Effects of NK1 receptor antagonists on vasodilation induced by chemical and electrical activation of sensory C-fibre afferents in different organs

Neurokinin-1 receptor desensitization to consecutive microdialysis infusions of substance P in human skin

The neuropeptide substance P is known to be localized in nerve terminals in human skin and substance P-induced vasodilatation is believed to be partially dependent on nitric oxide (NO) and H1 histamine receptor activation. Unlike other neuropeptides investigated in human skin, substance P-induced vasodilatation has been shown to decline during continuous infusion, possibly suggestive of an internalization of neurokinin-1 (NK1) receptors, which are highly specific to substance P. However, questions remain regarding these mechanisms in human skin. Fifteen subjects participated in this series of studies designed to investigate the effect of consecutive infusions and possible mechanisms of substance P-induced vasodilatation in human skin. Two concentrations of substance P (10 microm and 20 microm) were tested via intradermal microdialysis in two groups of subjects. Site 1 served as a control and received substance P only. Site 2 received substance P combined with 10 mm L-NAME to inhibit NO synthase. Site 3 received substance P combined with 500 microm pyrilamine, an H1 receptor antagonist. Site 4 received substance P combined with 10 mm L-NAME plus 500 microm pyrilamine. Red blood cell (RBC) flux was measured via laser-Doppler flowmetry to provide an index of skin blood flow. Cutaneous vascular conductance was calculated as RBC flux/mean arterial pressure and was normalized to maximal vasodilatation via 28 mm sodium nitroprusside. Substance P was perfused through each microdialysis fibre at a rate of 4 microl min(-1) for 15 min. The subsequent increase in skin blood flow was allowed to return to baseline (approximately 45-60 min) and a stable 5 min plateau was used as a new baseline (post-infusion baseline). A second dose of substance P was then delivered to the skin and skin blood flow was monitored for 45-60 min. Substance P produced a dose-dependent increase in skin blood flow with the concentrations of substance P tested, which was significantly attenuated in the presence of L-NAME and the combination of L-NAME plus pyrilamine. However, substance P-induced vasodilatation was unaffected in the presence of pyrilamine. There was no significant difference between the L-NAME-only sites and the L-NAME plus pyrilamine sites. Importantly, the second dose of substance P did not produce a significant increase in skin blood flow compared to the initial baseline or the post-infusion baseline. These data suggest substance P-induced vasodilatation delivered via microdialysis contains an NO component but does not contain an H1 receptor activation component at the doses tested. Additionally, these data provide evidence for NK1 receptor desensitization as there was no observable increase in skin blood flow following a second administration of substance P. This may provide a useful model for studying the role of substance P in the control of skin blood flow in humans.

Minimal role for H1 and H2 histamine receptors in cutaneous thermal hyperemia to local heating in humans

The precise mechanism(s) underlying the thermal hyperemic response to local heating of human skin are not fully understood. The purpose of this study was to investigate a potential role for H1 and H2 histamine-receptor activation in this response. Two groups of six subjects participated in two separate protocols and were instrumented with three microdialysis fibers on the ventral forearm. In both protocols, sites were randomly assigned to receive one of three treatments. In protocol 1, sites received 1) 500 microM pyrilamine maleate (H1-receptor antagonist), 2) 10 mM L-NAME to inhibit nitric oxide synthase, and 3) 500 microM pyrilamine with 10 mM NG-nitro-L-arginine methyl ester (L-NAME). In protocol 2, sites received 1) 2 mM cimetidine (H2 antagonist), 2) 10 mM L-NAME, and 3) 2 mM cimetidine with 10 mM L-NAME. A fourth site served as a control site (no microdialysis fiber). Skin sites were locally heated from a baseline of 33 to 42 degrees C at a rate of 0.5 degrees C/5 s, and skin blood flow was monitored using laser-Doppler flowmetry (LDF). Cutaneous vascular conductance was calculated as LDF/mean arterial pressure. To normalize skin blood flow to maximal vasodilation, microdialysis sites were perfused with 28 mM sodium nitroprusside, and control sites were heated to 43 degrees C. In both H1 and H2 antagonist studies, no differences in initial peak or secondary plateau phase were observed between control and histamine-receptor antagonist only sites or between L-NAME and L-NAME with histamine receptor antagonist. There were no differences in nadir response between L-NAME and L-NAME with histamine-receptor antagonist. However, the nadir response in H1 antagonist sites was significantly reduced compared with control sites, but there was no effect of H2 antagonist on the nadir response. These data suggest only a modest role for H1-receptor activation in the cutaneous response to local heating as evidenced by a diminished nadir response and no role for H2-receptor activation.

