Major metabolites of substance P degraded by spinal synaptic membranes antagonize the behavioral response to substance P in rats

The Significance of NK1 Receptor Ligands and Their Application in Targeted Radionuclide Tumour Therapy

To date, our understanding of the Substance P (SP) and neurokinin 1 receptor (NK1R) system shows intricate relations between human physiology and disease occurrence or progression. Within the oncological field, overexpression of NK1R and this SP/NK1R system have been implicated in cancer cell progression and poor overall prognosis. This review focuses on providing an update on the current state of knowledge around the wide spectrum of NK1R ligands and applications of radioligands as radiopharmaceuticals. In this review, data concerning both the chemical and biological aspects of peptide and nonpeptide ligands as agonists or antagonists in classical and nuclear medicine, are presented and discussed. However, the research presented here is primarily focused on NK1R nonpeptide antagonistic ligands and the potential application of SP/NK1R system in targeted radionuclide tumour therapy.

Glial cell type-specific changes in spinal dipeptidyl peptidase 4 expression and effects of its inhibitors in inflammatory and neuropatic pain

Altered pain sensations such as hyperalgesia and allodynia are characteristic features of various pain states, and remain difficult to treat. We have shown previously that spinal application of dipeptidyl peptidase 4 (DPP4) inhibitors induces strong antihyperalgesic effect during inflammatory pain. In this study we observed low level of DPP4 mRNA in the rat spinal dorsal horn in physiological conditions, which did not change significantly either in carrageenan-induced inflammatory or partial nerve ligation-generated neuropathic states. In naïve animals, microglia and astrocytes expressed DPP4 protein with one and two orders of magnitude higher than neurons, respectively. DPP4 significantly increased in astrocytes during inflammation and in microglia in neuropathy. Intrathecal application of two DPP4 inhibitors tripeptide isoleucin-prolin-isoleucin (IPI) and the antidiabetic drug vildagliptin resulted in robust opioid-dependent antihyperalgesic effect during inflammation, and milder but significant opioid-independent antihyperalgesic action in the neuropathic model. The opioid-mediated antihyperalgesic effect of IPI was exclusively related to mu-opioid receptors, while vildagliptin affected mainly delta-receptor activity, although mu- and kappa-receptors were also involved. None of the inhibitors influenced allodynia. Our results suggest pathology and glia-type specific changes of DPP4 activity in the spinal cord, which contribute to the development and maintenance of hyperalgesia and interact with endogenous opioid systems.

[Mechanism of spinal pain transmission and its regulation]

Morphine has been widely used for the treatment of acute, chronic, and cancer pain and is considered the strongest analgesic in clinical care. Conversely, morphine-induced analgesia may be accompanied by several side effects. Animal studies have demonstrated that low doses of morphine administered intrathecally can produce reliable analgesia for thermal, mechanical, and chemical nociceptive stimulation. On the other hand, high doses of morphine administered intrathecally may induce spontaneous nociceptive responses such as scratching, biting, and licking in mice as well as agitation and vocalization in rats. In addition, similar nociceptive responses including hyperalgesia, allodynia, and myoclonus have been observed in humans following intrathecal or systemic administration of high-dose morphine. It has been suggested that the spontaneous nociceptive behaviors evoked by high-dose morphine may be mediated by a non-opioid mechanism that is not yet fully understood. This review describes the mechanisms of spontaneous nociceptive behaviors evoked by high-dose morphine focusing on the neurotransmitters/neuromodulators released from primary afferent fibers.

Small constrained SP1-7 analogs bind to a unique site and promote anti-allodynic effects following systemic injection in mice

Previous results have shown that the substance P (SP) N-terminal fragment SP1-7 may attenuate hyperalgesia and produce anti-allodynia in animals using various experimental models for neuropathic pain. The heptapeptide was found to induce its effects through binding to and activating specific sites apart from any known neurokinin or opioid receptor. Furthermore, we have applied a medicinal chemistry program to develop lead compounds mimicking the effect of SP1-7. The present study was designed to evaluate the pharmacological effect of these compounds using the mouse spared nerve injury (SNI) model of chronic neuropathic pain. Also, as no comprehensive screen with the aim to identify the SP1-7 target has yet been performed we screened our lead compound H-Phe-Phe-NH2 toward a panel of drug targets. The extensive target screen, including 111 targets, did not reveal any hit for the binding site among a number of known receptors or enzymes involved in pain modulation. Our animal studies confirmed that SP1-7, but also synthetic analogs thereof, possesses anti-allodynic effects in the mouse SNI model of neuropathic pain. One of the lead compounds, a constrained H-Phe-Phe-NH2 analog, was shown to exhibit a significant anti-allodynic effect.

