Differential ability of tachykinin NK-1 and NK-2 agonists to produce scratching and grooming behaviours in mice

Biological evaluation and molecular docking studies of AA3052, a compound containing a μ-selective opioid peptide agonist DALDA and d-Phe-Phe-d-Phe-Leu-Leu-NH2, a substance P analogue

The design of novel drugs for pain relief with improved analgesic properties and diminished side effect induction profile still remains a challenging pursuit. Tolerance is one of the most burdensome phenomena that may hamper ongoing opioid therapy, especially in chronic pain patients. Therefore, a promising strategy of hybridizing two pharmacophores that target distinct binding sites involved in pain modulation and transmission was established. Previous studies have led to the development of opioid agonist/NK1 agonist hybrids that produce sufficient analgesia and also suppress opioid-induced tolerance development. In our present investigation we assessed the antinociceptive potency of a new AA3052 chimera comprised of a potent MOR selective dermorphin derivative (DALDA) and an NK1 agonist, a stabilized substance P analogue. We have shown that AA3052 significantly prolonged responses to both mechanical and noxious thermal stimuli in rats after intracerebroventricular administration. Additionally, AA3052 did not trigger the development of tolerance in a 6-day daily injection paradigm nor did it produce any sedative effects, as assessed in the rotarod performance test. However, the antinociceptive effect of AA3052 was independent of opioid receptor stimulation by the DALDA pharmacophore as shown in the agonist-stimulated G-protein assay. Altogether the current results confirm the antinociceptive effectiveness of a novel opioid/SP hybrid agonist, AA3052, and more importantly its ability to inhibit the development of tolerance.

Are biological actions of neurokinin A in the adult brain mediated by a cross-talk between the NK1 and NK2 receptors?

Mice lacking the NK(1) receptor (NK(1)R-/- mice) and selective, high-affinity, non-peptide, NK(1), NK(2) and NK(3) receptor antagonists were used to identify the tachykinin receptor subtype(s) mediating the central responses induced by neurokinin A (NKA). The peptides, substance P (SP), NKA and senktide and the antagonists were injected intracerebroventricularly (ICV) through an implanted cannula. NKA (50 pmol) was as potent as SP (50 pmol) in inducing grooming behaviour (face washing and hind limb grooming) in wild-type mice, but both peptides failed to induce behavioural responses in NK(1)R-/- mice. In wild-type mice, the NK(1) receptor antagonist, RP 67580 (2 nmol), effectively inhibited grooming behaviour elicited by SP, but was inactive against grooming induced by NKA, which in turn was abolished after pre-treatment with the selective NK(2) receptor agonist, SR 48968 (2 nmol). Unlike NKA, the selective NK(2) receptor agonists, (β Ala(8)) NKA 4-10 and (NLeu(10)) NKA 4-10, injected ICV at doses of 50 or 100 pmol did not elicit any behavioural response in wild-type mice. The NK(3) receptor antagonist, SR 142801, inhibited behaviours induced by the NK(3) receptor agonist, senktide, but did not alter behavioural responses to either SP or NKA in wild-type mice. The present findings demonstrate that central biological actions of SP and senktide are mediated by activation of NK(1) and NK(3) receptors, respectively. Our results also indicate that NK(1) receptors are essential for generating central actions induced by NKA, which are most probably mediated by a cross-talk between the NK(1) and NK(2) receptors.

Ablation of least shrew central neurokinin NK1 receptors reduces GR73632-induced vomiting

The neurocircuitry mediating the emetic reflex is still incompletely understood, and a key question is the degree to which central and/or peripheral components contribute to the overall vomiting mechanism. Having previously found a significant peripheral component in neurokinin NK-receptor mediated emesis, the authors undertook this study to examine the putative central component. Adult least shrews were injected intracerebroventricularly (icv) with saline or the blood-brain barrier impermeable toxin, stable substance P-saporin (SSP-SAP), which ablates cells expressing NK receptors. After 3 days, shrews were challenged intraperitoneally with the emetogenic NK agonist GR73632 at different doses, and vomiting and scratching behaviors were quantified. Ablation of NK1-bearing cells was verified immunohistochemically. Although SSP-SAP injection reduced emesis at GR73632 doses of 2.5 and 5 mg/kg, no injections completely eliminated emesis. These data demonstrate that there is both a major central nervous system component and a minor peripheral nervous system component to tachykinin-mediated vomiting. Side effects of the current generation of antiemetics could potentially be reduced by improving bioavailability of the drugs in the more potent central nervous system compartment while reducing bioavailability in the less potent peripheral compartment.