Determination of a novel substance P inhibitor in human plasma by high-performance liquid chromatography with atmospheric pressure chemical ionization mass spectrometric detection using single and triple quadrupole detectors

Methods based on high-performance liquid chromatography (HPLC) with atmospheric-pressure chemical ionization (APCI) mass spectrometric (MS) detection using either single (MS) or triple (MS/MS) quadrupole mass spectrometric detection for the determination of (2R)-[1(R)-(3,5-bis-trifluoromethylphenyl)ethoxy]-3(S)-(4-fluoro-phenyl)morpholin-4-ylmethyl]-5-oxo-4,5-dihydro-[1,2,4]triazol)methyl morpholine (Aprepitant, Fig. 1) in human plasma has been developed. Aprepitant (I) and internal standard (II, Fig. 1) were isolated from the plasma matrix buffered to pH 9.8 using a liquid-liquid extraction with methyl-t-butyl ether (MTBE). The analytes were separated on a Keystone Scientific's Javelin BDS C-8 2 mm x 4.6 mm 3 microm guard column coupled to BDS C-8 50 mm x 4.6 mm 3 microm analytical column, utilizing a mobile phase of 50% acetonitrile and 50% water containing 0.1% formic acid and 10 mM ammonium acetate delivered at a flow rate of 1 ml/min. The single quadrupole instrument was operated in a single ion monitoring (SIM) mode analyzing the protonated molecules of Aprepitant and II at m/z 535 and 503, respectively. The triple quadrupole mass spectrometer was operated in multiple reaction monitoring mode (MRM) monitoring the precursor --> ion combinations of m/z 535 --> 277 and 503 --> 259 for Aprepitant and II, respectively. The linear calibration range for both single and triple quadrupole detectors was from 10 to 5000 ng/ml of plasma with coefficients of variation less than 8% at all concentrations. Both single and triple quadrupole instruments yielded similar precision and accuracy results. Matrix effect experiments performed on both instruments demonstrated the absence of any significant change in ionization of the analytes when comparing neat standards to analytes in the presence of plasma matrix. Both instruments were used successfully to support numerous clinical trials of Aprepitant.

Skin reactions to histamine of healthy subjects after hypnotically induced emotions of sadness, anger, and happiness

BACKGROUND

The severity of symptoms in asthma and other hypersensitivity-related disorders has been associated with changes in mood but little is known about the mechanisms possibly mediating such a relationship. The purpose of this study was to examine the influence of mood on skin reactivity to histamine by comparing the effects of hypnotically induced emotions on flare and wheal reactions to cutaneous histamine prick tests.

METHODS

Fifteen highly hypnotically susceptible volunteers had their cutaneous reactivity to histamine measured before hypnosis at 1, 2, 3, 4, 5, 10, and 15 min after the histamine prick. These measurements were repeated under three hypnotically induced emotions of sadness, anger, and happiness presented in a counterbalanced order. Skin reactions were measured as change in histamine flare and wheal area in mm2 per minute.

RESULTS

The increase in flare reaction in the time interval from 1 to 3 min during happiness and anger was significantly smaller than flare reactions during sadness (P<0.05). No effect of emotion was found for wheal reactions. Hypnotic susceptibility scores were associated with increased flare reactions at baseline (r=0.56; P<0.05) and during the condition of happiness (r=0.56; P<0.05).

CONCLUSION

Our results agree with previous studies showing mood to be a predictor of cutaneous immediate-type hypersensitivity and histamine skin reactions. The results are also in concordance with earlier findings of an association between hypnotic susceptibility and increased reactivity to an allergen.

