Cloning of a C-terminally truncated NK-1 receptor from guinea-pig nervous system

Two C-terminal isoforms of Aplysia tachykinin-related peptide receptors exhibit phosphorylation-dependent and phosphorylation-independent desensitization mechanisms

Diversity, a hallmark of G protein-coupled receptor (GPCR) signaling, partly stems from alternative splicing of a single gene generating more than one isoform for a receptor. Additionally, receptor responses to ligands can be attenuated by desensitization upon prolonged or repeated ligand exposure. Both phenomena have been demonstrated and exemplified by the deuterostome tachykinin signaling system, although the role of phosphorylation in desensitization remains a subject of debate. Here, we describe the signaling system for tachykinin-related peptides (TKRPs) in a protostome, mollusk Aplysia. We cloned the Aplysia TKRP precursor, which encodes three TKRPs (apTKRP-1, apTKRP-2a, and apTKRP-2b) containing the FXGXR-amide motif. In situ hybridization and immunohistochemistry showed predominant expression of TKRP mRNA and peptide in the cerebral ganglia. TKRPs and their posttranslational modifications were observed in extracts of central nervous system ganglia using mass spectrometry. We identified two Aplysia TKRP receptors (apTKRPRs), named apTKRPR-A and apTKRPR-B. These receptors are two isoforms generated through alternative splicing of the same gene and differ only in their intracellular C termini. Structure-activity relationship analysis of apTKRP-2b revealed that both C-terminal amidation and conserved residues of the ligand are critical for receptor activation. C-terminal truncates and mutants of apTKRPRs suggested that there is a C-terminal phosphorylation-independent desensitization for both receptors. Moreover, apTKRPR-B also exhibits phosphorylation-dependent desensitization through the phosphorylation of C-terminal Ser/Thr residues. This comprehensive characterization of the Aplysia TKRP signaling system underscores the evolutionary conservation of the TKRP and TK signaling systems, while highlighting the intricacies of receptor regulation through alternative splicing and differential desensitization mechanisms.

Association of Neurokinin-1 Receptor Signaling Pathways with Cancer

BACKGROUND

Numerous biochemical reactions leading to altered cell proliferation cause tumorigenesis and cancer treatment resistance. The mechanisms implicated include genetic and epigenetic changes, modified intracellular signaling, and failure of control mechanisms caused by intrinsic and extrinsic factors alone or combined. No unique biochemical events are responsible; entangled molecular reactions conduct the resident cells in a tissue to display uncontrolled growth and abnormal migration. Copious experimental research supports the etiological responsibility of NK-1R (neurokinin-1 receptor) activation, alone or cooperating with other mechanisms, in cancer appearance in different tissues. Consequently, a profound study of this receptor system in the context of malignant processes is essential to design new treatments targeting NK-1R-deviated activity.

METHODS

This study reviews and discusses recent literature that analyzes the main signaling pathways influenced by the activation of neurokinin 1 full and truncated receptor variants. Also, the involvement of NK-1R in cancer development is discussed.

CONCLUSION

NK-1R can signal through numerous pathways and cross-talk with other receptor systems. The participation of override or malfunctioning NK-1R in malignant processes needs a more precise definition in different types of cancers to apply satisfactory and effective treatments. A long way has already been traveled: the current disposal of selective and effective NK-1R antagonists and the capacity to develop new drugs with biased agonistic properties based on the receptor's structural states with functional significance opens immediate research action and clinical application.

Ion Channels Involved in Substance P-Mediated Nociception and Antinociception

Substance P (SP), an 11-amino-acid neuropeptide, has long been considered an effector of pain. However, accumulating studies have proposed a paradoxical role of SP in anti-nociception. Here, we review studies of SP-mediated nociception and anti-nociception in terms of peptide features, SP-modulated ion channels, and differential effector systems underlying neurokinin 1 receptors (NK1Rs) in differential cell types to elucidate the effect of SP and further our understanding of SP in anti-nociception. Most importantly, understanding the anti-nociceptive SP-NK1R pathway would provide new insights for analgesic drug development.

Truncation of neurokinin-1 receptor-Negative regulation of substance P signaling

Substance P (SP) is a tachykinin peptide, which triggers intracellular signaling in the nervous and immune systems, as well as, other local and systemic events. The interaction between SP and its receptor, neurokinin-1 receptor (NK1R), results in major downstream cellular actions, which include changes in calcium fluxes, ERK, and p21-activated kinase phosphorylation and NFκB activation. Two naturally occurring variants of the NK1R, the full-length, 407 aa receptor (NK1R-F) and the truncated, 311 aa isoform (NK1R-T), mediate the actions of SP. Receptor truncation partially disrupts signaling motifs of the carboxyl tail, a critical site for mediating NK1R signaling, resulting in a "less-efficient" receptor. Although NK1R-F is the predominant isoform in the central and peripheral nervous systems, NK1R-T is expressed in several tissues and cells, which include monocytes, NK cells, and T-cells. The SP binding domain is not affected by truncation and this site is identical in both NK1R receptor isoforms. However, while cells expressing NK1R-F respond to nanomolar concentrations of SP, monocyte and macrophage activation, mediated through NK1R-T, requires micromolar concentrations of SP in order to elicit signaling responses. Elevated plasma levels of SP are associated with increased inflammatory responses and NK1R antagonists reduce inflammation and cytokine production in vivo. This mini review presents and discusses the novel hypothesis that the expression of NK1R-T on immune system cells prevents immune activation in a milieu, which usually contains low concentrations of SP and, thus, maintains immune homeostasis. In contrast, in the activated neuronal microenvironment, when SP levels reach the threshold at tissue sites, SP promotes immune activation and modulates monocyte/macrophage polarization.