Targeted liquid chromatography quadrupole ion trap mass spectrometry analysis of tachykinin related peptides reveals significant expression differences in a rat model of neuropathic pain

Animal models are widely used to perform basic scientific research in pain. The rodent chronic constriction injury (CCI) model is widely used to study neuropathic pain. Animals were tested prior and after CCI surgery using behavioral tests (von Frey filaments and Hargreaves test) to evaluate pain. The brain and the lumbar enlargement of the spinal cord were collected from neuropathic and normal animals. Tachykinin related peptides were analyzed by high performance liquid chromatography quadrupole ion trap mass spectrometry. Our results reveal that the β-tachykinin₅₈₋₇₁, SP and SP₃₋₁₁ up-regulation are closely related to pain behavior. The spinal β-tachykinin₅₈₋₇₁, SP and SP₃₋₁₁ concentrations were significantly up-regulated in neuropathic animals compared with normal animals (p<0.001; p<0.001 and p<0.05, respectively). In contrast, the spinal SP5₅₋₁₁ concentration in neuropathic animals revealed a significant down-regulation compared with normal animals (p<0.05). The brain β-tachykinin₅₈₋₇₁ and SP concentrations were significantly up-regulated (p<0.05 and p<0.001, respectively). Interestingly, no significant concentration differences were observed in the spinal cord and brain for NKA, β-tachykinin₅₈₋₇₁, SP₁₋₇ and SP₆₋₁₁ (p>0.05). The β-tachykinin₅₈₋₇₁, SP and C-terminal SP metabolites could potentially serve as biomarkers in early drug discovery.

Intrathecal substance P augments morphine-induced antinociception: possible relevance in the production of substance P N-terminal fragments

The present study sought to examine the mechanism of substance P to modulate the antinociceptive action of intrathecal (i.t.) morphine in paw-licking/biting response evoked by subcutaneous injection of capsaicin into the plantar surface of the hindpaw in mice. The i.t. injection of morphine inhibited capsaicin-induced licking/biting response in a dose-dependent manner. Substance P (25 and 50 pmol) injected i.t. alone did not alter capsaicin-induced nociception, whereas substance P at a higher dose of 100 pmol significantly reduced the capsaicin response. Western blots showed the constitutive expression of endopeptidase-24.11 in the dorsal and ventral parts of lumbar spinal cord of mice. The N-terminal fragment of substance P (1-7), which is known as a major product of substance P by endopeptidase-24.11, was more effective than substance P on capsaicin-induced nociception. Combination treatment with substance P (50 pmol) and morphine at a subthreshold dose enhanced the antinociceptive effect of morphine. The enhanced effect of the combination of substance P with morphine was reduced significantly by co-administration of phosphoramidon, an inhibitor of endopeptidase-24.11. Administration of D-isomer of substance P (1-7), [D-Pro(2), D-Phe(7)]substance P (1-7), an inhibitor of [(3)H] substance P (1-7) binding, or antisera against substance P (1-7) reversed the enhanced antinociceptive effect by co-administration of substance P and morphine. Taken together these data suggest that morphine-induced antinociception may be enhanced through substance P (1-7) formed by the enzymatic degradation of i.t. injected substance P in the spinal cord.