Light and electron microscopic study of the distribution of substance P-immunoreactive fibers and neurokinin-1 receptors in the skin of the rat lower lip

Cutaneous antidromic vasodilatation and plasma extravasation, two phenomena that occur in neurogenic inflammation, are partially blocked by substance P (SP) receptor antagonists and are known to be mediated in part by mast cell-released substances, such as histamine, serotonin, and nitric oxide. In an attempt to provide a morphological substrate for the above phenomena, we applied light and electron microscopic immunocytochemistry to investigate the pattern of SP innervation of blood vessels and its relationship to mast cells in the skin of the rat lower lip. Furthermore, we examined the distribution of SP (neurokinin-1) receptors and their relationship to SP-immunoreactive (IR) fibers. Our results confirmed that SP-IR fibers are found in cutaneous nerves and that terminal branches are observed around blood vessels and penetrating the epidermis. SP-IR fibers also innervated hair follicles and sebaceous glands. At the ultrastructural level, SP-IR varicosities were observed adjacent to arterioles, capillaries, venules, and mast cells. The varicosities possessed both dense core vesicles and agranular synaptic vesicles. We quantified the distance between SP-IR varicosities and blood vessel endothelial cells. SP-IR terminals were located within 0.23-5.99 microm from the endothelial cell layer in 82.7% of arterioles, in 90.2% of capillaries, and in 86.9% of venules. Although there was a trend for SP-IR fibers to be located closer to the endothelium of venules, this difference was not significant. Neurokinin-1 receptor (NK-1r) immunoreactivity was most abundant in the upper dermis and was associated with the wall of blood vessels. NK-1r were located in equal amounts on the walls of arterioles, capillaries, and venules that were innervated by SP-IR fibers. The present results favor the concept of a participation of SP in cutaneous neurogenic vasodilatation and plasma extravasation both by an action on blood vessels after binding to the NK-1r and by causing the release of substances from mast cells after diffusion through the connective tissue.

Substance P-induced vasodilatation is mediated by the neurokinin type 1 receptor but does not contribute to basal vascular tone in man

AIMS

Following intravenous administration of its prodrug, L-758,298, we assessed the pharmacodynamics of L-754,030, a novel and highly selective NK1 receptor antagonist, by examining systemic haemodynamics and the blood flow responses to intra-arterial substance P infusion.

METHODS

Sixteen healthy male volunteers participated in a double-blind, randomised, placebo controlled crossover trial of L-758 298. Forearm blood flow was measured using venous occlusion plethysmography during intrabrachial substance P infusion (0.125-128 pmol min-1 ). In part 1, eight subjects received substance P infusions before and during placebo, 0.25 mg, 1 mg or 5 mg of L-758 298. In part 2, eight subjects received substance P infusions 24 h after placebo or 1.43 mg of L-758 298.

RESULTS

L-758 298 caused dose dependent inhibition of substance P induced vasodilatation (P<0.001). Placebo adjusted differences (95% CI) in baseline forearm blood flow, mean arterial pressure and heart rate showed no relevant changes with 5 mg of L-758 298 (>1400-fold shift in substance P response): 0.00 (-0.49 to +0.49) ml 100 ml-1 min-1, 1. 0 (-3.2 to +5.2) mmHg and 1.9 (-5.9 to +9.7) beats min-1, respectively. Twenty-four hours after 1.43 mg of L-758,298, there was approximately 34-fold shift in response to substance P induced vasodilatation (P<0.008) at plasma L-754 030 concentrations of 2-3 ng ml-1. L-758 298 was generally well tolerated without serious adverse events.

CONCLUSIONS

Substance P induced forearm vasodilatation is mediated by the endothelial cell NK1 receptor in man but endogenous substance P does not appear to contribute to the maintenance of peripheral vascular tone or systemic blood pressure.

Involvement of neurokinins in antidromic vasodilatation in hairy and hairless skin of the rat hindlimb

