantly impede the dynamic characteristics of the arterial baroreflex concomitantly mediated by the stimulated nerve. Short-term AP regulation by the arterial baroreflex may be preserved during the baroreflex activation therapy.

Effects of different input pressure waveforms on the carotid sinus baroreflex-mediated sympathetic arterial pressure response in rats

Although the pulsatility of an input pressure is an important factor that determines the arterial baroreflex responses, whether the difference in the input waveforms can meaningfully affect the baroreflex function remains unknown. This study aimed to compare baroreflex responses between two distinct pressure waveforms: a forward saw wave (FSW) and a backward saw wave (BSW). In seven anesthetized rats, carotid sinus pressure was exposed to the FSW or the BSW with a mean of 120 mmHg, pulse pressure of 40 mmHg, and pulse frequency of 1 Hz. Changes in efferent sympathetic nerve activity (SNA) and arterial pressure (AP) during six consecutive saw wave trials (FSW1, BSW1, FSW2, BSW2, FSW3, and BSW3) were examined. The steady-state SNA value during FSW1 was 91.1 ± 1.9%, which was unchanged during FSW2 and FSW3 but significantly increased during BSW1 (106.6 ± 3.4%, P < 0.01), BSW2 (110.6 ± 2.5%, P < 0.01), and BSW3 (111.6 ± 2.3%, P < 0.01). The steady-state AP value during FSW1 was 98.2 ± 8.1 mmHg, which was unchanged during FSW2 and FSW3 but significantly increased during BSW1 (106.7 ± 7.4 mmHg, P < 0.01), BSW2 (105.6 ± 7.8 mmHg, P < 0.01), and BSW3 (103.8 ± 7.2 mmHg, P < 0.05). In conclusion, the FSW was more effective than the BSW in reducing mean SNA and AP. The finding could be applied to designing an artificial pulsatile pressure such as that generated by left ventricular assist devices.

NEW & NOTEWORTHY This study examined whether the waveforms of an input pressure alone can affect the baroreflex function by using a forward saw wave and a backward saw wave with the same mean pressure, pulse pressure, and pulse frequency. The forward saw wave was more effective than the backward saw wave in reducing sympathetic nerve activity and arterial pressure. The finding could be applied to designing an artificial pulsatile pressure such as that generated by left ventricular assist devices.

Cervical vagus nerve stimulation augments spontaneous discharge in second- and higher-order sensory neurons in the rat nucleus of the solitary tract

Vagus nerve stimulation (VNS) currently treats patients with drug-resistant epilepsy, depression, and heart failure. The mild intensities used in chronic VNS suggest that primary visceral afferents and central nervous system activation are involved. Here, we measured the activity of neurons in the nucleus of the solitary tract (NTS) in anesthetized rats using clinically styled VNS. Our chief findings indicate that VNS at threshold bradycardic intensity activated NTS neuron discharge in one-third of NTS neurons. This VNS directly activated only myelinated vagal afferents projecting to second-order NTS neurons. Most VNS-induced activity in NTS, however, was unsynchronized to vagal stimuli. Thus, VNS activated unsynchronized activity in NTS neurons that were second order to vagal afferent C-fibers as well as higher-order NTS neurons only polysynaptically activated by the vagus. Overall, cardiovascular-sensitive and -insensitive NTS neurons were similarly activated by VNS: 3/4 neurons with monosynaptic vagal A-fiber afferents, 6/42 neurons with monosynaptic vagal C-fiber afferents, and 16/21 polysynaptic NTS neurons. Provocatively, vagal A-fibers indirectly activated C-fiber neurons during VNS. Elevated spontaneous spiking was quantitatively much higher than synchronized activity and extended well into the periods of nonstimulation. Surprisingly, many polysynaptic NTS neurons responded to half the bradycardic intensity used in clinical studies, indicating that a subset of myelinated vagal afferents is sufficient to evoke VNS indirect activation. Our study uncovered a myelinated vagal afferent drive that indirectly activates NTS neurons and thus central pathways beyond NTS and support reconsideration of brain contributions of vagal afferents underpinning of therapeutic impacts.NEW & NOTEWORTHY Acute vagus nerve stimulation elevated activity in neurons located in the medial nucleus of the solitary tract. Such stimuli directly activated only myelinated vagal afferents but indirectly activated a subpopulation of second- and higher-order neurons, suggesting that afferent mechanisms and central neuron activation may be responsible for vagus nerve stimulation efficacy.

Aortic depressor nerve stimulation does not impede the dynamic characteristics of the carotid sinus baroreflex in normotensive or spontaneously hypertensive rats

Recent clinical trials in patients with drug-resistant hypertension indicate that electrical activation of the carotid sinus baroreflex can reduce arterial pressure (AP) for more than a year. To examine whether the electrical stimulation from one baroreflex system impedes normal short-term AP regulation via another unstimulated baroreflex system, we electrically stimulated the left aortic depressor nerve (ADN) while estimating the dynamic characteristics of the carotid sinus baroreflex in anesthetized normotensive Wistar-Kyoto (WKY; n = 8) rats and spontaneously hypertensive rats (SHR; n = 7). Isolated carotid sinus regions were perturbed for 20 min using a Gaussian white noise signal with a mean of 120 mmHg for WKY and 160 mmHg for SHR. Tonic ADN stimulation (2 Hz, 10 V, and 0.1-ms pulse width) decreased mean sympathetic nerve activity (73.4 ± 14.0 vs. 51.6 ± 11.3 arbitrary units in WKY, P = 0.012; and 248.7 ± 33.9 vs. 181.1 ± 16.6 arbitrary units in SHR, P = 0.018) and mean AP (90.8 ± 6.6 vs. 81.2 ± 5.4 mmHg in WKY, P = 0.004; and 128.6 ± 9.8 vs. 114.7 ± 10.3 mmHg in SHR, P = 0.009). The slope of dynamic gain in the neural arc transfer function from carotid sinus pressure to sympathetic nerve activity was not different between trials with and without the ADN stimulation (12.55 ± 0.93 vs. 13.03 ± 1.28 dB/decade in WKY, P = 0.542; and 17.37 ± 1.01 vs. 17.47 ± 1.64 dB/decade in SHR, P = 0.946). These results indicate that the tonic ADN stimulation does not significantly modify the dynamic characteristics of the carotid sinus baroreflex.

Ketamine-mediated afferent-specific presynaptic transmission blocks in low-threshold and sex-specific subpopulation of myelinated Ah-type baroreceptor neurons of rats

Background

Ketamine enhances autonomic activity, and unmyelinated C-type baroreceptor afferents are more susceptible to be blocked by ketamine than myelinated A-types. However, the presynaptic transmission block in low-threshold and sex-specific myelinated Ah-type baroreceptor neurons (BRNs) is not elucidated.

Methods

Action potentials (APs) and excitatory post-synaptic currents (EPSCs) were investigated in BRNs/barosensitive neurons identified by conduction velocity (CV), capsaicin-conjugated with Iberiotoxin-sensitivity and fluorescent dye using intact nodose slice and brainstem slice in adult female rats. The expression of mRNA and targeted protein for NMDAR1 was also evaluated.

