Intrathecal capsaicin enhances one-kidney renal wrap hypertension in the rat

The Vanilloid (Capsaicin) Receptor TRPV1 in Blood Pressure Regulation: A Novel Therapeutic Target in Hypertension?

Today's sedentary lifestyle with excess food and little exercise increases the number of people with hypertension, a major risk factor for stroke. New knowledge of treatments in this field is of utmost importance. In animal experiments, the activation by capsaicin of TRPV1-expressing sensory afferents evokes a drop in blood pressure by triggering the Bezold-Jarisch reflex. In hypertensive rats, capsaicin reduces blood pressure. Conversely, genetic ablation of the TRPV1 receptor results in elevated nocturnal (but not diurnal) blood pressure. These observations imply a therapeutic potential for TRPV1 activation in hypertensive patients. Indeed, in a major epidemiological study involving 9273 volunteers, dietary capsaicin was found to lower the risk for hypertension. New research indicates that the mechanism of action of capsaicin on blood pressure regulation is far more complex than previously thought. In addition to the well-recognized role of capsaicin-sensitive afferents in blood pressure regulation, TRPV1 seems to be expressed both in endothelial cells and vascular smooth muscle. This review aims to evaluate the therapeutic potential of TRPV1-targeting drugs in hypertensive patients.

Selective ablation of TRPV1 by intrathecal injection of resiniferatoxin in rats increases renal sympathoexcitatory responses and salt sensitivity

This study tested the hypothesis that selective ablation of transient receptor potential vanilloid type 1 (TRPV1)-positive nerve fibers by intrathecal injection of resiniferatoxin (RTX) enhances renal sympathoexcitatory responses and salt sensitivity. Intrathecal injection of RTX (1.8 μg/kg) to the levels of lower thoracic and upper lumbar spinal cord (T8-L3) increased mean arterial pressure (MAP) in rats fed a normal (NS, 1% NaCl) or high-sodium (HS, 8% NaCl) diet for 4 weeks compared to vehicle-treated rats (NS: 121 ± 2 vs. 111 ± 2; HS: 154 ± 2 vs. 134 ± 2 mm Hg, both P < 0.05), with a greater increase in HS compared to NS rats (9 ± 1% vs. 15 ± 1%, P < 0.05). TRPV1 contents were decreased in T8-L3 segments of spinal dorsal horn but not in corresponding dorsal root ganglia and the kidney following RTX treatment (P < 0.05). Selective activation of GABA-A receptors with intrathecal T8-L3 segment-injection of muscimol (3 nmol/kg) decreased renal sympathetic nerve activity and increased urinary excretion in both NS and HS rats, with a greater effect in RTX-treated compared to vehicle-treated rats (P < 0.05). Chronic activation of GABA-A receptors with muscimol (50 mg/kg/day × 2, p.o.) abolished RTX treatment-induced pressor effects in NS and HS rats. GAD65/67, a GABA synthetase, in the spinal cord was downregulated and tyrosine hydroxylase in the kidney upregulated in NS or HS rats treated with RTX (P < 0.05). Thus, selective ablation of TRPV1-positive central terminals of sensory neurons plays a prohypertensive role possibly via inhibition of spinal GABA system especially with HS intake, suggesting that activation of TRPV1 in central terminals of sensory neurons may convey an antihypertensive effect.

Renal Denervation Prevents Immune Cell Activation and Renal Inflammation in Angiotensin II-Induced Hypertension

RATIONALE

Inflammation and adaptive immunity play a crucial role in the development of hypertension. Angiotensin II and probably other hypertensive stimuli activate the central nervous system and promote T-cell activation and end-organ damage in peripheral tissues.

OBJECTIVE

To determine if renal sympathetic nerves mediate renal inflammation and T-cell activation in hypertension.

