Renal effects of intracerebroventricularly injected tachykinins in the conscious saline-loaded rat: receptor characterization

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.

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.

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

Tachykinins are involved in the central autonomic control of blood pressure. In the present study, we examined the i.c.v. cardiovascular effects of several tachykinin receptor antagonists in awake spontaneously hypertensive rats (SHR, 15 weeks old). Results showed that two tachykinin NK(3) receptor antagonists (R-820: 3-indolylcarbonyl-Hyp-Phg-N(Me)-Bzl and SB 222200: (S)-(-)-N-(alpha-ethylbenzyl)-3-methyl-2-phenylquinoline-4-carboxamide) caused a sustained and dose-dependent reduction of blood pressure when injected i.c.v. but not i.v. The stereoselective anti-hypertensive effect of SB 222200 peaked at 3 h and faded at 6 h post-injection (if injected at 07:00 h) or had a slower onset and peaked at 8 h post-injection (if injected at 13:00 h). The effect of R-820 was maximal at 24 h and lasted up to 48 h post-injection. Both antagonists failed to alter blood pressure in normotensive Wistar-Kyoto rats (WKY) and heart rate was not affected in both strains. The anti-hypertensive effect of SB 222200 was not associated with changes in plasma levels of catecholamines and vasopressin and it remained unchanged in SHR subjected to acute bilateral nephrectomy. In contrast, blood pressure was not affected by tachykinin NK(1) (RP 67580: (+/-) 7,7-diphenyl-2[1-imino-2(2-methoxy-phenyl)-ethyl]perhydroisoindol-4-one(3aR,7aR)) and NK(2) (SR 48968: (S)-N-methyl-N[4-(4-acetylamino-4-phenylpiperidino)-2-(3,4-dichlorophenyl)butyl]benzamide) receptor antagonists. Data suggest that brain tachykinin NK(3) receptors are implicated in the maintenance of hypertension in SHR. Hence, these receptors may represent promising therapeutic target in the treatment of arterial hypertension.

Implication of nigral tachykinin NK3 receptors in the maintenance of hypertension in spontaneously hypertensive rats: a pharmacologic and autoradiographic study

1. The role of nigral tachykinin NK(1), NK(2) and NK(3) receptors in central cardiovascular regulation was studied by measuring the effects of selective agonists and antagonists on mean arterial pressure (MAP) and heart rate (HR) after bilateral microinjection into the substantia nigra of spontaneously hypertensive rats (SHR). Quantitative in vitro autoradiography was also performed in the midbrain of SHR and Wistar-Kyoto (WKY) with the NK(3) receptor ligand [(125)I]-HPP-Senktide. 2. Tachycardia was elicited by the NK(1) ([Sar(9),Met(O(2))(11)]SP) and NK(2) ([betaAla(8)]NKA(4-10)) agonists at 25 and 100 pmol while the NK(3) agonist (senktide, 50 and 100 pmol) had no significant effect. The three agonists had no effect on behaviour, and increases in MAP were elicited by the NK(1) agonist only. 3. Whereas antagonists at NK(1) (RP 67580, 500 pmol) and NK(2) (SR 48968, 500 pmol) receptors had no significant effect on MAP and HR, the NK(3) antagonist (R-820, 500 pmol) reduced MAP for over 3 h in SHR. That anti-hypertensive effect did not occur after intracerebroventricular or intravenous injection of R-820. Also, R-820 had no cardiovascular effect in WKY. 4. The affinity (K(D): 0.7 nM) and densities of specific NK(3) receptor binding sites measured in the substantia nigra, ventral tegmental area, hippocampus and amygdala were not significantly different in SHR and WKY. 5. It is concluded that endogenous tachykinins exert a tonic activity on NK(3) receptors in the substantia nigra of SHR to maintain high blood pressure. Hence, nigral tachykinin NK(3) receptors may represent a promising therapeutic target in the treatment of arterial hypertension.