Results

Ketamine time-dependently blocked afferent CV in Ah-types in nodose slice with significant changes in AP discharge. The concentration-dependent inhibition of ketamine on AP discharge profiles were also assessed and observed using isolated Ah-type BRNs with dramatic reduction in neuroexcitability. In brainstem slice, the 2^nd-order capsaicin-resistant EPSCs were identified and ∼50% of them were blocked by ketamine concentration-dependently with IC₅₀ estimated at 84.4 μM compared with the rest (708.2 μM). Interestingly, the peak, decay time constant, and area under curve of EPSCs were significantly enhanced by 100 nM iberiotoxin in ketamine-more sensitive myelinated NTS neurons (most likely Ah-types), rather than ketamine-less sensitive ones (A-types).

Conclusions

These data have demonstrated, for the first time, that low-threshold and sex-specific myelinated Ah-type BRNs in nodose and Ah-type barosensitive neurons in NTS are more susceptible to ketamine and may play crucial roles in not only mean blood pressure regulation but also buffering dynamic changes in pressure, as well as the ketamine-mediated cardiovascular dysfunction through sexual-dimorphic baroreflex afferent pathway.

Modeling the differentiation of A- and C-type baroreceptor firing patterns

The baroreceptor neurons serve as the primary transducers of blood pressure for the autonomic nervous system and are thus critical in enabling the body to respond effectively to changes in blood pressure. These neurons can be separated into two types (A and C) based on the myelination of their axons and their distinct firing patterns elicited in response to specific pressure stimuli. This study has developed a comprehensive model of the afferent baroreceptor discharge built on physiological knowledge of arterial wall mechanics, firing rate responses to controlled pressure stimuli, and ion channel dynamics within the baroreceptor neurons. With this model, we were able to predict firing rates observed in previously published experiments in both A- and C-type neurons. These results were obtained by adjusting model parameters determining the maximal ion-channel conductances. The observed variation in the model parameters are hypothesized to correspond to physiological differences between A- and C-type neurons. In agreement with published experimental observations, our simulations suggest that a twofold lower potassium conductance in C-type neurons is responsible for the observed sustained basal firing, where as a tenfold higher mechanosensitive conductance is responsible for the greater firing rate observed in A-type neurons. A better understanding of the difference between the two neuron types can potentially be used to gain more insight about pathophysiology and treatment of diseases related to baroreflex function, e.g. in patients with autonomic failure, a syndrome that is difficult to diagnose in terms of its pathophysiology.

Translational neurocardiology: preclinical models and cardioneural integrative aspects

Neuronal elements distributed throughout the cardiac nervous system, from the level of the insular cortex to the intrinsic cardiac nervous system, are in constant communication with one another to ensure that cardiac output matches the dynamic process of regional blood flow demand. Neural elements in their various 'levels' become differentially recruited in the transduction of sensory inputs arising from the heart, major vessels, other visceral organs and somatic structures to optimize neuronal coordination of regional cardiac function. This White Paper will review the relevant aspects of the structural and functional organization for autonomic control of the heart in normal conditions, how these systems remodel/adapt during cardiac disease, and finally how such knowledge can be leveraged in the evolving realm of autonomic regulation therapy for cardiac therapeutics.

Decreased excitability and voltage-gated sodium currents in aortic baroreceptor neurons contribute to the impairment of arterial baroreflex in cirrhotic rats

Cardiovascular autonomic dysfunction, which is manifested by an impairment of the arterial baroreflex, is prevalent irrespective of etiology and contributes to the increased morbidity and mortality in cirrhotic patients. However, the cellular mechanisms that underlie the cirrhosis-impaired arterial baroreflex remain unknown. In the present study, we examined whether the cirrhosis-impaired arterial baroreflex is attributable to the dysfunction of aortic baroreceptor (AB) neurons. Biliary and nonbiliary cirrhotic rats were generated via common bile duct ligation (CBDL) and intraperitoneal injections of thioacetamide (TAA), respectively. Histological and molecular biological examinations confirmed the development of fibrosis in the livers of both cirrhotic rat models. The heart rate changes during phenylephrine-induced baroreceptor activation indicated that baroreflex sensitivity was blunted in the CBDL and TAA rats. Under the current-clamp mode of the patch-clamp technique, cell excitability was recorded in DiI-labeled AB neurons. The number of action potential discharges in the A- and C-type AB neurons was significantly decreased because of the increased rheobase and threshold potential in the CBDL and TAA rats compared with sham-operated rats. Real-time PCR and Western blotting indicated that the NaV1.7, NaV1.8, and NaV1.9 transcripts and proteins were significantly downregulated in the nodose ganglion neurons from the CBDL and TAA rats compared with the sham-operated rats. Consistent with these molecular data, the tetrodotoxin-sensitive NaV currents and the tetrodotoxin-resistant NaV currents were significantly decreased in A- and C-type AB neurons, respectively, from the CBDL and TAA rats compared with the sham-operated rats. Taken together, these findings implicate a key cellular mechanism in the cirrhosis-impaired arterial baroreflex.

Electrical stimulation of the aortic depressor nerve in conscious rats overcomes the attenuation of the baroreflex in chronic heart failure

Chronic heart failure (CHF) is characterized by autonomic dysfunction combined with baroreflex attenuation. The hypotensive and bradycardic responses produced by electrical stimulation of the aortic depressor nerve (ADN) were examined in conscious CHF and control male Wistar rats (12–13 wk old). Furthermore, the role of parasympathetic and sympathetic nervous system in mediating the cardiovascular responses to baroreflex activation was evaluated by selective β1-adrenergic and muscarinic receptor antagonists. CHF was induced by myocardial infarction. After 6 wk, the subjects were implanted with electrodes for ADN stimulation. Twenty-four hours later, electrical stimulation of the ADN was applied for 20 s using five different frequencies (5, 15, 30, 60, and 90 Hz), while the arterial pressure was recorded by a catheter implanted into the femoral artery. Electrical stimulation of the ADN elicited progressive and similar hypotensive and bradycardic responses in control ( n = 12) and CHF ( n = 11) rats, while the hypotensive response was not affected by methylatropine. Nevertheless, the reflex bradycardia was attenuated by methylatropine in control, but not in CHF rats. Atenolol did not affect the hypotensive or bradycardic response in either group. The ADN function was examined under anesthesia through electroneurographic recordings. The arterial pressure-ADN activity relationship was attenuated in CHF rats. In conclusion, despite the attenuation of baroreceptor function in CHF rats, the electrical stimulation of the ADN elicited a stimulus-dependent hypotension and bradycardia of similar magnitude as observed in control rats. Therefore, electrical activation of the aortic baroreflex overcomes both the attenuation of parasympathetic function and the sympathetic overdrive.

Current Approaches to Quantifying Tonic and Reflex Autonomic Outflows Controlling Cardiovascular Function in Humans and Experimental Animals

The role of the autonomic nervous system in the pathophysiology of human and experimental models of cardiovascular disease is well established. In the recent years, there have been some rapid developments in the diagnostic approaches used to assess and monitor autonomic functions. Although most of these methods are devoted for research purposes in laboratory animals, many have still found their way to routine clinical practice. To name a few, direct long-term telemetry recording of sympathetic nerve activity (SNA) in rodents, single-unit SNA recording using microneurography in human subjects and spectral analysis of blood pressure and heart rate in both humans and animals have recently received an overwhelming attention. In this article, we therefore provide an overview of the methods and techniques used to assess tonic and reflex autonomic functions in humans and experimental animals, highlighting current advances available and procedure description, limitations and usefulness for diagnostic purposes.

