Central anti-hypertensive effect of tachykinin NK3 receptor antagonists in rat

Genetic validation of neurokinin 3 receptor antagonists for ischemic heart disease prevention in men – A one-sample Mendelian randomization study

Background

Ischemic heart disease (IHD) is a leading cause of mortality, particularly for men. Few interventions have focused on protecting specifically men. Emerging evidence may implicate testosterone. Neurokinin 3 receptor (NK3R) antagonists, an existing class of drugs being considered as treatments for reproductive conditions in women, affect testosterone; this study addresses genetic validation of their use to prevent IHD in men.

Methods

A one-sample Mendelian randomization (MR) study using the UK Biobank cohort study, based on independent (r² < 0.005) genetic variants predicting testosterone in men (n = 157738) at genome wide significance in the target gene for NK3R antagonists (TACR3), was used to assess associations with IHD (cases=15056, non-cases=151964) and positive control outcomes (relative age voice broke, children fathered, hypertension) in men and a negative control outcome (IHD) in women using summary statistics. A two-sample MR study using the PRACTICAL consortium was used for the positive control outcome of prostate cancer.

Findings

Two relevant TACR3 genetic variants (rs116646027 and rs1351623) were identified in men. Genetically mimicked NK3R antagonists were inversely associated with IHD (odds ratio 0.54 per standard deviation lower testosterone, 95% confidence interval 0.31, 0.94) and with control outcomes (older relative age voice broke, fewer children and lower risk of hypertension and prostate cancer) as expected in men and in women (unrelated to IHD).

Interpretation

Genetic validation of a role of NK3R antagonists in IHD suggests their consideration as a new means of preventing IHD in men. Whether they protect against prostate cancer might bear further consideration.

Tachykinin neurokinin 3 receptor antagonists: a new treatment for cardiovascular disease?

Great progress has been made in reducing cardiovascular mortality over the past 50 years. Nevertheless, prevalence is rising in some settings and remains higher in men than in women, even with the same level of established risk factors. To gain new insights, researchers are now considering cardiovascular disease in relation to the well known evolutionary biology model of growth and reproduction trading off against longevity, with trials of calorie restriction underway. However, calorie restriction has not been as successful as expected in primates and it is increasingly realised that effects on the reproductive axis might also be important. In this paper, the modulation of the reproductive axis using existing agents that have such properties-tachykinin neurokinin 3 receptor antagonists-is proposed as a way of reducing cardiovascular disease and combating a leading cause of global morbidity and mortality.

Activation of tachykinin, neurokinin 3 receptors affects chromatin structure and gene expression by means of histone acetylation

The tachykinin, neurokinin 3 receptor (NK3R) is a g-protein coupled receptor that is broadly distributed in the nervous system and exerts its diverse physiological actions through multiple signaling pathways. Despite the role of the receptor system in a range of biological functions, the effects of NK3R activation on chromatin dynamics and gene expression have received limited attention. The present work determined the effects of senktide, a selective NK3R agonist, on chromatin organization, acetylation, and gene expression, using qRT-PCR, in a hypothalamic cell line (CLU 209) that expresses the NK3R. Senktide (1 nM, 10nM) caused a relaxation of chromatin, an increase in global acetylation of histone H3 and H4, and an increase in the expression of a common set of genes involved in cell signaling, cell growth, and synaptic plasticity. Pretreatment with histone acetyltransferase (HAT) inhibitor (garcinol and 2-methylene y-butylactone), that inhibits p300, p300/CREB binding protein (CBP) associated factor (PCAF), and GCN 5, prevented the senktide-induced increase in expression of most, but not all, of the genes upregulated in response to 1 nM and 10nM senktide. Treatment with 100 nM had the opposite effect: a reduction in chromatin relaxation and decreased acetylation. The expression of four genes was significantly decreased and the HAT inhibitor had a limited effect in blocking the upregulation of genes in response to 100 nM senktide. Activation of the NK3R appears to recruit multiple pathways, including acetylation, and possibly histone deactylases, histone methylases, or DNA methylases to affect chromatin structure and gene expression.

Blockade of tachykinin NK3 receptor reverses hypertension through a dopaminergic mechanism in the ventral tegmental area of spontaneously hypertensive rats

BACKGROUND AND PURPOSE

Intracerebroventricularly injected tachykinin NK(3) receptor (R) antagonists normalize mean arterial blood pressure (MAP) in spontaneously hypertensive rats (SHR). This study was pursued to define the role played by NK(3)R located on dopamine neurones of the ventral tegmental area (VTA) in the regulation of MAP in SHR.