METHODS AND RESULTS

Bilateral renal denervation using phenol application to the renal arteries reduced renal norepinephrine levels and blunted angiotensin II-induced hypertension. Bilateral renal denervation also reduced inflammation, as reflected by decreased accumulation of total leukocytes, T cells, and both CD4+ and CD8+ T cells in the kidney. This was associated with a marked reduction in renal fibrosis, albuminuria, and nephrinuria. Unilateral renal denervation, which partly attenuated blood pressure, only reduced inflammation in the denervated kidney, suggesting that this effect is pressure independent. Angiotensin II also increased immunogenic isoketal-protein adducts in renal dendritic cells (DCs) and increased surface expression of costimulation markers and production of interleukin (IL)-1α, IL-1β, and IL-6 from splenic DCs. Norepinephrine also dose dependently stimulated isoketal formation in cultured DCs. Adoptive transfer of splenic DCs from angiotensin II-treated mice primed T-cell activation and hypertension in recipient mice. Renal denervation prevented these effects of hypertension on DCs. In contrast to these beneficial effects of ablating all renal nerves, renal afferent disruption with capsaicin had no effect on blood pressure or renal inflammation.

CONCLUSIONS

Renal sympathetic nerves contribute to DC activation, subsequent T-cell infiltration and end-organ damage in the kidney in the development of hypertension.

Calcitonin gene-related peptide (CGRP) and its role in hypertension

Hypertension is still presently the number one "silent killer" in the Western World, and a major risk factor for the development of secondary diseases contributing to cardiovascular disease (CVD). However, despite a broad range of therapies, the mechanisms involved in the onset of hypertension remains unclear, therefore there is a real need to investigate the mechanisms involved. Calcitonin gene-related peptide (CGRP) is the most potent microvascular vasodilator known to date. Widely expressed in the nervous system, this peptide is considered to play a positive role in wound healing and protects against ischaemic and other traumas. However, whilst the protective mechanisms are not well understood, evidence indicates that these mechanisms become important in vascular-related stress. This review provides evidence that CGRP is both a potent vasodilator and hypotensive agent. However studies to date suggest that CGRP does not contribute to the physiological regulation of blood pressure. By comparing results from a range of human and animal studies, findings broadly suggest an association between CGRP and the pathophysiology of hypertension in terms of protective mechanisms, with possibly the RAMP1 component of the CGRP receptor playing a key role in the brain stem, in addition to peripheral receptors. The studies of agents that release CGRP agonists are at an early stage, with analogues for human use currently under development. However, at this stage, further research is required to establish the mechanisms by which CGRP is protective in the onset of hypertension, if novel and therapeutic modes of treatment are to be developed.

Role of anandamide transporter in regulating calcitonin gene-related peptide production and blood pressure in hypertension

OBJECTIVES

To explore the role of anandamide (AEA) transporter in regulating calcitonin gene-related peptide (CGRP) production and blood pressure.

METHODS AND RESULTS

Plasma levels of AEA, CGRP, asymmetric dimethylarginine (ADMA) and nitric oxide in patients with essential hypertension, spontaneously hypertensive rats (SHRs) and 2 kidney 1 clip hypertensive rats and the CGRP mRNA expression in dorsal root ganglion of rats were measured. Peripheral blood lymphocytes were isolated to examine the AEA transporter activity, the role of AEA transporter in regulating CGRP mRNA expression or the effect of exogenous ADMA on AEA transporter activity. In both hypertensive patients and SHRs, the plasma level of AEA was elevated, but the AEA transporter activity was attenuated concomitantly with decreased CGRP production. Moreover, plasma ADMA level in SHRs was elevated accompanied by decreased nitric oxide level. By contrast, the plasma AEA level was elevated accompanied by increased CGRP production in 2 kidney 1 clip hypertensive rats, and there were no significant changes in plasma levels of ADMA, nitric oxide and the AEA transporter activity. In vitro, exogenous administration of AEA upregulated CGRP mRNA expression in lymphocytes, which was inhibited by AEA transporter blocker, AM404, and the AEA transporter activity was reduced by ADMA.