The psychopharmacology of tachykinin NK-3 receptors in laboratory animals

The present article reviews the studies so far published on the psychopharmacological effects mediated by tachykinin NK-3 receptors in laboratory animals. Central administration of NK-3 receptor agonists has been reported to attenuate alcohol intake in alcohol-preferring rats and to evoke conditioned place preference. These findings suggest that NK-3 receptors may affect reward processes to drugs of abuse. Anxiolytic-like and antidepressant-like effects have been previously reported for NK-1 receptor antagonists, and anxiolytic-like effects for NK-2 receptor antagonists. More recently, it has been shown that NK-3 receptor agonists have anxiolytic-like and antidepressant-like effects in mice and rats, while an NK-3 receptor antagonist was reported to be anxiogenic in mice. These findings indicate that different TK receptor subtypes may be involved in anxiolytic-like and antidepressant-like effects in laboratory animals and raise interest for the possible role of NK-3 receptors in the control of anxiety and depression in man.

SR142801 behaves as a tachykinin NK-3 receptor agonist on a spinal nociceptive reflex in the rat

Effects of two commonly used tachykinin NK-3 receptor antagonists (SR 142801 and R820) intrathecally (i.t.) administered were assessed in the rat tail-flick test. SR142801 and its (R)-enantiomer SR142806 (1.3, 6.5 and 65 nmol) were found as potent as senktide and [MePhe7]NKB (NK-3 selective agonists) to induce transient antinociceptive effects. Naloxone (10 microg) and R820 (6.5 nmol) blocked reversibly the responses to 6.5 nmol senktide, [MePhe7]NKB, SR142801 and SR142806 when administered i.t. 15 min earlier. However, the antinociceptive responses induced by SR142801 and SR142806 were not affected by i.t. pretreatments with NK-1 (6.5 nmol SR140333) and NK-2 (6.5 nmol SR48968) receptor antagonists. In control experiments, the NK-1 and NK-2 antagonists prevented the hyperalgesic effects to NK-1 ([Sar9,Met(O2)11]SP) and NK-2 ([beta-Ala8] NKA(4-10)) receptor agonists (6.5 nmol i.t.), respectively. R820 had no direct effect on nociceptive threshold and failed to alter angiotensin II-induced antinociception. The data suggest that the antinociceptive effect of SR142801 is due to an agonist effect at NK-3 receptor in the rat spinal cord that involves a local opioid mechanism. These results can be best explained by the existence of inter-species NK-3 receptor subtypes.

Central injections of capsaicin cause antidiuresis mediated through neurokinin-1 receptors in rat hypothalamus and vasopressin release

Intracerebroventricular injections of capsaicin at 100-500 nmol elicited dose-dependent decreases in urine outflow volume in anesthetized, hydrated rats. The capsaicin (500 nmol)-induced antidiuresis was inhibited by pretreatment with CP96345 (30 nmol, a neurokinin-1-receptor antagonist), but not by that with phenoxybenzamine (20 nmol, an alpha-adrenoceptor antagonist), timolol (100 nmol, a beta-adrenoceptor antagonist) or atropine (300 nmol, a muscarinic antagonist) into the hypothalamic supraoptic nucleus (SON). Intravenous injections of d(CH2)5-D-Tyr(Et)VAVP (50 microg/kg, a vasopressin-receptor antagonist) completely blocked the antidiuresis. In intra-SON microdialysis experiments, acetylcholine concentration in the perfusate of the capsaicin-injected rats was not different from that of the vehicle-injected rats. These findings suggested that capsaicin stimulated substance P release in the SON and caused the antidiuresis as a result of the increased release of vasopressin into the circulation from the neurohypophysis mediated through neurokinin-1 receptors in the SON.

Further evidence that central tachykinin NK-1 receptors mediate the inhibitory effect of tachykinins on angiotensin-induced drinking in rats