Acute Response to Unilateral Unipolar Electrical Carotid Sinus Stimulation in Patients With Resistant Arterial Hypertension

Bilateral bipolar electric carotid sinus stimulation acutely reduced muscle sympathetic nerve activity (MSNA) and blood pressure (BP) in patients with resistant arterial hypertension but is no longer available. The second-generation device uses a smaller unilateral unipolar disk electrode to reduce invasiveness while saving battery life. We hypothesized that the second-generation device acutely lowers BP and MSNA in treatment-resistant hypertensive patients. Eighteen treatment-resistant hypertensive patients (9 women/9 men; 53±11 years; 33±5 kg/m(2)) on stable medications have been included in the study. We monitored finger and brachial BP, heart rate, and MSNA. Without stimulation, BP was 165±31/91±18 mm Hg, heart rate was 75±17 bpm, and MSNA was 48±14 bursts per minute. Acute stimulation with intensities producing side effects that were tolerable in the short term elicited interindividually variable changes in systolic BP (-16.9±15.0 mm Hg; range, 0.0 to -40.8 mm Hg; P=0.002), heart rate (-3.6±3.6 bpm; P=0.004), and MSNA (-2.0±5.8 bursts per minute; P=0.375). Stimulation intensities had to be lowered in 12 patients to avoid side effects at the expense of efficacy (systolic BP, -6.3±7.0 mm Hg; range, 2.8 to -14.5 mm Hg; P=0.028 and heart rate, -1.5±2.3 bpm; P=0.078; comparison against responses with side effects). Reductions in diastolic BP and MSNA (total activity) were correlated (r(2)=0.329; P=0.025). In our patient cohort, unilateral unipolar electric baroreflex stimulation acutely lowered BP. However, side effects may limit efficacy. The approach should be tested in a controlled comparative study.

Differences in the dynamic baroreflex characteristics of unmyelinated and myelinated central pathways are less evident in spontaneously hypertensive rats

The aim of the study was to identify the contribution of myelinated (A-fiber) and unmyelinated (C-fiber) baroreceptor central pathways to the baroreflex control of sympathetic nerve activity (SNA) and arterial pressure (AP) in anesthetized Wistar-Kyoto (WKY; n = 8) and spontaneously hypertensive rats (SHR; n = 8). The left aortic depressor nerve (ADN) was electrically stimulated with two types of binary white noise signals designed to preferentially activate A-fibers (A-BRx protocol) or C-fibers (C-BRx protocol). In WKY, the central arc transfer function from ADN stimulation to SNA estimated by A-BRx showed strong derivative characteristics with the slope of dynamic gain between 0.1 and 1 Hz ( Gslope) of 14.63 ± 0.89 dB/decade. In contrast, the central arc transfer function estimated by C-BRx exhibited nonderivative characteristics with Gslope of 0.64 ± 1.13 dB/decade. This indicates that A-fibers are important for rapid baroreflex regulation, whereas C-fibers are likely important for more sustained regulation of SNA and AP. In SHR, the central arc transfer function estimated by A-BRx showed higher Gslope (18.46 ± 0.75 dB/decade, P < 0.01) and that estimated by C-BRx showed higher Gslope (8.62 ± 0.64 dB/decade, P < 0.001) with significantly lower dynamic gain at 0.01 Hz (6.29 ± 0.48 vs. 2.80 ± 0.36%/Hz, P < 0.001) compared with WKY. In conclusion, the dynamic characteristics of the A-fiber central pathway are enhanced in the high-modulation frequency range (0.1–1 Hz) and those of the C-fiber central pathway are attenuated in the low-modulation frequency range (0.01–0.1 Hz) in SHR.

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.

An afferent explanation for sexual dimorphism in the aortic baroreflex of rat

Sex differences in baroreflex (BRx) function are well documented. Hormones likely contribute to this dimorphism, but many functional aspects remain unresolved. Our lab has been investigating a subset of vagal sensory neurons that constitute nearly 50% of the total population of myelinated aortic baroreceptors (BR) in female rats but less than 2% in male rats. Termed "Ah," this unique phenotype has many of the nonoverlapping electrophysiological properties and chemical sensitivities of both myelinated A-type and unmyelinated C-type BR afferents. In this study, we utilize three distinct experimental protocols to determine if Ah-type barosensory afferents underlie, at least in part, the sex-related differences in BRx function. Electron microscopy of the aortic depressor nerve (ADN) revealed that female rats have less myelin (P < 0.03) and a smaller fiber cross-sectional area (P < 0.05) per BR fiber than male rats. Electrical stimulation of the ADN evoked compound action potentials and nerve conduction profiles that were markedly different (P < 0.01, n = 7 females and n = 9 males). Selective activation of ADN myelinated fibers evoked a BRx-mediated depressor response that was 3-7 times greater in female (n = 16) than in male (n = 17) rats. Interestingly, the most striking hemodynamic difference was functionally dependent upon the rate of myelinated barosensory fiber activation. Only 5-10 Hz of stimulation evoked a rapid, 20- to 30-mmHg reduction in arterial pressure of female rats, whereas rates of 50 Hz or higher were required to elicit a comparable depressor response from male rats. Collectively, our experimental results are suggestive of an alternative myelinated baroreceptor afferent pathway in females that may account for, at least in part, the noted sex-related differences in autonomic control of cardiovascular function.

Rosiglitazone improves insulin sensitivity and baroreflex gain in rats with diet-induced obesity

Obesity decreases baroreflex gain (BRG); however, the mechanisms are unknown. We tested the hypothesis that impaired BRG is related to the concurrent insulin resistance, and, therefore, BRG would be improved after treatment with the insulin-sensitizing drug rosiglitazone. Male rats fed a high-fat diet diverged into obesity-prone (OP) and obesity-resistant (OR) groups after 2 weeks. Then, OP and OR rats, as well as control (CON) rats fed a standard diet, were treated daily for 2 to 3 weeks with rosiglitazone (3 or 6 mg/kg) or its vehicle by gavage. Compared with OR and CON rats, conscious OP rats exhibited reductions in BRG (OP, 2.9 ± 0.1 bpm/mm Hg; OR, 4.0 ± 0.2 bpm/mm Hg; CON, 3.9 ± 0.2 bpm/mm Hg; P < 0.05) and insulin sensitivity (hyperinsulinemic euglycemic clamp; OP, 6.8 ± 0.9 mg/kg · min; OR, 22.2 ± 1.2 mg/kg · min; CON, 17.7 ± 0.8 mg/kg · min; P < 0.05), which were well correlated (r(2) = 0.49; P < 0.01). In OP rats, rosiglitazone dose-dependently improved (P < 0.05) insulin sensitivity (12.8 ± 0.6 mg/kg · min at 3 mg/kg; 16.0 ± 1.5 mg/kg · min at 6 mg/kg) and BRG (3.8 ± 0.4 bpm/mm Hg at 3 mg/kg; 5.3 ± 0.7 bpm/mm Hg at 6 mg/kg). However, 6 mg/kg rosiglitazone also increased BRG in OR rats without increasing insulin sensitivity, disrupted the correlation between BRG and insulin sensitivity (r(2) = 0.08), and, in OP and OR rats, elevated BRG relative to insulin sensitivity (analysis of covariance; P < 0.05). Moreover, in OP rats, stimulation of the aortic depressor nerve, to activate central baroreflex pathways, elicited markedly reduced decreases in heart rate and arterial pressure, but these responses were not improved by rosiglitazone. In conclusion, diet-induced obesity impairs BRG via a central mechanism that is related to the concurrent insulin resistance. Rosiglitazone normalizes BRG, but not by improving brain baroreflex processing or insulin sensitivity.

