Actions Of Tachykinins Research Articles

Sequences analysis and pituitary actions of tachykinins in Chinese sturgeon (Acipenser sinensis)

Tachykinins belong to a large, evolutionarily conserved family of brain/gut peptides that are involved in a variety of physiological functions in mammals, such as reproductive regulation. However, little information was available about tachykinins in ancient fish lineage. In the present study, we firstly identified three tachykinin genes (named tac1, tac3 and tac4) and three neurokinin receptors (named nk1r, nk2r and nk3r) from Chinese sturgeon brain and pituitary. Sequence analysis showed that tac1 encoded substance P (SP) and neurokinin A (NKA), tac3 encoded neurokinin B (NKB) and NKB-related peptide (NKBRP), and tac4 encoded hemokin 1 (HK-1) and hemokin 2 (HK-2), respectively. The luciferase reporter assay results showed that NK1R preferentially selected asSP, NK2R preferentially selected asNKA, and NK3R preferentially selected asNKB. Tissue expression analysis showed that the three tac genes were highly detected in the telencephalon and hypothalamus, whereas nkr genes were widely expressed in peripheral tissues. Spatio-temporal expression analysis showed that all three tac genes were highly expressed in unknown sex individuals. Intraperitoneal injection experiments showed that both asSP and asNKB could stimulate luteinizing hormone (LH) release in Chinese sturgeon serum. At the transcriptional level, asSP and asNKB could significantly reduce pituitary follicle-stimulating hormone beta (fshβ) mRNA expression, but induce pituitary growth hormone (gh) mRNA expression. In addition, estradiol (E2) could stimulate tac3 mRNA expression in hypothalamus. Taken together, this study provided information on the tachykinin family in Chinese sturgeon and demonstrates that asNKB and asSP could be involved in reproductive and growth regulation in pituitary.

Tachykinins, new players in the control of reproduction and food intake: A comparative review in mammals and teleosts.

In vertebrates, the tachykinin system includes tachykinin genes, which encode one or two peptides each, and tachykinin receptors. The complexity of this system is reinforced by the massive conservation of gene duplicates after the whole-genome duplication events that occurred in vertebrates and furthermore in teleosts. Added to this, the expression of the tachykinin system is more widespread than first thought, being found beyond the brain and gut. The discovery of the co-expression of neurokinin B, encoded by the tachykinin 3 gene, and kisspeptin/dynorphin in neurons involved in the generation of GnRH pulse, in mammals, put a spotlight on the tachykinin system in vertebrate reproductive physiology. As food intake and reproduction are linked processes, and considering that hypothalamic hormones classically involved in the control of reproduction are reported to regulate also appetite and energy homeostasis, it is of interest to look at the potential involvement of tachykinins in these two major physiological functions. The purpose of this review is thus to provide first a general overview of the tachykinin system in mammals and teleosts, before giving a state of the art on the different levels of action of tachykinins in the control of reproduction and food intake. This work has been conducted with a comparative point of view, highlighting the major similarities and differences of tachykinin systems and actions between mammals and teleosts.

Novel Biology of Tachykinins in Gonadotropin-Releasing Hormone Secretion.

The tachykinin family of peptides, composed of the neurokinins A and B (NKA, NKB) and substance P are involved in the central control of gonadotropin-releasing hormone (GnRH) release through a variety of neuronal circuitries that mediate the activation of Kiss1 neurons and the synchronization of their activity within the arcuate nucleus. The major outcome of this role is the precise regulation of the pulsatile pattern of GnRH release. In addition, tachykinins are involved in the maturation of the reproductive axis by determining the optimal timing of puberty onset, as well as in the timing of the preovulatory luteinizing hormone surge in females. Therefore, the action of tachykinins in reproduction appears to extend to all the critical aspects required for the successful attainment and maintenance of fertility. In this review, we summarize the latest advances in our understanding of the biology of tachykinins in the control of GnRH release, addressing the existing controversies, open questions, and future perspectives.

Ablation of Tacr2 in mice leads to gastric emptying disturbance.