Role of Substance P Neuropeptide in Inflammation, Wound Healing, and Tissue Homeostasis

Substance P (SP) is an undecapeptide present in the CNS and the peripheral nervous system. SP released from the peripheral nerves exerts its biological and immunological activity via high-affinity neurokinin 1 receptor (NK1R). SP is also produced by immune cells and acts as an autocrine or paracrine fashion to regulate the function of immune cells. In addition to its proinflammatory role, SP and its metabolites in combination with insulin-like growth factor-1 are shown to promote the corneal epithelial wound healing. Recently, we showed an altered ocular surface homeostasis in unmanipulated NK1R^-/- mice, suggesting the role of SP-NK1R signaling in ocular surface homeostasis under steady-state. This review summarizes the immunobiology of SP and its effect on immune cells and immunity to microbial infection. In addition, the effect of SP in inflammation, wound healing, and corneal epithelial homeostasis in the eye is discussed.

The Therapeutic Potential of Targeting Substance P/NK-1R Interactions in Inflammatory CNS Disorders

The inflammatory responses of resident central nervous system (CNS) cells are now known to play a critical role in the initiation and progression of an array of infectious and sterile neuroinflammatory disorders such as meningitis, encephalitis, Parkinson's disease, Alzheimer's disease and multiple sclerosis (MS). Regulating glial inflammatory responses in a timely manner is therefore critical in preserving normal CNS functions. The neuropeptide substance P is produced at high levels within the CNS and its selective receptor, the neurokinin 1 receptor (NK-1R), is abundantly expressed by neurons and is present on glial cell types including microglia and astrocytes. In addition to its functions as a neurotransmitter in the perception of pain and its essential role in gut motility, this tachykinin is widely recognized to exacerbate inflammation at peripheral sites including the skin, gastrointestinal tract and the lungs. Recently, a number of studies have identified a role for substance P and NK-1R interactions in neuroinflammation and described the ability of this neuropeptide to alter the immune functions of activated microglia and astrocytes. In this review article, we describe the expression of substance P and its receptor by resident CNS cells, and we discuss the ability of this neuropeptide to exacerbate the inflammatory responses of glia and immune cells that are recruited to the brain during neurodegenerative diseases. In addition, we discuss the available data indicating that the NK-1R-mediated augmentation of such responses appears to be detrimental during microbial infection and some sterile neurodegenerative disorders, and propose the repurposed use of NK-1R antagonists, of a type that are currently approved as anti-emetic and anti-anxiolytic agents, as an adjunct therapy to ameliorate the inflammatory CNS damage in these conditions.

The expression of tachykinin receptors in the human lower esophageal sphincter

Mammalian tachykinins are a family of neuropeptides which are potent modulators of smooth muscle function with a significant contractile effect on human smooth muscle preparations. Tachykinins act via three distinct G protein-coupled neurokinin (NK) receptors, NK1, NK2 and NK3, coded by the genes TACR1, TACR2 and TACR3 respectively. The purpose of this paper was to measure the mRNA and protein expression of these receptors and their isoforms in the clasp and sling fibers of the human lower esophageal sphincter complex and circular muscle from the adjacent distal esophagus and proximal stomach. We found differences in expression between the different receptors within these muscle types, but the rank order of the receptor expression did not differ between the different muscle types. The rank order of the mRNA expression was TACR2 (α isoform)>TACR2 (β isoform)>TACR1 (short isoform)>TACR1 (long isoform)>TACR3. The rank order of the protein expression was NK2>NK1>NK3. This is the first report of the measurement of the transcript and protein expression of the tachykinin receptors and their isoforms in the muscles of the human lower esophageal sphincter complex. The results provide evidence that the tachykinin receptors could contribute to the regulation of the human lower esophageal sphincter, particularly the TACR2 α isoform which encodes the functional isoform of the tachykinin NK2 receptor was the most highly expressed of the tachykinin receptors in the muscles associated with the lower esophageal sphincter.

Primary afferent neurons containing calcitonin gene-related peptide but not substance P in forepaw skin, dorsal root ganglia, and spinal cord of mice

In mice dorsal root ganglia (DRG), some neurons express calcitonin gene-related peptide (CGRP) without substance P (SP; CGRP(+) SP(-) ). The projections and functions of these neurons are unknown. Therefore, we combined in vitro axonal tracing with multiple-labeling immunohistochemistry to neurochemically define these neurons and characterize their peripheral and central projections. Cervical spinal cord, DRG, and forepaw skin were removed from C57Bl/6 mice and multiple-labeled for CGRP, SP, and either marker for the sensory neuron subpopulations transient receptor potential vanilloid type 1 (TRPV1), neurofilament 200 (NF200), or vesicular glutamate transporter 2 (VGluT1). To determine central projections of CGRP(+) SP(-) neurons, Neurobiotin (NB) was applied to the C7 ventral ramus and visualized in DRG and spinal cord sections colabeled for CGRP and SP. Half (50%) of the CGRP-immunoreactive DRG neurons lacked detectable SP and had a mean soma size of 473 ± 14 μm(2) (n = 5); 89% of the CGRP(+) SP(-) neurons expressed NF200 (n = 5), but only 32% expressed TRPV1 (n = 5). Cutaneous CGRP(+) SP(-) fibers were numerous within dermal papillae and around hair shafts (n = 4). CGRP(+) SP(-) boutons were prevalent in lateral lamina I and in lamina IV/V of the dorsal horn (n = 5). NB predominantly labeled fibers penetrating lamina IV/V, 6 ± 3% contained CGRP (n = 5), and 21 ± 2% contained VGluT1 (n = 3). CGRP(+) SP(-) afferent neurons are likely to be non-nociceptive. Their soma size, neurochemical profile, and peripheral and central targets suggest that CGRP(+) SP(-) neurons are polymodal mechanoceptors.