Diet-induced obesity severely impairs myelinated aortic baroreceptor reflex responses

Diet-induced obesity (DIO) attenuates the arterial cardiac baroreceptor reflex, but the mechanisms and sites of action are unknown. This study tested the hypothesis that DIO impairs central aortic baroreceptor pathways. Normal chow control (CON) and high-fat-chow obesity-resistant (OR) and obesity-prone (OP) rats were anesthetized (inactin, 120 mg/kg) and underwent sinoaortic denervation. The central end of the aortic depressor nerve (ADN) was electrically stimulated to generate frequency-dependent baroreflex curves (5-100 Hz) during selective activation of myelinated (A-fiber) or combined (A- and C-fiber) ADN baroreceptors. A mild stimulus (1 V) that activates only A-fiber ADN baroreceptors induced robust, frequency-dependent depressor and bradycardic responses in CON and OR rats, but these responses were completely abolished in OP rats. Maximal activation of A fibers (3 V) elicited frequency-dependent reflexes in all groups, but a dramatic deficit was still present in OP rats. Activation of all ADN baroreceptors (20 V) evoked even larger reflex responses. Depressor responses were nearly identical among groups, but OP rats still exhibited attenuated bradycardia. In separate groups of rats, the reduced heart rate (HR) response to maximal activation of ADN A fibers (3 V) persisted in OP rats following pharmacological blockade of β(1)-adrenergic or muscarinic receptors, suggesting deficits in both parasympathetic nervous system (PNS) and sympathetic nervous system (SNS) reflex pathways. However, the bradycardic responses to direct efferent vagal stimulation were similar among groups. Taken together, our data suggest that DIO severely impairs the central processing of myelinated aortic baroreceptor control of HR, including both PNS and SNS components.

GABA(B)-mediated inhibition of multiple modes of glutamate release in the nucleus of the solitary tract

In the caudal portions of the solitary tract (ST) nucleus, primary sensory afferents fall into two broad classes based on the expression of transient receptor potential vanilloid type 1 (TRPV1) receptors. Both afferent classes (TRPV1+/-) have indistinguishable glutamate release mechanisms for ST-evoked excitatory postsynaptic currents (EPSCs). However, TRPV1+ terminals release additional glutamate from a unique, TRPV1-operated vesicle pool that is temperature sensitive and facilitated by ST activity to generate asynchronous EPSCs. This study tested whether presynaptic γ-aminobutyric acid (GABA)(B) receptors inhibit both the evoked and TRPV1-operated release mechanisms on second-order ST nucleus neurons. In horizontal slices, shocks activated single ST axons and evoked the time-invariant (latency jitter <200 μs), glutamatergic EPSCs, which identified second-order neurons. Gabazine eliminated GABA(A) responses in all recordings. The GABA(B) agonist baclofen inhibited the amplitude of ST-EPSCs from both TRPV1+ and TRPV1- afferents with a similar EC(50) (∼1.2 μM). In TTX, GABA(B) activation decreased miniature EPSC (mEPSC) rates but not amplitudes, suggesting presynaptic actions downstream from terminal excitability. With calcium entry through voltage-activated calcium channels blocked by cadmium, baclofen reduced mEPSC frequency, indicating that GABA(B) reduced vesicle release by TRPV1-dependent calcium entry. GABA(B) activation also reduced temperature-evoked increases in mEPSC frequency, which relies on TRPV1. Our studies indicate that GABA(B) G protein-coupled receptors are uniformly distributed across all ST primary afferent terminals and act at multiple stages of the excitation-release cascades to suppress both action potential-triggered and TRPV1-coupled glutamate transmission pathways. Moreover, the segregated release cascades within TRPV1+ ST primary afferents represent independent, potential targets for differential modulation.

TRPV1 marks synaptic segregation of multiple convergent afferents at the rat medial solitary tract nucleus

TRPV1 receptors are expressed on most but not all central terminals of cranial visceral afferents in the caudal solitary tract nucleus (NTS). TRPV1 is associated with unmyelinated C-fiber afferents. Both TRPV1+ and TRPV1- afferents enter NTS but their precise organization remains poorly understood. In horizontal brainstem slices, we activated solitary tract (ST) afferents and recorded ST-evoked glutamatergic excitatory synaptic currents (ST-EPSCs) under whole cell voltage clamp conditions from neurons of the medial subnucleus. Electrical shocks to the ST produced fixed latency EPSCs (jitter<200 µs) that identified direct ST afferent innervation. Graded increases in shock intensity often recruited more than one ST afferent and ST-EPSCs had consistent threshold intensity, latency to onset, and unique EPSC waveforms that characterized each unitary ST afferent contact. The TRPV1 agonist capsaicin (100 nM) blocked the evoked TRPV1+ ST-EPSCs and defined them as either TRPV1+ or TRPV1- inputs. No partial responses to capsaicin were observed so that in NTS neurons that received one or multiple (2-5) direct ST afferent inputs--all were either blocked by capsaicin or were unaltered. Since TRPV1 mediates asynchronous release following TRPV1+ ST-evoked EPSCs, we likewise found that recruiting more than one ST afferent further augmented the asynchronous response and was eliminated by capsaicin. Thus, TRPV1+ and TRPV1- afferents are completely segregated to separate NTS neurons. As a result, the TRPV1 receptor augments glutamate release only within unmyelinated afferent pathways in caudal medial NTS and our work indicates a complete separation of C-type from A-type afferent information at these first central neurons.

Methods of assessing vagus nerve activity and reflexes

The methods used to assess cardiac parasympathetic (cardiovagal) activity and its effects on the heart in both humans and animal models are reviewed. Heart rate (HR)-based methods include measurements of the HR response to blockade of muscarinic cholinergic receptors (parasympathetic tone), beat-to-beat HR variability (HRV) (parasympathetic modulation), rate of post-exercise HR recovery (parasympathetic reactivation), and reflex-mediated changes in HR evoked by activation or inhibition of sensory (afferent) nerves. Sources of excitatory afferent input that increase cardiovagal activity and decrease HR include baroreceptors, chemoreceptors, trigeminal receptors, and subsets of cardiopulmonary receptors with vagal afferents. Sources of inhibitory afferent input include pulmonary stretch receptors with vagal afferents and subsets of visceral and somatic receptors with spinal afferents. The different methods used to assess cardiovagal control of the heart engage different mechanisms, and therefore provide unique and complementary insights into underlying physiology and pathophysiology. In addition, techniques for direct recording of cardiovagal nerve activity in animals; the use of decerebrate and in vitro preparations that avoid confounding effects of anesthesia; cardiovagal control of cardiac conduction, contractility, and refractoriness; and noncholinergic mechanisms are described. Advantages and limitations of the various methods are addressed, and future directions are proposed.