Tacr2 is one of the G protein-coupled receptors(GPCRs) that mediate the biological actions of tachykinins. It is abundantly expressed in the gastrointestinal (GI) system and is thought to play an important role in GI motility, secretion, and visceral sensitivity. Previously, the physiological and pathophysiological functions of Tacr2 were mainly studied using Tacr2 selective agonists or antagonists. Here, we seek to investigate the effect of Tacr2 disruption in mice to provide further insights. The Tacr2 knockout mice were generated by homologous recombination and the phenotypic changes of the Tacr2-null mice were analyzed and compared with their wild type (wt) littermates. Increased food retention was detected in Tacr2-/- mice. The stomach of Tacr2-/- mice had thinner muscularis externa and less neurons in the myenteric plexus. The stomach and small intestine exhibited longer duration of electrical field stimulation (EFS)-induced inhibition in the gastric fundus and decreased frequency of migrating motor complex (MMC), respectively. Neuronal nitric oxide synthase (nNOS) and vasoactive intestinal polypeptide (VIP) were significantly up-regulated due to Tarc2 deficiency, contributing to enhanced nitric oxide (NO) signaling in the stomach of Tacr2-/- mice. Intraperitoneal application of 7-nitroindazole (7-NI) to Tacr2-/- mice effectively relieved the gastric emptying disturbance. Moreover, Creb and NF-κB signalings were involved in the regulation of these physiological changes initiated by Tacr2 deficiency. Tacr2 negatively regulated the expression of nNOS and VIP both in vivo and in vitro. Its ablation in mice elevated the expression of nNOS and VIP, enhanced NO signaling and changed the Creb and NF-κB signalings, finally leading to the gastric emptying disturbance of Tacr2-/- mice.

Molecular Recognition of Tachykinin Receptor Selective Agonists: Insights from Structural Studies

This Review deals essentially with the elucidation of structural features of Tachykinin family of neuropeptides, which are known to interact through three distinct GPCR subtypes, namely NK1 (Neurokinin 1), NK2 (Neurokinin 2) and NK3 (Neurokinin 3) receptors. In mammals, Tachykinins have been shown to elicit a wide array of activities such as powerful vasodilatation, hypertensive action and stimulation of extravascular smooth muscle and are known to be involved in a variety of clinical conditions including chronic pain, Parkinson's disease, Alzheimer's disease, depression, rheumatoid arthritis, irritable bowel syndrome and asthma. This broad spectrum of action of Tachykinins is attributed to the lack of selectivity of tachykinins to their receptors. All tachykinins interact with all the three-receptor subtypes with SP preferring NK1, NKA preferring NK2 and NKB preferring NK3. This lack of specificity can be accounted for by the conformational flexibility of these short, linear peptides. Hence, identification of structural features of the agonists important for receptor binding and biological activity is of great significance in unraveling the molecular mechanisms involved in tachykinin receptor activation and also in rational design of novel therapeutic agents. Understanding structure of the ligand-receptor complex and analysis of topography of the binding pocket of the tachykinin receptor is also crucial in rational design of drugs.

Action of tachykinins in the hippocampus: Facilitation of inhibitory drive to GABAergic interneurons

By acting on neurokinin 1 (NK1) receptors, neuropeptides of the tachykinin family can powerfully excite rat hippocampal GABAergic interneurons located in the CA1 region and by this way indirectly inhibit CA1 pyramidal neurons. In addition to contact pyramidal neurons, however, GABAergic hippocampal interneurons can also innervate other interneurons. We thus asked whether activation of tachykinin-sensitive interneurons could indirectly inhibit other interneurons. The study was performed in hippocampal slices of young adult rats. Synaptic events were recorded using the whole-cell patch clamp technique. We found that substance P enhanced GABAergic inhibitory postsynaptic currents in a majority of the interneurons tested. Miniature, action potential–independent inhibitory postsynaptic currents were unaffected by substance P, as were evoked inhibitory synaptic currents. This suggests that the peptide acted at the somatodendritic membrane of interneurons, rather than at their axon terminals. The effect of substance P was mimicked by a selective NK1 receptor agonist, but not by neurokinin 2 (NK2) or neurokinin 3 (NK3) receptor agonists, and was suppressed by a NK1 selective receptor antagonist. In contrast to substance P, oxytocin, another peptide capable of activating hippocampal interneurons, had no effect on the inhibitory synaptic drive onto interneurons. We conclude that tachykinins, by acting on NK1 receptors, can influence the hippocampal activity by indirectly inhibiting both pyramidal neurons and GABAergic interneurons. Depending on the precise balance between these effects, tachykinins may either activate or depress hippocampal network activity.