Utilization of the least shrew as a rapid and selective screening model for the antiemetic potential and brain penetration of substance P and NK1 receptor antagonists

Substance P (SP) is thought to play a cardinal role in emesis via the activation of central tachykinin NK1 receptors during the delayed phase of vomiting produced by chemotherapeutics. Although the existing supportive evidence is significant, due to lack of an appropriate animal model, the evidence is indirect. As yet, no study has confirmed that emesis produced by SP or a selective NK1 receptor agonist is sensitive to brain penetrating antagonists of either NK1, NK2, or NK3 receptors. The goals of this investigation were to demonstrate: 1) whether intraperitoneal (i.p.) administration of either SP, a brain penetrating (GR73632) or non-penetrating (e.g. SarMet-SP) NK1 receptor agonist, an NK2 receptor agonist (GR64349), or an NK3 receptor agonist (Pro7-NKB), would induce vomiting and/or scratching in the least shrew (Cryptotis parva) in a dose-dependent manner; and whether these effects are sensitive to the above selective receptor antagonists; 2) whether an exogenous emetic dose of SP (50 mg/kg, i.p.) can penetrate into the shrew brain stem and frontal cortex; 3) whether GR73632 (2.5 mg/kg, i.p.)-induced activation of NK1 receptors increases Fos-measured neuronal activity in the neurons of both brain stem emetic nuclei and the enteric nervous system of the gut; and 4) whether selective ablation of peripheral NK1 receptors can affect emesis produced by GR73632. The results clearly demonstrated that while SP produced vomiting only, GR73632 caused both emesis and scratching behavior dose-dependently in shrews, and these effects were sensitive to NK1-, but not NK2- or NK3-receptor antagonists. Neither the selective, non-penetrating NK1 receptor agonists, nor the selective NK2- or NK3-receptor agonists, caused a significant dose-dependent behavioral effect. An emetic dose of SP selectively and rapidly penetrated the brain stem but not the frontal cortex. Systemic GR73632 increased Fos expression in the enteric nerve plexi, the medial subnucleus of nucleus tractus solitarius, and the dorsal motor nucleus of the vagus, but not the area postrema. Ablation of peripheral NK1 receptors attenuated the ability of GR73632 to induce a maximal frequency of emesis and shifted its percent animals vomiting dose-response curve to the right. The NK1-ablated shrews exhibited scratching behavior after systemic GR73632-injection. These results, for the first time, affirm a cardinal role for central NK1 receptors in SP-induced vomiting, and a facilitatory role for gastrointestinal NK1 receptors. In addition, these data support the validation of the least shrew as a specific and rapid behavioral animal model to screen concomitantly both the CNS penetration and the antiemetic potential of tachykinin NK1 receptor antagonists.

Centrally administered hemokinin-1 (HK-1), a neurokinin NK1 receptor agonist, produces substance P-like behavioral effects in mice and gerbils

Hemokinin-1 (HK-1) is a recently described mouse tachykinin peptide whose biological functions are not fully understood. To date, a unique receptor for HK-1 has not been identified. Recent studies suggest HK-1 may have a role in immunological functions, but there has been little characterization of HK-1's effects in the central nervous system (CNS). In the present studies, we confirm that HK-1 is an endogenous agonist at all of the known tachykinin receptors, and is selective for the NK1 receptor over the NK2 and NK3 subtypes. CHO cells transfected with the human NK1 receptor released intracellular calcium in response to HK-1. In addition, HK-1 competed with substance P (SP) for binding to mouse NK1 and human NK1 receptors. In vivo central administration of HK-1 to gerbils and mice induced foot-tapping and scratching behaviors, respectively, similar to those observed following central administration of SP or the NK1 receptor agonist, GR-73632. Furthermore, these behavioral effects were blocked by the selective NK1 receptor antagonist, MK-869. Finally, a comprehensive expression analysis of HK-1 demonstrated that HK-1 mRNA is much more broadly expressed than previously reported with expression observed in many brain regions. Together these data demonstrate that HK-1 is a functional agonist at NK1 receptors and suggest that HK-1 may function both centrally and peripherally.