Intrathecal substance P (1–7) prevents morphine-evoked spontaneous pain behavior via spinal NMDA-NO cascade

Previous research has shown that injection of high-dose of morphine into the spinal lumbar intrathecal (i.t.) space of rats elicits an excitatory behavioral syndrome indicative of severe vocalization and agitation. Substance P N-terminal fragments are known to inhibit nociceptive responses when injected i.t. into animals. In this study, we investigated the effect of i.t. substance P (1-7) on both the nociceptive response and the extracellular concentrations of glutamate and nitric oxide (NO) metabolites (nitrite/nitrate) evoked by high-dose i.t. morphine (500 nmol). The induced behavioral responses were attenuated dose-dependently by i.t. pretreatment with the substance P N-terminal fragment substance P (1-7) (100-400 pmol). The inhibitory effect of substance P (1-7) was reversed significantly by pretreatment with [d-Pro2, d-Phe7]substance P (1-7) (20 and 40 nmol), a d-isomer and antagonist of substance P (1-7). In vivo microdialysis analysis showed a significant elevation of extracellular glutamate and NO metabolites in the spinal cord after i.t. injection of high-dose morphine (500 nmol). Pretreatment with substance P (1-7) (400 pmol) produced a significant reduction on the elevated concentrations of glutamate and NO metabolites evoked by i.t. morphine. The reduced levels of glutamate and NO metabolites were significantly reversed by the substance P (1-7) antagonist (40 nmol). The present results suggest that i.t. substance P (1-7) may attenuate the excitatory behavior (vocalization and agitation) of high-dose i.t. morphine by inhibiting the presynaptic release of glutamate, and reducing NO production in the dorsal spinal cord.

Characterization of the gene structures, precursor processing and pharmacology of the endokinin peptides

The endokinins represent several species-divergent and peripherally located mammalian tachykinins (hemokinin-1 in mouse and rat, endokinin-1 in rabbit and endokinins A and B in humans) and also the tachykinin gene-related peptides. These peptides are all encoded on the preprotachykinin 4 (TAC4) gene. Their complementary DNA sequences, gene structures and expression profiles have been determined from a number of different mammalian species. They are all flanked by adjacent upstream and downstream dibasic cleavage sites in their respective precursor proteins, except for human EKA/B that instead possesses a N-terminal monobasic cleavage site. Evidence for differential processing in the periphery at the N-terminal cleavage site of the tachykinins could explain why in humans the evolutionary pressure to maintain the N-terminal dibasic cleavage site of EKA/B has been lost. Furthermore, the TAC4 encoded tachykinins all exhibit a remarkable selectivity and potency for the highly species conserved tachykinin NK(1) receptor, similar to that of substance P. Particular consideration is also given to the potential interactions of the endokinins with the short NK(1) receptor isoform and to speculation of whether there could be an "endokinin-sensitive" NK(1) binding site.

New challenges in the study of the mammalian tachykinins

There is an expanding repertoire of mammalian tachykinins produced by a variety of tachykinin genes, gene splicing events and peptide processing. Novel tachykinin-binding molecules/receptors are proposed, but only, three tachykinin receptors are identified with certainty. The question remains - do more tachykinin receptors exist or is there just the need to reappraise our understanding of the known receptors? The tachykinin NK1 receptor, the preferred receptor for both substance P and the peripheral SP-like endokinins, exists in several tissue-specific conformations and isoforms and may provide some clues. This review addresses recent advances in this exciting field and raises challenging new concepts.

Further evidence on the anxiogenic-like effect of substance P evaluated in the elevated plus-maze in rats

Substance P (SP) and its preferred NK1 receptor are widely expressed throughout the fear-processing pathways of the brain and its role in the modulation of experimental anxiety has been demonstrated. SP, like other peptides, are cleaved by peptidases in two fragments: C-terminal (SP 6-11) and N-terminal (SP 1-7) that could be responsible for its anxiogenic-like response. In this study we investigate the effects of i.c.v. micro-injections of SP free acid (SPfa), which is resistant to enzymatic cleavage, the influence of the pretreatment with peptidase inhibitors (PIs), thiorphan and/or phosphoramidon, as well as the effects of SP 6-11 and SP 1-7 and the participation of NK1 and NK2 receptors on their behavioral effects. Adult male Wistar rats were treated with 10 pmol solutions of SP 6-11, SP 1-7 or 1 and 10 pmol of SPfa and evaluated in the elevated plus maze (EPM) test. Other experimental groups received thiorphan 0.2 pmol, phosphoramidon 2 pmol or both PIs 30 min prior SP 1-11, 10 pmol i.c.v. The C-terminal fragment (SP 6-11, 10 pmol) and SPfa (1 pmol) promoted an anxiogenic-like profile of action similar to 10 pmol of SP 1-11, i.e., a decrease of entries and time spent on the open arms, whereas the N-terminal fragment (SP 1-7) was inactive at the EPM. The effect of SP 6-11 was inhibited by pretreatment (100 pmol) with NK1 (FK 888) and NK2 (SR 48968) antagonists. Moreover, both PIs enhanced the SP effect when used alone, but their combination produced an apparent reversion of anxiogenic-like effect produced by SP. Altogether, our results give further support to the SP role in the modulation of experimental anxiety in rats.