CONCLUSION

Decreased plasma CGRP level in patients with essential hypertension or SHRs is likely due to the reduced AEA transporter activity, and the increased ADMA level may account for the reduced AEA transporter activity.

Calcitonin gene-related peptide and hypertension

Capsaicin-sensitive sensory nerves participate in the regulation of cardiovascular functions both in the normal state and the pathophysiology of hypertension through the actions of potent vasodilator neuropeptides, including calcitonin gene-related peptide (CGRP). CGRP, a very potent vasodilator, is the predominant neurotransmitter in capsaicin-sensitive sensory nerves, and plays an important role in the initiation, progression and maintenance of hypertension via: (1) the alterations in its synthesis and release and/or in vascular sensitivity response to it; (2) interactions with pro-hypertensive systems, including renin-angiotensin-aldosterone system, sympathetic nervous system and endothelin system; and (3) anti-hypertrophy and anti-proliferation of vascular smooth muscle cells. The decrease in CGRP synthesis and release contributes to the elevated blood pressure, as shown in the spontaneously hypertensive rats, alpha-CGRP knockout mice, Dahl-salt or phenol-induced hypertensive rats. In contrast, the increase in CGRP levels or the enhancement of vascular sensitivity response to CGRP plays a beneficial compensatory depressor role in the development of hypertension, as shown in deoxycorticosterone-salt, sub-total nephrectomy-salt, N(omega)-nitro-L-arginine methyl ester or two-kidney, one-clip models of hypertension in rats. We found that rutaecarpine causes a sustained depressor action by stimulation of CGRP synthesis and release via activation of vanilloid receptor subtype 1 (VR1) in hypertensive rats, which reveals the therapeutic implications of VR1 agonists for treatment of hypertension.

The vanilloid receptor and hypertension

Mammalian transient receptor potential (TRP) channels consist of six related protein sub-families that are involved in a variety of pathophysiological function, and disease development. The TRPV1 channel, a member of the TRPV sub-family, is identified by expression cloning using the "hot" pepper-derived vanilloid compound capsaicin as a ligand. Therefore, TRPV1 is also referred as the vanilloid receptor (VR1) or the capsaicin receptor. VR1 is mainly expressed in a subpopulation of primary afferent neurons that project to cardiovascular and renal tissues. These capsaicin-sensitive primary afferent neurons are not only involved in the perception of somatic and visceral pain, but also have a "sensory-effector" function. Regarding the latter, these neurons release stored neuropeptides through a calcium-dependent mechanism via the binding of capsaicin to VR1. The most studied sensory neuropeptides are calcitonin gene-related peptide (CGRP) and substance P (SP), which are potent vasodilators and natriuretic/diuretic factors. Recent evidence using the model of neonatal degeneration of capsaicin-sensitive sensory nerves revealed novel mechanisms that underlie increased salt sensitivity and several experimental models of hypertension. These mechanisms include insufficient suppression of plasma renin activity and plasma aldosterone levels subsequent to salt loading, enhancement of sympathoexcitatory response in the face of a salt challenge, activation of the endothelin-1 receptor, and impaired natriuretic response to salt loading in capsaicin-pretreated rats. These data indicate that sensory nerves counterbalance the prohypertensive effects of several neurohormonal systems to maintain normal blood pressure when challenged with salt loading. The therapeutic utilities of vanilloid compounds, endogenous agonists, and sensory neuropeptides are also discussed.