Tachykinins and their receptors: contributions to physiological control and the mechanisms of disease

The tachykinins, exemplified by substance P, are one of the most intensively studied neuropeptide families. They comprise a series of structurally related peptides that derive from alternate processing of three Tac genes and are expressed throughout the nervous and immune systems. Tachykinins interact with three neurokinin G protein-coupled receptors. The signaling, trafficking, and regulation of neurokinin receptors have also been topics of intense study. Tachykinins participate in important physiological processes in the nervous, immune, gastrointestinal, respiratory, urogenital, and dermal systems, including inflammation, nociception, smooth muscle contractility, epithelial secretion, and proliferation. They contribute to multiple diseases processes, including acute and chronic inflammation and pain, fibrosis, affective and addictive disorders, functional disorders of the intestine and urinary bladder, infection, and cancer. Neurokinin receptor antagonists are selective, potent, and show efficacy in models of disease. In clinical trials there is a singular success: neurokinin 1 receptor antagonists to treat nausea and vomiting. New information about the involvement of tachykinins in infection, fibrosis, and pruritus justifies further trials. A deeper understanding of disease mechanisms is required for the development of more predictive experimental models, and for the design and interpretation of clinical trials. Knowledge of neurokinin receptor structure, and the development of targeting strategies to disrupt disease-relevant subcellular signaling of neurokinin receptors, may refine the next generation of neurokinin receptor antagonists.

Functional organization of autonomic neural pathways

There is now abundant functional and anatomical evidence that autonomic motor pathways represent a highly organized output of the central nervous system. Simplistic notions of antagonistic all-or-none activation of sympathetic or parasympathetic pathways are clearly wrong. Sympathetic or parasympathetic pathways to specific target tissues generally can be activated tonically or phasically, depending on current physiological requirements. For example, at rest, many sympathetic pathways are tonically active, such as those limiting blood flow to the skin, inhibiting gastrointestinal tract motility and secretion, or allowing continence in the urinary bladder. Phasic parasympathetic activity can be seen in lacrimation, salivation or urination. Activity in autonomic motor pathways can be modulated by diverse sensory inputs, including the visual, auditory and vestibular systems, in addition to various functional populations of visceral afferents. Identifying the central pathways responsible for coordinated autonomic activity has made considerable progress, but much more needs to be done.

Visceral hypersensitivity in symptomatic diverticular disease and the role of neuropeptides and low grade inflammation

BACKGROUND

  Recurrent abdominal pain is reported by a third of patients with diverticulosis, particularly those with previous episodes of acute diverticulitis. The current understanding of the etiology of this pain is poor. Our aim was to assess visceral sensitivity in patients with diverticular disease and its association with markers of previous inflammation and neuropeptides.

METHODS

  Patients with asymptomatic and symptomatic diverticular disease underwent a flexible sigmoidoscopy and biopsy followed 5-10 days later by visceral sensitivity testing with barostat-mediated rectal distension. Inflammation was assessed by staining of serotonin (5HT) and CD3 positive cells. mRNA levels of tumor necrosis factor alpha (TNF α) and interleukin-6 (IL-6) were quantitated using RT-PCR. Neuropeptide expression was assessed from percentage area staining with substance P (SP) and mRNA levels of the neurokinin 1 & 2 receptors (NK1 & NK2), and galanin 1 receptor (GALR1).

KEY RESULTS

  Thirteen asymptomatic and 12 symptomatic patients were recruited. The symptomatic patients had a lower first reported threshold to pain (28.4 mmHg i.q.r 25.0-36.0) than the asymptomatic patients (47 mmHg i.q.r 36.0-52.5, P < 0.001). Symptomatic patients had a higher median overall pain rating for the stimuli than the asymptomatic patients (P < 0.02). Symptomatic patients had greater median relative expression of NK1 and TNF alpha mRNA compared with asymptomatic patients. There was a significant correlation between barostat VAS pain scores and NK 1 expression (Figure 4, r(2) 0.54, P < 0.02).

CONCLUSIONS & INFERENCES

  Patients with symptomatic diverticular disease exhibit visceral hypersensitivity, and this may be mediated by ongoing low grade inflammation and upregulation of tachykinins.

Cellular localization of receptors using antibodies visualized by light and dual labeling confocal microscopy

Immunocytochemistry can be used to visualize the binding of specific site-directed antisera to receptors in tissue sections and permits the precise identification of cell types expressing a particular receptor when viewed using a conventional light microscope or by confocal microscopy. Protocols are also described for the dual labeling of cells in the same section using primary antisera raised in two different species (one to the receptor of interest, the second to an immunogen such as a cell-specific marker or the endogenous ligand) with the corresponding secondary antisera conjugated to different fluorescent dyes.The technique has a range of applications. Subtypes of receptors can be identified and distinguished prior to the development of selective agonists or antagonists, which is particularly important for mapping orphan receptors, where the identity of the endogenous ligand in not yet known. The deletion of genes encoding receptors, particularly in mice, has emerged as a powerful tool in understanding the role of a specific receptor in physiological processes. Receptor immunocytochemistry can be used to analyze the resulting phenotype in whole body sections of mice without preselection of the tissue to be studied.

Implication of Substance P in myocardial contractile function during ischemia in rats

Evidence suggests that substance P (SP) participates in the pathology of acute myocardial ischemia and infarction but the profiles of the peptide in regulation of cardiac functions are still elusive. The aim of this study was to investigate the role of substance P in regulation of cardiac functions and its association with adrenergic mechanism in acute myocardial ischemia and infarction with rodent models. The experiments were carried out in Sprague-Dawley rats. SP and norepinephrine were significantly up-regulated in myocardium at 15min, 30min and 60min of coronary artery occlusion. Pretreatment of the rats with a specific antagonist of neurokinin-1 receptor, D-SP, significant increased+dp/dt and decreased -dp/dt, compared with the controls, pretreated with 0.9% saline. Pretreatment of the isolated CAO hearts with substance P (10(-7)mol/L) significantly increased left ventricular end diastolic pressure. SP producing no effects on cardiac functions when given alone to isolated (non-CAO) heart caused significant attenuation of the changes in the contractility and diastolic functions induced by norepinephrine, when given with norepinephrine. SP attenuated the increase in the activity of PKA provoked by norepinephrine in cultured myocytes. In conclusion, the findings may indicate SP regulates cardiac functions via modulation of adrenergic activity, through suppression of over-activation of PKA.