Localization of the mRNA encoding prolyl endopeptidase in the rat brain and pituitary

Prolyl endopeptidase (EC 3.4.21.26, PEP), a serine protease that hydrolyzes peptides at the carboxyl side of proline residues, is involved in the breakdown of several proline-containing neuropeptides and, thus, may contribute to the regulation of behavioral activities. In this study, the distribution of PEP mRNA was investigated in the central nervous system and pituitary of rat by means of quantitative reverse transcriptase-polymerase chain reaction analysis and in situ hybridization histochemistry. High densities of PEP transcripts were found in cerebellar Purkinje and granule cells, within most hypothalamic nuclei, in pyramidal neurons of the Ammon's horn, in granule cells of the dentate gyrus, and within the basolateral complex of the amygdala. Moderate levels of PEP mRNA were observed in layers 3-5 of the cerebral cortex, the anterior thalamic group, the septal region, the substantia nigra, the magnocellular neurons of the red nucleus, and the motor nuclei of the cranial nerves. Low concentrations of PEP mRNA were detected in the deep mesencephalic nuclei, the reticular formation, the pretectum, and the tectum. A high density of PEP mRNA was found in the intermediate and the anterior lobes of the pituitary, while the neural lobe was devoid of labeling. In several brain regions, the distribution pattern of PEP mRNA overlapped that of various neuropeptide receptors, suggesting that PEP is actually involved in the inactivation of regulatory neuropeptides.

The role of spinal nitric oxide and glutamate in nociceptive behaviour evoked by high-dose intrathecal morphine in rats

Injection of high-dose of morphine into the spinal lumbar intrathecal (i.t.) space of rats elicits a nociceptive behavioural syndrome characterized by periodic bouts of spontaneous agitation and severe vocalization. The induced behavioural response such as vocalization and agitation was observed dose-dependently by i.t. administration of morphine (125-500 nmol). Pretreatment with naloxone (s.c. and i.t.), an opioid receptor antagonist, failed to reverse the morphine-induced behavioural response. The excitatory effect of morphine was inhibited dose-dependently by pretreatment with 3-((+)2-carboxy-piperazin-4-yl)-propyl-1-phosphonic acid (CPP), a competitive N-methyl-D-aspartate (NMDA) receptor antagonist and MK-801, a non-competitive NMDA receptor antagonist. The non-selective nitric oxide (NO) synthase inhibitor N(G)-nitro L-arginine methyl ester (L-NAME) inhibited dose-dependently the behavioural response to high-dose i.t. morphine (500 nmol), whereas D-NAME was without affecting the response to high-dose i.t. morphine. In the present study, we measured NO metabolites (nitrite/nitrate) in the extracellular fluid of rat dorsal spinal cord using in vivo microdialysis. The i.t. injection of morphine (500 nmol) evoked significant increases in NO metabolites and glutamate from the spinal cord. Not only NO metabolites but also glutamate released by high-dose morphine were reduced significantly by pretreatment with L-NAME (400 nmol). Pretreatment with CPP and MK-801 showed a significant reduction of the NO metabolites and glutamate levels elevated by high-dose i.t. morphine. These results suggest that the excitatory action of high-dose i.t. morphine may be mediated by an NMDA-NO cascade in the spinal cord.