The neurobiology and control of anxious states

Fear is an adaptive component of the acute "stress" response to potentially-dangerous (external and internal) stimuli which threaten to perturb homeostasis. However, when disproportional in intensity, chronic and/or irreversible, or not associated with any genuine risk, it may be symptomatic of a debilitating anxious state: for example, social phobia, panic attacks or generalized anxiety disorder. In view of the importance of guaranteeing an appropriate emotional response to aversive events, it is not surprising that a diversity of mechanisms are involved in the induction and inhibition of anxious states. Apart from conventional neurotransmitters, such as monoamines, gamma-amino-butyric acid (GABA) and glutamate, many other modulators have been implicated, including: adenosine, cannabinoids, numerous neuropeptides, hormones, neurotrophins, cytokines and several cellular mediators. Accordingly, though benzodiazepines (which reinforce transmission at GABA(A) receptors), serotonin (5-HT)(1A) receptor agonists and 5-HT reuptake inhibitors are currently the principle drugs employed in the management of anxiety disorders, there is considerable scope for the development of alternative therapies. In addition to cellular, anatomical and neurochemical strategies, behavioral models are indispensable for the characterization of anxious states and their modulation. Amongst diverse paradigms, conflict procedures--in which subjects experience opposing impulses of desire and fear--are of especial conceptual and therapeutic pertinence. For example, in the Vogel Conflict Test (VCT), the ability of drugs to release punishment-suppressed drinking behavior is evaluated. In reviewing the neurobiology of anxious states, the present article focuses in particular upon: the multifarious and complex roles of individual modulators, often as a function of the specific receptor type and neuronal substrate involved in their actions; novel targets for the management of anxiety disorders; the influence of neurotransmitters and other agents upon performance in the VCT; data acquired from complementary pharmacological and genetic strategies and, finally, several open questions likely to orientate future experimental- and clinical-research. In view of the recent proliferation of mechanisms implicated in the pathogenesis, modulation and, potentially, treatment of anxiety disorders, this is an opportune moment to survey their functional and pathophysiological significance, and to assess their influence upon performance in the VCT and other models of potential anxiolytic properties.

Neurokinin(1) receptor antagonists as potential antidepressants

Selective, nonpeptide antagonists for tachykinin receptors first became available ten years ago. Of the three known tachykinin receptors, drug development has focused most intensively on the substance P-preferring receptor, neurokinin(1) (NK(1)). Although originally studied as potential analgesic compounds, recent evidence suggests that NK(1) receptor antagonists may possess antidepressant and anxiolytic properties. If confirmed by further controlled clinical studies, this will represent a mechanism of action distinct from all existing antidepressant agents. As reviewed in this chapter, the existing preclinical and clinical literature is suggestive of, but not conclusive, concerning a role of substance P and NK(1) receptors in the pathophysiology of depression and/or anxiety disorders. The ongoing clinical trials with NK(1) receptor antagonists have served as an impetus for much needed, basic research in this field.

Different subtypes of tachykinin NK(1) receptor binding sites are present in the rat brain

(2-[(125)I]iodohistidyl(1))Neurokinin A ([(125)I]NKA), which labels "septide-sensitive" but not classic NK(1) binding sites in peripheral tissues, was used to determine whether septide-sensitive binding sites are also present in the rat brain. Binding studies were performed in the presence of SR 48968 (NK(2) antagonist) and senktide (NK(3) agonist) because [(125)I]NKA also labels peripheral NK(2) binding sites and, as shown in this study, central NK(3) binding sites. [(125)I]NKA was found to label not only septide-sensitive binding sites but also a new subtype of NK(1) binding site distinct from classic NK(1) binding sites. Both subtypes of [(125)I]NKA binding sites were sensitive to tachykinin NK(1) antagonists and agonists but also to the endogenous tachykinins NKA, neuropeptide K (NPK), and neuropeptide gamma (NPgamma). However, compounds of the septide family such as substance P(6-11) [SP(6-11)] and propionyl-[Met(O(2))(11)]SP(7-11) and some NK(1) antagonists, GR 82334, RP 67580, and CP 96345, had a much lower affinity for the new NK(1)-sensitive sites than for the septide-sensitive sites. The hypothalamus and colliculi possess only this new subtype of NK(1) site, whereas both types of [(125)I]NKA binding sites were found in the amygdala and some other brain structures. These results not only explain the central effects of septide or SP(6-11), but also those of NKA, NPK, and NPgamma, which can be selectively blocked by NK(1) receptor antagonists.