Neurokinin-1 receptor: functional significance in the immune system in reference to selected infections and inflammation

The G protein-coupled receptor (GPCR), neurokinin-1 receptor (NK1R), and its preferred ligand, substance P (SP), are reviewed in relationship to the immune system and selected infections. NK1R and SP are ubiquitous throughout the animal kingdom. This important pathway has unique functions in numerous cells and tissues. The interaction of SP with its preferred receptor, NK1R, leads to the activation of nuclear factor-kappa B (NF-κB) and proinflammatory cytokines. NK1R has two isoforms, both a full-length and a truncated form. These isoforms have different functional significances and differ in cell signaling capability. The proinflammatory signals modulated by SP are important in bacterial, viral, fungal, and parasitic diseases, as well as in immune system function. The SP-NK1R system is a major class 1, rhodopsin-like GPCR ligand-receptor interaction.

Substance P (SP) enhances CCL5-induced chemotaxis and intracellular signaling in human monocytes, which express the truncated neurokinin-1 receptor (NK1R)

Substance P (SP) is a potent modulator of monocyte/macrophage function. The SP-preferring receptor neurokinin-1 receptor (NK1R) has two forms: a full-length NK1R (NK1R-F) isoform and a truncated NK1R (NK1R-T) isoform, which lacks the terminal cytoplasmic 96-aa residues. The distribution of these receptor isoforms in human monocytes is not known. We previously identified an interaction among SP, NK1R, and HIV viral strains that use the chemokine receptor CCR5 as a coreceptor, suggesting crosstalk between NK1R and CCR5. The purpose of this study was to determine which form(s) of NK1R are expressed in human peripheral blood monocytes and to determine whether SP affects proinflammatory cellular responses mediated through the CCR5 receptor. Human peripheral blood monocytes were found to express NK1R-T but not NK1R-F. SP interactions with NK1R-T did not mobilize calcium (Ca2+), but SP mobilized Ca2+ when the NK1R-F was transfected into monocytes. However, the NK1R-T was functional in monocytes, as SP enhanced the CCR5 ligand CCL5-elicited Ca2+ mobilization, a response inhibited by the NK1R antagonist aprepitant. SP interactions with the NK1R-T also enhanced CCL5-mediated chemotaxis, which was ERK1/2-dependent. NK1R-T selectively activated ERK2 but increased ERK1 and ERK2 activation by CCL5. Activation of NK1R-T elicited serine phosphorylation of CCR5, indicating that crosstalk between CCL5 and SP may occur at the level of the receptor. Thus, NK1R-T is functional in human monocytes and activates select signaling pathways, and the NK1R-T-mediated enhancement of CCL5 responses does not require the NK1R terminal cytoplasmic domain.

Differences in the length of the carboxyl terminus mediate functional properties of neurokinin-1 receptor

The neurokinin-1 receptor (NK1R) has two naturally occurring forms that differ in the length of the carboxyl terminus: a full-length receptor consisting of 407 aa and a truncated receptor consisting of 311 aa. We examined whether there are differential signaling properties attributable to the carboxyl terminus of this receptor by using stably transfected human embryonic kidney (HEK293) cell lines that express either full-length or truncated NK1R. Substance P (SP) specifically triggered intracellular calcium increase in HEK293 cells expressing full-length NK1R but had no effect in the cells expressing the truncated NK1R. In addition, in cells expressing full-length NK1R, SP activated NF-kappaB and IL-8 mRNA expression, but in cells expressing the truncated NK1R, SP did not activate NF-kappaB, and it decreased IL-8 mRNA expression. In cells expressing full-length NK1R, SP stimulated phosphorylation of PKCdelta but inhibited phosphorylation of PKCdelta in cells expressing truncated NK1R. There are also differences in the timing of SP-induced ERK activation in cells expressing the two different forms of the receptor. Full-length NK1R activation of ERK was rapid (peak within 1-2 min), whereas truncated NK1R-mediated activation was slower (peak at 20-30 min). Thus, the carboxyl terminus of NK1R is the structural basis for differences in the functional properties of the full-length and truncated NK1R. These differences may provide important information toward the design of new NK1R receptor antagonists.

Substance P depolarizes striatal projection neurons and facilitates their glutamatergic inputs

The striatum is the main basal ganglia input nucleus, receiving extensive glutamatergic inputs from cortex and thalamus. Medium spiny striatal projection neurons (MSNs) are GABAergic, and their axon collaterals synapse on other MSNs. Approximately 50% of MSNs corelease substance P (SP), but how this neurotransmitter controls MSN activity is poorly understood. We used whole-cell recordings to investigate how SP affects MSNs and their glutamatergic inputs. SP elicited slow depolarizations in 47/90 MSNs, which persisted in the presence of tetrodotoxin (TTX). SP responses were mimicked by the NK1 receptor agonist [Sar9,Met(O(2))11]-substance P (SSP), and fully blocked by the NK1 receptor antagonists L-732,138, or extracellular zinc. When intracellular chloride was altered, the polarity of SP responses depended on the sign of the chloride driving force. In voltage-clamp, SP-induced currents reversed around -68 mV and displayed marked inward rectification. These data indicate that SP increased a ClC-2-type chloride conductance in MSNs, acting through NK1 receptors. SP also strongly increased glutamatergic responses in 49/89 MSNs. Facilitation of glutamatergic responses (which was observed both in MSNs directly affected by SP and in non-affected ones) was reduced by application of L-732,138, and fully blocked by coapplication of L-732,138 and SB222200 (an NK3 receptor antagonists), showing that both NK1 and NK3 receptors were involved. SP-induced facilitation of glutamatergic responses was accompanied by a marked decrease in paired-pulse ratio, indicating a presynaptic mechanism of action. These data provide an electrophysiological correlate for the anatomically known connections between SP-positive MSN terminals and the dendrites and somata of other MSNs.