Pro-nociceptive effects of neuromedin U in rat

The neuropeptide neuromedin U (NMU) has been shown to have significant effects on cardiovascular, gastrointestinal and CNS functions. The peptide was first isolated from the porcine spinal cord and later shown to be present in spinal cords of other species. Little is known about the distribution of neuromedin U receptors (NMURs) in the spinal cord and the spinal action of the peptide. Here we report on the expression of NMURs and a potential role in nociception in the rat spinal cord using a combination of behavioral and electrophysiological studies. Receptor autoradiography showed that NMU-23 binding was restricted to the superficial layers of spinal cord, a region known to be involved in the control of nociception. In situ hybridization analysis indicated the mRNA of NMUR2 was located in the same region (laminae I and IIo) as NMU-23 binding, while the mRNA for NMU receptor 1 was observed in a subpopulation of small diameter neurons of dorsal root ganglia. Intrathecal (i.t.) administration of neuromedin U-23 (0.4-4.0 nmol/10 microl) dose-dependently decreased both the mechanical threshold to von Frey hair stimulation and the withdrawal latency to a noxious thermal stimulus. Mechanical allodynia was observed between 10 and 120 min, peaking at 30 min and heat hyperalgesia was observed 10-30 min after i.t. administration of NMU-23. A similar mechanical allodynia was also observed following i.t. administration of NMU-8 (0.4-4 nmol/10 microl). A significant enhancement of the excitability of flexor reflex was induced by intrathecal administration of NMU-23 (4 nmol/10 microl). Evoked responses to touch and pinch stimuli were increased by 439+/-94% and 188+/-36% (P<0.01, n=6) respectively. The behavioral and electrophysiological data demonstrate, for the first time, a pro-nociceptive action of NMU. The restricted distribution of NMU receptors to a region of the spinal cord involved in nociception suggests that this peptide receptor system may play a role in nociception.

Effect of S 17092, a novel prolyl endopeptidase inhibitor, on substance P and alpha-melanocyte-stimulating hormone breakdown in the rat brain

In the present study, we have investigated the effects of a novel prolyl endopeptidase (EC 3.4.21.26, PEP) inhibitor, compound S 17092, on substance P (SP) and alpha-melanocyte-stimulating hormone (alpha-MSH) metabolism in the rat brain. In vitro experiments revealed that S 17092 inhibits in a dose-dependent manner PEP activity in rat cortical extracts (IC50 = 8.3 nm). In addition, S 17092 totally abolished the degradation of SP and alpha-MSH induced by bacterial PEP. In vivo, a significant decrease in PEP activity was observed in the medulla oblongata after a single oral administration of S 17092 at doses of 10 and 30 mg/kg (-78% and -82%, respectively) and after chronic oral treatment with S 17092 at doses of 10 and 30 mg/kg per day (-75% and -88%, respectively). Concurrently, a single administration of S 17092 (30 mg/kg) caused a significant increase in SP- and alpha-MSH-like immunoreactivity (LI) in the frontal cortex (+41% and +122%, respectively) and hypothalamus (+84% and +49%, respectively). In contrast, chronic treatment with S 17092 did not significantly modify SP- and alpha-MSH-LI in the frontal cortex and hypothalamus. Collectively, the present results show that S 17092 elevates SP and alpha-MSH concentrations in the rat brain by inhibiting PEP activity. These data suggest that the effect of S 17092 on memory impairment can be accounted for, at least in part, by inhibition of catabolism of promnesic neuropeptides such as SP and alpha-MSH.

Degradation of nociceptin (orphanin FQ) by mouse spinal cord synaptic membranes is triggered by endopeptidase-24.11: an in vitro and in vivo study

We analyzed spinal metabolic pathway of nociceptin/orphanin FQ related to pain-transmission or modulation in the both in vitro and in vivo experiments. Nociceptin was degraded by spinal synaptic membranes. Major metabolites of nociceptin were free phenylalanine, nociceptin (1-13) and nociceptin (14-17). Both the degradation of nociceptin and the accumulation of the major cleavage metabolites, nociceptin (1-13) and nociceptin (14-17), were strongly inhibited by a metal chelator and also by specific inhibitors of endopeptidase-24.11, thiorphan and phosphoramidon. Furthermore, purified endopeptidase-24.11 hydrolyzed nociceptin at the cleavage site (Lys(13)-Leu(14) bond) identical to that by spinal synaptic membranes. Recently, we have found that nociceptin, injected intrathecally at small doses (fmol order) elicits a behavioral response consisting of scratching, biting and licking in mice. In the present study, we have examined the effect of peptidase inhibitors on the behavioral response elicited by intrathecal injection of nociceptin in mice. Phosphoramidon simultaneously injected with nociceptin additively enhanced nociceptin-induced behavioral response, whereas the nociceptin-induced behavioral response was unaffected by either bestatin, an aminopeptidase inhibitor or captopril, an angiotensin-converting enzyme inhibitor. However, the nociceptin effect was potentiated by combined injection of phosphoramidon and bestatin, indicating that inhibition of aminopeptidase may also contribute to inducing the behavioral response to nociceptin. These data suggest that endopeptidase-24.11 plays a major role in initial stage of nociceptin metabolism at the spinal cord level in mice.