Action sequencing is impaired in D1A-deficient mutant mice

The role of dopamine in the production of behaviour is multifarious in that it can influence different aspects of movement (e.g. movement initiation, sensorimotor integration, and movement sequencing). A characteristic of the dopamine system which seems to be critical for the expression of this diverse influence is its varied receptor population. Previous studies have shown that specific receptor subtype activation leads to specific behavioural responses or alterations of selective aspects of movement. It is known that one of the important influences of dopamine includes sequential co-ordination of 'syntactic' patterns of grooming movements because moderate loss of the dopaminergic nigrostriatal projections specifically disrupts these patterns without affecting grooming actions in a general fashion (Berridge, K.C. Psychobiology, 15, 336, 1989). The specific receptors of the dopamine family which play a key part in this co-ordination of movement sequences is not known. In the present study, we examined the serial order of particular syntactic sequences or chains of grooming actions in mice lacking D1A receptors to explore the relationship between this receptor subtype and movement sequencing. Mutant mice had shorter grooming bouts and a disruption of the organization of sequential patterns compared with wild-type littermate controls. Sequential disruption was reflected in the failure of D1A mutants to follow the syntactic pattern of grooming to completion. This sequential disruption deficit appeared to be specific, as mutant mice initiated more syntactic chains than wild-type controls even though they were less likely to complete them. These results support the hypothesis that D1A receptor activation plays a part in the sequencing of natural action. This conclusion has important implications for the understanding of the functional heterogeneity of dopamine receptor subtypes and of the aetiology of symptoms observed in patients with basal ganglia disease.

Small sensory neurons in the rat dorsal root ganglia express functional NK-1 tachykinin receptor

The tachykinins substance P (SP) and neurokinin A, released by the C-type primary afferent fibre terminals of the small dorsal root ganglion (DRG) neurons, play important roles in spinal nociception. By means of non-radioactive in situ hybridization and whole-cell recording, we showed that the small rat DRG neurons also express the NK-1 tachykinin receptor. In situ hybridization demonstrated that the positive neurons in rat DRG sections were mainly small cells (85.9%) with diameters less than 25 microm. The remaining positive neurons (14.1%) were cells with medium diameters between 26 and 40 microm. No positive large neurons (diameters > 40 microm) were observed. Expression in small DRG neurons (diameter < 21 microm) was confirmed by in situ hybridization of isolated cells, which were demonstrated to express NK-1 receptor mRNA at a very high frequency (> 90% of small DRG neurons) and therefore were subjected to whole-cell recording. In 57 of 61 cells recorded, SP or the selective NK-1 receptor agonist [Sar9, Met(O2)11]SP (Sar-SP, 1 or 2 microM) produced a delayed vibrating inward current (50-300 nA) with a long duration of 0.5-2 h. These currents were blocked by co-application of the NK-1 receptor antagonist L-668, 169 (1 microM), but were not affected by the NK-2 antagonist L-659, 877 (2 microM). Both current-clamp recording and cell-attached single-channel recording demonstrated that the long-lasting response was due to the opening of a channel with an inward current. Employment of non-Ca2+ and Ca2+ + choline solutions revealed that this channel might be a Ca2+-permeable, non-selective cation channel. The prolonged NK-1 tachykinin response exhibited extreme desensitization. This work suggests that presynaptic NK-1 autoreceptors may be present on the primary afferent terminals in the spinal cord, where they could contribute to the chronic pain and hyperalgesia.

Spinal calmodulin inhibitors reduce N-methyl-D-aspartate- and septide-induced nociceptive behavior

The effect of two calmodulin inhibitors, W-7 (N-(6-aminohexyl)-5-chloro-1-naphtalenesulfonamide) and calmidazolium, on the nociceptive behavior induced by the intrathecal injection of NMDA (N-methyl-D-aspartate), AMPA (alpha-amino-3-hydroxy-5-methyl-4-iso xazolepropionic acid) or of septide is described. Lumbar intrathecal injection of NMDA, AMPA or septide induced a caudally directed nociceptive reaction (biting, scratching and licking). The nociceptive behavior induced by NMDA (4 microg) was dose dependently inhibited when W-7 (0.25-1 micromol/rat) or calmidazolium (0.12-0.5 micromol/rat) was coinjected. Biting, scratching and licking produced by AMPA (2 microg) were unaffected by intrathecal calmodulin inhibitors. Finally, septide-evoked nociceptive behavior (2 microg) was antagonized by W-7 (0.12-0.5 micromol/rat) and calmidazolium (0.06-0.25 micromol/rat). Thus, calmodulin inhibitors prevent the nociceptive reaction evoked by drugs that modify intracellular Ca2+, NMDA and septide, without affecting the nociceptive response induced by AMPA, for which Ca2+ is not the main second messenger.