The order of potency of tachykinin (TK) receptor agonists suggests that TK NK-1 receptors mediate their inhibitory effect on water intake induced by intracerebroventricular (i.c.v.) injection of angiotensin II (AngII) in rats. The present study was aimed at further evaluating which TK receptor subtype mediates the effect, using selective antagonists for the TK receptor subtypes. Pulse i.c.v. injection of the TK agonist neuropeptide gamma (NP gamma), 31-250 ng/rat, markedly inhibited AngII-induced water intake. The i.c.v. injection of the NK-1 receptor antagonist SR14033, 0.5 microgram/rat, significantly reduced, while 1 microgram/rat completely abolished the inhibitory effect of NP gamma, 125 ng/rat. The selective NK-2 receptor antagonist SR48968 and the selective NK-3 receptor antagonist R820 were devoid of any effect up to the i.c.v. dose of 2 micrograms/rat. On the other hand, i.c.v. injection of SR140333, 1 microgram/rat, did not increase drinking induced by i.c.v. injection of AngII, 0.1-10 ng/rat, and did not increase drinking in water sated or water deprived rats. The results of the present study confirm that central TKergic mechanisms inhibit AngII-induced drinking in rats, and provide further evidence that TK NK-1 receptors mediate the effect. Failure of i.c.v. injected SR 140333 to increase AngII-induced drinking, as well as water intake in sated or deprived rats suggests that brain NK-1 receptor mechanisms apparently do not exert a tonic control on AngII-induced drinking and, in general, on water intake in rats. From a pharmacological point of view, the inhibitory effect of TKs on the dipsogenic action of AngII can represent a functional test for activity at central NK-1 receptors in rats.

Renal effects of intrathecally injected tachykinins in the conscious saline-loaded rat: receptor and mechanism of action

1. The effects of intrathecally (i.t.) injected substance P (SP), neurokinin A (NKA), [beta-Ala8]NKA (4-10) and [MePhe7]neurokinin B (NKB) at T13 thoracic spinal cord level were investigated on renal excretion of water, sodium and potassium in the conscious saline-loaded rat. Antagonists selective for NK1 (RP 67580), NK2 (SR 48968) and NK3 (R 820; 3-indolylcarbonyl-Hyp-Phg-N(Me)-Bzl) receptors were used to characterize the spinal effect of SP on renal function. 2. Saline gavage (4.5% of the body weight) enhanced renal excretion of water, sodium and potassium over the subsequent hour of measurement. Whereas these renal responses were not affected by 0.65 nmol SP, the dose of 6.5 nmol SP blocked the natriuretic response (aCSF value 3.9 +/- 0.8; SP value 0.7 +/- 0.3 micromol min(-1), P<0.01) as well as the renal excretion of water (aCSF value 48.9 +/- 5.8; SP value 14.5 +/- 4.0 microl min(-1), P<0.01) and potassium (aCSF value 4.8 +/- 0.6; SP value 1.5 +/- 0.6 micromol min(-1), P<0.01) at 30 min post-injection. SP had no significant effect on urinary osmolality. The SP-induced renal inhibitory effects during the first 30 min were abolished in rats subjected to bilateral renal denervation 1 week earlier or in rats injected i.t. 5 min earlier with 6.5 nmol RP 67580. In contrast, the co-injection of SR 48968 and R 820 (6.5 nmol each) did not affect the inhibitory responses to SP. On their own, these antagonists had no direct effect on renal excretion function. Since SP induced only transient changes in mean arterial blood pressure (-18.8 +/- 3.8 mmHg at 1 min and +6.3 +/- 2.4 mmHg at 5 min post-injection), it is unlikely that the renal effects of SP are due to systemic haemodynamic changes. 3. NKA (6.5 nmol but not 0.65 nmol) produced a transient drop in renal excretion of water (aCSF value 31.2 +/- 5.1; NKA value 11.3 +/- 4.2 microl min(-1), P<0.05), sodium (aCSF value 1.7 +/- 0.8; NKA value 0.4 +/- 0.2 micromol min(-1), P<0.05) and potassium (aCSF value 4.1 +/- 0.7; NKA value 1.5 +/- 0.4 micromol min(-1), P<0.05) at 15 min post-injection. However, the same doses (6.5 nmol) of selective agonists for tachykinin NK2 ([beta-Ala8]NKA(4-10)) and NK3 ([MePhe7]NKB) receptors were devoid of renal effects. 4. This study provided functional evidence that tachykinins may be involved in the renal control of water and electrolyte excretion at the level of the rat spinal cord through the activation of NK1 receptors and the sympathetic renal nerve.