No Pain, No Gain

Myocardial ischemia, the major cause of mortality and morbidity in the United States, accounts for nearly 20% of all deaths.1 The prognosis of patients with acute myocardial infarction is strongly related to the amount of tissue destroyed during ischemia/reperfusion; hence, limiting this injury is of paramount importance for reducing the morbidity and mortality associated with ischemic heart disease.2,3 Over the past 40 years, several hundred pharmacological or nonpharmacological therapies have been reported to alleviate ischemia/reperfusion injury in experimental animal models; unfortunately, few of these results have been reproducible, and, with the exception of early reperfusion, none have been translated into clinical therapies.4 These sobering facts provide a cogent rationale for a reassessment of the models and methodologies that have been used heretofore to test the efficacy of putative cardioprotective interventions. The obvious translational failure in this field calls for a new paradigm in which preclinical studies must be done as rigorously as their clinical counterparts, that is, using clinically relevant animal models, appropriate statistical methods, and a multicenter, randomized, blinded design using centralized core laboratories for data analysis.4 This paradigm would also improve our understanding of the mechanism of cardiomyocyte death during ischemia/reperfusion. Article p 3617 One facet of ischemic biology that has received relatively little attention is the role of cardiac sensory nerves. Myocardial ischemia/reperfusion is known to cause the release of bradykinin and protons, both of which activate the vanilloid receptor 1 (VR1, also known as transient receptor potential vanilloid type 1 [TRPV1]) on the capsaicin-sensitive cardiac sensory nerves (unmyelinated [type C] or thinly myelinated [A-δ] fibers).5 The VR1 is a nonselective cation channel that plays an important role in the polymodal detection of noxious stimuli such as low pH and heat.6 It can also be activated by chemicals such as capsaicin …

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.

Investigation of neurokinin-2 and -3 receptors in the human and pig bladder.

To investigate the role of neurokinin (NK)-2 and -3 receptors in mediating the contraction of detrusor muscle strips from human and pig, to determine whether the pig is a good model for the study of tachykinin receptors in the human bladder, as the biological actions of tachykinins, e.g. substance P and NKA are mediated via three distinct receptor subtypes, NK-1, -2 and -3. Strips of detrusor muscle were obtained from the bladder dome and neck of female pigs and from patients undergoing cystectomy. Cumulative concentration-response curves to NKA were obtained in the absence and presence of either the NK-2 receptor-selective antagonist SR48968 or the NK-3 receptor-selective antagonist SB223412. NKA produced concentration-dependent contractions in the human and pig detrusor muscle; the curves were shifted to the right by SR48968, with high affinity (pKB 8.9, 8.3 and 8.0 in the human, pig dome and pig neck, respectively), whereas SB223412 had a minimal effect (pKB 5.8, 5.8 and 6.3, respectively). These data confirm that the NK-2 receptor subtype mediates NKA-induced contraction of the human and pig detrusor muscle. The NK-3 receptor appears to have no role in detrusor contraction of either species. The results also provide evidence that the NK-2 receptor in human and pig are the same, and the latter may be an appropriate species to study tachykinin-induced contractions in human bladder.

The tachykinin peptide family.

The tachykinin peptide family certainly represents one of the largest peptide families described in the animal organism. So far, more than 40 tachykinins have been isolated from invertebrate (insects, worms, and molluscs), protochordate, and vertebrate (skin, gastrointestinal tract, peripheral and central nervous system) tissues. Substance P (SP), first identified by bioassay as early as 1931 but sequenced only in 1971, several years after the elucidation of the structure of eledoisin from molluscan tissues and of physalaemin from amphibian skin, may be considered as a prototype of the tachykinins. Hitherto, as many as 19 tachykinins have been isolated from amphibian integument, and eight additional peptides have been isolated from amphibian gut and brain. Counterparts of skin tachykinins in mammalian tissues are SP, neurokinin A, and neurokinin B. Three main receptor subtypes for the tachykinins have been identified (NK1, NK2, and NK3), but their number is probably destined to increase. It is obvious that the peripheral and central effects of the tachykinins may substantially vary depending on the activation of different receptor subtypes. Matters are further complicated by the frequent capacity of the single tachykinins to bind, although with different affinity, to more receptors. It has been recognized that tachykinins have a variety of effects in physiological and pathological conditions, and there is evidence suggesting intrinsic neuroprotective and neurodegenerative properties of these neuropeptides. This review provides an update on the current body of knowledge regarding tachykinin occurrence and distribution in the animal kingdom, from the lowest invertebrates to man, and the physiological and pharmacological actions of tachykinins outlining the pregnant importance of this large peptide family.