Further evidence for the involvement of tachykinin receptor subtypes in formalin and capsaicin models of pain in mice

The intradermal (i.d.) injection of NK1 receptor antagonists GR 82334 and FK 888 (1-50 pmol/paw), in association with formalin, produced graded inhibition of the early but not the late phase of the formalin test. The NK2, SR 48968 and NK3 SR 142801 receptor antagonists (1-50 pmol/paw) were effective in inhibiting both phases of the formalin model. Co-injection of NK1, (FK 888, GR 82334), NK2 (SR 48968) or NK3 (SR 142801) receptor antagonists with capsaicin dose-dependently attenuated capsaicin-induced licking. In addition, all antagonists were more efficacious when compared with response in the formalin test. The antinociception caused by i.d. injection of the NK3 receptor antagonist SR 142801 against both phases of the formalin test, but not that of NK1 and NK2 receptor antagonists, was significantly reversed by intraperitoneal (i.p.) injection of naloxone (5 mg/kg). Intracerebroventricular (i.c.v.) injection of NK1, NK2 or NK3 receptor antagonists (15-500 pmol/site), all produced significant and dose-dependent inhibition of both phases of the formalin and capsaicin tests. With the exception of the response of SR 48968, which was equipotent in both models of nociception, FK 888, GR 82334 and SR 142801 were about 2-25-fold less potent at the ID50 level against the capsaicin-induced pain. The antinociception caused by i.c.v. injection of NK1, NK2 or NK3 receptor antagonists was reversed by i.p. injection of naloxone (5 mg/kg). These results indicate that tachykinin receptor antagonists, acting through NK1, NK2 and NK3 receptors, produce powerful antinociception when injected i.d. or by i.c.v. route against both formalin- and capsaicin-induced licking, being more efficacious against the latter model of nociception. The action of NK3 receptor antagonist given i.d. was mediated through an opioid mechanism sensitive to naloxone. However, when injected i.c.v., the antinociception caused by NK1, NK2 or NK3 receptor antagonists was largely reversed by naloxone when assessed in the formalin test, suggesting a distinct mechanism of action.

Effects of central administration of tachykinin receptor agonists and antagonists on plus-maze behavior in mice

This study assessed the effects of intracerebroventricular administration of selective agonists and antagonists for tachykinin NK1 and NK2 receptors on performance of mice in the elevated plus-maze, an ethological model of anxiety. Mice were treated with either vehicle (5 microliters) or 1, 10, 100 or 500 pmol of substance P, neurokinin A, the selective NK1 receptor agonist substance P methyl ester, or the selective NK2 receptor agonist, [beta-Ala8]neurokinin A-(4-10). Other mice received similar doses of FK 888, i.e., N2-[(4R)-4-hydroxy-1-(1-methyl-1 H-indol-3-y)carbonyl-L-prolyl]-N-methyl-N-phenylmethyl-3-(2-naphthyl)-L- alaninamide, or SR 48968, i.e., (S)-N-methyl-(N-[4-acetylamine-4-phenylpiperidine)-2-(3, 4-dichlorophenyl)buthyl]benzamide, selective antagonists of tachykinin NK1 and NK2 receptors, respectively. Injections of substance P, neurokinin A, substance P methyl ester or [beta-Ala8]neurokinin A-(4-10) significantly reduced the frequency of open arm entries, and [beta-Ala8]neurokinin A-(4-10) also enhanced the percentage of entries into enclosed arms. Conversely, the NK1 antagonist FK 888 and the NK2 antagonist SR 48968 each increased the time spent in the open arms, and SR 48968 also increased the frequency of entries into the open arms. None of the tachykinin receptor agonists or antagonists modified motor performance and coordination on the rotarod apparatus or ambulation in an activity cage. Together, these results suggest that centrally administered NK1 and NK2 receptor agonists and antagonists can modulate anxiety, as evaluated in the elevated plus-maze test in mice. Stimulation of either tachykinin NK1 or NK2 receptors induces anxiogenic-like responses, whereas the reverse occurs following their blockade. The anxiolytic-like profiles of action of both tachykinin NK1 and NK2 receptor antagonists suggest that central tachykinin mechanisms are tonically involved in the modulation of anxiety.