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.

Immunohistochemical localisation of the NK1 receptor in the human amygdala: preliminary investigation in schizophrenia

The amygdala has a role in the modulation of moods and emotion, processes that are known to be affected in people with psychiatric disorders such as schizophrenia and depression. The tachykinin NK(1) receptor is known to be expressed in the amygdala. However to date, there is limited knowledge of the distribution of the NK(1) receptor in this region. This study used immunohistochemistry to analyse the distribution of the NK(1) receptor in fixed human amygdala tissue in control subjects with no history of psychiatric illness and matched subjects with a diagnosis of schizophrenia (n=4 pairs). The NK(1) receptor was observed sparsely distributed in cell bodies in all amygdaloid nuclei with the basolateral and lateral having a greater relative density of NK(1) receptor-immunoreactive cell bodies than the other nuclei. Double labelling with antibodies to microtubule associated protein and the NK(1) receptor revealed that the NK(1) receptor is expressed by large pyramidal, small stellate and large bipolar neurons. Interestingly, the basal nucleus of Meynert, which is just dorsal to the amygdala, was observed to have a significantly higher relative density of NK(1) receptor-immunoreactive cell bodies compared to any of the amygdaloid nuclei. Preliminary analysis of the density of NK(1) receptor-immunoreactive cell bodies in the major amygdaloid nuclei and the basal nucleus of Meynert revealed no significant differences between schizophrenia and control subjects. Real-time PCR showed that the mRNA for both the short and long isoforms of the NK(1) receptor was expressed at low levels in fresh frozen human amygdala tissue from control subjects and that this was not different in matched subjects with schizophrenia (n=11 pairs). In conclusion, this study has demonstrated that the NK(1) receptor is widely distributed in the amygdala, and has shown for the first time a high relative density of NK(1) receptor-immunoreactive cell bodies in the basal nucleus of Meynert.

Full-length and truncated neurokinin-1 receptor expression and function during monocyte/macrophage differentiation

The substance P (SP)-preferring receptor neurokinin-1 receptor (NK-1R) has two forms: a full-length receptor consisting of 407 aa and a truncated receptor consisting of 311 aa. These two receptors differ in the length of the C terminus of NK-1R. We studied the undifferentiated and phorbol myristate acetate (PMA)-differentiated human monocyte/macrophage cell line THP-1 to investigate the expression and function of NK-1R. The expression of full-length and truncated NK-1R in this cell line was determined by using real-time PCR and immunofluorescence staining. Undifferentiated THP-1 cells expressed only truncated NK-1R. The differentiation of THP-1 cells with PMA to a macrophage-like phenotype resulted in the expression of full-length NK-1R, which was functionally accompanied by an SP (10(-6) M)-induced Ca2+ increase. In contrast, the addition of SP (10(-6) M) did not trigger Ca2+ response in undifferentiated THP-1 cells; however, SP did enhance the CCR5-preferring ligand RANTES (CCL5)-mediated Ca2+ increase. When a plasmid containing the full-length NK-1R was introduced into undifferentiated THP-1 cells, exposure to SP triggered Ca2+ increase, demonstrating that the full-length NK-1R is required for SP-induced Ca2+ increase. The NK-1R antagonist aprepitant (Emend, Merck) inhibited both the SP-induced Ca2+ increase in PMA-differentiated THP-1 cells and the SP priming effect on the CCL5-mediated Ca2+ increase, indicating that these effects are mediated through the full-length and truncated NK-1R, respectively. Taken together, these observations demonstrate that there are unique characteristics of NK-1R expression and NK-1R-mediated signaling between undifferentiated THP-1 cells and THP-1 cells differentiated to the macrophage phenotype.

Immunohistochemical localization of substance P NK1 receptor in guinea pig distal colon

BACKGROUND

Neurokinin receptors facilitate tachykinin mediated intestinal motility and secretion. Distribution of Substance P (SP) neurokinin 1 receptor (NK1r) immunoreactivity (IR) has been previously characterized in guinea pig ileum, but not colon. This study localizes NK1rs in guinea pig distal colon.

METHODS

Neurons were double labelled for NK1r and either acetylcholine transferase (ChAT), calbindin (calb), neuropeptide Y (NPY), nitric oxide synthase (NOS) or SP. The NK1r endocytosis was induced by 10(-5) mol L(-1) SP, septide, [SarMet] SP or neurokinin A.

RESULTS

In guinea pig distal colon, NK1r-IR was present on 70% of submucosal neurons. Sixty-threepercent of the NK1r-IR submucosal neurons were ChAT-IR, 16% calb/SP-IR, 19% NPY-IR and 14% NOS-IR neurons. The NK1r-IR was present on 5% of myenteric neurons. Of these 63% were ChAT-IR, 16% calb-IR neurons and 25% NOS-IR. The NK1rs were also on myenteric plexus interstitial cells of Cajal and on circular muscle.

CONCLUSION

In guinea pig distal colon, NK1rs were on 70% of submucosal neurons including all three secretomotor neuron subtypes and sensory neurons, suggesting NK1rs have a major role in neuronal control of mucosal reflexes. The NK1rs were on few myenteric neurons but were dense on muscle cells, suggesting NK1rs affect motility through neuro-muscular rather than neuro-neuronal transmission.