Loss of dipeptidylpeptidase IV activity in chronic rhinosinusitis contributes to the neurogenic inflammation induced by substance P in the nasal mucosa

In this study, we have found that dipeptidylpeptidase IV (DPPIV) plays in vivo an active role in the modulation of the inflammatory response of chronic rhinosinusitis. Human nasal mucosa expresses DPPIV-like immunoreactivity in submucosal seromucus glands, leukocytes, and endothelial cells of blood vessels. DPPIV enzymatic activity in nasal tissue biopsies taken from patients suffering from chronic rhinosinusitis was correlated inversely with the density of inflammatory cells in the nasal mucosa, and the DPPIV activity rose when chronic rhinosinusitis was treated. By using a pig animal model, we have shown that the intranasal administration of recombinant DPPIV decreased the vasodilatation induced by exogenous substance P (SP), a proinflammatory peptide released by sensory nerves. In contrast, an inhibitor of DPPIV enhanced the vasodilatatory effect at low doses of SP. SP5-11 was 100- to 1000-fold less potent than SP as a vasodilator of the nasal mucosa. The vasodilatatory effect of SP was abolished by a NK1 receptor antagonist. In conclusion, these results suggest a new pathophysiological pathway for rhinitis based on clinical observations in humans, indicating the involvement of an enzyme to modulate non-adrenergic and non-cholinergic substrate that occurred during nasal dysfunctions.

Investigation of substance P transport across the blood-brain barrier

The transport of substance P (SP) was investigated using the bovine brain microvessel endothelial cell culture model of the blood-brain barrier (BBB). The samples were derivatized precolumn with naphthalene dialdehyde, then analyzed by cyclodextrin-modified micellar electrokinetic chromatography with laser-induced fluorescence detection. SP crossed the BBB in both the apical-to-basolateral and basolateral-to-apical directions through an active transport mechanism. The transport of SP from the apical side was demonstrated to be via transcytosis. The N-terminal (SP(1-4)) and C-terminal (SP(3-11)) fragments were also found to permeate the BBB from the apical side.

Investigation of the metabolism of substance P at the blood-brain barrier using capillary electrophoresis with laser-induced fluorescence detection

Substance P (SP) metabolism was investigated upon exposure to a monolayer of bovine brain microvessel endothelial cells (BBMECs), a cell culture model of the blood-brain barrier. SP was incubated with the BBMECs and its metabolism was followed as a function of time over a 5-h period. The resulting samples were derivatized with naphthalene-2,3-dicarboxaldehyde (NDA)/cyanide, separated, and detected using cyclodextrin-modified electrokinetic chromatography with laser-induced fluorescence detection (CDMEKC-LIF). Upon exposure to the BBMEC monolayer, SP rapidly degraded to produce the N-terminal (1-9), (1-4) and (1-7) and C-terminal (2-11) and (3-11) fragments. These results were compared with those in an earlier report from our laboratory, where SP metabolism was investigated in vivo by microdialysis sampling in rat striatum.

Investigation of the metabolism of substance P in rat striatum by microdialysis sampling and capillary electrophoresis with laser-induced fluorescence detection

The metabolism of substance P (SP) was investigated in rat striatum using in vivo microdialysis. Substance P was perfused for 5 h at 0.2 microl/min, and its metabolism was followed for over 13 h. The resulting samples were derivatized precolumn with naphthalene-2,3-dicarboxaldehyde (NDA)/cyanide, separated and detected by cyclodextrin-modified electrokinetic chromatography with laser-induced fluorescence detection (CDMEKC-LIF). Substance P rapidly degraded to form the fragments (3-11), (1-9), (1-4) and, to a lesser extent, (1-7). The metabolites reached steady-state levels 2-3 h after addition of SP.