Enhancement of neurokinin A-induced smooth muscle contraction in human urinary bladder by mucosal removal and phosphoramidon: relationship to peptidase inhibition

Neurokinin A (NKA) is potent in contracting the human detrusor muscle. Here, we have investigated whether these contractile responses are influenced by the presence of the mucosa, by the peptidase inhibitor phosphoramidon or by possible modulators, prostaglandins and nitric oxide. Contractile responses to neurokinin A were unaffected by indomethacin or N-ω-nitro- l-arginine, but were significantly reduced in strips containing mucosa. Phosphoramidon, an inhibitor of neutral endopeptidase 24.11 (neprilysin, CD10), was ineffective at 10 μM, but at 100 μM, significant increase in the maximum response was achieved by neurokinin A in detrusor strips with and without mucosa. In immunohistochemical studies, neutral endopeptidase immunoreactivity occurred in peripheral nerve trunks in the detrusor and in a fibrous meshwork in the subepithelial lamina propria. Our data indicate that neutral endopeptidase is present in bladder mucosa and detrusor, and support the concept that this metalloprotease and/or related enzymes are important in regulating the actions of tachykinins.

Effects of tachykinins on uterine smooth muscle.

1. Sensory nerves supplying the mammalian uterus have been shown to contain substance P (SP) and neurokinin (NK)A. This review presents some of the advances that have led to a greater understanding of the effects of tachykinins on uterine smooth muscle. 2. The cell-surface peptidase neprilysin (EC.3 24.11, endopeptidase 24.11, enkephalinase, CALLA, CD10) has been shown to play a major role in regulating the actions of tachykinins on both rat and human myometrium. Because this peptidase is known to be regulated by steroids and pregnancy, its effects may be of physiological relevance. 3. Tachykinins produce contractions of isolated myometrial preparations from non-pregnant rats and mice. The NK2 receptor mediates these effects in rat uterus, while the NK1 receptor may mediate these effects in the mouse uterus. 4. The effects of tachykinins have been examined on myometrial preparations obtained at Caesarean section from near-term pregnant women. In the presence of the peptidase inhibitors (thiorphan, captopril and bestatin), the mammalian tachykinins SP, NKA and NKB produced concentration-dependent uterine contractions. 5. The order of agonist potency NKA > SP = NKB suggested that NK2 receptors mediate uterine contractions in the human. This was confirmed using the stable analogues [Sar9,Met(O2)11]SP, [Lys5MeLeu9Nle10]NKA(4-10) and [N-MePhe7]NKB, which are NK1, NK2 and NK3 receptor selective, respectively. Only [Lys5MeLeu9Nle10]NKA(4-10) produced concentration-related contractions of human uterine smooth muscle. 6. The experimental findings described in the present review, taken together with results published previously in the literature, indicate that tachykinin peptides may play a physiological or pathophysiological role in regulating uterine smooth muscle activity. However, more extensive research will be required to confirm such a role for these peptides.

Peripheral actions of tachykinins

Tachykinins mediate a variety of physiological processes in the gastrointestinal, pulmonary and genito-urinary tract mainly through the stimulation of NK1 and NK2 receptors. Preclinical evidence obtained through the use of selective tachykinin receptor antagonists indicates that endogenous tachykinins are involved in augmented smooth muscle contraction, vasodilatation, chemotaxis and activation of immune cells, mucus secretion, water absorption/secretion. Recent evidence also suggests that endogenous tachykinins released at the peripheral level may play a role in visceral inflammation, hyperreflexia and hyperalgesia. Possible mechanisms underlying the stimulation of primary afferent neurons by tachykinins may involve a direct excitation of these neurons and the release of mediators which sensitise or stimulate sensory nerves. Tachykinin receptor antagonists could have a clinical utility in several human diseases such as irritable bowel syndrome, asthma, and in micturition disturbances characterized by a hyperactive bladder.