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.

Tachykinin peptides and receptors: putting amphibians into perspective

The tachykinins form one of the largest peptide families in nature. In this review, we describe the comparative features of the tachykinin peptides and their receptors, focusing particularly on amphibians. We also summarize our systematic studies of the localization, characteristics, and actions of bufokinin, a toad substance P-related peptide, in its species of origin. In addition, we discuss the establishment of multiple isoforms of the NK1-like receptor in the toad, and their structure, pharmacology and tissue distributions. We conclude that tachykinin peptides and receptors are well conserved in terms of their structures, physiological functions and coupling mechanisms during tetrapod evolution.

Post-stimulus potentiation of transmission in pelvic ganglia enhances sympathetic dilatation of guinea-pig uterine artery in vitro

Vasodilatation produced by stimulation of preganglionic neurones in lumbar and sacral pathways to pelvic ganglia was studied using an in vitro preparation of guinea-pig uterine artery and associated nerves in a partitioned bath allowing selective drug application to the ganglia or artery. Arterial diameter was monitored using real time video imaging. Vasodilatations produced by hypogastric nerve stimulation (HN; 300 pulses, 10 Hz) were significantly larger and longer in duration than with pelvic nerve stimulation (N = 18). Stimulation of ipsilateral lumbar splanchnic nerves or ipsilateral third lumbar ventral roots also produced prolonged vasodilatations. Blockade of ganglionic nicotinic receptors (0.1-1 mM hexamethonium) delayed the onset and sometimes reduced the peak amplitude of dilatations, but slow dilatations persisted in 16 of 18 preparations. These dilatations were not reduced further by 3 microM capsaicin applied to the artery and ganglia, or ganglionic application of 1 microM hyoscine, 30-100 microM suramin or 10 microM CNQX. Dilatations were reduced slightly by ganglionic application of NK1 and NK3 receptor antagonists (SR140333, SR142801; 1 microM), but were reduced significantly by bathing the ganglia in 0.5 mM Ca2+ and 10 mM Mg2+. Intracellular recordings of paracervical ganglion neurones revealed fast excitatory postsynaptic potentials (EPSPs) in all neurones on HN stimulation (300 pulses, 10 Hz), and slow EPSPs (3-12 mV amplitude) in 25 of 37 neurones. Post-stimulus action potential discharge associated with slow EPSPs occurred in 16 of 37 neurones (firing rate 9.4 +/- 1.5 Hz). Hexamethonium (0.1-1 mM) abolished fast EPSPs. Hexamethonium and hyoscine (1 microM) did not reduce slow EPSPs and associated post-stimulus firing in identified vasodilator neurones (with VIP immunoreactivity) or non-vasodilator paracervical neurones. These results demonstrate a predominantly sympathetic origin of autonomic pathways producing pelvic vasodilatation in females. Non-cholinergic mediators of slow transmission in pelvic ganglia produce prolonged firing of postganglionic neurones and long-lasting dilatations of the uterine artery. This mechanism would facilitate maintenance of pelvic vasodilatation on stimulation of preganglionic neurones during sexual activity.

Autocrine proliferation of neuroblastoma cells is partly mediated through neurokinin receptors: relevance to bone marrow metastasis

Despite intensive therapy, approximately 60-80% of children who are diagnosed with metastatic neuroblastoma (NB) succumb to the disease. NB preferentially metastasizes to the bone marrow (BM). In the present study we used SY5Y and CHP212 (NB cell lines) to study the roles of the preprotachykinin-I (PPT-I) gene and the natural receptors for PPT-I peptides, neurokinin-1 (NK-1) and NK-2, in the biology of NB. PPT-I, NK-1 and NK-2 were constitutively expressed in the NB cells. Functional studies, with specific NK receptor antagonists, showed that PPT-I peptides mediate autocrine proliferation of the NB cells through both NK-1 and NK-2 receptors. Full-length and truncated NK-1 receptors were detected in NB cells. Since there is one copy of the NK-1 gene, we used NK-1-specific siRNA to suppress the expression of NK-1. The NK-1-deficient NB cells showed phenotypes consistent with cell differentiation. Suppression of NK-1 did not appear to cause cell death, as demonstrated by trypan blue exclusion and by undetectable active caspase. NK-1 suppression reduced the proliferation of the NB cells beginning by 10-fold at day 1 and reached a 10(5)-fold reduction by day 10. The NK-1 deficient cells did not proliferate when they were placed as cocultures with BM stroma, which suggests that NK-1 signaling is important for the survival of NB cells in the BM. The results show potential roles for NK receptors in the proliferation of NB.

Vascular NK-1 receptor occurrence in normal and chronic painful Achilles and patellar tendons: studies on chemically unfixed as well as fixed specimens

It is not known as to whether the Achilles and patellar tendons contain neurokinin-1 (NK-1) receptors. This is a drawback when considering the fact that pain symptoms are frequent in these and as recent studies show that the pain symptoms might be cured via interference with blood vessel function. In the present study, the human Achilles and patellar tendons were examined concerning immunohistochemical expression of the NK-1 receptor. Chemically unfixed and fixed specimens, TRITC and PAP stainings and a battery of NK-1 receptor antibodies, including antibodies against the C-terminus and the N-terminal region, were utilized. NK-1 receptor immunoreaction could be detected in inner parts of the walls of large blood vessels and in the walls of small blood vessels. To some extent, NK-1 immunoreaction was also detectable in small nerve fascicles and in tenocytes. It was found to be of utmost importance to apply both chemically unfixed and fixed specimens. The use of chemically unfixed tissue was found advantageous in order to depict the immunoreactions in the blood vessel walls. The observations represent new findings and are of relevance as substance P (SP) is known to be of importance where neurogenic angiogenesis contributes to diseases and as SP on the whole has profound effects concerning blood vessel regulation.