Elevated angiotensin II in rat nodose ganglia primes diabetes-blunted arterial baroreflex sensitivity: involvement of NADPH oxidase-derived superoxide

Clinical trials and experimental animal studies have confirmed the contribution of arterial baroreflex impairment in causing excess morbidity and mortality in type-1 diabetes. Our previous study has shown that angiotensin II (Ang II)-NADPH oxidase-superoxide signaling is associated with the reduced cell excitability in the aortic baroreceptor neurons (a primary afferent limb of the arterial baroreflex) from diabetic rats. In this study, we examined whether above-mentioned signaling might contribute to the blunted baroreflex sensitivity in streptozotocin-induced diabetic rats. Using Ang II (125)I radioimmunoassay and lucigenin chemiluminescence method, we found Ang II concentration, NADPH oxidase activity, and superoxide production in the nodose ganglia were enhanced in diabetic rats, compared to sham rats. As an index of the arterial baroreflex sensitivity, the reflex decreases in blood pressure and heart rate evoked by unilateral steady-frequency aortic depressor nerve stimulation were attenuated in diabetic rats. Local microinjection (50 nl) of losartan (an AT(1) receptor antagonist, 1 nmol), apocynin (a NADPH oxidase inhibitor, 1 nmol), and tempol (a superoxide dismutase mimetic, 10 nmol) into the nodose ganglia significantly improved the arterial baroreflex sensitivity in diabetic rats. In addition, these three chemicals also normalized exogenous Ang II-attenuated arterial baroreflex sensitivity in sham rats. These results indicate that overactivation of the Ang II-NADPH oxidase-superoxide signal pathway in the nodose ganglia contributes to the blunted baroreflex sensitivity in diabetes.

Primary afferent activation of thermosensitive TRPV1 triggers asynchronous glutamate release at central neurons

TRPV1 receptors feature prominently in nociception of spinal primary afferents but are also expressed in unmyelinated cranial visceral primary afferents linked to homeostatic regulation. Cranial visceral afferents enter the brain at the solitary tract nucleus (NTS) to control the heart, lungs, and other vital organs. Here we identify a role for central TRPV1 in the activity-dependent facilitation of glutamatergic transmission from solitary tract (ST) afferents. Fast, synchronous ST-NTS transmission from capsaicin-sensitive (TRPV1+) and -insensitive (TRPV1-) afferents was similar. However, afferent activation triggered long-lasting asynchronous glutamate release only from TRPV1+ synapses. Asynchronous release was proportional to synchronous EPSC amplitude, activity, and calcium entry. TRPV1 antagonists and low temperature blocked asynchronous release, but not evoked EPSCs. At physiological afferent frequencies, asynchronous release strongly potentiated the duration of postsynaptic spiking. This activity-dependent TPRV1-mediated facilitation is a form of synaptic plasticity that brings a unique central integrative feature to the CNS and autonomic regulation.

Optical tracking of phenotypically diverse individual synapses on solitary tract nucleus neurons

The solitary tract nucleus (NTS) is the termination site for cranial visceral afferents-peripheral primary afferent neurons which differ by phenotype (e.g. myelinated and unmyelinated). These afferents have very uniform glutamate release properties calculated by variance mean analysis. In the present study, we optical measured the inter-terminal release properties across individual boutons by assessing vesicle membrane turnover with the dye FM1-43. Single neurons were mechanically micro-harvested from medial NTS without enzyme treatment. The TRPV1 agonist capsaicin (CAP, 100 nM) was used to identify afferent, CAP-sensitive terminals arising from unmyelinated afferents. Isolated NTS neurons retained both glutamatergic and inhibitory terminals that generated EPSCs and IPSCs, respectively. Visible puncta on the neurons were stained positively with monoclonal antibody for synaptophysin, a presynaptic marker. Elevating extracellular K(+) concentration to 10 mM increased synaptic release measured at individual terminals by FM1-43. Within single neurons, CAP destained some but not other individual terminals. FM1-43 positive terminals that were resistant to CAP could be destained with K(+) solution. Individual terminals responded to depolarization with similar vesicle turnover kinetics. Thus, vesicular release was relatively homogenous across individual release sites. Surprisingly, conventionally high K(+) concentrations (>50 mM) produced erratic synaptic responses and at 90 mM K(+) overt neuron swelling--results that suggest precautions about assuming consistent K(+) responses in all neurons. The present work demonstrates remarkably uniform glutamate release between individual unmyelinated terminals and suggests that the homogeneous EPSC release properties of solitary tract afferents result from highly uniform release properties across multiple contacts on NTS neurons.

Comparison of baroreceptive to other afferent synaptic transmission to the medial solitary tract nucleus

Cranial nerve visceral afferents enter the brain stem to synapse on neurons within the solitary tract nucleus (NTS). The broad heterogeneity of both visceral afferents and NTS neurons makes understanding afferent synaptic transmission particularly challenging. To study a specific subgroup of second-order neurons in medial NTS, we anterogradely labeled arterial baroreceptor afferents of the aortic depressor nerve (ADN) with lipophilic fluorescent tracer (i.e., ADN+) and measured synaptic responses to solitary tract (ST) activation recorded from dye-identified neurons in medial NTS in horizontal brain stem slices. Every ADN+ NTS neuron received constant-latency ST-evoked excitatory postsynaptic currents (EPSCs) (jitter < 192 micros, SD of latency). Stimulus-recruitment profiles showed single thresholds and no suprathreshold recruitment, findings consistent with EPSCs arising from a single, branched afferent axon. Frequency-dependent depression of ADN+ EPSCs averaged approximately 70% for five shocks at 50 Hz, but single-shock failure rates did not exceed 4%. Whether adjacent ADN- or those from unlabeled animals, other second-order NTS neurons (jitters < 200 micros) had ST transmission properties indistinguishable from ADN+. Capsaicin (CAP; 100 nM) blocked ST transmission in some neurons. CAP-sensitive ST-EPSCs were smaller and failed over five times more frequently than CAP-resistant responses, whether ADN+ or from unlabeled animals. Variance-mean analysis of ST-EPSCs suggested uniformly high probabilities for quantal glutamate release across second-order neurons. While amplitude differences may reflect different numbers of contacts, higher frequency-dependent failure rates in CAP-sensitive ST-EPSCs may arise from subtype-specific differences in afferent axon properties. Thus afferent transmission within medial NTS differed by axon class (e.g., CAP sensitive) but was indistinguishable by source of axon (e.g., baroreceptor vs. nonbaroreceptor).

Electrophysiological and neuroanatomical evidence of sexual dimorphism in aortic baroreceptor and vagal afferents in rat

Evidence for sexual dimorphism in autonomic control of cardiovascular function is both compelling and confounding. Across healthy and disease populations sex-associated differences in neurocirculatory hemodynamics are far too complex to be entirely related to sex hormones. As an initial step toward identifying additional physiological mechanisms, we investigated whether there is a sex bias in the relative expression of low-threshold-myelinated and high-threshold-unmyelinated aortic baroreceptor afferents in rats. These two types of afferent fibers have markedly different reflexogenic effects upon heart rate and blood pressure and thus the potential impact upon baroreflex dynamics could be substantial. Our results, using a combination of a patch-clamp study of fluorescently identified aortic baroreceptor neurons (ABN) and morphometric analysis of aortic baroreceptor nerve fibers, demonstrate that females exhibit a greater percentage of myelinated baroreceptor fibers (24.8% vs. 18.7% of total baroreceptor fiber population, P < 0.01) and express a functional subtype of myelinated ABN rarely found in age-matched males (11% vs. 2.3%, n = 107, P < 0.01). Interestingly, this neuronal phenotype is more prevalent in the general population of female vagal afferent neurons (17.7% vs. 3.8%, n = 169, P < 0.01), and ovariectomy does not alter its expression but does lessen neuronal excitability. These data suggest there are fundamental neuroanatomical and electrophysiological differences between aortic baroreceptor afferents of female and male rats. Possible explanations are presented as to how such a greater prevalence of low-threshold myelinated afferents could be a contributing factor to the altered baroreflex sensitivity and vagal tone of females compared with males.