Role of calcitonin gene-related peptide in the phenol-induced neurogenic hypertension in rats

Previous investigations have demonstrated that capsaicin-sensitive sensory nerves are involved in the development of hypertension in some rat models of hypertension. To determine the role played by calcitonin gene-related peptide (CGRP; the predominant neurotransmitter in capsaicin-sensitive sensory nerves) in a rat model of neurogenic hypertension, in which hypertension was induced by injecting 50 microl of 10% phenol in the lower pole of the left kidney, systolic blood pressure (SBP) was monitored by the tail-cuff method throughout the experiment. Fifteen days after injection of phenol, mean arterial pressure (MAP), concentrations of CGRP in the plasma, the expression of CGRP mRNA in dorsal root ganglia (DRG) and CGRP content in laminae I and II of the spinal cord were measured. SBP was significantly increased 5 days after the intrarenal injection of phenol (164+/-7 mm Hg, p<0.01). At the end of experiment, blood pressure (BP) was significantly elevated in the phenol-injected rats compared with the controls (SBP: 187+/-6 vs. 122+/-4 mm Hg, p<0.01; MAP: 157.56+/-3.02 vs. 103.80+/-2.04 mm Hg, p<0.01). Treatment with capsaicin, which selectively depletes neurotransmitters from the capsaicin-sensitive nerves, failed to enhance the development of hypertensive responses to the intrarenal injection of phenol. Intravenous administration of CGRP(8-37), the specific CGRP receptor antagonist, also failed to increase the already elevated MAP. The expression of CGRP mRNA (both alpha- and beta-CGRP isoforms), the content of CGRP in laminae I and II of the dorsal horn of the spinal cord and the concentration of CGRP in the plasma was decreased in the rats treated with phenol. These results suggest that CGRP does not play a counterregulatory role in the phenol-induced hypertensive rats, and support the hypothesis that reduction of CGRP (alpha and beta isoforms) could contribute to a blood pressure elevation in this setting.

Vanilloid receptor 1 expression in the rat urinary tract

Previous findings have shown that the capsaicin sensitivity of sensory fibres is due to the expression of a specific membrane protein, the vanilloid receptor type 1 (VR1). In the present work we studied the distribution, morphology and the neurochemical content of nerve fibres expressing this receptor in the rat urinary tract. Immunolabelling was performed against the VR1 and the positive fibres were examined by light and electron microscopy. Colocalisation of VR1 and substance P or calcitonin gene-related peptide immunoreactivities, and isolectin B4 binding, was evaluated under the confocal microscope. In addition, the effect of intravesical administration of resiniferatoxin, an ultra-potent vanilloid receptor agonist, in the receptor expression in the bladder was also studied. Numerous VR1-immunoreactive fibres were found in the mucosa and muscular layer of the entire urinary tract except the kidney. In the bladder, most fibres were also substance P- or calcitonin gene-related peptide-immunoreactive but did not bind isolectin B4. Under the electron microscope VR1 immunoreactivity was confined to unmyelinated axons and varicosities containing small clear and large dense-core synaptic vesicles. They occurred beneath or among epithelial cells or closely apposed to smooth muscle cells. Intravesical resiniferatoxin decreased VR1 immunoreactivity transiently. These data indicate that primary sensory fibres expressing VR1 are extremely abundant in the rat urinary tract and that, in contrast to the skin, they belong almost exclusively to the peptide-containing sub-population of primary afferents. As capsaicin-sensitive bladder afferents are involved in nociception and reflex micturition control, the numerous free terminal nerve endings expressing VR1 in the mucosa seem more adequate to accomplish the former function. However, the close apposition between VR1-expressing fibres and smooth muscle cells suggests that they may also encode the tonus of the muscular layer.

Role of renin-angiotensin-aldosterone system in salt-sensitive hypertension induced by sensory denervation