NK1 receptor activation in rat rostral ventrolateral medulla selectively attenuates somato-sympathetic reflex while antagonism attenuates sympathetic chemoreflex

The effects of activation and blockade of the neurokinin 1 (NK1) receptor in the rostral ventrolateral medulla (RVLM) on arterial blood pressure (ABP), splanchnic sympathetic nerve activity (sSNA), phrenic nerve activity, the somato-sympathetic reflex, baroreflex, and chemoreflex were studied in urethane-anesthetized and artificially ventilated Sprague-Dawley rats. Bilateral microinjection of either the stable substance P analog (pGlu5, MePhe8, Sar9)SP(5–11) (DiMe-SP) or the highly selective NK1 agonist [Sar9, Met (O2)11]SP into the RVLM resulted in an increase in ABP, sSNA, and heart rate and an abolition of phrenic nerve activity. The effects of [Sar9, Met (O2)11]SP were blocked by the selective nonpeptide NK1 receptor antagonist WIN 51708. NK1 receptor activation also dramatically attenuated the somato-sympathetic reflex elicited by tibial nerve stimulation, while leaving the baroreflex and chemoreflex unaffected. This effect was again blocked by WIN 51708. NK1 receptor antagonism in the RVLM, with WIN 51708 significantly attenuated the sympathoexcitatory response to hypoxia but had no effect on baseline respiratory function. Our findings suggest that substance P and the NK1 receptor play a significant role in the cardiorespiratory reflexes integrated within the RVLM.

Substance P and the neurokinin-1 receptor in relation to eosinophilia in ulcerative colitis

Substance P (SP) has been implicated in the pathophysiology of ulcerative colitis (UC) and it has been suggested that blocking of its effect would be advantageous in this disease. Eosinophils have also been implicated in the pathophysiology of UC. In the present study, specimens from the sigmoid colon of UC patients were investigated by the use of antisera against SP and the neurokinin-1 receptor (NK-1R) and staining for demonstration of eosinophils. The degrees of SP innervation and NK-1R immunoreaction, as well as the levels of eosinophil infiltration, varied between different patients. Interestingly, NK-1R immunoreaction in the epithelium was often seen to be the most marked where there were numerous eosinophils in the underlying mucosa and where the mucosa showed a marked morphologic derangement. The observations suggest that there are marked fluctuations in effects of SP and eosinophils during the disease. The infiltrating eosinophils may be involved in the destruction of the mucosal tissue. Furthermore, for the majority of cases where there is marked derangement of the mucosa, it is apparent that there is an upregulation of the NK-1 receptor in the epithelium in parallel with the infiltration of the eosinophils.

Comparison of immunoblotted delta opioid receptor proteins expressed in the adult rat brain and their regulation by growth hormone

It has previously been suggested that exogenous growth hormone (GH) affect quality of life and higher brain functions through the endogenous opioid system. Recently, we showed that GH down-regulate 72 and 48 kDa delta opioid receptor (DOR) proteins in the adult rat cerebral cortex and cerebellum. In the present study, we found that an antiserum raised against the N-terminus of the DOR also recognizes a 36 kDa protein, not recognized by a C-terminus-directed antiserum. We aimed to investigate the identity of the 72, 48 and 36 kDa proteins and to further study the effects of GH on their expression in different brain regions. The expression was studied in hypophysectomized (Hx) and untreated normal female rats. One subgroup of Hx rats received GH as a daily subcutaneous injection for 19 days. Our data show that treatment with GH in Hx rats normalized the expression of the 72 kDa protein in the cerebral cortex, whereas no significant effect were observed for the 48 or 36 kDa proteins. However, GH significantly reduced the ratio between the 72 and 36 kDa proteins in different brain regions of Hx rats. Our data suggest that GH reduces the levels of a 72 kDa DOR that likely represents a dimeric form of a 36 kDa DOR post-translationally truncated at the C-terminus, and that altered receptor dimerization may be involved in GH induced effects in the central nervous system.

Eledoisin and Kassinin, but not Enterokassinin, stimulate ion transport in frog skin

In frog skin, tachykinins stimulate the ion transport, estimated by measuring the short-circuit current (SCC) value, by interacting with NK1-like receptors. In this paper we show that Kassinin (NK2 preferring in mammals) increases the SCC, while Enterokassinin has no effect. Therefore, either 2 Pro residues or 1 Pro and 1 basic amino acid must be present in the part exceeding the C-terminal pentapeptide. Eledoisin (NK3 preferring in mammals) stimulation of SCC is reduced by CP99994 and SR48968 (NK1 and NK2 antagonists) and not affected by SB222200 (NK3 antagonist). None of the three antagonists affects Kassinin stimulation of SCC.

Molecular identification and characterization of three isoforms of tachykinin NK1-like receptors in the cane toadBufo marinus

The tachykinin peptide bufokinin, isolated from the cane toad intestine, is important in intestinal and cardiovascular regulation in the toad. In this study, three tachykinin NK1-like receptor isoforms, bNK1-A, bNK1-B, and bNK1-C, encoding proteins of 309, 390, and 371 amino acids, respectively, were cloned from the toad brain and intestine. These isoforms differ only at the intracellular COOH terminus. The bNK1-A and bNK1-B isoforms are similar to the truncated and full-length forms of the mammalian NK1receptor, whereas bNK1-C is unique and does not correspond to any previously described receptor. RT-PCR studies demonstrated that three isoform transcripts are widely distributed in the toad with high expression in gut, spinal cord, brain, lung, and skeletal muscle. When expressed in COS-7 cells, bufokinin showed similar high affinity (IC500.6–0.8 nM) in competing for125I-labeled Bolton-Hunter bufokinin binding at all receptors, but the binding affinities of substance P (SP) and neurokinin A (NKA) were very different at each isoform. When expressed in Xenopus oocytes, the truncated isoform, bNK1-A, was inactive, whereas bNK1-B and bNK1-C produced changes in chloride current when stimulated by tachykinins (minimum concentrations: bufokinin, 0.1 nM; SP, 1 nM; and NKA, 10 nM). A marked desensitization of the response was seen to subsequent applications of tachykinins, as experienced by the mammalian NK1receptor. In summary, our study describing three isoforms of NK1-like receptor from the toad suggests that the alternative splicing of NK1receptor is a physiologically conserved mechanism and raises a fundamental question as to the physiological role of each isoform.