Organization and properties of GABAergic neurons in solitary tract nucleus (NTS)

Cranial visceral afferents enter the brain at the solitary tract nucleus (NTS). GABAergic neurons are scattered throughout the NTS, but their relation to solitary tract (ST) afferent pathways is imprecisely known. We hypothesized that most GABAergic NTS neurons would be connected only indirectly to the ST. We identified GABAergic neurons in brain stem horizontal slices using transgenic mice in which enhanced green fluorescent protein (EGFP) expression was linked to glutamic acid decarboxylase expression (GAD(+)). Finely graded electrical shocks to ST recruit ST-synchronized synaptic events with all-or-none thresholds and individual waveforms did not change with greater suprathreshold intensities--evidence consistent with initiation by single afferent axons. Most (approximately 70%) GAD(+) neurons received ST-evoked excitatory postsynaptic currents (EPSCs) that had minimally variant latencies (jitter, SD of latency <200 micros) and waveforms consistent with single, direct ST connections (i.e., monosynaptic). Increasing stimulus intensity evoked additional ST-synchronized synaptic responses with jitters >200 micros including inhibitory postsynaptic currents (IPSCs), indicating indirect connections (polysynaptic). Shocks of suprathreshold intensity delivered adjacent (50-300 microm) to the ST failed to excite non-ST inputs to second-order neurons, suggesting a paucity of axons passing near to ST that connected to these neurons. Despite expectations, we found similar ST synaptic patterns in GAD(+) and unlabeled neurons. Generally, ST information that arrived indirectly had small amplitudes (EPSCs and IPSCs) and frequency-dependent failures that reached >50% for IPSCs to bursts of stimuli. This ST afferent pathway organization is strongly use-dependent--a property that may tune signal propagation within and beyond NTS.

Theoretical and electrophysiological evidence for axial loading about aortic baroreceptor nerve terminals in rats

Arterial baroreceptors are essential for neurocirculatory control, providing rapid hemodynamic feedback to the central nervous system. The pressure-dependent discharge of carotid and aortic baroreceptor afferents has been extensively studied. A common assumption has been that circumferential deformation of the arterial wall is the predominant mechanical force affecting baroreceptor discharge. However, in vivo the arterial tree is under significant longitudinal tension, leading to the hypothesis that axially directed forces may contribute to baroreceptor function. To test this hypothesis, we utilized a combination of finite element modeling methods and an in vitro rat aortic arch preparation. Model formulation utilized traditional analytic constructs available in the literature followed by refinement of model material parameters through direct comparison of computationally and experimentally generated pressure-diameter curves. The numerical simulations strongly indicated a functional role for axial loading within the region of the baroreceptive nerve terminal. This prediction was confirmed through single-fiber recording of baroreceptor nerve discharge under conditions with and without longitudinal tension in the vessel preparation. The recordings (n = 5) demonstrated that longitudinal tension significantly (P < 0.02) lowered both the pressure threshold (P(th), mmHg) for baroreceptor discharge and sensitivity (S(th), Hz/mmHg). The effect was nearly instantaneous and sustained; i.e., under longitudinal tension average P(th) was 84 +/- 3 mmHg and S(th) was 0.71 +/- 0.15 Hz/mmHg, which immediately increased to a P(th) of 94 +/- 4 mmHg and a S(th) of 1.20 +/- 0.32 Hz/mmHg with loss of axial tension. Possible explanations of how an abrupt change in axial loading could result in a synchronized increase in afferent drive of the baroreceptor reflex, and the potentiating effect this could have on neurogenically mediated orthostatic intolerance are discussed.

Baroreflexes of the rat. V. Tetanus-induced potentiation of ADN A-fiber responses at the NTS

In a long-term neuromuscular blocked (NMB) rat preparation, tetanic stimulation of the aortic depressor nerve (ADN) enhanced the A-fiber evoked responses (ERs) in the cardiovascular region, the nucleus of the solitary tract (dmNTS). The potentiation persisted for at least several hours and may be a mechanism for adaptive adjustment of the gain of the baroreflex, with functional implications for blood pressure regulation. Using a capacitance electrode, we selectively stimulated A-fibers and acquired a stable 10-h "A-fiber only" ER baseline at the dmNTS. Following baseline, an A+C-fiber activating tetanus was applied to the ADN. The tetanus consisted of 1,000 "high current" pulses (10 trains; 300 mus, 100 Hz, 1 s), with intertrain interval of 9 s. A 10-h A-fiber only posttetanic test phase repeated the stimulus pattern of the baseline. Fourteen tetanus experiments were done in 12 rats. Compared with the baseline before tetanus, the A-fiber ER magnitudes of posttetanus hours were larger [F(13, 247) = 3.407, P < .001]; additionally, the 10-h posttetanus magnitude slopes were more positive than during 10 h before tetanus (df = 13; t = -3.47; P < 0.005); thus, an ADN A+C fiber-activating tetanus produced increases in the magnitude of the A-fiber ERs in the dmNTS that persisted for several hours. In an additional rat, application of an NMDA receptor antagonist, prior to the tetanus, blocked the potentiation effect. The stimulus protocols, magnitude and duration of the effect, and pharmacology resemble associative long-term potentiation (LTP).

Electrophysiological and pharmacological validation of vagal afferent fiber type of neurons enzymatically isolated from rat nodose ganglia

An unavoidable consequence of enzymatic dispersion of sensory neurons from intact ganglia is loss of the axon and thus the ability to classify afferent fiber type based upon conduction velocity (CV). An intact rat nodose ganglion preparation was used to randomly sample neurons (n=76) using the patch clamp technique. Reliable electrophysiological and chemophysiological correlates of afferent fiber type were established for use with isolated neuron preparations. Myelinated afferents (approximately 25%) formed two groups exhibiting strikingly different functional profiles. One group (n=10) exhibited CVs in excess of 10 m/s and narrow (<1 ms) action potentials (APs) while the other (n=9) had CVs as low as 4m/s and broad (>2 ms) APs that closely approximated those identified as unmyelinated afferents (n=57) with CVs less than 1m/s. A cluster analysis of select measures from the AP waveforms strongly correlated with CV, producing three statistically unique populations (p<0.05). These groupings aligned with our earlier hypothesis (Jin et al., 2004) that a differential sensitivity to the selective purinergic and vanilloid receptor agonists can be used as reliable pharmacological indicators of vagal afferent fiber type. These metrics were further validated using an even larger population of isolated (n=240) nodose neurons. Collectively, these indicators of afferent fiber type can be used to provide valuable insight concerning the relavence of isolated cellular observations to integrated afferent function of visceral organ systems.