Distinct regulations by septide and the neurokinin-1 tachykinin receptor agonist [pro9]substance P of the N-methyl-D-aspartate-evoked release of dopamine in striosome- and matrix-enriched areas of the rat striatum

The effects of septide (a short substance P C-terminal analogue) and of the neurokinin-1 receptor agonist [Pro9]substance P on the N-methyl-D-aspartate (50 microM)-evoked release of [3H]dopamine (continuously synthesized from [3H]tyrosine) were investigated in the absence or the presence of the selective neurokinin-1 receptor antagonist RP 67580 in selected striosome- and matrix-enriched areas of the rat striatum. Experiments were performed in vitro using a microsuperfusion procedure described previously. At a concentration of 0.1 microM, septide and [Pro9]substance P stimulated the spontaneous release of [3H]dopamine in striosome-enriched areas similarly. However, in this compartment, these peptides induced larger and opposite effects on the N-methyl-D-aspartate (50 microM)-evoked release of [3H]dopamine (estimated in the absence of magnesium). Indeed, septide markedly enhanced the N-methyl-D-aspartate response, while [Pro9]substance P largely reduced the N-methyl-D-aspartate-evoked release of [3H]dopamine. Septide also enhanced the N-methyl-D-aspartate response in the matrix, but [Pro9]substance P was without effect. When used alone, at 0.1 or 1 microM, RP 67580 reduced by about 33% the N-methyl-D-aspartate-evoked release of [3H]dopamine in striosome-enriched areas. In contrast, in the matrix, the N-methyl-D-aspartate response was enhanced in the presence of a low concentration of the antagonist, while the higher concentration was ineffective. In striosomes, the reducing effect of [Pro9]substance P and the enhancing action of septide on the N-methyl-D-aspartate response were respectively blocked in the presence of low and high concentrations of RP 67580, while the stimulatory effect of septide on the N-methyl-D-aspartate response in the matrix was prevented with both concentrations of the neurokinin-1 receptor antagonist. Finally, the co-application of [Pro9]substance P (0.1 microM) with septide (0.1 microM) abolished the enhancing effect of septide on the N-methyl-D-aspartate-evoked release of [3H]dopamine in both striatal compartments. Altogether, these results suggest that substance P and eventually one of its metabolites, substance P(6-11) or another endogenous tachykinin released under the action of N-methyl-D-aspartate, contribute to the regulation of [3H]dopamine release in both striatal compartments. They also extend previous observations which allowed us to demonstrate that the local circuits contributing to the presynaptic regulation of [3H]dopamine release differ in striosome- and matrix-enriched areas. Furthermore, in agreement with observations made in some peripheral tissues, the present results support the existence of "septide-sensitive" tachykinin receptors in the rat striatum or alternatively of septide sensitive sites on tachykinin neurokinin-1 receptors distinct from those sensitive to neurokinin-1 receptor agonists, coupled to distinct transducing systems, and thus leading to biological responses which differ from those evoked by neurokinin-1 receptor agonists.

Comparative behavioural profile of centrally administered tachykinin NK1, NK2 and NK3 receptor agonists in the guinea-pig

1. The NK1 tachykinin receptor agonists, septide, [Sar9,Met(O2)11]SP and [Pro9]SP produced locomotor hyperactivity (10-20 min) when injected intracerebroventricularly (i.c.v.) in the guinea-pig. The most potent in eliciting this hyperactivity was septide (from 0.63 to 5 micrograms), compared to [Sar9,Met(O2)11]SP, which was active at 2.5 and 5 micrograms and [Pro9]SP which induced a non-significant increase even at 10 micrograms. 2. Wet-dog shakes were elicited by septide, [Sar9,Met(O2)11]SP and [Pro9]SP injected by the i.c.v. route in the guinea-pig. [Sar9,Met(O2)11]SP, active from 0.16 to 2.5 micrograms was more potent than septide (active at 1.25 micrograms) and [Pro9]SP (active at 0.63 micrograms) in eliciting such behaviour. To a lesser extent, grooming was also observed after injection of these agonists. 3. The NK2 tachykinin receptor agonist, [Lys5,MeLeu9,Nle10]NKA(4-10), up to the dose of 10 micrograms i.c.v. had no effect in the guinea-pig. It neither modified locomotor activity nor induced a characteristic behavioural response. At higher doses (20 micrograms), some toxic effects were noted. 4. The NK3 tachykinin receptor agonist, senktide, contrasts with the NK1 receptor agonists in that it elicited only wet-dog shakes, at doses ranging from 0.32 to 1.25 micrograms. It neither modified locomotor activity (1 microgram) nor induced grooming (up to 5 micrograms) in the guinea-pig. 5. To our knowledge, these results are the first demonstration that the guinea-pig could be useful to differentiate tachykinin agonists on the basis of their behavioural profile, distinct from those obtained in mice and rats.