Pharmacological evaluation of α and β human tachykinin NK2 receptor splice variants expressed in CHO cells

In the present study, we have investigated, by binding and functional experiments, the pharmacological profile of a new human tachykinin NK(2) receptor splice variant named beta isoform. Neurokinin A, nepadutant, SR48968 [(S)-N-methyl-N[4-(4-acetylamino-4-phenyl piperidino)-2-(3,4-dichlorophenyl) butyl]benzamide] and substance P have been tested for binding on the receptor expressed in whole CHO transfected cells. Only SR48968 binds, but with an affinity about sixfold lower in respect to the alpha isoform. Moreover, neurokinin A was unable to inhibit the [(3)H]SR48968 binding to the beta isoform up to microM concentrations. In cells expressing the human tachykinin NK(2) receptor beta isoform, contrary to those expressing the alpha isoform, natural or selective tachykinin receptor agonists (1 microM) were unable to produce a significant activation of inositol phosphate (IP) production or increase of intracellular calcium concentration [Ca(2+)](i). The recently discovered tachykinins, endokinins C and D, did not activate IP production or [Ca(2+)](i) increase in cells expressing the alpha or beta isoform of the human tachykinin NK(2) receptor. The present data indicate that the human tachykinin NK(2) receptor beta isoform is poorly or not expressed on the cell membrane surface and that it may possibly act as a regulator of tachykinin NK(2) receptor function. We cannot exclude the possibility that this receptor could interact with other presently unknown ligands.

A 25 year adventure in the field of tachykinins

Several aspects of our 25 year adventure in the field of tachykinins will be successively described. They concern: substance P (SP) synthesis and release in the basal ganglia, the identification and pharmacological characterization of central tachykinin NK(1), NK(2) and NK(3) binding sites and their topographical distribution, the description of some new biological tests for corresponding receptors, the identification of tachykinin NK(1) receptor subtypes or conformers sensitive to all endogenous tachykinins (substance P, neurokinin A (NKA), neurokinin B (NKB), neuropeptide gamma (NP gamma) and neuropeptide K (NPK)) and finally, the functional involvement of these receptors and their subtypes in tachykinin-induced regulations of dopamine and acetylcholine release in the striatum.

Tachykinins and tachykinin receptors: a growing family

The peptides of the tachykinin family are widely distributed within the mammalian peripheral and central nervous systems and play a well-recognized role as excitatory neurotransmitters. Currently, the concept that tachykinins act exclusively as neuropeptides is being challenged, since the best known members of the family, substance P, neurokinin A and neurokinin B, are also present in non-neuronal cells and in non-innervated tissues. Moreover, the recently cloned mammalian tachykinins hemokinin-1 and endokinins are primarily expressed in non-neuronal cells, suggesting a widespread distribution and important role for these peptides as intercellular signaling molecules. The biological actions of tachykinins are mediated through three types of receptors denoted NK(1), NK(2) and NK(3) that belong to the family of G protein-coupled receptors. The identification of additional tachykinins has reopened the debate of whether more tachykinin receptors exist. In this review, we summarize the current knowledge of tachykinins and their receptors.

Neurokinin-1 and -3 receptor blockade inhibits slow excitatory synaptic transmission in myenteric neurons and reveals slow inhibitory input

Recent studies have shown that tachykinins mediate slow synaptic transmission to myenteric AH (afterhyperpolarising) neurons via neurokinin-3 receptors (NK(3)R). This study investigated a similar role for neurokinin-1 receptors (NK(1)R) and compared the effect of selective receptor antagonists on non-cholinergic slow excitatory post-synaptic potentials (EPSPs) recorded in myenteric AH neurons of the guinea-pig ileum. Slow EPSPs evoked by electrical stimulation of circumferentially oriented presynaptic nerves were mimicked by application of senktide, an NK(3)R agonist. [Sar(9),Met(O(2))(11)]-substance P, an NK(1)R agonist, depolarised a smaller number of neurons. SR142801, a selective NK(3)R antagonist (100 nM), inhibited slow EPSPs and responses to senktide, but had no effect on depolarisations evoked by forskolin, an activator of adenylate cyclase. SR140333, a selective NK(1)R antagonist, inhibited slow EPSPs in a subset of neurons and blocked responses to [Sar(9),Met(O(2))(11)]-substance P, but not to senktide or forskolin. Slow EPSPs that were predominantly mediated by NK(1)R had significantly shorter latencies than those due to activation of NK(3)R. After blockade of slow EPSPs, slow hyperpolarizing responses to presynaptic nerve stimulation were revealed in one-third of neurons. These events, which were associated with a decrease in input resistance and blocked by tetrodotoxin, were equated with slow inhibitory postsynaptic potentials. They were abolished by the 5-hydroxytryptamine(1A) receptor antagonist 1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl]-piperazine (NAN-190), but unaffected by phentolamine, an alpha-adrenoceptor antagonist. In conclusion, these results provide the first direct evidence that NK(1)R mediate some slow excitatory synaptic input to myenteric AH neurons, and suggest that NK(1)R and NK(3)R activate distinct signal transduction pathways. These results also demonstrate that slow inhibitory synaptic transmission, which may be mediated by 5-hydroxytryptamine, is more prevalent in the myenteric plexus than previously indicated.