Hierarchical recruitment of the sympathetic and parasympathetic limbs of the baroreflex in normotensive and spontaneously hypertensive rats

The arterial baroreflex acts to buffer acute changes in blood pressure by reciprocal modulation of sympathetic and parasympathetic activity that controls the heart and vasculature. We have examined the baroreflex pressure-function curves for changes in heart rate and non-cardiac sympathetic nerve activity (SNA, thoracic chain T8-12) in artificially perfused in situ rat preparations. We found that the non-cardiac SNA baroreflex is active over a lower range of pressures than the cardiac baroreflex (threshold 66 +/- 1 mmHg versus 82 +/- 5 mmHg and mid-point 77 +/- 3 versus 87 +/- 4 mmHg, respectively, P < 0.05, n = 6). This can manifest as a complete dissociation of the baroreflex limbs at low pressures. This difference between the cardiac and non-cardiac SNA baroreflex is also seen in end-organ sympathetic outflows (adrenal and renal nerves). Recordings of the cardiac vagal (parasympathetic) and the inferior cardiac (sympathetic) nerves identify the cardiac parasympathetic baroreflex component as being active over a higher range of pressures. This difference in the operating range of the baroreflex-function curves is exaggerated in the spontaneously hypertensive rat where the cardiac component has selectively reset by 20-25 mmHg to a higher pressure range (threshold of 104 +/- 4 mmHg and mid-point 113 +/- 4, n = 6). The difference in the pressure-function curves for the cardiac versus the vascular baroreflex indicates that there is a hierarchical recruitment of the output limbs of the baroreflex with a sympathetic predominance at lower arterial pressures.

Baroreflex responses to electrical stimulation of aortic depressor nerve in conscious SHR

Baroreflex responses to changes in arterial pressure are impaired in spontaneously hypertensive rats (SHR). Mean arterial pressure (MAP), heart rate (HR), and regional vascular resistances were measured before and during electrical stimulation (5-90 Hz) of the left aortic depressor nerve (ADN) in conscious SHR and normotensive control rats (NCR). The protocol was repeated after beta-adrenergic-receptor blockade with atenolol. SHR exhibited higher basal MAP (150 +/- 5 vs. 103 +/- 2 mmHg) and HR (393 +/- 9 vs. 360 +/- 5 beats/min). The frequency-dependent hypotensive response to ADN stimulation was preserved or enhanced in SHR. The greater absolute fall in MAP at higher frequencies (-68 +/- 5 vs. -38 +/- 3 mmHg at 90-Hz stimulation) in SHR was associated with a preferential decrease in hindquarter (-43 +/- 5%) vs. mesenteric (-27 +/- 3%) resistance. In contrast, ADN stimulation decreased hindquarter and mesenteric resistances equivalently in NCR (-33 +/- 7% and -30 +/- 7%). Reflex bradycardia was also preserved in SHR, although its mechanism differed. Atenolol attenuated the bradycardia in SHR (-88 +/- 14 vs. -129 +/- 18 beats/min at 90-Hz stimulation) but did not alter the bradycardia in NCR (-116 +/- 16 vs. -133 +/- 13 beats/min). The residual bradycardia under atenolol (parasympathetic component) was reduced in SHR. MAP and HR responses to ADN stimulation were also preserved or enhanced in SHR vs. NCR after deafferentation of carotid sinuses and contralateral right ADN. The results demonstrate distinct differences in central baroreflex control in conscious SHR vs. NCR. Inhibition of cardiac sympathetic tone maintains reflex bradycardia during ADN stimulation in SHR despite impaired parasympathetic activation, and depressor responses to ADN stimulation are equivalent or even greater in SHR due to augmented hindquarter vasodilation.

Role of unmyelinated fibers in electroacupuncture cardiovascular responses

The afferent fiber type responsible for the transmission of sensory neural traffic to the central nervous system during acupoint stimulation is uncertain. Several early studies evaluating compound action potentials have suggested that myelinated fibers contribute to the afferent input of the autonomic reflex adjustments during electroacupuncture (EA). Our more recent data, employing single unit recordings of somatic afferents, show that both myelinated and unmyelinated fibers are stimulated by EA, although more finely myelinated than unmyelinated fibers are activated by low frequency, low current stimulation. We hypothesized in this study that unmyelinated group VI fibers also contribute significantly to the inhibitory influence of EA on cardiovascular pressor responses. We found that neonatal capsaicin-treated rats depleted of substance P from primary afferents were insensitive to the inhibitory EA effect during gastric distention. Thus, EA at P5-P6 reduced gastric distention-induced pressor responses from 19+/-3 to 11+/-2 mmHg in eight untreated rats while capsaicin-treated rats (n=9) were unresponsive to EA. Substance P containing neurons in dorsal root ganglion cells at Ti-T5 were significantly decreased in the capsaicin-treated rats from 47+/-4 to 22+/-4 cells. Treated compared to untreated rats also demonstrated a significantly (P<0.03) reduced number of group IV fibers identified with single unit recording techniques. This study demonstrates that the inhibitory effect of EA at P5-P6 on cardiovascular autonomic excitatory reflexes involves unmyelinated group IV fibers of the median nerves.

Purinergic and vanilloid receptor activation releases glutamate from separate cranial afferent terminals in nucleus tractus solitarius

Vanilloid (VR1) and purinergic (P2X) receptors are found in cranial afferent neurons in nodose ganglia and their central terminations within the solitary tract nucleus (NTS), but little is known about their function. We mechanically dissociated dorsomedial NTS neurons to preserve attached native synapses and tested for VR1 and P2X function primarily in spindle-shaped neurons resembling intact second-order neurons. All neurons (n = 95) exhibited spontaneous glutamate (EPSCs) and GABA (IPSCs)-mediated synaptic currents. VR1 agonist capsaicin (CAP; 100 nm) reversibly increased EPSC frequency, effects blocked by capsazepine. ATP (100 microm) increased EPSC frequency, actions blocked by P2X antagonist pyridoxalphosphate-6-azophenyl-2', 4'-disulfonic acid (PPADS; 20 microm). In all CAP-resistant neurons, P2X agonist alphabeta-methylene-ATP (alphabeta-m-ATP) increased EPSC frequency. Neither CAP nor alphabeta-m-ATP altered EPSC amplitudes, kinetics, or holding currents. Thus, activation of VR1 and P2X receptors selectively facilitated presynaptic glutamate release on different NTS neurons. PPADS and 2',3'-O-(2,4,6-trinitrophenyl)-ATP blocked alphabeta-m-ATP responses, but P2X1-selective antagonist NF023 (8,8'-[carbonylbis (imino-3,1-phenylene carbonylimino)]bis-1,3,5-naphthalenetrisulfonic acid) did not. The pharmacological profile and transient kinetics of ATP responses are consistent with P2X3 homomeric receptors. TTX and Cd(2+) did not eliminate agonist-evoked EPSC frequency increases, suggesting that voltage-gated sodium and calcium channels are not required. In nodose ganglia, CAP but not alphabeta-m-ATP evoked inward currents in slow conducting neurons and the converse pattern in myelinated, rapidly conducting neurons (n = 14). Together, results are consistent with segregation of glutamatergic terminals into either P2X sensitive or VR1 sensitive that correspondingly identify myelinated and unmyelinated afferent pathways at the NTS.

Nociception attenuates parasympathetic but not sympathetic baroreflex via NK1 receptors in the rat nucleus tractus solitarii

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 GABAA 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.