Respiratory actions of tachykinins in the nucleus of the solitary tract: effect of neonatal capsaicin pretreatment

1. The respiratory response to microinjection of capsaicin and tachykinin receptor agonists into the commissural nucleus of the solitary tract (cNTS) was investigated in adult, urethane-anaesthetized rats which had been pretreated with capsaicin (50 mg kg(-1) s.c.) or vehicle (10% Tween 80, 10% ethanol in saline) as day 2 neonates. 2. Microinjection of capsaicin (1 nmol) into the cNTS of vehicle-pretreated rats, significantly reduced respiratory frequency (59 breaths min(-1), preinjection control, 106 breaths min(-1)) without affecting tidal volume (VT). In capsaicin-pretreated rats, the capsaicin-induced bradypnoea was markedly attenuated (minimum frequency, 88 breaths min(-1); control, 106 breaths min(-1)). 3. In vehicle-pretreated rats, microinjection of substance P (SP, 33 pmol), neurokinin A (NKA, 33 pmol) and NKB (330 pmol), and the selective NK(1) tachykinin receptor agonists, [Sar(9), Met(O(2))(11)]-SP (33 pmol) and septide (10 pmol), increased VT (maxima, 3.60 - 3.93 ml kg(-1)) compared with preinjection control (2.82 ml kg(-1)), without affecting frequency. The selective NK(3) agonist senktide (10 pmol) also increased VT (3.93 ml kg(-1)) which was accompanied by a bradypnoea (-25 breaths min(-1)). The selective NK(2) agonist, [Nle(10)]-NKA(4-10) (330 pmol) increased VT slightly but significantly decreased frequency (-12 breaths min(-1)). In capsaicin-pretreated rats, VT responses to SP and [Sar(9), Met(O(2))(11)]-SP were increased whereas the response to septide was abolished. Both the VT and bradypnoeic responses to senktide and [Nle(10)]-NKA(4-10) were significantly enhanced. 4. These results show that neonatal capsaicin administration markedly reduces the respiratory response to microinjection of capsaicin into the cNTS. The destruction of capsaicin-sensitive afferents appears to sensitize the NTS to SP, NKB, [Sar(9),Met(O(2))(11)]-SP, senktide and [Nle(10)]-NKA(4-10). Moreover, the loss of septide responsiveness in capsaicin-pretreated rats, suggests that 'septide-sensitive' NK(1) receptors may be located on the central terminals of afferent neurons.

Respiratory actions of tachykinins in the nucleus of the solitary tract: characterization of receptors using selective agonists and antagonists.

1. The respiratory response to microinjection of tachykinins and analogues into the commissural nucleus of the solitary tract (cNTS) of urethane-anaesthetized rats was investigated in the presence and absence of selective tachykinin NK(1), NK(2) and NK(3) antagonists (RP 67580, SR 48968 and SR 142801, respectively). 2. All tachykinins, except for the selective NK(2) agonist, [Nle(10)]-NKA(4-10), increased tidal volume (VT). The rank potency order of naturally-occurring tachykinins was neurokinin A (NKA)> or =substance P (SP)>>NKB, whereas the rank order for selective analogues was senktide> or = septide>> [Sar(9),Met(O(2))(11)]-SP>>[Nle(10)]-NKA(4-10). Septide (NK(1)-selective) and senktide (NK(3)-selective) were 22 fold more potent (pD(2) approximately 12) at stimulating VT than SP (pD(2) approximately 10.5). 3. Tachykinin agonists produced varying degrees of respiratory slowing, independent of changes in VT. At doses producing maximum stimulation of VT, agonists induced either a mild (<10 breaths min(-1) decrease; SP and septide), moderate (10 - 25 breaths min(-1) decrease; NKA, NKB and [Sar(9),Met(O(2)]-SP) or severe ( approximately 40 breaths min(-1) decrease; senktide) bradypnoea. [Nle(10)]-NKA(4-10) produced a dose-dependent bradypnoea without affecting VT. 4. RP 67580 significantly attenuated the VT response to SP (33 pmol) and NKA (10 pmol) but not NKB (100 pmol). In the presence of RP 67580, the mild bradypnoeic response to NKB was significantly enhanced whereas SP and NKA induced a bradyapnea which was not observed in the absence of RP 67580. SR 48968 had no effect on the VT response to SP or NKB, markedly enhanced the VT response to NKA and completely blocked the bradypnoeic response to [Nle(10)]-NKA(4-10). Only SR142801 attenuated the VT response to NKB. 5. The present data suggest that all three tachykinin receptors (NK(1), NK(2) and NK(3)) are present in the cNTS and are involved in the central control of respiration.