To define the role of the renin-angiotensin-aldosterone system in a novel salt-sensitive model, neonatal Wistar rats were given capsaicin (50 mg/kg sc) on the first and second days of life. After weaning, male rats were divided into the following six groups and treated for 3 wk with: control + normal sodium diet (CON-NS), CON + high-sodium diet (CON-HS), CON + HS + spironolactone (50 mg x kg(-1) x day(-1), CON-HS-SP), capsaicin pretreatment + NS (CAP-NS), CAP-HS, and CAP-HS-SP. Radioimmunoassay shows that plasma renin activity (PRA) and plasma aldosterone level (PAL) were suppressed by HS, but they were higher in CAP-HS than in CON-HS and CON-HS-SP (P < 0.05). Both tail-cuff systolic blood pressure and mean arterial pressure were higher in CAP-HS than in all other groups (P < 0.05). Urine water and sodium excretion were increased with HS intake, but they were lower in CAP-HS than in CON-HS (P < 0.05). Western blot did not detect differences in adrenal AT1 receptor content. Therefore, insufficiently suppressed PRA and PAL in response to HS intake by sensory denervation may contribute to increased salt sensitivity and account for effectiveness of spironolactone in lowering blood pressure in this model.

Degeneration of capsaicin-sensitive sensory nerves leads to increased salt sensitivity through enhancement of sympathoexcitatory response

We have previously shown that neonatal degeneration of capsaicin-sensitive sensory nerves renders a rat responsive to a salt load with an increase in blood pressure and a decrease in natriuretic response. To test the hypothesis that the enhanced sympathoexcitatory response to a high salt intake contributes to the development of hypertension in this model, newborn Wistar rats were given 50 mg/kg capsaicin and/or 80 mg/kg guanethidine subcutaneously. Control rats were treated with vehicle. After the weaning period, male rats were grouped as the following and given a high sodium diet (4%) for 2 weeks: capsaicin and guanethidine coadministration (CAP-GUA), capsaicin only (CAP), guanethidine only (GUA), and vehicle control (CON). Norepinephrine concentrations in the atrium were significantly lower in CAP-GUA and GUA than in CON rats (P<0.05). Twenty-four-hour urine and sodium excretions were significantly lower in CAP than in CAP-GUA, GUA, and CON rats (P<0.05). Mean arterial pressure (mm Hg) was significantly higher in CAP (180+/-10) than in CAP-GUA (106+/-1), GUA (133+/-5), and CON (122+/-3) rats (P<0.05). Thus, sympathectomy restores the natriuretic response to a high salt intake and prevents the development of salt-sensitive hypertension induced by sensory denervation. These data indicate that sensory nerves counterbalance the prohypertensive effect of the sympathetic nerves to maintain blood pressure within normal range during salt loading.

Salt-sensitive hypertension induced by sensory denervation: introduction of a new model

To test the novel hypothesis that neonatal degeneration of capsaicin-sensitive sensory nerves causes the rat to respond to a salt load with a significant and sustained rise in blood pressure, newborn Wistar rats were given 50 mg/kg capsaicin subcutaneously on the 1st and 2nd day of life. Control rats were treated with vehicle. Immediately after the weanling period, male rats were divided into 4 groups and fed different sodium diets for 2 weeks: capsaicin pretreatment plus high sodium diet (4%, CAP-HS), capsaicin plus normal sodium diet (0.5%, CAP-NS), control plus high sodium diet (CON-HS), and control plus normal sodium diet (CON-NS). Both tail-cuff systolic blood pressure and mean arterial pressure with anesthesia were significantly higher in CAP-HS than in CAP-NS, CON-HS, and CON-NS (P<0.05), but they were not different among the latter 3 groups. Radioimmunoassay revealed that levels of calcitonin gene related peptide in dorsal root ganglia were markedly decreased by capsaicin treatment (P<0.05). Twenty-four-hour urine volume and urine sodium excretion were significantly lower in CAP-HS than in CON-HS but were higher in CAP-HS and CON-HS compared with CAP-NS and CON-NS (P<0.05). Urine potassium excretion was not different among the 4 groups. Thus, this study provides the first evidence that neonatal degeneration of capsaicin-sensitive sensory nerves renders the rat salt-sensitive in terms of blood pressure regulation. Furthermore, our data suggest that neonatal capsaicin treatment may impair renal sodium and water excretion responses to high sodium intake. This model will provide a novel experimental paradigm for exploring underlying molecular mechanisms linked with salt-sensitive hypertension and sensory nerve function.