By intravenous application of the specific neurokininl receptor antagonist SR 140333 and the specific calcitonin gene-related peptide receptor antagonist CGRP8-37 we tested to what extent neurokinins (substance P, neurokinin A) and calcitonin gene-related peptide are involved in mediating antidromic vasodilatation in skin of anaesthetized Wistar rats. The lumbar sympathetic chain was sectioned bilaterally between ganglia L2 and L3 to remove ongoing vasoconstrictor activity to the hindquarter. The left dorsal root L5 was stimulated electrically at 1 Hz with 20 pulses supramaximal for activating C-fibres to evoke antidromic vasodilatation which was measured with laser Doppler flowmetry on the glabrous plantar skin and the hairy skin of the lower hindlimb within the left L5 territory. Stimulation-induced vasodilatation was tested after applying SR 140333 (0.1 mg/kg) and CGRP8-37 (0.3 mg/kg) alone or in combination. SR 140333 delayed the onset of the vasodilatation, but did not change its amplitude. CGRP8-37 reduced the amplitude and duration of the vasodilatation, but did not affect the latency of its onset. In combination, SR 140333 potentiated the effect of CGRP8-37 on the amplitude of the vasodilatation in glabrous but not in hairy skin and CGRP8-37 potentiated the delayed onset produced by SR 140333 in both cutaneous tissues. Antidromic vasodilatation in glabrous skin was almost totally blocked by SR 140333 (0.1 mg/kg) in combination with CGRP8-37 (0.45 mg/kg), but a substantial dilatation remained in hairy skin. It is concluded that in rat glabrous skin the vasodilatation evoked by a low level of activity in small-diameter primary afferents is likely to result from the release and synergistic action of neurokinins (substance P and/or neurokinin A) and calcitonin gene-related peptide, while in hairy skin neurokinins are involved to a minor extent only.

The L-arginine/nitric oxide pathway contributes to the acute release of tissue plasminogen activator in vivo in man

OBJECTIVE

Effective endogenous fibrinolysis requires rapid release of endothelial tissue plasminogen activator (t-PA). Using the nitric oxide synthase inhibitor, L-NG-monomethylarginine (L-NMMA), we examined the contribution of endogenous nitric oxide to substance P-induced t-PA release in vivo in man.

METHODS

Blood flow and plasma fibrinolytic and haemostatic factors were measured in both forearms of 8 healthy male volunteers who received unilateral brachial artery infusions of substance P (2-8 pmol/min) and L-NMMA (1-4 micrograms/min).

RESULTS

Substance P caused dose-dependent increases in blood flow (P < 0.001) and plasma t-PA antigen (P = 0.04) and activity (P < 0.001) concentrations confined to the infused forearm, but had no effect on plasminogen activator inhibitor type I (PAI-I) or von Willebrand factor concentrations. In the presence of L-NMMA, substance P again caused significant increases in blood flow (P < 0.001) and t-PA antigen (P = 0.003) and activity (P < 0.001) concentrations but these increases were significantly less than with substance P alone (P < 0.001, P = 0.05 and P < 0.01, respectively). L-NMMA alone significantly reduced blood flow in the infused arm, but had no measurable effect on t-PA or PAI-1 concentrations.

CONCLUSIONS

The L-arginine/nitric oxide pathway contributes to substance P-induced t-PA release in vivo in man. This provides an important potential mechanism whereby endothelial dysfunction increases the risk of atherothrombosis through a reduction in the acute fibrinolytic capacity.

The tachykinin NK1 receptor. Part II: Distribution and pathophysiological roles

The tachykinin NK1 receptor is widely distributed in both the central and peripheral nervous system. In the CNS, NK1 receptors have been implicated in various behavioural responses and in regulating neuronal survival and degeneration. Moreover, central NK1 receptors regulate cardiovascular and respiratory function and are involved in activating the emetic reflex. At the spinal cord level, NK1 receptors are activated during the synaptic transmission, especially in response to noxious stimuli applied at the receptive field of primary afferent neurons. Both neurophysiological and behavioural evidences support a role of spinal NK1 receptors in pain transmission. Spinal NK1 receptors also modulate autonomic reflexes, including the micturition reflex. In the peripheral nervous system, tachykinin NK1 receptors are widely expressed in the respiratory, genitourinary and gastrointestinal tracts and are also expressed by several types of inflammatory and immune cells. In the cardiovascular system, NK1 receptors mediate endothelium-dependent vasodilation and plasma protein extravasation. At respiratory level, NK1 receptors mediate neurogenic inflammation which is especially evident upon exposure of the airways to irritants. In the carotid body, NK1 receptors mediate the ventilatory response to hypoxia. In the gastrointestinal system, NK1 receptors mediate smooth muscle contraction, regulate water and ion secretion and mediate neuro-neuronal communication. In the genitourinary tract, NK1 receptors are widely distributed in the renal pelvis, ureter, urinary bladder and urethra and mediate smooth muscle contraction and inflammation in response to noxious stimuli. Based on the knowledge of distribution and pathophysiological roles of NK1 receptors, it has been anticipated that NK1 receptor antagonists may have several therapeutic applications at central and peripheral level. At central level, it is speculated that NK1 receptor antagonists could be used to produce analgesia, as antiemetics and for treatment of certain forms of urinary incontinence due to detrusor hyperreflexia. In the peripheral nervous system, tachykinin NK1 receptor antagonists could be used in several inflammatory diseases including arthritis, inflammatory bowel diseases and cystitis. Several potent tachykinin NK1 receptor antagonists are now under evaluation in the clinical setting, and more information on their usefulness in treatment of human diseases will be available in the next few years.