Actions of Tachykinins Within the Heart and Their Relevance to Cardiovascular Disease

Substance P and neurokinin A are tachykinins that are co-localized with calcitonin gene-related peptide (CGRP) in a unique subpopulation of cardiac afferent nerve fibers. These neurons are activated by nociceptive stimuli and exhibit both sensory and motor functions that are mediated by the tachykinins and/or CGRP. Sensory signals (e.g., cardiac pain) are transmitted by peptides released at central processes of these neurons, whereas motor functions are produced by the same peptides released from peripheral nerve processes. This review summarizes our current understanding of intracardiac actions of the tachykinins. The major targets for the tachykinins within the heart are the intrinsic cardiac ganglia and coronary arteries. Intrinsic cardiac ganglia contain cholinergic neurons that innervate the heart and coronary vasculature. Tachykinins can stimulate NK3 receptors on these neurons to increase their excitability and evoke spontaneous firing of action potentials. This action provides a mechanism whereby tachykinins can indirectly influence cardiac function and coronary tone. Tachykinins also have direct effects on coronary arteries to decrease or increase tone. Stimulation of NK1 receptors on the endothelium causes vasodilation mediated by nitric oxide. This effect is normally dominant, but NK2 receptor-mediated vasoconstriction can also occur and is augmented when NKi receptors are blocked. It is proposed that these ganglion stimulant and vascular actions are manifest by endogenous tachykinins during myocardial ischemia.

Characterization of the mechanism of action of tachykinins in rat striatal cholinergic interneurons

The mechanism by which substance P depolarizes cholinergic interneurons in the rat striatum was studied using whole-cell recording techniques. In all cases the effects of substance P were mimicked by the neurokinin 1 receptor agonist [Sar 9, Met(O 2) 11] substance P and were antagonized by the neurokinin 1 receptor antagonist SR140333. [Sar 9, Met(O 2) 11] substance P was found to depolarize cholinergic interneurons by the induction of a calcium-activated inward current at −60 mV. This inward current was irreversibly potentiated by photolysis of caged GTP γS within neurons implicating the involvement of a G-protein. The [Sar 9, Met(O 2) 11] substance P-induced inward current was inhibited by the phospholipase C inhibitor U-73122, and by the inclusion of the inositol-1,4,5-triphosphate receptor antagonist heparin in the electrode solution. These findings suggest that neurokinin 1 receptors depolarize cholinergic interneurons in the rat striatum primarily through a phosphoinositide signalling pathway.

Mechanism of action of tachykinins in the frog adrenal gland.

Mechanism of action of tachykinins in the frog adrenal gland.

Actions of tachykinins and neurotensin and tachykinin-like immunoreactivity in human colon

Actions of tachykinins and neurotensin and tachykinin-like immunoreactivity in human colon

Actions of tachykinins on the ion transport across the frog skin

The tachykinin-dependent stimulation of ion transport across frog skin was studied. Tachykinin stimulation was due to interaction with an NK1-like receptor as [Sar 9-Met(O 2) 11]-Substance P (a very selective NK1 agonist) strongly stimulated SCC, whereas [β-Ala 8]-Neurokinin A 4–10 (a very selective NK2 agonist) did not. The rank order of tachykinin potency was: PG-KI > Uperolein > Hylambatin > Kassinin > Phyllomedusin > [Sar 9-Met (O 2) 11]-Substance P > Ranatachykinin A > Physalaemin > Ranakinin > Substance P and Eledoisin ≫ Neurokinin A. Neurokinin B, Scyliorhinin I, Urechistachykinin I and Urechistachykinin II had no effect. We conclude that the minimal structural requirements for stimulating SCC in the frog skin were the presence of: a) the C-terminal sequence Phe-X-Gly-Leu-Met-NH 2; b) at least one Pro residue in the N-terminal sequence.