EXPERIMENTAL APPROACH

SHR (16 weeks) were implanted permanently with i.c.v. and/or VTA guide cannulae. Experiments were conducted 24 h after catheterization of the abdominal aorta to measure MAP and heart rate (HR) in freely behaving rats. Cardiovascular responses to i.c.v. or VTA-injected NK(3)R agonist (senktide) and antagonists (SB222200 and R-820) were measured before and after systemic administration of selective antagonists for D(1)R (SCH23390), D(2)R (raclopride) or non-selective D(2)R (haloperidol), and after destruction of the VTA with ibotenic acid.

KEY RESULTS

I.c.v. or VTA-injected SB222200 and R-820 (500 pmol) evoked anti-hypertension, which was blocked by raclopride. Senktide (10, 25, 65 and 100 pmol) elicited greater increases of MAP and HR when injected in the VTA, and the cardiovascular response was blocked by R-820, SCH23390 and haloperidol. VTA-injected SB222200 prevented the pressor response to i.c.v. senktide, and vice versa, i.c.v. senktide prevented the anti-hypertension to VTA SB222200. Destruction of the VTA prevented the pressor response to i.c.v. senktide and the anti-hypertension to i.c.v. R-820.

CONCLUSIONS AND IMPLICATIONS

The NK(3)R in the VTA is implicated in the maintenance of hypertension by increasing midbrain dopaminergic transmission in SHR. Hence, this receptor may represent a therapeutic target in the treatment of hypertension.

Contribution of the central dopaminergic system in the anti-hypertensive effect of kinin B1 receptor antagonists in two rat models of hypertension

Kinins are neuroactive peptides that could play a role in central autonomic control of blood pressure. Whereas kinin B1R binding sites were increased in specific brain areas of spontaneously hypertensive rats (SHR) and Angiotensin II (AngII)-hypertensive rats, the contribution of kinin B1R in hypertension remains controversial. The aims of the study were to determine: (a) the effects on mean arterial blood pressure (MAP) of centrally and peripherally administered B1R antagonists in SHR (16weeks) and AngII-hypertensive rats (200ng/kg/minx2weeks, s.c.); (b) the contribution of central dopamine in the effects of SSR240612. The rationale is based on the overactivity of the dopaminergic system in hypertension. In both models, SSR240612 (1, 5 and 10mg/kg, gavage) reduced dose-dependently MAP (-75mm Hg at least up to 6-8h) and this therapeutic effect was resolved after 24h. At the dose of 5mg/kg, SSR240612-induced anti-hypertension was prevented by two dopamine receptor blockers, namely raclopride (0.16mg/kg, i.v.) and haloperidol (10mg/kg, s.c.). I.c.v. SSR240612 (1mug) decreased rapidly MAP in both models (1-6h) via a raclopride sensitive mechanism. In comparison, peripherally acting B1R antagonists (R-715 and R-954, 2mg/kg, s.c.) caused shorter and very modest decreases of MAP (from -20 to -30mm Hg). Centrally or peripherally administered B1R antagonists had no effect on MAP in control Wistar-Kyoto rats. Data provide the first pharmacological evidence that the up-regulated brain kinin B1R contributes through a central dopaminergic mechanism (DA-D2R) to the maintenance of arterial hypertension in genetic and experimental animal models of hypertension.

Trafficking of tachykinin neurokinin 3 receptor to nuclei of neurons in the paraventricular nucleus of the hypothalamus following osmotic challenge

Tachykinin neurokinin 3 receptor (NK3R) is a G-protein (GTP binding protein) -coupled receptor that is heavily expressed by magnocellular neurons of the paraventricular nucleus of the hypothalamus (PVN). Osmotic challenge is reported to activate NK3R expressed by magnocellular neurons and cause the NK3R to be internalized to the cytoplasm and perhaps the cell nucleus. In this study we show using immuno-electron microscopy that isolated nuclei from neurons in the PVN of osmotic challenged animals (rats) show a robust labeling for the NK3R. NK3R immunoreactivity was detected by Western blot in isolated nuclei of PVN neurons following the 2 M NaCl injection. No nuclear NK3R immunoreactivity was detected in control animals. NK3R antibody specificity was confirmed by small interfering (SI) RNA technology. This study establishes that the NK3R is trafficked to the nucleus of PVN neurons following a peripheral osmotic challenge.