Neurogenic vasodilatation and plasma leakage in the skin

1. Primary afferent nerve fibers control cutaneous blood flow and vascular permeability by releasing vasoactive peptides. These vascular reactions and the additional recruitment of leukocytes are commonly embodied in the term neurogenic inflammation. 2. Calcitonin gene-related peptide (CGRP) acting via CGRP1 receptors is the principal transmitter of neurogenic dilatation of arterioles whereas substance P (SP) and neurokinin A (NKA) acting via NK1 receptors mediate the increase in venular permeability. 3. Neurogenic vasodilatation and plasma protein leakage play a role in inflammation because many inflammatory and immune mediators including interleukin-1 beta, nitric oxide, prostanoids, protons, bradykinin, histamine, and 5-hydroxytryptamine can stimulate peptidergic afferent nerve fibers or enhance their excitability. 4. Neurogenic inflammatory reactions can be suppressed by alpha 2-adrenoceptor agonists, histamine acting via H1 receptors, 5-hydroxytryptamine acting via 5-HT1B receptors, opioid peptides, and somatostatin through prejunctional inhibition of peptide release from vasoactive afferent nerve fibers. CGRP, SP, and NKA receptor antagonists are powerful pharmacological tools to inhibit neurogenic inflammation at the postjunctional level. 5. Imbalance between the facilitatory and inhibitory influences on afferent nerve activity has a bearing on chronic inflammatory disease. Impaired nerve function represents a deficit in skin homeostasis while neuronal overactivity is a factor in allergic and hyperreactive disorders of the skin.

Binding of neuroleptic drugs (trifluoperazine and rimcazole) to vanilloid receptors in porcine dorsal horn

Neuroleptic drugs were reported to modulate [3H]resiniferatoxin binding to vanilloid receptors in the spinal cord, with marked differences between rat and man. In the present study, we have used a [3H]resiniferatoxin binding assay using porcine dorsal horn membranes to explore further species differences in the interaction of neuroleptic drugs at spinal vanilloid receptors. Specific binding of 13 pM [3H]resiniferatoxin to porcine dorsal horn membranes (corresponding to a 7% fractional receptor occupancy) was affected by trifluoperazine in a bi-phasic fashion, with an initial 90% enhancement of binding preceding inhibition: a fit to the modified Hill equation yielded a cooperativity index of 1.8 and a Ki of 5 microM. Under similar conditions, rimcazole, by contrast, had a monophasic effect: it enhanced but, up to 100 microM, did not inhibit [3H]resiniferatoxin binding. These results are in accord with previous findings in human spinal cord but contrast with those in the rat. In experiments in which the concentration of [3H]resiniferatoxin was varied, 20 microM trifluoperazine reduced the Bmax by 33% (from 181 +/- 9 fmol/mg protein to 121 +/- 5 fmol/mg protein) without a measurable change in affinity or cooperativity. In parallel experiments, by contrast, neither capsaicin nor capsazepine (both at a concentration of 10 microM) affected the Bmax or cooperativity but, as expected, reduced the affinity from 61 +/- 8 pM to 120 +/- 11 pM or to 101 +/- 7 pM, respectively. Whereas vanilloid receptor agonists (resiniferatoxin and capsaicin) affected [3H]resiniferatoxin binding at low (approximately 7%) fractional receptor occupancies by the radioligand in a bi-phasic fashion, the competitive vanilloid receptor antagonist capsazepine failed to induce the initial binding enhancement. Thus, capsazepine appears to bind to vanilloid receptors in a non-cooperative fashion, or at least with much reduced positive cooperativity in this system. The mechanism by which neuroleptic drugs modulate resiniferatoxin binding is yet to be clarified and is clearly complicated as well as species-dependent; nonetheless, the reduced Bmax at higher concentrations suggests that it may at least in part be non-competitive.