Vanilloid-sensitive afferents activate neurons with prominent A-type potassium currents in nucleus tractus solitarius

Cranial visceral afferents innervate second-order nucleus tractus solitarius (NTS) neurons via myelinated (A-type) and unmyelinated (C-type) axons in the solitary tract (ST). A- and C-type afferents often evoke reflexes with distinct performance differences, especially with regard to their frequency-dependent properties. In horizontal brainstem slices, we used the vanilloid receptor 1 agonist capsaicin (CAP; 100 nm) to identify CAP-sensitive and CAP-resistant ST afferent pathways to second-order NTS neurons and tested whether these two groups of neurons had similar intrinsic potassium currents. ST stimulation evoked monosynaptic EPSCs identified by minimal synaptic jitter (<150 microsec) and divided into two groups: CAP-sensitive (n = 37) and CAP-resistant (n = 22). EPSCs in CAP-sensitive neurons had longer latencies (5.1 +/- 0.3 vs 3.6 +/- 0.3 msec; p = 0.001) but similar jitter (p = 0.57) compared with CAP-resistant neurons, respectively. Transient outward currents (TOCs) were significantly greater in CAP-sensitive than in CAP-resistant neurons. Steady-state currents were similar in both groups. 4-Aminopyridine or depolarized conditioning blocked the TOC, but tetraethylammonium had no effect. Voltage-dependent activation and inactivation of TOC were consistent with an A-type K+ current, I(KA). In current clamp, the activation of I(KA) reduced neuronal excitability and action potential responses to ST transmission. Our results suggest that the potassium-channel differences of second-order NTS neurons contribute to the differential processing of A- and C-type cranial visceral afferents beginning as early as this first central neuron. I(KA) can act as a frequency transmission filter and may represent a key target for the modulation of temporal processing of reflex responsiveness such as within the baroreflex arc.

Vanilloid receptors presynaptically modulate cranial visceral afferent synaptic transmission in nucleus tractus solitarius

Although the central terminals of cranial visceral afferents express vanilloid receptor 1 (VR1), little is known about their functional properties at this first synapse within the nucleus tractus solitarius (NTS). Here, we examined whether VR1 modulates afferent synaptic transmission. In horizontal brainstem slices, solitary tract (ST) activation evoked EPSCs. Monosynaptic EPSCs had low synaptic jitter (SD of latency to successive shocks) averaging 84.03 +/- 3.74 microsec (n = 72) and were completely blocked by the non-NMDA antagonist 2,3-dihydroxy-6-nitro-7-sulfonyl-benzo[f]quinoxaline (NBQX). Sustained exposure to the VR1 agonist capsaicin (CAP; 100 nm) blocked ST EPSCs (CAP-sensitive) in some neurons but not others (CAP-resistant). CAP-sensitive EPSCs had longer latencies than CAP-resistant EPSCs (4.65 +/- 0.27 msec, n = 48 vs 3.53 +/- 0.28 msec, n = 24, respectively; p = 0.011), but they had similar jitter. CAP evoked two transient responses in CAP-sensitive neurons: a rapidly developing inward current (I(cap)) (108.1 +/- 22.9 pA; n = 21) and an increase in spontaneous synaptic activity. After 3-5 min in CAP, I(cap) subsided and ST EPSCs disappeared. NBQX completely blocked I(cap). The VR1 antagonist capsazepine (10-20 microm) attenuated CAP responses. Anatomically, second-order NTS neurons were identified by 4-(4-dihexadecylamino)styryl)-N-methylpyridinium iodide transported from the cervical aortic depressor nerve (ADN) to stain central terminals. Neurons with fluorescent ADN contacts had CAP-sensitive EPSCs (n = 5) with latencies and jitter similar to those of unlabeled monosynaptic neurons. Thus, consistent with presynaptic VR1 localization, CAP selectively activates a subset of ST axons to release glutamate that acts on non-NMDA receptors. Because the CAP sensitivity of cranial afferents is exclusively associated with unmyelinated axons, VR1 identifies C-fiber afferent pathways within the brainstem.

Carotid and aortic baroreflexes of the rat: I. Open-loop steady-state properties and blood pressure variability

To characterize the baroreflex in central nervous system-intact neuromuscular-blocked rats, we measured the vascular and cardiac responses and compared direct stimulation of the aortic depressor nerve (ADN) with a capacitance electrode (differentially activating either A or A + C fibers) to carotid sinus pressure with a micro-balloon (SINUS). One-thousand-two-hundred-ninety-seven open-loop measurements of systolic blood pressure (SBP), heart rate, venous pressure (VBP), and mesenteric (msBF), femoral (fmBF), and skin (skBF) blood flow were completed; the linear range of the effects was determined for each response and stimulus mode. The rats were sinoaortic denervated (SAD). The open-loop stimulation effect was very stable; e.g., the mean effect of 790 ADN stimulations during >7 days was -9.8 mmHg, with an average drift of +0.001 mmHg/h. In contrast, there was large variability of the SBP baseline (e.g., SD = +/-10.9), which was due to SAD (+/-6.3 to +/-16.3 mmHg, t = -13. 9, df = 4, P < 0.0002) and was reversed by ganglionic block (+/-10.8 to +/- 2.9 mmHg, t = -12.9, df = 3, P < 0.001). The ADN stimuli produced larger depressor responses than sinus stimuli (-66 vs. -45 mmHg); all component responses paralleled the magnitude of the SBP effect, except interbeat interval (IBI), for which the ADN DeltaIBI was approximately 10 times that of SINUS. For all stimuli, fmBF increased and msBF did not. Mesenteric and femoral vascular conductance both increased, whereas VBP decreased and skBF followed SBP. We found that for all baroreflex response components, with the exception of SINUS-elicited DeltaIBI, there was an orderly, substantially linear, relationship between stimulus strength and response magnitude.

Carotid and aortic baroreflexes of the rat: II. Open-loop frequency response and the blood pressure spectrum

To determine the relationship between blood pressure (BP) variability and the open-loop frequency domain transfer function (TF) of the baroreflexes, we measured the pre- and postsinoaortic denervation (SAD) spectra and the effects of periodic and step inputs to the aortic depressor nerve and isolated carotid sinus of central nervous system-intact, neuromuscular-blocked (NMB) rats. Similar to previous results in freely moving rats, SAD greatly increased very low frequency (VLF) (0.01-0.2 Hz) systolic blood pressure (SBP) noise power. Step response-frequency measurements for SBP; interbeat interval (IBI); venous pressure; mesenteric, femoral, and skin blood flow; and direct modulation analyses of SBP showed that only VLF variability could be substantially attenuated by an intact baroreflex. The -3-dB frequency for SBP is 0.035-0.056 Hz; femoral vascular conductance is similar to SBP, but mesenteric vascular conductance has a reliably lower and IBI has a reliably higher -3-dB point. The overall open-loop transportation lag, of which </=0.1 s is neural, is approximately 1.07 s. Constrained algebraic solution, over a range of frequencies, of the pre- and postSAD endogenous noise spectra and the independently determined relative frequency and absolute lag measurements was used to calculate the absolute gain for the open-loop TF. The average gain at 0.02 Hz, the frequency of maximum sensitivity, was 1.47 (95% confidence interval = +/-0.48), which agrees well with estimates for the dog reversible sinus. We found that, in the NMB rat, the effects of SAD on the BP noise spectrum were accounted for by the open-loop properties of the baroreflex.