Upregulation of tachykinin NK-1 and NK-3 receptor binding sites in the spinal cord of spontaneously hypertensive rat: impact on the autonomic control of blood pressure

1 Effects of intrathecally (i.t.) injected tachykinin NK-1 and -3 receptor agonists and antagonists were measured on mean arterial blood pressure (MAP) and heart rate (HR) in awake unrestrained spontaneously hypertensive rats (SHR,15-week-old) and age-matched Wistar Kyoto rats (WKY). Quantitative in vitro autoradiography was also performed on the lower thoracic spinal cord of both strains and Wistar rats using specific radioligands for NK-1 receptor ([(125)I]HPP[Arg(3),Sar(9),Met(O(2))(11)]SP (3-11)) and NK-3 receptor ([(125)I]HPP-Asp-Asp-Phe-N-MePhe-Gly-Leu-Met-NH(2)). 2 The NK-1 agonist [Sar(9),Met(O(2))(11)]SP (650 and 6500 pmol) decreased MAP and increased HR in WKY. The fall in MAP was blunted in SHR and substituted by increases in MAP (65-6500 pmol) and more sustained tachycardia. The NK-3 agonist senktide (6.5-65 pmol) evoked marked increases in MAP and HR (SHR>>>WKY), yet this response was rapidly desensitized. Cardiovascular effects of [Sar(9),Met(O(2))(11)]SP (650 pmol) and senktide (6.5 pmol) were selectively blocked by the prior i.t. injection of LY303870 (NK-1 antagonist, 65 nmol) and SB235375 (NK-3 antagonist, 6.5 nmol), respectively. Antagonists had no direct effect on MAP and HR in both strains. 3 Densities of NK-1 and -3 receptor binding sites were significantly increased in all laminae of the spinal cord in SHR when compared to control WKY and Wistar rats. The dissociation constant was however not affected in SHR for both NK-1 (K(d)=2.5 nM) and NK-3 (K(d)=5 nM) receptors. 4 Data highlight an upregulation of NK-1 and -3 receptor binding sites in the thoracic spinal cord of SHR that may contribute to the hypersensitivity of the pressor response to agonists and to the greater sympathetic activity seen in this model of arterial hypertension.

Agonist and hypertonic saline-induced trafficking of the NK3-receptors on vasopressin neurons within the paraventricular nucleus of the hypothalamus

The neurokinin 3 receptor (NK3R) is colocalized with vasopressinergic neurons within the hypothalamic paraventricular nucleus (PVN) and intraventricular injections of NK3R agonists stimulate vasopressin (VP) release. Our objectives were to test the hypotheses that intraventricular injections of the selective NK3R agonist, succinyl-[Asp6, N-Me-Phe8] substance P (senktide), activate NK3R expressed by vasopressinergic neurons within the PVN, and see whether NK3R expressed by vasopressinergic neurons in the PVN are activated by hyperosmolarity. NK3R internalization was used as a marker of receptor activation. Immunohistochemistry revealed that NK3Rs were membrane-bound on VP immunoreactive neurons in control rats. Following senktide injection, there was a significant increase in the appearance of NK3R immunoreactivity within the cytoplasm and a morphological rearrangement of the dendrites, indicating receptor internalization, which was reversible. Furthermore, pretreatment with a selective NK3R antagonist, SB-222200, blocked the senktide-induced VP release and internalization of the NK3R in the PVN. These results show that the trafficking of the NK3R is due to ligand binding the NK3R. In a subsequent experiment, rats were administered intragastric loads of 2 or 0.15 M NaCl, and NK3R immunohistochemistry was used to track activation of the receptor. In contrast to control rats, 2 M NaCl significantly increased plasma VP levels and caused the internalization of the NK3R on VP neurons. Also, NK3R immunoreactivity was located in the nuclei of vasopressinergic neurons after senktide and 2 M NaCl treatment. These results show that hyperosmolarity stimulates the local release of an endogenous ligand in the PVN to bind to and activate NK3R on vasopressinergic neurons.

Intraventricular injections of tachykinin NK3 receptor agonist reduce the gain of the baroreflex in unrestrained rats

The tachykinin neuropeptides acting at NK3 receptors affect mean arterial pressure (MAP) through both neuroendocrine and neural mechanisms. NK3 receptors are found in brainstem nuclei that mediate the baroreflex, but the effects of NK3 receptor stimulation on baroreflex function are unknown. The present study tests the effects of intraventricular injections of senktide, a selective NK3 receptor agonist, on the sensitivity of the baroreflex in three stains of rats: Charles River Laboratory, Long-Evans, and Brattleboro rats, which lack the ability to synthesize vasopressin. Rats with lateral ventricle cannulas were administered injections of isotonic saline, 100 ng, or 200 ng senktide, and 5 min later arterial baroreceptor-heart rate (HR) function was examined by constructing full-range blood pressure-HR curves using alternating doses (5-20 microg kg min) of phenylephrine and nitroprusside to raise and decrease blood pressure approximately 50 mm Hg over a period of 1 min, respectively. Intraventricular injections of 200 ng senktide had no significant effect on baseline MAP, but significantly decreased the gain of the baroreflex in all three rat strains whereas the 100 ng dose had no effect on the baroreflex. These results show that NK3 receptor stimulation modulates the baroreflex that is independent of any action of vasopressin.